Morris Minor Owners Club

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...!

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.

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

John ;-)

Not even a polite 'fart'

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

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.

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...

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

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?

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

Chris

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

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

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

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

Oldmogman wrote: ↑Wed Aug 14, 2024 1:50 pm 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).

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).