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Cut And Shut Plenum

AMR33

By AMR33,
in Engines & Forced Induction

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Ben C34
Ben C34

Maybe we need a pro and cons list. pros less pipe work easier to change plugs cons unknown effect on air distribution better chance of a defect then it's just a matter of weighing it u

ARTZ
ARTZ

but bro, a bunch of internet people just told me it wouldn't work with no factual evidence to back up their claims.. how can you argue with that? reading through some toyota forums and they have the

ARTZ
ARTZ

Ok.. Lets discuss. "Nissan designed it that way for a reason" Nissan designed alot of things for a reason, most of which are modified by some of you at some point...so maybe we should stop modifying

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GTSBoy Grand Master

GTSBoy

Posted
Posted

I am not going to try any more. I am not an internet theorist. I am an engineer whose work rather deeply involves aerodynamics, particularly flow in ducts. You can tell when someone who doesn't know abotu air flow at all starts posting abotu this stuff because they start talking as if boost has something to do with it (ie there's a difference between turbo and NA air flow - which there isn't really until you start getting into the detail of flow inside the ports in the head). Or they start to carry on about how their tuner didn't notice X or Y, but when asked if the tuner had individual exhaust temperature probes, the answer is always "no".

Cut open the upper plenum of an RB, or an old 7M, or even the plenum off a 2J or SR20. Look at the strange lumps and bumps involved in the casting on the inside of the chamber. If you can't point to a bolt or other clearance issue on the outside of the plenum as the reason for a bulge on the inside, then it's a fair bet that some switched on OEM engineer put that lump there to alter the air flow inside the plenum to achieve a goal of some sort - probably to balance the flow between runners. There is a hell of a lot that goes into designing this stuff properly that people who are uninformed of it would never imagine is possible. I spent $100k of CFD computer and engineering time doing a plenum design job for a kaolin calcination kiln to get the air into the process in the best possible way. Alright, I didn't spend the money. The client spent the money. I did the work.

ARTZ Grand Master

ARTZ

Posted
Posted

That's nice that you are educated GTSboy im sure your mummy is proud of you too...

But believe it or not I do know about the lumps inside the plenum and what they are there for..and i didn't even finish high school. :woot:

I also know there is plenty of cars around making power with these plenums so unfortunately your argument, while somewhat "educated" is not really being proven 100% correct yet

Now the only point I was trying to make is that there is alot more pressure in the plenum of a boosted car than that of a NA..i don't remember ever hearing of an NA car blowing the welds on the intake...doesn't it make sense that this pressurized air is going to make its way out of or into any opening (hopefully the valves) that it can make its was into...

Now while.it might not technically be the most efficient way of doing things it seems to me that it can indeed work well enough as I am yet to hear of an engine that blew up just because it had a cut n shut plenum...

GTScotT Grand Master

GTScotT

Posted
Posted

Without individual cylinder temperature probes it would be difficult to know if its working right or not. The AF at the tailpipe is basically an average across all 6 cylinders, and you need the temp probes to tell you which is literally running hotter (leaner) than the others.

In this case I know there are cars where this is known to work FINE, and others where it is not so fine.. I am unsure on the verdict for an RB25.

What I do know is I am about to start fixing a car with this EXACT plenum cut and shut job. I will take photos when I do... The motor has been looked after but has gradually spun a bearing AND fried the fire rings on one or more cylinders. I am looking forward to seeing the piston crowns once I crack it open... I am fearful that some cylinders will present leaner than others.

I will report back in due time. If anyone is specifically wanting to know about this (long term) and doesn't hear back from me in 3 months drop me a PM and I will update my findings at that stage.

GTSBoy Grand Master

GTSBoy

Posted
Posted

But believe it or not I do know about the lumps inside the plenum and what they are there for..and i didn't even finish high school. :woot:

Now the only point I was trying to make is that there is alot more pressure in the plenum of a boosted car than that of a NA..i don't remember ever hearing of an NA car blowing the welds on the intake...doesn't it make sense that this pressurized air is going to make its way out of or into any opening (hopefully the valves) that it can make its was into...

Now while.it might not technically be the most efficient way of doing things it seems to me that it can indeed work well enough as I am yet to hear of an engine that blew up just because it had a cut n shut plenum...

Responses to the various paragraphs in your post.

1) OK, then do tell what you know about the function of these lumps and bumps.

2) No. Wrong. Pressure has NO EFFECT. Air flow is air flow. Until you can grasp that, you are labouring under a massive misapprehension as to how this stuff works. Sure, the absolute value of the static pressure in a flowing gas will change the gas properties a bit, which will tend to influence the thickness of the boundary layer and a few other such things. But it does not change the nature of the flow patterns at all. Not one bit.

3) A motor does not have to blow up to be working not as well as it could if the plenum were working properly.

GTSBoy Grand Master

GTSBoy

Posted
Posted

No. RBs are "freaking easy to kill" when you do things the wrong way. Limiter bashing is the wrong way. Smashing ceramic turbines into the engine is the wrong way. Leaning out on old fuel pumps is the wrong way. Revving them hard without dealing with the oil pump drive or the CAS scatter is the wrong way. Randomly upsetting the flow into the inlet manifold runners is the wrong way. Sure, some people's bodgy mod might work OK. Others might not. Unless you've properly tested the result of a mod to the plenum you can't claim it is fine.

I could suggest a lot of mods to plenums that I would be happy to implement without testing because it is a very fair bet that they won't upset the flow. Making the plenum very large, for example, will almost certainly do nothing bad for distribution. Might not be great for response, but that's not the point. Making it smaller, or randomly aiming the incoming air at some other point in the plenum are not what I would call sensible without an instrumented test afterwards. Hell, I wouldn't even trust a Plasmaman type plenum unless they could show me that they'd done some reasonable testwork during development (and preferably that they'd had problems with distribution on some prototypes and found solutions to them - it's hard to trust "we got it right the first time"). Sure, there may be a big installed base of engines out there with Plasmaman plenums on that you can point to as proof that it works......but I'd still like to see at least one of them report all the exhaust temps (and show that the injectors were all within a little margin of each other and there were no cylinder trims in effect) before believing that they aren't all limping along with some flow distribution problem that is being masked by the tuning.

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