Max RPM for Head Flow

[KnightEngines]

We know that you can open the intake valve before TDC and we know that a collector extension alone can be worth in the neighborhood of 60 ft lbs below tq peak so overlap is clearly important but would an actual # make a difference?

Bingo.

[groberts101]

Then add the simple fact that testing cylinder heads at higher depressions than is the "industry norm" almost always results in improved power output. In at least one aspect, it forces greater idealization of short turns design and overall efficiency by illuminating issues that lower test depressions will blindfold the test equipment from "seeing and reporting". In fact, I don't EVER recall seeing, hearing, or even reading about any intake port tested @10"(and then converted to 28") beating another which was tested and refined @28". There are of course going to be diminishing returns but I have met and talked with many pro's who will tell you that it can be an exceptional and eye opening learning experience to test at much higher depressions. This has been well known for a very long time.

If that wasn't actually the case?.. there would likely be far fewer resources being expended/consumed to build more capable test equipment and we wouldn't even have some of the cylinder head designs we see today. Every average Joe would have shop vac assisted 10" benches churning out nearer to world class cylinder head designs from their garage.

I also think Tony nailed it too. Sensor speed/resolution and specific/multiplied testing locations would be key to sussing out pressure data that could then be interpreted towards further port refinement/capability. You don't need to fully understand the physics behind it to see and follow the glaringly obvious design trends.

[twl]

I would like to point out that virtually no cylinder filling occurs during overlap. There in a small chamber with open valves on each end. The only place the mixture can go is out the exhaust pipe.

Additionally, the valves are not open much at all during overlap, in most cases. So there can be only extremely small actual flow happening. There is no way that the engine is flowing on the ports with the valves only just cracked open.

It seems apparent that the purpose of overlap is to scavenge the chamber, and to get a strong flow signal into the intake tract, so that flow can get going strong early in the intake cycle. How well it accomplishes these tasks would seem to be reflected by increased power production.

I feel that 28" flow testing a valid test regime, because this is an approximation of conditions with the valves more open, and the intake is flowing more on the port, instead of choked off by the barely open valve. This is not to say that testing at higher depressions is not valid, because I think it can help expose some flow problems that are harder to detect at 28".

I'm in favor of all testing, and using valuable data. However, I try not to lean too hard on any single testing regime alone. I try to keep the big picture in mind.

Just my 2 cents.

[groberts101]

I would like to point out that virtually no cylinder filling occurs during overlap. There in a small chamber with open valves on each end. The only place the mixture can go is out the exhaust pipe.

Additionally, the valves are not open much at all during overlap, in most cases. So there can be only extremely small actual flow happening. There is no way that the engine is flowing on the ports with the valves only just cracked open.

It seems apparent that the purpose of overlap is to scavenge the chamber, and to get a strong flow signal into the intake tract, so that flow can get going strong early in the intake cycle. How well it accomplishes these tasks would seem to be reflected by increased power production.

I feel that 28" flow testing a valid test regime, because this is an approximation of conditions with the valves more open, and the intake is flowing more on the port, instead of choked off by the barely open valve. This is not to say that testing at higher depressions is not valid, because I think it can help expose some flow problems that are harder to detect at 28".

I'm in favor of all testing, and using valuable data. However, I try not to lean too hard on any single testing regime alone. I try to keep the big picture in mind.

Just my 2 cents.

Maybe those things are true on a less developed street engine with full exhaust.. but that is certainly not the case on a well developed considerably higher rpm powerplant. If you're basically saying that additional intake port pressures created via inertial ramming, and don't forget pressure build up in the enlarged bowl area, is bending 180° and going straight out the exhaust only?.. that is entirely incorrect. Sure much of any additional flow may be lost out the exhaust in the process of trying to cheat more air in with inertial ram/exhaust scavenge.. but that's just the price we pay to get a few more grams of mass packed into the cylinder before IVC.

And if you do the math for available cross flow real estate in a bigger valved/high duration cam?.. that amount of opening is not insignificant at all. Look at what that opening does to a carburetors metering when pressures are greatly reduced(figuratively speaking) at idle. What do you think could happen as port pressures rise/fall considerably in both induction and exhaust at much higher rpm?

Well developed motors spit fire out the exhaust for a very good reason. They literally waste fuel to the point that the inducted gains are well worth the fuel efficiency losses.

[Stan Weiss]

I do sometimes wish that polls could be done on this forum. Because I believe that here like on most forums the street / strip type engine builds out number the NASCAR type engine builds by a huge amount.

Stan

[hoffman900]

Physics is physics.

I agree with Tom's post.

[Stan Weiss]

Physics is physics.

I agree with Tom's post.

Bob,
That is true.

But how well / long will that NASCAR engine be with a full street legal exhaust going from stop light to stop light in center city Phila with the AC on in the middle of the summer?

Stan

[hoffman900]

Physics is physics.

I agree with Tom's post.

Bob,
That is true.

But how well / long will that NASCAR engine be with a full street legal exhaust going from stop light to stop light in center city Phila with the AC on in the middle of the summer?

Stan

It's Speedtalk, not Parktalk!

In all seriousness, I think most on here would gain more from mitigating the effects of reversion than anything. Actually even the serious builds would too, but that's Calvin's department.

[Fastvette94]

I would like to point out that virtually no cylinder filling occurs during overlap. There in a small chamber with open valves on each end. The only place the mixture can go is out the exhaust pipe.

Additionally, the valves are not open much at all during overlap, in most cases. So there can be only extremely small actual flow happening. There is no way that the engine is flowing on the ports with the valves only just cracked open.

It seems apparent that the purpose of overlap is to scavenge the chamber, and to get a strong flow signal into the intake tract, so that flow can get going strong early in the intake cycle. How well it accomplishes these tasks would seem to be reflected by increased power production.

I feel that 28" flow testing a valid test regime, because this is an approximation of conditions with the valves more open, and the intake is flowing more on the port, instead of choked off by the barely open valve. This is not to say that testing at higher depressions is not valid, because I think it can help expose some flow problems that are harder to detect at 28".

I'm in favor of all testing, and using valuable data. However, I try not to lean too hard on any single testing regime alone. I try to keep the big picture in mind.

Just my 2 cents.

This link of my simulation shows significant intake flow/cylinder filling during overlap
https://youtu.be/KR6SCnZxby8

Left side is intake port.

[GARY C]

Actual video inside combustion chamber, not a simulation! https://www.youtube.com/watch?v=sEf8va1S7Sw

Reduce the speed as you watch the video of air rushing in while piston sits still. one thing you will notice at max lift is that the wet fuel drops become bigger and more evident.

[twl]

I would like to point out that virtually no cylinder filling occurs during overlap. There in a small chamber with open valves on each end. The only place the mixture can go is out the exhaust pipe.

Additionally, the valves are not open much at all during overlap, in most cases. So there can be only extremely small actual flow happening. There is no way that the engine is flowing on the ports with the valves only just cracked open.

It seems apparent that the purpose of overlap is to scavenge the chamber, and to get a strong flow signal into the intake tract, so that flow can get going strong early in the intake cycle. How well it accomplishes these tasks would seem to be reflected by increased power production.

I feel that 28" flow testing a valid test regime, because this is an approximation of conditions with the valves more open, and the intake is flowing more on the port, instead of choked off by the barely open valve. This is not to say that testing at higher depressions is not valid, because I think it can help expose some flow problems that are harder to detect at 28".

I'm in favor of all testing, and using valuable data. However, I try not to lean too hard on any single testing regime alone. I try to keep the big picture in mind.

Just my 2 cents.

This link of my simulation shows significant intake flow/cylinder filling during overlap
https://youtu.be/KR6SCnZxby8

Left side is intake port.

Not saying that there is no flow.
Just saying that the cylinder filling is limited to the capacity of the combuston chamber, and perhaps some small amount of cylinder volume present at the top of the stroke. And the flow is limited by the relatively small valve lift around TDC.

Yes, certainly with very long overlap periods there can be more cylinder volume exposed for filling after TDC.
However, this will be somewhat off-set by the losses from the very powerful depression in the exhaust tract until the exhaust valve closes, and actual gas trapping can begin.

My point is that cylinder filling is limited by available volume around TDC, and flow is very limited by small valve openings around TDC, so after scavenging is complete, the main purpose of overlap is to send the flow signal up the intake so that the intake flow starts as rapidly as possible when the piston is descending(giving more cylinder volume), and valves are further open (permitting more flow).

The pressure traces clearly show me that the optimum exhaust valve closing point is when the exhaust depression weakens and the piston descent takes over, and real cylinder fill/trap begins. Not implying that wave effects and exhaust depressions are unimportant.

[Stan Weiss]

I would like to point out that virtually no cylinder filling occurs during overlap. There in a small chamber with open valves on each end. The only place the mixture can go is out the exhaust pipe.

Additionally, the valves are not open much at all during overlap, in most cases. So there can be only extremely small actual flow happening. There is no way that the engine is flowing on the ports with the valves only just cracked open.

It seems apparent that the purpose of overlap is to scavenge the chamber, and to get a strong flow signal into the intake tract, so that flow can get going strong early in the intake cycle. How well it accomplishes these tasks would seem to be reflected by increased power production.

I feel that 28" flow testing a valid test regime, because this is an approximation of conditions with the valves more open, and the intake is flowing more on the port, instead of choked off by the barely open valve. This is not to say that testing at higher depressions is not valid, because I think it can help expose some flow problems that are harder to detect at 28".

I'm in favor of all testing, and using valuable data. However, I try not to lean too hard on any single testing regime alone. I try to keep the big picture in mind.

Just my 2 cents.

This link of my simulation shows significant intake flow/cylinder filling during overlap
https://youtu.be/KR6SCnZxby8

Left side is intake port.

Not saying that there is no flow.
Just saying that the cylinder filling is limited to the capacity of the combuston chamber, and perhaps some small amount of cylinder volume present at the top of the stroke. And the flow is limited by the relatively small valve lift around TDC.

Yes, certainly with very long overlap periods there can be more cylinder volume exposed for filling after TDC.
However, this will be somewhat off-set by the losses from the very powerful depression in the exhaust tract until the exhaust valve closes, and actual gas trapping can begin.

My point is that cylinder filling is limited by available volume around TDC, and flow is very limited by small valve openings around TDC, so after scavenging is complete, the main purpose of overlap is to send the flow signal up the intake so that the intake flow starts as rapidly as possible when the piston is descending(giving more cylinder volume), and valves are further open (permitting more flow).

The pressure traces clearly show me that the optimum exhaust valve closing point is when the exhaust depression weakens and the piston descent takes over, and real cylinder fill/trap begins. Not implying that wave effects and exhaust depressions are unimportant.

Since we are talking about dream builds for most people and not realistic builds.

Let look at a Comp Cams DRS Drag Roller

The DRS Drag Rollers are the "evil cousin" of the DR drags. These are designed for very stiff valve train systems incorporating
60mm or larger cam journals. The major intensity of these are less than 30°, and these lobes provide more area under the curve
than any previous designs.

Code: Select all

Camshaft Type Lobe # Rated Duration Duration in Degrees Lobe Lift Tappet Lift @ TDC Theoretical Valve Lift @ "0" Lash Rocker Arm Ratio
_____________________________________@ .050 @ .200____________________106°__110°______________________1.8______1.9_______2.0

1982____________________ 320___________ 291___ 217_______.585________.213___.192_____________________1.053____1.112_____1.170

► .192 * 2 = .384 - lash = lift @TDC

Should have flow at that lift.

Stan

[statsystems]

Seems Isky used to print the VOTDC in their catalogs.

I actually measured my cam and without pulling the file, IIRC VOTDC was .155 at the lifter so with a 1.6 rocker that would be .248 minus lash of .014 so .234 intake valve opening at TDC. Seems a bit big for a street strip deal.

I know what my heads flow around that lift, and I'm damn glad I uses a 50* seat.

[Fastvette94]

Yes the overlap is there to take advantage of the exhaust stream rapidly moving away from the cylinder, thus giving the intake a major head start.

At TDC the int/ex valve lift is at .283/.197 respective. The intake flow is already at 210cfm (64grams per second) and exhaust is flowing -248CFM (-39 grams per second). The outbound exhaust column is providing the energy to setup a strong intake flow even before the piston moves downward.

22 degrees later the lift is .435/.049". The intake flow is now 293cfm (113gps) and exhaust -48cfm (-10gps). Again, the exhaust did most of the work to set this up.

BTW: cam specs for this is .688/.648 100 CL 104LS 261/262 degrees at .05"(from a past thread on a forum).

Let me qualify this that this is a single cylinder simulation (no header collector interactions)

[hoffman900]

I'll post graphs from EngMod4t showing pressure and mass flow traces as well as Cam Doctor data later..