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MG MGB Technical - Exhaust Tailpipe Diameter

I’ve just fitted a ‘GES’ stainless system, which I purchased...errrm, about 20 years ago.

The tail pipe seems a bit on the small side. It measures 1-5/8” OD, whereas the rear mount bracket has an ID of approximately 2”.

The old system was a very stylish 1970s spiral stainless steel affair, which was a much better fit in the rear bracket IIRC.

Do I have the wrong rear bracket, or is my tail pipe too small?

Dave O'Neill 2

Your tail pipe is the same size as mine, and the saddle clips are a snug fit. The V8 tail pipe is a shade under 2" and there is a larger saddle clip to fit that. Looks like you have the V8 saddle clip with the roadster tail-pipe.

The main hanger attached to the body, cotton reels and the hanger bracket are the same for both roadster and V8.
paulh4

Thanks Paul.
Dave O'Neill 2

My MGA tailpipe is (now was) 1-1/2" and that on my MGB 1-5/8".

When I fitted a mildly worked 18V engine (ported head, 9:81 compression, but standard cam grind) engine to my MGA about 18 months ago, both the cylinder head shop and my mechanic independently advised a 2" exhaust. The standard MGA system goes to progressively smaller diameters from the manifold to the exhaust pipe to the tailpipe.
For the life of me I can't find the site now, but I was also given a link to an exhaust calculator by a friend who races and rallies (Morgan Plus 8 and BMW Alpina). This site suggested the appropriate diameter for an 1800 engine of similar specs to our MGB engines to be 1-7/8".

I resisted the change for a while, wanting to keep my MGA superficially looking original.
Just recently however (after being told, now by a fourth individual) that my MGA "felt strangled" I finally had a 2" diameter exhaust fitted, from the original manifold through to the tailpipe.

I must say, I am extremely surprised how very obvious the difference is. The car was instantly more responsive the moment I drove the car out of the exhaust installer's workshop, and on the road goes much more eagerly and revs out much more easily.
I wanted the car still to look (and sound) as close to standard as possible, and the first muffler fitted was too loud. It's now been replaced by something much closer to the original in note and volume. Despite being "toned down" the car still goes palpably much better with the larger system.
It's interesting too to hold your hand a few inches behind the tailpipe with the car idling. The gas pulse pressure is palpably much less.

I always think the original MG engineers in Abingdon were very astute and experienced engineers, and, mostly, setting up a car exactly as per factory is best. For whatever reason, in the choice of exhaust systems, personally I have to conclude, bigger IS better, but perhaps this comes down to my higher compression ratio.

I've owned my 1965 MGB now for over 48 years. It's absolutely standard spec throughout, apart from again a slightly worked head (but standard compression ratio). On its third trip round the odometer (about 240,000 miles now) the car has always been a delightful performer. But suddenly I find myself wondering about a slightly less restrictive exhaust system....
T Aczel

You really need before and after rolling-road results to be sure, I'm always suspicious that after spending lots of money on enhancements the temptation is to convince yourself that they are better. As a start you could replace the centre box with a pipe without spending too much money, and see what that does.

A pal has a super-charger (which DOES make an obvious difference ...) but on an RR removing a straight tail-pipe trim which slides over the tail-pipe apparently gained 5 BHP, which I find difficult to believe. I was always under the impression that a certain amount of back-pressure is needed for engines in road trim to run most effectively.
paulh4

Paul this all started for me because my 1622 MGA 1600 Mk II that I bought about 8 years ago felt decidedly gutless, especially devoid of low speed response compared to my long term ownership and very familiar MGB. This was despite being told by the seller that the engine had been fitted with "a high lift camshaft and a "professionally modified" cylinder head". It also proved to be 40 overboard: 1667cc, approximately.

So, eventually I took my MGA to have it dyno'ed. It confirmed my impressions, delivering a limp 53.5HP at the rear wheels. Fiddling with the advance curve got the power up to a still underwhelming 55HP! Needless to say, despite the minor lift, it still felt underpowered.

So then started a careful lookat the engine. It measured 125lb/sq in compression. Inspecting the head showed that the inlet ports had been opened up to a hugely oversize diameter. (Bigger is NOT always better). Combustion chambers had been taken out so far the head gasket was overlapping into the combustion chambers! etc. Carefully modifying the head improved things, but I was stuck with the oversize ports. Next, flat top pistons went in, (original Australian spec was dished pistons, 8.3: compression ratio) and the end result was a hair over 185lb/sq in compression. With this the car livened up tremendously, even sounding more crisp. But it still didn't match my MGB, though it was getting pretty close over 4,000rpm.

So I decided to start all over again, with a MGB V-series 1798cc engine out of a rubber bumper MGB. Even this was sold as "ready to install, I've been building MG engines for over 20 years". Fortunately I lifted the head before installing the engine. Let's just say this engine too had to be completely stripped down, the block re-decked, straight this time, the opportunity taken to balance components etc The MGA now, finally, comfortably matched my lovely MGB for performance. The exhaust has lifted things further, though I "hear you" that only formally "dynoing" the car will document the benefits. But I now have a car which performs as I want it to, so I'm happily to leave things be, finally.

Incidentally, unlike an MGB, an MGA only has a rear silencer, and lacks the centre resonator of the MGB.

Unfortunately this is the best photo I could find with the tailpipes of both cars in view (before the new exhaust went onto the MGA). A difference is evident on my computer but will probably not be visible in this lower resolution view via the BBS.


T Aczel

Dave and "TA" (Sorry, not sure whether it's Tim, Tom, Tobias ...?) - thank you for posting your experiences with A and B, as your comments transfer nicely across to mine with a '73 BGT. It seems that professional builders - in this country, though possibly not in the UK - vary widely in skill and in opinion of their own work. I've had my BGT for several years now and after a body stripdown and repaint it serves well as a daily driver; it has the Supra 5-speed gearbox and has had some headwork done in the past; it's quite lively at touring speed but "could do better" on acceleration. Your comments prompt me to think that at some stage it would be worth having an "experienced professional" look at everything from carburetion to bottom and top end, as well as exhaust, and then let the dyno do the talking. Apropos the original post, the PO (a keen enthusiast) fitted a 2" exhaust when modifying the head, though when I fitted new exhaust recently, it now has an annoying ring, I suspect from the resonator.
PS - the block on mine is probably an 18V, but it carries a strange engine number (DBL 230 71036 E where only the E is stamped, the rest reverse-pressed)- I suspect of UK origin, but not traced for origin when I asked a question on the BBS a few years ago.
Keep up the good work ... regards, John.

J P Hall

Sorry John' I really should indicate my name.
"T"="Thomas", aka "Tom".

As for your engine block and number, it was obligatory in all Australian states for the engine number to be physically stamped into the block for all vehicles. Despite the fact that the Australian BMC facility built engines (and gearboxes, and body panels and rear axle/differentials etc etc etc here, right down to casting the blocks and heads locally), in the case of the MGB, the engine and gearbox was imported fully assembled and bolted together. The British engine number plate was wired to the dipstick when the engine/gearbox was crated up for shipping to Australia. The engine number on this plate was then stamped into the block here in Australia, into the location where the engine number plate was fitted for the UK built cars.
T Aczel

Ha! - thanks Tom. Yes, I experienced our Aussie system when I returned from a work stint in the UK in 1983, bringing with me a '73 B roadster I'd been driving around Wimbledon. The authorities in Melbourne removed the id plate from the block and stamped the number on the plinth. Benefit of hindsight, I should have asked them if I could keep the aluminium tag as a souvenir.
That is what confuses me about the current BGT - the id plate is intact on the block! There is at least 1 other DBL 230 ... plate in England, and 2 I think in the USA. So I have no idea how it came to be in Australia with the plate still in place. I may one day carefully remove it and see if there is anything underneath.
J P Hall

This thread was discussed between 08/05/2018 and 13/05/2018

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