Torque, Differential Ratios, Quarter Mile Time

[zgeezer]

I just had a long bench racing session with my computer addicted brother, who has "designed" and built three 10 second street cars... that are actually licensed and driven on the street. Two Falcons and one Austin Healy. Two of the cars were powered with 331 Ford Windsor strokers and one with a 427 351 stroker in a Falcon. Two vehicles with Ford's C4 auto and one (Austin Healy) with an late Ford overdrive.

 

The thing is that both of the Falcons were fitted with a Ford 9 inch 3.24 (I think that is the right ratio, the stock one just below the standard 3.0) with a detroit locker; the AH is an open 3.54. The AH is light... lighter than most of our Z (2200#); the Falcons were both in the 2900# area.

 

The Falcons used 14 inch wheels and soft radials.

 

His position, pretty much backed up with time slips, is that the gear ratio is only a torque multiplier. If you are running a low torque engine that builds 7,500 rpm, then you will probably need shorter gears. He uses the '57 Chevy with a 270hp/283 ci engine as an example: it was not a torque engine and needed 4.56 or so to get moving and had the rpm potential to carry the 1/4 with those gears. If you have a high torque engine then you can use the longer gears. His basic assumption is that if you put down enough torque to smoke the tires through the first two gears, then you need to either get bigger and sticker tires or back out of all that torque. One way to do that is to go to longer gears. The side benefit is longer gears are much more usable on the street.

 

He has run his computer programs [desktop dyno is one of them) to estimate hp on a stock LM7, LS1, and the high compression 6.0 liter truck engine [Advertised HP 345] in a Z that weighs in at 2550#, running 205x60x14 tires/wheels and an open rear end.

 

Here's the kicker: he calculates that each of these three vehicles will turn faster 1/4 mile times with a 3.54 than a 4.11 or 3.9. The truck engine would be as quick with a 3.15 as a 3.36.

 

This really runs counter to what I would like to believe. He cautions me that this is only for 1/4 mile drag times, where the variables are somewhat limited, and not to road racers, drifters, or other track cars.

 

For me, the practical side is I will crank, sometime in February, my mostly stock LM7 [2001 LS6 cam & springs cold air intake, and shorty headers] and I am choosing my differential gear set now. The car will run the stock AT; with one of these three R-200s : 3.9, 3.54 or 3.36 [all CLSD].

 

What has been the experience of the LS1 swaps?

 

How does one go about rationally choosing a rear end ratio?

 

Z

Edited December 15, 2009 by zgeezer
spelling grammar

[jc052685]

Pretty much all that was said I have always thought true. V8 Z is gonna do god with a 3.54 or 3.36

[Michael]

Simple quarter-mile performance calculators assume that horsepower application is a constant. Then they multiply by a penalty factor to crudely account for the engine's torque curve, traction losses and gear shifts.

 

What this means in practice is that "proper gearing" is that gearing for which the engine is kept maximally close to its power peak for as much of the race as possible. Too steep a gear would require an upshift too early, while too shallow a gear means loss of time while the engine struggles at rpm below its power peak.

 

The problem with all of this reasoning is that it applies to skillful drivers who shift correctly, and to well-sorted cars which achieve predictable traction. Being a lazy and conservative driver I almost invariably prefer steep gears, because they produce the grin-inducing (but not necessarily race-winning!) burst of acceleration on demand, even in the wrong gear.