General Cylinder Head Flow Numbers discussion

[SSDiv6]

Originally Posted by HawkBrosMav

Let me add a little more specific question to this:

.444 valve lift cam.

Lift -------- Head A -------------- Head B

.050 -------- 50.8 -------------- 35.1
.100 -------- 81.0 -------------- 57.6
.150 -------- 108 --------------- 88.2
.200 -------- 129.9 ------------ 117.12
.250 -------- 142.3 ------------ 136.6
.300 -------- 157.0 ------------ 158.0
.350 -------- 161.8 ------------ 172.6
.400 -------- 166.8 ------------ 175.6
.450 --------- 166.4 ----------- 175.6
.500---------- 165.8 ----------- 174.5

Which head would you rather have? Head B is a bit better up top but Head A is substantially better down low. I would still say looking at it from an area under curve stand point that Head B would be slightly larger but very small difference.

You need to take into consideration the intake manifold and throttle body flow also. No matter how much air flow you have on the cylinder heads, the intake manifold and throttle body/carburetor, can become a restriction.

[FED 387]

exhaust port flow is going to come into play in this too

[Dissident]

When doing comparative testing, it is imperative to use the same radius inlet guide (not rolled up clay method) as the specific entry has influence on results. On the exhaust side, one should always use a short length of pipe for the same reason.

The methodology of area under the curve is best evaluated by graphics and using the cfm/sq in rating. One should have a graph of the cam / valve lift for that evaluation as well. It is in these types of details that gains are sqeezed out of otherwise run of the mill components.

After the heads are evaluated, then one can begin sorting out manifolds and carbs and placement of same.
Notes, lots of notes so you can take a look at what items worked the best. This is particularly important when you are evaluating valve jobs.
Lots of patience, study, and thoughts with coffee will help get the job done.

Regards,
HB2
Dissident

[HawkBrosMav]

Originally Posted by Dissident

When doing comparative testing, it is imperative to use the same radius inlet guide (not rolled up clay method) as the specific entry has influence on results. On the exhaust side, one should always use a short length of pipe for the same reason.

The methodology of area under the curve is best evaluated by graphics and using the cfm/sq in rating. One should have a graph of the cam / valve lift for that evaluation as well. It is in these types of details that gains are sqeezed out of otherwise run of the mill components.

After the heads are evaluated, then one can begin sorting out manifolds and carbs and placement of same.
Notes, lots of notes so you can take a look at what items worked the best. This is particularly important when you are evaluating valve jobs.
Lots of patience, study, and thoughts with coffee will help get the job done.

Regards,
HB2
Dissident

I'm playing with those curves now. That is one reason I am asking the questions I'm asking. Trying to determine if area under the curve if just area under the curve and doesn't matter where it is.. obviously where the curve moves will determine other pieces like cam and timing events, possibly converter as well and potentially effect header lengths and sizes, but I have to choose a set of heads first to match those things to the flow curve.. If area doesn't matter as long as the other parts are matched well to the system as a whole... great.. if having the flow low but giving up some up top is better.... great... if wanting the great numbers around peak lift while giving some up early is best... great.. just thought I'd pick the brains of others that have had to think about this stuff before I did.

[Billy Nees]

Velocity is your friend.

[SSDiv6]

Originally Posted by Billy Nees

Velocity is your friend.

Correct.

[Brad Hawk]

I don't know how much of their experience comes from working w/ lower-lift Stocker heads, but cylinder head guys like Darin Morgan and Chad Speier emphasize port designs that remain stable w/o any backing up in flow at lifts well above the actual valve lift used, even if it reduces some of the flow #s seen at lower lifts.

However, those recommendations are when they're talking about ports remaining stable to 1.0" when the valve lift might be .700". They're saying this, IIRC, based on findings from flow testing well above the standard 28" H2O pressure since the higher test depressions can reveal different flow behavior than seen at 28". How that might need to be "adjusted" (as much as can be w/in the constraints of the class guidelines) when the peak lift is .400"-.500", I dunno... just throwing that out there for consideration. Thx

[HawkBrosMav]

Originally Posted by SSDiv6

You need to take into consideration the intake manifold and throttle body flow also. No matter how much air flow you have on the cylinder heads, the intake manifold and throttle body/carburetor, can become a restriction.

I get that, but would you run a set of heads you knew flowed less air intentionally knowing the TB and intake may be a restriction larger than the heads or would you put the "best" flowing heads on regardless?

I think I'm more curious as to what people are more concerned with if area under the curve is similar between 2 sets of heads. Having the larger peak numbers or giving up a little up top to have much better down low numbers. Obviously there are other factors to consider... mainly possible restrictions before the port and then the relation to exhaust port flow as mentioned in the next reply. But at this point for a simplistic questions and answer.. would you prefer a head that peak flows more or flows down low more given the overall "area under the flow curve" is generally equal.

Billy seems to lean towards low flow potentially with his response.

[SSDiv6]

Originally Posted by HawkBrosMav

I get that, but would you run a set of heads you knew flowed less air intentionally knowing the TB and intake may be a restriction larger than the heads or would you put the "best" flowing heads on regardless?

I think I'm more curious as to what people are more concerned with if area under the curve is similar between 2 sets of heads. Having the larger peak numbers or giving up a little up top to have much better down low numbers. Obviously there are other factors to consider... mainly possible restrictions before the port and then the relation to exhaust port flow as mentioned in the next reply. But at this point for a simplistic questions and answer.. would you prefer a head that peak flows more or flows down low more given the overall "area under the flow curve" is generally equal.

Billy seems to lean towards low flow potentially with his response.

I always flow the intake manifold first and pick the best. Many years ago I flowed lots of Ford 5.0 intakes and the biggest variation was the lower manifold. As I recall, there are 4 different lowers from different foundries with the same part number and the difference was the last revision letter on the casting number. I believe I flowed around 22 lower intakes and picked the best 4 and the remainder I took to the aluminum recycler.

Having the baseline number from the intake allowed me to then work on the heads, knowing the Delta the intake would effect on the cylinder heads. The brand of valves makes a difference the same as the cut and angles.