Rear end Ratios for 427

[Hurricobra95]

I was thinking of buying and building Hurricane Cobra. What would the best rear end ratios be? I am planning on using a 427W from Pro-Performance Unlimited with 550 hp and 560 TQ with a close ratio Tremec T-56 Magnum with 26 inch tires. I am between 3.55 or 3.27. Which would be best for a fast car for weekends or ocasional driving?
Best Regards

[bobcowan]

I have a 5 speed, with a .64 5th. I find 3.55 to be the perfect gear. Lowest speed in 5th is about 60mph. As I cruise down the highway, even when traffic slows considerably, I don't need to down shift. When I drive the back roads, I can generally use 5th without any problems. Cruising at 80mph, I generally run about 2600 rpm's.

My rpm differance between 4th and 5th is about 1,000 rpm's. That's a lot.

Since you'll have 2 OD gears, 3.55 might work well for you, too. Easy cruising at 80mph. But not a huge drop if you do need to down shift.

In a street car, I like a low first gear ratio; mine has a 3.29 1st gear. It makes the car very easy to drive in traffic, in parking lots, into the garage, onto a trailer, etc. Although I could, I rarely start out in second.

[mpanten]

I had a 3.27 and am in the process of switching to a 3.73. The 3.27 was ok, just wanted more umph. I did find that i was often cruising between gears, if that makes any sense. i need to be above 2k for the car to run right. I would go 3.55 but a lot depends on how your car runs at certain rpms and the driving you will be doing.

[Hurricobra95]

Thank you very much. I guess I'll go for 3.55 for more drivable. Best Regards

[charles roybal]

My 408w w/530 hp 535 ft.torque 3:27..17's with 315's and I have a sever traction issue in 1st &.2d..some times 3d. I can spin my 315's in 4th at 60mph just by pushing the go pedal 2/3's down. I think 308's would be better or change over to drag radials.

[mpanten]

Originally Posted by charles roybal

My 408w w/530 hp 535 ft.torque 3:27..17's with 315's and I have a sever traction issue in 1st &.2d..some times 3d. I can spin my 315's in 4th at 60mph just by pushing the go pedal 2/3's down. I think 308's would be better or change over to drag radials.

You have some seriously lousy tires

[fordracing65]

Dont fear the gear!


!

[bobcowan]

Originally Posted by charles roybal

My 408w w/530 hp 535 ft.torque 3:27..17's with 315's and I have a sever traction issue in 1st &.2d..some times 3d. I can spin my 315's in 4th at 60mph just by pushing the go pedal 2/3's down. I think 308's would be better or change over to drag radials.

Originally Posted by mpanten

You have some seriously lousy tires

You have a significant problem, and it's not your gears.

I'm putting out 510hp on the chassis dyno, and I don't have a serious traction problem (unless I want one). I use a 285 rear tire.

[Bruce M]

Originally Posted by Hurricobra95

I was thinking of buying and building Hurricane Cobra. What would the best rear end ratios be? I am planning on using a 427W from Pro-Performance Unlimited with 550 hp and 560 TQ with a close ratio Tremec T-56 Magnum with 26 inch tires. I am between 3.55 or 3.27. Which would be best for a fast car for weekends or ocasional driving?
Best Regards

I have a wide ratio 2.52 first gear with a 3.70 RA ratio which is a great compromise as long as your engine has enough power to stay above the torque curve when changing gears.

[sspano01]

I have recently gotten my Cobra on the road. I have a 521 inch, 3.5:1 tru-trac rear, and a T56-Magnum with the 2.94:1 first gear. I'm running 295/50/15 cooper cobra's right now.

I have been pushing my car a little harder now and thankfully have not had any traction issues. I can start in 1st,2nd,3rd gear. I have been normally starting in 3rd and shifting to 5th. Car runs very fast and seems quite controllable. I can lay rubber in 1st or 2nd, slip the clutch a bit and slap the gas and it lights the tires up.

Check my other thread that I posted called "scary cobra stories".

[gsharapa]

496 FE with TKO 600 (5th is .82) and 3:31s

[charles roybal]

Originally Posted by mpanten

You have some seriously lousy tires

Nitto NT555 315-35-17 ZR

[bobcowan]

That may not be the best street traction tire on the market, but it is pretty darned good. I use the same tire in a 285 width, and I don't have a traction problem. I can spin the tires when I want to, but it only happens when I want it to. If you have a "sever traction problem", there is something wrong somewhere. Your car will be a lot more fun to drive when you figure that out.

There's no such thing as too much power, only not enough control.

[mpanten]

Originally Posted by bobcowan

That may not be the best street traction tire on the market, but it is pretty darned good. I use the same tire in a 285 width, and I don't have a traction problem. I can spin the tires when I want to, but it only happens when I want it to. If you have a "sever traction problem", there is something wrong somewhere. Your car will be a lot more fun to drive when you figure that out.

There's no such thing as too much power, only not enough control.

I would have to agree, im doing about 450 at the wheels with 275's and dont have issues to that point, even with eagle gt's.

[Excaliber]

Pffffttt,,,, I'm running SLICKS and can burn all the way through 2nd and have to be careful in third.

TKO with a 2.97 1st gear turning a 3.31 rear gear with those tall tires. Some of us,,, really do have plenty horse power, some of us.

Oh wait, I just figured it out, this is a small block thread?

[mpanten]

Originally Posted by Excaliber

Pffffttt,,,, I'm running SLICKS and can burn all the way through 2nd and have to be careful in third.

TKO with a 2.97 1st gear turning a 3.31 rear gear with those tall tires. Some of us,,, really do have plenty horse power, some of us.

Oh wait, I just figured it out, this is a small block thread?

Much easier to spin the tires with a big block dumping oil all over the road

[sspano01]

Hi
I posted this other thread "scary cobra stories" asking some more questions about all these tire spin issues I read about from time-to-time.

I have recently gotten my 521 Cobra on the road and it (so far) has behaved very nicely. Fast fast acceleration, and I can spin the tires...but I have to make it do it (i.e. rolling start, slip the clutch, hit the gas fast, dump the clutch/etc).

During the build process, I spent time doing some simple modelling of the engine, car, tires,weight/etc. I did this in a 'fancy' engineering program (I own an engineering company) to try to deterime the effects of static/dynamic friction and torque (grip or slip points). My kids and I were actually outside in the summer with some different tires to make static-friction measurements on the street...

Anyway....I was not able to create a situation using a single torque vector that would produce tire spin. Meaning, I could only simulate a tire spin if the applied torque was something very large like 4000 foot-pounds (figure a 500ft-pound engine, through the tranny, and rear ~ you get alot of torque multiplication).

So this got me to another point, which may be more to the root issue. Its actually called "jerk" and here is a quick link on it.

Jerk (physics) - Wikipedia, the free encyclopedia

Jerk...is basically the rate of change of acceleration. When you apply this to a rotating object, like a tire, it becomes the rate of change of torque essentially. If I start creating a model of an engine that has some quick changes in its torque output, then the model I made spins the tires all over the place.


So....it would be really interesting to see a chassis dyno report on some of these engines to see what their torque curves look like. I think it could be that some of the smaller engines that have been cammed-up and stroked, may have some fast transitions in their torque curves.

So lets say your engine is @ 200 foot pounds at 1500 RPM and then goes to 400 foot pounds at 2500 RPM. This is a 200 foot pound change in 1000 rpm. Now lets say it takes 1/2 second to get from 1500 to 2500 RPM. You now basically have (400-200) / (0.5 * 0.5) which is 800 footpounds of dynamic torque IN addition to the 400 foot pounds of torque you reach a 2500 rpm. Once you get to 2500 RPM (and lets assume you stay at 2500 RPM), the 800 footpounds of dynamic torque goes away because there is no more change in torque output.

So I think in many of these cases...there may be rapid pulses of dynamic torque that snap the tires loose. One the tires break loose, the friction drops significantly and then the tires are much easier to spin.

From the reports I have seen on big blocks with stroker setups (like my 521) they are supposed to have a fairly flat torque curve from idle and up. So with a flatter torque curve, you limit these fast torque "jerks" on the tires.

Of course...my simulation model could be wrong as well....I need to meet more folks around here with Cobra's to do some burnouts with for scientific analysis :P

[charlandk]

I have a roush 427 with similar hp to your motor. I am also using the t56. I am running a 3.91 rear end. I love the grunt it gives and it still will cruise on the highway in 6th at very low rpms. The only down side is that 1st gear comes and goes way too fast. The 1st gear is almost useless. That said, I would go with the 3.55 rear end.

[Slick61]

... running 3.73 on my aluminum 427W (612 hp/615 tq), TKO-600 with the 0.82 overdrive. Has a fairly nasty cam, so is set up just about perfect... 75 mph @ ~3000 rpm... right in the sweet spot.

[D-CEL]

4.10

:d