A recent debate has come up and the issue of gearing is in question.

First the myth: Gears kill traction. Too much power does that. Gears for those that have them, allow the car to rehook faster than the faster rotating tires with stock gears would if traction is lost. 60's tell that story.

The evidnece of this is with tuner cars that I have seen that run 3.73's with 700rwhp in the nines and manuals with 3.90's that run 9's. Cartek's R&D car with TT's ran 9.9's with 3.90's on a Z51 transmission.

Chris at ECS ran 9's consistenly with 3.73's on his auto with s/c and an N2O hit.

So many guys dont think gears will allow them to run faster trap speeds.

One guy recently said his car would run hotter in 6th gear with 3.90's than 3.42's. Any input on this last one? I am N/A and run 176degrees in 6th with 4.10's on the hiway. I dont understand a car running a lower load in a slightly higher rpm is going to run hotter. What am I missing.

Obviously it is more credible to have the gears installed to make an observation/statement than a guy with a theory and stock gears.

Thanks in advance for sharing...

 

as to the question of temp increase with higher (numerically) gear ratio, I don't really know that answer. Maybe he was running the car harder with his new gears and noticed an increase in coolant temp?


I can tell you for sure that your car will ET and MPH better with a lower (numerically) gear in a F/I application. The tuner cars you mentioned above all ran great, but I bet if you asked them, they would each tell you that the car was still spinning the tires going down the track. By dropping to a 3.15 in any of those combinations, the tire will slow down just enough to re-gain traction, and transfer the wheelspin into forward motion. It's usually worth 3-4 tenths and 4-5 MPH in the 1/4

I have made this change on several cars that ran consistant 9.90's and dropped to 9.50-9.60 after nothing but the gear change. The M/T cars you mentioned probably did benefit a little from the 3.73-3.90 ratio on the gear shift like you said, but with over 700 HP, it's not long after the shift that the power will overtake the traction and give you some wheelspin.

Hope this helps you, it's just my .02 but I've seen it work time and time again,

Ed

 

Well said Ed,
The car we ran the 3.73's with was only because of them being in there already before the big power and we thought they would help so we didnt change them, well we thought wrong. I can only amagine what that car would have ran with a 3.15 It made over 800rwhp with the 3.73's and only ran 9.7's, today we would be in the 9.3's or better with that power level, but that's progress and the learning curve.

 

Too bad you didnt then we would have seen for sure.

 

I credit the gains you said you had however your comment on the rotation speed of the tire is incorrect.

At any given rpm the tires spin slower with numerically higher gears. At redline for instance a 3.42 geared car with Z51 is spinning at 53 mph in first gear and at redline with 4.10's they are spinning at 43mph. Sower rotations per minute. Thats why 60's drop with numerically higher geared car and sane HP. Using 3.15's on a manual car if you check has a final gear ratio equal to starting in second gear with the higher ratio...ie starting in second with 3.90's has the same ratio as starting in 1st with 3.15's.

Rotation speed is what gives the faster rehook. If you launch at 2000rpms with 3.90's the tires are spinning slower with 3.90's then they are with 3.15's by over 25%. 2000rpms in 1st with 3.90's is 20mph and its 26 with 3.15's. The car is hooking when the car gets to the speed of the rotating tire.

My dart, while not FI is 765rwhp and 800+rwtq and couldnt hook at until I got to 6.13's. (solid roller-10,000 redline). The 60's and ability to hook werent so good even at 5.88's. In this example while the transmissions are vastly different they are still both 1:1 in 4th. The trap speed for this 6.13 geared car is 138 (about the same as 3.90's in a vette) and obviously at 10k rpms the tires are moving about 1/2 the speed (rate of rotation) than the modeled car you speak of with 3.15's. The speedo doesnt work in the car so I cant say for sure but at 5k rpms in 1st gear is about 18mph. The tires are spinning way slower than If I had 3.15's at that same RPM.

The entire concept of overdrive is to have extremely fast spinning tires for a low rpm. 3.15's allow a higher top speed and thus spin way faster than the 3.90 counterpart. A tire is moving slower in 1st gear at redline than at second at redline which is slower than 3rd at redline...ect. 3.90's is like a gear under 1st so its slower than 3.15's in first. I cant think of any more ways to illustrate it.

If this still isnt agrred upon I will do it in rotations per minute with the final drive ratios for each gear set and a given tire for first gear.

Think about it. My car would never hook with a 6k launch with 3.15's. The tires are spinning 2x as fast. What you said makes no sense to me.

One thing I have to agree with is that a car with the ability to ET at 9.3 would have a trap speed higher than 3.42's could give you. The 60's would not be as good but the 3.15's would be the correct gear.

Are you saying that with 450rwhp on a typical head cam combo I will gain 3-4 tenths (or any amount) by going to 3.15's from 4.10's or 3.90's? Most guys gain by gearing numerically higher.

 

Any ATI kit guys in the 550rwhp range have any track experience with this?

 

I think your sentence above answers the question. You are correct in your thinking on SANE HP levels ( N/A ) but an F/I car acts a different way. It makes xxx amount of power untill x,xxx RPM when boost comes in, then the power level increase dramatically, and with numerically higher rear gear, the increase in power easily overtakes the traction available.

On your head & cam example above, the total power output is not enough to break traction at higher RPM, so the 3.15 gears would hurt ET. Now if you added a supercharger or turbo making 600-700 RWHP you would see that you would start to spin the tires shortly after reaching full boost (with a 3.42 gear), but with a 3.15 gear, you won't see that wheel spin.

I probably should have specified that this practice only applies to drag racing F/I cars that are above 600RWHP.

Ed

 

At600whp I roast the tires when the boost comes on in 3rd gear and will immeadiately bump the limiter if I'm not careful and can easily spin 4th if I don't peddle.with my 427 build that is under way, I expect to be around 800whp and will have no choice but to go with a lower numeric gear.

 

Maybe Im missing something here, but its not just the tires rotating speed at any given rpm. At 50mph with my car (and crap sports), I would have rather dropped the hammer at 4500rpms than 5400rpms where I am making about 70rwhp more.. I do however understand you're point at coming off the line, Im talking about after the car is moving..

Looking soley at the rotating velocity of the tires at redline makes conditions look like they are better in 1st and second, but when I look at the vehicles velocity as a constant I see the opposite..

When you doin that blower?