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MG MGB Technical - SU 'trumpets'

Stephen Strange gave a good account of what intake lengths do for 'torque' placement within the rev range, but I am not entirely sure how it applies; Spoke to the gentleman whom sold me my current GT. At the time he had two for sale, I settled for the beat-up one because there was plenty to do! But he was telling me that he had borrowed some intake rams for his standard su carburettors, and found them impressive. 'I said, Are you running without filters? yes!Now this raises immediate concern for the quality of air going into the engine. Next, will the charge be weak? He maintains performance has increased, I'll admit induction roar must be up!But now more scientifically, a long induction track is good for low down torque? A short induction track good for high rev torque? Or is this too much a generalisation? Mike
J.M. Doust

Mike an MG with SUs has a fairly long induction tract anyway. I think it was, as you say, the noise that impressed him. I did a few pulls on the dyno with my supercharged B with a 2" ram a 1.5" and no ram. Could not read any difference. I do however use the 1.5" in my cold air intake because it should smooth the air flow and its not going to hurt. Denis
D M HILL

The biggest problem is the siamesing of the inlet ports. When one inlet valve is flowing, if the other valve is closed the area of the split to the valve acts as an 'idler branch' as in the 1+4 ( and 2+3)) branches of a 4-2-1 ex system. The idle branch acts as a resonator. When both inlet valves are open and you get the charge robbing, goodness only knows what is happening in the ports! To successfully pulse tune one needs separate ports, both inlet and exhaust.


To give an idea of the problem, when we ground the split back like an A series head, power was good to 6200 or so at which time the rwp dropped to about 10! We could only think a standing wave was set up across the inlet valve to inlet valve area propogated by the lack of port split. Weslake must have discovered this the hard way when he developed the A and B engines.

However the length of pipe to main jet does have an impact on fuel pick up.


Peter
Peter Burgess Tuning

Sounds too complicated to me, I stick to the present system! Mike
J.M. Doust

Thinking on you very last comment Peter;Length of pipe to main jet has an effect on fuel pick-up,' so like a venturi if a long pipe might be allowing an air column a chance of picking up velocity(?) then would this air column draw more fuel into its stream? Thus might (and let's assume we have a separate intake throat per cylinder)this give the impression of more power since we are drawing in more fuel??? I should make a flow bench shouldn't I?? Mike
J.M. Doust

It will make more power if a richer mix is required, if it is not rquired it may well lose power. trial and error on an engine dyno/rolling road. I think a dynamic rather than a flow bench approach is needed.

Peter
Peter Burgess Tuning

This thread was discussed between 13/02/2012 and 15/02/2012

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