PDA

View Full Version : DO I NEED A 58mm THROTTLE BODY on my TPI 350?



grumpyvette
05-30-02, 03:32 PM
"DO I NEED A 58mm THROTTLE BODY on my TPI 350 to get better performance"
I get this question about once a week in my E-MAIl

ok time for a math lesson, please try to follow along,(yes Im well aware some of you know even more than I do, but this is for the new guys)the stock throttle body flows about 670cfm,
your 350 in the vette has a stock cam that has about 210 dur and lets be even more generous and say the heads flow 210cfm and your TPI intake also flows 210 cfm (to keep things simple, trust me, your stock heads, intake and cam don,t flow that well combined) now that means that at a steady flow rate with all the ports open at max cam lift for 360 degress of engine rotation that possiable durring the intake stroke your flowing 210cfm per intake port, correct (again being very generous) now in a standard stock engine theres 360 degees available in an intake cycle if the valve never closes (bad for the compression stroke) so about the most those ports can flow if the valves open instantly and stay open the full 210 degs at max lift is 210cfm/360 degs x 210degs the cams open or about 122.5 cfm x 8 cylinders=980cfm but since those valves open and shut durring the cycle your only able tp flow at about 65% of the max for a flow rate of about 637cfmthat well under the stock throttle bodys flow,
lets look at it from a second angle, your TPI 350 spinning at about 5500rpm (where it starts to run out of airflow big time) if the cylinders fill completely on every intake stroke(which remember is every 2nd stroke)is 350cid x 5500/2=962500 cubic inches or divided by 1728 to get cfm=557 cfm
now even if we add a 30% better flow due to exhaust scavageing, and ram tuning the intake pressure waves and a more realistic 35% reduction in intake flow due to the valve needing to open and close ,(not being instantly fully open the whole time)thats still an average of (557+637=1194cfm+10%=1313/2=656cfm,is about max the engine can use and in the real world its slightly less in a stock engine, and thats not counting for airflow volume losses due to heat expansion or restrictive losses due to the screens on your M.A.F if you still have them and are not running a speed density TPI or the air filter restrictions or the catalitic converters partly screwing up the exhaust scavageing.
now for the good news, unlike a carb system that relies on air flow to carry the fuel air mix, EFI can and does run better with a max possiable intake pressure behind that intake valve, adding a 52mm or 58mm throttle body will effectively add slightly to your potential air flow and reduce the systems airflow restrictions and unlike a carbed system its extremely difficult to add a throttle body thats so large that it screws things up,so if you want to add a larger throttle body ,go right ahead, it may not help (on a stock engine) but its almost never going to hurt either, but after much testing Ive got to say, if your talking about a stock engine spend your money on a 383 stroker kit, better heads,a better flowing intake manifold, headers, and a larger cam in the 220-230 durration range before worrying about a throttle body restricting your hp!

johnny
06-04-02, 04:17 PM
Thanks! Now I am convinced that I need a 58mm.:D