ChucksZ06

I am getting these numbers from car and driver article with the risk that that entails. The ratios are nice and close until the 4th to 5th shift which is almost double the mph increase as the first 4 gears. 126 to 169mph. That happens to be the range where the cars acceleration slows more than it should in my opinion (downforce, etc.) It also has to do with the big ratio jump. Because of the high rear end ratio of 2.41 the transmission ratio changes are exaggerated over say a 3.42 rear gear. I was wondering if the trans is so configured because of its wide use in the trucks? It would have been nice to use one of the overdrive gears to keep the engine in its sweet spot up to 150mph instead of having to pull that gear all the way to 169. I would think changing the rear end ratios to even a 3.08 would really help this car for track use. Road race and drag strip both.

Thoughts?

fdxpilot

No way. 1st gear is 4.56. With a 3.08 rear, you would have a 14.0 to 1 overall gear ratio. Your top speed in 1st would be somewhere around 30 mph. 5th gear would now be almost the same ratio as 4th is now (3.91 vs 4.07,) topping out around 130. 6th would be almost the same ratio as the current 5th (3.08 vs 3.06.) Possibly a 2.56 or 2.73 rear might help, but given the super low ratios of the A8 tranny, 3.08 or 3.42 is way to much gear. Just my $0.02 worth

LS1LT1

Not even something as (relatively) steep as 3.08s would be needed. 2.93s (GM has used that ratio in some 6 speed automatic models in the past) or as mentioned above, even just 'the more readily available' 2.73s would be more than enough to make for a nice jump.

GTB/ZR-1

169 in 5th, should cover every US track... lol

But I agree--a bit too much rev drop 4 to 5.

I went M7 & it won't have that spread.

0Bobby @ LG Motorsports

I believe the rear gear to be a 2.41 correct me if I am not right on it.
for the auto the gears are:

1. 4.56:1
2. 2.97:1
3. 2.08:1
4. 1.69:1
5. 1.27:1
6. 1.00:1
7. .85:1
8. .65:1

Sure it gives it a 2000rpm cruise speed at 100mph and a theoretical top speed of like 312 mph, but on track the RPM drops are not ideal at all.

They had that many gears to choose from, it should have been spaced a bit better.


I have not seen the 7spd ratios to know if they are any different than the base cars.

0Bobby @ LG Motorsports

Also, changing the diff ratio does not change the RPM drops.

If you want to change that, the only way to change it is to change ratios' in the tranny itself.

Greg Quillen

Base and Z06 8speed





Z51 8 speed

Greg Quillen

Z06 7 speed manual

http://www.dragtimes.com/2013-Chevro...tton=Calculate

Zentraedi

You can thank the two overdrive gears for that.

fdxpilot

This is from one of the GM webpages

oicw

Looking at Greg's graph, the 1st to 2nd drop is actually the biggest - 6500 to 4300 rpm.

2nd to 3rd is 6500 to 4600 rpm. 3rd to 4th 6500 to 5300 rpm. 4th to 5th is 6500 to 4900 rpm. 5th to 6th 6500 to 5100 rpm.

So while it's a bit larger than 3rd to 4th, it's still very tight generally speaking.

Also, don't forget the tremendously fat power band of the LT4. Looking at Vegence Racing's dyno, at 4900 rpm the engine only drops 35 hp from peak - 550 vs 585 hp! As soon as you hit 5300 rpm you're back at peak hp.

This means a shift drop to 4900 rpm actually cost very little in propulsion force. I'm sure the GM engineers thought of this when doing the design.

Greg Quillen

with so little power drop the rpm drop doesnt mean much..

JoesC5

Looking at the GM engine dyno graph....


It appears that the 650 HP at 6500 RPM drops to 580 at 4900 RPM(70 horspower drop) and doesn't get back to peak HP until it hits 6400 RPM, not 5300 RPM(where it's only back up to 610 HP)

Greg Quillen

on dyno graph and real power its pretty flat..
drops to 540whp at 4700rpms on shift then back to peak pretty much by 5300rpms

ChucksZ06

I only mentioned this because the car accelerated poorly at the Atlanta track from 120 mph on up. It has been mentioned that this is because of the downforce. Regardless it would help lap times noticeably if it pulled harder at speed. We only need one overdrive gear with a 2.41 axle ratio. lol that is like an overdrive all by itself.

AzDave47

Minor number issue if the calcs were done with a 345 tire as the new Z06 comes with 335/25-20 tires although the 26.73 diameter is close to the 26.6 +- of the 335s

Mopar Jimmy

That's awesome that out of the hole she will hit like she has 4.56 gears in her making her a real terror at the dragstrips (and I love drag racing) and my left knee is getting more arthritic, so A8 for me!

3 Z06ZR1

I looked at that too. The stick looks to have the better ratio
tigher gears. Much better 1/2 mile gears!

3 Z06ZR1

So low it.
Might be hard to hook anything on street tires.
Seems to be small handicap at the track for the fastest a6 vs a8
too low of gear to hook.

Greg Quillen

The auto has way tighter gears than the manual. Look at graph I posted