Plenum design has a maths to the volume size for petrol capacity, vacuum only this has to be with petrol as they work in vacuum 90% of the time, a turbo diesel works near 100% of the time in pressure. But for N/A diesel you can adjust this for diesel considering the higher compression ratio and vacuum associated with this mechanic. This mod to raise the lid 50mm is more about slowing the incoming pressurised air so it has a chance to distribute along the plenum log which isn't ideal for this. The other thing it addresses is to allow enough volume to distribute to the end runners or cylinders. What happens here over and above the STD log manifold is the air has a bit more room above the dividers where considerable turbulence happens which chokes off air passage to 1 and 6 cylinder. For example from the bench which I cannot exactly remember set pressure I think it was 4 psi or 1 psi but regardless the STD manifold flows 173 cfm 1 and 6 and 205 for number 4 which is the highest obviously due to the intake sitting over the top of number 4 runner. With the lid lift your get all the cylinders flowing very close to 235 cfm there is only one or 2 points separation different. As you can see it allows more even distribution and some more flow for this set pressure. Obviously as the pressure gets higher or more pumped volume the values get more separation hence a larger lid lift or more plenum volume is required. As I said before my bench is old very old and the new benches are very much better at this sort of accurate values I was only after change and the increase of change hence I am very reluctant to state values because they are only indicative to my bench which was running with a calibration system that is not ideal.Quote:
Originally Posted by Robo
I do have a results sheet for a super flow 110 bench of the STD manifold in blow through format and my 20+ year old results do show the same spread of values but mine are a bit lower output values. From all this testing and stuffing about I didn't get results to suggest the plenum could be to large as the spread results just got better as the inlet got further away from the runner/ dividers. but the biggest change didn't happen until a 50mm lid lift hence my suggestion here after 50mm the spread results didn't really change. The reason for this is the turbo's we fit mostly except for the HT18 have a bigger capacity to pump volume than the runner can flow so we always have a over supply except that pressure will overcome flow into the runners to a point where diminishing returns happen this is in my best guess and happens at about the 150-160rwkw mark or about 22 psi in the real world.
You have to remember here this is for outright KW's I have never been about this I have always been about torque response and torque rise to make the engine produce torque over a wider rev range so a flatter curve which is much more fun to drive and if you are about acceleration times this is the stuff that makes a diesel really fun to drive on road or off. KW's are just a product of torque so it is what it is.
So far in my search I have not seen a pressure drop due to volume capacity from the turbo's I have used but I have seen it with the tiny STD HT18 but to be fair its not really the turbo's fault its a product of a really REALLY crap induction system and air box system. Letting this little turbo breath with out restriction it can flow enough volume to not see plenum pressure drop when this turbo is set to 12 psi and within its map sweet spot. So I really cannot suggest if there is a optimum plenum size for a given turbo size as I think the runners and head start to be the limiting factor for that given pressure capacity of that turbo.
To note a front mount has other issues that cause pressure drop and due to the length of tubing and its size its a trade off against response due to tube restriction bends and the sheer volume in the induction system. And yes this can be offset with turbo selection and its pressure ratio etc etc so you can design a front mount without detectible lag for a diesel.. Its just that for me I like the simplicity of the induction system top mount W/A system and the advantage of minimal volumes in comparison, so response to me is paramount and tube length, bends and huge volume Air to Air front mounts are just to much to compromise, My opinion only of course.
