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Velocity stacks or Trumpets in intake plenums

LaurelPWR

By LaurelPWR,
in Engines & Forced Induction

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LaurelPWR Rising Star

LaurelPWR

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I've been looking into one of these "billet" intake manifolds that have been doing the rounds from Otaku Garage and a few other lads who all get them through Alibaba I assume....

I wanted a nismo plenum but I can't justify $2k+ for a plenum.... Especially not when these are going for $700-$1k

One particular seller is offering them much cheaper than others I've seen and is also offering an option of having velocity stacks inside the plenum.

I'm only really interested in the rb26 individual throttle plenum as I run GTR throttles on my rb25.

I'm aware hypertune, plasmaman and JUN all run internal trumpets in their plenums and thats enough for me to believe it's a good thing to have.

I've been told by another seller who doesn't offer the stacks option that that are a hindrance to flow and performance inside a plenum..

What's your lads thoughts on all this? Stacks or no stacks?

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GarettH Enthusiast

GarettH

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They have flow tests of these manifolds without the velocity stacks vs stock manifold and it was extremely pleasing all cylinders with were very close compared to the oem which didnt supply enough to cyl 5 and 6. They dont have flow tests with velocity stacks so id probably opt out on them

GTSBoy Grand Master

GTSBoy

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Let's be clear.  Trumpets inside a plenum do NOTHING to improve distribution between cylinders.  The only reason to have them is to obtain the total runner length that you are after.  If you are so fussy that an extra 10mm of runner length is enough to make you put trumpets inside a plenum to change the tuned rpm of the inlet by, what, maybe 200rpm) then go for it.  But in general it is a total WOFTAM.

Many very scientific (and by that I don't mean MightyCarMods scientific, I'm talking Society of Automotive Engineers type scientific) tests have been done that prove that best inlet treatment is generally just a rolled edge on the floor of the plenum.  That's like the top of those trumpet bells that roll all the way back on themselves.  So it's like having half a donut around each runner opening.  The donut being sliced longways through its equator.

The next best is just a decent bellmouth buried completely in the floor of the plenum.

Unless a plenum is a big box and the air flow is coming from somewhere different (to what it does in a typical RB plenum) then trumpets sticking up into the flow can usually only cause turbulence and mess.  Unless someone can show you serious physical modelling/testwork or good quality CFD results proving that they don't hurt, it is safest to assume that they will.

The biggest and most important part of a plenum is that it should be large enough and shaped such that it will do the job it is asked to do - being to distribute the flow evenly.  My comments about testwork or modelling from above apply here too.  Unless it is a direct copy of a known good design, it may be safest to assume that they don't do all that they are supposed to.

sneakey pete Grand Master

sneakey pete

Posted
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Would caution against looking into "flow" numebrs of manifolds.

Eg somewhere it's claimed that the 26 ones flow twice as much as a stock 26 one.

That's nice but think about it unless you have 800kw are you going to have that much aie moving through your plenum?

No.

What is important is how it behaves, which may or may not have relationship with the maximum flow.

GarettH Enthusiast

GarettH

Posted
Posted
Would caution against looking into "flow" numebrs of manifolds.
Eg somewhere it's claimed that the 26 ones flow twice as much as a stock 26 one.
That's nice but think about it unless you have 800kw are you going to have that much aie moving through your plenum?
No.
What is important is how it behaves, which may or may not have relationship with the maximum flow.

Yea of course, the maximum numbers dont mean a hole lot but more so that they claim all 6 cylinders are very closely balanced as apposed to the oem one.
LaurelPWR Rising Star

LaurelPWR

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I'll have mine running a stock plenum with per cylinder egt's in a few weeks hopefully, will be able to see how the stock one performs decently well then



This will be valuable info man! Awesome stuff... What's your engine specs?

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