How much horsepower is reasonabe for an IRS coupe?

[ttpmmx]

I've been spending too much time looking at the Coyote 5.0 and other newer mustang motors, superchargers and turbos. Now, I've found the new 500 hp Ford Boss crate motor. Is this too much for the coupe? I'm pretty sure I'm going with the irs rear end with traction lock. Fuel Injection is on the list and I'm still undecided on the transmission. I'm thinking I'd like some crazy fun horsepower, but am I going too far?

[vnmsss]

Welcome to the Forum.....The Coupe chassis can easily handle a 500 HP-rated motor, however, if you're considering any serious track or dragstrip use, and are looking for long term reliability, I'd use the best quality IRS system you can find (02-04 Cobra, no T-Bird)...Even then, plan to check, change, and refresh parts regularly.....For normal, day to day street use, 500 HP (say 440 wheel HP) could be managed. We have some pretty big HP Coupes on this forum, and folks seem to enjoy them.....I'd think that with 500 HP you'd likely have more issues with sound and cockpit heat than the IRS.

K

[Bob Cowan]

A lot depends on how you use it. High numerical differential gear, big sticky drag tires, and 3,500 rpm launches will kill it pretty quick. Mine seems to be holding up just fine - but I don't drag race.

[Stengun]

Howdy,

The 8.8" IRS is very strong and it will handle all the power you can put into a street motor.

My Spyder has over 500hp/torque and I'm running the 8.8" IRS and I'm not worried about it.

Paul

[68GT500MAN]

500hp is doable, but as Karen mentiones, there is a LOT of heat (or was it the 100+ Fresno Summer).
Doug

[ttpmmx]

I never considered drag racing (tried it once, didn't do it for me). I just liked the idea of having more power than needed, but I didn't consider heat being an issue. Of course this is compounded (or minimized) by the fact that I'm in the Phoenix area... Perhaps I should stop by Levy racing. I would think being in the valley, he may have some insight on hot motors in this hot part of the world.

Sound has always been a concern, any thoughts on toning these things down and keep the dual sidepipes?
Thanks for the input

[turbonut48]

I have 530 hp at flywheel. HP is heat, and noise.
I have the 3 link rear suspension tho.

The Nut

[Bob Cowan]

Heat is an issue in every performance car. Fortunatly, there are some answers.

Coat the headers and side pipes with a good ceramic coating. Not only blocks the heat, but makes the exhaust more efficient. Some people also use header wrap with good results.

Use a good insulation on the foot boxes. I like the ceramic brush on stuff, but there are a number of good choices.

Noise control is (fairly) easily handled as well. Cut the standard side pipes apart and weld in some new mufflers from Classic Chambered Exhaust. I did that two years ago, and it's a dramatic improvement.

[Roy Hewson]

Just another opinion, I have a 347 with 425hp/400 tq through a Tremec TKO 600 into an 8.8 IRS with Michilin 335 PS2 on the rear. After 24,000 miles there have been NO problems with the drive train set up except some bugs in the engine it self which are now solved. I have done a few track days and some very spirited starts on a regular basis, but that never includes a high rpm clutch dump. I just never have been able to make my self abuse my equipment in that manner also 335 are expensive. Heat has not been a problem but I don't live in Arizona.

[Colonist]

Use a good insulation on the foot boxes. I like the ceramic brush on stuff, but there are a number of good choices.



Are you willing to share your specific product recommendations?

[Bob Cowan]

I used Lizard Skin. You can do the horizontal surfaces all in one thick layer. But the vertical surfaces will run and sag if it's too thick. I bought about 6 cheap brushes. Every night for a week, I went out and brushed on a new layer. Then I threw the brush away. Only takes about 30 minutes or so to do each time.

The down side is that it produces a hard surface. For most places that's fine. But under your feet you'll want some kind of pad beneath the carpet.

[loeffler1]

I also used Lizard Skin. According to the manufacture it should be sprayed on not brushed, reason, the ceramic particles do not apply well in brushing. However there is an alternative and the one I used, a foam applicator 2" wide and stir the mixture with a paint stirrer in an electric drill quite often to prevent the ceramic particles from settling. This information came about as I dont have a spray gun for undercoating,etc and called the manufacturer as to what the alternative methods might be. They were a great help. I actually had to apply only 2 coats, but I'm sure the more coats the more resistance to heat. Stuff works great. I also have fabricated aluminum heat shields with silicone sided Koolmat applied to the inner wall of the heat shield and mounted to the footboxes with 1/2" aluminum spacers. On the driver's side I used the forward large hood scoop with a 2" short piece of tailpipe cut at an angle and welded to a light gauge sheet steel and fiberglassed to the underside of the hood at the scoop. To this I attached a piece of flexible hose (the same as the hoses supplied with the AC/heater ductwork) and aimed at the gap between the footbox and the shield and partially at the right header. After running the engine at a medium RPM for roughly 1/2 hour in the garage the temperature of the interior footboxes were the same as the ambient temperature in the garage, even though the headers were very hot and the exterior of the heat shield was also very hot. I would guess that I shouldn't have an heat issues in the footboxes. Lastly I'm located in Maine not arizona so there could be a difference.

Bill

[Colonist]

Interesting -- I looked at Lizard Skin but had kind of dismissed it because of the necessity of spraying. Good to know it can be effectively brushed as well with some attention to keeping the ceramic suspended. Thanks for the info.

[Jacob McCrea]

I am in the process of rebuilding an aluminum 8.8" differential from a Lincoln Mark VIII, which fortunately came from the factory with 3.27:1 gears. I'm resurrecting this old thread to mention a few components which should make the differential a little better able to handle big horsepower or abuse. I have no idea how much they'll really help, but I have to think that they're worth the money.

Here are some ARP carrier bearing studs.

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And here is a Ford Motorsports heavy duty cast aluminum differential cover, which has recently become available as a single part (rather than having to buy the whole housing). It has preload "screws" for the carrier caps, as well as an inlet and outlet for a pump and oil cooler setup (separate from the fill and drain plugs). Compared to the flimsy stock cover it is very heavy.

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[vnmsss]

Thanks for ressurecting this Jacob.....I also think the Boss intake will create a hood clearance issue.....

K

[tirod]

It's probably a bit late for the OP, but the real issue is appreciating that even with a 350-400 hp motor, the car is going to be quick - much quicker than the average daily driver.

It's not about the hp, it's about the hp to weight ratio. These cars run in the single digit pounds per hp, from 10 down to 6 pounds. A normal daily driver econo car or SUV can run up to 20 to 1.

That's the reason a stock drivetrain Mustang donor suddenly shaves seconds off it's 0-60 and 1/4 mile times - it's shed of almost 1000 pounds of dead weight. Getting that much out of a stock Mustang unibody would cost nearly as much as simply dropping it into a kit.

There's also a necessary amount of caution, too. Sure, 500hp would be crazy fun - and Youtube is full of crazy fun cars driving thru garage doors, kitchen walls, bouncing off parked cars, etc. And forums are full of posts about those 500hp cars and their grabby clutches, tuning problems, poor handling - and having to wear earplugs behind the wheel. One thing too many give up for a crazy fun car are the amenities of a quiet ride or shelter from the weather. It makes the money spent deliver a very restricted return - one some don't want. They sell them off.

You get what you pay for - emphasizing hp over the total result as a sorted out car tilts the odds it won't be. Take time to look over the whole picture - trading off a few hp to install a/c or power steering and getting a pleasant car that can tour in a grand style meets the requirement. It's what GT really means - not chop rod with motor that cost more than the kit.

[Jacob McCrea]

^ I don't disagree. A decade of motorcycle racing showed me that more power doesn't always get you around a track faster.

Anyway, a few words of caution for anyone who puts one of these FMS heavy duty differential covers, or any of the other ones, for that matter, on their Coupe or Roadster. First, check to make sure that the rear mount fits. The Ford cover's rear mount is machined a little differently on the passenger side than the standard MN-12 cover. To get it to fit initially, I had to wedge it into place, which you can see below by looking at how the frame mounting "tabs" are spread apart. I didn't like that so I took it out and made it fit properly.

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Here you can see how I had to remove material with a 5" air grinder to get it to fit without forcing it.

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Here is the "fitted" cover.

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And another photo, with some trick hollow chromoly bolts from Racemaster Industries and some safety wire done in a somewhat novel way - essentially it sits down in the bolt threads and blocks the nut from backing off, rather than securing the nut in its eact location. I am using some lightweight nuts from Racemaster and they are too small to drill and wire.

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The other problem is that this thing fit through the IRS cage with ZERO room to spare. I had some masking tape on the frame and it got ground off pushing the differential up into place. The problem is that the carrier bearing caps prevent the differential from moving back as far it would with a stock cover. This makes an otherwise tight fit next to impossible. You can see all the scrapes where the cover interferes with the frame mount as I tried to rotate it into place. I tried all kinds of angles and thought I would have to remover the cover, put the diff and cover in the frame separately, then bolt them together, but it eventually "scraped" through the frame and into place. I never had a problem putting a diff with a stock cover in there, but if I ever take this thing out, I will be removing and installing it one piece at a time, as ridiculous as that sounds. So, be advised of what an unbelievable hassle this is!

[Jacob McCrea]

Here are a few photos of the custom tools I made to rebuild the differential and install a Torsen ring gear carrier: 1) a case spreader for installing the last two carrier bearing shims (my understanding is that this is only needed for aluminum cases); 2) a pinion flange holder and 3) two devices for holding the differential in a vise, one horizontally and the other vertically. What I'll call the "horizontal" diff holder looks like a football goal post and has a big mounting surface for a dial indicator, which makes checking the backlash a little easier. If I had to do it again, I'd build the "goal post" holder so that the front of the differential was pointed down a little. A little gravity would make the carrier less likely to fall out and break your foot while you are playing with the shims.

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[vnmsss]

Nice work, Jake, and glad to see the progress you are continuing to make in your build.

Just to clarify my earlier post regarding the resiliency of the stock IRS...We've not experienced any issues with the differential (gears, carrier bearings, etc.)...Our issues were all centered on the axles and hubs, as well as the rear LCA's breaking under sustained load/torque.....Very few folks will ever come close to taxing/stressing an IRS system to the levels which we have visited (Three 25 hour races, in addition to three full seasons of competition).....As noted, under "normal" use, the stock IRS system seems to hold up well.

Karen

[john8642]

Nice work, Jake, and glad to see the progress you are continuing to make in your build.

Just to clarify my earlier post regarding the resiliency of the stock IRS...We've not experienced any issues with the differential (gears, carrier bearings, etc.)...Our issues were all centered on the axles and hubs, as well as the rear LCA's breaking under sustained load/torque.....Very few folks will ever come close to taxing/stressing an IRS system to the levels which we have visited (Three 25 hour races, in addition to three full seasons of competition).....As noted, under "normal" use, the stock IRS system seems to hold up well.

Karen

Hello, Karen. Did you run the 25hr this year, or were you still tuning/adjusting?

[David]

Jake, those look great! Man, that case spreader is trick! Im running a cast iron pumpkin this time around, so luckily wont need to go there :)

David