steve cowan wrote: ↑
Mon Nov 25, 2019 1:22 pm
i think Randy's question -how much cfm do you need in a running engine??
was valid as different depressions have not really been mentioned here,please bare with me as i am not the smartest person around.
some calculations i made (only rough )
cfm demand at peak piston speed approx 76deg atdc
i would assume that demand varies to piston position and rpm through the range.
port velocity will vary through rpm/valve lift sequence etc
if port is stable and airspeeds stable port in theory should keep flowing till choke ??
flowbench flow air one way at a time-engine dont but i think we would like it to flow one way from intake to cylinder.
i wont put all the numbers up but i played around with a set of sportsman 2 heads a while back.
198cc as cast
227cfm@.600''@28'' turbulent @ 0.400''
these heads had a 3 angle valve job previously and at 86% throat
i set pinch to 2.3''
blended throat to 87%
narrowed guide boss
did not touch SSR just blended through port and polished apex of SSR
so lifts above 0.500'' it picked up 12% cfm
260cfm @ 0.600''@ 28''
port was smooth through the lift range and reckecked at 202cc
so then i tested at 35'' depression
wont bother with all the numbers
apex ssr 398
what i am trying to say is flow and velocity increase with depression,how much depression does running engine have at peak demand ??
i have read Larry Meaux early readings about sonic choke
can a engine choke from to much cfm??
could be a stupid question (maybe)