Mitchy260

Can someone explain the pros and cons if i were to fit a shorter ratio'd gearbox. I know it will improve acceleration but limit top end speed.

Does short gearing mean 3rd is needed for 60 and 5th needed for 100?? Why does it give faster acceleration??

I know all the raleigh cars, colin mcrae etc can do 60 in sub 3 secs, how would they manage that with 3 gear changes??

Help please.....Confused about the principle behind short and long ratio'd gearboxes.

Can someone go back to basics for me please

Mitchy260

31 views and no answers......?? Does that mean another 30 people out there don't know either?? or are they not just being helpful??

Somebody must know the answer to a simple engineering question? I thought that was a basic easy question to answer??

greasemonkey

Little explanation necessary, sounds like you've got a grip on the general principles already.

That would obviously depend on exactly what set of ratios you were talking about, and on what car. If you're on about putting the short ratio Type R/RA gearing (which involves a higher final drive ratio) on a car with a 6500rpm red line, then yes, third would be needed for 60, and most likely fifth for 100

Because the ratios are lower. Your car accelerates harder in second than it does in third, yes? All a shorter ratio gearbox does is lower some, or possibly the entire set of ratios. For example, fifth gear on a late model STi Type R is about the same overall gearing as fourth gear on a UK Impreza Turbo.

Erm, even assuming that they needed third to get to 60mph, that'd be two changes (first to second and second to third), not three. In any case, it would be incorrect to assume that all modern rallycars would need to take third to do this speed.

As to why they're so quick to accelerate, that's a combination of a hugely torquey engine, gearbox that changes gear much quicker than any roadcar's, excellent traction and so-on.

You sound like you've got the details in your head already. Don't forget that the shorter geared Japanese market Imprezas are designed for use in different conditions (notably a 112mph maximum speed), while the STi engines rev to 8000rpm instead of 6500. The higher power band is a large part of the reason why these cars need lower overall gearing.

BTW, why are you asking? Unless you have a specific usage in mind (rallying, hillclimbing, sprinting etc.), it is unlikely you will benefit from lowering the gearing of your own car.

Don't take the p*ss. It's not really cricket coming up with a comment like that in the first place, let alone less than 24 hours after posting your original question (and at the weekend too).

As it turns out, I was writing a reply to your post when you came up with this comment. Don't forget that Scoobynet is a public forum, and that nobody is obliged to answer your post if they don't feel like doing so.

If it was that easy a question, why didn't you do a bit of research and find the information you needed all by yourself?

[Edited by greasemonkey - 2/1/2004 3:24:20 PM]

sg72

Hi Mitchy!

What Greasemonkey says is of course correct.

However as is pointed out. It depends, what you want to do.

I have often found that driving on B/C roads. The jump between 3rd and 4rth to be to great and visa/versa.
4rth is a V/useful gear on A roads.

Overall I can see what they say about Unsuitability of the JAP gearing on UK roads.
Unless you just want to dragrace (As Said Have to be in 3rd to reach 60!

Steve.

Mitchy260

Cheers for that grease monkey, i didn't mean to sound sarcastic. Im just trying to get a little more educated in the way short/long gearing works.

Cheers for your response.

Mitchy260

It may sound like a silly question but if i dont ask ill never know..

We all know the car accelerates quicker in 1st gear then 2nd and so on. Would it be possible to design a gearbox with a very long 1st and 2nd gear, i mean just for drag racing and not general road use?? Like 60+ in 1st and 100 in 2nd. Or am i not understanding the principles??

sg72

You could. Easily . But you'd Baulk on the line.Doh!

greasemonkey

Yeah, it's certainly technically possible to do something like that. Out and out drag cars normally only use two or three speed transmissions.

Don't forget that the basic principle behind a gear reduction system is just two gears with different numbers of teeth. By varying the numbers of teeth you can produce whatever combination of ratios you want.

john banks

The key advantage to shorter gearing comes when the rev limit is increased and additional power is made higher up the rev range, so the maximum road speed in each gear is still generous but torque multiplication occurs through the gearing.

Consider the M3, VTEC, VVTi, GT3 and Ferrari engines and you find relatively high power in BHP compared to torque in lbft. They all use variable valve timing to enable then to hold torque to a high RPM thus making lots of power. They use short gearing to make the most of this but retain similar in gear speeds.

The opposite is a lazy V8 such as the older BMW 4.4 V8 in the 540i which makes less power in BHP compared to torque in lbft, and has a relatively low RPM limit. Or a diesel engine. They tend to have longer gearing to again reach similar in gear speeds, but rely on torque from capacity or turbocharging to make power at lower RPM.

So you can offset displacement and boost by using RPM if you can get the cams and the engine breathing/integrity to do it.

Short gearing with the same rev limit will generally mean you are just changing gear more often, but if the ratios are closely stacked with a peaky engine it may help you avoid dropping out the power band. By shortening the gearing you can get slower times due to spending a lot of time changing gear.

I've just switched from a five speed long geared UK box which really suited my torquey 2.5 to a six speed which really has you working the lower four gears because they are quite short. I can't take the rev limiter safely over 7000 RPM as stock because I will most likely break a conrod. It feels even more manic in the lower gears than it did, but honestly I think it would go far better with a four speed autobox with widely spaced ratios, I don't need it to drop back to 5500 RPM after a 7000 RPM gearchange - 4000 RPM would actually be fine! The peak torque on this engine is at about 4000 RPM and peak power is at about 5500 RPM. The same turbo on a 2.0 has peak torque at about 5000 RPM and peak power at about 6900 RPM, so it suits an 8000 RPM limit and goes as quick if you keep it on the boil.

Forgot to say that by using smaller capacity/higher RPM limit/shorter gearing you can go as fast by making similar power and often weighing less.

Compare the in-gear times for an M3 (3.2 six short geared c.8000 RPM limit) and M5 (4.9 eight long geared c.7000 RPM limit) and you will see that despite 100lbft less torque the M3 is not ashamed.

Similarly compare a 2.5 engine with 7000 RPM limit with a 2.0 engine with 8000 RPM limit and the differences are not always what they seem.

Some people deliberately destroke their engine to allow it to rev higher without breaking conrods. It is only with high revving motors that you get 120 BHP/litre from normally aspirated engines. Short gearing goes well with this.

Subaru seem to invariably do three things when they fit shorter gearing to a car:

Higher rev limit
Wilder cams and/or larger port heads
Larger turbo

They all go together nicely

[Edited by john banks - 2/1/2004 10:22:32 PM]

911

The gentlemen above are perfectly correct. To add a real experience to this: I have a hill climb Stiv3. The car must be able to blast between corners!
I've removed the stock transmission (4.44 diffs etc) and fitted an early RA, 4.11 and the short ratios. The engine is stock save the full decat, so it screems to 8000rpm.
The car used to pull 4250 at 80mph, it now pulls 5500. The accelleration is much sharper, though not astoundingly so in comparison, but you can find a gear better and keep the engine in the turbo band of 4000 to 7000+.
Hope it reduces my times, and the dream that RA boxes are stronger than most...
I used to drag race in the 70's, and the AA fuelers use the wheel spin and the enlargening of the tyre diameter to effect a variable gear ratio on the hard launch for the first few meters....
Graham.

K9VYN

JB,

just read your post - very interesting. i seem to have gone completely the wrong way according to what you have written. orginally i drive MY95 WRX (TD05) and replaced the knacked box with UK spec & rear diff. This seemed to work fine and I increased RPM limit to 8K with Scoobyecu @ 16+ psi.

now i have STi 2 ra + APS FMIC (still TD05) with the current map @ 1.3bar boost (still running in though). i still have the UK box. once the car is run in and mapped appropriately (perhaps more boost), will the longer UK box with 8K RPM make the car feel 'lardy' in your opinion or will the now increased boost potential/capability make up for this lower down? perhaps we can map around this?

i like the longer gears - useful when overtaking, but i'm a lover of responsiveness and don't want to be hampered by the box (which may or may not last long depending on how much torque i generate) making the car feel sluggish . obviously i have (over time) uprated fuelling and breathing, so the car is now at a fairly high spec.

thanks in advance for any advice,

Kevin

911

Kevin, i changed my Sti trans for an early RA for competition purposes, but for pure road driving, the Sti ratios take a LOT of beating. IMHO the factory got it spot-on.
Dump the uk trans and get an Sti set-up (no, I'm NOT trying to sell mine!)
Graham.

stevebt

iTrader: (8)

[qoute]know all the raleigh cars, colin mcrae etc can do 60 in sub 3 secs[/quote]


thought they only made bikes have an RA gearbox in mine and 80 hits 3500rpm but top end is still normal and acceleration is

john banks

Kevin you may find that 2nd gear is a bit tall for instant response out of slow corners with a TD05. Certainly T-uk found this. The shorter STI gearing has you at higher revs in 2nd ready to play in slow corners. Changing into first when moving is not so practical

K9VYN

911/JB,

i don't know that the UK box will hold the torque i may generate, but there is no point dumping it until it dies i guess. i haven't really got a target bhp figure in mind, but am aware that the RA closed deck block is capable of upwards of 400hp.

i have yellow injectors (440cc?) and uprated fuel pump, adjustable reg etc so think the fuelling and TD05 are capable of taking me to at least 350hp. i trust a UK box can handle up to around that amount of power and relative torque (no more money in the pot just now for an unecessary change of box)?

i may perhaps pick up an STi box (and diff) later on just as a spare in case the UK trans fails - although it is still low(ish) milage. 2nd gear was not too bad with the 260PS WRX engine, but may be a tad lardy with the RA one? however 3rd was quite magical and the car picked up even more strongly than 2nd - 4th was/is great for 60 - 120+ in a short space of time. it may just mean a slight change of driving style... but will there really be that much difference seen from the engine swap?

also, will the ECU be able to control boost to the point where i can make more in the lower rev range (in an attempt to increase low down torque) to combat the higher gearing?

thanks for the advice,

Kevin