Diff Gears, How they affect acceleration
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.
Thanks
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.
The Wagon on Wheels..
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.
Originally Posted by Blown V6 Hatch
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Reaper
Nil Bastardo Carborundum
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.)
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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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More trouble than u think
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.
CruznCalais
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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.
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Nil Bastardo Carborundum
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.
More trouble than u think
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
CruznCalais
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Projecting
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.
Nil Bastardo Carborundum
For the type of driving you do, the diff ratio in the car at present is probably the most suitable. My reference tells me its 3.08, which is definitely biased towards economy and would account for the sluggish take off.
If you wanted to improve the acceleration slightly without sacrifcing too much economy, your next option would be the 3.46:1 .
I wouldn't recommend going any lower than that.
More trouble than u think
ok mate thanks for the input, much appreciated.
CruznCalais
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i have a vs v6 which i just got 3.9 gears in
the acceleration is definitely quite a bit better, fuel economy hasn't really been affected, first tank it was even with what i was getting on the old diff, this tank i got 1L per hundred more, but i think my driving might account for that
i dont do much highway driving though, doing 400kms a weekend on the highway i think it would probably get on your nerves, going through more petrol, and sitting on higher revs. at 100 mine sits on about 2200-2300rpm from memory
the only time i really notice the difference is in 1st and 2nd. the only times it gets into first is when stopped or nearly stopped, but 1st has so much torque, way more acceleration. if you get gears, id get either a spool or lsd as well. i got an lsd, but without it traction would be a big problem in 1st (and lets face it single spinners are embarrassing)
would i reccomend it? **** yeah.
if your doing that much highway driving I wouldnt bother changing them, but as said, if you really want a little more acceleration but dont wnt to compromise your highway driving too much, go for the 3.45's but honestly I dont really see too much point in that
Searching for the billion
Geez, this thread has gone a little crazy.
The answer to the original question is that lower diff ratio will ALWAYS give faster accelleration...... thats until you hit the red line in top gear doing about 150, and your mate with the stock ratio catches you and fly's right by.
Nil Bastardo Carborundum
Did you alter your speedo gearing to match the diff? If not, your speedo, odo and trip computer will be giving you false (optimistic) readings.i have a vs v6 which i just got 3.9 gears in
the acceleration is definitely quite a bit better, fuel economy hasn't really been affected, first tank it was even with what i was getting on the old diff, this tank i got 1L per hundred more, but i think my driving might account for that
i dont do much highway driving though, doing 400kms a weekend on the highway i think it would probably get on your nerves, going through more petrol, and sitting on higher revs. at 100 mine sits on about 2200-2300rpm from memory
the only time i really notice the difference is in 1st and 2nd. the only times it gets into first is when stopped or nearly stopped, but 1st has so much torque, way more acceleration. if you get gears, id get either a spool or lsd as well. i got an lsd, but without it traction would be a big problem in 1st (and lets face it single spinners are embarrassing
would i reccomend it? **** yeah.
yeah im using the jaycar speedo corrector. the speedo odo and trip all read correctly
well I just got my 3.7's fitted and it seems like theres no difference to what it was before...
at 100 it does like 2000 which is what it was doing before I swear. Anyone with manual and 3.7's got any info??
Banned
There may be some confusion here in what is meant by a higher ratio, the original poster rates 3.9 as high and 3.08 as low, where others talk about it the opposite way around. The diff is a speed reducing or force multiplying device so its possible better to refer to diffs that create a lower output speed as lower ratios, that's what is most typical so Ill go with that.
Its been written above that by changing to a lower diff ratio then the acceleration in any given gear will be higher. this is true, but by itself gives the misleading idea that the car will accelerate faster overall, untrue.
The car will accelerate at a higher rate in a given gear with a lower diff ratio, the trade off being that it will be at a lower road speed, and basically the same rate of acceleration as a previous lower gear at the same road speed. There is nothing magical about putting in another diff ratio, no more than putting on smaller wheels.
The only real factor on acceleration at a given road speed is the rpm that the engine is doing which as mentioned by several is just due to the overall gearing created by the whole drive train.
Given that most of us at least have four gears to play with, Choice of diff ratio really is only a consideration for not getting the car bogged down in 1st gear and at the opposite end of the spectrum having the revs low at cruising speed.
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What info are you after?
DANJA'S CLEAROUT 2010 : Various VT-VZ parts, short shifters, performance parts. Check it out!
It will be reving at more than 2000 with 3.7s, It'll be on 2400 or so if you really have 3.7s. There nothing really to be gained acceleration wise with a stock cammed 304 by changing diff gears so don't expect to go much faster
yeh I was cruising on the freeway after I installed it last night and was doing 100 when in actual fact ot was only doing lie 80. I realised today when I was doing 100 and everyone was flying past me hahaha. just needa put my speedo corrector in now.
CHE Tuned 156.8rwkw
Calabar, if i went to 3.08, would that make much difference from 2.87 then?
Ive worked out that there is a 7% difference between the two diffs.
The Blue One
Nil Bastardo Carborundum
Hi Paul,
If you are unhappy with your 2.87, which is a really tall diff, and want better acceleration, the 3.08 won't make much difference - it is still a tall ratio orientated towards economy rather than performance. If you dropped to 3.46:1, you would certainly notice a difference in performance,but also economy and engine noise.
2.87 seems extremely highly geared for an SV6 - are you sure that's what your running at present?
CHE Tuned 156.8rwkw
Im actually going to buy Andyman's 3.08 out of his VZ wagon, Reading what Danja said earlier about the 3.73:1 7 how much of a difference there was, Thats wat he mentioned earlier, ill double check, but im pretty sure its 2.87:1.
The Blue One
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