gearing

Hey guys, new member but long-time owner. I've had #079 for a little over a year. Love the car, biggest surprise has been how comfortable the car has been (limited visibility notwithstanding). Please bear with a short analysis, I do have a simple question at the end. Great forum by the way, glad to be here!

Recently I had the occasion to compare two cars, the Ford GT and the new Porsche Turbo S, regarding 2nd gear acceleration. It's a fair and interesting comparison, I think, MSRP's are similar (for the "S" Turbo). I'm comparing 2nd gear acceleration as a measure of (straight line) street performance, more meaningful I think than 1/4 mile.

As we know, motor Torque is multiplied by the appropriate gear ratios and finally translated to Force through the radius of the powered tires. Here's the comparative specs :

Ford GT Torque : 500 ft-lbs
Porsche Turbo S Torque : 457 ft-lbs
(we'll make the approximation that torque is flat across RPM's of interest)

Ford GT, 2nd gear ratio : 1.71
Porsche Turbo, 2nd gear ratio : 2.05

Ford GT, final drive ratio : 3.36
Porsche Turbo, final drive ratio : 3.44

Ford GT tire radius : 1.21 feet (315/40ZR19)
Porsche Turbo tire radius : 1.04 feet (295/30ZR18)

Few observations : Ford GT Torque wins ... at the motor. But those large diameter tires are certainly not your best friend for torque multiplication. In fact, the corresponding forces at the road/tire surface are :

Ford GT : Force (2nd gear) = 2374 pounds
Porsche Turbo : Force (2nd gear) = 3099 pounds

Ultimately, of course, we must factor in weight (mass) to determine acceleration resulting from this force. But there's little doubt which car will plant your butt in the seat when you stand on it in 2nd gear.

Now I know the GT is not designed for drag racing. But the chosen gearing is a bit questionable, to say the least. OK ... 0-60 times look impressive, largely due to being able to reach 60mph in first gear (no shift lag). But I respectfully submit that the gearing in this car is just not optimized for street use. Heck you can drive all day, at speeds far in excess of any posted speed limit, without ever reaching higher than 3rd (probably 2nd!) gear. The car is just geared too darn tall!

Finally my question :

Is a rear gear swap possible in this transmission? Say, something in the ~4.50's?

I know there's some exciting new discussions of pulley swaps and blower upgrades, but I think the car doesn't need more power ... just shorter gears!

Anyway, thanks for listening. I've searched, but nobody seems to discuss this 'dark secret' of the GT. Please correct me if I'm wrong. Again, thanks in advance, great to be here!

Thanks for the responses guys! That's a great link centerpunch ... very much what I've been thinking lately. I think the GT is a perfect example of a car whose gearing seems heavily influenced by the marketing of 0-60 times.

And wonka is correct, of course, about how apparent this issue becomes as soon as you drive the car. First gear takes you to over 60 mph ... hmmm. It's kinda funny to me that you still hear people saying things like ... "Man that car is so fast, you can almost reach 100mph in second gear!" Ummmm ... that's not a good thing. The quick response should be something like ... "Well, I wonder what all those other gears are for then?" Heck, back in the 70's (yeah, I'm kinda old) we used to put ~4.10 rear gears in street cars with only four speed transmissions (remember those?).

I wonder what the typical enthusiast would rather do : be able to brag that he only ever needs 2 out of the 6 gears, or be able to release all that power and smoke the tires in just about any gear he picks? Guess you guys know what camp I'm in.

You know, I kinda don't like my first couple posts sounding so negative. I really do love the car, it's a fantastic machine :thumbsup I'd just love it alot more if I could stand on it in third gear and break those huge rear tires loose!

centerpunch said:

That chart is from here:
http://www.roushind.com/news_downloads/tech_papers/2004-01-1252.pdf

I think the production cars are electronically limited to 205 mph, but I could be wrong. And I bet the top speed in 5th is faster than in 6th.

(Me, I'm a 50-year-old wuss, and 130 or so is as fast as I'll be going, even on a track.)

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centerpunch ... how much fun would the car be with a rear-end ratio that yields a top speed of 130mph in 6th (or 5th) gear? I know, do you really need a car that's traction-limited in 4th gear? Ummmm .... yes, please :biggrin

Too bad Ford didn't offer a rear-ratio option at purchase. I still wonder if a swap is possible with this tranny :confused

centerpunch said:

I think about a 20% change would be great. It wouldn't reduce the top speed.

Say, a change from 3.36 to 4.20 or so........

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It would be, hands down, the first modification I'd make.

That's a great graph ... a rear-ratio swap would simply "rescale" the axis (axes?). The tractive force on the y-axis would scale directly in each gear (in the good direction), and the vehicle speeds on the x-axis would scale down proportionately. The force require for speed would stay the same of course, but there's plenty of "room" in the gearing to scale forces up, scale speeds down, and still achieve an impressive top speed.

I wouldn't mind a 20% increase in tractive force across all gears with no motor modifications whatsoever :biggrin

Actually centerpunch, I think I understand your graph. I read "Tractive Effort" as the force generated at the tire/road surface. The second gear curve, for example, lines up pretty well with the "back-of-the-envelope" number I calculated (2374 lbs). But if so, there's a small mistake in the graph. The units on the y-axis should be pounds (lbs), not lbft. In other words, the y-axis is a measure of force, not torque.

The peak number for first gear, "3745 Lbs Force", looks correct.

I gotcha ... very informative! My only minor nit-pick is the y-axis units are pounds (i know, it's the original chart, not yours).

So short gearing of 4.20 is not a clear winner. But it occurs to me that we're only a short step away from calculating the OPTIMAL rear-end ratio, for optimizing time-to-any-given-speed, or time-to-distance. In other words, what's the best rear for 1/4 mile? Or say 0-100? That'll be a fun exercise I'll have to do one of these days :biggrin

Can you whip-up a little chart with a 4.56 rear? I think that's one of the old favorites ...

centerpunch said:

The only way to improve the gearing on the car would be to make the gears closer together, and yes, you could lower first while you were doing that.

I now don't see how any final drive change could help at all.

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Please try this : plot a graph with a final drive of 5.20 (OEM is 3.36) ... actually, any number over 5.13. See if there's any speed where the new gearing doesn't provide more tractive force, and therefore stronger acceleration. Of course, we'll see a top speed penalty ... but how significant for the street?

Sound like a crazy number? Maybe ... but not by much. Remember those huge diameter rear tires ...

This is really helpful centerpunch, thanks for your time. Please indulge me this ~5.20 request. If I'm right, and tractive force is improved over all speeds, perhaps a smaller drive number would still help alot, while not being quite so "extreme".

Thanks for indulging me centerpunch! A few key points, if I may :

1. Yeah I picked 5.20's for a reason ... it scales 2nd gear to look like first. Yep, just like adding a gear below first.

2. Note that with crazy 5.20 gears, the new gearing shows more (or equal) force all the way to 100mph except for about 80-90 mph. I'm willing to bet that 5.20's will get you to 100mph quicker.

3. Of course, what isn't shown in this graph (although it is very informative) is shift lag ... which takes two or three tenths from time-to-speed numbers. So there is definitely some loss associated with using more gears.

4. What's really kinda cool ... and if I may, I think it's what centerpunch has been suggesting ... is that gearing won't buy you more power, of course. Where does power show up in this graph? Well, power is the product of force and speed. Power being a constant means force times speed is constant, or force equals a constant divided by speed. It's a classic "1/x" curve, and it's apparent in these graphs that centerpunch made if you connect all the right-most endpoints of the curves. Nothing you can do with gearing will get you off the curve connecting the right-most endpoints.

5. As a consequence of #4, if we had many, many gears available, rear end ratio wouldn't matter at all ... only the "horsepower curve" generated by connecting endpoints. But with only a few gears at our disposal, rear-end tweaks can often buy you some performance on the street, where you are typically only using a few gears.

Oh hell, I just want to break the tires loose in 1st, 2nd, 3rd, ... :biggrin

Finally, centerpunch I disagree (I think?) with the stress comment. The motor and drivetrain (including clutch) before the final gear won't experience any more stress ... the motor power hasn't changed. The drivetrain components after the rear gear will see the stress ... axles, tires.

centerpunch said:

Uhh, yeah, you are right. I guess my brain was just tired after doing all those graphs for you!

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Thanks again centerpunch ... you're a gentleman and a scholar :thumbsup