Head flow number vs cam shaft profile

[SappySE107]

Ill post up some stock flow numbers. Im doing a set of heads right now and have been trying to figure out what to do about the intake to exhaust flow ratio. Im gonna copy/paste what i found.

"Single or Dual-Pattern?


A good rule of thumb is that normally aspirated engines whose heads have exhaust-to-intake flow-ratios under 70 percent (i.e., the exhaust flow is 70 percent that of the intake) like a dual-pattern cam to crutch the weak exhaust port. Over 80 percent, you're in single-pattern territory. Blower, turbo, and nitrous motors generally prefer dual-pattern grinds because of the larger-than-normal exhaust volume they generate.

Then there's that 70-80-percent "gray" area in between. The only way to find out is to try both styles of cams, different ratio rockers, or (if the cam is mechanical) lash-loops."

Stock exhaust flows over 90% of the intake for quite a few of the same lift areas, with it only dipping under 75% at the higher lifts on the exhaust. If I do any sort of porting on the exhaust, the flow keeps climbing vs flatlining early. I still haven't found any explanation to the % ever given. I doubt anyone would want their exhaust port flow lowered compared to stock, but im wondering if that could be a good thing. I will post up the current intake flow rates on the heads im doing. Only way I could possibly get more flow out of them would be to open it up quite a bit in certain areas but I feel the velocity hit would cause more negative aspects for an NA setup.

Also, its for a nitrous setup and I noticed the copied article portion mentions more exhaust then normal with boost and nitrous. Purhaps the extra exhaust flow would help on the bottle?

On topic with the title, would the higher exhaust flow be fine with a properly ground cam? If so, id have no problem boosting the exhaust flow numbers knowing that the cam can keep things in check. Would make sense to me that it would be possible to have the exhaust valve lift closer to BDC (what larry widmer of theoldone.com talks about doing) to make full use of the combustion event.

Ill take speculation over nothing right now...as thats all I have for documented proof on % ratios right now.

[SappySE107]

RE: Head flow number vs cam shaft profile

"A head with a ratio of less than 60 percent is not a very effective air mover, particularly since it indicates that it can move air in, but not out of the cylinders. A head with a ratio of over 90 percent, although rare, is capable of extremely good power numbers so keep an eye out for it. "

ok....so how about over 100% in some areas:P Taken from

http://superchevy.com/technical/engines_drivetrain/cams_heads_valvetrain/0506sc_major/

[Guest]

RE: Head flow number vs cam shaft profile

This is one of those discussions that have as much voodoo surrounding it as cam discussions.

I could see using the cam to control flow, (hey isn't that what it's job is? ), in a combination that isn't ideal or may not be axactly what was desired.

I have not seen any data that would support a 1:1 ratio of exhaust to intake flow, either helping or not, but I believe velocity will have an effect here, on an actual running engine. Flow benchs don't make power, they just give us a guide line to go by and try and match all ports.

That being said, I'd suspect that on any power adder engine somewhere between 85 and 95% would probably be ideal, sinc there is a lot being squeezed in (in most power adder engine anyway. ), It doesn't really have the ability to have the same force behind it, or does it? How does the piston effect exhaust flow, but that adds a shit load of other variables, like piston speed, dwell time, piston accel/decell, etc all effected by stroke and rod ratios..... Hmmmmm

BTW, what's a "lash loop"?

[SappySE107]

RE: Head flow number vs cam shaft profile

I dunno what a lash loop is, but I guess some people may wanna check into it:P Yeah, i know the flowbench isn't going to tell everything. Ive preached that long enough on here and even argued with someone on the validity of my port work before the bench. I haven't really done any testing other than the flowbench though with the pushrod heads so it will be interesting to see the dyno results once the owner has it all together. Granted it will have a lot more than port work done...

It seems to me that the exhaust flow should be less at lower lift and more at mid/higher lifts to keep the intake charge with the fuel mixture from flowing out the exhaust port during overlap. Any thoughts on this?

[SappySE107]

RE: Head flow number vs cam shaft profile

http://www.ls1tech.com/forums/showthread.php?t=410676&page=1&pp=20

I posted in that thread so perhaps we will have some more information. Pretty good discussion over there so far.

[SappySE107]

Since I have cross posted this, I am posting a reply I got.

s8n on realfierotech.com
"Posted: Fri Nov 25, 2005 3:47 am Post subject:

--------------------------------------------------------------------------------

I don't know if this will help, it's taken from a Race Flow Development at LS1tech.com:
"The exhaust ports, .. we tend to look at velocity maps through the port, ..
and at the valve seat area as more important then just a cfm number
from the flow bench. In exhaust ports, cfm numbers compared to
the intake just doesn't work the same.

Two things to remember with an exhaust port, .. when the valve cracks
off the seat the air is over 600 - 700 lbs of pressure and very hot and expanding.
Also the design of the exhaust valve seat area has to help the port flow go sonic.
There's MANY more dynamics to exhaust port flow to look at."

Here's the link to the entire post:

http://www.ls1tech.com/forums/showthread.php?t=280923&page=1&pp=20

"
-------------------------------------------

Velocity has been the most important aspect on both sides of the head for this set. The flow numbers are higher than a set I did for testing that was hogged out more in some areas so I am confident that they will have good velocity on the intake side so far. Intake manifolds are also getting ported only where it helps.

Exhaust side. Other than some anti reversion measures, I don't have to hog it out either since GM did an excellent job on design. Here are the current numbers, ported intake vs stock exhaust. The exhaust didnt have a shroud around it, which I now use. Numbers don't flatline at 146, more like 153.


...................CCFM.................PERCENT OF
....Lift........No. 1......No. 2.......No. 1.......No. 2
--------......-------.....-------.....-------.......-------
0.0500.......32.1.......24.6........100.00.......7 6.64
0.1000.......58.3.......51.2........100.00.......8 7.82
0.1500.......84.2.......76.2........100.00.......9 0.50
0.2000......109.2......96.9........100.00.......88 .74
0.2500......135.0......114.5.......100.00...... 84.81
0.3000......158.4......131.2.......100.00......82. 83
0.3500......180.1......138.5.......100.00......76. 90
0.4000......199.3......140.9.......100.00......70. 70
0.4500......213.2......142.1.......100.00......66. 65
0.5000......225.5......144.7.......100.00......64. 17
0.5500......231.0......144.1.......100.00......62. 38
0.6000......231.3......146.4.......100.00......63. 29
--------....-------.......-------........-------......-------
Average:...154.8....112.6.........100.00......76.2 9


And this is compared to a ported exhaust on a different head. I will probably change the design to preserve the low lift flow numbers.

CCFM PERCENT OF
...Lift.........No. 1.....No. 2.........No. 1.....No. 2
--------.....-------.....-------.......-------.....-------
0.0500.......32.1.......22.7........100.00.......7 0.72
0.1000.......58.3.......46.5........100.00.......7 9.76
0.1500.......84.2.......77.6........100.00.......9 2.16
0.2000......109.2......104.6.......100.00.......95 .79
0.2500......135.0......124.8.......100.00.......92 .44
0.3000......158.4......138.5.......100.00.......87 .44
0.3500......180.1......151.4.......100.00.......84 .06
0.4000......199.3......164.2.......100.00.......82 .39
0.4500......213.2......171.8.......100.00.......80 .58
0.5000......225.5......174.4.......100.00.......77 .34
0.5500......231.0......177.9.......100.00.......77 .01
0.6000......231.3......180.0.......100.00.......77 .82
--------.....-------......-------.......-------.......-------
Average:...154.8.......127.9.......100.00.......83 .13

[SappySE107]

Im CCing the ports now I can also add some velocity number from the flowbench since the software provides me the info vs a cross section measurement.

All measurements are with the ports from above.

Stock Intake: 110.8cc
Ported Intake: 112 cc
Stock Exhaust: 69.6 cc
Ported Exhaust : 73.9 cc

I haven't experimented with the exhaust side for porting and flow so that measurement is just 1 example of porting and flow. Ill start measuring the valve margins and such and get some cross section measurements to post up velocity. Im pretty happy with intake cc difference vs flow though right now.