Can you explain something to me?

I've owned stick 'Vettes for 14 years, since the C5 in 2000 and one of the first C6 in 2005. And, I'm just recently (had Museum Delivery and drove it 2,100 miles home) the proud owner of a totally equipped Stingray Convertible (LT3, Z51 and everything else I could think of). And, I've been to Bondurant School of Performance driving last year, racing C6-Z06's for two days. All of which is: Lucky me!

But, in all that time, I have no idea what I should make of Transmission Ratios. For one example: 2nd gear in the Stingray has Trans Ratios of: 1.783 in the Manual, 2.36 in the Auto, 2.066 in the Z51 Stick and 2.36 in the Z51 Auto.

And I know that those are all connected somehow to the Axle Ratios (in same order as above, 3.42, 2.56, 3.42 NS 2.73) and probably also related to tire size (I have 19/20 in Z51 vs 18/19 in non-Z51).

Practically, what does all that mean in terms of speed/acceleration? Why are they designed the way they are?

Any other light you can shed for me?

Thanks!

Separately and BTW, as an FYI… after having the C5 and C6 for a total of 14 years, I can say the Stingray is a fundamentally different and much better car and drive. I could go on and on about why!

 

Read these links and see me in the morning.

http://www.datsuns.com/Tech/tech_gearing.htm

http://en.wikipedia.org/wiki/Close-ratio_transmission

http://www.f-body.org/gears/

http://www.howstuffworks.com/gears.htm

http://www.lotustalk.com/forums/f25/...le-terms-3914/

 

Take the transmission ratio and the differential ratio and multiply them together and then do the comparison. The higher tranny ratio of the automatic comes with a lower diff ratio.

Now, if you want high precision, also take rear wheel rolling radius and multiply it into the Tranny ratio and diff ratio (in case rear wheel sizes are different.)

 

Transmission ratio is just the engine speed divided by the output speed, and the same with the differential (input RPM/output RPM)
The higher the ratio, the quicker the acceleration, but the lower the top speed.

To get final drive ratio, divide engine RPM by gear ratio, then rear end ratio.
6500 RPM/1.783/3.42 =1065 wheel RPM

Overdrive gears are less than 1.0, so engine is turning slower than drive shaft.

 

Unlike cars with the transmission attached to the engine, the C5/6/7 always has the driveshaft RPM equal to the engine RPM, no matter what the gear ratio(s) of the transmission or differential is.

Now, the RPM of the axles(half shafts) will be different from the engine RPM on the C5/6/7.

 

Should have said the output of the transmission will be turning faster than engine RPM.
Here is a spreadsheet to play with

Copy of MPH Calculator.xls

 

Thanks, everyone. That all explains to me the calculation method. Very useful.

But, what I'm really interested in is what it means in actual driving!

For example, the specs say the Z51 Trans ratio for 2nd gear, Stick is: 2.066 and for Auto is: 2.36. Why are they different? Is the performance the same?

 

I think this graph helps answer your question

 

Visit a local transmission shop, they will show you if they are nice.
Stand at the bottom of a staircase. Proceed to climb the stairs 3 steps at one time. Run down the staircase taking one step at a time. In both cases, you are in the wrong gear. How do you know if you are in the right gear on your bicycle? Count your crank revolutions and compare it to your rear wheel revolutions in each instance. Measure the daimeter of the rear wheel and the sprocket diameters. Simple arithmetic will display the ratios in math terms.
Every transmission has a different design. Autos have epi-cyclic gears and manual transmissions have constant mesh gears.
In the same way, Wheel diameters, rear axle ratios, gear ratios are matched to engine performance of your car. Max acceleration will occur at shift points which coincide with the max power rpm of the engine and not the max torque rpms. That's another story. The new 8 speed auto is designed to keep the rpm as low as possible at low acceleration to save gas. Ideally, a transmissions should keep engine rpms constant, either at max power, max torque or idle RPMs. See above graph.

 

Very nice thanks!

 

The stock trans ratios in Corvettes whether manual or Auto trans keep getting lower and lower---
In the earlier days it was typical to see a Corvette with ratios in the 3's
When the C6 came out the autos dropped even lower
C7's have now hit an all time low of 2.56 with an auto with an optioned 2.73

Starting with the C5 a manual came with a 3.42 and an auto either a 2.73 or optioned 3.15 So why ??? Look at the performance 1st --Even though a C5 auto could come with a 3.15 rear the NEW C7 with a 2.56 is by far quicker and faster than ever before !! Approx one whole second faster in the 1/4 mile !!!!
This all comes down to EPA fuel mileage requirements ---Getting higher and higher
So GM has come up with the technology to run a C7 at a lower RPM cruising speed than ever before without sacrificing performance but actually exceeding previous performance where Corvette had much taller gears ! That s an amazing accomplishment
C7's (autos) now cruise on the freeway at 65-70 at only 1500 RPM's compared to C5's that were around 2000---- But now with the advent of VVT they have enough power to easily run at 1500 without lugging or pinging----PLUS the C7 TQ converter is so re -designed with it's multi disc set-up and smaller diameter that it basically now is a stock stall converter----ALSO the new A6 tranny is re-designed as well--A C5--C6 auto trans had about 20-22 % drivetrain loss--However the NEW A6 has now only about 10% drive train loss---Now equal to a manual trans !!!!
Simply put---EPA fuel mileage regulations forced GM to come up with technology that allows a C7 to get 29 MPG yet still run 12.0 in the 1/4 mile--the fastest stock Corvette ever !!!! Axle ratios are NOT relevant between a 60's--70's 80's 90's or even a C6 Corvette to a C7 You're comparing apples and oranges---As you can't compare them equally when a major change in technology happens---

 

Notice in the chart how the Z51 gear ratios have been optimized to go from 6500 RPM at redline down to 4500 RPM or so at each up shift for gears 1 to 5 which are the only gears you would ever use on track, since the car does not have enough power to get to redline in 5th gear where it would be going over 200 mph. For the Stingray gearing, the ratios are all lower allowing the engine to loaf at lower RPM while cruising, and the ratios drop to engine to all different RPM's at each upshift. However the drop in RPM from 5th to 6th is huge on the Stingray, and much less in the Z51. The drop in RPM from gear to gear is less in the Stingray, so that provides less abrupt shifting than in the Z51. One has to assume the ratios have been optimized to match the transmission's characteristics to improve fuel economy. 6th and 7th gears are for fuel economy only as the lower the RPM, the less fuel used, as well as for quieter cruising.

 

These are the same gears in the C6 and C6 z51 only change is the added 7th gear!

 

Not to high-jack the OP's thread, but curious your opinion on a CVT?

 

A M7 and A8 are tending toward such an ideal xmission. The most efficient rpm would be at max torque, the max acceleration rpm would be at max power. Best gas mileage would be at the lowest rpm. The A8 will let you get to 80mph never exceeding 1500rpm. At max acceleration, you could keep the rpm within 1800rpm of max power up to 6th gear with the A8 and 5th gear with the M7.
The duty of a PAX car in city driving requires infinitely variable rpms and acceleration rates. The size of the car will further compound design objectives in cold and hot climates at varying altitudes.
Vari cams, direct injection, turbos, etc make exploration of a suitable CVT design less attractive than current transmission trends.
The most efficient fossil fuel vehicle on the planet are diesel electric locomotives. They have constant RPM diesels engines that drive generators which power electric motors. If you built a C7 with this system and drove it in San Francisco, your buddy with his A8 C7 would get better gas mileage.

 

Is this not too generalized in which engine volumetric efficiency is ignored as a very big part of the overall equation? Also CVT would be far superior if weight/durability and power transmission losses could be made equal to an A8. Also most trends are towards torque converter less transmissions if looking at fuel efficiency (lower losses).

Uhm, here is a diesel electric fact http://www.factcheck.org/2008/07/fue...reight-trains/ . Also if looking at single passenger, a Boing 787 can get 80 passenger miles per gallon… and that’s moving at a good clip.

So with above generalizations how does A8 look? Not so good, systems have be looked at in their entirety.

 

You have to look at both the transmission gear ratios and the differential gear ratio.
For example you can have a transmission 1st gear of 4:1 and a differential gear ratio of 2:1 and the engine will be turning 8 times faster than the axles. total gear reduction of 8:1.

Then you can have a transmission 1st gear of 2:1 and a differential gear ratio of 4:1 and the engine will also be turning 8 times faster than the rear axles. Same total gear reduction on 8:1 as in my first example.

 

How does Vari cam, DI, turbo, blower not address VE? I'm quite sure that Pre Z06 doesn't want a thesis.
A8 looks damned good and this is the 'trend', my friend in vehicles in general from light trucks to luxury cars.
Sports cars may be a different matter. CVT so far is impossible to design for a Vette and there is always the question of slippage.
A torque converter will do a better job both in comfort and endurance for general purpose vehicles. Drive a Lexus and then drive a Panamera. GM engineers have taken ZF and Asianne gearboxes and vastly improved the design. Keep your eye on it's development. It won't replace the sequential on the race car however.
Foreign high performance cars use DCTs but in the real world, there are penalties such as I have mentioned plus excessive mass and redundance. Ultimate performance comes from sequential gearboxes and so far, the LFA is the only car that chose it.
Yamaha designed the engine and gearbox with some Honda patents, (Seamless ratchets) It is not necessary for a CVT, but a LFA type sequential with a TC or without, could address all high performance applications.

 

I seriously doubt that the A6 (basically a carryover from the C6) has 10% drivetrain loss. For that matter, I doubt that even the stick cars have that low of a drivetrain loss. Care to produce a source of your info?

 

considering the A6 puts down less power than the M7.....