I have set out to try to figure out when a 305 will need bigger intake parts. I am still looking into this but this is what I have so far...


VE 100% @5000rpm = 885cfm
VE 90% @5000rpm = 796cfm
VE 80% @5000rpm = 708cfm
VE 70% @5000rpm = 619cfm

I have no idea what VE my engine is yet but I do know power, displacement, and rpm. I am still looking into this. If anyone has any other ideas about this let me know. I am still looking into this. Do these numbers seem right? They seem kinda high to me.

 

The formula I have seen for CFM is as follows. CFM= RPMxdisplacement / 3456.

So at 5000 rpm your 305 would need 442 CFM by my calculations.

 

But doesn't cfm depend greatly on things like valve size? compression and a few other variables?

Seems like the stock TPI intake flows around 650 cfm? I can't remember for sure...but thats plenty over 442 and we all know that above 5000 rpm the TPI starts falling off no matter what cam/heads you have (unless you're making very little power)

 

Valve size/compression/head flow change your VE. I always thought that a less efficient motor(lower VE) needed more to produce less... if the intake only flows 650, I guess thats why our stock TB(approx 780 cfm) works so good...

 

Yeah the stock TB should be good for 450 or so horsepower fairly easy. I believe its the same TB on my LT1 car 48mm twin blade...I've seen plenty of LT1 cars dyno 400+ on factory TB's. I too have always read that valve size, head flow etc change the volumetric effeciency.

 

They do change the VE. Just beware of the "self fulfilling prophecy" problem, however. If you put on parts that don't flow enough you will lower the engine's VE- and then that same too-small part looks like it's just the right size since you have such a low VE. Around and around you go.

If you can keep good velocity (ports and runners that aren't too big) there is basically no such thing as "too much" flow.

 

If you want to determine the restriction of the intake then you can just connect a line from a vacuum gauge to your plenum and measure the pressure drop when you are at WOT.

You were talking about figuring VE, but keep in mind that VE is for the entire system which includes restrictions in the heads and exhaust, so it will not be a good way to measure just the restriction of your intake.

If your goal is to improve your VE, then the part to concentrate on is the area causing the most restriction, and that will be around the valve seat area of the cylinder heads.