OK, lets look at some numbers

[crobin4]

A GTM weighs 2450lbs. with a 505hp zo6 engine it has a P/W ratio of 4.8 to one.
0-60 is 3.0 sec. 0-100 is 6.6 sec. skidpad 1.05g
with a 405hp engine it has a P/W ratio of 6.0 to one
The GTM is awesomely fast.

The 818's target weight is 1800lbs. It has a gearing advantage over the GTM, as well.
a 400hp engine will give it a P/W ratio of 4.5 to 1
a 375hp engine will give it a P/W ratio of 4.8 to 1
a 300hp engine will give it a P/W ratio of 6.0 to 1
a 265hp engine yields 6.8 to 1
With a 375hp engine it will accelerate faster , turn faster, and brake faster than the GTM "deluxe" , unless it's neutered at the design stage.
I don't think of the 818 as a street machine. I think of it as a track/autocross weapon that you can drive on the street.
This most certainly not a toy car.

[riptide motorsport]

best of both worlds I believe...............Steven

[PhyrraM]

I wonder what a 'safe and sane' level of power for 1800 pounds really is? While I've driven competitively at rallycross and autocross, neither was in a really high power-to-weight ratio car.

Years ago I had a MX-3. FWD, 2500 pounds, V-6, 2.5 liters. About 180HP. While it was certainly safe, if was difficult to control wheelspin. So much so, the fun was lost at times. (Yes, I know. FWD)

I've had a 2011 5.0 Mustang for a rental a few times. While still what I would call safe, if your not careful when the traction control is turned off you will pay for it. That thing is about 3800 pounds @ 400HP. And a pig too.

I don't really think there is anything to be worried about, but I hope that 250ish HP in the 818 is still street cruise-able. I don't want a car that is so fast and twitchy that all the 'street fun' has been sucked out of it and can only be enjoyed at the track.

[Benji]

I've had a 2011 5.0 Mustang for a rental a few times.

I'm sorry I'm going to have to de-rail this thread for a second.

Where did you get a rental 5.0?! When I was over there last year for work I got a brand new 2010/2011 V6 Mustang but no where seemed to have the 5.0!

I'm moving over permanently sometime in June/July and want to rent a 5.0 for a month or so to see if I really do want one :D

[crobin4]

I wonder what a 'safe and sane' level of power for 1800 pounds really is? While I've driven competitively at rallycross and autocross, neither was in a really high power-to-weight ratio car.

Years ago I had a MX-3. FWD, 2500 pounds, V-6, 2.5 liters. About 180HP. While it was certainly safe, if was difficult to control wheelspin. So much so, the fun was lost at times. (Yes, I know. FWD)

I've had a 2011 5.0 Mustang for a rental a few times. While still what I would call safe, if your not careful when the traction control is turned off you will pay for it. That thing is about 3800 pounds @ 400HP. And a pig too.

I don't really think there is anything to be worried about, but I hope that 250ish HP in the 818 is still street cruise-able. I don't want a car that is so fast and twitchy that all the 'street fun' has been sucked out of it and can only be enjoyed at the track.
Me neither if ends up a bit much you can always turn down the boost (especially at 2500-4000 rpm) so you don't get the wall of torque as lean into the throttle. Spring Rates and alignment will be the contributors to "twitchiness" , those can be changed also.

[PhyrraM]

I'm sorry I'm going to have to de-rail this thread for a second.

Where did you get a rental 5.0?! When I was over there last year for work I got a brand new 2010/2011 V6 Mustang but no where seemed to have the 5.0!

I'm moving over permanently sometime in June/July and want to rent a 5.0 for a month or so to see if I really do want one :D

Avis at the Ontario, California airport. Most of the airport Avis and Hertz have specialty cars, at least here in California.

[Benji]

Avis at the Ontario, California airport. Most of the airport Avis and Hertz have specialty cars, at least here in California.


Home this was Avis in San Antonio airport, they only had the V6's in :(

Thanks anyway, I'll check with them when I move.

[bromikl]

A 4.5 P/W ratio is nothing new to anyone with a sportbike.

My Buell weighs 400 lbs (dry) plus my 175 lbs weighs about 575 lbs. Stock Hp is 103 Hp. That's for an air-cooled, fuel-injected, 1200cc twin-cylinder engine with emissions controls. Some people have added turbos and other mods bringing up Hp to the 140 - 150 range.

Power-to-weight for those turbo monsters can be a 3.8. My 100% stock bike is about a 5.6 - and that's on two wheels! You can't spin wheels or skid without dumping it, so you learn to pay close attention to acceleration - starting, stopping and turning.

My stock bike does 50 MPH in 1st gear and four seconds.

Is a car that performs like a bike "safe"? Depends on the driver. I'm afraid the "safety" of having a body and a frame around you will encourage more reckless behavior. Though it's not a deal-breaker, I feel AWD is much more appropriate (and safer, AND more fun) when you have power-to-weight numbers in the 4's and 5's.

[crobin4]

A 4.5 P/W ratio is nothing new to anyone with a sportbike.

My Buell weighs 400 lbs (dry) plus my 175 lbs weighs about 575 lbs. Stock Hp is 103 Hp. That's for an air-cooled, fuel-injected, 1200cc twin-cylinder engine with emissions controls. Some people have added turbos and other mods bringing up Hp to the 140 - 150 range.

Power-to-weight for those turbo monsters can be a 3.8. My 100% stock bike is about a 5.6 - and that's on two wheels! You can't spin wheels or skid without dumping it, so you learn to pay close attention to acceleration - starting, stopping and turning.

My stock bike does 50 MPH in 1st gear and four seconds.

Is a car that performs like a bike "safe"? Depends on the driver. I'm afraid the "safety" of having a body and a frame around you will encourage more reckless behavior. Though it's not a deal-breaker, I feel AWD is much more appropriate (and safer, AND more fun) when you have power-to-weight numbers in the 4's and 5's.

Yep, depends on the driver. With power, the necessity for skill and common sense becomes paramount. Unfortunately, our culture breeds neither now a days.
If one has a vehicle with extraordinary capabilities, it's up to the owner to develop the skills to use it properly in what ever situation they find themselves in.

[Gollum]

The veyron must be dangerous as hell!!!

In all seriousness, a lot of how fun and safe the car is to drive at those lower power/weight ratios of around 4.5-3.0 has to do with the tune as much as anything. There's a lot you can do these days regarding engine tune, and you can program boost curves if you want to get real fancy. You can even make boost targets dynamic based upon pedal position so that 0-50% throttle opens the wastegate at 0-5psi, 50-75% shoots up to 10psi, while reserving the last 25% of pedal travel for full boost for those nice long straights.

That's the beauty of turbos. They're completely dynamically controllable and basically turn off when not in use.

I'll be expecting some Veyron performance figures pretty swiftly after these things start getting built by people. Sub 2.5 0-60 figures are within reason with enough tire I suspect.

Oh, and back on the safety thing real quick, I've driven cars which have flirted with 9's in the 1/4 mile that could be driven by pretty much anyone. Full interior, heat/AC, easy to get in and out of, and perfectly tame on the street. The right turbo for your goals makes ALL the difference.

[bromikl]

Oops - I did my P/W calculations wrong. I added the rider, which isn't the case when calculating P/W on four wheels.

My bike is a 3.9
Modified bikes: 2.7

[PhyrraM]

This conversation reminds me of an old car guy saying:

"It's more fun to drive a slow car fast, than a fast car slow."

Not that we want slow cars.....;)

[BrandonDrums]

The veyron must be dangerous as hell!!!

In all seriousness, a lot of how fun and safe the car is to drive at those lower power/weight ratios of around 4.5-3.0 has to do with the tune as much as anything. There's a lot you can do these days regarding engine tune, and you can program boost curves if you want to get real fancy. You can even make boost targets dynamic based upon pedal position so that 0-50% throttle opens the wastegate at 0-5psi, 50-75% shoots up to 10psi, while reserving the last 25% of pedal travel for full boost for those nice long straights.

That's the beauty of turbos. They're completely dynamically controllable and basically turn off when not in use.

I'll be expecting some Veyron performance figures pretty swiftly after these things start getting built by people. Sub 2.5 0-60 figures are within reason with enough tire I suspect.

Oh, and back on the safety thing real quick, I've driven cars which have flirted with 9's in the 1/4 mile that could be driven by pretty much anyone. Full interior, heat/AC, easy to get in and out of, and perfectly tame on the street. The right turbo for your goals makes ALL the difference.

Yeah, the stock Wrx wastegate and turbo dynamics tables in the ECU allow you to control and target boost really easy based on throttle% and rpm.

However turbo spool is somewhat a function of engine load, 1st and 2nd gear don't often see 100% of your max target boost because the engine is swallowing air since it increases revs faster than the turbo can. The STI ECU allows for gear-specific wastegate values to counter some of the lack of spool in lower gears and over-boosting risk in higher gears. But each turbo has it's boost 'signature' which is pretty difficult to change even with all sorts of trickery in wastegate duty to make numbers.

With this car being so lightweight, I'm interested to see how the turbo selection process works out. Shooting for 375-400 hp in this car might make for a pretty laggy torque curve given the car simply won't have the inertia to build turbo spool in lower gears. To make big power you need a big turbo. But a big turbo takes time to spool. Something the size of an 18G or so would probably be about as laggy as most people would want in a car this light I imagine. Above that and people might end up having to sacrifice some of the boost/torque smoothing tactics in order to try and make 375hp+ worth of boost before slapping into the rev-limiter in gears 1-3, doing so will give you the twitchy wall of torque effect.

So, with that in mind, a smaller turbo that hits target boost sooner but might not have as much top end would be easier to tune for. Having a snail that CAN hit target boost levels of 18+psi in the low to mid 3k range would have enough time for a factory WRX ECU to "smooth" the torque delivery and reduce the power surge that risks spinning this car out of control.

I guess this brings back the H6 discussion to some extent. This Turbo lag and turbo power surge issue with just RWD will become a giant topic of debate and discussion. An N/A H6 will become even more attractive as people get competitive in this platform.

Just my take, I hate turbo lag but some guys don't mind waiting until 4000-4500 rpm for power. I prefer it much....much sooner than that.

[0100]

^I agree. With the weight of the car a NA engine is almost a better choice. Really have no experience with a H6. How much hp can be had from these?

I don't think some realize how fast this car is going to be.

A Radical SR8 has a PWR of 3.4 and laps Nordschleife in 6:55

A ZR1 has a PWR of 5.2 and laps Nordschleife in 7:26

Just to give you an idea how bananas this car is going to be. :)

[Wayne Presley]

Oops - I did my P/W calculations wrong. I added the rider, which isn't the case when calculating P/W on four wheels.

My bike is a 3.9
Modified bikes: 2.7


It's pretty wild having a car with a sub 4lb/hp, really keeps you attention. My FFR roadster weighs 2521 and makes 830ish FWHP (705 RWHP) for a 3.03 lbs/HP and 3.26lbs/HP with me in it. The Lotus Elise with a turbo that I have in the shop is putting out
255 FWHP and weighs 1993 for a P/W of 7.8lbs and its a blast to drive

[BrandonDrums]

^I agree. With the weight of the car a NA engine is almost a better choice. Really have no experience with a H6. How much hp can be had from these?

I don't think some realize how fast this car is going to be.

A Radical SR8 has a PWR of 3.4 and laps Nordschleife in 6:55

A ZR1 has a PWR of 5.2 and laps Nordschleife in 7:26

Just to give you an idea how bananas this car is going to be. :)

I'm sure you can eek out close to 300 hp from an N/A H6. The guys who have done the swap that I know of get around 280 hp and 275tq. The difference is the torque is available from 1500 rpm on just about.

You can make a lot more power from the turbo 4 easy but the number that's more important is average hp as well as having good throttle response.

For example,
With an over-boosted car, HP can vary quite a bit depending on where the engine is revving when you change gears. For example, I can hit target boost in 2nd gear on my wrx if I give it the full beans while the engine sits at 3k or less RPM. If I downshift from 3rd in a roll where the engine is sitting at 3500 or 4k rpm I won't come anywhere close to full boost in 2nd and my acceleration is noticeably slower.

Now I could get a bigger turbo to get around that problem but then I face lag in the first situation where I downshift into 2nd and sit at 3k rpm after the shift. I'd need to be in 1st instead of 2nd because I'd have to wait until 3500 or 4k to see full spool and for part of that time I might even see negative pressure in the manifold vs. atmospheric which just makes the engine bog down.

So with this situation I'd want shorter gearing to keep the car in the power band. However, shorter gears mean more shifting, even less engine load in lower gears so again you fight the spool game but have more torque multiplication with the gearing to get around some of the lag issue.

You don't have that issue with an N/A car to that extent. I'm pretty sure that's why so many Porche's use a N/A engine, put out less HP than another similarly priced supercar but end up spanking the competition. They just make their torque and power everywhere along the band, and that's a big bonus.

[Bob-O]

hey, wasn't there a certain Mr. Chapman that made cars famous for being light weight? (Lotus of course)

[bromikl]

I've never had a car with a turbo. But I often see discussions about spool, boost, and RPM. From what I understand, it seems a tricky task to get a turbo set up exactly as you want it. I also know a supercharger puts out the same PSI all the time, so balancing everything would appear to be simpler. So I must conclude there are some good reasons people choose a turbo over a supercharger. Anyone care to enlighten me?

Thanks!

[mn_vette]

I've never had a car with a turbo. But I often see discussions about spool, boost, and RPM. From what I understand, it seems a tricky task to get a turbo set up exactly as you want it. I also know a supercharger puts out the same PSI all the time, so balancing everything would appear to be simpler. So I must conclude there are some good reasons people choose a turbo over a supercharger. Anyone care to enlighten me?

Thanks!

You have to be careful here. A roots/twin screw type blower will give constant boost, but a centrifigle supercharger gives boost based on RPM. The big down fall of the supercharger vs the turbo is that the supercharger takes more horsepower to spin the compressor compared to a turbo at the same boost. All though it would be pretty cool to have a twin charged or dual charged setup with a roots style supercharger for low end torque and then run a large turbo once it gets spooled up. There are some people out there running this setup with some WILD numbers out of the engine.

[BrandonDrums]

I've never had a car with a turbo. But I often see discussions about spool, boost, and RPM. From what I understand, it seems a tricky task to get a turbo set up exactly as you want it. I also know a supercharger puts out the same PSI all the time, so balancing everything would appear to be simpler. So I must conclude there are some good reasons people choose a turbo over a supercharger. Anyone care to enlighten me?

Thanks!

I'll take a shot.

There are tradeoffs for both turbochargers and superchargers. Superchargers are mechanically driven by the engine to create boost. The benefit is for the most part they make their boost throughout the RPM range and instantaneously create that boost right along with throttle inputs, however because it's a mechanical linkage they also take a good amount of HP to run on the high-end to create that boost so it limits the overall output more than a turbocharger would.

Super chargers come in a couple different varieties, roots-type and centrifugal type. A roots type like a twin-screw is the most common, they sit right on top of the engine and suck in air directly. A centrifugal type is basically the business end of a turbo that is mechanically driven instead of exhaust driven. A roots type like what mn_vette said gives low-end boost and tends to run out of steam up top. A centrifugal type is like a turbo where it makes boost at high rpm, down low you experience mechanical instead of induction lag which makes centrifugal type superchargers feel similar to turbos

A turbo spins independently from the engine using exhaust to generate the boost. They tend to be much easier on an engine and give their numbers with far less sacrifice to fuel economy and reliability when compared to a supercharger. A turbo multiplies it's output with it's own output, the boost puts more air in the engine, the engine gives out more exhaust, the exhaust spins the turbocharger.

A supercharger doesn't have the benefit to take advantage of it's own output like that, it takes HP to run and somwhere along the curve the HP it takes the engine to spin it will outweigh the HP it creates causing a drop off on the top end for roots-type blowers, and mechanical lag down low for centrifugal type. However, because superchargers are mechanically linked the boost follows the engine instantly and throttle response is preserved.

Turbo's overall are a better and much more cost-effective way to achieve power numbers. However, a Supercharger will allow you to go more balls-out on power while sacrificing less throttle response at the expense of being expensive, much higher in maintenance and much harder on the engine.

There's a good section that summarizes all of this in this wiki article http://en.wikipedia.org/wiki/Supercharger

All in all, I believe you can make higher HP with a supercharger which is why drag engines tend to use those instead of turbos. But for a track car, turbo's allow for more reliable output as a supercharger would eventually eat the engine in most cases.

[bromikl]

I'm getting that a turbo is a positive-feedback loop of pressurized air. Run the turbo faster, and more air goes into the engine => more hot air coming out => spins the turbo faster. With no mechanical linkage, the boost is related to, but not exactly a direct ratio of engine RPM.
The turbo is powered by the difference in engine exhaust pressure (compared to the other end of the tailpipe) - energy which would be lost anyways.

A centrifugal supercharger would operate much like a turbo, but a root supercharger has a more steady boost over the RPM range. The power draw of the superchargers are higher compared to turbos, because they are driven by the crankshaft - so less power is available to the wheels.

Turbos are cheaper to install, but more complicated to operate. Turbos (in most cases) will provide all the boost you could want - and sometimes too much. I'll have to take your word for it that superchargers are more prone to failure. So that explains why people go through the hassle of running turbos over superchargers.

Thank you.

[PhyrraM]

Actually, many OEMs went to (positive displacement) superchargers first. Mainly because of reliability concerns and the fact they mirror a normally aspirated power curve. It's only recently, in this world of CAFE and extreme efficiency requirements, that the turbo is taking on a decided lead. While a turbo is not 'free' power, it uses a lot less than a crank driven PD supercharger. Also, from an OEM cost standpoint, if you have 100HP robbed from the crank to drive a supercharger (vs. turbo) then you have to build the bottom end to withstand 100HP more than you will get out of it.

[mn_vette]

Turbos are cheaper to install, but more complicated to operate. Turbos (in most cases) will provide all the boost you could want - and sometimes too much. I'll have to take your word for it that superchargers are more prone to failure. So that explains why people go through the hassle of running turbos over superchargers.

Thank you.

I wouldn't make these assumptions. There are alot of superchargers that are very cost effective to add instead of a turbo. And the superchargers are less prone to failures depending on how you have it set up because it is not getting blasted with several thousand degrees of exhaust heat. It really depends on the application.

Here is one fairly definate statment though, per pound of boost turbos will give you more horsepower to the wheels. Which is why alot of drag guys use them, and why they are banned from top fuel dragsters(they make enough power already).

[Wayne Presley]

All though it would be pretty cool to have a twin charged or dual charged setup with a roots style supercharger for low end torque and then run a large turbo once it gets spooled up. There are some people out there running this setup with some WILD numbers out of the engine.

I have that in my car, twin turbos feeding a roots SC. Makes 5psi of boost at 1500 RPM and 22psi at 6500

[riptide motorsport]

If lag is a concern , why not just go with 2 small turbos? that should give the power as well as the response...Steven

[Wayne Presley]

Choosing the correct AR housing on the exhaust and going with a ball bearing turbo helps greatly with spool up time. 2 turbos for this small displacement/HP target isn't really feasible.

[Kasmodean]

The flow energy associated with an engine's exhaust is the same regardless of the turbos used. It takes power to compress air and since power-in equals power-out, the potential boost will remain the same. A larger turbo has more rotational inertia, but twin turbos have increased friction.

The reason twin turbo setup are used frequently are for twin setups or sequential. In the case of switching from 1 turbo to 2 faster spooling turbos, the two turbos are probably spooling faster because they output less pressure than the single larger turbo which is okay depending on the application and driver's desires. When going for max boost, generally one large turbo is used.

[Gollum]

^^^ Quality to the point information right there.

The main downside to using a roots twin charge system is that placement could be extremely difficult if the intercooler is sitting on top of the motor.

[bromikl]

I just finished watching the video walk-around of the Mclaren MP4-12C. What an awe-inspiring piece of engineering.

The 3.8L twin-turbo V-8 makes 592 bhp in a car that weighs 2,866 lbs. That gives it a P/W ratio of 4.84 - which we could surpass with the right setup.

The performance numbers of this car are off the charts! Quarter mile: 11 sec. Top speed: 200+. Acceleration: 0-124 MPH in :D 8.9 seconds??

The most impressive number is the stopping distance: from 62MPH to 0 in 98 feet, or seven car lengths. I'm sure this is only possible because of the active rear air brake (spoiler). I'd really like to see this feature. This car could be a McLaren killer!

[Gollum]

An active rear brake would kill the budget I suspect, as it's not the simplest thing to just throw onto a car. But I for one would love to see someone provide one, or even make it myself.

Air brakes are fabulous on the track too, not just in emergency stopping. The air brake requires no traction dedication from the tires, allowing them to focus on what they need to do, corner!

[bromikl]

Re: active air brake: I can't imagine it's that hard to do. A solenoid activated by brake travel should do it. Return force would be the wind pushing it back into place.

[John F]

Air brakes are fabulous on the track too, not just in emergency stopping. The air brake requires no traction dedication from the tires, allowing them to focus on what they need to do, corner!


It would have to be setup as a front and rear wing to work. Otherwise you severly unload the front and upset the car. If it was really worth doing, you would see it in F1 where all the money is spent.

[bromikl]

It would have to be setup as a front and rear wing to work. Otherwise you severly unload the front and upset the car. If it was really worth doing, you would see it in F1 where all the money is spent.

The front is always loaded during braking, as weight shifts forward during deceleration. The air brake would simply add loading to the rear during braking. (You might want that in the corners.)

Random thought to avoid a quick, unpredictable shift in handling: Make air brake travel a direct ratio of brake pedal travel. One possible way to do this: Add second master cylinder under the dash and a slave cylinder under the rear deck. Balance the air brake wing so there is little force felt at the pedal, or use a second power brake booster. (Considered a fifth caliper under the rear deck, but we wouldn't want an air brake malfunction to cause brake failure.)

[Benji]

It would have to be setup as a front and rear wing to work. Otherwise you severly unload the front and upset the car. If it was really worth doing, you would see it in F1 where all the money is spent.

Whilst it's a good general rule of thumb it doesn't necessarily always hold true. Certain things are banned in the rules from F1 that would give a performance advantage.

Anyway, here is the Aeromotive S2 active (split) rear wing, cheap at only 73% the cost of the FFR 818 kit at the target price (I'll let you do the maths):

http://aeromotions.com/products/s-series/

You could go for the non-split version (I'm not entirely convinced about split wings anyway) which is also cheap at only 36.5% of the cost of the FFR 818 kit (again, you do the maths):

http://aeromotions.com/products/r-series/

Additionally I think they also actuate at the same ratio as the pedal travel like bromikl suggests.

[Gollum]

Yea I'm pretty sure air brakes are banned in F1. There's a reason why many of the high dollar supercars are now adding air brakes.

The reason I say I have my doubts is that just the simple hydraulic system and reinforcement of a rear panel/wing could easily add $300 to the kit. I could see it as a possible option, but part of a base kit. Then there also has the be the market for it.

And adding an air brake doesn't load/unload the brakes much at all, because the traction seen at the tires doesn't change unless there's DOWN force due to the air brake. The key to a good air brake is getting that panel as straight as your hydraulic system allows. There's also a good reason most of the cars using air brakes only put them in the rear. Because they're using air friction to stop the car, opposed to ground friction, putting a brake too large up front could make it near impossible to keep the car facing the right way. Any time you brake hard the back end will be wanting to come around on you. By adding a rear brake only the car will naturally pull itself straight, which can actually be a handy safety feature to boot! With air brakes, all of the sudden braking the in the rain becomes an entirely different situation.

[Kasmodean]

Re: active air brake: I can't imagine it's that hard to do. A solenoid activated by brake travel should do it. Return force would be the wind pushing it back into place.

Air brakes are great as they don't depend on the tires, but careful testing/analysis should be done before slapping an air brake on a car. The air brake could kill downforce, increase lift and turn you into a plane in certain situations if not carefully designed.


The front is always loaded during braking, as weight shifts forward during deceleration.

This is incorrect. In general deacceleration on an object, the direction of the torque (hence weight shift) generated by braking on an object depends on the location of the breaking force relative to the location of the CoG. The only reason cars transfer weight forward during breaking is because their braking force interface is the tire & road. This is below the CoG so weight shifts forward.

With an air brake, if the braking force is applied above the CoG then weight transfer is to the back. This is dangerous because unlike tire & road front braking where traction is limited, in extreme cases if the braking force with an air brake is large enough you could flip backwards. Even if you don't flip, you are taking weight off the front tires and that will cause a decreased steering effectiveness. Plus you have to remember, the higher the air brake is mounted the more effective it is. If you lower the air brake or make it small enough to not cause this issue, it might not have enough airflow hence effect to be worth the cost of installing and testing it.

P.S. These are just facts, not a personnal attack :)

[mn_vette]

I'm not sure we would want a hydraulic air brake, It would be a bit of a waste to use is while going through the McDonalds drive through. I would think that you would want some kind of speed limit, on the low side for sure, perhaps on the high side so you don't rip the thing off the back of your car at 150mph.

Perhaps you could have a control solenoid would tip the brake when you have so much pressure in the brakes(hard braking) and were within certain speed perameters. But it would spring back into place as soon as you let up on the brake.

And this is for the racer version only right? I'm not sure I'd want the extra complexity on my street car. Actually I don't really want a big fin on my steet car.

[bbjones121]

Turbo lag is something you learn to deal with over time and learn to love or hate it. Your average joe wil not have much fun with the lag, but an experienced driver along with a good clutch will have a blast.


Yeah, the stock Wrx wastegate and turbo dynamics tables in the ECU allow you to control and target boost really easy based on throttle% and rpm.

However turbo spool is somewhat a function of engine load, 1st and 2nd gear don't often see 100% of your max target boost because the engine is swallowing air since it increases revs faster than the turbo can. The STI ECU allows for gear-specific wastegate values to counter some of the lack of spool in lower gears and over-boosting risk in higher gears. But each turbo has it's boost 'signature' which is pretty difficult to change even with all sorts of trickery in wastegate duty to make numbers.

With this car being so lightweight, I'm interested to see how the turbo selection process works out. Shooting for 375-400 hp in this car might make for a pretty laggy torque curve given the car simply won't have the inertia to build turbo spool in lower gears. To make big power you need a big turbo. But a big turbo takes time to spool. Something the size of an 18G or so would probably be about as laggy as most people would want in a car this light I imagine. Above that and people might end up having to sacrifice some of the boost/torque smoothing tactics in order to try and make 375hp+ worth of boost before slapping into the rev-limiter in gears 1-3, doing so will give you the twitchy wall of torque effect.

So, with that in mind, a smaller turbo that hits target boost sooner but might not have as much top end would be easier to tune for. Having a snail that CAN hit target boost levels of 18+psi in the low to mid 3k range would have enough time for a factory WRX ECU to "smooth" the torque delivery and reduce the power surge that risks spinning this car out of control.

I guess this brings back the H6 discussion to some extent. This Turbo lag and turbo power surge issue with just RWD will become a giant topic of debate and discussion. An N/A H6 will become even more attractive as people get competitive in this platform.

Just my take, I hate turbo lag but some guys don't mind waiting until 4000-4500 rpm for power. I prefer it much....much sooner than that.

[Gollum]

Air brakes are great as they don't depend on the tires, but careful testing/analysis should be done before slapping an air brake on a car. The air brake could kill downforce, increase lift and turn you into a plane in certain situations if not carefully designed.



This is incorrect. In general deacceleration on an object, the direction of the torque (hence weight shift) generated by braking on an object depends on the location of the breaking force relative to the location of the CoG. The only reason cars transfer weight forward during breaking is because their braking force interface is the tire & road. This is below the CoG so weight shifts forward.

With an air brake, if the braking force is applied above the CoG then weight transfer is to the back. This is dangerous because unlike tire & road front braking where traction is limited, in extreme cases if the braking force with an air brake is large enough you could flip backwards. Even if you don't flip, you are taking weight off the front tires and that will cause a decreased steering effectiveness. Plus you have to remember, the higher the air brake is mounted the more effective it is. If you lower the air brake or make it small enough to not cause this issue, it might not have enough airflow hence effect to be worth the cost of installing and testing it.

P.S. These are just facts, not a personnal attack :)

These are all actually very well explained facts too!

If you really wanted to counter the levering act of the air brake above the CoG then you could add a small fromt air brake/spoiler to keep the nose firmly planted. If they're run by the same system it wouldn't take much to fine tune it to find a good balance front/rear.

I for one though, think that if the CoG is low enough, and the roof line is slim enough then the air brake won't lift the front end enough to matter really. The Veyron and the Merc SLR get away with rear only air braking just fine.

Edit: Oh yea, I might be wrong on this, but I think you also have to consider the mount point for the air brake, not the location of the panel itself. The braking force is transmitted to the chassis at the mount point, and that's the point you want as close to the CoG height as possible, to minimizing pitch or dive effect.

[Kasmodean]

Edit: Oh yea, I might be wrong on this, but I think you also have to consider the mount point for the air brake, not the location of the panel itself. The braking force is transmitted to the chassis at the mount point, and that's the point you want as close to the CoG height as possible, to minimizing pitch or dive effect.

Correct. Air pushes on panel, panel pushes on mounting hardware/point, mounting hardware pushes on car frame.

Also the CoG moves up and down as the front and rear suspension travel so you have to make sure the brake is stable in all possible suspension/body positions.

[bbjones121]

All research I did on rebuilding the toyota 2zz engine for a FI install in a lotus elise pointed out that it is actually a turbo that can achieve greater hp than a supercharger. This may not be true for high displacement engines, but I think drag cars use them because they have the traction for high torque off the line, not because they are more powerful. Superchargers would probably be more consistent, which is extremely important when counting milliseconds.


I'll take a shot.

There are tradeoffs for both turbochargers and superchargers. Superchargers are mechanically driven by the engine to create boost. The benefit is for the most part they make their boost throughout the RPM range and instantaneously create that boost right along with throttle inputs, however because it's a mechanical linkage they also take a good amount of HP to run on the high-end to create that boost so it limits the overall output more than a turbocharger would.

Super chargers come in a couple different varieties, roots-type and centrifugal type. A roots type like a twin-screw is the most common, they sit right on top of the engine and suck in air directly. A centrifugal type is basically the business end of a turbo that is mechanically driven instead of exhaust driven. A roots type like what mn_vette said gives low-end boost and tends to run out of steam up top. A centrifugal type is like a turbo where it makes boost at high rpm, down low you experience mechanical instead of induction lag which makes centrifugal type superchargers feel similar to turbos

A turbo spins independently from the engine using exhaust to generate the boost. They tend to be much easier on an engine and give their numbers with far less sacrifice to fuel economy and reliability when compared to a supercharger. A turbo multiplies it's output with it's own output, the boost puts more air in the engine, the engine gives out more exhaust, the exhaust spins the turbocharger.

A supercharger doesn't have the benefit to take advantage of it's own output like that, it takes HP to run and somwhere along the curve the HP it takes the engine to spin it will outweigh the HP it creates causing a drop off on the top end for roots-type blowers, and mechanical lag down low for centrifugal type. However, because superchargers are mechanically linked the boost follows the engine instantly and throttle response is preserved.

Turbo's overall are a better and much more cost-effective way to achieve power numbers. However, a Supercharger will allow you to go more balls-out on power while sacrificing less throttle response at the expense of being expensive, much higher in maintenance and much harder on the engine.

There's a good section that summarizes all of this in this wiki article http://en.wikipedia.org/wiki/Supercharger

All in all, I believe you can make higher HP with a supercharger which is why drag engines tend to use those instead of turbos. But for a track car, turbo's allow for more reliable output as a supercharger would eventually eat the engine in most cases.

[Wayne Presley]

A turbo will always make more power than a supercharger at the same boost level. This is due the HP required to drive the SC. On a centrifugal SC on a small block 302 making 450 RWHP (14psi), the SC takes between 75-80 HP to run the blower. A turbo car at the same boost level makes 500-510 RWHP.

[Gollum]

Air Temp/Density also matters when comparing the two. A turbo pushing peak HP numbers on an application that's actually in the sweet spot of it's compressor map can be over 75%, much higher than any roots style supercharger I've seen, and also rivals the best screw type as well. Then when you combine the fact that very few roots or screw type supercharging systems do anything to cool the air after the outlet it's no wonder a turbo has very little challenge beating out the conventional supercharger setups.

Now, if we're going to compare an 80's turbo without an intercooler to an OEM roots (like an eaton), I bet PSI for PSI they're nearly equal in HP.

Now, centrifugal superchargers are well suited to using an intercooler, but will still require a small HP loss to be driven, and usually to avoid absurd whine have heavily helical cut gears, which increase rotational friction. Just like how a straight cut gear in a tranny will be stronger and have less power loss, the gearing in a centrifugal supercharged is no different. There's always going to be a balance of noise and power loss. Ever notice how the most efficient centrifugal's on the market are also the noisiest? (ATI Prochargers) Granted the compressor engineering has a lot to do with it, but reduced friction is a good part of it too.

[BrandonDrums]

All research I did on rebuilding the toyota 2zz engine for a FI install in a lotus elise pointed out that it is actually a turbo that can achieve greater hp than a supercharger. This may not be true for high displacement engines, but I think drag cars use them because they have the traction for high torque off the line, not because they are more powerful. Superchargers would probably be more consistent, which is extremely important when counting milliseconds.

I buy that, sorry about the bad info. Another guy even pointed out that turbo's are banned from top-fuel dragsters. The added power is probably why.

Fact fail!

[wjfawb0]

I prefer roots superchargers on street cars for quick torque and less shifting. I suspect driving a large V8 with a turbo is a lot more satisfying than a small four cylinder, though. The two STIs I had were fun, but the turbo lag even on a stock turbo wasn't nearly as enjoyable as having 450 lb-ft of torque instantly at 2500rpm. If I had an 1800lb car with a WRX drivetrain, I would want it tuned using the stock solenoid system and a nice linear power increase to make the car more predictable and less likely to swap ends suddenly going down the road. In my cars I've always foudn right foot control to be the most effective way to keep it under control. :D

[iamnottelling]

I actually feel smarter as a result of reading this Forum...

Thanks for all the great explanations (CoG with breaking & Turbo vs. Supercharger)

- Just another young engineer -

[Twinspool]

I actually feel smarter as a result of reading this Forum...

Thanks for all the great explanations (CoG with breaking & Turbo vs. Supercharger)

- Just another young engineer -

Wait until you get to upper division classes. Heh.

I'd be very curious about the demographics of the average FFR buyer. I gather that many of the owners are, ahem, well seasoned. From a business standpoint, the 818 is utterly brilliant in its more youthful focus and could expand the FFR customer base far outside of the range it has been in.

A thousand thanks for making this car. You have me seriously interested. Based on conversations I had with GTM and Roadster builders, I assumed that an FFR kit was something that I wouldn't be able to do until I had $50k and idle time a plenty.

[PhyrraM]

I'd be very curious about the demographics of the average FFR buyer. I gather that many of the owners are, ahem, well seasoned. From a business standpoint, the 818 is utterly brilliant in its more youthful focus and could expand the FFR customer base far outside of the range it has been in.


I've often wondered the same thing. When the original 'single donor C**ra' came out I seriously wanted one, however in my mid-twenties, he 10K price simply wasn't possible. In today's money that is....$7K? A significant discount vs. the O.G. Roadster. So a (time adjusted) lower kit cost would open the doors of opportunity even sooner for many would be builders.

Also consider that the 818 is likely to be even less restrictive than the original Roadster in terms of an appropriate donor.

IIRC, the roadster need a very specific year range of Mustang ('87-'93?) for the required suspension parts, motor mounts, wheels, etc. to work with the kit without modification. As the years progressed FFR started adding options and variations to the kit to support other Mustang generations. If the 818 kit mirrors the Roadster or GTM coupe as far as reused donor car parts (a big 'if' at this point), almost any Subaru will do. If you can forgo the turbo motor (only 1800lbs, remember?), you can pick up a running Legacy/Impreza for as little as $500. That puts the cost as low as $11,000 for the supreme budget builder.

Here's an idea:
FFR can offer a less complete kit for (no seats, wiring, instuments, lighting, etc). Basically strip out anything not related to the chassis or basic body and sell it for...what?....$7,000? That would really open up the gates to younger, less established buyers. Of course, there would eventually be tons of unfinished kits on Ebay. I suppose that wouldn't do much for the 'FFR is not a kit car' image they have spent a lot of time developing.