General engine intake question
Thread Starter
Racer
Joined: Dec 2002
Posts: 498
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From: Utica NY
General engine intake question
OK, this is a question I think about from time to time and I can't come up with an answer... so hopefully some one else can come up with it for me 
if a tpi runner/base stock can flow like 190 CFM and theres 8 runners... why couldnt you theortically throw a 1500 CFM throttle body on a stock system and realize huge gains? I know this doesnt work.... but I dont know WHY it doesnt work... anyone know??
John
if a tpi runner/base stock can flow like 190 CFM and theres 8 runners... why couldnt you theortically throw a 1500 CFM throttle body on a stock system and realize huge gains? I know this doesnt work.... but I dont know WHY it doesnt work... anyone know??John
Pro
Joined: Dec 2008
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From: Lubbock for now, headed for Oklahoma
OK, this is a question I think about from time to time and I can't come up with an answer... so hopefully some one else can come up with it for me if a tpi runner/base stock can flow like 190 CFM and theres 8 runners... why couldnt you theortically throw a 1500 CFM throttle body on a stock system and realize huge gains? I know this doesnt work.... but I dont know WHY it doesnt work... anyone know??
John
if a tpi runner/base stock can flow like 190 CFM and theres 8 runners... why couldnt you theortically throw a 1500 CFM throttle body on a stock system and realize huge gains? I know this doesnt work.... but I dont know WHY it doesnt work... anyone know??John
Because in effect you are only using one of those runners at a time
In reality it is a little more than that. You'll have multiple cylinders drawing at once, but not all of them. Some of what you are thinking about is why we have plenums and a mass of stored air above the cylinders.
Thread Starter
Racer
Joined: Dec 2002
Posts: 498
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From: Utica NY
that makes perfect sence actually... and it was so simple! thanks Im guessing since there's 4 strokes and there's 8 cylanders, its reasonable to assume at any given time 2 could be in the intake stroke at a time... maybe slightly more with a high duration cam?
Max G’s
Joined: Nov 2008
Posts: 2,751
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From: Monroe OH
NCM Sinkhole Donor
Here's an explanation I've read before. If you have a stock plenum and runners and base and add a higher volume throttle body, at only 1/4 throttle you'll be taking in more air than the intake manifold can flow, so your computer will only inject enough fuel for 1/4 throttle, therefore your mixture will be extremely lean and bring you too close or at detonation. As well it will make the cylinders very hot.
Melting Slicks
Joined: Feb 2006
Posts: 2,753
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From: backwoods upstate ny
intake duration is very similar to exhaust...with anything "sweeter" than stock mufflers, at idle you can hear the individual cylinder exhuast period, and even at redline there is evidence of sound pulses... gives ya a rough idea of what is happening in the intake tract
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
" stored" so to speak.
However you may lose throttle response at low revs with a larger TB because you have lost flow velocity
Race Director
Joined: Nov 2003
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From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
If you use another ("better breathing") intake, the "needs" of 350ci @ 6k rpms should still be 600cfm or less. 75cfm per cylinder is required, but it needs to be filled during 1/4 of the 4-stroke cycle. (e.g., 4 times faster). 4x75=300cfm. Multiply that times 2 cylinders = 600cfm.
Interesting.... that's the size they picked for a TB. Keep in mind though that it's rediculously rare to find a motor with 100% VE. IOW, you don't get the cylinders 100% full of air, so a 350 really requires less than 600cfm with the best designs.
For me, I've yet to be convinced you ever need a larger-than-stock TB for any NA 350 motor. But, you see people do it all the time.
gp
Edit: Looks like I've finally received evidence that a 52mm is ideal for some 350 builds. Another forum member sent me an engine simulation using a 350 motor, AFR 195 heads, 1.75 headers, and a short-runner intake. It shows VE rising above 100% and a CFM requirement of 650ish cfm.
So, you MIGHT need a larger TB -- if your mods are extensive enough!!!!
Last edited by GREGGPENN; Dec 28, 2008 at 05:23 PM.
