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

 

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.

 

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?

 

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.

 

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

 

You can't take in more air into the intake than what is going out the exh.There is no place for the air to be
" stored" so to speak.
However you may lose throttle response at low revs with a larger TB because you have lost flow velocity

 

You're gettin' it! Multiply 190cfm (from your orig post) x 2 = 380cfm. Even if a little extra is being pulled from another cylinder, you can see why there's plenty of head-room for the stock 600cfm TB (on a stock motor).

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!!!!