catpat8000

merely use the same manual transmission in both
the Z51 and base models and offer a steeper rear
gear (3.90) in the Z51?

If you multiply the transmission gear ratios in the Z51
by the rear axel ratio (3.42), you'll see they are pretty
much equivalent to the base transmission ratios
multiplied by a 3.90 gear set with one big difference
in the first 4 gears: 4th gear will have 15% more
torque multiplication.

What would be the result? Same driving experience
as today except that 4th would pull harder than it
does today, resulting in better 1/4 ets and trap speeds.

No need to stock two transmissions and better numbers
in magazine tests.

Why not?

Pat

Violet

so you'd have to stock two rearends instead of two transmissions? i don't see the "benefit" of one over the other there.

and, with the lower gearing in the first three of the transmission, but leaving the other gears (esp the Overdrive ratios alone) you get:

1) better acceleration in the area 99% of corvette drivers are gonna notice it.

2) no big hit in highway mileage for people to complain about when in overdrive.

of course, that's all just conjecture and guessing on my part, but it makes sense to me.

Scissors

GM chose the better way.

Why? Because they used 4th gear to maximize dyno numbers, 6th gear to maximize fuel savings, and 5th gear to maximize top speed.

All of these would suffer with your suggestion.

catpat8000

My point was that the base transmission coupled with a
3.90 rear gear has effectively the SAME gearing in the
first three gears as a Z51 transmission with a 3.42 rear
gear. So acceleration would be exactly the same in the
first three gears. Acceleration in the 1/4 mile would be
better in my scenario, because 4th gear would then
effectively put 15% more torque to the rear wheels and
you spend roughly 3 seconds in 4th gear in a 1/4 mile
run on this car.

The difference in the highway mileage would be
negligible. A 3.90 rear gear would bump the rpm at
65 mph by probably 200 rpm.

Pat

catpat8000

By this you are implicitly suggestion that optimizing
dyno numbers is more important than optimizing 1/4
et and trap speeds because that's the trade-off they've
made. Personally, I find this hard to fathom.

I won't argue about the top speed issue because that's
purely a marketing issue and it's perfectly obvious GM
believes top speed is a useful metric to optimize.

It's not at all clear to me mileage suffers significantly in
6th gear. I'm sure there's a hit but 6th is so low numerically
already that the 3.90s only add 200 or so rpm at 65mph.

Pat

Scissors

No, optimizing dyno numbers and better gas milage and top speed are more important than optimizing 1/4 mile times.

And your suggestion wouldn't optimize 1/4 mile times anyway.

The C6 is designed to do a lot of things well, not just one thing. It has to do well on the road course and on the drag strip.

Not to mention the inherent benefits to a 1:1 4th gear.

LS1LT1

Might have to do with strength (warranty) as well, I would think a 3.42 gear is inherently stronger, even if only slightly, than a 3.90 gear all else being equal.

catpat8000

I think I must not be explaining myself clearly. I
apologize for this.

What I am suggesting doesn't change 4th gear. It
is still 1:1. I am suggesting using the base C6
transmission, which has a 1:1 4th gear.

The change I suggested results in an appearance
of NO DIFFERENCE in the first 3 gears. The torque
seen at the rear wheels is the same and the mph
@ redline is the same. Further, it results in 4th gear
being STRONGER than today, which benefits everyone
(not just drag racers) and which DOES result in a
better 1/4 mile et and trap speed.

I don't see it affecting fuel economy significantly
though I may be wrong about that.

As for a 3.90 rear gear being weaker than a 3.42 gear,
I'll leave that to one of our many structural experts.

Pat

Scissors

Total multiplication (base, base w/ 3.9:1, Z51)
1: 9.0972 10.374 10.1574
2: 5.8140 6.6300 7.0794
3: 4.4460 5.0700 4.8906
4: 3.4200 3.9000 3.4200
5: 2.5308 2.8860 2.4282
6: 1.7100 1.9500 1.9494

As you can see the base w/ 3.9:1 rear gear has a shorter 1st, which means more wheel spin on stock tires but better accleration on drag tires. 2nd gear, however, is still owned by the Z51. 2nd is the strongest gear not involved in a launch, so it's very important. A lot of the trap speed is made here. 3rd gear goes back to the base w/ 3.9:1, but only by a little. 4th gear is barely touched in a 1/4 mile.

Meanwhile, the base transmission w/ 3.9:1 still sucks for road racing. Just look at the RPM drops for the base w/ 3.9:1 and for the Z51:

Shift: RPM drop (base and Z51)
1-2: 36.09% 30.30%
2-3: 23.53% 30.92%
3-4: 23.08% 30.07%
4-5: 26.00% 29.00%

You can see that the Z51 gaps are more consistent, which is pretty damn close to optimal. The base transmission RPM drops are all over the place--not what you want for road racing.

On top of all of that, the 3.9:1 rear gear has quite a bit more drive train loss than the 3.42:1. The Z51's losses over the base transmission, however, are quite a bit less.

TEAShea

Just changing the rear end would lower the top speed.

TEAShea

Scissors

I used my 2005 C6 Z51 template in my drag race software and changed only the gears and rear axle ratio.

The simulation shows the Z51 w/ 3.42 having an advantage of 0.31 seconds in the 1/4 miles versus the base transmission w/ 3.9:1. That's significant.

However, the 3.9:1 base transmissioned car shows a 1 MPH advantage on its trap speed.

Edit: When I switch to race tires, the simulation shows both cars having virtually identical E/Ts and Trap Speeds. 0.027 second advantage to the base w/ 3.9 and a 0.2 MPH advantage as well.

catpat8000

Well, as is probably no surprise, I agree with some of what
you wrote and not with all of it

1. True, first is shorter in base+3.9 but only by 2%,
hardly significant at all, probably not even detectable
by driver or by wheelspin.

2. True, second is taller in base+3.9, by 6%.. This is
significant, as you say.

3. Third is shorter in the base+3.9 by 4%, clearly an
advantage to the base+3.9 car and almost as big an
advantage as the Z51 has in second. Significantly, more
time is spent in 3rd in the 1/4 mile than in 2nd so this
effect has more time to make itself felt.

4. Fourth is 14% shorter in the base+3.9 car which is
a big difference as this translates directly into 14% more
rear wheel torque in 4th. Fourth gear, in either the Z51
or base+3.9 probably accounts for 3 of the 13 seconds
(to one significant figure) in a 1/4 mile run, which is
almost 25% of the time. That's a long time to have 15%
more torque available.

5. As far as road racing, I am afraid I come to the opposite
conclusion as you. The base+3.9 1-2 shift is a bigger
drop than the Z51 but from then on, the rpm drops are
less, in every shift up to fifth. In effect, the base+3.9 has
a close ratio spread while the Z51 has a wide ratio spread.
Close is better than wide for road racing, drag racing, and
almost all kinds of racing. And ignoring the 1-2 shift,
which will be common only on certain, low speed courses,
the drops are not all over the place. The most common
shifts will be 2-3 and 3-4 and these are both the same
in either case.

6. The top gear rpm at any given speed would be identical
between the two so highway mileage would be identical.

7. I'll take your word for it that driveline losses are higher
with a 3.9 rear but I'd like to ask about your source. I
don't see why it should be significantly different (emphasis
on the word 'significant').

While we'll probably never have a controlled experiment
to answer this question, I would be willing to bet you
dinner that a base+3.9 car will take the Z51 in the 1/4,
both in et and trap speed.

Further, I'd be willing to bet if the Z51 bits were applied
to a base+3.9 car, that car would be faster around 85%
of all race tracks.

Pat

catpat8000

I posted my response above before I saw this note. Your
simulation sw sounds very interesting. Can you tell me
about it? Is it a commercial product?

Pat

Corvette Junkie

So they could get the Z51 equipped C6 to attain 300 kph.

Scissors

Drag 2000 Simulator. And there's Dyno 2000 as a companion product.

They have fewer inputs than another simulation program that I have, yet they still wind up being more accurate.

And, yes, they're retail products.