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Rb26 Plenum Adopter Kit For Rb25Det

alvinT

By alvinT,
in Engines & Forced Induction

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GravyGarage Community Regular

GravyGarage

Posted
Posted

I heard the other forward facing ones did that aswell? Would be worse with the single throttle body at the front?

Got limited info out of him on the weekend. Seems that the workshops he has been to say they will have to draw up the design manually as they don't have CAD cameras?

It's seems the first plate that they produce will be the most expensive and could be used afterwards as a template for more at a cheaper cost.

Didn't give me an exact number but the template will probably be a bit over 1k. Ryan and I (gravy garage) will be getting the plates back and taking it to a machinist Ryan knows. Will then weigh up the costs to see if it's worth producing a few of them.

Will keep you guys updated!

Hanaldo Grand Master

Hanaldo

Posted
Posted

Or if you really want to spend some money, get the GTR plenum mated onto your neo head: http://www.skylinesa...ad/page__st__40

Wouldn't it be easier and cheaper to get an aftermarket plenum? The gtr unit has been known to run cylinder 5 and 6 leaner due to its internal design...

Lol, so you tell those of us wanting to get an aftermarket plenum to just get the GTR one; and those of us wanting to get a GTR plenum you tell to go aftermarket? You need to make up your mind, sir! :P

Bsa Mentor

Bsa

Posted
Posted

I recon you can't go wrong with the gtr plenum just costs a fair bit to mate to the rb25. Also the throttle body designs are much better then most aftermarket plenums it's almost impossible to get 100% flow equal to all cylinders. Iv done so much research on this stuff I'm currently making a plenum for a rb25 and the gtr plenum is one of the best designs going

SargeRX8 Grand Master

SargeRX8

Posted
Posted

Lol, so you tell those of us wanting to get an aftermarket plenum to just get the GTR one; and those of us wanting to get a GTR plenum you tell to go aftermarket? You need to make up your mind, sir! :P

Hahahah i just read this thread and read what I wrote lol. I think they would all do it, the boost will always push the air through he path of least resistance, ie the back of the chamber. f**k THAT IS AMAZING CONTRACTION LOL.

I'm opposed to both now!

Hanaldo Grand Master

Hanaldo

Posted
Posted (edited)

Hahahah i just read this thread and read what I wrote lol. I think they would all do it, the boost will always push the air through he path of least resistance, ie the back of the chamber. f**k THAT IS AMAZING CONTRACTION LOL.

I'm opposed to both now!

Yes, but that's where the design of the plenum itself comes into it. I'm no expert on plenums by any means, but from what research I have done, I understand that the rear of the plenum is narrower and the front wider to create a more even airflow, eg. like this:

intake_plenum_skyline1.jpg

That shape means the path of least resistance is no longer the back of the chamber.

As I said though, I'm no expert. Anyone with a better understanding care to correct me or elaborate?

*EDIT* I'm a university man, so gotta leave my source :P

http://horsepowercal...manifold-design

And to quote said source:

Consideration #2: air mass….

Your engine consumes the air…

At the throttle body 100% of the air mass is flowing (say 375 cfm of air for 250hp)

After runner #1 , a quarter of the air has gone into the engine… that leaves you with 280 cfm of air (or 75%)

After runner #2 , another quarter is consumed leaving you with 50% or 187cfm

After runner #3, another quarter is consumed leaving you with 25% or 93cfm of air

Now if you look at the air velocity

Say you have a 4″ diameter tube as your plenum or 12.56 square inches….

your air velocity will drop as you go down the tube because you have less CFM flow divided by the same area tube

Here are the velocities:

#1 = 375 cfm * 1728 / 12.56 = 51,592 inches per minute or 23 meters per second

#2 = 23 * 3/4 = 18 meters per second

#3 = 23 * 1/2 = 12 meters per second

#4 = 23 * 1/4 = 6 meters per second

So what happens is the air slows down as it approaches cylinder 4

Now what this does in a traditional style intake (same diameter) is that cylinder #1 runs the leanest (gets the most air) and cylinder #4 runs the richest (gets the least air) with all 4 cylinders running exactly the sameinjector duty cycle (unless you have a good ECU that can do individual cylinder fuel trimming).

Now racers that know this build a cheap equal diameter manifold, and then just make sure that they tune based on the air fuel readings of cylinder #1 … if cylinder #1 is safe , then cylinder #4 will be rich, some power is wasted but there is no chance of blowing things up… if by mistake you tune to an oxygen sensor in cylinder #4… and you make it perfect… then #1 will run lean and you may lose th motor …

So what the smarter people do (or the people who have the ability to fabricate a slightly more complex shape plenum) is to taper the plenum towards cylinder #4 at almost exactly that ratio of 100%, 75%,50%,25% going from runner 1 down to runner 4…

These are ratios of area … so if you work it back to ratios of diameters you get this

100% diameter @ runner #1

86% diameter @ runner #2

70% diameter @ runner #3

50% diameter @ runner #4

Edited by Hanaldo
Hanaldo Grand Master

Hanaldo

Posted
Posted

The source is just above the quote.

Which part of it didn't make sense to you Roy? As I said, I don't know how much of this is fact etc. I could go to my uni library and research it but I think that's getting a bit involved lol. The above quote made sense to me, especially considering the brands with a lot of R&D in them utilize that shape.

Might be getting a little off topic though...

GTSBoy Grand Master

GTSBoy

Posted
Posted

The assumptions are certainly wrong in that quote.

A same sized plenum is more likely to run the last cylinder lean than the first cylinder. But it so much depends on the layout of the plenum that it's impossible to make general statements that apply to everything.

People always tend to forget that air is a fluid with mass, and that it is a compressible fluid. So when you try to make air turn a corner it will tend to crowd the outside of the bend (ie the density will increase at the outside of the bend, decrease at the inside of the bend, and the velocity distribution will tend to be biased towards lower velocity near the inside and higher towards the outside. So when you have something like the Plazmaman plenum with the TB set at an angle to end of the plenum, you have to appreciate that the idea is to direct the air towards the first couple of inlet runners. If instead you have the air coming in at a line that is at 90deg to the runners, then it will tend to want to continue past the first couple of runners. And when it all piles up in the back end of a plenum, that's why the last cylinder tends to run lean. Because the air stagnates, turning it's velocity pressure into static pressure, just waiting to squirt down the last runner every time the inlet valves open.

So the Plazmaman plenum shape is all about tapering the plenum volume down towards the far end (to try to keep the velocity across the runners as close to the same as possible) and also to use the initial inlet configuration to assist the front couple of runners to grab their fair share.

Ideally, a plenum is so big that it works as a plenum properly (as in the definition of a plenum). In that case it is a huge volume that allows the incoming air to just sit around waiting for a suction pulse from an inlet runner. But in reality out plena have to be small enough to fit into the available space. And the result is that the internal aerodynamics of them become significant.

I can look at a plenum (for any configuration of engine) and come up with an idea of how well it would distribute the flow. But that's because I've spent years and years working as an aerodynamicist (of sorts) with industrial combustion modelling. But I would still hedge my bets on any given plenum I looked at. That's because there are always tricks hiding in the flows that pop up to surprise, and there are flow treatments (lumps and bulges and so on) that OEM engineers can cast into them to cause the flow to do something non-obvious. It is just not possible to say just by looking how well they flow. You can only either do dyno testing with individual exhaust temp measurements or some careful flowbench style testing to be sure. And very few people do either of these.

The reason I am particularly suspicious of the converted OEM RB manifolds (ie the ones where the TB is just moved to the front end of the plenum) is that that whole plenum was designed to receive the flow from the central location opposite #3&4. There must have been some tricky stuff done by the Nissan boys to stop #3&4 from running too lean - ie to try to force the air to distribute out to the ends of the plenum as well. Just banging the air into the end of the plenum seems fraught with the chance of it not working, because all the OEM flow treatments are still there inside the plenum. And the stock plenum is too small to be able to soak up too much abuse. But then, you get people saying it works. The thing I wonder about them though, is, how much testing of the flow balance have they done? Probably none, and so they can only say that they haven't blown their motor up yet.

The Greddy plena are obviously a simple (and obvious) straightforward design with as much internal volume as you can squeeze into it. And they probably went to some effort to make sure that the flows were balanced - they'd be foolish not to. Whether the copies of same have successfully copied any internal casting tricks is not known (by me, anyway).

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