We haven't done it on either of our Vantages. I have sold 2 of these so far, not sure how many the race team has gone through but I think we made 10 sets initially. We haven't done anything software-wise.

One of these that was installed for a road car was reported to be *slightly* louder than stock, but as you pointed out earlier that is probably affected mainly by the quality of installation.

 

Ok, just to check my understanding, this optional final drive is the equivalent of driving in a lower gear, so instead of being in 6th it feels like 5th, 3rd feels like 2nd etc., but 1st is lower than anything you have now?

So, why not just drive in a lower gear than you would normally, at anything over about 20mph you'd get the same effect. Under 20mph you wouldn't be able to accelerate as well as a car with lower final drive, but once you reach 20, just stay in a lower gear and you're there.

Have I missed something?

 

All gears are shifted down, including first. Another way to think of it is that there are a greater number of revolutions for the rear wheels for the same engine rpms in any given gear. It also makes it easier to keep the engine at the higher rpms where the most power is.

 

In some ways yes, in some ways no. We were just comparing the affect of the discussed final drive change on engine RPMs at cruising speed and that was a convenient example because it is easy to interpret when you're trying to figure out whether that would bother you or not.

Changing final drive is like riding a bicycle. Imagine that the gearbox is the small gears on the driven wheel at the back, and the differential/final drive is the front cogs attached to the pedals.

Changing to a shorter final drive ratio like we're discussing here is like changing from the small front cog to the big one. For each revolution of the pedals, the back wheels will make less revolutions. Think about what happens whey you do this on a bicycle and then start pedalling.... with the same amount of force applied to the pedal (torque applied by the engine) you accelerate up to your maximum velocity faster.

Now, given a narrow example like that, could you achieve a similar end by selecting 5th gear on the back wheel(gearbox) instead of 6th? Yes, the two would be roughly equivalent.

So why is this any different than just grabbing a lower gear? Because when you using 2 gear sets in combination, like on a bike, or theoritically on a car when you're talking about changing the final drive, the function is not arithmetic. When you change from one cog to the next, it doesn't simply shift every gear downwards by a fixed integer amount (say 500rpm), it shifts every gear down by a relative amount.

Take a look at the gearing chart I posted above for example. If you accept that my calculations are correct (and I did fail Calculus 101, although in my defense it was my first semester and I was dumb enough to schedule it at 8:30, probably missed half the lectures!) then let's look at the differences between 5th & 6th gear....

For the 4.55:1 final drive, at 100MPH in 6th gear, the engine is turning at 4471RPM. For the 3.9:1 this figure is 3841. Total difference? 630RPM

Look at the same figures for 5th gear - 5394-4634RPM = 760RPM.

So as you travel down through the gears, the effect of a final drive change is *multiplied* through the gear set rather than *added* to it.

What does that all equate to? The critical difference is that it applies the output of the engine more efficiently over a given MPH range. Think about all 6 gears and the range that they cover spread out from 0MPH to 178MPH in the standard car. Now take 6 gears and spread them out between 0MPH and 163MPH. What it really means is that across all gears you are getting the engine into its highest output range (powerband) more quickly and that you can keep it there for longer. You are maximizing the mechanical advantage of the gears for acceleration.

If you're looking at cars with forced induction sometimes this doesn't apply as well, partly because they have much broader torque curves, so they don't need to be kept 'on the cam' quite as much, and also because they need some time to see load and build boost (not so much turbo lag as there used to be, but longer gearing works better in turbo cars than normally aspirated)

The image below illustrates this reasonably well (sorry, best I could find). Imagine that the only line you can see is the bold blue one. When you change the final drive, the start point of the graph stays the same on the left, but the end point on the right moves back towards zero, and the whole graph shifts accordingly.



Another way to accomplish this, and one that is common in racing is the use of a close ratio gearbox. With this method, you don't necessarily lose top speed, you might keep 6th gear exactly the same, but shift all the other ratios upwards. So you might end up with a 1st gear that is like 2nd or even 3rd gear in a road car because you virtually never come to a stop or travel that slow. Then the ratios are clustered closely together. On the graph above, rather than shifting the whole thing to the left, each of the individual peaks would shift further to the right, with the one on the end staying fixed.

The disadvantage of this is that it requires changing all of the ratios in the gearbox (or most of them) rather than just the one in the differential.

I think that makes sense...? I might have screwed one or two things up, not enough coffee this morning, too much beer last night but I think that gives you the gist of it.

 

I just re-read that, it was a little long winded. The short version? Shorter gearing = more torque applied at the wheels. Changing down a gear will accomplish something similar under fixed conditions.

If you limit the conditions (say, from 0-163MPH), the change in final drive means you are applying more torque at the wheels over that RPM range than you would be with the original final drive.

 

Here's another way to look at it. You're objective for maximum acceleration is to keep max HP going to the wheels. Having a "Higher Ratio Rear end", get's you up into the max HP band sooner in 1st. Then for every up-shift, which in our Astons should occur just at Redline, you stay higher up on the HP curve after you shift up--Just what you want to accomplish.

 

Thanks for all the great responses, it's definitely on my shopping list. Interestingly I was driving a Lamborghini today and noticed its gearing was 2,700 rpm at 60mph.

 

Thanks for all the replies guys, I think I get it now. It isn't that it just shifts everything down by 1 gear, it brings all the ratios closer together, and you just lose the higher gearing that 6th give you.

Can see why that would be appealing in a car with limited low-down torque.

 

For what it's worth... I would definitely consider other mods as a first step before changing the CWP. As much as it is a very cool mod, if it were my car I would do other things first.

 

And that is over in the US where fuel is almost free. It seems like it would be a very unpopular mod in Europe.

 

Can you elaborate Stuart? From what I can tell, the CWP would bring the best bang for the buck in terms of performance as compared to other mods, such as cats & a tune.

 

Curiously, I've been looking at the maps in the AMV8 ECU and we never get wide open throttle until 5K RPM. There is a 2D map throttle overrider that limits max throttle opening vs RPM which is independent of accelerator pedal position! Same on Mustangs and a common thing to change I believe.

If you don't believe it, datalog your OBD2 with RPM and relative throttle position(%) and go WOT.

 

A little awkward for me, feel like I'm about to 'unsell' our product...

It is a $7000 item (or items to be more exact.) Plus shipping and installation cost it is closing in on 10Gs. Installation is something that requires a well qualified technician. Not just any guy who turns a wrench can do this kind of work. AM main dealers don't even rebuild diffs. If one failed they would just install a new one. It is also not as reversable if you ever decided to sell the car and wanted to put the OEM parts back on and sell some of your upgrades seperately.

$10K is half a supercharger.... or exhaust, cats, ECU, filters and some carbon goodies... etc. etc.

Don't get me wrong, I'm not saying it is not a good mod, I'm just saying that for me personally there are other things I would do first. It depends entirely upon your criteria. Some people think sports exhausts are too loud, the OEM is fine, but maybe they want the acceleration and gearing changes - more power to them.

For me, sound is a huge part of the driving experience and enjoyment factor. Okay, so an exhaust only makes a handful of BHP, but it puts a smile on my face and my enjoyment factor goes through the roof. So I guess it depends on how you define bang for your buck, what constitutes a 'bang'... uhhhhhh....

 

Okay, didn't realize the expense involved, or that this mod is not reversible. Thanks.

 

Yeah its a little pricier than I'd like but tooling costs a lot of money...

It is reversible, it is just a much more invasive and expensive install than a lot of other parts, so equally, same cost etc. if you wanted to reverse it.

Say it is $10K all in, for that money you could have our Vantage C Power Kit - up around 35-36RWHP, down over 50lbs in weight, awesome sound, with a set of lowering springs and a carbon front splitter.... that's where I'd put $10K of mod money if I had a Vantage.

Again it depends what peoples priorities are, some people think loud exhausts and carbon add-ons are for 'chavs' so diff'rent strokes...