Low and medium lift Flow

[frnkeore]

We all see heads advertised and we usually focus their high lift valve flow but, should we be focusing just as much, on the low and mid lift numbers?

What does the low and mid lift flow do to the HP curve?

What will happen if your head flow peaks at .500 lift and levels off or, even decreases a little as your cam goes to .650 lift, for one or both valves?

Is there any dyno testing that address this question?

[jed]

Read the foot notes just below the Post Reply.
Then read those foot notes. When you get through reading all the foot notes your question will have Answer's.

[MadBill]

Read the foot notes just below the Post Reply.
Then read those foot notes. When you get through reading all the foot notes your question will have Answer's.

Yes, many answers and not all in agreement, but much to consider.

[vortecpro]

We all see heads advertised and we usually focus their high lift valve flow but, should we be focusing just as much, on the low and mid lift numbers?

What does the low and mid lift flow do to the HP curve?

What will happen if your head flow peaks at .500 lift and levels off or, even decreases a little as your cam goes to .650 lift, for one or both valves?

Is there any dyno testing that address this question?

You need to think about where the piston is when your trying to fill the cylinder.

[GARY C]

We all see heads advertised and we usually focus their high lift valve flow but, should we be focusing just as much, on the low and mid lift numbers?

What does the low and mid lift flow do to the HP curve?

What will happen if your head flow peaks at .500 lift and levels off or, even decreases a little as your cam goes to .650 lift, for one or both valves?

Is there any dyno testing that address this question?

You need to think about where the piston is when your trying to fill the cylinder.

I did a mock up a while back and it was interesting to see how far the valve was opened before the piston reached TDC. I wish I had the equipment and the desire to do a video on that.

[mag2555]

My question to those who have replied and to those who may still reply with knowledge, or atleast a good informed guess to the person that posted this would be this.

I know the OP of this thread is dealing with a non high performance iron head factory head, so in light of that would it not make the best sence to capitalize on the lift range where the head can be reworked to flow the best while maintaining port wall longevity, and then have a Cam ground to help out the short coming's that are left to be dealt with?

It seems that to many post talk about untimate air flow numbers and little is mentioned about for instance ram tuning at the end of the Intake cycle ( which by the way takes place at low lift!) pressure wave tuning and Exh scavenging starting off the Intake flow into the cylinder while the piston it still parked at TDC during the overlap phase.

[travis]

Like so many things related to the ICE, the correct answer would be “it depends”. I’ve read so many different discussions on the subject, and learned a ton along the way...including that there is no one size fits all answer.

What is the application and intended usage?

[frnkeore]

I read that 13 page, "Low Lift Flow and Torque" thread, although much of it was over my head.

A question was asked, at about page 5, that wasn't addressed and it apply's to my head. The question was about low lift flow, related to the ex side, only. Most of the 13 page thread, dealt with in flow and reversion problems, related to the intake, valves and port.

It seems to me, that LLF (low lift flow), should be beneficial at blow down and maybe at over lap, to savage the cyl. Mag has been helping, as much as he can, in this project (I've never ported a head) and the ex port has excellent low a mid lift numbers for a SBF.

Also I have a sim that I compared HP & TQ, using measured head flows, between two heads and the same cam and CI engine. A 385 Ford, against a FE. P51 (385) against TF, for the FE. The high flow numbers, were 384/227 vs 333/220 cfm but, the low and mid flow, was up on the TF, to somewhere between .3 & .4 lift. But, the ex numbers favored the TF heads, all the way to .6 lift. A lot at low/mid and from there on up the P51 caught up gradually at .600.

So, because I should have good LLF, on my ex ports, that is why I'm asking this question. Below is the cam events that will be used on my engine.

CC 7409 x 7411 lobes
------------------------- valve lift @
Lobe-.020-.050-.200-Lift -106° - 1.6 lift
7409 278 -252 168 .373 .118 - .597 net .580
7411 282 -256 172 .380 .126 - .608 net .591

Cam timing, 106° LSA, ICL @ 102°
24/48 x 58/18

[steve cowan]

We all see heads advertised and we usually focus their high lift valve flow but, should we be focusing just as much, on the low and mid lift numbers?

What does the low and mid lift flow do to the HP curve?

What will happen if your head flow peaks at .500 lift and levels off or, even decreases a little as your cam goes to .650 lift, for one or both valves?

Is there any dyno testing that address this question?

I have only done SBC cylinder heads but things I consider~
Shooting for little to no turbulence at all lift points at different test depressions.
Testing at as high a depression as possible, my bench will go to 45"-50 ".
Not making exhaust port to big just to chase cfm.
Valve seat and top cut angles.
Valve angles - nail head or tulip shape valves.
Trying to keep consistent CSA from throat to flange, this is difficult to measure accurately.
I have yet to see a as cast port (intake or exhaust) not have some form of turbulence, talking about 23 deg head.
In my opinion if you are Testing at low depressions you will not learn much.

[PRH]

I look at it sort of like........
Low lift flow is great....... when thats all you have to work with.

Like in many of the milder Stock Eliminator combos that are using small heads with small valves and low lift cams.
Some of those things make amazing power, all things considered.

When you don’t have high lift, you don’t worry about it.

Whether having more low lift flow in combinations that aren’t relying solely on that to make power........ helps or hurts.......is definitely one of those things that falls into the “it depends” category.

[digger]

I’d worry about the top 1/3 of the valve lift curve that’s where all the heavy hitting is done.

[ptuomov]

In my opinion, the curve that is intake flow(intake valve lift(degrees)) should be as close to rectangular as possible for the best results at the tuned rpm.

On the opening side, even if the piston is not moving down yet, a tuned header will pull the gas moving from the intake port to the combustion chamber. You either want the valve closed or as open as possible, in my opinion.

On the closing side, after the BDC all the energy is in the flowing charge and its waves that the engine can put into them. As long as the cylinder keeps filling you want the valve as open as possible, and you want it closed as quickly as possible when the gas starts flowing in the wrong direction.

This in my opinion leads to an either-or situation. You can get good results with either very high "low-lift" flow, like in a four valve head, or with no low-lift flow at all, but you can get sucky results with moderate amount of low-lift flow. And I think that you want the flow to transition as abruptly as possible from no low-lift flow at all to a lot of low-lift flow at some valve lift.

just opinions.

[David Redszus]

We all see heads advertised and we usually focus their high lift valve flow but, should we be focusing just as much, on the low and mid lift numbers?

What does the low and mid lift flow do to the HP curve?

What will happen if your head flow peaks at .500 lift and levels off or, even decreases a little as your cam goes to .650 lift, for one or both valves?

Is there any dyno testing that address this question?

Flow numbers at maximum lift have very little meaning since peak air demand occurs at maximum piston velocity.
If peak piston speed is at 74 deg ATC, then whatever valve lift is present at that crank angle will determine air flow.

On the exhaust side things are a little different. Maximum exhaust gas mass flow occurs from EVO to BDC. Therefore, low valve lit is critical. If EVO is at 75 deg BBC, then the critical valve lift area is from 0 to 75 deg.

[digger]

We all see heads advertised and we usually focus their high lift valve flow but, should we be focusing just as much, on the low and mid lift numbers?

What does the low and mid lift flow do to the HP curve?

What will happen if your head flow peaks at .500 lift and levels off or, even decreases a little as your cam goes to .650 lift, for one or both valves?

Is there any dyno testing that address this question?

Flow numbers at maximum lift have very little meaning since peak air demand occurs at maximum piston velocity.
If peak piston speed is at 74 deg ATC, then whatever valve lift is present at that crank angle will determine air flow.

On the exhaust side things are a little different. Maximum exhaust gas mass flow occurs from EVO to BDC. Therefore, low valve lit is critical. If EVO is at 75 deg BBC, then the critical valve lift area is from 0 to 75 deg.

David the intake runner tuning and lag mean that mass flow is not highest when piston speed is highest. At low rpm it might be true but at peak hp rpm the peak mass flow is not at peak piston speed

[ptuomov]

We all see heads advertised and we usually focus their high lift valve flow but, should we be focusing just as much, on the low and mid lift numbers?

What does the low and mid lift flow do to the HP curve?

What will happen if your head flow peaks at .500 lift and levels off or, even decreases a little as your cam goes to .650 lift, for one or both valves?

Is there any dyno testing that address this question?

Flow numbers at maximum lift have very little meaning since peak air demand occurs at maximum piston velocity.
If peak piston speed is at 74 deg ATC, then whatever valve lift is present at that crank angle will determine air flow.

On the exhaust side things are a little different. Maximum exhaust gas mass flow occurs from EVO to BDC. Therefore, low valve lit is critical. If EVO is at 75 deg BBC, then the critical valve lift area is from 0 to 75 deg.

David the intake runner tuning and lag mean that mass flow is not highest when piston speed is highest. At low rpm it might be true but at peak hp rpm the peak mass flow is not at peak piston speed

Digger — Should you plot the velocity or mass flow in that graph, which one is more relevant? Best, Tuomo