Morris Minor Owners Club

Would I be right in assuming that a lcb with an outlet dia. of 2" is too large for a 1098 with 295 head. If so does anyone know what size I should be looking for and where to get it.
Ian.

2" is too large for your head Ian. The diameter should be the same ID as the OD of the port. Going too large or small creates an imballanced gas flow and hence an inefficient exhaust. I have an Armstrong manifold together with twin SU's if its any good for what you are wanting to achieve.

Hi Ian,
Do you already have the LCB? If so - do you have any plans for it?
Ray.

Ray, No I was bidding on ebay. Went to see it tonight and the outlet looked rather large for 1098. I have been advised in a mini site to go for 1.75" outlet from manifold. I didn't have a tape to measure it exactly so suppose it could have been 1.75". But still looks too big too me.
Ian.

If the price is right - grab it anyway. It won't be too bad on your 1098 with modded head and big carb - certainly better than the standard exhaust. If you get it - and then decide not to keep it - I will take it off your hands straight away !
Jonathon - any chance of a pic of the 'Armstrong' manifold ?? Not a name I recognise.

No probs BM, I'll take some pics tommorow,but will need to e-mail them to you. Can you please PM your details.

Certainly - bmcecosse(at)hotmail.com. Maybe Ian would like a look at the pics too !

It is an exact science I am led to believe. Vizard touches on it in his book and Maniflow offer one of two sizes for the Minor depending on your engine - so I guess they have done their sums too.

Oh yes of course for the perfectionist there is an 'exact' size to suit each engine capacity and state of tune. I very much suspect that a normal person will not be able to detect any significant difference - certainly a 'too small' exhaust with back pressure is definitely NOT a good idea - and that's what I have at the moment!

Hello all,

as a matter of interest, Philip H. Smith in his book, 'Scientific design of exhaust and intake systems' proposes that the primary pipe internal diameter should be a minimum of 1\8" greater in diameter than the valve throat, i.e. the port diameter directly below the head of the valve. Unless the port tapers outward this would give a larger bore in the manifold flange than the port outlet!

Alec

Thats what I meant to say, der! the ID of the tube manifold being the same as the OD of the manifold flange. Smaller restricts flow, larger creates pooling of the gases as they leave the engine .

But having a larger bore on the manifold is sometimes done intentionally - to create vortexes - which prevent reverse flow of exhaust gas. That's the theorey anyway!
I can certainly say that too large is much less of a problem than too wee - size matters !

BMC, if you go too large then you loose the particle inertia effect which helps 'suck' more of the gas out when the valve opens by having a partial vacuum ready at the exhaust valve. It's all explained in Vizard's book. Sometimes a SLIGHTLY restrictive system performs better than one which is too large. As the 'Nova' boys with their massive pipes find out.

On a 1098 I doubt if you will notice the difference between the two manifold sizes as they won't be THAT different. For optimal tuning though it IS an exact science and pipe length also plays a part with shock wave tuning which is something often overlooked as we can't vary the length much.

Best I ever had was lcb and open pipe on my 998 Mini - ended just at the rear subframe front crossbar - it shrieked at high revs and the car went like stink - obviously not on the road!
All this 'tuned length' stuff on an A series engine is pretty much guesswork - mainly because of the shared exhaust port for 2 and 3. However - backpressure on the engine with inefficient exhaust gas scavenging is definitely not a good idea - especially with a long period cam.

It is not guesswork! Vizards didn't dream it did they? I do know the standard setup is quite restrictive, but surely there have been people thinking about that, when developing the minor!

What bore is the standard Mini btw? Because that doesn't have the lovely fart does it?

Standard Mini 1000 (& 850) should be the same as the Minor 1000 (1 1/4" or 32mm). Not sure about the 1275 Minis as I've never owned one with a standard exhaust. Mine were Mini 1000s with engine transplants and LCBs!

BMC, there was a Mini round here about 15 years ago with a straight copper pipe! It too went very well, but the driver had to change the exhaust to a quieter one after the police heard him drive past at full chat!

Also I don't think you're allowed to use copper in exhausts unless it was original equipment because it encourages the reduction of CO2 to CO.

I don't think this lad knew or cared personally. He changed his mind though (and his exhaust) after the local plod had a word or two.

1275 Minis have twin pipe exhaust manifold.
Copper is banned from exhausts because it resonates.

What bore is the standard Mini btw? Because that doesn't have the lovely fart does it?

The Mini doesn't fart because the silencer is at the back and prevents the resonance that the Minor has with a single, central silencer.