intake manifold modification

[OldMav]

Think I'm reading you, given the preferred radius and---
so adding volume/space by raising the top by 50mm gives the air flowing inwards room to become less turbulent and thus allows for a larger air volume to enter & flow though the runners.
and at the same time keeping inlet manifold size optimal to not suffer pressure drop.

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.

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.

[OldMav]

Found the Super flow 110 results that were sent to me 10" H2O so 1 psi so my results must have been for 1 psi also.
#1 184
#2 185
#3 181
#4 218
#5 192
#6 183

And from another Super flow forgot what model done at 15"
#1 204
#2 207
#3 192
#4 241
#5 215
#6 203

PS obviously when you lift the lid you would by logic fit the inlet exactly centre so this fixes the issue with number 3 cylinder. And is the obvious reason why #3 cylinder fails with the STD manifold when you try to inject too much fuel sometimes, or a logical reason why we see failed pistons with 1, 3 and 6..

[Robo]

Thanks heaps.

Info like this gives us novices a chance to make better informed decisions.
Champion Oldmav.*****

[OldMav]

Well I found some numbers that will make sense to compare to the STD manifold. Its for my manifold I am using now with cooler. At 1 psi about 10 inch H2O. on my bench.

#1 274
#2 274
#3 275
#4 275
#5 274
#6 274

This is about the best you can get with the std runners you can half this again plus if you have vertical dividers to the runners.. which is more than the head runners can handle..

[Ben-e-boy]

Well I found some numbers that will make sense to compare to the STD manifold. Its for my manifold I am using now with cooler. At 1 psi about 10 inch H2O. on my bench.

#1 274
#2 274
#3 275
#4 275
#5 274
#6 274

This is about the best you can get with the std runners you can half this again plus if you have vertical dividers to the runners.. which is more than the head runners can handle..

I like seeing figures like this, what does the actual head runners flow?

[OldMav]

MMMmmm Benny this will confuse you but any way I will spill out the data at about 10" the head port without valve will flow about 295 but with valve at .4 inch lift it will flow pretty close to 100 cfm just for interest the exhaust flows about 80 at .4 lift cannot remember or find what it flows open. This is going to be difficult to explain if you ask why lol. Mmmm But just think of this stuff is about total Kw's and what we are after is filling the cylinder more completely within the valve open duration. And remember this is about continuous flow and not how the engine operates.