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Head flow, Positive boost vs. Flow Bench

Posted By: turbobitt

Head flow, Positive boost vs. Flow Bench - 12/13/09 05:13 PM

Here is a question that is bothering me. How does flow bench under vaccuum relate to positive boost pressure flow ? Example, if my head flow flattens out at .600 lift on a flow bench, could I expect a different reaction under boost pressure ? Would flow increase ?
Allan G.
Posted By: Cab_Burge

Re: Head flow, Positive boost vs. Flow Bench - 12/13/09 05:58 PM

Quote:

Here is a question that is bothering me. How does flow bench under vaccuum relate to positive boost pressure flow ? Example, if my head flow flattens out at .600 lift on a flow bench, could I expect a different reaction under boost pressure ? Would flow increase ?
Allan G.

Thank about that You can force more air thru a hole under pressure,same thing as liquids when you turn up the pressure on a water hose
Posted By: Anonymous

Post deleted by Defbob - 12/13/09 06:19 PM

Posted By: turbobitt

Re: Head flow, Positive boost vs. Flow Bench - 12/13/09 08:03 PM

I undestand the relation between HP and boost. Just wondering if I should consider more lift on my cam.
AG.
Posted By: gregsdart

Re: Head flow, Positive boost vs. Flow Bench - 12/13/09 08:12 PM

Maybe another way to look at the situation is absolute pressure. 0 being a total vacuum, 14.7 psi is normal atmospheric pressure at sea level, 30 psi (14.7 plus 15.3 psi boost) about two atmospheres of pressure. Horespower will rise about the same as absolute pressure, if the efficiency of the blower/intercooler is high enough.
The valve lift can remain the same, heads can remain the same, the extra pressure will put more air in the cylinder.
Posted By: blownzoom440

Re: Head flow, Positive boost vs. Flow Bench - 12/13/09 11:06 PM

i have read where going .050 past peak flow will keep the port in peak flow window longer.mine is.
Posted By: polyspheric

Re: Head flow, Positive boost vs. Flow Bench - 12/13/09 11:37 PM

Can't really answer the question, because some of the blower discharge is expanded into volume at low pressure (only slightly higher than normal low vacuum in the manifold), but some may still be under higher pressure between boost pressure and zero, depending on manifold volume and boost level. A 3-71 producing 5 psi into a 500" RB with a tunnel ram manifold will get the entire flow as CFM at zero to low vacuum, a 14-71 producing 30 psi into a 300" LA with a simple base plate to the manifold will have over 20 psi.
Flow benches don't test density.

HP will not increase linear to boost regardless of blower efficiency, because there is heating and expansion just due to the temperature rise in the engine.
The Wallace calculator simply multiplies existing power by (14.696 + your boost input) ÷ 14.696, and it's wayyy too high (yes, the same mistake appears elsewhere, even on blower manufacturer sites - and still wrong).

Read my article: http://victorylibrary.com/mopar/blower-c.htm
Posted By: Cab_Burge

Re: Head flow, Positive boost vs. Flow Bench - 12/14/09 12:04 AM

Quote:

I undestand the relation between HP and boost. Just wondering if I should consider more lift on my cam.
AG.

Anything you can do to improve the airflow into and out of the motor will normally make more HP and torque. When it comes to boost and intake manifold temps, lower air temps makes the air a lot denser than hotter air temps. My message is a inter cooled motor will make a lot more power with the same amount of boost as a roots blown motor with no intercooler or alcholol or water injection to cool the inlet air temps. down Same thing on any supercharged or blown motor. I'm hoping my next boosted motor will be turocharged instead of a crank driven supercharger You have to drive the air compressors and that eats up a certain % of the power gained from the boost, not so on a turbo motor
Posted By: dodgeboy11

Re: Head flow, Positive boost vs. Flow Bench - 12/14/09 02:23 AM

Flow would increase, but you're thinking about it wrong. The motor doesn't know it's boosted mechanically or not. Vacuum is often used incorrectly. The engine doesn't see vacuum, it sees reduced pressure. N/A the engine sees atmospheric pressure. If you add 1 bar under boost, that's a total of 2 atmospheres. So the flow bench would have to have the pressure drop increased. I've done it out of curiosity and a port that went turbulent at 28in did indeed flow more air at 35in. I don't know what the numbers are exactly, but more pressure will force more air through the same size hole UNTIL you've got the port saturated. Which is why I go for volume on engines I know are going to be boosted.
Posted By: Shaker223

Re: Head flow, Positive boost vs. Flow Bench - 12/14/09 02:49 AM

My car has run a best of 117mph in the qtr @ 3,500lbs. The stock slant six intake flows around 110-120cfm and the exhaust flows around 90cfm. Using various formulas this calculates to around 450 FWHP. This is at 29psi. Increasing from 22psi to 29psi of boost netted 7mph in the qtr. I can't wait to see what a good flowing head and camshaft will do!
Posted By: dodgeboy11

Re: Head flow, Positive boost vs. Flow Bench - 12/14/09 03:23 AM

I expect it to drop your boost numbers and increase your mph:)
Posted By: turbobitt

Re: Head flow, Positive boost vs. Flow Bench - 12/14/09 04:31 AM

Quote:

Flow would increase, but you're thinking about it wrong. The motor doesn't know it's boosted mechanically or not. Vacuum is often used incorrectly. The engine doesn't see vacuum, it sees reduced pressure. N/A the engine sees atmospheric pressure. If you add 1 bar under boost, that's a total of 2 atmospheres. So the flow bench would have to have the pressure drop increased. I've done it out of curiosity and a port that went turbulent at 28in did indeed flow more air at 35in. I don't know what the numbers are exactly, but more pressure will force more air through the same size hole UNTIL you've got the port saturated. Which is why I go for volume on engines I know are going to be boosted.


Im looking at different lobe profiles that have more lift at the same duration. My goal would be to increase flow at boost without sacrificing low end performance at vaccum or no boost.
Allan G.
Posted By: DakFink

Re: Head flow, Positive boost vs. Flow Bench - 12/14/09 05:54 AM

Quote:

Flow would increase, but you're thinking about it wrong. The motor doesn't know it's boosted mechanically or not. Vacuum is often used incorrectly. The engine doesn't see vacuum, it sees reduced pressure. N/A the engine sees atmospheric pressure. If you add 1 bar under boost, that's a total of 2 atmospheres. So the flow bench would have to have the pressure drop increased. I've done it out of curiosity and a port that went turbulent at 28in did indeed flow more air at 35in. I don't know what the numbers are exactly, but more pressure will force more air through the same size hole UNTIL you've got the port saturated. Which is why I go for volume on engines I know are going to be boosted.



THIS ANSWER RIGHT HERE IS ABOUT AS CLOSE TO ANY I HAVE HEARD COME FROM PROFESSIONAL TURBO RACERS ABOUT HEADS!!!!

I've had the privilege to hear and pay attention when questions about building Turbo Motrs were brough up and HEADS was always the HOT TOPIC!!

Everyone of them said your heads is where it's at! Buy the best flowing heads you pocket $$$$ can stand and go from there. The turn radius and stagnant port flow that N/A guys worry about doesn't apply as much to Boost.

Look for Port Volume and Flow the more the better.

I wish more people would flow their heads like dodgeboy11 did, especially when they know they are being set-up for Boost.
Posted By: Shaker223

Re: Head flow, Positive boost vs. Flow Bench - 12/14/09 03:04 PM

Quote:

I expect it to drop your boost numbers and increase your mph:)



Believe me...I realize the stock cam and head are an obstruction!! I've always wondered bow much of the boost was making it into the cylinders with a .380"(after lash cam)
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