Morris Minor Owners Club

Hi all, can anyone tell me the bore size of the lcb manifold that ESM are selling? Cheers.

Generally the larger exhaust manifolds and systems are 1 1/2" bore so I would assume theirs are the same size.

It seems like 1.5" primary i.d. is a little on the large side unless the head has been heavily ported and the engine modified to suit. The average driver who usually stays under 5000rpm would be better suited to 1.25inch primaries, and a 4-2-1 design, and as long of a primary as possible. The stock exhaust port is something like 1.1xx inches in diameter so a 1.5 inch i.d. is a very big step.

But how would you fit a 4-2-1 manifold to a 3 port Moggy head?

It seems like 1.5" primary i.d. is a little on the large side unless the head has been heavily ported and the engine modified to suit.

I think you have missed the point as the question was about the bore size, no mention was made about what size engine was in place.
And anyway many owners of otherwise standard cars have found quite an improvement with fitting the larger 1 1/2" system (which is the standard upgrade) along with the correct manifolds and a slightly bigger carb as the original manifold / exhaust system is very restrictive.

Ahhh sorry i was thinking of my 4age project which has 4 exhaust ports. Im just saying that from a strictly theoretical standpoint 1.5 inch primaries is slightly large. Based on Bernouilli's principle i believe, a gas will flow faster through a smaller diameter tunnel, but the downside is that volume of gas flowing through decreases. But what is more important at the typical Rpm range that most 1275's run at?. It will benefit much more from high gas velocity. This will provide more low end horsepower and especially torque from a 1.25 inch header for a more or less stock engine. Exhaust headers are very temperamental and unless tuned specifically to the engine aren't worth it.

I'm not an expert at all, but have built my fair share of exhaust headers for both cars and mini bikes. I can cite all of this information as well as what i ihave read in books and on the internet.

Kevin, he was obviously talking about a 948, 1098, or a 1275. It;s a morris minor forum, he would have otherwise specified.

As i see it the question is asking for the bore at the down pipe or after the collecter. 1.5" primaries would be a tad overkill IMHO

oh wow i feel stupid now

Based on Bernouilli's principle i believe,

Fair enough but its easier to read David Vizard book where the A series testing is mainly done using flow benches over a period of many years, in case you are not familiar with him.
http://www.amazon.co.uk/Tuning-Engine-D ... 915&sr=8-1

Many of the bigger Mini engines run with 2" exhausts - the 1.5 will barely be enough for a good 1275! The object of the exercise is to minimise backpressure on the exhaust valves - with the A series 3 port exhaust you can pretty much forget about shock-wave tuning - and just let the exhaust out as freely as possible! And it's well worth opening up the exhaust valve throats in the engine - especially where they turn the corner! As standard (940 head) yyou can barely get a finger through the gap - there's lots of room for improvement with a grindstone in a leccy drill.

Our 1380 has a 'large bore' LCB manifold with 1 1/2" bore (1 5/8" O/D) primaries into a 1 7/8" bore system (2" O/D), running right through at the same size to the rear of the car. Seems to work pretty well.

I have the second edition of Vizard, and there look to be some typo's around the suggested LCB exhaust pipe sizes, particularly the middle size system he is detailing.

The original, reasonably well modified 948 engine had an LCB that had a 1 1/2" O/D outlet pipe size, so fitting a twin box exhaust system that went over this (1 5/8" O/D) also seemed to work pretty well. Sounded nice too!!

Hello all,

another point or two.

Minors, in common with several other engines have rectangular exhaust ports so the circular pipe is shaped to that. 1.5" bore pipe will adapt to 1 1\8" by 1 1\4" approximately which I would guess is pretty close.
Also in Smith' book, 'Scientific design of intakes and exhausts' he proposes that it is better to have the manifold a slightly larger size than the exhaust port.

Alec

Larger manifold port size to head port size makes sense for exhaust systems. The back pressure issue is a myth, or at least the part about back pressure being needed. In fact you want the best flow rate you can get. A gas moving from a port into a larger diameter manifold will see a slight pressure drop, and that will improve the flow rate. No need to mirror polish the ports either. Removing any rough casting helps though.

A 1.5" LCB sounds ideal to me.

Larger is not always better. You will see from Maniflow's web site that they do two sizes for the Minor. There is an optimum size for the state of tune of your vehicle based on gas exit speed. This is vital to provide good scavaging of the cylinders (efficient removal of waste gasses). For an LCB Vizard quotes a 5% increase in bhp above 4000 rpm for a-series in standard tune. Below 4000rpm there is imperceptible difference over a standard manifold. 5% increase represents poor value for money in my book when huge increases can be made elsewhere for much less outlay.
Allsaid, however I do believe we have buried ourselves too far into Vizardland for the original Q.

I never said that an engine needs backpressure. What i am talking about is the most efficient way to expel back pressure to esnure that the most horsepower and torque is available the motors optimum rpm range. Heres another thing to look at which has also been proven; if the primary is too big based on Bernoulli's principle the gas will flow slower. The gas gets more time to cool. Cool air is heavy and dense and its up to the newly exiting exhaust to try and push this heavy mass out of the system.

Another thing to think about is that the biggest header i have since for the Toyota 4age is 1.5 inch primaries, even ones making over 180hp.

http://www.nsxprime.com/FAQ/Miscellaneo ... theory.htm

Larger manifold port size to head port size makes sense for exhaust systems

Yes but too big can negate the efficiency of the exhaust

Yes - the larger manifold port match is intention - the idea is that reverse flow pulses meet up with the mismatch, while exit pulses get an easy passage. But we are talking of the last 2 bhp here on a race tuned engine! For general road use - free flow is what's required - so the at 1.5 going into a 2" system - hopefully with only one staright through silencer should work ok.
I did run a 'full race' Mini with lcb and a short open pipe which ended under the car - I crawled under a Downton works car and measured the length! And that worked well for me - and sounded awsome at full revs!