raj

With real-world accounts of the C7 Z06's acceleration capability starting to pop up, I decided to perform a quick analysis of the gearing of the A8. (FYI, I did a similar exercise on the C5's gearing and published my thoughts on this very forum some 15 years ago, followed by a comparison of the gearing and power curves of the C6 Z06 and the STR10 Viper - boy, how time flies! )

The chart below compares the RPM vs. speed of the A8 Z06 vs. the C6 ZR1. I used the ZR1 as the reference here but it could be replaced by the M7 Z06 since it has exactly the same 1-6 gear ratios as the ZR1.



The A8's short first gear is in evidence here as it provides massive torque multiplication giving the Z06 the advantage provided traction is not an issue. In fact, the Z06's 2nd gear's effective gearing is only slightly taller than the ZR1's first as can be seen by the comparable slopes.

Similar story comparing the A8's 3rd gear and the ZR1's 2nd. Beyond 3rd is where the choice of gear ratios on the A8 do not make sense to me. The 3-4 drop is super short (~19%) and 4th is only good for ~126 mph with the 2.41:1 rear end, marginally higher than the ZR1's 3rd. This necessitates a precipitous drop from 4th to 5th, violating the best-known-method of progressively shorter drops required to maintain acceleration as velocity increases and aerodynamic drag force follows as v². True, the RPM drop as the A8 shifts into 5th at this speed can be easily overcome by the LT4's prairie flat torque band; however, the resultant impact to acceleration due to weak torque multiplication cannot be denied as the majority of the triple digit run to 160 mph is accomplished in this gear.

The subsequent drop into 6th gear at 168 mph is also deeper than the ZR1's 4-5 drop and not ideal for top end acceleration.

I also compared the Z06 A8 gear ratios vs. the 991 GT3's PDK to illustrate the point. The GT3's gear ratios are basically ideal and exactly what an engineer would choose if there were no other hardware/physical constraints.



Note the progressively shorter RPM drop with each higher gear designed to minimize the inevitable acceleration loss with increasing speeds. It is not clear to me why GM didn't choose a gear ratio set for the A8 that would accomplish the same, and I would like to hear your opinions on what may have been the motivation behind their choice or the constraints that lead to what I believe is an un-optimized gear set. Especially given the significant down-force created by the stage 3 Z07 bits, GM could've been much more thorough as well as aggressive in defining these ratios. After all, the battle with the EPA on fuel economy was long lost as far as the A8 was concerned, AFM notwithstanding.

I will also leave you with the acceleration comparison between the Z06/Z07 A8 vs. the M7 Z06 and the C6 ZR1, all based on C&D's test data (obviously, the ZR1 was not tested on the same day as the other two).



While gearing will explain some of the acceleration difference between the Z06 and ZR1, there is obviously an aerodynamic drag component that's responsible for the Z06 M7's relative deficiency at triple digit velocities compared to the ZR1.

All comments are welcome.

*C7*

Thanks for all your work and the great comparison charts. It looks like GM really messed up the A8 ratios. I can see no reason at all for the ratios they used.

It would be very interesting to hear the engineers comments on this subject.

As the A8 was a new design by GM I think they really missed a great opportunity by using these haphazard ratios. They should have looked a little closer at the Porsche.

darth g-f

I noticed the same thing. Your analysis is well explained and detailed.

I don't understand why big companies like GM use stupid gear ratios like that. Why have an eight speed or 7 speed transmission if the last 3-4 ratios are not good for anything?

It looks like the last 3 gears are maxed out on the M7 and A8. Having a long first gear helps with traction, but from then on close gear ratios would help acceleration a lot, especially with the track focus of the Z06 and the extra aero. The A8 shifts super quickly, use it! Make top speed reachable in the second last gear and make the last ratio very long for fuel efficiency/EPA.

Porsche certainly understands that. The relatively low power output of the GT3 make choosing good gear ratio selection very important.

I'm sure engineers at GM know that. Why they decided for these ratios is puzzling.

ChucksZ06

Thanks for posting what I posted 2 days ago. Your charts prettied up the 4 to 5th ratio gap in the a8 that I refered to.

ChucksZ06

BTW op your avatar sucks...it is not a corvette.

l888apex

Can you please do the same for the Manual?

WA 2 FST

M7 in the C7Z has the same 1-6 ratios as the C6ZR1... so you can use the chart at the top.

OP... nice work.

G35c6MT

Could it have something to do with non-performance, around-town driving or freeway cruising? I'm guessing its geared for all-around usage profiles.

Nosferatu

I agree...even for example the base C7 Z51 M7

1st gear around 47mph
2nd gear around 72mph
3rd gear around 103mph
4th...why oh why at stupid high speeds is this gear now going to pull me to 150mph??? 4th should have been a bit closer.

At least my C6 M6 is more appropriate....53-->82-->115-->150

Stunt

Could it be because GM plans to use that same transmission in their trucks and those higher gear ratios work better in that application?

JoesC5

The A8 was designed for GM's trucks, not for the Corvette. They just adapted it to be used in the Corvette. That explains it all

Now if you were to also use the ratios from Tremec's 7 speed TR-9007 DCT instead of the A8's ratios.......

3.14
2.05
1.43
1.10
0.86
0.68
0.56

or

3.24
2.02
1.45
1.08
0.81
0.63
0.50


Both with 3.42 rear gear.

JoesC5

The only way to change the 4th gear ratio is to redesign the complete transmission. If you want a different(effective) ratio in 4th gear then it has to be accomplished by changing to a different ratio ring gear & pinion.

You want the Z06 transmission with 3.90 rear gears to accomplish what you are looking for in the base C7(with or without Z51). Of course you will also be looking at a GG tax with a 3.90 rear gear.