Morris Minor Owners Club

Can somebody explain what pressure wave tuning is?

Never heard of it BUT found this on the web seems to make sense.

The idea is to tune the pressure wave in the intake port to arrive at the intake valve at a pre determined moment. The reasoning is if the + pressure wave arrives at the valve just before the valve closes the momentum of the air will continue to flow into the cylinder aiding in volumetric efficiency.

It's very difficult to acieve with an A-Series engine because the intake ports are siemesed (shared) for much of thier length.

Oddly enough I've been reading vizard on this recently.

Basically the concept is that as the gas exits the cylinder it travels along the exhaust at the 'local speed of sound' (different to the normal speed of sound due to the exhaust gas temperature, pressure and impurities in the 'air') ... as the pressure wave of gas hits the exit of the exhaust, the pressure difference acts as a kind of mirror reflecting the pressure wave back along the pipe (this is a similar process to looking up through water, where the water/air junction acts as an optical mirror)

This reflected pressure wave will be of -ve pressure and this travels back along the pipe(s) to the manifold ... if the tuning of the exhaust length is 'just right' then the -ve pressure wave will arrive just as the exhaust port is closing and the inlet port is opening, this has a dual effect in that it scavenges the remainder of the exhaust gases out of the cylinder, and if you get everything setup just right it can even help to pull the fresh charge into the cylinder.

Overall vizard seems to say (so far, I've not finished the chapter yet) that the LCB (Long Centre Branch) exhausts seem to work very well for this on the A-Series.

One important thing to remember though is that the exhaust and intake systems should be treated as a whole, the design of the exhaust needs to match the design of the intake ... so if you havent flowed your head, opened up your ports, uprated your carb & switched to a K&N filter then adding a wide-bore exhaust may actually reduce the available power ... similarly doing lots of work on your intake without paying any attention to the exhaust may also produce sub-optimal results.

Fantastic, thank you.

And - as said above, because the inlets are siamesed, and the firing order is 21 then 34 - there is a double pull on each inlet port, followed by a lull on each port (2 and 3 are often seen to be 'weak' while 1 and 4 are 'rich') - and so it's almost impossible to get any sort of 'pressure wave' going because of the stop-start flow in the inlet ports. Similiar story in the exhausts - although here only 2 nad 3 are siamesed, and their pulses are equally spaced, so some benefit indeed can be made with a good tuned length exhaust. These problems spawned the development of the 8 port head for the A series engine - where each inlet and each exhaust has it's own port. Makes a BIG difference !

I think you may be thinking of resonant tuning - where you setup a standing wave in the exhaust system, which I'll agree is basically impossible with a 5 port head.

What vizard appears to be talking about is basic wave mechanics ... the way he explains it is that the current cylinder is acting on itself - he says that the pulse propagates at around 1300-1500 feet/s giving it plenty of time to pass from one end of the exhaust to the other & back again before the exhaust valve closes.

I've finished the chapter now and he seems to conlude that overall the LCB manifold produces the best results, even if it is only a couple of bhp at either end of the power band.

I think he compares the 3 into 1 Long branch LCB - against the more normal 2 into 1 type LCB - where 1and 4 exhausts meet at a Y, and then further along they meet up with the flow from cylinders 2 and 3 at a second Y. I'm not aware of a 3 into 1 Long LCB made for the Minor ?

In my copy he is comparing the 2-1 style LCB (the two outer ports join symetrically at the top of the manifold and then this has a long tail on it which joins up with the long centre branch) against the cooper 3 into 1 as well as the 3-1 LCB ... overall the 2-1 LCB seems to produce the best results.

Lukey wrote:Can somebody explain what pressure wave tuning is?

Good grief glad I didnt jump in with an answer as I thought it was something you could get done at the hairdressers