Sideways

[QUOTE=afwfjustin,May 28 2006, 04:11 PM] You go faster because you get your torque sooner.

Race Miata

As I mentioned in the 4-page long thread Sideways linked, 4.77 makes you faster because it's easier to reach peak HP at the start and it makes the gear ratios statistically shorter, not because of the improved wheel torque from each gear and definitely not because you can run out of each gear sooner (which is actually hurting your performance, more details later).

In reality, you run out of each gear MORE THAN 16% sooner because besides the higher wheel torque you get from each gear, the road speed range for each gear is also shorter (i.e. you don't accelerate to 43mph in 1st gear any more, you accelerate to just 37mph). That's why gears give very dramatic FEEL in improvement.

However, running out of gear sooner means you also need to run a higher gear (lower torque) sooner, which cancel out the improved wheel torque in the previous gear. The disadvantage of 4.77 over 4.56 for 1/4-mile performance is NOT mainly the delay shifting to 5th, it's because you simply have to run 5th gear with its lower wheel torque.

Gregg Lee

And you have to upshift 16% earlier. Once you upshift you have less wheel torque than stock gears. Only when the stock car upshifts at a higher speed do you get ahead again.

And of course every gear "feels" better, at every RPM. But at the same RPM you aren't going the same speed as the stock gears.

Lower gears have an advantage off the line, but after that the torque versus speed curves cross back and forth as the shifts occur at different speeds. And lower gears require more shifts.

Lower final drive gears will improve 0-30. But on average they have no effect on track performance - you will just be running in higher gears - 4th rather than 3rd for example.

Race Miata

Almost true but don't forget that gears bring ratios statistically closer so you're like having a closer-ratio gearbox with shorter final drive. For example, with 4.77, 5th gear ratio is very similar to stock 4th gear. But because 4th-5th is closer to 3rd-4th, your 4th-to-5th shift with 4.77 (at road speed which would've required 3rd-to-4th shift stock) will have a lower RPM drop and improve performance.

Sideways

But be in a slightly lower thrust range in 5th ameloriating most of the advantage of the closer gear ratios of 4-5. Everything is a trade off. Gain a little here, loose a little there.

The way I would use to determine the best gear ratio & tire size for your particular car is to graph out the senarios and calculate the acceleration vs speed.

Race Miata

Oh yeah, for sure. And that's why my word "statistically".

Don't discount the importance of close ratios especially for track performance. When you're accelerating on the track, it's not necessarily on a flat straight line. That means you might have to stay longer within an RPM range not in favor of best acceleration possible. In that case, a closer ratio setup will statistically improve performance more significantly.

SpitfireS

After reading this post and the other linked post I think I understand.

Less shifts with stock.
More rev drop.
Total area under the line is (only slightly) less => less acceleration.

More shifts with 457 (or 477)
Less rev drop.
More area under the line => more accelaration.

Only IF all shifts are perfect.
1 bad shift and advantage is gone.
IMO revs are in relation with HP => more revs more HP.
HP per second is acceleration force.

Sideways

Looks like you are on the right tract.
Please excuse this chart, it is oriented sideways (no pun intended ) It is thrust vs rpm in gear.
In the current orientation as you shift you go one line to the right and thus drop in thrust.
As you can see the shift always puts you into a lower thrust situation so maximum rpms in each gear are beneficial to obtain maximum thrust.
With lower gears you end up going to the lower thrust line sooner. With the 4.77 vs 4.10 example it is 16% in first gear, 32% in second gear, 48% in 3rd gear, 64% in 4th gear 80% in 5th gear and 96% in 6th. Sixth gear with 4.77s is very similar to 5th gear with 4.10s (4% difference.) If I charted this information with the 4.77s in place of the 4.10s the 6th gear line would be almost superimposed over the 5th gear line in this chart and the car would be top gear limited to the lower speed.



The car with taller (numerically smaller) gearing leaves the car in the higher thrust for a longer period and will give greater acceleration during the period where the car with lower gears has already shifted to a higher gear (that 16% in first, 32% in second, etc.)

Eventually the concept will become clear. There are several different ways to look at it and several different terms used to explain it.

If you calculate the trust you can divide it by vehicle weight to obtain G forces. One G is 22mph/second acceleration so you can chart out acceleration curves minus the drag and traction variables. Physics was never this much fun in Jr. Highschool.

Race Miata

Your pic is not right. With just shorter final drive (without changing the actual gears in the tranny) the RPM drop on upshift doesn't change. What's changed is that each RPM drop (each shift) occurs at a lower road speed. In your graph (which won't illustrate the benefit of closer ratios), the X-axis will just squeeze tighter. For an engine with a given HP, the lower the road speed, the faster the acceleration and therefore the shorter time it takes the engine to get up the RPM and therefore the more RPM drop that the setup can tolerate. This is exactly why a performance manual transmission runs closer ratios on higher gears.

When you run a shorter final drive you pull the closer-spaced high gears lower. At the speed where you would've upshifted from 3rd to 4th (dropping to 7054rpm from 9000rpm), you'll now be shifting from 4th to 5th (dropping to 7526rpm, closer to peak HP). I know your new 5th gear does not have the exact overall gear ratio as the old 4th but the trend is there and that's why I say statistically (when you have to do a drag for more than a single upshift).

afwfjustin

Every gear set has it's ups and downs. My explanation was a very, very simplified one. You all went pretty nuts with the math and graphs, I'm impressed!

On the street, I will say that, despite this and that and shifting and speeds and rpms and etc, having gears makes you pull away from other cars much, much faster, it's as simple as that.

I've run a stock S on the street once since I got my gears and it was a complete blowout. Yes, I have some power adders and weight savings, but the difference was drastic.