tweenierob

I am currently lookin into some different gear ratios but after reading through a recent thread i am unsure what to do...
I would like a good ratio for Drag racing but do not want to be bouncing off the rev limiter on the motorway

Harvey you have A LOT more power than tim but a lower top speed??. Can the gear ratios really have that much affect??
Would you say that your gearing is better suited for drag racing??

Tim, can you compare your gearing to a WRX (for example) does your acceleration suffer for the longer gearing??

It has been suggested as an idea that i run with RA 1st and 2nd, WRX 3rd and UK 4th and 5th.

Please discuss.

Rob

David_Wallis

Interesting, Havent got a clue...

Also consider front and rear diff ratio's, rolling radius of the wheels..

David

NotoriousREV

How about downloading the free DOS based version of cartest and playing with the ratios in that? It'll give you some idea of the effects on changing diffs, ratios and wheel sizes.

Dan B

Rob: consider an Sti 5th. I cruise at and indicated 85mph at only 3250rpm -> thus getting that mpg I think 5th is 0.738, which come to think of it is the same as the Uk turbos.... ignore me...

tweenierob

David, any more info on Diff ratio's??

Where can i get that program??

Rob

AlanG

Rob

YHM

NotoriousREV

Rob,

Visit http://home.earthlink.net/~patglenn/ct.html

Scroll to the bottom of the page and click on Download Cartest 4.5

R19KET

Rob,

The choice of gear ratio's, and final drive choice for top speed, will have a lot to do with the power curve of the car.

Ideally, we would choose the gear ratio's based on the RPM peak power is achieved, so that the RPM drop between gears keeps you in the best power band.

So, if you make good torque from say, 4500rpm, it's pointless having a ratio that will drop the RPM to 4000rpm when you change up. Like wise, you wouldn't want the rpm to drop to 5500rpm either.

Once you've got a rough idea from the gear ratio's available, you can work out what final drives will best suit the speeds you want to achieve, then tweeking the gear ratio's, to get the best compromise.

Mark.

tweenierob

Thanks Mark very informative

I was hoping to get a reply from Harvey TBH, it seems as though top speed is not his aim and his 1/4 times are pretty much what i would like to get or better.
Harvey are you running STI gearing??
Tim W, what 1/4 times are you getting?? I dont really want to go for Uk gearing as top speed is not really my thing but feel that longer gearing may help.
My way of thinking is that the longer gearing means that i would be on power for longer, but the RA gearing accelerates quicker??

Excuse my stupid questions, i'm sure i will make sense of it one day!!

Andy F, were you running full stock RA gearing for your 11 sec 1/4?? what diff ratios??

So many questions!!

Thanks people.

Rob

Bob Rawle

Rob, Harvey would certainly reply as and when he sees this thread, you would be better emailing him directly though rather than relying on him viewing Scoobynet, he is not one who spends alot of time in front of his PC.

tweenierob

Thanks Bob
Rob.

Tim W

iTrader: (1)

Rob, I've know idea what my 1/4 mile times are, the last time I rana 1/4 the car had 25k miles on it and only had a backbox and filter change! Back then I got a 13.768s 1/4 with a terminal speed of 99.9 mph not bad for a standard UK car really

Now it's been modded I'm too frightend to try and launch it properly for fear of destroying the gearbox, afterall it wasn't designed to take nigh on 100lb/ft of torque up on the standard rating

Having looked at what I had planned for the car though I was going to stick with UK ratios, because with an extra 1000rpm to play with at the top end it would be really flexible on track, which is where my car will be used the most, I'm not really into becoming a 'drag queen'

tweenierob

Lol thanks for the reply Tim.
Are you saying that you now have an extra 1000rpm due to a raised rev limit or because you have 1k over what an STI car has for the same speed??
Excuse me if i sound stupid.. I am a drag queen

Rob

Tim W

iTrader: (1)

Rob, your excused, especially as you seem to like flouncing around in straight lines

1k extra rpm becuase I'll be having an engine with an 8k rev limit rather than the 7k I'm limited to with my piston slapper of an 87k mile uk engine.

I might be tempted to try slightly shorter 2nd, 3rd and 4th though...got to think about it sensibly, but only when I can afford a gearbox from LAteral Performance.

tweenierob

LMFAO!!!!

Tim, was it you i was talking to at the end of PE?? could have questioned you then

Rob
Still none the wiser..
Gonna have a go with the data, thanks Alan

Tim W

iTrader: (1)

Rob, twas indeed me with Mark and Pat at PE last weekend, I was there with my new trusty daily driver of a Focus diesel the scoob is now for weekends and pleasure only...it's still in pieces although the gearbox is back in, for the 3rd time in the last month [img]images/smilies/mad.gif[/img]

Today I small mostly be removing the inlet manifold to modify the injector rails, fit the 550cc injectors, pressure reg and fit a 70mm dia induction pipe

Then maybe it can run

harvey

iTrader: (48)

Sorry, I just found your post as I have not been on the net much this last week, but here is the e.mail I sent you.
Regards,
Harvey

PS: The weight of your car can also be a consideration.

Getting the gearing right on a lower powered car has a greater effect on results than on a big power car. Generally, the more the power the taller the ratios the engine will support and it is a balance between the gear ratios, the number of gear changes to a given speed or distance and aspirations for top speed or cruising speed.


Hi Rob,

I saw you posted a question about gearing and I meant to answer it but did not do it immediately and then could not find it.

Basically, I know about my own gearing and that of any other car I drive but it is difficult to tell you exactly what to do without driving the car first. There is so much to take into consideration, the actual gearbox and hence the ratios. It is very important to identify the exact ratios and this can be done by assessing what you think the gearbox is, finding the assumed ratios from a technical data sheet which I probably have, and then running the car to say 3500rpm in each gear and then calculating the geared speeds and comparing it against what was expected to confirm that the original assessment on gearing is correct. It is also important to accurately identify the diff ratio which could be 4.44, 4.11, or 3.9. Lots of owners think that they have one particular diff ratio and actually have a different one.

The next thing is to look at your bhp and torque curve. What power do you have and where abouts in the rev. range is it?

The next thing is to accurately determine what you want. This could be, relaxed long distance cruising or alternatively geared for maximum top speed or seperately best possible accelleration but if that is the objective you have to determine over what distance.The gearing for 400m may be different to 500m believe it or not and will certainly be different for say 1 mile.

It is a pretty complex subject and needs you to define your goals and answer as many of the above questions as possible. Once that is done I am happy to help you and if you want to talk this through further either then or now to get a bit more background from me, my mobile is 07860 302020 but do not phone when the Grand Prix is on !!!!. After 2200 and the answer might not make a lot of sense!!!!

Regards,


Harvey