mark kibort

just think, a 4.45 rear end could get you to a 25mph 1st, then the 3.09 39mph 2nd gear equiv. and the rest would be the same to 118mph. now, there could be an argument on why a 6500rpm at 26mph would be useless. (and an effective top speed of the vehicle of 118mph in the new 5th gear with a 4.45)

its not about just going lower and lower, its about matching the vehicles power and grip, and better said, maximizing the power to the wheels overthe operational range of speeds used.

Mk

m21sniper

The 3.09 will finish the 1/4 about one full gear 'ahead' of the 2.20 car.

Ie, the 3.09 car will have used a larger total % of it's potential power output vs the 2.2 car.

Remember, for a drag car ideal is redline(or max HP) in top gear at the traps.

So while a 5spd 2.2 may finish at redline in 3d, the 3.09 car will be at redline in 4th, having used fully 20% more of it's total potential power.

This is STILL not ideal, as all of 5th is left unused. Therefore a 4.09 would be perfect, leaving you at redline in 5th at the traps.

A 4.09 Auto trans would be even better though-Much faster (and fewer) shifts.

mark kibort

We have already discussed, that in all racing , you want to be near max hp as much of the time as possible and at near redline at the end of any straight (drag or road racing)

what it seems like you are failing to recoginize with your (20% number) is that the ONLY change is in the 1st gear. (gain of effectively 26% in that gear to 39mph) after that, there is NO difference in power to the wheels NONE!! this is BECAUSE after 39mph, both cars (ie 3.09 to 2.2) would have the exact same ratios moving toward the finish line and approximate speeds of near 115mph, or near redline in the FINAL gear , whatever you want to number designate, or call it!!! First of all, do you understand this??

next, going to the 4.45 (which would be still another 1.4x gear change and follow the stock spacing ) you would then shift the gear set down again, and have a 26mph 1st, 39mph 2nd, and then the same as the 2.2 1st gear 55mph 3rd, 80mph 4th and 115mph 5th. the difference in this case would be the 1st 2 gears now, and then after 39mph AGAIN, nothing would change. i think all would agree that this 4.45 in our gear spacings would be useless. Ive already concieded that in a drag situation, the 3.09 would have the advantage in the new 1st based on the 26% more hp to the wheels based on "perfect" grip. the grip factor if reduced or not taken advantage of, will only drop that 26% gain in only that particular speed range to 39mph.

you are getting confused of the ratio multipliers and close ratios scenareos.
the gear box has a fixed spacing, all you are doing with the 3.09 or a 4.45 is shift the entire box down a full gear space. again, after 39mph, there would be NO difference in a 3.09 vs a 2.2

if you disagree show your work on how you got your 20% gain over the entire speed range in a drag. as I calculate and have shown the work, i get 26% gain in 1st gear to 39mph and the rest is equal. taking the rate of doing work (ie HP) over time, you can see that 26% for .5 seconds vs the entire speed run of 14 seconds, would have a total effect on the average power of less than 5%this is why you would only be able to see a .5 second or less differecne at the finish line with a 3.09 vs a 2.2 stock s4.

It also comes down to common sense too. If you were right, then an average hp gain to the wheels of 20% would be equivilant to near 400hp vs a 320hp car just with this gear box change. obviously, this is not the case.

ideal wouldnt just be using all the gears, indescriminately, it would be developing a new set of gears and spacing. so, if 1st is best to be at the 3.09 12.5:1 ratio, then you could get 4 more gears all the way to 120mph evenly spaced. that would be the way to do it.

Sorry,

mark

Imo000

0.5 sec in the 1/4 mile is the same as adding ~50hp to the engine or taking ~500lb off the car. Half a second is alot in drag racing. Depending on the trap speed but it can be as much as 2 car lenghts.

mark kibort

I understand, my number was just an approximation.

again, what i do know, is the 3.09 vs 2.2 as it only effects 1st gear up to 39mph, is only like a 60hp change to 39mph. after that, there is NO diff. how that effects average hp to the wheels is not that hard to ball park. the actual difference is that 60hp for .5 seconds and the rest of the run is the same. after that, you can average the 60hp gain for that .5 seconds over the entire 12-14 second run. you can easily see that the net net of the gain for the 3.0 is less than 20hp, average over the entire run. (and thats being generous)

Mk


we are talking a couple of tenths here, and yes it would be faster based on grip equality. its not the 20% overall as sniper has mentioned. AGAIN, the difference is only in the first 39mph in the "new " 1st gear with the 3.09. after that, as we ALL know, its SAME SAME SAME.



mk

Imo000

Ok, so let me try to understand this. Let’s say there are two identical 5-speed 928s, one with the 3.09 and the other with the stock 2.20. If they are for example, both drive next to each other at 50mph and both engines are close to the same rpm. When they both floor it, will they be accelerating at the same rate?

mark kibort

yes, this is the way it is. BECAUSE, both cars at 50mph would be in near idential ratios. IN fact, bring the cars back to 40mph, and you would have the same situation. However, below 40mph (ie 39mph and lower) the 3.09 would have the 26% hp advantage due to the 12.5 1st vs the 8.9:1 1st gear (* that is until 39mph!!! )

Mark

6.0-928S

And if both cars were cruising side by side at the beginning of fifth gear (albeit at different RPM's) & they both stood on it the 3.09 would walk away from the 2.20. Period.
Hammer

6.0-928S

One tenth is a car length in drag racing.
Hammer

Imo000

DOH.... I should have looked that up before posting it!

GlenL

This is just fuzzy thinking.

I believe what you're saying is that going from shift to shift (to redline and back) more times will get more power out of the engine. That's just not the way it works. What matters, from a power perspective, is what is the median engine output over the time period. Whether you use 3, 4 or 5 gears is immaterial. Now if the gears themselves were closer the engine could be run in a higher power band and that'd would be an improvement.

The reason a higher ratio rear end is better off the line is two fold.
1) During launch the engine has a larger mechanical advantage over the wheels
2) After it is hooked up the engine is running more RPMs and producing more power

Imo000

After reading all these theories, there is only ONE way to settle this argument!

GlenL

Thumbsies?

6.0-928S

Right on
The Rixter is helping to do a service to the 928 community. The gear vendor has patterns for manual trans 3.09 ring & pinions. He doesn't have them for autos. Rixter has volunteered to disassemble an automatic diff. to ship me a ring & pinion. I will in turn ship it to the vendor at my expense. He will draft up a blueprint & call me with a price. I asked him about a group buy & he said we can discuss that when he calls me back. If we pull this off & I can get one in my car I'd be more than willing to put my 3.09 into the staging lights with a certain car with a 2.20 or whatever is in there. Breaking the laws of Physics-priceless!!

Hammer

Old & New

But what of that? It means you have more torque in the 3.09 since you are running at higher rpm's, of course. If the 2.2 drops a gear, then they are about equal again.

If a driver wants drag race performance (and invests in getting the torque to the ground), the 3.09 will help in an otherwise stock 928. After spending howmuch(?) accomplishing this, they may need to change gears again if they decide to increase engine output.

At a certain point, more low gear torque is not desirable. My first gear requires careful modulation to avoid wheelspin, and second gear breaks free when the tach reachs the top third of its travel. This is with centrifugal boost; I imagine the turbo and positive displacement boost cars would be affected even more negatively.

I'm not sure I'd like seeing the tach so high when cruising at 80. Drive a few days in fourth to get a taste of what it would be like to run 3.09's. IMO, spend a couple G's more and boost power instead of lowering gear ratio.