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CFM drop

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cfm drop
3K views 17 replies 3 participants last post by  Old as Dirt 
#1 · (Edited)
Anyone that does a lot of bench testing.. can you give me some kind of ballpark idea how much CFM you lose when a Holley 750/Single plane like a Vic Jr. is bolted up to an AFR 185cc runner head that flows 295CFM @ .550 lift? Need some numbers to plug into a program to see if I am sort of close to the 260CFM piston demand required at the peak HP rpm to spit out some VE%..
Minimum Intake CSA numbers are 2.025"/305FPS @ 6125rpm
Thanks..
 
#2 · (Edited)
You're probably looking at something very close to your 260 number but without putting your heads, intake and carb on a bench there's really no way to know. Then try another bench and start over.
How are you calculating peak piston demand?
Where did you get 305 fps @ 6125?
What's the bore/stroke of the engine you're working on and what's the cam intake @ .05 duration? Assuming SB Ford?
 
#3 · (Edited)
Appreciate your reply Scott.. Yes, 10.6:1 347 SBF w/SFT cam@ 4.030 x 3.400.. I'm using Larry Meaux's "Pipemax" program to calculate piston demand and the 305fps numbers came from figuring the Min CSA off AFR's Website for this set of heads.. dropdown menu for figuring the preferred port size in Min CSA in Larry's program.. has a VE% calculation dependent on head flow.. but it comes up with a figure that seems pretty high to me.. I'd be amazed if this engine goes more than about 103- 105%.. even that is a tough range to make.. duration @ .050 on the intake is 232
At 103VE%=435.2HP
At 105VE%=448.5HP

EDIT: Made a mistake.. was using 7500rpm on the FPS calc.. should be 250fps@6125rpm
 
#13 ·
did I read that right,that 185cc head is a little big for the 347 inch stroker:confused:I run 195cc Canfields on my 347 with more bottom end then I know what to do with and plan on going bigger cc some day..cam is not an alphabit one tho,or off shelf..
 
#14 · (Edited)
little more top end..

Was advised that if I wanted a bit more top end.. to go with a 1.7 rocker ratio.. so it wont kill off the bottom and mid range, but will add a little top.. seems like sound advice, so I opted to leave the duration alone.. do the Crane rockers have the right fulcrum length for these heads @ 1.7 over the stock 1.6 ratio?
Like to buy them once.. an not have to try 5 sets of rockers..
 
#15 ·
Was advised that if I wanted a bit more top end.. to go with a 1.7 rocker ratio.. so it wont kill off the bottom and mid range, but will add a little top.. seems like sound advice, so I opted to leave the duration alone.. do the Crane rockers have the right fulcrum length for these heads @ 1.7 over the stock 1.6 ratio?
Like to buy them once.. an not have to try 5 sets of rockers..
might sound crazy,but sometimes SBC rockers lay down nicer on the SBf and better geometry...
 
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