915 Gears - 2.5 Ltr Eng

[Ben M]

I recently built a 2.5 liter short stroke engine and mated it to a stock 915 transmission. Not a good match. This is a race application. Should I change to ring and pinion to a 7/31 or new 3rd, 4th & 5th gears;
3rd - 21/31
4th - 22/29
5th - 24/27

I use 16 inch rims, although I have a set of 17 inch rims which in accordance with the charts would work assist on tracks with long straights.

Appreciate your thoughts and recommendations.

Ben

[KTL]

Need more info on the tracks you run. With a 27/24 5th gear (1.125, same ratio as my 4th) I think you'd have a rather low top speed, even with a 8:31 std. ring & pinion.

What tire size do you run. That's another key in knowing what your speeds are going to be

As an example of a compromise gearbox that does a fair job of being able to run short tracks and long tracks, here's my gears

8:31 R&P
19:32 2nd
22:29 3rd
24:27 4th
26:26 5th

I have a 7300 rpm redline so I can get up to just over 140. Which is good enough to run Road America or Road Atlanta since I only have ~220 rwhp.

That spacing you propose is very tight. That would certainly keep you high in the rev range if that's what you need. Switching to a 7:31 will just lower your peak speeds in each gear. Won't do anything to change the spacing of gears (obviously) and you may still find spots where the engine is "off-cam."

[Ben M]

Thanks for the reply and recommendation.

I run 255/50/16 and have about 240 rwhp and shift at 7,200, redline is 7,800. I was at Road American last weekend and had to shift into 2nd gear all the time, going into third the rpm's dropped off so I was out of the torque curve, it sucked. Never even used fifth.

I agree the 140 is a good top end, maybe 135 is OK if I gain in torque on the rest of the track? My though was to run 17 rims on the long tracks and 16's on the short tracks, the 17's add 5 mph to the top end. At least according to the charts.

I'm also chasing gremlins in a new electronic fuel injection management system, so more adjustments may be necessary to air fuel ratios.

[KTL]

Yep that's one of the disadvantages of having a higher shift point/redline. When you take advantage of that, it stretches the already compromised street-based gears even further. Proof of that is you never used 5th at Road America! My car by comparison, i'm afraid will run out of gear at Road America at 7500 in 5th and that's why i'm always on the lookout for a 0.96 5th.

The larger rolling dia. 17 in. setup will stretch your short gearing but it'll also kill your acceleration in each gear. HP is king at Road America so you want to find a balance of accel and top speed. Do a search here and you'll find Bill Verburg has shared some nice info on thrust caused by tire size choice. That, and calculating your optimum shift point with a dyno plot can make a considerable difference.

[ned911]

With my mostly stock 2.4 T and S 225/50/15's made a huge difference on acceleration versus 225/50/16's. Was very happy when PCA finally dropped the minimum rear tire OD rule.

Maybe try 15's.