what diff ratio

poo slinger · Post by **poo slinger** » Fri Nov 21, 2008 3:30 pm

i want to know what diff retio will be better for my car.

i'm going to be using an lt77 gearbox out of a twin plenum sd1.

i have a choice of 3.44 and 3.75 lsd.

the car is just going to be for road use.

CastleMGBV8 · Post by **CastleMGBV8** » Fri Nov 21, 2008 5:36 pm

I would go or the 3.44/1 ratio, any lower (higher numerically) and first gear will be useless and economy will suffer.

Even 3.44/1 is a bit low for road use unless you are running large wheels and tyres.

What axle do you have?

Kevin.

poo slinger · Post by **poo slinger** » Fri Nov 21, 2008 6:00 pm

13" wheels and lil tyres

CastleMGBV8 · Post by **CastleMGBV8** » Fri Nov 21, 2008 6:24 pm

Thats not going to work then, what is the axle perhaps you can get a higher ratio or change your wheels and tyres but not to low profile ones or you will defeat the object.

Kevin

topcatcustom · Post by **topcatcustom** » Fri Nov 21, 2008 6:43 pm

Jus found this as I'm trying to decide too!
http://www.csgnetwork.com/gearratcalc.html

topcatcustom · Post by **topcatcustom** » Fri Nov 21, 2008 6:51 pm

Scrub that- its crap!

poo slinger · Post by **poo slinger** » Fri Nov 21, 2008 7:52 pm

an atlas axle, i have a choice of them 2 lsd ratio's as they are the cheapest.

i run a 3.89 on the escort at the mo with a type 9 gearbox and its great.

CastleMGBV8 · Post by **CastleMGBV8** » Fri Nov 21, 2008 9:54 pm

I have an Excel programme that allows you to work out everything in relation to gear ratios, final drive and tyre size.

I would post it on the forum but cant find a way of doing it. If anyone can tell me please do.

If you guys would like it PM me your e-mail addresses,

Kevin.

JSF55 · Post by **JSF55** » Fri Nov 21, 2008 10:18 pm

Try this one http://www.bgsoflex.com/auto.html best differential gear

topcatcustom · Post by **topcatcustom** » Sun Nov 30, 2008 8:03 pm

Guys, I need to start looking for a diff for the mx5, it will have to be an LSD, I am aiming for about 300bhp and will naturally be quite torquey, LT77 'box with normal SD1 ratio's and the rear wheels are about 1950mm circumference.
I am more interested in acceleration than top end (as you can use it!!!) but want to be able to have some fast fun on the strip or track too, so am thinking about 150mph top end?

p.s. I rekon rev limit will be about 6000ish

Any ideas what diff ratio I should be looking for then?! Will probably be a cossie or bmw unit so may be a little limited on choice, but I need something to aim towards!

CastleMGBV8 · Post by **CastleMGBV8** » Sun Nov 30, 2008 11:30 pm

Tom,

Just sent you an e-mail with a gearing program.

Did you get the head pics.

Kevin.

topcatcustom · Post by **topcatcustom** » Mon Dec 01, 2008 3:04 pm

Cheers kevin, yes got the pics- look very nice- are they new or shotblasted?

topcatcustom · Post by **topcatcustom** » Mon Dec 01, 2008 4:02 pm

That program's brilliant! Only there arent many 4:1 diffs out there! There are loads of 3.6's (M3's etc) but that would give top speed of about 170

yea right! What sort of top end would you think is realistic in a fairly streamlined car with 300bhp that weighs in at just under a tonne? -Still want acceleration though which a high top end isnt going to help!

CastleMGBV8 · Post by **CastleMGBV8** » Mon Dec 01, 2008 4:19 pm

Tom,

The heads were not mine and I didn't get any shots of them when they were being done so just sent some shots of heads showing appropriate work.

Re; gearing,

An MGB with approx 230BHP was said to be good for 140MPH, mine will have approx 260BHP but think I would run out bravery well before the car reaced it max speed.

What size rear tyre are you going to use as any rear end ratio higher numerically than 3.54 will make 1st gear useless.

Kevin.

topcatcustom · Post by **topcatcustom** » Mon Dec 01, 2008 6:49 pm

The rears are 215/45/17 but may go to 55's to make them a bit bigger still. That is the problem I dont want to have- 1st being a completely useless gear! I think 140 would be enough but for that it says I really need about a 4:1 ratio- which as you say may just make 1st a tyre shredding gear! I think I will try and get a 3.62 BMW M3 LSD as it cant be far out, and if it is I will just try another!