CFM drop
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CFM drop

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    Muffler City Old as Dirt's Avatar
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    Default CFM drop

    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..
    Last edited by Old as Dirt; 04-25-2016 at 04:41 PM.
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    Quote Originally Posted by Old as Dirt View Post
    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..
    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?
    Last edited by scott foxwell; 04-26-2016 at 05:37 AM.
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    Quote Originally Posted by scott foxwell View Post
    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?
    Appreciate your reply Scott.. Yes, 10.6:1 347 SBF w/SFT [email protected] 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 [email protected]
    Last edited by Old as Dirt; 04-26-2016 at 06:59 AM.
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    Quote Originally Posted by Old as Dirt View Post
    Appreciate your reply Scott.. Yes, 10.6:1 347 SBF w/SFT [email protected] 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 [email protected]
    Using 6100 rpm, I get a peak piston demand of 237cfm @ 100%ve. 103% would be 244 and 105% would be 248.
    I get a MCSA of 1.72 sq in. for the airspeed I would target.
    What rpm do you want to make peak power? With a 2.1 minimum you can see your airspeed is slow @ 6100. This will hurt your VE. You would also want to know what the lift is @ ~76* crank rotation (approx. peak piston demand) and then compare head flow at that lift to see if you meet the demand.
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    Quote Originally Posted by scott foxwell View Post
    Using 6100 rpm, I get a peak piston demand of 237cfm @ 100%ve. 103% would be 244 and 105% would be 248.
    I get a MCSA of 1.72 sq in. for the airspeed I would target.
    What rpm do you want to make peak power? With a 2.1 minimum you can see your airspeed is slow @ 6100. This will hurt your VE. You would also want to know what the lift is @ ~76* crank rotation (approx. peak piston demand) and then compare head flow at that lift to see if you meet the demand.
    I'd need a VE% OF 125 to get the heads flowing what the piston demands.. that's like a 9:1 super rubbed on miracle headed super fuel monster built by Aliens, with a ton of duration and lift.. and a blower.. probably split the block in two.. crap, guess I will have to settle with a mild, reliable stroker in the 400HP range for the little hotrod then.. appreciate your time Scott.. and the learning lesson.. looks like I would need 7500 peak hp rpm to see close to a target of 300fps and use these heads on the 347..
    Last edited by Old as Dirt; 04-26-2016 at 02:26 PM.
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    Quote Originally Posted by Old as Dirt View Post
    I'd need a VE% OF 125 to get the heads flowing what the piston demands.. that's like a 9:1 super rubbed on miracle headed super fuel monster built by Aliens, with a ton of duration and lift.. and a blower.. probably split the block in two.. crap, guess I will have to settle with a mild, reliable stroker in the 400HP range for the little hotrod then.. appreciate your time Scott.. and the learning lesson..
    You're going to be fine. Heads are a little on the large side for what you're doing but it should run good. Heads will meet the demand with no problem, the airspeed will just be a little slow.
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    Quote Originally Posted by scott foxwell View Post
    You're going to be fine. Heads are a little on the large side for what you're doing but it should run good. Heads will meet the demand with no problem, the airspeed will just be a little slow.

    Ordered the Cam Friday/two week lead time.. sent a message to him to see if I could add a bit of duration and lift on the core he is ordering.. bump it up so the peak HP is 6400rpm instead of 6000rpm.. got to help the airspeed(252 vs 238) in the port by revving it a bit more.. good rods and pistons, so no issue with the bottom end.. just hope it don't tick him off..
    Last edited by Old as Dirt; 04-26-2016 at 06:02 PM.
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    Quote Originally Posted by Old as Dirt View Post
    Ordered the Cam Friday/two week lead time.. sent a message to him to see if I could add a bit of duration and lift on the core he is ordering.. bump it up so the peak HP is 6400rpm instead of 6000rpm.. got to help the airspeed(255 vs 235) in the port by revving it a bit more.. good rods and pistons, so no issue with the bottom end.. just hope it don't tick him off..
    More duration will hurt airspeed in the port. The air has more time...it can walk now, instead of run.
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    Quote Originally Posted by scott foxwell View Post
    More duration will hurt airspeed in the port. The air has more time...it can walk now, instead of run.
    You sure have a way with words to help guys like me to understand.. thanks.. would more compression help the air run? pull harder on the intake tract?
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    Quote Originally Posted by Old as Dirt View Post
    You sure have a way with words to help guys like me to understand.. thanks.. would more compression help the air run? pull harder on the intake tract?
    I'm glad it's helpful and not discouraging...I don't mean to be.
    Compression is always your friend when it comes to making power, but lower compression tends to like a little more cross section. Your induction will determine/dictate where you will make peak power as much as anything. You may make peak closer to your 6400 than you think, even if you're "cammed" for 6000.
    I'll run this by Chris and see what he says. A little more lift and duration might not hurt.
    Last edited by scott foxwell; 04-26-2016 at 06:23 PM.
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    By the way...237cfm @ 100% ve (6000 rpm) through a 2.1 minimum is 271 fps which is not that bad.
    Last edited by scott foxwell; 04-26-2016 at 06:24 PM.
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    Thanks for the conversation Scott.. enjoyable.. you can pm me about Chris' s response if you would like.. think that might be even a little bit better.. as the MCSA is 2.025, not 2.100.. every little bit helps when it's all you got....
    Last edited by Old as Dirt; 04-26-2016 at 08:26 PM.
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    did I read that right,that 185cc head is a little big for the 347 inch strokerI 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..
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    Default 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..
    Last edited by Old as Dirt; 04-27-2016 at 01:11 PM.
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