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MG MGA - Exhaust Back Pressure

Hi all,

With the combination Fast Street cam, MGB 18 head, and less so, a supercharger, I have a great deal of backpressure from the exhaust. After the cam is when it really increased.

I am running a pettern Walker exhaust, aluminzied steel, 1-1/2 bore. The system is still in physically good condition, but I think the muffler is the problem (too restrictive).

I posted a similar question on the supercharging forum and the Falcon big bore exhaust was one reccommendation. I did have one of these on my B some years ago...

While researching the system I noticed that the muffler was costructed 'with the same backpressure and baffles as the original'. However, I also found a statement that the muffler was a straight-through design from another site. Which is correct?

I hope I can just find a muffler so I can re-use the rest of my system. Been on the car 10 years (6 actually on the road) and it would be a shame to waste the pipes and hardware!

Any other ideas?


Tom Baker

The Falcon stainless steel exhaust sold by Moss (and others) has 1-3/4-inch main pipe and straight through glass pack muffler. It is reasonably quiet with a nice mellow sound when new, quite nice. By 40,000 miles it may blow the fiberglass packing out of the muffler, after which it is loud and raspy and irritating.

Your fast street cam and supercharger should have no problem with the back pressure of the 1-3/4-inch system. Beyond that it would be more important to toss the original exhaust manifold in favor of a good set of tubular headers (not the junky short ones). With good headers you might find some small increased benefit with 2-inch exhaust, but expect it to be louder.
Barney Gaylord

Tom, how are you measuring the exhaust back pressure?
How much is it ?
A Pearse

Tom, Based on just your email address your car sounds interesting, how about attaching a picture or two for our viewing pleasure. Your email also reminded me that my exhaust diameter is down to about 1 inch after running over ruts and bumps in the fields at the car shows; ergo, let me know if you decide to sell your used exhaust....Dennis

Dennis Suski


The Falcon seems pretty good. This is no racer so I think the added benefit (cost and hassle) of adding headers isn't justified. The straight through muffler will free up enough pressure and a bore larger that 1-3/4 just isn't necessary. I never cared for the louder big bore exhausts, anyway.

I didn't actually measure the back pressure per se, I just noticed while standing behind the car, the exhaust was much, much stronger than before even at idle. Strong enough to actually hurt my leg as vice just blow the hair around. While driving it really seems to strugle to get out of the muffler as well....with distinctive pop-pop-pops. This was never the case with the stock engine setup and all was well. If I had not made any modifications, I would not consider changing out the exhaust until this one rotted out. I thought it would have by now....


I'll let you know. It's old and I'd feel guilty selling it (same reason) but we may be able to work out an arrangement.

Pictures to come.....
Tom Baker

This thread was discussed between 10/02/2009 and 12/02/2009

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