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Making My Custom Plenum.

47une

By 47une,
in Engines & Forced Induction

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ISL33P Proficient

ISL33P

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ps.....from memory you have previously placed much emphasis on that variance being due to water temps between the front and rear of the head, but now its the plenum ???

Edited by ISL33P
47une Collaborator

47une

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haha yeah, if I was doing it for a buisness and for a reputation, I would also flow test mine to prove, but I have no one to prove my results to, its a stinking RB20, it if blows because of my dodgy plenum, then so be it, I will build or replace the engine for next to nothing and try a different style of plenum.

juggernaut1 Proficient

juggernaut1

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Have you considered the Hermholtz reasonance theory when designing your manifold - here's an overview plucked from a US thread in relation to the sr20.

Long runners make more "midrange" as Def says, "focused powerband" because of:

http://en.wikipedia.org/wiki/Helmholtz_resonance

This is the key here. The long runners make more airflow available to each cylinder... but ONLY During a very specific, narrow, RPM Range... depending on the size and shape of the runners. the SR20 intake runners get a mid-range bump in the 4500-5500 RPM range.. and thats about it... Anything ABOVE or UNDER that RPM range will suffer, because hermholtz also works backwards, subtracting available airflow from each cylinder, anytime its not in its peak operating range. So VE will ramp up quickly just into boost coming out of the 3000-3500RPM range, right in peak VE range thanks to hermholtz resonance theory (4500-5500RPM) at which point the vehicle has gained more and more acceleration due to this design, but as you pass 6000RPM hermholtz ruins your top end VE... killing torque.. and thus power... but the SR20's stock turbo starts running on the bleeding edge after that spot anyways... so VE is dropping but so is flow through the turbine and compressor... the turbo has to flow less now because it moves less air through the motor at the same boost, because of hermholtz, this complements the stock turbo perfectly since by lowering its flow at high RPMS it can level off without overworking itself. so the drop in torque is barelly noted on the stock setup (after hermholtz is removed, at high RPMS), but the loss of the long runner intake IS noted because of the huge dip in mid-range you lose by not having hermholtz there anymore.

This is what our thread op has experienced. And as mentioned, cams are a huge factor here. They allow the motor to breath after hermholtz has been removed, taking advantage of a shorter runner intake manifold, the way the stock one cannot do, at all. Sure you can slap on a bigger turbo and bigger cams and keep the stock intake... but its a huge waste, you will be fighting hermholtz right when your large(er) than stock turbo comes on boost... this is why we change the stock intake. it isnt about volume or flow (although those help) its about getting rid of the hermholtz problem when it becomes a problem. It also depends on application, a longer runner intake may help the turbo get moving, as VE is punched right when it comes on boost briefly (obviously depending on the size of the turbo) so the car feels like it responds quicker... and it does... but overall power is lacking compared to others with the same setup because you are always going to feel that drop in torque after hermholtz starts robbing you, no matter how big your cams or turbo is.

esky_mad Proficient

esky_mad

Posted
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sydney kid you do realise were talking about a plenum for an RB20 ?

there are no "cheap" aftermarket plenums for RB20's budget at least $1000 for an off the shelf replacement

theres only the somewhat suspect , throttle to the front of the stock plenum.

i brought mine off SAU for $400, was a private sale. joe il send you some pics when i get the chance.

i only skipped though the most recent posts so i could be wrong here but.... dont swap the 20 for the 25. 20's are tough as.

47une Collaborator

47une

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dont know if you boys have seen this it was in the Fabrication http://www.skylinesaustralia.com/forums/R3...t191089.htmlbut

But yeah thats is has been helping me make my plenum

Thanks Matt,

Yeah I have had a good read though that one already, I will be using some of the information from there for sure. Mine is going to be a little more basic though.

Joe.

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