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MG MGB GT V8 Factory Originals Technical - what diff ratio to use

Hi what diff ratio should I use in a B with a more or less standard V8 Rover it has RV8 headers and will have a holly carb other than that it is standard running through a sd1 5sp box using 15 inch wheels with 205/50/15 tyres so what ratio do You guys recomend I want it to pull well giving a good 0-60 time but I dont want it to rev out at 80 MPH being told the standard rear is no good the donor car is a 1972 with overdrive dont know what ratio this is if the ratio is ok will it take the power.
Thanks
p Dorner

You will get widely varying info on this, everyone has tried something different.

I am looking at either 3.08 or 3.27:1 in mine. I have found that with our stock diam tires our cars end up geared one gearset below a mustang, at least with the mustang T5 I am using. Most of the T5 guys go 3.55 or better for better pulling off the line, which would mean 3.27's. But with our light cars I am thinking of dropping one additional ratio because we will not have issues getting rolling, which puts me at the 3.08's. That should make a nice low cruise rpm also.
Larry Embrey

I'm going to say around 3.27. If you look at the gearing formula on Larrys website, and plug in the numbers for the sd1 gearbox ratios, and our existing 3.90 rear ends, you see that first gear craps out around 15-20 mph. That's really annoying. Sd1 boxes have a .83 5th gear, and with a 3.27 rear end that should put you around 2900 rpm's at 80 mph. No it's not the lowest, but it still gives you useable grunt in 1-2 gear. So once again, I like 3.27, unless you have a stupid amount of power, then use 3.08, or 2.73 and let the motor do its thing.

Justin
Justin

Our American colleagues fortunately do not have to pay £35 each time they fill their tanks,.... if you want cruising fuel consumption anywhere near 20mpg with a Holley, you should look to 3.08, there's bags of torque with the Rover engine even at low revs, and it doesn't like high revs in standard form.

Ive got a 3.07 SD1 axle, 195x65x15 tyres, and I can get 30mpg on the motorway out of my Fi engine.

Good luck with the conversion,

Mike
Michael barnfather

That is a very good question!
My take on this subject is, what is the purpose of the swap? Gas mileage, performance oriented, or just a daily driver. or all of the above.
All of the above then performance oriented driving will have to be sacrifice a bit.
1. What are the specs of the camshaft? This play a big role on diff. gear ratios.
2. Type of induction
3. Transmission gear ratios; first, second etc.
4. Weight of car (we know that one)
All of these play a big rol;e on the correct gear ratio for an all around driver.

Gearing for the best mpg the engine would be of a low compression, the cam would have to have very short duration (@.50 lift) 190 to 204 max and about 350 to 390 lift both the intake and the exhaust. tire dia. of 23 to 24 inches, a wide ratio trans with 5 gear ratio .78 with a final of 3:0
And the list goes on.
Good performance and mpg can be achieved by matching the right cam to the final gear ratio.

A good compromise would be 9:1 cr a camshaft @50 lift 204 to 214 duration, 450 lift with a lobe separation of 112 to 114 degrees, dual plane manifold, and a 390 to 500 cfm carb. All of this match to a trans with close ratio gear spacing, and a .78 5 gear with a final diff 3:27 to 3:40 High vacuum from the camshaft and the 5 gear can achieve a very good mpg and crisp performance.
The right foot would be the big factor on mpg with this set up.
Bill Guzman

I have an 8" Ford that will have a 3.27 posi in it by the end of the summer. I was leaning towards a 3.55 until I read some of the posts on this site as well as discussed same with Ted Lathrop of FastCars, Inc.

I amd using a T5 with 0.63 final drive.

rick ingram
rick ingram

I'll describe my recent experience with this issue since I recently put a 8.8 ford rearend into my car.
THEN:
1977 MGB
215 Buick high performance cam, 11:1 compression pistons
headers through fender well
dual exhaust, turbo mufflers
MGB rearend(3.9 ratio), wire wheels, 175/80/14
gas mileage on the highway=28mpg

NOW:
Changed to narrowed 8.8 ford, 3.27 ratio
wheels: 15x7 panasport
tires: 205x55x15
gas mileage on the highway 22-24 mpg

What happened? mileage is worse.

As for the ratio to choose I'd say personal preference is the rule. For me the 3.27 took away to much of the acceleration. I might go to 3.55.
Rick

Rick, that's my point. Before your right foot only had to move a bit to get the engine going due to the final diff ratio (3:90)High engine vacuum. Once you change the ratio your foot has to stay longer on the gas thus lower vacuum on accelaration
If you can install a 3:55 or a 3:73 you would be happier in overall performance. You have a nice comvinacion and your 5 gear is correct.
Bill Guzman

Mike,
What is an sd 1 diff?
Which fuel injected engine do you have?
Thanks,
Chris
c l sorenson

Chris

SD1 was the last model Rover saloon to use the V8 engine, (1976-1986) sold as Rover 2000, 2300, 2600 and 3500 in UK along with Vanden Plas and Vitesse models.
Michael barnfather

Chris.....
to continue (must have hit wrong button)
All my running gear (engine, gearbox, axle and brakes) is from a 1983 SD1 Vitesse ...this came with a Lucas/Bosch flap valve analogue system, rated at 190bhp in the Rover saloon, which at that time was the fastest production saloon available, used by UK motorway patrols, and govt. ministers.

Mike
Michael barnfather

Running the 15 inch tyres really means that you should go for the 3.3 to 1 MGC diff - niot easy to find and expect to pay around £400 for one.
Chris Betson

Mike,
Thank you for your reply,
What is the displacement of the engine in your car? Is it 4.1?
I don't see myself running across an sd1 rearend in my local, Minnesota junk yard. Out of curiosity, is this rear end posi? What are your thoughts on the need (or lack thereof) for a posi rear end on a car used for fast touring, freeway driving and rallies? I never expect to race someone from a stand still, but I the 30mph to 100 mph interval of my motorcycle and the camero I used to have.
How is your driving enhanced or hindered by your choice of a posi or non-posi rearend?
Thanks again,
chris
c l sorenson

Chris Sorenson...I briefly looked through the posts, what transmision are you currently using?

Popular differential exchanges here in the States at thr Ford 8" and the Ford 8.8", along with the Chevy S-10 (available from D&D Fabrication in Michigan).

I pulled a Ford 8" from a 1979 Ford Granada in a salvage yard, had it narrowed 6", and just this past weekend had Ted Lathrop install an Auburn 3.27 posi. I run a BW T5 transmission with 0.63 final drive...will be running 15" tyres.

Hope this helps..

rick
rick ingram

rick,
I will be using a BW T15 as well with 14 inch wheels.
I am hoping to get by with my "B" rearend. I suppose there is no harm in using it until it wears out. How much does it cost to have a ford 8 or 6.8 narrowed and posi added?
Thanks,
Chris
c l sorenson

Chris,

Yes, you can get by. I run the stock rearend with a T-5 that has the .63 overdrive. I have 205/60-14s and the tach reads about 2500 at 75 MPH.

Rick,

Ditch the .63, get a .73. Thank me later. :) You don't neeed it with your gears. With the .63 the gap between 4th & 5th really bugs me. There is something like an 1100 RPM difference. Ask Mike Moor, he feels the same way that I do. If I ever change from the MG rear the .63 overdrive is outa here.
Carl

I have a 8 ford rear ready to go on the B what size of wheel cylinder should I be using to match the brakes on the B
any info is helpful
thanks
Joe Walsh

Carl and Rick,
I have a factory nos set of 3.07 gears that I plan to replace my 3.9 gear set. I have thought also that I will need a .73 od. My other concern is will first be too tall with the 3.07. At present I can start from a dead stop in second with no effort to slip the clutch, so my opinion is that the 2.93 first with the 3.07 rear will be fine. Opinions????
James Johanski

James,

I would agree that you'll be okay with that combo. Others have used it with good results.
Carl

This thread was discussed between 10/08/2003 and 27/08/2003

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