Effect of final drive on acceleration
 

I've been having a discussion with a friend on the effects of a shorter R&P on two otherwise equal 5 speed gearboxes. Given two boxes with the same gearsets, one with a tall 2.20 final drive and one with a 2.73 final drive, my understanding is the shorter 2.73 will be more fun to drive and will offer quicker acceleration. He disagrees.

I equate the final drive to the front chainrings on a bicycle. You accelerate quicker through the gears when on the small chainring than the larger.

What are the real world effects, tangible and intangible, of a shorter final drive?

Quicker acceleration, lower top speed.

It's possible the 2.73 will have a higher top speed.

If the 2.20 is too tall for the engine to be in its optimal rev range at top speed, the 2.73 may have the higher top speed, at the expense of fuel mileage.

The 2.20 is very tall. Probably to maximize fuel economy.

You are correct

The shorter rear end may not be more fun, if the gears are too close together you're shifting too often....so 'generally' you are correct, but I don't think it's a hard and fast rule.

There are a lot of factors, gear ratios in the transmission, wheel diameter and torque band.

On my 84 GTV-6 I got the Bionic Transmission! With only a 2.5 some mods cams etc. My Buddy Jeff and I decided to place the gears of the 84 into a diff from an 82 very tall and WOW what a cruiser. Gone a 140MPH not today, been in the car for over 20 years. Not LSD but not much of an upgrade in the 116's
My acceleration has been altered not enough to hinder the performance, but when I go into 5th Watch out!!

Taller gears are better!

The objectiive answer is that 2.73 will offer quicker acceleration. If that is what you mean by "fun" then there's your answer. In my driving environment, quick acceleration does indeed = fun. If you live in wide open spaces where driving at 140+ mph is possible, then that may be more fun for you.
I had a truck driver friend who drove what I think was an IH or a Peterbuilt. It had a 5 speed+Brownie+2 speed rear axle. He could hit ten gears in the space of a city block and not exceed the speed limit. He was having fun.

My 85 928 has a euro ring and pinion. It is great off the line, and particularly in slower corners it makes second gear much more suitable. For some cars, particularly track cars, it won't matter once you are off the line, as long as you have a gear for every corner you drive.

If I wanted to supercharge my 85, I'd have to gear it back up again.

The final drive ratio should be part of the over all gear ratio selection. Generally it depends on what the car will be used for such as a NASCAR Sprint cup car on an oval that hardly ever shifts you want to have a rear end ratio that will peak the RPM/power at the end of the longest straight. For a road race car you want to be able to come out of the corners with maximum acceleration and a gear spread that will allow the car to quickly get to top speed by the next corner. The lower ratio in the given example may feel faster but it is not much lower so probably little real world performance will result. Today's average drivers would never tell the difference.

As an example, my 914-6 race car had stock R&P but two different gear boxes, one was M-S-X the other K-Q-V. When racing the street circuit races in Mexico I put the K-Q-V box in and on at least one corner would have to shift down into 1st gear but the acceleration out of the turns was great. There was a '68 Camaro with a 355 NASCAR detuned engine and he could never catch me unless the straight was really long. He had a stock geared transmission and a 4.11 rear setup.

Endless discussions of this in the muscle car world. My Mach 1 has the stock 3.55. I've been told 4.10 will really make it scoot.

Thanks for all the input. Mr. Dudley's car is, to some degree, what generated the initial discussion.

My 91 928 has a 4 speed auto with a 2.54 final drive. I've never actually driven my car so with my block back from the machine shop, I've been thinking about what do with the car after I rebuild the motor. That means I had to drive some 928s.

Drove a 90 S4 that the owner, a Rennlister 928 enthusiast, kept up mechanically but was cosmetically horrible. That car made me want to never drive a 928. It was incredibly slow...my 94 E320 is quicker. And while he said it had new rear shocks, the car rode like it had 10 year old tires and the shocks were frozen. The rear hatch rattled like it was going to fly off at any second. It recently sold on BAT for $9100 or so. Horrible car.

1990 Porsche 928 S4 | Bring a Trailer


Then I drove an 87 5-speed S4. Taking on and off ramps, respecting that it’s not my car and I didn’t know the tires, was intoxicating. Effortless speed through them. It drove like a 928 should with bank vault like solidness. I fell in love. An 87 has a tall 2.20.

But that car made me realize that if I want to keep my 928, I would have to make it a 5-speed.

With that in mind, I did some research and the only transmissions worth putting into it would be the 2.73 G28/10 in Mr. Dudley's car or an uber rare G28/55 found in 89-91 GTs and the 88 Club Sport. I have found one, in fact it's part of an entire conversion kit.

I am driving a 91 S4 auto in pristine condition tomorrow as a reality check.

I do have access to a 90 GT with the motor in pieces so in theory I could rebuild my motor, put my car back together, sell it and buy the GT and rebuild that motor. Then I'd have a truly collectible 928 with a 5 speed. But I'd also be rebuilding another motor.

Where I live fun equals 2nd and 3rd all day long. I think a 2.73 will be a lot more fun than a 2.20.

In general higher numerical gearing gives more push at the same speed, but for less range.

Changing cwp has the same effect as changing tire heights,

Here's a comparison that was handy comparing 2 rear tire sets on an SC, the shorter 275/35 x15 gives more push(AKA acceleration) in each gear at every speed, but the top speed in each gear is also shorter, aero affects are also reflected in the curves
http://forums.pelicanparts.com/uploa...1477004403.jpg

here the data only reflects a change in CWP from 3/444 to 4.00
http://forums.pelicanparts.com/uploa...1477004521.gif

Another way of looking at it... My stock 2nd gear is lower than the guy with low ratio gearbox in 3rd gear - so more acceleration. And he had to do an extra gear shift to get there too.

I think it's really all been covered, but in general, I would think that a lower (higher number) final drive than 2.2:1 would make your 928 more fun to drive.

I'm assuming that you'll be doing most of your driving in the 40-80mph range or maybe 30-100. What's the theoretical top speed of your car, 140, 150, 170 (if it had enough power)? Are you looking for low gears to maximize fuel economy on really long drives? I assume your car isn't going to have peak power at a very low RPM. Yeah, a little higher should give you a bit more fun at normal speeds.

My understanding is that automatic transmissions usually have lower gears than manual transmission and I think part of the reason is because the torque converter can actually function as a torque multiplier under some circumstances.

From wikipedia
"The key characteristic of a torque converter is its ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. This is a feature beyond that of the simple fluid coupling, which can match speeds but only provides the same torque, thus less power."

I think 928s are "safe" up to 160 or so. I have no interest in going to jail however; even with Waze and an empty highway, I wouldn't try that.

I only want to maximize fun in 1st, 2nd and 3rd.

I was simply amazed at how fun the 87 5 speed was well under 60.

I have a Bilstein and Hypercoil suspension I'm sending to Bilstein to rebuild.

I'm sticking with 16" D90s and may try 245/45s at all 4 corners. I've heard that's a good combination.

I would like to do some DEs next summer with it. Putting in an early sump pick-up and a pan spacer just for those occasions.

2.73 over 2.20 = Cruising at a higher RPM given the same highway speed = Less fun sometimes.....Depends on how you use the car.....
I luv the short gears in my 73.5 with a high compression 3.2. Fun around town and on backroads....Not so much on the open road at 70-80....

Just a basic check using one of the online calculators.

Assuming that there's no overdrive on the trans, the difference between 2.2 and 2.73 at 70 mph would be 2070rpm vs 2570. I can't imagine that the 500 rpm would be that big a deal, especially since I suspect the transmission would actually have some overdrive on the final gear which would drop the RPM lower.

Another check, 6000rpm 245-45-16 tires, 2.73 rear with no OD, speed would be 163mph. The same rpm with a 2.2 rear would yield 202mph. I don't think you've got anything to worry about on the top end. I have no idea what the redline of a 928 is, but I assume it's at least 6k. And how often would you find yourself at 160 thinking "if I only had another 40mph!"

Years ago i drove a 4.7 litre 928S (Euro, so 300bhp) automatic with stock cwp that had been stripped of all interior except the 2 seats and the instruments, so it was way lighter than when it left the factory). With 2 people in it was spinning its wheels in all gears in the rain(with good, rather than great, tyres) but still controllable and it was the most fun to drive of any road car that I had driven up until I acquired my Clio 172 Cup ( which is lethal in the rain but a hero on twisties in the dry).