This discussion started on another thread, so I thought I would start its own thread and give my 2 cents


The modern combustion engine is designed to achieve peak horsepower and torque at specific RPM’s. Therefore in a theoretical vacuum a seven or eight speed transmission would be more effective at keeping the engine within said horsepower range that a six speed transmission.
But the devil is always in the details.
Using the 4.3 Aston Martin V-eight engine in a hypothetical exercise.
Maximum horsepower is achieved at 7000 RPM with a maximum engine speed of 7300 RPM. Maximum torque occurs at 4500 RPM and stays flat to approximately 6000 RPM
Assuming, for simplicity, the RPM drop between gears on the six speed manual transmission is 400 RPM.
Driving it like you stole it would give you the best acceleration shifting at 7000 RPM through all the six gears. After each gear change the engine’s RPM would drop from 7000 to 6600 RPM. (Difference of 400 RPM)
Using this same model on a seven speed transmission you would still shift at 7000 RPM but after each gear change the engine’s RPM would drop from 7000 to 6656 RPM. A difference of 344 RPM.
An eight speed transmission would produce engine RPM drop between gears from 7000 RPM to 6700 RPM a difference of 300 RPM
Note the difference in engine RPM drops between the six speed and seven speed is 56 RPM.
Difference in engine RPM drops between the seven speed transmission and the eight speed transmission is 44 RPM.
It worth noting that when Chrysler came out with an eight speed automatic transmission a few years ago, a test by a major car magazine on an automatic eight speed Chrysler never saw the car shift into eighth gear.
This is a classic example of diminishing returns, the more gears you add to the transmission the less returns you get.
In the real world there is the human factor. Gear changes take approximately ½ to ¾ of a second.
Paradoxically, the more gears in the transmission does not automatically translate to going faster.
Fact: Changing gears with a clutch slows the vehicle.
During the period when the engine is disengaged from the transmission via the clutch being depressed, vehicle acceleration is slowed.
The 427 Cobra blistering 0-60 MPH numbers were achieved because the car would accelerate to 60 MPH plus in first gear. The Corvette ZR1, Viper, Shelby GT 500, (and others) are all designed to exceed 60 MPH in first gear primarily to generate terrific 0-60 times.
On the question of whether a seven or eight speed manual transmission would enhances this cars acceleration?
No I don’t believe it would.
The difference in RPM drops between gears of a seven and eight speed transmission is 56 to 44 RPM. That is not exactly taking the engine out of its peak power band. That is not enough to overcome the time it takes to shift gears and regain momentum lost at shifting.
Does seven and eight speed transmission have a future? Absolutely, but they will be in automatic transmissions that can shift in milliseconds.
Note that the current seven speed transmission in the corvette is an overdrive gear, for fuel economy only. Looking at the gear ratios on this transmission it appears fifth, sixth and seventh gears are all overdrive gears at less than 1:1.
Interestingly the six speed manual transmission in my 2007 Vantage shows fifth gear at 0.94:1 and sixth gear as 0.76:1 (both less than 1:1)
Ironically, all six speed manual transmission cars that I have read about attempting top speed all attain said top speed in 5th gear.

 

You generally go past peak power before shifting to the next higher gear, since even though power is now falling, it is still higher than the power in the NHG after the drop in rpm.

 

Power climbs to redline with a tune on the 4.3L, and I'd imagine it's the same with the 4.7L, too.

A long first gear is fine if you've got the gobs of torque to push it. You're never in first gear when driving - only when accelerating from a complete stop. And even then, you can survive without 1st gear. I've only ever driven one car that I couldn't start from a dead stop in 2nd - that was an SR20-swapped 240SX with a race clutch. It was practically binary - it was either engaged or not lol. My buddy that owned it said I was one of two people to not stall it the first time driving it.

Adding gears adds complexity, and likely adds weight and cost. I'd guess it also adds to clutch wear, since you're shifting more often.

If the 7-speed becomes more commonplace (4's were replaced by 5's, and then 6's became the norm, so it's not unimaginable that 7's won't be more widespread), I'm going to venture a guess and say it's for fuel economy more than anything else.

I'm surprised the 6-speed manual doesn't have an overdrive gear. Maybe that's why we're only rated at 19 mpg on the highway hahaha

 

If AM do go to a seven speed it will certainly be for added fuel economy, not performance. At 3000 RPM I am doing about 79 MPH in the Vantage. At 80 MPH my Infinity FX is turning 2500 RPM

 

I hope not. Six gears is just about right.

 

Umm, I don't know if you guys are aware, but Astons have been available with 7 and 8 speed transmissions for quite some time. OK, so not a fully "manual" gearbox, but all Sportshift cars (Vantage) have 7 gears. Also, all Touchtronic III cars (Vanquish and Rapide S) come with 8 gears.

 

You mean my car has seven gears? I never knew...

Seriously, it's too many. These are sports cars, not trucks. Call me a Luddite, but the obsession with shaving off a few more fractions of fuel economy with more gears, electric steering, etc. is tainting the driving experience for many great cars (911 for example). Same goes for turbocharging. I like the feel of pulling through the rev range in a given gear - who cares if I'm a few tenths slower to 60? In my DD (XFR-S) with 8 gears I sometimes lose track of what gear I'm in. For that car, a ZF 6 speed auto with the technology improvements of the 8 speed (quicker shifts, rev matching) would be ideal. Probably the same could be applied to the Rapide or Vanquish.

 

In early Vantages, fifth and six are overdrive ratios...

 

I am willing to bet that seventh and eighth gears are also overdrive gears. It would appear that 4 speeds are all that's necessary to get maximum acceleration from just about all v-8 cars