home made plenum

[JoKer]

as for volume (dont think it's been covered yet)

dontcha need the same volume of air in the plenum as the capacity of your motor? (or enuff to fill 2x cylinder at any one time ie half) I can never remember

edit : some copy/pasta from some very knowledgable guys I am allowed to hang around

http://bbs.scoobynet.com/projects-40/420866-intake-manifold-more-top-end-power.html

I stumbled upon this thread where Andy F is developing his intake manifold. Interesting discussion on the plenum size in particular. Bigger is better due to induction overlap between cylinders on the boxer 4. I hadnt considered this before

Unfortunately the thread turns political, but we know what it ends up looking like

Main points were pairing cylinders 1,2 and 3,4 for induction cycle separation. Larger Tb. And also having a plenum big enough for more than one cylinder to draw air from.

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http://mrtperformance.com.au/images/stories/resources/technical/MRT-InletManTech.pdf

worth a read - more on later manifolds

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great read!!!!!!!!!!!

from what i understand is that you want a plenium big enough for two cylinders worth of air. please correct me if i am wrong as i am going to be doing a cutom intake at this stage for something a little different and to help the piping to go over the alternator easily.

Is there a great deal of perfomance gains in dual pleniums considering cars like gobstopper only has single plenium ( not that i am going to do dual plenium )

what difference does the length and width of the runners have?

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Dual plenums isnt necessary, you might get slightly better throttle response from having two throttle bodies, but the extra effort doesnt reap any real rewards from what ive seen

RAC showed good gains from increasing plenum size from the v3/4 to v1/2 manifolds. V1/2 manifolds typically require a lot of honing out of the runners as the casting is poor. EJ25 manifolds are much tidyier inside and have the sameish size plenum, so prob best option

runner size influences gas speed and harmonics, it's all a trade off. long runners for low rpm power but less high rpm power, short runners for the opposite

large diameter to flow more gas but loose air speed and therefore torque, smaller diameter to keep air speed up but cause a resitriction at high flow rates limiting power.

in your situation there's little issue in starting off with a v3/4 manifold, get tuned and running, then look at switching to a manifold with a bigger plenum. Retune costs will be negligible as usually only fuelling needs to be slightly tweaked

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might be subaru specific (sorry) or apply here