Final ratio - Tarmac rally car

[Grundo]

Hello,

I have a VX race car which has a LSD, larger carb, lightened flywheel, Guy Croft pistons which has the engine being re-built at the moment. I've also purchased (you might have seen it on the Lancisti website) the 4.36 final drive (off a 77 berlina). This question is taken off the Guy Croft website where I had no response so am looking to the wise heads of this forum to help.

A question I have in general for anyone with any knowledge is how much torque through a front wheel drive car is too much? The reason for my question is I want to install the 4.36 final drive in my gearbox which significantly changes the drive force of the wheels (standard is 3.26) and incidently replicates the performance of the car (expected) to that of the Group 4 rally car from 1975. The only difference I guess though is the influence of the supercharger.

The Group 4 rally car had:
190 DIN a 7700 rpm. (195 bhp)
22 kgm @ 5500 rpm
Drive ratio 13/58 (4.462:1) (from Beta 1300)
1st 55 km/h @ 7000 rpm
2nd 78 km/h "
3rd 102 km/h "
4th 127 km/h "
5th 151 km/h "

My car will rev up to 7000 rpm and given the changes to the engine it is uncertain what the engine performance will be, going by Guy's original book I'm thinking approx 25hp improvement along with better breathing for higher revs. The standard engine has peak torque at 3000 rpm at almost the same torque as the rally car - 21kgm.

Developing drive force/speed cascades and assuming higher revs and higher engine power due to the larger carb I derive:
165hp at 6500 rpm
Drive ratio 4.36
1st 44 km/h @ 6000 rpm
2nd 68 km/h "
3rd 100 km/h "
4th 133 km/h "
5th 165 km/h "

As it isn't a close ratio gearbox, the drive force in first is quite high (peaks at 2438 lbft in first) which strikes me as a potential issue although the speeds etc seem comparable.

So any thoughts from people as to whether I am way off the mark, will end up with an undrivable front wheel drive car or will actually be quite good?

Thanks,
Adam

[Neil-yaj396]

I can't see that the car will be undrivable. As you are point out you are pretty much replicating the old Beta rally car. More power has gone through the Beta set up. Sound like it should be pretty good.

[RickyJ]

a bit of a generalisation, but recent Hot hatches with silly power/torque outputs have used a LSD to put that much power to the front wheels - Focus RS, Integra Type-R e.t.c. If it works for them then why not? Also, wasn't John Day's Racing Beta from years ago putting out over 200bhp through the front wheels? I know that most of the front end was removed and braced but the basic car was a regular FWD Beta.

[RickyJ]

Not relevant to the thread (sorry!) but just found some footage of the John Day car here:

http://video.google.com/videoplay?docid=-1385131338336845133#

[Grundo]

Thanks for the replies, its a bit of no-mans land I find myself in, the engine hasn't gone in yet so don't fully know its capabilities - its all a bit of a guess and I don't want to limit the top speed to something too low.

Sounds like a suck it and see approach!

[lanciamad]

Hi Adam,
As mentioned I don't think the power/torque your be putting through will be so much of a problem, the gearboxes are pretty bomb proof, and the LSD should keep everything driveable for you, without an LSD i'd say you'd be struggling, because I certainly was at tight and twisty Llandow, and i'm running standard engine/box! What series are you running the car in? Surely the question regarding top speed is what speed your opposition are achieving in your class/series, once you know that (roughly), you can decide on what gears(box) will give you the best compramise for top speed and acceleration.
As also mentioned, just look at some of the hot hatches of today running stupid power through the front wheels, BUT, they have the chassis to cope with it (well most), I presume you've got a roll cage in it? The front struts will certainly need stregthening IMO, preferably with bracing running through the bulkhead onto the cage and a decent strut brace set-up (if you havn't already done so), as well as the engine mounts and subframe all in good order.
Like a lot of things, if someone's not already been there and done it, you've got to give it a go yourself
Marcus.

[75coupe]

Hi,

I would say that ratio would be too low for your engine, the vx will have considerably more torque low down than the original 1756cc rally car did - which would have had negligible torque at low revs and probably only came "on cam" after 4000 rpm.

My 2 refreshed 2 litre with twin 45's, 11:1 CR, performance cams etc, revs to 7500 rpm, runs the standard 3.78:1 diff which is still lower than the VX item and I think it is too low!!! I run out of revs very quickly in 1st and traction can be a problem if giving it everything even with sticky Toyo R888 tyres.It easily runs past 200 KPH.

Your VX will have more low down torque - remember it is torque that gives acceleration not power, you may get frustrated with such low gearing. Maybe compromise with a 4.07 or 3.78 diff, still lower than the VX.

That's my opinion anyway - I'm sure someone can argue against it!!