Z06

What affect on a race track would gear ratio changes make?..Is there a real gain?

How would you determine the proper or near perfect gear ratio?

Has anyone changed there gear ratio?........Was it worth it?

Geoffrey

I have changed the 2-5 gear ratios on my turbo and it made a huge difference. It was probably the single biggest gain of any individual mod. While this was a turbo car, it applies equally to a N/A car and allows the engine to remain in the torque and minimizes the torque drop between gears.

I picked my desired speed at redline in 5th gear and picked the 5th gear ratio. I then used a combination of a dyno sheet showing torque and HP and a spreadsheet I developed that shows the torque drop with various gear ratios, then worked backwards.

JackOlsen

For a novice driver, it won't have very much effect. As you start to squeeze the last few Nths out of your car, though, it makes an important difference.

There are two (somewhat overlapping) schools of thought on how to do it. One is to map out the torque curve of the particular motor, and gear accordingly -- the idea being to keep the motor in its torque sweet spot more of the time.

The simpler way is to look at the maximum speed you'll be using on the tracks you drive (it's lower than you think) and set that as the ceiling, setting up evenly-spaced intervals down to 2nd gear.

My car has a 5-speed 915. My redline in 5th is set at 136 mph. I have a taller-than-stock 2nd, a stock 3rd, and shorter-than-stock gearsets for 4th and 5th.

There are trade-offs, of course. Lower gearing means a lower top speed, should you want to go out and set a land speed record. Cruising on the freeway at 80 in my car means the revs are very close to 4K, which is not great for my ears or my gas mileage.

Also, a taller 2nd compromises my car's 0-60 capabilities, since the jump from 1st to 2nd is longer.

In a perfect world, you'd have a gearbox for each track you drive.

Z06

I was thinking about the maximun speed on the Track i most often use and work back from there, 120 mph at the braking point, i could go a little bit quicker...maybe 124 max mph top of 4th, still getting used to the car, but not much more then that (124 mph) and just under 160 mph in 5th on a different track.


Peak hp is at 7300 rpm, GT3 guys think we should be shifting at 8000 rpm, and peak torque is around 5250 rpm.

shifting at 8000 rpm from

1 gear drop to 2 gear 4500 rpm -8000@rpm = 78 mph
3 gear 5800 rpm 108
4 gear 6200 rpm 139
5 gear 6600 rpm 168
6 gear 6800 rpm 197
These are very close to the actual numbers.

Stock gearing does not land on peak torque # in the higher rpms/speeds.

These cars have variable camshaft timing which i think increases the rpm band, should i be looking to land higher in the band 6000 to 6500 rpm as it is stock in the higher gear.

Adam Richman

I think it will really come down to budget at an understanding of what you want to gain. Its not uncommon at all for SCCA Production cars to have multiple (and multi-thousand dollar) gearboxes for the tracks they visit. In Improved Touring (rules reasons prevent changing gear sets, only FD can be changed) based on the tracks they run, folks might either carry multiple final drives or they might run two differing tire sizes (both affect the same thing by and large - cost of 2 sets of rims vs. 2 transmissions ?? but no affect on handling ... tradeoffs).

IMO, FD is cheaper for a track only car provided someone is making one for it. The tire height change would be doable if its only a minor change you are looking for. Since tire height or final drive cannot effect the RPM drop between gear changes (presuming shift points are the same), I would look at your drop between 4th / 5th (or 6th if you have) and see if you really want to be topped out in your top gear - in the early 944 N/A for instance, its like a 1.07 to .73 drop to 5th so I'd prefer a larger FD (lower number I mean) and make the car effectively a 4 speed. In my 944S (actually the CRX is even bettah ), its a much smaller differential between 4th and 5th (think 1.03 to .83) so I would look to top out 5th at a track like LMS (fastest top speed I see around here).

Basically, by either changing the gears themselves or changing the final drive ratio, you can see some big gains at some tracks and even some disadvantages at others. There is no way to find the perfect gears and fd ratio for all tracks (unless you drive a F500) - if you can find one that works at the most important or most enjoyable to you; that's about as good as you can do with one box IMO.

carreracup21

I'm not an expert in this, but I've read somewhere that hp was more important than torque once you got going faster so in the higher gears it may be better to stay in the high hp band rather than going down to 5250 for peak torque ? That said, I agree there is no sense in gearing for a speed you will never reach unless you are looking for improved mileage. ( not likely on this board )

Sam Lin

The posts here concentrated mainly on getting max speed at redline for each track - that's one aspect of gearing to the track, but really only involves top gear and final drive. What's more important for each track is how the lower gears fit the track - where is the shifting being done, where is the car's power band?

For example, if the track has a long sweeper where you can just hold a speed that's just a tad too high for one gear, you'd be better off with either a taller lower gear or a shorter next gear, since with your current setup you'd need to shift up and be at the lower end of your power band exiting.

If the track has a series of turns (chicane) where your car can take the turns at the upper end of one gear, but the short straighs in between require a shift up, then you would be better off changing gearing to save the 2 shifts each turn (one up exiting, one down entering the next).

There are endless iterations of these types of situations, where your current gearing has you making an upshift only to downshift very shortly after, or where your car's speed through a corner opening onto a straight means you're stuck in the bottom of your power band at the front straight. It's these situations where optimizing gearing for each individual track will net large gains.

Sam

Bill L Seifert

There is another way of changing overall gear ratios, other than changing the ring and pinion. Try changing tire size. I don't know what tires are available in for GT3's, but Hoosier makes a bazillion tire sizes as does Khumo. On my 944, I had bought a 4:11 rear gear for my car, and was shocked at what it was going to cost to put a $150 gear in my transmission. I used to run 225-50-16's or 245-45- 16's or if I ran 15" wheels, I used 225-50 15. All three of those tires have approximately the same diameter, so the gear ration is the same. Then I discovered the Hoosier 225-45-15. It is about the same width as the above sizes, but has a lot smaller diameter, effectively lowering my over all rear end ratio to 4:11 to one. I solved my problem, by buying tires which I already needed, and didn't have to spend the bucks to take my transmission apart.

Talk to a Hoosier, or Khumo rep, and he may be able to get you the rear ratio you need.

Bill Seifert

1987 944S Race Car

Geoffrey

Here is an example of what I use to determine gear ratios for a given engine. Torque comes from the dyno, all other values are computed.

What it shows is the axle torque for a given gear ratio, so as you shift, your new axle torque will be the engine torque * final drive ratio. Laying it out like this helps you understand the true torque that is realized. Black numbers mean you lose torque, red minus numbers mean you gain torque.

mds

If you look at the GT3's HP versus RPM curve, there is a relatively larger drop in HP below 6500rpm, so ideally I think it makes sense to keep RPM above this point after each shift. The 996 Cup car and in particular the 997 Cup car do this in 3rd through 6th gears by using an adjacent gear ratio in the range 120 to 125%. The street car on the other hand uses this range only for 5th and 6th gears, but on the track, at least in CA, you are rarely if ever going fast enough to use those gears. There is a much larger gap between the lower gears so RPM drops quite a bit below 6500rpm in these lower gears. I think switching to the Cup gearing would be a good choice.

tdf360

One thing I noticed when I got a ride in a GT3 at Buttonwillow a few weeks back was that the gearing was much taller than my 360. And in R&T's recent road test of the 997S, I noticed the gearing was much more similar to the 360. I find that strange.

GT3 speed in gears:

1 - 46
2 - 82
3 - 113
4 - 144
5 - 171
6 - 200

997S Speed in gears:

1 - 41
2 - 69
3 - 100
4 - 125
5 - 148
6 - 182

360 Speed in gears:

1 - 49
2 - 75
3 - 100
4 - 127
5 - 157
6 - 189

Gary

JackOlsen

Changing final gear ratio (via tires or ring & pinion) is a completely different thing than creating close-ratio gearsets. The former just lowers gearing, which has some benefit, assuming you aren't shooting for top speed. The latter changes the space between gears, allowing shorter 'steps' in up- and down-shifting.

mitch236

i was told not to change my gears as the replacemerit gears are not manufactured to Porsche quality specs. i had a chance to drive a similar car as mine but with close ratio gears and what a difference! i felt like i was always in the power band

JCP911S

If you are racing a stock class PCA car for example, gearing changes are the single biggest advantage you can get IMHO (lets forget for the moment that it is illegal).

The key is fully utilizing the powerband at the exits of key turns. The tow of the boot at the Glen is a classic example. It is uphill leading to a straight. 2nd is loo low, so you have to shift and lose momentum. 3rd is to high so the car bogs the whole way up the hill.

In an SC for example, a gear right in the middle is worth a solid three car lengths or more. Not only does this yield faster lap times, more importantily, it gives you a virtually guaranteed passing opportunity down the straight...

That one change could be the difference between 5th place and the podium

JCP911S

Oppps... I meant TOE of the boot.... there is little drafting opportuinity here