2802NR Posted September 9, 2011 Share Posted September 9, 2011 years ago I started a tapered runner intake manifold project. http://forums.hybridz.org/index.php/topic/41439-custom-intake-progress-pics/page__p__340610__fromsearch__1#entry340610 I had big plans with alot of research backing it up. As many projects do, this one took a back seat for far too long but i'm finally back at it full of motivation. I dont have access to a CNC machine shop these days but i'm quickly learning that big bad fabrication tools arent nearly as important as a small selection of hand tools combined with some creativity, tenacity and a little patience. everything is coming along nicely but I think the plenum is off. its around 2.25 liters (liquid volume), tapers too small at the ends and needs to extend past the outside runners a little more. my question is this, Whats a good way to figure plenum volume? I have read many conflicting opinions on the subject. Some say 100-150% of displacement works best for high rpm. some say 80% is sufficient for NA engines and 150% is adequate for boosted applications. I've also read that bigger plenum's will make more power all day on the dyno but, if too big, can barely move the car down the street. i had an engine management book that had a formula based on RPM, CFM, and displacement for proper plenum size but i cant find it anywhere now. Basicly I want more RPM than my stock FI manifold can provide, without loosing too much low end. its going on a 10.4-1 NA L28 but if I decide to go turbo later on, it'd be nice to have an adequate plenum for that setup too. Thats not too much to ask right? hahaha any thoughts, formulas, refferences or ideas would be a huge help. there's gotta be a happy medium. Quote Link to comment Share on other sites More sharing options...
madkaw Posted September 9, 2011 Share Posted September 9, 2011 Interested in this thread! I have also studied a bit about plenum size and I believe I have read the same numbers you have found. I like your design with the center inlet, just makes it tougher to fit in our cars. I am currently making an airbox for my 2.4, I get confused on the math to figure volume size of the airbox compared to engine size. Quote Link to comment Share on other sites More sharing options...
johnc Posted September 9, 2011 Share Posted September 9, 2011 Well, on a no limit NA build I had a custom manifold that was exactly 1.5 times the total displacement of the engine. Engine builder went to a lot of work to make sure that was the volume, even calculating for the loss in volume from the 6 TWM velocity stacks inside the plenum that fed the runners. EDIT: He also included 1/6 of the total runner volume in his calcs. Quote Link to comment Share on other sites More sharing options...
Leon Posted September 9, 2011 Share Posted September 9, 2011 In my past FSAE engine project, we did engine dyno testing of 3 different plenum volumes (something like 1x, 2x, 4x displacement, need to look it up) and had the largest plenum tested give us the best results. Of course, the testing was done on a restricted 450cc single-cylinder engine, and may not mean anything to you. Honestly, the only way to really know is to make your plenum modular and go to the dyno. Good luck! Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 9, 2011 Share Posted September 9, 2011 Recall my myriad stories of the small-plenum conversion I did just not putting me back in the seat at 137mph like the large plenum did. Made my decision easy: In my case Bigger=Better. I'd have to dig the numbers on plenum up, but the actual plenum portion was 3/4 the previous models size...I think my small plenum was 192CID (on a 168CID Engine) and the larger one was around 268 CID. Damn, that's really close to John C's setup. I just realized that. I must have read it in a book someplace when making the first plenum (and copying the HKS Plenum Box...) cutting that 2" off the bottom of the box just made the response suck. Seriously. Quote Link to comment Share on other sites More sharing options...
madkaw Posted September 9, 2011 Share Posted September 9, 2011 Okay, let me see if i am adding this up right. My air box dimensions are 3X5X22 which figures to be 330 cubic inches. So I am covered for my 150 cubic inch p-30---right? Quote Link to comment Share on other sites More sharing options...
madkaw Posted September 11, 2011 Share Posted September 11, 2011 Anyone want to verify my math??? Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 11, 2011 Share Posted September 11, 2011 No, math makes my head hurt. Oh, waitaminit, that is the beer, rum, and Jonny Walker Black... Quote Link to comment Share on other sites More sharing options...
2802NR Posted September 11, 2011 Author Share Posted September 11, 2011 interesting stuff. john and tony, how did you feel about your powerband with those setups? i know my setup is probly way different but just wondering. low end vs. top end, you know. madkaw your numbers look good i do wonder if my water measuring method is any good for measuring the volume of something to be filled with air... anyway, my fabd one measures 2.25 roughly 80% of displacement. i also measured a fwd SR20 plenum out to be 2.0 liters, right on 100% displacement. no idea what my n37 is but my best guess would be quite a bit smaller than even 80%. anyone know what any of the stock L series FI plenums measure out at? by the way, i`m not including any runner length like johns engine builder did but my air horns on the runners only extend past the plenum wall .25in Quote Link to comment Share on other sites More sharing options...
2802NR Posted September 11, 2011 Author Share Posted September 11, 2011 oh and i have definetly considered making it modular so i can play with different setups. seems to make the most sense Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 12, 2011 Share Posted September 12, 2011 "i do wonder if my water measuring method is any good for measuring the volume of something to be filled with air..." Volume is Volume, you want to get something that doesn't effervesce like pouring a coke 2L into the thing would not be recommended, nor would Litros of Red horse Extra Strong Lager... But if you have a known volume, and fill carefully to avoid turbulence, you can get very accurate measurements. A 100CC graduated flask or even a cooking cup will do to give you decent CC approximation and from there direct calculation is a cinch. If you want maths, and are some sort of new-age masochist, then by all means figure in cups, tsp, tbs, and etc and convert from Pecks Dry to Gwarfhals Preen on the Newbian Parsec Market exchange, then back to metric to get a good number. You never hear of a guy with a Turbo Buick 3.8 going: "MAN I dig my one-gallon engine!" (Think about it! It's what it is!!! ) Quote Link to comment Share on other sites More sharing options...
johnc Posted September 12, 2011 Share Posted September 12, 2011 Powerband was typical high horsepower L6. Nothing below 4,000 rpm and then a whole bunch until 7,500 when it started to fall off. BTW... in all racing classes where the NA engine is essentially unrestricted except for an inlet restrictor, the builders make as large a plenum as possible. I once saw a VG30DE engine make 423hp on an engine dyno through one 35mm restrictor. That engine went on to win the LMP2 class during the ALMS race at Road Atlanta (Petit LeMans). Quote Link to comment Share on other sites More sharing options...
2802NR Posted September 20, 2011 Author Share Posted September 20, 2011 so the new one i came up with measures roughly 3.5 liters, 125% displacement. deffinetly interested to see how it turns out. i'll update the thread with some dyno numbers eventually, hopefully before the last track day of the year in oct. Quote Link to comment Share on other sites More sharing options...
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