How much HP can a stock Subaru transaxle handle?

[veloce2]

I see a lot of big figures tossed out 200, 250, 300 , 380 , 420, 450 Stage 1 2 or 3 this leads me to ask how much HP can a stock Subaru transaxle handle?

Are there upgrade kits for the transaxles?

Any input appreciated

[PhyrraM]

There a lots of opinions, and tons of post already dedicated to this topic.

However, the real answer is that nobody really knows yet. The 2WD conversion and the much lighter curb weight throw too many variables to give anything more than an educated guess at this point. I would sit back and wait for a few builds to be finished, only then will we have a somewhat real answer.

[veloce2]

Well I understand in regards to applying it to the 818, but to get a base line I would like to hear what Subaru WRX owners get. Stock I have to imagine is just fine, what about cars that have been modified? There should be some pretty good knowledge out there about the limit of these transaxles. This could help a lot of new 818 buyers.

[bbjones121]

Subaru owners can't really answer your question, because the donor cars are AWD. I have close to 500 crank horsepower in my Subaru Legacy. Whether or not only the front wheels could handle that is not a question that can be answered.

[bbjones121]

Google subaru fwd conversion s. Company's typically tell you how much power their kit handles.

[metalmaker12]

The Subaru transaxles are all actually pretty strong, but what they can handle, well we will see. I would see no problem with 300-350whp, after that it will handle it, but a missed shift here, grind there.."More power more problems". In Awd trim one of my previous Sti's had about 450whp and a stock tranny/flywheel with a stage 2 act clutch. I had a friend Wrx with upgraded turbo tuned to 395whp, and the tranny was stock and it held up. If you abuse gears with a powerful car, you will not have gears very long and your tranny will blow apart no matter which one you have.

[veloce2]

Yea that yields very little info, so far I don't find the Subaru data that assessable. I do read where the STI 6 speed is much more rugged. Is that a easy swap out ?

[Erik W. Treves]

getting a STI tranny converted to front wheel drive will be problematic I believe.

[metalmaker12]

No the 6 spd is longer, by 3 inches, but in time we might be able to make it happen. The body is almost touching the linkage of the 5 spd so the 6 would have issues. In time we will get one to fit though. I would not worry much about this, the car will be a ferrari competitor with 250whp.

You can convert the STI to front drive, It has not been done too many times, I will have to conference with some tuners.

[veloce2]

I am tossing out the body anyway, I have my own design so the longer transaxle is not a issue.

[metalmaker12]

I will take your body?

[veloce2]

Thanks good to know

[PhyrraM]

Some of the issues that have been talked about, at length, both here and on Subaru forums - with no clear concesus or proof of any resolution or definitive answer:

Case design may allow gears to "push" apart under high torque.

Long, "skinny", unsupported input shaft twists under load and separates from syncro on rest of cluster.

Front ring and pinion is not designed for 100% power.

Long narrow shaft that drives the pinion runs thought the center of the layshaft and may not react in a predictable manner under high power.

I'm sure there are others I can't remember off the top of my head.

[bnr32jason]

I've only owned one Subaru (a GC8 STi about 5 years ago) but from what enthusiasts told me is that it all depends how you drive it. If you try to drive it like a Honda (powershifting and "banging gears") the transmissions can have problems have even stock power levels. However if you drive it like a sane person there are people running 400+ without problems.

[philly15]

^i agree with this 100% all my wrxs have been near or above the 300/300 whp and tq levels which is where a lot of people claim they break and i never once had an issue my 2nd wrx is still running great sold it to a friend has 175k miles on the original clutch and gets dragged and auto crossed, i really do believe it is how you drive it my last wrx was pushing over the 300whp/tq level and i autocrossed it a lot last year had probably 50-60 launches on it at least sold it to an idiot who likes to slam gears do high rev clutch drops etc etc blew the hell out of 1+2 gear in 2 months of owning it in my mind if you keep up with proper maintanence and drive correctly any subaru will last well over 200k miles even modified

[bbjones121]

Yea that yields very little info, so far I don't find the Subaru data that assessable. I do read where the STI 6 speed is much more rugged. Is that a easy swap out ?

Subaru information is not easily accessible? For more subaru information than you could ever possibly need, google "subaru forums" join some. Start with NASIOC. Also, on first google search on subaru transmission conversions, this came up, http://www.bremarauto.com/products/subaru-2wd-conversion-kit/.

[bbjones121]

also, there are some comparisons out there on gear strength as well. A Legacy GT 5 speed transmission is equally as strong as an STI.
http://flatironsrally.typepad.com/faq/2010/05/comparison-of-subaru-gear-sets-wrx-legacy-gt-sti-ra-and-more.html

[Drew P]

It definitely comes down to how you drive. But it also is a bit situation. My WRX was set up with about 300WHP and under normal conditions the tranny was fine. Even while doing auto crosses it held up. However, at one spot on the course there was a bit of a crest where the car un-weighted. I happened to be grabbing second gear and put my foot in it. The shift was smooth, but with the car unweighted and got some wheelspin and all 19.5 pounds of boost. When the car re-weighted, second gear blew up.

I have since built a new tranny with the RA close ratio gearset as well as a new engine with more HP and a lot more torque (the old engine blew at 190K after 4 years of AX and an aggressive tune the whole time). The new tranny has held up great with that set up. I of course am much more careful when the course takes us over that crest now though.

I unfortunately hit a deer on the freeway and insurance totaled it. I bought it back and now have my donor sitting in the garage with a bomb proof drive train waiting to drop into an 818.

[veloce2]

great story look forward to the rebirth.

[Movieman]

Something tells me that with the low weight of a 818 if you stuck a 400HP Subaru engine in it you could shift it like your grandmother and it would still walk away from any Ferrari..:D

[shinn497]

Yeah this has been debated to DEATH.

Some say the lack of grip will keep the tranny from breaking others (primarily me) say the impulse from the engine will subject the tranny to the same force regardless of weight.

[veloce2]

Lets hear it

[Turboguy]

The 2WD conversion and the much lighter curb weight throw too many variables to give anything more than an educated guess at this point.


The Cobra guys went down a similar road looking at "how much HP a tranny can take" as they took TKO and T5 trannies from rustangs and put them in Cobra replicas.

Searching for a definitive answer, I actually called Tremec and talked to the engineer in charge of the TKO line. For what it's worth, he told me that significantly reducing the overall weight of the car (like taking a transmission from a FOX-body Mustang and putting it in a Shelby Cobra) only marginally increases the transmission's HP or torque rating - he used 10-15% as an example. He also said that torque is a far more limiting factor than horsepower, and that it's the real killer.

[bbjones121]

Torque is the only actual measurement of force, horsepower is not. Since horsepower is calculated from torque and the RPM. Forces on the gear teeth are never calculated using a horsepower measurement. The gear strength topic has been discussed a lot. It is an impulse force that damages the tranny. You only have a damaging impulse if your tires have enough traction to apply an opposite force in your tranny.

The best guess of how much less stress the tranny will take in a lighter car is by figuring out what torque it takes for a 4000lb chevy to break the tires free at a dead stop to how much torque the 1800lb 818 takes to break them free. If I had to guess, the 818 would need more than a 10-15% reduction in force to keep the wheels from spinning. If it takes 300ftlbs of torque to reach the slip of the tires on the 4000lb car, but takes only 150ftlbs of torque to start slipping on the 818, than there is half the impulse force on the gears of the same transmission in the lighter 818. There is obviously way more to it than just that over the total acceleration, but that can give you a good guess of when the force matters most at first engagement. You could figure out the ratio of traction during gear shifts as well to guess at reduced stress during those events.

[veloce2]

Then at speed there is that big wing pressing the rear tires to the pavement changing the amount of force it takes to spin the rear tires, correct?

[bbjones121]

At what points are you shocking the transmission. At first start and gear shifts. I highly doubt the transmission would fail at continous load. That would be a crazy wing to give you more than 2000lbs of downforce at speed.

[metalmaker12]

We have beat this to death in two- three other threads, hp/ torque force etc. It has had four pages of theory in fact. Bottom line is that we must put these mostly used transmissions in a much lighter car, and hope they last. Should they, yes, will they, let's wait and see, they do last in the wrx's. Until we put the inner tranny parts, with cover plate and do countless hole shots we will not really know it's true potential. It is like when engineers build a new car and put it through a endurance road coarse with bumps and all kinds of nasty stuff. They in theory have built a car to make it through with good results, but sometimes theory is just theory.

[bbjones121]

+1 to that. sorry for rambling on.

[metalmaker12]

No it's perfectly fine, we just have to wait and see, or if we keep speculating we might get all fired up over nothing. That's all, you made good sense.

[veloce2]

I appreciate the input, because I had no idea the transmission could take numbers into or above 300, knowing that range is helpful. I can see owners putting more HP in and wanting good sources for parts and upgrades though.

I built a Beck 550 spyder back in the 90's, but it only had 140 HP, at 1400 lbs it moved pretty good. I remember going to Chucks shop and in the corner he had a Subaru Boxer unit that he was testing. He didn't like all the plumbing so he took a 911 engine cut the block removing the center pistons building a overhead cam 4 banger that made some pretty big HP. He took it to run and Gun that year beating all the cobra's in every event but the drags. 550's were always known as giant killers.

[metalmaker12]

The Beck 550 spyder is a sweet machine!!15623

[veloce2]

Very well built, chuck kept his molds in perfect condition, as a result the gel coat was perfect. The technology was pretty old though, so it was hard to make many upgrades for handling. My car was the first one to have the head rest like a A version I built it off of old pictures, Chuck liked it so much I sent it to him to copy for later cars.

[Silvertop]

The Beck 550 spyder is a sweet machine!!15623

I love the 550! I very nearly selected the Thunder Ranch version (which IS the Beck design) as my project car. But after sitting in company owner Tom McBurnie's personal car a few years ago, I found it just a little bit too hard to get in and out of for my aging bones. I then considered the Thunder Ranch Speedster. But then FFR came up with the 818 idea, and I knew I had found what I was looking for. LOL, I may find that egress in and out of the 818 isn't all that great either -- but I'm committed.

We're wandering a little afield here, though. Sorry about the hijack..............

[SixStar]

HP doesn't kill transmissions, TQ and drivers do. I tossed my OEM trans with only 250/250 and I tossed a Rallispec RA box with only 260/260. In the end you'll be fine with around that power simply for the fact that there is a lot less strain on the parts from 2 less wheels and 1,000 less LBS.

[Desertrunner]

No the 6 spd is longer, by 3 inches, but in time we might be able to make it happen. The body is almost touching the linkage of the 5 spd so the 6 would have issues. In time we will get one to fit though. I would not worry much about this, the car will be a ferrari competitor with 250whp.

You can convert the STI to front drive, It has not been done too many times, I will have to conference with some tuners.

You can get a lock for the centre diff on a 6 speed, it will enable the gearbox to be , FWD, RWD or fixe 4WD, they come from here,

http://www.possumbourne.co.nz/trans-and-diff-parts.html

Tony

[metalmaker12]

I had that link under bookmarks awhile ago, Thank you for reuniting me. The thing is they are not affordable lol.

[veloce2]

Those are Aussie dollars right? Aren't they like a hundred to one?

[Subarugears]

I can give you SOME real-world feedback on how the 2WD transmissions hold up in different cars. We have been selling 2WD Subaru 5 speed conversion parts and also reverse ring and pinions for Subaru 5 speed transmissions for 3 years now. So this feedback is based on customers that have run our parts in mid-mounted 2WD cars (e.g. Porsche 914 and Ferrarri 308's with Subaru STi motors) and the many 2WD reversed transmissions customers have run in beach buggies, beetles, kombis and vanagons.

I have heard of one Subaru standard ring and pinion break (not a Subarugears reversed one). Fitted to a mid engined dune racing buggy with 33" tyres and doing a demo in the middle East, hitting multiple whoop-dee-do's at very high speed. The shock loading from all that wrecked the pinion gear. This guy ended up moving to a Weddle transmission with 10" ring gear. They actually placed second in a long-distance dune buggy race with the 5 speed Subie trans a few weeks earlier but doing this demo/practise run saw it break. Extreme conditions and shock loading.

We have plenty of EJ20, EJ25 and EZ30 motors powering our reversed 2WD transaxles in VW cars and have not had a breakage. The cars weigh from 800kg (approx 1750 lbs) to around 1900kg (approx 4,000 lbs) and those motors put out from 150hp to 230hp. They are plenty quick too. We also have a couple of customer cars running EJ20 turbo and EJ25 turbo STi motors that have not had issues, running reversed 2WD transaxles for between 12 and 24 months so far. This includes OBX LSD's in some cases as well.

There is significantly less shock loading in 2WD format and a lighter weight car. If you make sure your Subaru trans is in good condition, with sharp dogs and good syncros, it should give you good service in an 818. I have to have a little snicker at some people asking if it will hold 350hp, 450hp whatever. Don't worry about it. At 250hp you have a supercar. The rest is just wheelspin.

As you say, the 6 speed STi trans can be run in FWD as well, with the Possum Bourne locker (pricey) but I find this is a much larger, heavier, longer trans for no real gain. If you want a tough 6 speed, take a look at the splitcase 6 speed (6 speed version of the 5 speed) - this has a massive first gear and turbo thickness 2nd through 6th gear. We are producing 2WD locking spools and rear plates for these as they become more popular as well. (oh and they are factory cable shift).

Hope that gives you some real-world feedback. You should be confident that your Subaru 5 speed 2WD transaxle will serve you well.

[Oppenheimer]

I can give you SOME real-world feedback...

Great! This is the closest to rear-world 818 knowledge as we've seen yet. Very valuable info. It should prove very applicable to 818 usage.


At 250hp you have a supercar. The rest is just wheelspin.

This^. Wheelspin will be the safety valve that protects the trans from excessive loads. The question will simply be at what size, and compound, does traction reach a level that this valves threshold exceeds the trans ability to cope with it. If that traction threshold only occurs at tire sizes beyond the practical limit of what will fit the 818, street going 818's should all be fine.

[longislandwrx]

good to know... was wondering if I needed to make blast plates.

[BrandonDrums]

Unlike an engine, it doesn't make sense to spend money on it to prevent a failure unless you have funding and a race is on the line.

An engine can fail catastrophically to the point that repair is not an option and it can even take out other stuff at the same time, a little money spent to prevent an engine failure can save you money.

Upgrading a transmission to prevent failure just means avoiding failure. You can make sure you spend the money by spending the money, if you don't spend the money, well you might not ever have to.

I'm not sure how it's so hard to figure out but weight is absolutely a BIG factor in determining how much stress a transmission goes through even with power remaining constant. It's been beaten to death for sure but there's no argument. I think SubaruGear's REAL world feedback confirms that.

[veloce2]

Take note there are people who know and this gives us a good example. I have a feeling we will all be learning a lot ......

[Xusia]

I'm not sure how it's so hard to figure out but weight is absolutely a BIG factor in determining how much stress a transmission goes through...

+1 :)

[RM1SepEx]

The forces involved are ultimately determined by the mass you accelerate and the traction that you can get with the road surface. Heavier AWD Subarus are very good at getting that power hooked up and many drive them aggressively or in anger... clutch dumping starts that apply shock loading are very tough on the drive line. The 818 will not be capable of hooking up and just jumping off the line, tires will spin, the shock loads will be greatly reduced, parts will last longer, parts can and will still break on occasion.

[Turboguy]

I'm not sure how it's so hard to figure out but weight is absolutely a BIG factor in determining how much stress a transmission goes through...

+1 :)

According to an engineer who designs transmissions for TREMEC, substantially lightening a car will only marginally increase a transmission's capacity.

See my earlier post.

[Xusia]

I saw that post, and what he said matters is shock loading. The tires will spin more easily on a lighter car, lessening the shock loading. Sure, you could still put a huge instantaneous shock load into the transmission and do some damage before the tires can break free, BUT that's going to be harder to do. I think you'd have to pretty much have to be trying to do that, or be driving very aggressively (such as racing). For the average person on the street (spirited, but non-aggressive driving, with purposeful thought to avoid damage to the car), I think the lighter weight is more likely to result in wheelspin than transmission damage when a shock load is applied unintentionally.

[rjh2pd]

According to an engineer who designs transmissions for TREMEC, substantially lightening a car will only marginally increase a transmission's capacity.

See my earlier post.

Your earlier post also went from being a 2 wheel drive car to still a 2 wheel drive car. Going from AWD to 2 wheel drive will be a huge factor as well as weight. By going to two wheel drive you are losing around 50% of tire grip. The transmissions shouldn't have problems with breaking, because the wheels will let go a lot sooner in this car than in the wrx. the one thing that i would look at is the differential, because those were only designed to have 50% of the torque where as all the torque went through the transmission.

ps. the change in acceleration is called jerk.

[Turboguy]

No offence, but at the end of the day you guys are all just offering your thoughts and opinions about what you THINK will make a difference without having any education or experience in the area to substantiate it.


Why not simply call a design engineer at a major transmission manufacturer -or better yet, the company who manufactures the Subaru box- and get the low down from a professional?

[longislandwrx]

Why not simply call a design engineer at a major transmission manufacturer -or better yet, the company who manufactures the Subaru box- and get the low down from a professional?


sounds good, do you know either?

[RM1SepEx]

I am a mechanical engineer, it is a basic engineering problem.

As I posted above ultimately you can not apply more torque than you can apply in that specific situation. If you have the drive wheels in the air virtually no load will be seen by the drivetrain, you can only put down that which is required to rotate the driveline mass.

for a simple example how much torque can you apply to a loose wheel nut? ans only what is required to offset the inherant friction.

with no load an engine spins up very easily and develops little torque (measured) and HP always a simple calculation.

So with less weight and less available traction the drivetrain will see lower shock loads. It simply has to...

Now it will transfer the full torque and HP, over time, if applied at a rate that doesn't cause the tires to slip. Most transmission issues occur due to shock loads, over time, exacerbating stress risers in areas of transition of the various assemblies. These would be areas where shaft diameters change or have machined areas, areas where castings change shape, thrust surfaces etc... oiling issues due to Gee forces could cause longer term issues etc...

ALL life cycle calculations take into account loading and rotational quantity (revolutions for a car might be what is expected for say 60,000 miles in top gear for example) A ton of fudge/safety factors are plugged in as well offsetting mfg variations, materials variability etc...

AWD Subarus routinely get abused off the line, they have awesome traction capabilities and 3500 lbs JUMP off the line. An 818 will not be able to do that, the tires will need to be babied a bit off the line, will hook up, and then go past the heavier car. Shock loads will be lower as the tires will spin...

[BrandonDrums]

No offence, but at the end of the day you guys are all just offering your thoughts and opinions about what you THINK will make a difference without having any education or experience in the area to substantiate it.


Why not simply call a design engineer at a major transmission manufacturer -or better yet, the company who manufactures the Subaru box- and get the low down from a professional?

No need to call. Thanks, RM1SepEx.


I am a mechanical engineer, it is a basic engineering problem.

As I posted above ultimately you can not apply more torque than you can apply in that specific situation. If you have the drive wheels in the air virtually no load will be seen by the drivetrain, you can only put down that which is required to rotate the driveline mass.

for a simple example how much torque can you apply to a loose wheel nut? ans only what is required to offset the inherant friction.

with no load an engine spins up very easily and develops little torque (measured) and HP always a simple calculation.

So with less weight and less available traction the drivetrain will see lower shock loads. It simply has to...

Now it will transfer the full torque and HP, over time, if applied at a rate that doesn't cause the tires to slip. Most transmission issues occur due to shock loads, over time, exacerbating stress risers in areas of transition of the various assemblies. These would be areas where shaft diameters change or have machined areas, areas where castings change shape, thrust surfaces etc... oiling issues due to Gee forces could cause longer term issues etc...

ALL life cycle calculations take into account loading and rotational quantity (revolutions for a car might be what is expected for say 60,000 miles in top gear for example) A ton of fudge/safety factors are plugged in as well offsetting mfg variations, materials variability etc...

AWD Subarus routinely get abused off the line, they have awesome traction capabilities and 3500 lbs JUMP off the line. An 818 will not be able to do that, the tires will need to be babied a bit off the line, will hook up, and then go past the heavier car. Shock loads will be lower as the tires will spin...

[BrandonDrums]

According to an engineer who designs transmissions for TREMEC, substantially lightening a car will only marginally increase a transmission's capacity.

See my earlier post.

It's also a bit of a trick question. Technically speaking, the transmission's capacity doesn't change whatsoever. Every gear and syncro etc. has a breaking point that stays stagnant regardless of what car it's in. What changes is the forces the transmission is subjected to when weight is reduced and the marginal decrease in subjected force decreases as gear ratios increase.

In other words, as leverage works against the engine in the higher gears with ratios greater than 1:1 the effect of the weight of the car on how much torque gets applied through the transmission diminishes. You still get better relative durability for a specific output but the increase is a smaller percentage than the lower gears. However, it's not the higher gears we're worried about, shock-loading aka, impact is what breaks things and in the higher gears the ratios work to minimize shock loading which is why they rarely break.

Make sense?

[Desertrunner]

Removed post due to it being a waste of time.

Tony

[philly15]

^good thing were all using 6 speeds....lol....i agree the 6 speed is bullet proof..if you know how to drive..ive seen people blow em up not even putting down that much torque im also curious how you think thats a big tire? and since we are all using 5 speeds that was an irrelevant post

[boostinsidewayz]

Why doesn't anyone address actual performance? Almost every post mentions tire spin. Well what about people that don't want tire spin.
My 818 will see the drag strip and it will see sticky tires.

[philly15]

and a good LSD...

[Oppenheimer]

Why doesn't anyone address actual performance? Almost every post mentions tire spin. Well what about people that don't want tire spin.
My 818 will see the drag strip and it will see sticky tires.

At some point, for any car, power exceeds traction and you get wheel spin. If you are interested in drag racing, then you look for things that improve traction. With something like the 818, 2wd, light, limited tire size, and an engine with serious power potential, the point at which power will exceed traction should be all too easy to attain. Hence all the talk about wheel spin.

[boostinsidewayz]

That is a very pointless post. Yes even top fuel dragsters can get wheel spin. So since there isn't much load, you could put a Wrx Trans there right?

Fwd civics can dead hook with 400 to the wheels. I'm pretty sure a car with the same weight but rwd will be able to hook too.

[Silvertop]

That is a very pointless post. Yes even top fuel dragsters can get wheel spin. So since there isn't much load, you could put a Wrx Trans there right?

Fwd civics can dead hook with 400 to the wheels. I'm pretty sure a car with the same weight but rwd will be able to hook too.

My opinion -- There's nothing pointless about it. Tire adhesion (or lack of it) is a significant factor in determining shock load. Add enough raw power, or continue to increase adhesion -- Its pretty much a guarantee that you will ultimately reach a point where something will break. Exactly where is that point? Given that the 818 is not even in production yet -- we don't really know. Those who want to push the limits will discover it for us.

In the meantime -- we should try to keep our responses to other people's posts a little more civil. This thread is starting to get a little edgy.

[boostinsidewayz]

Well define -all to easy to attain-
That is the only opinion you stated, and I think you're opinion is to broad to mean anything. I can put spare tires on the 818 and spin through 3rd gear with a na 2.0 liter. Or you can make 300 to the wheels and spin good street radials. I don't think you'll find traction s big issue once you put slicks on the car. Then the actual question of -how much power can the Trans take- comes into play.
Your post is absolutely pointless. What person on here doesn't know why you get tire spin?
The post is about how much power the Trans can take, not the amount of wheel spin the car will get.

[Silvertop]

Well define -all to easy to attain-
That is the only opinion you stated, and I think you're opinion is to broad to mean anything. I can put spare tires on the 818 and spin through 3rd gear with a na 2.0 liter. Or you can make 300 to the wheels and spin good street radials. I don't think you'll find traction s big issue once you put slicks on the car. Then the actual question of -how much power can the Trans take- comes into play.
Your post is absolutely pointless. What person on here doesn't know why you get tire spin?
The post is about how much power the Trans can take, not the amount of wheel spin the car will get.

Actually, the "point" of my post had a lot more to do with civility than it did with wheelspin.

[Movieman]

Well said Silvertop!
Civility before anything else.
That way we learn more with less time wasted peeing at one another..
Ok, I'll leave and let the geniuses continue the debate..:D

[longislandwrx]

A great little video on the 5mt, gears syncros and abuse.


https://www.youtube.com/watch?v=Uf9NzVRWySE&feature=player_embedded

[flynntuna]

Isn't you tube great, and +1 for New Yawkers. :cool:

[Oppenheimer]

That is a very pointless post. Yes even top fuel dragsters can get wheel spin. So since there isn't much load, you could put a Wrx Trans there right?

If it has wheels and tires from a WRX (or other likely 818 wheel/tire combos) mounted on the top fuel drag racer, and it isn't putting any more weight on these tires than an 818 would, and you don't rev the engine past around 7K rpm's, then yeah, you could put a WRX trans in there.

[Desertrunner]

The single weakest point is the clutch, the standard unit will let go before the transmission fails. One of the heavy duty failed as well when I was running the 5 speed in my car before I went to the 6 and clutchs were the biggest issue. Anyone planning to do any sort of drag racing or high rev launch will kill the clutch before you know it. Other issue I had was I pulled a pull bearing apart on the clutch. Seems as you go to a heavier clutch the pull out bear can't stand the load and the seperates.
Tony

[philly15]

The single weakest point is the clutch, the standard unit will let go before the transmission fails. One of the heavy duty failed as well when I was running the 5 speed in my car before I went to the 6 and clutchs were the biggest issue. Anyone planning to do any sort of drag racing or high rev launch will kill the clutch before you know it. Other issue I had was I pulled a pull bearing apart on the clutch. Seems as you go to a heavier clutch the pull out bear can't stand the load and the seperates.
Tony

i agree i blew threw two clutches last year autocrossing my 03 wrx in SM (i also had a co driver) i had an excedy stg 1 from the previous owner that went after the second event replaced it with an ACT sb3-hdss full face organic and blew threw that pretty quick too im think im going right to a 6 puck or 4 puck this time around thats crazy that the bearing pulled right out!

[Desertrunner]

I may have missed it but I figure the big issues not mentioned are what quality of gear replace the centre diff and will it take the sudden increase in power going its way.
Also worth considering is how good is the gear change, nothing distroys a gearbox faster then poor gear changes.
Tony

[Matty_STi]

Clutches are far easier to replace than a tranny..

In subarus it isn't torque that is killing the gears but shocking loading. There are some insane drag cars that run multiple seasons with out failures of their drivelines (we're talking 800+whp and torque figures well above 600lb-ft). Discussing my transmission requirements for some of my projects with various vendors and race teams the ones who are eating gears aren't the crazy torque figure drag monsters but the rally cars. They are constantly eating transmissions and rebuilds and gear replacements are a common thing (this coming from both smaller rally teams and big teams running albins, hewland, ppg, modena, kaps gears). None of those gear sets or transmissions are cheaper (often we are talking about the 6mt gear sets too). Shock loads are the killer, and there are a tonne of different ways to get shock loads. Rally cars produce insane amounts of shock loading thus why they eat transmissions. Asking what power or torque range will kill a 5mt is impossible to say. Vehicle weight, tire grip, suspension setup, clutch setup, axle flex, down force, torque, road surface, driving style, etc. etc... All play a factor. And note the rally cars that are eating tranny's aren't doing it off the line, but in the middle of stages. I will say this, there are guys that are going to destroy their tranny's at power levels that others will be able to abuse their cars in.. There are too many variables to really say what the breaking point is.. Sure you can determine the forces involved on killing the transmissions directly but modeling those forces in a real world car would take some computing power at a level I doubt anyone here has access to let alone seen. For a nice round number I would say 400whp and similar torque figures with smooth shifting and no clutch dump take offs or crazy hoonage and the 5mt will probably be alright... No one can say right now.. Lets first see some guys blow their transmissions up being dumb with boost before we get too crazy here, need real world data of this application..

-Matt

[metalmaker12]

+1, kind if what some of us have been saying rolled into one. Yea I know some rally guys and builders, and they eat built trannys all the time. It is an expensive sport to say the least.