Front Facing Stock Plenum

[m0rt]

Hey guys,

After going throu bout 40 odd pages of search resaults on plenums and not finding what i was after thought i would make a post.

I'm lookin at doin the front mounted throttle body to a stock plenum. Main question/ thing troubling me is;

People saying that air will be "forced" to cyl 6 and 5 and less air avail to 1 an 2

I don't think this would happen as aren't gases ment to expand\ contract to equalise the area they're in??

And just after some general feedback from people with\ had one any dramas, or noticable changes

[The Mafia]

under pressure, distribution of air is completly equal.

[rob82]

Hey guys,

After going throu bout 40 odd pages of search resaults on plenums and not finding what i was after thought i would make a post.

I'm lookin at doin the front mounted throttle body to a stock plenum. Main question/ thing troubling me is;

People saying that air will be "forced" to cyl 6 and 5 and less air avail to 1 an 2

I don't think this would happen as aren't gases ment to expand\ contract to equalise the area they're in??

And just after some general feedback from people with\ had one any dramas, or noticable changes

Just because air isn't as heavy as most fluids doesn't mean that it doesn't display properties of fulids. It has a mass and even though it is very small it will still take time to deviate from its path so if you think of it as a fluid then in front mounted throttle body the majority of flow is perpendicular to the direction you want it to head. eg1 even though there is a large negative pressure differential in cylinder 1 the air still has to turn a 90deg bend in order to fill that cylinder. This is still the case even when on boost as the air within the plenum is never stagnent unless it has a massive volume.

If you want more proof look up various SAE articles about manifold design and you will see that there are large air flow differences usually in the last few cylinders. This is probably one of the reasons rb26's run lean in the num6 cylinder.

[GeeTR]

under pressure, distribution of air is completely equal.

Unfortunately incorrect in relation to a forward facing plenum, including RB26's.

As Rob says, air has inertia. Its why most people fit their highest flowing injectors in the last two cylinders. Its why most ppl det the last two cylinders (well, and less head coolant circulation back there) People use the Greddy jobby for trying to equalize flow. Mines R34 uses the Nismo one, which if you look at its overall size, isn't used because of its outright flow.

Mort, as you might have read, many people have measured big flow differences over a myriad of pressures with a mod'd 25 plenum. I think URAS has actual flow numbers.

Its not "too" much of an issue as long as you take measures to trim AFR's across the cylinders... this isn't a complete solution though, as the flow differences between runners, change with flow rate, ie - RPM.

They're just the issues with long plenum's, the issue is made even worse by mod'd the 25 unit (i read) having distribution fins or ribs inside it, that assist the function when the TB is mounted factory midway.

Iv never done the mod, nor seen the results with my own eyes, though would figure as long as you don't lean on the motor too hard, you should be fine.

Edited February 20, 2008 by GeeTR

[Spook's_Skyline]

ive got a rb25 with a modified stock plenum facing forward

had it for a few years, motor is fine

apparently there is evidence of low rpm pinging, knock never shows up on powerfc

made working on the engine easier

engine is down a few rwkw on what it should be though, dont know if thats related to plenum, just this motor itself or its something else

[bubba]

I'm making one for my 30 out of an rb26 plenum and stock runners, hopefully it'll be fine. Having to do it out of necessity as opposed to any other reason.

[dano4127]

what happened to your RIPS one bubba?

[m0rt]

So its best advised to do this with an aftermarket ecu, after injectors have been replaced\ flow tested

or don't risk it and leave it be and save for a proper one

have a spare plenum in the shed, might have a looksy tomorrow for fins or anything

cos the one thing i always thought was if your gonna move the intake to the front and "lean" out 5&6 then why at the stock position don't say 1&6 "lean" as the air is coming in hitting the far wall and i'm guessing going left and right from there and one would think 1&6 would get more air, oh well as stated will have a look for ribs or fins tomorrow

[GeeTR]

One could theorize that the air "slug" has been split in half, and is now a little more turbulent from the 90⁰ turn. This aids in more even distribution. All guess work of course, until someone CAD's it up, and runs it through a FEA.

You don't necessarily need an aftermarket ECU, its like umm, like a pack of snow dogs, the team as an entity, will only be as fast as the slowest dog; that is, your tuner will stop winding boost and ignition into the motor when those more sensitive cylinders start complaining, loosing you maybe 20kw that you would have got if you could tune to the max of the less sensitive ones.

As long as a decent safety margin is kept, you should really be no worse off.

You need some responses from people who have actually tuned an RB with the mod done i think.

Edited February 21, 2008 by GeeTR

[bubba]

what happened to your RIPS one bubba?

The guy turned out to be a bit flakey, at least I hadn't transferred money yet!

[sonoramicommando]

They're just the issues with long plenum's, the issue is made even worse by mod'd the 25 unit (i read) having distribution fins or ribs inside it, that assist the function when the TB is mounted factory midway.

i dont know about RB25DET ones but I ripped out my RB20 one, it has jack inside, no fins or ribs (well, there are real shallow ribs but they are more structural than flow-related). There is nothing in there to stop two middle cylinders getting more air than other cylinders, 2 & 5 cylinders getting less air and 1 & 6 even less air.

[GeeTR]

As i mentioned, I've NFI if a 20 is the same as a 25, I've never eyeballed one.

EGT's from mod'd 25 plenums tell the real story.

Check out the "Do i keep my 6 TB" thread ATM in Forced Induction. Theres some interesting talk, and per cyl AFR results.

[Adriano]

For the cost of tuning the engine with 6 o2 sensors, you could easily fit a greddy plenum.

Also in regards to tuning to the "worst" cylinder theory, there is no way to know if one cylinder is running vastly leaner than the rest with a single o2 sensor in the dump pipe, it reads an average of all the cylinders

[GeeTR]

...? hence whys its inconceivable without EGTs, which im sure are within most ppls reach

[**RB2530**]

under pressure, distribution of air is completly equal.

This is true but only in a static sense...As soon as flow occurs you have friction on the walls of the plenum as well as turbulence and the pressure distribution will be different...

[**RB2530**]

For the cost of tuning the engine with 6 o2 sensors, you could easily fit a greddy plenum.

Also in regards to tuning to the "worst" cylinder theory, there is no way to know if one cylinder is running vastly leaner than the rest with a single o2 sensor in the dump pipe, it reads an average of all the cylinders

Has anyone ever tried using a non contact temperature gun (too early in the morning for me to remember the exact name) to measure individual cyclinder EGTs...should be easy with a high mount manifold...Just point at each runner....

[GeeTR]

Infared temp gun? lol

Always been looking for an excuse to get a good one.

Edited April 15, 2008 by GeeTR

[Sydneykid]

The cheapest method is to pull the plugs and check their colour after a dyno run.

Cheers

Gary

[**RB2530**]

Infared temp gun? lol

Always been looking for an excuse to get a good one.

Yep thats the one. (hate getting in to work early)..I was talking to a guy a while ago who had a sprint car and thats what he uses...He just had a case where an engine he had built had a cracked piston ring from assembly...It wasn't making expected power and there was a slight miss...they did a static comp test and all cylinders were within specs...They ended up getting hold of an infra red temp gun and 1 cyl was down in temp...they pulled the engine down and found they had a broken piston ring in the colder cylinder...

If you get one I would get a good one...I also wouldn't expect it to work as well on the stock manifold because it is cast iron (higher thermal conductivity) and thick walled so the temperature gradients due to different EGTs would be lower..Although if you had another rb25 with similar exhaust setup you could comapre the two...

[**RB2530**]

I am gonna try and explain in lamans terms why the Greddy and other plenums are shaped the way they are and a better option...

Btw way i would not use any other plenum for a foward facing TB!! I know I am going to get quoted back to me that person x, y and z use a modified rb25 manifold and make x, y and z rwkW so they do work, BUT consider this

Just because an engine doesn't suffer detonation or hasn't blown up yet doesn't mean it is running perfectly...It just may be that the tuning is overly rich and the leanest cylinder is still rich enough to keep EGTs respectable...It also may just be that the variation of between cylinders is just under the precipice of being acceptible and the engine 'is' holding together ...only just!!

Dont forget that the standard way of tuning is by using one O2 sensor that gives an averaged reading and for turbo cars target AFRs used by tuners are always conservative when compared to NA tuning...

The aim of any engine design is to have as much eveness between cylinder as possible...It is a known fact that multi cylinder engines that have even perfomance in each cylinder will make more power than those that don't...all else being equal...

This why we go to the trouble of cc ing combustion chambers etc etc to ensure the comp ratio is even...Now I doubt any engine deign would be perfect as real life is never perfect...But factory setups would be designed, prototyped and bench tested to ensure variations are within fairly tight limits...

So unless you do everything you can attain eveness throughout the cylinders, even if the engine is holding together and making decent power, you are still leaving horsepower on the table..

As far as the Greddy plenum shape is concerned....

Consider that:

Airflow into manifold = airflow out of manifold..

Gas flowing through any enclosure expereinces friction so you get pressure drop between inlet and outlets..AND pressure drop is proportional to cross section area and volumetric flow..

Volumetric airflow is proportional to flow velocity x flow cross sectional area.. at constant pressure...so for the same pressure if you reduce the cross sectional area by 50% flow velocity will roughly double...

Now if air enters the plenum at 6 volumetric units per second it needs to be divided up 6 ways...To do this you need the pressure at each outlet to be even...If you had a parallel plenum 6 units of air would enter, some would exit to cyl number 1 and the rest would continure to along the plenum...The air continuing throught the plenum would be 6 units minus air that exited through cyl number 1... Due to friction in plenum you would have pressure drop...due to reduced volumetric flow (some air has gone to cyl number 1) you would also have lower flow velocity (dont forget plenum is parallel at this stage) which also changes friction which changes pressure drop as well...BUT

If after cylinder number 1 you reduced the cross section of the plenum to so that the air velocity is increased again, the pressure would be reinstated..This repeats for each cylinder until number 6 is reached...

I think this is nearly correct....please feel free to pull it too pieces..