which diff ratio?

[V8chevette]

Hi Chaps,

I'm currently in the process of fitting a 4ha Scimitar axle to my Chevette, and was wondering what diff ratio to go for.

The old axle was a 3.9 and I was finding 1st and 2nd gear to short, the new axle has a 3.3 fitted bit I do have the option of a 3.58.

Engine is a 3.9 Rover attached to a LT77 gear box and its only in a Chevette so there's not much weight to drag along the car is driven on the street and at the drag strip.

Any advice is much appreciated

[sidecar]

Hi Chaps,

I'm currently in the process of fitting a 4ha Scimitar axle to my Chevette, and was wondering what diff ratio to go for.

The old axle was a 3.9 and I was finding 1st and 2nd gear to short, the new axle has a 3.3 fitted bit I do have the option of a 3.58.

Engine is a 3.9 Rover attached to a LT77 gear box and its only in a Chevette so there's not much weight to drag along the car is driven on the street and at the drag strip.

Any advice is much appreciated

I've spent a long time messing about with diff ratios on my Cob replica.
The rear wheel size is going to have an affect, my wheels are 17" jobbies.

I run Sierra LSD's, I've tried a 3.64, the acceleration was good but cruising on motorways was annoying because the revs were too high. I've now got a 3.36 fitted, motorways are fine but the acceleration over 100 mph is not as good (private test track). Cobs are not very aero dynamic at all. My motor is a 4.6 stage III (285 BHP) I run an R380 box which has similar if not the same ratios as your LT77. The car weighs about 1000 kg

Try to find one of whose on-line calculators so that you can factor in your wheel size. I guess that it would need to be geared for about 115-120 max MPH. That should mean that its not over geared and therefore sluggish in top gear. If you gear it for a lower top speed you will find that the revs in top are too high at 70 MPH.

Oh, with my setup first gear is totally useless, I never use, it and the wheels light up in second all the time! I guess this is the problem when building a car from bits and bobs from other cars. (A one-off built to order gear box would sort it out!!!!)

[V8chevette]

My tyres are 195/50/15.

I've been using quaifs gear ratio calculator and with the 3.3 diff cruising at 70mph would be 2712 rpm compared to 2942 for the 3.38 and 3205 for the 3:9.

I just don't want to sacrifice to much acceleration through the lower gears but you can't have the nest of both worlds

[kiwicar]

Hi
2700 at 70 will still be on the low side for motorway cruising for more than about an hour (my rover 75 is geared about there and it weighs alot more than a chevette and has about 50 bhp less than your engine). 1st will still be pretty much useless but 2nd should be OK. It should be usable for drag racing but really probably 3.1 ish would bring 1st more into play. If your main consideration is drag racing then you want the gear you launch in (fist or second) to take you to about 55 to 65, the next gear to take you to 80 or 90 and the third gear you grab to take you to the line and only use a fourth gear if you have to. Try putting the all the ratios in your box into a calculator with the red line you will use and your choices of final drive ratio and see what you come out with try this one if you don't have one that does a whole box http://precisionsite.com/auto-x/tools/
Best regards
Mike

[V8chevette]

These are the speeds from the quaife calculator.

3.3 DIFF

Top Gear gives 25.805 MPH/1000 RPM and a top speed of 154.829 MPH at 6000 RPM

Engine speeds in top gear:-
30 MPH = 1163 RPM 40 MPH = 1550 RPM 50 MPH = 1938 RPM 60 MPH = 2325 RPM
70 MPH = 2713 RPM 80 MPH = 3100 RPM 90 MPH = 3488 RPM 100 MPH = 3875 RPM

Top Speed in 1 gear = 36.924 MPH
And changes into 2 gear at 3771 RPM dropping 2229 RPM
Top Speed in 2 gear = 58.756 MPH
And changes into 3 gear at 4013 RPM dropping 1987 RPM
Top Speed in 3 gear = 87.840 MPH
And changes into 4 gear at 4298 RPM dropping 1702 RPM
Top Speed in 4 gear = 122.625 MPH
And changes into 5 gear at 4752 RPM dropping 1248 RPM
Top Speed in 5 gear = 154.829 MPH

3.1 DIFF

Top Gear gives 27.470 MPH/1000 RPM and a top speed of 164.818 MPH at 6000 RPM

Engine speeds in top gear:-
30 MPH = 1092 RPM 40 MPH = 1456 RPM 50 MPH = 1820 RPM 60 MPH = 2184 RPM
70 MPH = 2548 RPM 80 MPH = 2912 RPM 90 MPH = 3276 RPM 100 MPH = 3640 RPM

Top Speed in 1 gear = 39.306 MPH
And changes into 2 gear at 3771 RPM dropping 2229 RPM
Top Speed in 2 gear = 62.547 MPH
And changes into 3 gear at 4013 RPM dropping 1987 RPM
Top Speed in 3 gear = 93.507 MPH
And changes into 4 gear at 4298 RPM dropping 1702 RPM
Top Speed in 4 gear = 130.536 MPH
And changes into 5 gear at 4752 RPM dropping 1248 RPM
Top Speed in 5 gear = 164.818 MPH

I'm not sure if a 3.1 diff is available for ther 4ha axle but there is a large choice out there so there will be one close to it.

And for comparison my old 3.9 diff

Top Gear gives 21.835 MPH/1000 RPM and a top speed of 131.009 MPH at 6000 RPM

Engine speeds in top gear:-
30 MPH = 1374 RPM 40 MPH = 1832 RPM 50 MPH = 2290 RPM 60 MPH = 2748 RPM
70 MPH = 3206 RPM 80 MPH = 3664 RPM 90 MPH = 4122 RPM 100 MPH = 4580 RPM

Top Speed in 1 gear = 31.243 MPH
And changes into 2 gear at 3771 RPM dropping 2229 RPM
Top Speed in 2 gear = 49.717 MPH
And changes into 3 gear at 4013 RPM dropping 1987 RPM
Top Speed in 3 gear = 74.326 MPH
And changes into 4 gear at 4298 RPM dropping 1702 RPM
Top Speed in 4 gear = 103.759 MPH
And changes into 5 gear at 4752 RPM dropping 1248 RPM
Top Speed in 5 gear = 131.009 MPH

[kiwicar]

Hi
As you already have the 3.3 you could put it in and try it, if you get on OK with it, it will have not cost you any more than you have spent. However I think somthing around 3.1 will suite it better. The ratio calculator shows you could do a 1/4 mile run with only 2 changes on track with the higher ratio diff as your terminal speed would be around 95 to 100 with your engine if you over rev it a couple of hundread revs. The 3.3 diff would definatly require a change into top which would cost you 1/2 a second or so.
Best regards
Mike

[V8chevette]

Cheers for that Mike

Probably best to try it first, I'm looking for a Jag powerlock LSD at the moment so I think I'll see if I can get one with something close to 3.1 so I have the choice to swapping the cwp around.

[russell_ram]

There are a couple of Jag ratios (year dependant) around 3.1 (3.07/3.05 for instance). You'll have to look fairly hard to find a PL in those ratios though because PL was only standard fit in V12s cars (which were all 2.88 ) and XJS (either 2.88 or 3.54). So you're looking for an 'option' fit unit. The are plenty of Jag specialist breakers who will have you over a barrel if they have one though. Gears from an XJ40 unit will also fit with a mod to the input shaft nose.

For what it's worth, I think something around the 3.1 number is the right one for you. I would think something around 30mph/1000rpm is ideal for an RV8 installation. My own car is at 32/1000 and can still manage a mid 11's 1/4 with RV8 power in road trim.

A slightly trickier problem, when fitting a GKN PL unit to a Scimitar axle, is that you'll need the 'special' halfshaft separating spacer that isn't fitted in the Jag IRD units - it's totally different to the one fitted to the standard open Scimitar diff. I can sketch what you'll need to have machined up when you get there.


Russ

[V8chevette]

Hi Russ,

A sketch of the the spacer would be appreciated, I'm an engineer by trade so making them won't be a problem

A 2.88 diff would give me 29.5mph/1000rpm

You wouldn't know how to get the diff out of a Scimitar axle would you?
I believe you need a casing stretcher but is there another way as I don't have one.

Cheers Jonathan