911racer

Hi,

What is the best way to select gears. Is there an online guide.

I have a 74 915 that I use with my race 2.7 L in a gt4 car.

Here are my current stock gears.
11-35
18-33
23-29
27-25
21-29

What is a good source for new/used gears also.

I currently hit the shift light (set at 6800) at the end of the back straight on Mid-ohio in fourth gear. I never use 5th gear right now.

By my calculations (and assuming I am going to leave 1 & 2 alone) My new gears should be:

11-35
18-33
22-30
25-26
28-23

This is about a .75 drop between all gears instead of the larger .68 step between second and third.

What should I be looking at on my dyno sheet to lead me to a new gear range.

Looking at my dyno sheets. I see that my max torque comes in at 4900 and my max hp comes in at 6500. Should I be tailoring my gearing to take advantage of max torque or hp.

Thanks

Ed

Steve Weiner-Rennsport Systems

Hi Ed:

When selecting gear ratios, one must factor several things to make intelligent and effective choices.

1) Engine RPM range. This means peak torque RPM and and peak power RPM. One also looks at the dyno chart to see how much HP falls away after the peak value. "Area Under the Curve" is very important, not, just peak HP.....

2) Ring and pinion

3) Rear tire diameter

4) Usage: this means what tracks you use the most.

I will also tell you that you really must change 2nd gear as this is so important for the slow corners. The whole point here is to reduce the RPM drops at each shift. Given the near 3000 RPM drop from 1st to 2nd, this is critical so you get effective use of 3rd gear.

If you are budget limited (and who isn't), leave 1st gear alone.

Depending on your engine's setup & configuration, look for a 1400 to 1600 RPM drop at each shift.

911racer

Hi Steve,

In 1), are you saying that I should chase torque or HP. Looking at the hp curve of my chart, I maintain within 8hp between 5500 and 7000 rpm. (6800 to 7000 is my shifting range)

2) I am running a 7:31 R&P

3) My rear tire size is 315 35 17's. So my tire diameter is 25.6. This would mean that my 7000 rpm speed with 28:23 gears would be 147mph. Yes, I reallize that my gear choices are mixed up because I was looking at a list of gears for the 901.

4) I mostly run at nelson ledges and mid ohio. I do like to travel, so that gets me everywhere from Watkins Glen, Mosport, VIR, Gingerman, and IRP.

I use my current second gear at nelson ledges and at Gingerman at one point in the track. Are you saying I should move my second gear taller or shorter.

I was targeting a .75 ratio move in gears. that would give me am 1800 rpm drop based on the 7000rpm shift point. If you are suggesting 1500 nominal drop, then I would move up to a .78 ratio move in gears.

Thanks

Ed

Bill Verburg

You want to gear for the hp curve

here is a summary of what you have, what you propose, and another idea. The question becomes what top speed do you need to gear for? Any idea?

Steve Weiner-Rennsport Systems

Hi Ed:

You don't "chase" either one: the goal is to keep the RPM between peak torque and peak power. This results in maximum acceleration and you can see how a wider torque range helps accomplish that.

Bill's gear idea is much better than what you proposed even though its more expensive,...
Since your motor appears to have a narrow range of usable power, you should maintain your splits to a 1500 RPM drop or less.

Gear choices are also affected by the tracks so I'd ask what RPM you are at in 2nd gear at those tracks you mentioned. Unles you are willing to build up a "full-active" tranny by raising 1st gear to make it usable, choose your second gear so that you are neither lugging nor screaming the engine in the slowest corners.

Just remember; everything will be a compromise not matter what you choose unless you are wealthy enough to build up some spare gearboxes with different ratios,...

Bill's program looks cool; I use a special proprietary bit of software for doing gearboxes that makes it easy to see the RPM drops.

Bill Verburg

Here's the drops


and a graph