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MG MGB GT V8 Factory Originals Technical - V8/Auto Rear Ratio
|I'm getting close to installing a 3.5 with the BW automatic and was curios as to the ideal rear axle ratio I should be using. I have found someone to adapt a ford 8.8 for a reasonable cost, but he's asked what gear ratio I want. Any info would be appreciated. Thanks in advance.|
The B and the C were fitted fitted with BW automatic gearboxes on order and had a 3.7 rear axle then. If your 3.5 is one of the 'hotter' ones it would should be in harmony wit a 3.3 to 3.5, i think. If it is a Range rover unit with 135 BHP, than the 3.7 should be the right axle.
I think you might be better with a higher rear axle ratio, the Rover SD1 with that gearbox used a 2.84/1 ratio to give a reasonable cruising RPM at 70MPH = approx 2700 RPM and a 3.07/1 = 2900 at at MPH still pretty high revs but obviously no overdrive or 5th gear as with the manual.
Presumably you are talking about the BW35 3speed which was the original fitment behind the Rover SD1 3.5 engine any lower ratio will give you very high revs at cruise and lousy consumption figures.
|I think you'll find something around 2.8:1 will give you a good comprimise between power and cruising rpm for reasonable mileage. Should be plenty of these around for the 8.8 as they are later axles and most used rather high ratios for better fuel economy. I ran a 2.43:1 axle in my midget with the 13" tires but it was a little high with the 5 speed so I dropped to a 3.43 ratio.|
I would be looking to have about 1700 / 1800 RPM at 60 MPH in top gear.
This will give you a decent touring speed at reasonabaly economical rev range and a 3.5 V8 will easily provide the needed torque and it will be well above the torque converter stall speed.
This would suggest a ratio around 3.08 or so depending on tyre choice and availability for the 8.8 Ford diff.
There are on line calculators for working this stuff out the quick and easy way.
Remember too this will alter you speedometer reading so that will need to be adjusted as well.
Cheers , Pete.
|It all depends on the top gear ratio of the auto. If it's 1/1 as in the BW65 & 66, then you need a 3.08 or 3.07/1. Rover used a 3.07 in the SD1 but I think 3.08 is a more common Ford ratio. Barrie E|
|A 2.84/1 or similar ratio will give you approx 26MPH per 1000RPM in direct top with the auto.|
It would be worth finding out the indirect ratios for the BW box to find out the potential speeds in first and second gears.
|C.E. You need ALL the gearing you can get! I am putting together a 3.5 with a sd1 auto & I am going to run a 195/65/15 tire & minilight wheel with at least a 2.8 to a 2.4 (I will run a 2.4 if I can find) rear. 3.07 with 15" wheel 63 mph = 3000 rpm., this I know.|
|The automatic MGC used a 3.301:1 differential.|
Many V8 conversions on this side of the pond use a 3.08:1 differential
|maybe this will be of help?|
|What really sets the rule to choose a rear axle gearing is you camshaft. What is the minimum rpm of the cam and what is the cruising rpm of the cam.|
This info should be on the cam performance card, or the manufacturer can tell you this.
Some street performance cams have a minimum of 2000 rpm and a cruising rpm range of 2200-2500.This is a pretty healthy street cam.
now, knowing this, match the gearing ratio to the cam at hwy average speed of 60 mph this is the speed used to calculate the gearing (the standard)
Now go and use Mr, Dan Masters program (a very nice tool) and figure your final gearing. Your engine will be on the power range at cruising speed and it will be more fun to drive and perform better. One downshift to 4 gear will pull you out of anywhere and from
anyone.:-) If interested, you mpg will be better.
|I tried a 2.84 diff on my conversion ( 5 speed manual box), but found 5th gear too high ( under 800rpm @ 30 mph), and quickly switched to the 3.08.|
Maybe it would work with an auto box, as it would hold in an intermediary gear presumably.
I have driven a Vitesse auto but it was some years ago, I can't remember if it got into top gear at 30 or not.
|Don't forget to take into account your tire size. What someone else runs satisfactorily with one tire size may be quite unsatisfactoy with a different tire. For example, a 3.5 rear axle with a 195-60-14 tire will have the same overall ratio as a 4.3 rear axle with a 245-65-16 tire. Quite a difference in the two tire sizes I used as an example, but it does illustrate the point. Tire size counts!|
|I have a 79 MGB 4 speed (no over drive). My question is there a different rear end ratio I can install so the engine is not reving so high at 70 MPH|
I would first determine what your objectives are. Do you want alot of low end torque? High top end speed? Will you be going on extended trips on the expressway? Is gas mileage important? Secondly, you will need to know the transmission final drive ratio and tire size. The link that Dan Masters mentions will help you selecting the best ratio for your intended usage.
My goal was to have as much low end torque as possible, but still be able to drive the car at 70 mph and keep the RPM's below 3400. In my case the best choice was a diff with a 3.55 ratio.
|Bill: If you still have the original 1800 engine in your 79 B, your cheapest option is to find an overdrive tranny, the best option is a Hi-Gear 5 speed conversion. If you are running a BOP 215 with the Saginaw 4 speed, get a T-5.|
This thread was discussed between 09/06/2007 and 20/06/2007
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