Club Cobra - Gearing Selection Advice

there is no free lunch, all gearing is some compromise. I too have an SPF, and have the 600 with the .82, but a 3.27 rear diff ratio.

You can run the speeds and rpm's on Mike Stenhouse's gearing selector as above. I think for typical american driving, where you won't be cruising at 90 mph and up, it really is the bet choice with the 15 inch wheels. If you run the 17's, it might be good to consider the 3.08, as they are smaller. One of the nicest things with my set up is the long legs i have in the gears, i can downshift at 80 mph to third, and have 40 more mph available before my 6K redline.

Gearing is a compromise between excessive clutch slip from a slow street start, cruise rpm, engine wear, MPG, and whatever characteristics you have from your engine, plus where your highway speeds will probably be.

here is something i wrote for the SPF forum recently:

Having worn out the Ford Traction-Loc differential clutches in my SPF’s 8.8 differential, I decided to have the anti-slip replaced with a more sturdy unit, and settled on the Torsen, as that seems to have held up very well in the SPF race cars run by the Olthoff’s. Since the diff had to come out, it was time to think about a more useable gearing. With the Tremec 3550’s first gear being a stubby 3.27, pushing the standard-for-SPF 3.73 rear gear, first gear was not very useful, as my car would effortlessly overpower the tires, yielding a paltry 39 mph at my 6000 rpm redline. In fact, my car was faster in the quarter mile if I just barely got moving in first gear, and then short shifted to second, where I could put down at least some of my engine’s power. At the other end of the shifting, there was a huge rpm drop from forth to fifth, so much so that acceleration well above a hundred mph (at the timed mile at Maxton, NC) was fairly leisurely.

I drive my car almost every day, and mostly in town, so that my usual speeds are 30-50 mph, and this seemed to unnecessarily rev my motor in forth, yet not give sufficient rpm in fifth for traffic. So, trying to carefully plan my personal driving compromises, I wanted a gearing that would be more useful in city driving, and have a “tight” close ratio thru all five gears, without excessive rpm changes on upshifts and downshifts. I also wanted a gearing more suited for the occasional race track event, where fewer shifts mean more time on the throttle. I also wanted to avoid unnecessary rpm at my usual highway cruise.

I cut and pasted the gearing calculator Mike Stenhouse has put on Second Strike’s web site, and compared every possible rpm/speed combination with different gearing. I decided that the close ratio Tremec 600 with the .82 “close ratio” fifth gear, coupled with the 3.27 differential was the best compromise for my driving enjoyment.

Dennis Olthoff installed this set-up in my car, and the driving experience was incredibly different, and vastly improved for me and my beloved SPF. The Torsen has provided absolutely even traction at the dragstrip, with no wheel hop or sideways slip. First gear at 2.87 in the 600 has made first gear quite useful, giving 51 mph at the same 6K rpm, which is a hugely noticeable difference, and has not required any more clutch slip from a start than previously. Each of the higher gears gives even more mph at any rpm, third pulling to 115 mph, vs. 95 before, a full 20 mph more. In fact, I can now run the traps at the quarter mile in third at about 6200 rpm, avoiding an upshift to forth, gaining .3 seconds and about 5 mph in trap speed without having to do anything to my motor’s output.

What is best about the new gearing is that fourth gear is now very useful, and I no longer have to over rev in fourth, or “lug” in fifth at less than highway speeds. Even at Interstate speeds, I can drop to forth, or even to third for the occasional need to merge in front of any Porsche or ‘Vette, and have plenty of useful rpm for acceleration. On winding back roads, forth gear is simply superb for the curves and hills that are encountered, with out the need to constantly shift between fifth and forth. I just drove on the Blue Ride Parkway, with its 45 mph limit, and mostly stayed in forth, easy and relaxed at about 2K rpm, yet with great torque for the hills. The previous gearing would have dropped to about 1600 rpm in fifth, a bit low for the hills and curves. At highway cruise, I am turning almost the same rpm as before, about 2000 rpm at 60 mph, so there wasn’t any real sacrifice in wear and tear on the Interstate.

In essence, my new gearing seems like it has both given me another 1000 rpm in each gear for an equivalent speed, or dropped my engine’s wear and tear noticeably at any desired speed. It just seems like the engine has endless acceleration in each gear, before it is time to upshift. And downshifts are just a quick heel-and-toe and a blip away, without the chasm between forth and fifth with the 3550 box. Interestingly, the rpm precisely converts to car speed, varified with a GPS, so that in fifth, 2K rpm is 60 mph, 2500 is 75 mph, 3K is 90 mph and so forth . Handy to know.

A nice feature of the 600 is the very direct, very mechanical, absolutely no “slop” in shifting, with the incredibly short throw of the new shifter, so that with the nearly imperceptible movement of the lever across the “H”, it seems as if the 600 shifts with just a fore and aft movement of the lever.

So, in summary, I have found for me this is the ideal set-up, with virtually no compromises. I have a stroked Windsor, and run 15 inch tires. If one has the smaller diameter 17 inch wheels, then perhaps the 3.08 diff would be a consideration, and if one’s SPF has a 460 motor, with 15 inch tires like me, it too might warrant a look at the 3.08. But for 351 based motors, sitting on 15 inch tires, I think it can’t be beat for utility, wear and tear, and usable rpm/mph.