MG Sports Cars

How much power can a stock MGA rear axle take? I am doing a 2.3 ford duratec swap and I want to know if I have to replace the rear axle or not. Also, are there any rear disc brake conversions for the MGA? I havent looked too in depth but I havent really found any. Any input would be helpful


John

Glad to hear you are working on an MGA. Not a lot of info out there on things like this but some good experts on this website. I am doing a 3.5 conversion and while I don't have an answer for you when I run through the rear end I will let you know. Have not seen a rear disc brake conversion but am still looking myself. Brian McCullough has a lot of background on these issues as well as a bunch of the other guys. Good luck. Richard

Also Bill Young and Curtis Jacobson are good sources of information and very helpful on these kinds of issues. You can shoot them a PM in case they dont catch this listing. Richard

John, the stock MGA axle should hold the power of a duratec, but the stock ratios (4.1 or 4.22) aren't too good for your conversion. Those rears are somewhat sought after by some restorers and racers as well, so it might be better to swap it out and sell it. I'd advise checking with one of the online gear ratio calculators such as this one I use from the GM F body site. [www.f-body.org] putting in your desired rpm for highway speeds as well as the transmission ratios along with the tire size and see what rear axle ratio would be best. If it's around 3.9:1 then I think I'd just swap in a late MGB rear axle. They are strong enough, are a bolt in, and lots of them around for next to nothing. If the 3.9 is too low then you're going to have to have an axle modified for length and fabricate the spring perches etc.
I like to keep as many "maintenance" type parts such as brake drums or rotors, linings or pads, etc un modified so that I can easily get replacements without having to visit the machine shop. For that reason I chose a Chevy Monza rear for my Midget as the lug pattern was the same as the original and I wouldn't have to have the axles redrilled along with the drums. For a MGA or MGB if you're keeping the original lug pattern in front then the best option if you can find one is the Ford 8" from an early six cylinder Mustang or Maverick which has the same bolt pattern on the axles and can be narrowed to fit quite easily. There is also a great selection of ratios available for that axle. If you plan on changing the front hubs then the options grow.



Edited 1 time(s). Last edit at 04/16/2009 11:28AM by Bill Young.

Do you think that the stock brakes will be ok for the speed of the car? I mean, there is obviously going to be some difference between a 50hp engine and a 130hp one, but would the brakes still be ok?

also, is there any kind of limited slip differential available for either the A or B rear axle?



Edited 1 time(s). Last edit at 04/16/2009 05:09PM by Seventy7INa45.

Google around and read about Quaife differentials... they're available for the MGB "Salisbury" axle, but they're not inexpensive!

Are MG brakes sufficient? Arguably, horsepower isn't really an issue when discussing brakes. You're not appreciably changing the car's weight. On public roads, your speed is far more limited by speed limits and by traffic than it is by horsepower. At least that's good theory until you get off onto a road where turns are spaced so closely together that the brakes overheat and "fade".

We're still talking about rear axles, right? The rear brakes only do a fraction of the work of the front brakes anyhow because weight shifts forward to the front tires when brakes are applied.

Here's the thing... if you change rear axles, you'll need to check that you haven't changed the "bias" between front and rear brakes. You may need to add or adjust a proportioning valve, or change other variables such as brake pad composition. (For MGB, it's interesting to note that the convertible and the GT came with two different brake cylinder diameters at the rear. You may find you can use this to your advantage...) The issue is that you want the rear brakes to do as much work as you can get them to do WITHOUT reaching the point of locking-up BEFORE the front brakes lock up. If your rear brakes lock-up when the car isn't traveling straight, the rear of the car will come around. In other words, you need to be a little concerned about getting the rear brakes "too good". If, however, the rear brakes aren't doing their fair share of the work than the front brakes will tend to overheat and fade too easily.

Don't forget if you are thinking of switching axles but want to stay in the MG family there are also 3.7, 3.3, and 3.07 ratios available from the MGC which uses pretty much the same axle as the B. They are getting a little harder to find but they are still out there, and with so many people switching to mega HP engines and domestic drive lines you might start seeing more of them up for sale.