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Diff Gears, How they affect acceleration

tanner892

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G'day people, just wanting to know the basic concept of diff gears and how they affect acceleration, will a ratio of 3.9:1 cause a car to accelerate faster then the same car running 3.08:1?
I've been talking recently with some people who seem to think that ratios of 3.7 or 3.9 will be no quicker in accelerating to 100kmh than that of a car that runs 3.08 or 3.46 for example. Is this true?

I always thought that a higher ratio caused faster acceleration (i.e. 0-100kmh) but therefore lower top speed, which is why drag cars run high diffs because they cant get to a high enough speed for that to matter when running a 1/4.

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shyun

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I think the main factor is 'It depends'. A higher dif will make in gear acceleration higher but this means youre changing gears more often- i spose with a dsg/sequential gearbox that changes in less than a tenth of a second, gear changing legths aren't realy a huge issue. So over multiple gear changes there isn't the same advantage. The higher the gearbox gear the lower the acceleration. A gear multiplies the torque from the engine, while sacrificing speed. So if you have 2X the torque you will have 1/2 the speed. If the gearing is too close, the more frequent gear changing can slow you down, or the spinning of the wheels in first gear (as there is not enough grip to cope with the torque). If the car doesnt have much torque down low, the shorter gearing can get it out of that rev range and up higher quicker too.

Also high reving engines are going to use high gear ratios/dif ratios because they can change gears less frequently (as they rev higher). so a car that revs twice as high can use twice the dif ratio, while mantaining the same actual speed (of the car) , assuming they have the same gearbox.
Cars that make their power through revs, so to speak, will need shorter gearing as they usually have lower engine torque. Whereas the same power engine with less revs will make more engine torque and will use wider gearing, so they end up changing gears at the same actual speed (of the car). So in effect they end up having the same power at the wheels (casue power= torque X revs). So your commodore is gunna use higher gearing than the falcon, as they have the same power output, but the falcon has more torque. That way theyl have the same power at the wheels for a given driving speed.

The drag cars are probably reving fairly high, and have sticky tyres, so they can use high gearing to their advantage. Their engines are also tuned to operate in the higher rev range, as once you're out of first gear you wont see the low rev range ever again in a drag race. I assume their gearbox's change gears very fast also.
 

VrWagz1

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Having no personal experiance but just thinking out laterally. Obviously 3.9's are gonna get you off the mark way quicker and definatly up to 60-80k's you will be quicker. As for up to 100k's i would imagine so, but there is going to be a point where you are going to run out of gearing and you will start to lose out as a car with lower gearing and better top speed will take you.
 

Calaber

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Diff ratio's by themselves don't affect accelleration - transmission ratios play a major part too.

If you have a low ratio diff (say 3.89 like early Holdens) but only three speed trannies, you need a low first gear to assist with take off, then have relatively large gaps between the three gears, because top has to be as high as possible (usually 1:1) to enable reasonable cruising speed and economy.

If you add more gears to the transmission, you close the gap between gears and by the time you have five speed trannies, you can start thinking about either having a low ratio diff with an overdrive, which will bring the overall ratio back to 1:1 or slightly higher, or you can raise the diff ratio and have an overdrive around 0.3 -0.5:1, giving you slightly slower accelleration but better (and quieter) cruising capability and lower consumption.

The ratios in the diff and trans will also be determined by the power available. The early Camira 1.6 had only 63kw, yet ran an optional five speed manual with 0.54:1 fifth gear. The four speed ran the same lower ratios but had a 1:1 top gear. I think the diff ratio was around 4.11:1. The low power means that, even with the low ratio diff, accelleration was pretty ordinary and the cars had to be revved hard for reasonable performance.

Depending on the power available, the accelleration difference between 3.08 and 3.46 would not be that noticable, but between say 3.08 and 3.70, you would start to feel the difference (and hear it, because the engine would always be revving harder at any given speed.)
 

danja

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Calaber is on the money.

The diff ratio (or final drive ratio as it's more commonly known) works with the gearbox to give the overall gear ratio from the engine to the wheels. The overall ratio is essentially a trade off between acceleration and top speed.

Working with your VZ SV6 as an example (assuming manual transmission, but same concept for autos). Your 1st gear ratio is 4.475:1 (meaning the engine turns 4.475 times for every turn of the driveshaft) and your diff ratio is 2.87:1 (driveshaft turns 2.87 times for every turn of the wheel). Multiply these numbers together, and you find out your engine has to turn 12.84 times to turn the wheels once.

Now lets say you changed to diff ratio of 3.73, the engine then has to turn 16.69 times to turn the wheels once. That's about 30% more times it has to turn to achieve the same wheel speed.

So what does that mean? Well, two things. One, it means that when you hit redline at 6,000rpm, your wheels are going 30% slower (so if your top speed in 1st was 60km/h it's now more like 40km/h) BUT, you've also got about 30% more torque being transmitted to the wheels, because gears are torque multipliers. More torque translates to more acceleration, so you'll also get to 40km/h faster.

So thats the long-ish answer. The short answer, will a higher diff ratio accellerate faster (all else being equal), the answer is yes, in a given gear. The question becomes more complex if you ask "will the 0-60 time be better". In the above example, the stock ratio would probably be faster to 60km, simply because you wouldn't waste time changing gear.
 

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this is fairly decent info guys.
how about for example overtaking? country driving throws a few trucks at ya, so you accelerate harder to a slightly higher speed to pass them in time. will certain diff gears cause this accelration to be poor, or because it is affecting acceleration in general, it'll accelerate at the same, if not quicker, rate? sorry, im a real noob when it comes to mechanicals and how the engine/trans/diff etc work.
 

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this is fairly decent info guys.
how about for example overtaking? country driving throws a few trucks at ya, so you accelerate harder to a slightly higher speed to pass them in time. will certain diff gears cause this accelration to be poor, or because it is affecting acceleration in general, it'll accelerate at the same, if not quicker, rate? sorry, im a real noob when it comes to mechanicals and how the engine/trans/diff etc work.
In the same gear, it will accelerate quicker. It has the same relative effect on all gears (thus the name "final drive ratio")

You will however find that on the highway, you will be cruising at a higher engine speed. Going back to my previous example, if with a stock SV6 you'd cruise at 110km/h in top gear doing 1500rpm, with a 3.73 diff you'd be doing about 2000rpm. This is going to put a small amount of extra wear on the engine, and also use slightly more fuel. How much of a difference it makes depends on your exact situation, how much highway driving vs city driving you do, what distances, speeds etc. These are the factors you need to weigh up when deciding what ratio you want.
 

Calaber

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this is fairly decent info guys.
how about for example overtaking? country driving throws a few trucks at ya, so you accelerate harder to a slightly higher speed to pass them in time. will certain diff gears cause this accelration to be poor, or because it is affecting acceleration in general, it'll accelerate at the same, if not quicker, rate? sorry, im a real noob when it comes to mechanicals and how the engine/trans/diff etc work.
Your accelleration will also be affected again by power and torque available at a given road speed.

Depending on the characteristics of your engine, the accelleration with a lower diff ratio may work for or against you, depending on what revolutions the engine is doing as you commence to overtake.

If your engine is running within its torque band, that is, the range of revolutions where it produces its greatest amount of torque, or twisting motion, the engine will respond well and the car will accellerate well. If, however, you are driving a car with a highly tuned and comparitively small capacity engine, say a two litre Euro, the chances are that the car might not be operating at the optimum point within its torque band and may require downchanging of gears to get the best accelleration. Most engines operate at their most economical outside their maximum torque band, so if you are cruising economically, the chances are that you are not operating at the revolutions where the engine will respond best for rapid accelleration.

Overall, it's a very complex issue and is affected by the engine capacity, output, torque, overall gearing and even weather conditions. On a hot, dry day, the engine is likely to produce less power than an overcast, cool and wet day, because the increased humidity in the air assists combustion.
 

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cheers danja and calaber, useful info there. i've been meaning to do some research on it, cos mine feels sluggish at times off the line, but i dont want to be sacrificing too much top end power cos i do a lot of highway km's every weekend. its easily 400kms highway per weekend, and 100 if i'm luck in town.
my engine is a stock V6, save for a catback 2.5 inch exhaust, and it'll stay almost that way. maybe some extractors at some stage and a CAI.
what diff gears should i be looking at if i want decent acceleration and power up to about 150kmph? and what else should i look at upgrading? what are some of the downsides from this?
apologies to the OP, i feel like i'm stealing the thread. My bad lol
CC
 

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No real input as most of the brains behind it have already been said above.

But I went from 3.45's (standard) to 3.9's in my vl and I really regret having them at times. Mainly because of my revs in 5th on the highway, sitting on 110 I end up sitting at about 2.5/3k, ideally I'd like to be sitting at a lesser rpm. Not to mention other problems like my speedo being effected.

It does however seem to be very touchy and accelerate faster. Apparently my next gearbox choice will bring it back to normal.
 
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