Pttrader

Sometime soon I will be getting lots of exercise replacing the tt couplers and I was considering as far as driveability is concerned it might help to go to a lower gear say 3:73, 3:90, Or 4:10.
My reasoning is with a modified car (About 100hp over stock) with a monster 3.5 clutch is when I am in traffic and especially on an incline I find the car lurching ahead or even sliding sideways alittle when I over do it with the gas. My excuse is to have the rpms up enough not to stall it and look like an idiot. I have drove stick cars before and stock Z06s are much easier to launch.
I had a 86' IrocZ and went from 3:23s to 4:10s and yes, the car had more zip, but the 1st gear was almost useless unless you were drag racing so I would usually launch the car in 2nd in traffic. The C5 Z06 has some very low 1st and 2nd gears already and the shifts from 1st to 2nd and 2nd to 3rd are already very short!
What are your experiences? I like the RPM Stage2 Differential?

Pumba

Well in my experience, you have two problems:

• As you have already stated, the C5 Z06 MN12 transmission has a poor gear set. The MN12 gear set causes your RPMs to down below the torque peak during the 1-2, 2-3, 3-4, and the 5-6 gear changes. This the poorest performance of any of the C5 - C6 manual transmissions. The C6 Z06 corrected some of the problem by going to a MN6 trans.

• GM chose to change trans gear set, rather than offer a steeper final drive gear to improve performance of the manual transmission Corvettes.

I have improved the situation in my Z06 by swapping another C5 owner my MN12 trans for his MN6 trans.





Next, I had RPM Transmissions install one of their Quaife 4.10:1 differential.





The combination is pleasant to drive around town, on the track, on the highway, and it can still get 28 mpg fuel economy.

Good luck with your Corvette.

ctusser

If the car has a lightweight aluminum flywheel, changing to a heavier flywheel will help a lot. When I did my clutch I installed a lightweight flywheel b/c they show up as a 7-10hp increase on a dyno, but they suck for launching the car and stop and go driving. If I had to do it over again I wouldn't go lightweight.

z06sicz

Pumba not sure I understand your graph. The MN12 looks fairly flat to the peak (5000) for the fist four gears until the overdrives of fifth and sixth.

tlove32

I'm not quite sure I understand it either. The M12 trans gearing in the first 3 gears is the equivalent to an M6 trans with 3.90 rear gears.

Pumba

What the graph shows is what your RPMs drop to if you shift at 7,000 RPMs.

What should happen is that your RPMs should stay above the PEAK TORQUE line. The MN12 is the poorest of the four transmissions shown.

The MH3, which is from the new C6 ZR1, is the best.

z06sicz

Ok now it makes sense.
But most guys are shifting 6600-6800 and that puts you even lower under the peak.
Seems odd the fifth gear transition is so tight to fourth. I would have expected a flat one through four and a drop thereafter.

With all the R&D and race development I wonder what their (GM's) rational was for the choice of ratios?
Fuel consumption standards/ guzzler tax?
Better press numbers 0-60 etc?

BRISLS1

Thank you for this explanation!
I thought I was crazy. My Z16 actually bumps, or slows down a tick, when I shift up into 2nd or 3rd. I drive myself crazy trying to figure out if I should keep into the throttle more to keep the engine rotating at a fast enough speed and not lose RPM whenever I depress the clutch.

Somebody posted here that rev-matching on the upshift might do damage? Is the answer, to try to rev up above that torque line before the shift even in casual driving? Now that roads are hot, I can do that without any trouble, but in winter, I will struggle with traction if I always let it spool up.

Thanks

winters97gt

I have a spec 3 clutch in mine with the RPM 2 rear end and 3.90's. It's really a nice gear for these cars.

BRISLS1

I know this does not sound smart, but here is my best analysis:
My 2002 C5 Coupe would go into second gear with an engine slow down, at any speed or RPM. You would feel the lugging as the engine tried to push a smaller cog.
In this Z06, it is almost like the shift to second from first causes the gear to force the engine to speed up. I am confounded by this phenomenon and spend hours trying to imagine what is happening.
Best I can guess, is the ratio is SO close, that any time at all on the clutch causes the engine to lose too much rotation for the forward movement of the car. And your upshift feels like a downshift, unless it is into 4rth :-( So I shift as quick and as predictable as I can.
I don't want to switch into some tame habits, as just when I do, some jerk in an M3 will approach from the side.

ptindall

Your "torque peak" line is in the wrong place for a stock LS6. The LS6 makes peak torque at 4800 rmps (3/5 of the way between your 4500rpm and 5000rpm lines) or..... pretty much exactly where the MN12 sits on your graph.

GTO2Vette

Pumba

Incorrect

If you drop your shift point to the 6,500 rpm level OF A STOCK LS6 your RPMs remaining after a shift will be even lower than shown on the graph.

Don't delude yourself, the MN12 is a very poor choice in transmissions.

AU N EGL

One should shift 12-1500 rpms above peek torque, which is just about peek HP

peek torque 46-4800 rpms, shift point 6,000 to 6500 rpms

SO when the shift is complete, then you are AT peek torque for best acceleration.

the old GM motorsports gear ratio and the new ZR1 gear ratio both a close ratio box is great. but inlight of the cost of a GM Motorsports rebuilt gear box, many ppl have pulled the MN12 out put in a MN6,but changed the 5th gear to .85 and removed 6th gear all together.

Pumba

I have reformatted the graph to show the RPMs remaining after a 6,500 rpm shift rather than a 7,000 rpm shift:






And here is a chart showing the RPM loss after a 6,000 rpm shift. Only the MH3 trans does acceptable well at these low shift speeds:




The MN12 would still be my last choice for a transmission.

tlove32

Pumba,

I can certainly see where you're going with the graphs, but one thing to keep in mind (which the graph doesn't show) is the rate of acceleration due to gearing. I would argue that all things being equal (car weight, hp, torque, driver), the M12-trans will accelerate faster than the M6 trans due to its more aggressive gearing in first thru third.

Your thoughts???

ptindall

So? Did I say anything about shift points or best transmissions? No. I pointed out the FACT that the peak torque line on you graph is in the WRONG PLACE for a stock LS6. There was nothing "incorrect" about my statement.

Pumba

You tried to mix apples and oranges.

You cannot state that the MN12 falls on a 4,800 rpm torque peak, when the graph was for an LS6 that would rev to 7,000 rpms.

With the shift point lowered to 6,500 rpm to represent a STOCK LS6, the MN12 drops the RPMs below the 4,800 rpm peak torque.

ptindall

No, I didn't. I simply stated two facts. I made no comparisons. Read my post again carefully. I state peak torque is 4800rpms for a stock LS6. FACT. I then state 4800rpms happens to be where the MN12 is on your graph. Also a FACT. That's all I say. Nothing in my post was incorrect. You chose to dispute conclusion I never made about those FACTS. Nothing about my post was "incorrect."



If I change the rev limiter on my stock LS6 to 7000rpms, then the MN12 falls on a 4800 rpm torque peak, doesn't it?



The graph was for a non stock LS6 that would be shifted at 7000rpms and had a peak torque of 5000rpms. If you wanted to use that graph as proof of anything, then you needed to qualify it as a graph of a non stock engine with a higher than stock peak torque. Stock is always the assumption unless stated otherwise.

It is not a graph for any LS6 that would rev to 7000rpms. I could go and change my rev limiter to 7000rpms, the peak torque on my stock LS6 is still 4800rpms. Then if I choose to shift my stock LS6 at 7000rpms, the MN12 would happen to land right on the peak torque. If I did that, my engine would be both apples and oranges, wouldn't it? Isn't a stock LS6 with a 7000rpm redline the standard for T1 cars? Mixed apples and oranges might not be so rare afterall.

Pumba

Yes, and the MN12 is still the poorest choice.