SNW Vette

Hi,
In the ‘60’s muscle car era a close-ratio transmission was a good thing and the standard transmission was a wide-ratio. Now, because of ZeeOh mania, the wide-ratio is the in-thing!

For racing I think an M6 is a better transmission because of the tighter gear spacing between 2nd-3rd & 3rd-4th. Why? Because the lower drops will produce a higher average power thru the gears.

Here is an example:
M6 w/3:90’s vs. M12 w/3:42’s using a ’02 LS6 power curve up to 6600 rpm.
This gives about the same 1st gear ratio. You may not want this much 1st gear depending on you power-to-weight and the tires you will be running. If your car is already rear-wheel traction limited, why add more 1st gear?

Total gearing looks like this:


Gear drops look like this:


Power vs. speed looks like this:


There is a ~2.5% average power advantage (~9 HP out of 360) with the M6 from 20 to 125 mph. If your engine has a peakier power curve than the very flat LS6 power curve used here, the M6’s advantage will be even greater.
Good luck,
Steve
:seeya

Miles in Michigan

Your last graph has a typo. Both the lines are labeled "M6"

Other than that, I agree with what you are saying. Before the Z06 came out, when preliminary information suggested the possibility of lower gearing, many of use suspected a 3.73 rear gear. Then, when it actually came out with a 3.42 and new gears 1-3,5-6, but the same 4th gear, it was remarked that keeping the same 4th gear (which has to be 1:1) screwed up the rpm drop when upshifting into 4th (from 3rd) and 5th (from 4th), and downshifting into 4th (from 5th) and 3rd (from 4th).

It'd be nice if the rpm drops where more consistant than they are (M6 has the curved pink line), like in a Porsche 911, for example. I did a similar plot for the 911 gearbox when the Z06 first came out, and not only does the 911 near redline at top speed in 6th gear, but the ratios are nearly perfectly picked to have the same rpm drop on upshifting at redline in any gear.

Oh well, what can you do...

SNW Vette

Hi,
Thanks for catching the typo, Miles. I’ve corrected it.

It looks like the General only uses 6th gear for best mileage. The Germans have the Autobahn and we have CAFÉ! A modern Vette is still an amazing car as it has both the highest top speed and the best mileage of any car I have ever owned!
Thanks again,
Steve
:seeya

kewlbrz

Nice work SNWVette.

So a MN6 with 3.90 is the optimal setup for the C5?

Miles in Michigan

Except from other posts I've read, there are (might be?) problems with anything larger than a 3.73 rear gear in a C5.

here's a link: http://forums.corvetteforum.com/zerothread?id=312529

C5Noir

Looks like the only solid way to go then would be with an M12 :( (to achieve the 3.90 ratio)

kewlbrz

Miles , I agree that there are concerns about the 3.90+ gears stressing the case too much and cracking it. I was just curious if on paper that was the optimal setup.

C5Noir, based on SNW's graphs, I dont think the M12 is the better solution. I think even a getrag 3.73 with the MN6 is a better overall setup.

Also, I wonder what a M12 and 3.73 graph would look like. SNW?

Miles in Michigan

I'll agree with that, not so much because one will necessarily be faster than the other, but because it would annoy me having gear ratios that weren't spaced right.

The same as M12 with 3.42, except compressed along the X axis, towards the left. :lol:

Really, I don't think the rear gear matters much, except for a few special cases:
1) the launch, when racing from a start
2) if racing a fixed distance or to a fixed speed, to end at rpm closest to redline as possible, in whatever gear the race ends in (which means sometimes higher is better and sometimes lower is better)
3) when on a road course, to avoid an unnecessary shift (which means sometimes higher is better, sometimes lower is better)

C5Noir

My only rationale for considering the M12 is to give the acceleration of a ZO6 and beat it (a stock one or lightly modified one) by a good amount considering my mods without worrying about breaking the cases, etc. Is this a viable, sleeper approach in regards to the gearing? Thanks.

SNW Vette

Hi,
Thanks Kewlbrz,
I am interested in the case of modified C5’s for the street. The example above holds to ~125mph trap speed which is faster than most. I think there are several interesting cases you can look at.

300 RWHP; For a mostly stock C5, the M6/3:42’s are too tall both in 1st & 4th for ¼ mile work. The M12/3:42 is a better choice but 4:10’s are needed for best performance. I think the M6 would show a small advantage in this case with 4:10’s. These engines have a very broad power band which diminishes the advantage of a close ratio. It seems that they also want an aluminum flywheel to soften the hit on EMT’s with the stiffer gears.

350 RWHP; Stock ZO6’s are perfect. Ask the man that owns one. They would be slightly more-perfect with an M6 and 3:90’s as shown above.

400 RWHP; ZO6’s have real traction problems with stock tires in this power range. An aluminum flywheel seems like it would help the launch. C5’s in this range are the case above. They need non-EMT's and aluminum flywheels. Andy of A&A has this drivetrain setup. If he ever stops changing cams, I hope we will hear how it launches!

Bye for now,
Steve
:seeya

Andy@A&ACorvette

I'm going to stop switching cams after this weekend .I SWEAR I am!! Maybe :D