flamin-roids

In order to get the highest ratios for fastest acceleration would I match up the following? S4 TII transmission with a S5 GTUs rear end (4.30 viscous LSD). Seeing as how my car will never see 140mph+ I'm not too worried and I don't want custom gears. From what I understand and experienced the S4 TII transmission pulls the fastest. Unless I'm mistaken. I've already got the S4 TII transmission in my car with the original 3.90 open rear diff. I'm going to rebuild the trans and replace the rear end. So is this the fastest acceleration (factory equipment) setup I can run?

powrdby13B

s4 tii's had a 4.1 fdr...
and yes that is the fastest rear end.

Evil Aviator

Choosing gearing isn't quite as simple as that. The gearing needs to be matched to the torque curve (some match it to the hp curve) of your particular engine setup, and to the particular race track. If you are concerned about drag racing, I think most drag racers use the 4.10 gearing. If you are just concerned with what "feels" the fastest, then yes, the 4.30 will feel the fastest, even if your race times suffer.

flamin-roids

I don't understand how times would suffer. Would having different higher gears affect the cars powerband?

Evil Aviator

Then why don't you use a 4.88 or 5.13 differential?

The powerband of the engine remains constant regardless of gearing (if you ignore ram air effects on the intake, intercooler, etc.).

All of the cars in the below link are using 4.10, although your particular setup may require a different differential. I think it's best to get some help from one of your local rotary drag racers if you want a good setup.
http://www.rx-7.com/events/062799.html

Hi_Im_Rob

I run 4.30's and have turned a more impressive time in my full interior T66 powered RX7@14lbs on 555r's than Steve did running ET streets.

*shrugs*

EDIT: Just stirring the pot.

flamin-roids

Thats odd. I was always under the impression Ari's car was faster than Karl's. I guess it isn't. I did get help from a local rotary drag racer. He told me to run a TII trans with a 4.30 rear end if I want to stay N/A. At least that way my gearing would somewhat make up for my lack of power. When I originally asked karl what to do with my automatic because I didn't want to do a swap at the time. Seeing as how I have the 3.90 open diff on my car a 4.30 would gain me 1/2 a second on the 1/4 mile. With a manual of course. So I got advice. I just hate pestering him with dumb questions.
*edit* Nevermind that was old news. I guess ari is running 9's now.

Evil Aviator

Well, then that's probably a good idea if you also use the tire size he stated.

My point is that I just want you to know that the 4.30 is not the universal answer to good 1/4 mi times.

flamin-roids

Ok would it hurt at all to have a 4.30 instead of the 3.90 I have now? I'm aware that top end would be affected (like my car would even have enough power) but isn't the point to have faster acceleration? And I'm not going pro drag. I just want to modify my drivetrain before the engine. I was told my car would pull alot faster with 4.10's or 4.30's. And if 4.30's will pull faster then I might as well go that route since I'm replacing my 3.90.

andrew lohaus

you should think about what type of trap speeds you are getting. if dropping down to the 4.3 ring makes you shift one more time before the end of the 1/4, the benefits of the shorter gears will probobly be lost. faster pulls+more shifts is not neccisarily faster than slower pulls+less shifts. ive beaten a number of cars that could "pull on me" in gear becuase s5 na's are geared so tall and you dont have to shift as much.

sit down and multiply out what your max speed in each gear will be for the different gearings you have. dont just go for the tallest, it may not be the fastet, at least over a fixed distance.

andrew lohaus

long story short, whichever gear will get you the closest to redline in 3rd gear with out shifting to 4th by the end of the 1/4, will almost gauranteed be the faster gear. now if you want to over rev with the 4.3, thats something to think about since it will help you out of the whole. may not help the trap speed much but could shave the et down as long as you dont need an extra shift.

flamin-roids

Well I have a TII trans matched up to a 3.90 open diff. I pulled a 1.92 60ft (But could have done better. Had really shitty tires) But trapped at only 73mph. I solved the problem. My ports were stuck and I had a disconnected sensor. I take 3rd all the way to damn near 95mph at 8000rpm. 3rd gear pulls really hard in these TII trannies. From 40mph in 3rd I can pull up to 80mph pretty quick. But I have been told that my rear diff was to tall when I let a pro drive my car. He's the one who recomended a 4.30. Basically with the power I have I will trap right as my 3rd ends with a 4.30. A 4.30 would drop me to around 83mph@7800rpm wouldn't it?