Ford LSD 7"axle ratios, which is best for my 5.0 Marcos

leylandracer · Post by **leylandracer** » Fri Dec 17, 2010 2:34 pm

Winter depression setting in i reckon

I'm toying with the idea of adding a LSD to the Marcos (LM500, 5.0 RV8)

Ford 7" dif Sierra, bolt in drive shafts.

The options are stick with the standard 3.14:1 dif & have a flange type ATB lsd fitted, Price circa £775 + vat:shock:

or

I'v noticed a few Sierra 7" 3.62 lsd difs around? much much cheaper

Q? What diference would there be to the gearing using a 3.62 to the standard 3.14? The Marcos as a long top gear.

Q? Would it be a straight forward fit if the 3.62 sounded a good option.

Any advice appreciated.

Cheers, Steve

Mark · Post by **Mark** » Sat Dec 18, 2010 2:53 pm

Can you not use the viscous LSD from a 3.62 axle and have it fitted to your 3.14 ?

The 3.62 - 7 inch lsd was fitted to 2.8 and 2.9 XR4x4 Sierras, all Escort Cosworths and all the Cosworth 4x4 Sapphires so there are plenty about.

Mark

leylandracer · Post by **leylandracer** » Sat Dec 18, 2010 2:58 pm

Thats a thought Mark, maybe worth considering?

still curious what sort of difference there would be using a 3.62

kiwicar · Post by **kiwicar** » Sat Dec 18, 2010 4:17 pm

At the risk of stating the obvious it will be geared 15% lower, if it was doing 2500 revs at 70 in top with the 3.14 it will now do 2875 revs with the 3.62, all your max revs in gears will arrive 15% earlier, if first got you to say 40 with the 3.14 it will get you to 35 with the 3.6. whether this is a good thing or not is really down to where all the gears end up you may find that you are trying to change from second to third in a corner with the 3.6 where before you could leave it to the exit, you may find 1st just is too low to be of any use, however you may find it places 3rd spot on at 60 say for overtaking where you were having to use second to get the car accelerating then snatch 3rd just as you pulled out. How are the ratios placed now? are you always grabbing a low gear to get the car moving when you really just want to floor it? is pulling away in traffic a painfull exercise in clutch slip? does it struggle on hills cruising on the motorway?
Personally I think you will find the change would result in first being just too low for the engine, but it isn't my car and I don't drive it on the roads you drive on so it really comes down to what do you think of what you have now and will lower gearing help?
Best regards
Mike

leylandracer · Post by **leylandracer** » Tue Dec 21, 2010 7:49 pm

Thanks for the info Mike

sidecar · Post by **sidecar** » Wed Dec 22, 2010 10:17 am

I have been messing about with Ford 7.5" diffs in my Cobra replica for some time now. Changing the ratio of the diff does make a big difference and not always for the better!

Obviously the gearbox ratios, wheels sizes, engine BHP/torque and 'slipperiness' of the body shape all have an effect.

My Cob is about as aero dynamic as a brick, the engine makes around 285 BHP, the wheels are 17" jobbies and the gearbox is a R380 from a Morgan. (Pretty much the same ratios as an LT77)

With a 3.64:1 diff the car had good acceleration at over 100 mph and would pull round to about 135 mph very quickly, it was more or less at peak revs at this point,or even over revving slightly. First gear was useless and the revs were annoyingly high at 70 mph to the point where it needed to be sorted out.

With a 3.36:1 diff the car still accelerates will up to 100 mph but it takes a lot longer to get to 130 mph. First gear is still useless! The revs are OK at 70 mph. (400 RPM less than with the other diff I think)

I've ended up staying with the 3.36:1 and fitted NOS. I've only done a few test runs, 140 mph came up pretty quickly.

Sticking with an LSD diff is a good move, your speedo will need to have some sort of adjustable function if you change the ratios.

I used some one line calculators to help work out how things would change.

ALL OF MY TESTING WAS ON A PRIVATE TEST TRACK!