Welcome to our Site for MG, Triumph and Austin-Healey Car Information.
MG MGB Technical - Exhaust ports
|So, back to this thread, Your assitance please all. Having read Peters books, I am now extrapolating; If I have eliminated a lot of the exhaust guide boss gases are going to flow better? but is the gas velocity going to decrease? and then is the incoming charge not going to come into the cylinder as well as before/ OR on the contrary, are the exhaust gases going to exit much quicker,so then also drawing in the new charge but with the risk of some of that new charge also exinting with the exhaust? Then fuel consumption goes up, and possible charge ignition in the exhaust system ie back fires? Of course this depends on the cam ie that period when exhaust valve just opens and inlet valve also opens as exhaust just about to close??? Just thinking am I on a sound track? Mike|
I took most of the exhaust guide bosses off my head when I did it, but made no other changes to the port shapes apart from general smoothing, and most importantly the tight inner radii. Three angle seats of course.
Mine has a Piper 285 cam, K&N pancake filters (the ones people say don't work) using the original filter base castings at the base fitted outside the filter which has a bigger hole cut to match. LCB manifold.
103 RWHP on Peter's old rolling road and perfectly happy pottering about in town. No backfires.
So don't worry, this will work well.
I think you will find the gas velocities will be similar, just happen earlier and finish earlier due to better flow, this includes flow going the wrong way. You may well lose a little power at very low rpms. Will also, as you say, depend on cam. For max efficiency at low rpms we developed the Econotune head.
|A question for Peter. What's your view on the practice of making the inlet ports slightly larger than the mating inlet manifold tracts, and the opposite with the exhaust ports/header. i.e., header tracts slightly larger than the head ports. The theory, as I understand it, is that the "step" dicourages reverse flow.|
|Hi Allan, as long as the steps are not too pronounced it may well help discourage flow reversal in the exhaust and encourage turbulence to aid mixture homogeneity in the inlet tract. Better the step in the right direction than the wrong one! |
|Thanks David and Peter that has answered my query. Your foto David looks good and my head is looking very similar, I have still got sort the hole in the inlet port ventilating the pushrod tube!. Mike|
You will have more of an effect on velocities enlarging the inlet port.
This thread was discussed between 04/04/2011 and 05/04/2011
MG MGB Technical index
This thread is from the archives. Join the live MG MGB Technical BBS now