Head Flow Numbers

B

bdomit01

New Member
#1
#1
What kind of flow numbers are others seeing on their heads? I've been playing with 27mm intake valves recently and the last couple heads I've been topping out in the mid 70's for the CFM. Here's the flow data for the last head I did. Anybody have any tips to share or have I pretty much reached the potential for a 27mm intake valve and it's time to step up to a larger valve.

 
ole4

ole4

#3
#3
That is a higher than I am seeing, but you may be a more skilled porter than me. Do you know what the minimum cross section of your port is? You can tell what your CD is if you know it. Closer to .9CD the better job you did. A CD of 1 (optimal) will flow 145 approx. CFM per sq inch.
 
xBrent92

xBrent92

New Member
#4
#4
On the subject of head flow numbers..i have a question..
Assuming this is the 212 predator, How much flow would be too much? Or could there be too much?
My assumption would be too much flow would mimic the effects of a camshaft with too much duration and lift combo.
 
B

bdomit01

New Member
#5
#5
MCSA is around 0.70 sqin, I didn't enlarge the port that much, just the bowl and SSR work, then some chamber work and valve job. Around 105 CFM per sqin.

Any body care to share what they've been able to get with larger valve sizes? I'm going to do a 28.5 next and need a goal to shoot for.
 
Prowler08

Prowler08

New Member
#6
#6
Same here I do not enlarge the port... I do make a "V" around the guide, and enlarge the valve head area for the larger valve, and usually cut the guide down a bit...

I just did a quick port on a 19cc Champion head that comes stock with a 27/25 combo and put a 28.5mm valve in.
I am seeing 80-83cfm at .350-.400 lift...

The head is a different combustion shape also like a half moon shape...
 
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ole4

ole4

#7
#7
Could I ask you guys how you calibrate your benches? I calibrate mine with the bore adapter off. If I calibrate it with the bore adapter I get values from 5 to 10% higher. I also use a exhaust extension when flowing the exhaust.
The best 28.5 head I have flowed 85CFM It was a old round port Honda head done by MDB a long time ago. If I raise the depression over 30"'s it does go turbulent and gives out a horrible screech. He really lays back the short side till it is almost gone and I think the air is going over the back of the valve. I had flowed a head for MISFIT that he has a thread on here and it was a 32mm valve and hit coil bind at .365 lift but it flowed 88 CFM at .350 and at .365 was at 91. The larger the valve the more low lift flow as well as at the higher lifts. most heads past .25 of valve diameter (point where area of valve equals the valve opening) do not increase flow much.
For how much flow you need I use this formula and a VE value of 100%
Required aur flow CFM = ( CID * RPM * .000978474 * Ve% ) / ( Cylinders * 127.5 )
 
B

bdomit01

New Member
#8
#8
Great thanks guys, sounds like I should shoot for the mid 80's on the next one.

Ole I haven't actually tested my bench with a calibration plate. Any idea where I can pick one up for a good price? But from what I've seen of others numbers for stock heads it seems to be good. I get repeatable numbers so I'm not super concerned if they're a little off, since really I use it as a tool to determine if a change helps to port flow more or not.

I would think you would want to calibrate the bench without the bore adapter though. The way I look at it is the bore is just part of the induction system and acts as a diffuser to help slow the air, not a part of the bench itself.
 
B

bdomit01

New Member
#9
#9
xBrent92 said:
On the subject of head flow numbers..i have a question..
Assuming this is the 212 predator, How much flow would be too much? Or could there be too much?
My assumption would be too much flow would mimic the effects of a camshaft with too much duration and lift combo.
Click to expand...
You can have a head that flows too much for your desired RPM range. Look at the equation Ole posted above and you can get a good idea. Having a head that flows more than that is just going to move you powerband up out of your desired RPM.
 
ole4

ole4

#10
#10
Here is a one inch sharp edged plate which should flow 71.7 cfm on ebay.

Cameron Nuflo 6"x 1 00" Orifice Plate 316 SS New | eBay

I use one that is 1.250 and set my 100% on manometer equal to 112 cfm which the plate flows assuming a CD=.62. Also I use plates equal to 90cfm, 60cfm, 40cfm and 20cfm. If you like I could lend them to you. I also figured using the cal plate on the bore adapter made more sense but the Bettis book and also bruce from PTS forum said that is not the correct way. He said the vena contrata effect causes inaccurate results.
 
ole4

ole4

#13
#13
if your mcsa is .7 the move from a 27 to 28.5 will not increase max flow that much as with a 27mm valve if the seat opening is lets say 1" the opening already has .79 sq inches of area (minus stem diam) however low lift flow increases as the area at low lifts are much greater around the valve. If the low lift flow goes up a lot and then flattens at .25 of valve diameter and above (slight increases above that are normal but that is the point where curtain area is equal to valve diameter and valve is not longer a restriction to flow unless you have shrouding issues. If that is what you see you can then start enlarging the MCSA a bit at a time to increase high lift flow.
 
ole4

ole4

#14
#14
By the way 105CFM per sq inch is very respectable. Darren Morgan in a seminar said he gets a bit over 130CFM per sq in on his best pro stock heads which have a much straighter and higher angle to the valve than our little heads. My best 28.5 head was one of MDB's early honda heads with round ports i had it on here but photo gallery is down it was in the 84cfm range. My 28.5 heads on my bench are in the 70 to 75 cfm range Once I started taking port molds and saw how bad the bowl and turn were I started filling in the floor and opening up the roof to try to get a smooth transition from port to bowl.
 
B

bdomit01

New Member
#15
#15
Thanks Ole. I opened up the CSA on them proportionally, went from about .7 to .8 sqin. I'm hoping for the mid 80s but we'll see once I get it on the bench.

I have another 27mm head I'm about halfway done with now. Been thinking about sending it over to you once it's done. I figured you can flow it on your bench and then we could compare numbers. You could also take a mold of it if you wanted to get a sense of how I've been porting them and see if you can pick up anything from it.
 
ole4

ole4

#19
#19
I'm sure Barry has done that but keep in mind building an engine requires a close match of the parts. Can't put a huge head on a otherwise stock engine and expect anything but a soggy bottom and maybe less total power. I just bought some new equipment to allow me to install oversize seats accurately (instead of using a boring bar in my mill) and built a test head with a 32mm valve. here are the results. A while back I converted my bench from a lfe to a orifice style and also started calibrating the bench without the bore adapter. Since then if I test a calibration orifice on the bore adapter after first doing it on the bench top I get about 10% more flow on the bore adapter. The delta gets slightly smaller with smaller calibration orifices. My numbers were kind of disappointing but I will reflow my MDB reference head and see what the delta is now from the LFE.
 
delray

delray

Well-Known Member
#20
#20
i know you guys enjoy doing most if not all your head work, but have any of you look into having a head CNC ported and seeing what kind of flow numbers you would get. i also see there is different kinds of finishes or you could say textures you can leave in the cnc porting operation.
 
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