varkwso

I remember reading somewhere, cannot recall the source - exactly, the rear gear ratio was the last tuning item for most road race cars and it would be selected to hit redline for the longest straight at the brake point in 4th gear.

For a C5 running (~6500 redline) at VIR, Road Atlanta or Roebling wouldn't that put the ratio at ~3.25 or so.

Or is this hypothesis incorrect? I do know I hated driving at 3.90 geared C5Z at Sebring 12 hour course.

mgarfias

Knowing nothing about selecting gears for this sort of application (i just run what i have), I'd think that you'd want to setup the gears so you have maximum torque out of the corner leading to the fastest section of the course.

But I'm probably sniffing glue.

AU N EGL

3.42 the stock diff ratio seams to work best.

then the MN12 5th gear or .87 could be used on RAs back straight if needed.

on shorter tighter course 3.90

davidfarmer

the "maximum torque out of the corner" argument doesn't work out for me, as in most cases, yoiu have enough torque to spin the tire (oversteer) regardless of rear gear, expect really fast corners.


I go by the top speed in 4th argument, or 5th depending on your trans. If you have a close ration Racing trans, this could be a 4.10 using 5th gear. However, since most of us don't have those, you have to run what you have.

If you want to stick to a 6500rev limit, 3.30 OEM diff would be a good selection, especially with an MN6 trans. If you run to 6900rpm, which most of us would, the 3.42 is actually very good, again using an MN6 is advantageous since 2-3 gears are much tighter.

mr.beachcomber

When running C2/C3Corvettes in SCCA club racing in the 70's (BP) and 80's (GT1), the rear end ratio was the last item to be optimized at each track we ran. The racer's maxim stated above probably worked for formula classes where everyone essentially ran on an equal footing; however, for the small block Vettes you had to consider other items when selecting the rear end ratio such as your engine's torque/HP curve, track elevation changes (we carried more weight than the formula classes), time spent on straight versus time spent in the twisty parts, and whether you were using a close ratio or wide ratio gearbox.

I can remember using a 3.08 posi at Daytona (BBC AP Vettes used a factory-provided 2.73 posi there), a 3.36 posi at Sebring, and a 3.55 posi at Road Atlanta/Mid-Ohio (giving up some top end speed for more torque/HP in the twisty parts and elevation changes).

All these ratios weren't optimal using the racer's maxim, but it's what we had to work with from the factory. BTW. our shift points weren't based upon redline. We tried to short shift to keep the engine within our engine's torque/HP band. (This usually meant shifting short of redline, but past peak HP. The shift points were dependent upon the gearbox ratio we were using. This practice keep us in the engine's maximum acceleration band up through 4th gear.)

Hope this info helps!

drivinhard

tempting to try 3.90 or 4.10 as this would put 5th at redline at peak speed at a lot of the high speed tracks we run (VIR/RA/Lowes) and would take 4th up to the rev limiter at CMP before the kink.

then for instance at RA or VIR, you'd come off the slow corner (which both are too slow for 3rd, and too fast for 2nd-with the M12) in 3rd gear with more gear, and have 3-4-5.

C&M Racing

We change the rear gear for the track. Oh, we have a quick change

Gray Ghost GS

I tried the 4.10 and 3.73 and was NOT happy with their performance at VIR - required too much shifting as well. I ended up going back to the stock 3.42 and found that gear to be the best.

wallyman424

or you could get a close ratio box w/ 3.90s

sperkins

M6 5th sucks beyond belief. It's by far the worst aspect of my car.
If the rules would allow it, I'd swap to the M12 5th in a heartbeat.

ninjavette

I don't have experience at any of the tracks you mentioned but I have ran 4.10s in my C6 Z51 (same trans as C5Z) at Monticello. It's basically a 3rd/4th gear track and the 4.10s were fine for the most part except in the longer straights. On the first long straight I had to shift a few hundred feet before the braking zone or I'd just be banging off the rev limiter and not gaining any speed/time. The long back straight had me shifting to 5th just before braking again. This is with Z51 gearing and a 7k rev limiter. Since my car is 99% street driven the cost to swap gears again isn't worth it to me but I think anyone who tracks their car regularly, 4.10s are probably going to be a little too much for most tracks. JMO

drivinhard

For sure in ST2/TTS the poor man's close ratio box would be an M6 with an M12 5th, and your rear gear of choice.

In PTA, we (C5Zs) are stuck with the M12 and whatever rear gear we want.

not all of us have a 427

what were the lap time differences?

The "too much shifting" wouldn't bother me, at least for TT "hand grenade" laps.

The tell tell would be checking data (or running 2 cars side by side) from a 45 mph roll, one with 3.42's and one with 3.90 or 4.10, and see which one covers 4000 ft in a straight line quicker. If there is no gain, then the advantages of less shifting win.

wallyman424

Well I'm getting a zr1 ratiod box and 390 rear. I've got decent data from vir and summit pt to compare to, so well see what its worth

Oh, and zr1 ratiod box isn't that much more expensive than a MN6 + MN12 5th, and the ratios are way better

drivinhard

already got M6 and M12 parts laying around here though

those ratios should be sweet

only thing is those tall gears (1st) will make this clutch become a real Pelosi tooling around the paddock/grid (depending on the rear gear)

Bill Dearborn

A number of years ago I was attending a TCC race at Watkins Glen with Danny Kellermeyer. He was using the M12 transmission with 3:73 gears. This gave him quicker acceleration off the corners in 4th and 5th was similar to what 4th was in a car that had the stock 3:42s.

A guy I know who used to race at Watkins back in the 60s and 70s said he geared his 66 Yenko Corvette to accelerate quickly up the back straight Vs trying to hit top speed at the end of the straight. He said it was just like the start on a drag strip, build a lead coming out of the turn and even though he spent a fair amount of time running at the red line and the other car hit a higher top speed at the end of the straight he got there first. It is very hard for cars that are closely matched power wise to make up even a short lead that is gained at the start of an acceleration race.

Bill

drivinhard

exactly

Brent Dalton

In my LS1 RX-7, I have a 4.10 rear with a stock F Body T-56(4th is 1.0 and 5th is .74) which I believe are the same ratios for the MM6. I *should* have solid traqmate data on it after next weekend if anyone is interested. So far(only had the new car on track there for a limited time), Road Atlanta has been a mostly 4th and 5th track where I occasionally go into 3rd. With my current tires size of 275/35/18 and a redline of 6500rpm's, I max 4th out at 120ish as per the traqmate data. 5th is calculated to redline at 162mph. My main concern will be how long it will take 5th gear to get to 160mph.

drivinhard

TTS car right? any TM data would be great as the power/weight and tire height and rpms are all very similar.

Brent Dalton

yep.

varkwso

I am focused more on the MN6 since my primary track car has one and its diff currently sounds a bit like the one TimZO6 had in his first race at RA. A ~3.30 gear would allow a practical use of second gear out T7 and T10a - giving the acceleration desired. I revved my "built" motor more then 6500 with a forged bottom end and right now it is mixing oil and water and makes interesting noises (after 2+years of track use).


Luckily for us in the LS1 and LS6 world the torque band is not as peaky as it was when I was running an L88. But your point is valid.


I really, really do not like the 5th to 4th downshift in the T56 - I do it but I do not like it as a common shift. I have seen more then one LS motor sent to an early grave with this shift.

Funny - I have run both, a lot, and except for the really crappy 5th I much prefer the MN6. I think a lower numerical rear gear would allow more effective use of 2nd.

I assume these are available at the normal sources? Since a pro rebuilt MN6 and MN12 are in the ~$2500 range and a hybrid Mn6/12 is more then the ZR1 box is in the ~$5000 range?

When I ran my 3.73 geared CMC Mustang at Charlotte against the 3.23 gears of my competitors they sailed by me quite nicely as I was at valve float in 5th and they passed me on the oval. I killed them on the infield and onto the oval though. Races are won on the straights and lost in the corners.

Brent - lots of experience with the MN6 and 3.42 gears. My guess it will take a long time to get to 160 even with 4.10s. I would be interested where you were in third gear though.