What's the optimum exhaust diameter for a 1275 Moggie?

[Oldmogman]

Could anyone advise on the optimum exhaust diameter for a 1275-powered Moggie?

While Vizard offers guidance on LCB tube size, I can’t see anything about the exhaust system. I understand too small a diameter can cause too much back pressure while too large a diameter can slow gas speed, but I don’t know what might be best for my set-up.

The engine is mildly tuned with a gas-flowed head, higher compression, Kent Megadyne 256/266(?) cam, HIF44 on an MG Metro manifold and an LCB with a 1 ½-inch outlet pipe.

The existing exhaust comprises a Mk3 Cortina front box and pipe (1 ¾ inch OD) and (I think) a Renault 18 rear section (modified to clear the radius arms) running over the axle.

It’s getting a bit tired so I’d like to replace it and would welcome any advice on pipe size, material (stainless or mild steel) and design.

I’d prefer to keep the single-silencer set-up because the rear silencer on twin-box systems I’ve seen runs a little too close to the fuel tank for my liking. And if I could get a bit of a rasp on the overrun...!

[philthehill]

I would suggest that 1 5/8" is adequate for the state of tune of your engine.
I run a steel set up which has held up well. I have a T.I. Industries straight through silencer which is highly thought of by Vizard and is much cheaper than the BMC/BL Special tuning exhaust. The T.I. silencer s featured in the Vizard book.

[Sleeper]

Does it still give the " fart" on the overrun?

John ;-)

[philthehill]

Not even a polite 'fart'

[Chipper]

I have a full Maniflow system, including LCB exhaust manifold (1.5", IIRC) fitted to my 1970 Traveller with 1275cc MG Midget engine. Certainly quite an improvement over the cobbled-together previous cheap stainless steel 'standard' system I had on there previously.

It sounds very similar to an MG Midget now, and has lost the over-run 'fart', but still sounds nicely raspy when giving it the beans, yet quiet enough for general use. Looks good too, with the chromed tailpipe.

I think this is the system I have:
https://www.maniflow.co.uk/index.php?vi ... oduct=1496

And the LCB:
https://www.maniflow.co.uk/index.php?vi ... oduct=1494

[philthehill]

Chipper
You may have inadvertently posted the wrong links but I would have thought that as you have a 1275cc engine you would have fitted the manifold in the link below:-
https://www.maniflow.co.uk/index.php?vi ... oduct=1495
https://www.maniflow.co.uk/index.php?vi ... oduct=1498
not the manifold and exhaust suitable for the 948cc/1098cc engine.

[Chipper]

Yes, Phil, you may well be right there, or at least, I hope so! Anyway, it certainly does the job.

However, the OP does say he has a 1.5" diameter LCB fitted, so the 1.5" system may well be a good match...

[MMT1968]

My 1275 Marina engine is tuned quite similar to the one mentioned in the 1st post. I use a Maniflow 41mm LCB with a Maniflow 41mm exaust kit which is almost straight through. It works well and sounds great. A bit louder than the original exaust at idle and „like a much bigger engine“ on the drive. To my opinion it’s a good match.

Chris

[Oldmogman]

Thanks guys

I'll check out the Maniflow system, which has the 1-5/8 diameter Islip Minor recommends.

However, I’m nervous about the back box running close to the fuel tank – how much clearance is there?

[MMT1968]

It’s enough clearance even if gas shocks are installed. No problem.

Chris

[MMT1968]

But! You must use the 1 5/8“ LCB on a 1275 engine. The 1 1/2 ist too small, that one is suitable for a 1098.

Chris

[philthehill]

I do not see a source reference to comments made by Islip Minor in this thread. Please enlarge.

[Oldmogman]

Ah, my mistake - I'd been looking at another post by Islip Minor earlier!

[philthehill]

Thank you for the clarification.
Unfortunately Islip Minor passed away over the covid period and his knowledge and advice is greatly missed.
Phil

[oliver90owner]

Ah, my mistake - I'd been looking at another post by Islip Minor earlier!

Richard’s Minor most certainly had some ”get up and go”. He treated my brother and me to a fairly comprehensive demonstration of how impressive it performed, around some of his local roads. I was sorry to hear of his passing as we got on well and I would certainly have visited again, as he only lived a few miles from my brother (but about 75 miles from me).

[Oldmogman]

Sorry to hear that - he was a nice guy and certainly knew his stuff (he once helped me sort a rough running problem in minutes).