TonyG

The cup cars are running either a 690 or 710 in the rear. In my avatar... the rear is a 690/330 tire (330 wide). The front is much shorter.

The front is much shorter. I'll have to look and get you the numbers on the front, but it's a much lower profile tire.

And yes.... gearing, specifically the R&P, is different on cars running the taller rear tires. The gearing change due to the tall rear tires is not trivial. That's why I brought it up.

TonyG

Duke

The latest cup's run 27/65 front and 31/71 rear. But given their weight distribution it's not the right car to take specifications from and adapt to our cars. Better to look at other FIA GT3 cars such as the Z4 GTR, Camaro GT, SLS AMG GT3. The Camaro runs 31/71 front/rear and both the Z4 and the SLS run 30/68 + 31/71 front/rear.

I'm pretty sure they don't set rake based on tire diamter. That is way too old school

CyCloNe!

Dang Tony that puppy is a work of art, I think the fender flares would look awesome if you do them, especially the smooth more arched ones.

V2Rocket

How high are you planning on revving the motor? Wring it out or just past the power peak? To figure top speed in gear.

333pg333

Oh for sure. Not comparing their setup or trying to emulate it, just curious what they run. I am still of the opinion that square on these cars is a good starting point.

odurandina

if it's a question of the top 4 gears of a 6 speed vs the top 3 or 4 gears of the 5 speed, with the regular stroke, you can run a '68 box and have the right gear/tire combo to come out of the corners in 3rd close to max torque (the stock 968 ratios look good for a race v8). i think a stroker engine will see a rebuild sooner which is why i'd take the lower stroke (if a '68 box can even take that level of punishment)... but the big-stroke Chevy isn't a big deal to rebuild.... and it'll be cool to see that '44 box pushed hard.
.

TonyG

It's a big trade off between a 968 box and a 951 box.

The boxes in terms of strength of about the same looking at the gears and bearings, shafts, etc...

The problem is that both boxes have a small 1/2 gear on the main shaft. Thus those are definite weak links. So.. as you said, it's 951 box (using a S2 R&P) with a 3/4/5 or a 968 box with a 3/4/5/6.

So what would you rather have... a potential situation where 3rd is too tall coming out of a slow corner, or a 3rd in the correct spot, but having an additional shift?

From my experience, I'd rather not have the extra shift. That's because all the cars in the top of the class I run in have sequential no-lift shift boxes. Every time I shift, they pull ahead.

So to take advantage of my wider power band, I prefer to use the wider gearing. This works out to be approximately 1 shift by me, for every 2 shifts by them using a 951 box.

And since there's only typically 1 or 2 turns on any track where I'm a little too slow for 3rd (I'm talking coming out of the corner at 3200-3300 rpms), it's a good trade off.

On the other side... with a 968 box... it's every straight section on any track that I'd have an extra shift ( vs 1 or 2 corners being a little slow for 3rd on a 951 box).

That's why with a V8 it's faster with a 5 speed than a 6.

TonyG

333pg333

I would also say that with your big jump in power and great loss of weight, that you won't be nearly as susceptible to bogging down than you used to be. Your car is going to leap out of corners compared to before. Having a longer geared transmission sounds like a good idea to me. I'd even say skip the S2 R&P in my estimation. As you say, the less you're shifting compared to your competitors the better.

TonyG

With the taller rear tires, the S2 R&P is a must. Without it you'd never be able to pull 3rd gear out of a slower turn.

TonyG

333pg333

I've not done the numbers, just making assumptions. That's why I think your power to weight is going to be phenomenal. Having driven a road car 3000lbs+ powered by a 300bhp 2.5L motor and the 5 speed with S2 r&p I can tell you that even on that car we changed gear pretty often. It's a great mod for 90% of the cars on this board. Not sure that your car falls into that category.

TonyG

Here's the problem with the tall 710 tires and the 951 gearing:

TonyG

333pg333

Yes the S2 rp will shorten the final drive but the space between the gearing will remain the same. Either way, it's going to be a rocket!!

TonyG

Yeah... the space between the gearing isn't really the issue.

The gears simply become too tall.

3/4/5 all become real tall.

5th, topping out at 192mph at 6500rpm is way too tall (at best we'd be lucky to hit 170-173mph with the 5th at the HP peak).

So with the S2 R&P, it's pretty close still.

Even with the S2 R&P, we still might need to go to the short 951 5th gear, but until we test we won't know for sure.

TonyG

333pg333

Where is peak power and tq in your engine?

TonyG

This was the engine using the smaller RH headers and the dual 2.5" exhaust.

PS > The engine now has much larger headers and a dual 3" exhaust, but this is not reflected on this dyno

TonyG