Is there a limit to head flow

[RL]

It was interesting how the 4 cam grinders had 4 different methods for working out cams, but if you raced the 4 cams in 4 cars I wouldn't be supprised if they were all very close

I think there are too many people thinking in black and white

I wondering if port velocity comes into cam selection

I noticed Larrys work on CI SBC ,041, 462 castings, he made 581HP. From memory the ports were very fast , and the cam had a lot of duration(280º) for 7300rpm. It seems you can get away with more duration if the port velocity if fast, maybe it's a time area problem, maybe the reversion can't over come the velocity. Then compare the cam to a largeish slower port, would the cam have less duration more lift

I wondering if the flow at low lifts could affect the LSA also

[Bad95Gts]

You need to be very careful to differentiate
between Theoretical Calculated Velocity
and measured Velocity.

I agree.

Too great a depression = equals greater pumping losses
too little not enough velocity and ram effect , VE% suffers.

I really like this analogy.
Very easy to understand.

I guess the tough part is determining where both sides of this spectrum are.
Port velocity (measured and calculated) and CSA is spoken of so much that it tends to be a benchmark most use to determine the engines operating range with very little mention to valve size.

i can re-Post the Dyno results if you want ?

I would like to see the dyno sheet.
I spent several days dynoing that same class engine for a customer testing parts.
That particular engine was able to still have 511 hp at 8500 rpm and the peak was around 575 at 6900/7000 rpm.
These engines see some pretty steep port velocities.
Super Stock engines are very interesting.

[CamKing]

I wondering if port velocity comes into cam selection

It is very much a part of my calculations.

Then compare the cam to a largeish slower port, would the cam have less duration more lift

That's exactly what we have to do with the sprint cars that run too big of a port. We decrease the duration, and increase the lift.

[GregGood]

So both pairs of Heads with the 2.000 and the 2.500
have the same 276 FPS Velocity and CFM/SqInch
yet at 5930 RPM the 2.500 Valve Head w/564.5 CFM will be too much Flow
and not cause enough Cylinder depression soon enough
and low enough...to create the necessary ram effect
at the intake valve closing point.



Too great a depression = equals greater pumping losses
too little not enough velocity and ram effect , VE% suffers.

Excellent post, and one that everyone needs to understand and put in their memory bank. Too much flow early in the intake stroke can hurt you sometimes. It's easy to get into that situation today with some of the heads that have very large intake valves.

[MaxFlow]

That was an excellent post from Larry. Its a simple concept that's hard to nail down. I've seen several engine builders lost in this area. I guess that's why there's head guys and there's engine guys.

"Flow Saturation" nice term.

[randy331]

At what crank deg. after TDC do we want to quit limiting flow?
Just saying not a lot of low lift flow wouldn't be acurate, since the valve lift at TDC varys with different cam durations, and aggressiveness, so there must be a point ATDC on the intake stroke that you want to stop limiting flow, and start gaining flow quickly.
I'm guessing, but this point seems piston position related, not valve lift related. (ie not good flow after X amount of valve lift, but good flow after X deg. ATDC, or good flow after the piston reaches X % of it's max piston speed at peak power, or maybe peak TQ)

So at what deg. ATDC do we want to start having good flow?

Randy

[jeffmckc]

Randy in the pipe max program it shows at what angle the crank is at and cfm @28 how much is needed

[Stan Weiss]

This is what I get for the same engine (bore, stroke, rod length) but at different RPM

[MadBill]

At what crank deg. after TDC do we want to quit limiting flow?
Just saying not a lot of low lift flow wouldn't be acurate, since the valve lift at TDC varys with different cam durations, and aggressiveness, so there must be a point ATDC on the intake stroke that you want to stop limiting flow, and start gaining flow quickly.
I'm guessing, but this point seems piston position related, not valve lift related. (ie not good flow after X amount of valve lift, but good flow after X deg. ATDC, or good flow after the piston reaches X % of it's max piston speed at peak power, or maybe peak TQ)

So at what deg. ATDC do we want to start having good flow?

Randy

Just want to point out that anything that beneficially limits low flow on the opening side of the intake event will do so during the 'ramming' phase on the closing side as well, where it may or may not be a plus, depending on IVC timing, closing rate, etc...

[randy331]

Randy in the pipe max program it shows at what angle the crank is at and cfm @28 how much is needed

I know, I have pipemax. What I was asking is how far behind piston cfm demand, and to what piston position/piston speed, do you want to stay to have improved cylinder filling at, and after BDC.

Randy

[randy331]

Just want to point out that anything that beneficially limits low flow on the opening side of the intake event will do so during the 'ramming' phase on the closing side as well, where it may or may not be a plus, depending on IVC timing, closing rate, etc...

So we need to desighn a valve with a retractable 30* undercut, not there when the valve is opening, but shows up while the valve is closing?

Randy

[Troy Patterson]

MadBill made a great point regarding designing a port to reduce low lift flow also affecting high rpm low lift flow on the closing side of the intake event.

But let's add some other variables because so far, it seems the discussion has assumed "generic" valve jobs with predetermined - common flow rates / values and "generic" induction systems, exhaust systems and so forth. Perhaps I missed part of the discussion - I apologize if I did.

Take an optimal cam for a given application but fairly conventionally, change one part of the build, say the valve job and now may have increased or decreased "effective" overlap. Not the measured at the cam / valve duration of overlap, but both the duration of time and volume of measurable and meaningful flow while both intake and exhaust valves are open. What to do then? I don't have Pipemax yet, I'm sure it's a great program, but I haven't seen in this conversation acknowledgment or compensation for such variables.

Other examples would be increasing the size of the carburetor / induction to reduce the vacuum seen in the intake track, changing the fuel curve to alter combustion efficiency thereby changing the burn rate therefore cylinder / exhaust pressure / flow rate / reflection waves - inertial scavenging, etc., etc.

All of these things have a very real effect on "actual" flow therefore effectively increasing or decreasing the scavenging of the cylinder / draw on the intake track during the overlap cycle. These things necessitate changing the designed overlap of the cam accordingly.

Troy Patterson TMPCarbs.net TMP Carbs

[RL]

Lets say I have a cam with 280º@0.050" but my valves have very bad low lift flow is my EFFECTIVE flow duration lower eg 260º. Then again , the piston plays into the effective flow duration

[GregGood]

Just want to point out that anything that beneficially limits low flow on the opening side of the intake event will do so during the 'ramming' phase on the closing side as well, where it may or may not be a plus, depending on IVC timing, closing rate, etc...

True, but if you fail to get the air column moving all the mid and low lift flow in the world won't fill the cylinder on the back end of the event.

[Bad95Gts]

Having a port that is large enough to allow complete filling before BDC negates the loss of poor low lift flow numbers during the ram effect filling after BDC.
The shorter period the intake valve is open coupled with the highest volume of air moved into the cylinder(during that time event) creates a very high inertial charge which helps in residual filling after BDC even though the initial calculated port velocity may be too low for efficient filling.