Bench Flowing

[chris adams 0]

ryt ive come across an artile about bench flowing and how useless it actually is. and that the 30% less rule is better. by 30% less i mean reducing the dead space in the cylinder head and increasing the velocity of air traveling through the inlet port of the cylinder head. does any 1 have any opinions on this.?

[DRTDVL1345402287 0]

flow benches are like dyno's, every reads differently and they are setup differently but are a good tuning tool...

 

we use to flow test ported heads to help get the port to flow better to suit the cams being used (matching peak flow to peak lift of the cam - no point having a head that flow 400cfm at 11mm lift but only 120cfm at 8mm where your max lift is), and to try and optimize flow/velocity with the valves in, most cfm figures are without the valves with can hugely vary the figures.

 

Measuring velocity in the ports creates it's own problems due to the fact you need to put something into the port (pitot tube) which in itself will effect the flow/velocity/turbulence of the port.

 

I think if you look at it as a tool and not a be all then its ok.

 

I'm skeptical figures as measuring at 10,25, or 28 inches of water will give different results on the same head, also the shaping of the clay on the port mouth can hugely vary the results of the flow test (we could easily swing the results by 10 - 15cfm by playing with the clay), but like a dyno testing the same head over and over on the same bench after changes are made can help baseline and measure improvements if you ignore the "actually figures" and look at the changes in values

 

Can you please tell me more about the 30% rule?

Edited March 27, 2011 by DRTDVL

[chris adams 0]

basically its actual valve size minus some number or what ever the equation was but it ended up being 30% percent less. that is the port entry up to the valve stem area. i need to learn how to do links if i can find the article. without the pics i dowt u will get what im trying to say as im no good at describing things. basically by reducing this area to create more volicity the whole engine made a broarder power band right to red line

[petert1345402284 0]

There's the 30% rule for valve lift. ie cam lift = valve dia x 30%

[chris adams 0]

no its not that rule. it basically reduces the throat at the valve guide area to 30% of the valve. i need to find the articl to show u guys.