500-540 ft/sec is subsonic, isn't it?

 

Mach 1 (speed of sound) is about 1100 ft/sec at sea level and standard temp, so this is less than M 0.5.

 

Max port velocity in the cup engines is reaching 600ft/sec.
That's where we run into problems. The longer the rod, the lower the max port velocity with the same port.

 

Are you sure?
What's the barometric pressure and temp in the port?

 

Yep but the ports in Cup can be made bigger and slowed down if desired and the rods and decks are shorter than ever. They must "like" having this velocity and port size with the rpm they are turning or they would use even larger heads or run the tallest decks possible which they do not. You are totally right about the max instantaneous port speed being higher with the shorter rods of course but people still aren't running heads as big as they can even now. There are Cup heads that are bigger than what is commonly run and flow 450+ cfm but almost no one uses them at this point since they can turn "only" 10,000 rpm. If you are stuck with an insufficient head then the longer rod might help you but that's the only place it seems to help pushrod stuff as everything else gets worse with longer rods.

 

What about IHRA PS which makes peak power at around 6750 FPM average piston speed and turn over 7500 FPM!

They (IHRA PS), make peak power at 6600-6900 FPM mean piston speed with a 1.35-ish to 1 R/S ratio and turn up to 7500 FPM max piston speed which is what you are listing up there.

Their stroke is nearly 6 inches or about 5.7 inches on average so their rpm is almost the same as their piston speed in FPM!

Since they turn much lower rpm they can run quite a bit crazier cam profiles of course which allows the same heads to breath at higher pistons speeds than if this same piston speed was gained with RPM.

Kind of gives you a pretty good window into what is limiting airflow more in these super high rpm engines, valvetrain or ports!

 

I believe density and pressure are a linear dependency and cancel each other out. If the temperature is 200°F, the speed of sound would be even higher..around 1300-1400 ft/sec.

 

Pressure has no effect on the Speed of Sound; only temperature.

Often the charge temp in NA inlet runners is less than ambient, especially after the fuel is added.

600 fps is near M 0.55 which seems to be where flow gets into trouble in an engine's inlet ports.

 

How would that be choke flow?

 

How would what?

 

Restriction caused by the flow going sonic. From Wikipedia:

 

Only in an ideal gas like air.
When you add fuel, things change. The density and the pressure no longer cancel each other out.
The air is elastic, the fuel is not.


κ is the adiabatic index also known as the isentropic expansion factor and sometimes called γ (Greek letter gamma). It is the ratio of constant-pressure to constant-volume heat capacities of the gas (Cp / Cv), and arises because a classical sound wave induces an adiabatic compression, in which the heat of the compression does not have enough time to escape the pressure pulse, and thus contributes to the pressure induced by the compression.
p is the pressure.

 

Is this a good time to discuss flow separation?

 

Probably not. However, this is an interesting discussion on the long rod's affect on airflow.

 

Do you have a reference to an article about the speed of sound in a gas with a liquid in solution?

The range of gamma for this still seems to be in the range of 1.3-1.4,
which should be less than 5%.

 

No, but let me see what I can find.
A friend of mine that helps me in this area is a lead engineer on the Scramjet project.
We were just talking about how the pressure differentials of both sides of an orifice change the choke flow of the orifice.

 

Camking, I know you are in the business for real so I know you already know the answer to this question but I will ask it to you anyway since on this board and others there is sometimes still some argument about it at times.

What I am saying is that valvetrain is what is limiting 99 percent of the super high RPM pushrod engines such as Cup or PS right now. It limits reliability, it limits power and it limits airflow. Right now we have heads in PS and Cup that are not even run although they are bigger and better but the valvetrain and valve sizes and mass right now limit us in RPM more than the heads and their airflow do. The cam and valvetrain are now more important than anything else in the quest for more power through RPM.

You could lame out the lift and intensity of the cam but then power will fall even as rpm rises which means you are slower. If you run larger duration tamer lobes you start inceasing overlap and bleeding off compression on intake closing too. You also will increase reversion so again you will lose power by simply running longer durations with lower lift if you are trying to make good VE. Anyone can turn more rpm by running a lower lift cam with a lot tamer lobe and 600 PSI seat pressure and turn rpm to the moon but the power will plummet whether you are turning more rpm or not because the airflow has too.

You could run bigger heads with bigger ports but that means bigger valves as well which are heavier and must be opened further so again the cam must be appropriately more intense at the same rpm to get this bigger valve open and open further as well if you want to fully utilize these new heads and really make more power. The problem though is as valves get bigger they also get heavier. This means that if you were already on the edge of valvetrain control you will now lose valvetrain control at a lower rpm due to the newer but larger and also higher mass intake valve.

So at some point you are stuck at how big a head to run VS. how radical you can keep the cam and the valvetrain and hold it all together. It's basically a compromise between all these things. Almost anything you do to make more power through rpm also limits your RPM. This is why there is always a limit to the rpm that these engines seem to turn and make good power at. Right now the valvetrain is where that limit is at.

 

Erik, I thought we were focused on the affects of rod/stroke ratio on flow, rather than valvetrain or porting.

This section of the thread really got started when CamKing indicated that the rod/stroke ratio has an affect that is compenstated for with valve timing. Specifically, that the (slightly) higher peak piston speeds from a lower rod/stroke ratio could cause choked flow in high rpm engines. That a higher rod/stroke ratio reduced this at the expense of lower end torque. And that a reduction in IVC compenstated for that.

This led to a discussion of whether the intake port or curtain area had sonic flow, a requirement for choked flow.

Are you saying the choked flow is past the valve at low lift? The peak piston speed occurs close to peak valve lift.

 

David,

The problem is we can agree here on how the world works.... We are talking about pressure here, but everyone seems to think that velocity of the port is directly linked to the velocity of the piston. The pressure differential between the port and the cylinder is what matters, but everyone thinks that air/fuel is some magical string that's attached to the top of the piston and when the piston goes faster so does the air/fuel charge. Everything has inertia and the higher you spin the motor the bigger gap between peak piston velocity and peak port velocity.... I think Adrenaline Z entered this "Is this a good time to discuss flow separation?" at the right time in the discussion since the flow separation of air and fuel is one of the many things that causes the limiting port velocity of most ports. Things are just going too fast for the fuel to make the turn with the air. This is considered the choke point of the motor... you can cause this to happen with a lot of changes to the system, the rod length (not rod stroke ratio) might have enough impact on this IF you have a perfectly matched system to start with. For what we are talking about most times that's far from whats going on.

Eric,

There is some odd ball thing that is happening at very high piston speeds 5500ft/sec+ but the answers I have heard about that are more akin to walking in the house, seeing a pile of dog **** on the carpet and then yelling at one of the many dogs and sending him outside. Obviously it was one of the dogs that put the pile there but I think that they are yelling at the wrong dog. If I had a answer for this that I thought was satisfactory I would go with it, but I don't and from what I have seen nobody else does either. We are here to explain how the world works not tell it how it works, mother nature don't care what we have to say.

As for the RPM limit, I think you hit the nail on the head more here than most times.... IT'S EVERYTHING! Once you change one part of a motor, everything else changes. If you are not pushing everything to the limit you are not doing it right and that's what we have been "discussing" between us for years now. I think we can agree it's not just one thing that limits these motors we discuss.

That's why this rod length question is retarded.... how can anyone say that just the rod length change in a motor is causing something to happen? What else did you have to change to get the new rod in the motor? The piston, the ring pack, the deck height, now add on all the other things that have to change with every one of those changes.... maybe a different shaped piston skirt design, maybe a different gas port arrangement or different scavenging system. If you changed the deck height you would have so many variables in your hands it would be crazy. Nothing is a stand alone specification in the motor. So these discussions are retarded, and nobody knows the real answer. Maybe one combination worked better than another but to pin point it to something like rod length is crazy... yeah theoretically it can cause certain things relative to port size etc... but by it self rod lengths effect on power is minimal. That is by far not even the sratching of the surface on this topic.

Camking,

First off, ya gotta start writing for the masses a little better. When the people on here who are smart enough to understand this take a few times of reading it to understand it, you don't have a clue who you are really writing for on here.

"κ is the adiabatic index also known as the isentropic expansion factor and sometimes called γ (Greek letter gamma). It is the ratio of constant-pressure to constant-volume heat capacities of the gas (Cp / Cv), and arises because a classical sound wave induces an adiabatic compression, in which the heat of the compression does not have enough time to escape the pressure pulse, and thus contributes to the pressure induced by the compression. p is the pressure."

FWIW, were are not concerned with a sound wave causing adiabatic compression, it's a pressure wave, but for some reason you keep talking about the pressure and not the sound wave. So obviously you get it, but it's kind of like talking in different tenses in the same sentence.

Maybe using a link would work that much better... http://en.wikipedia.org/wiki/Adiabatic_index

Anyways... this idea of the speed of sound and the pressure wave I think are all getting confused here. The pressure wave is not moving at the speed of sound ( I could be wrong here ), and the air/fuel certainly isin't. The theory that the pressure waves compression of the charge increases the temp because the volume can't change and react seems off to me. The volume of charge can move so the temp change doesn't happen as much, it's more likely that the pressure increases. IF the temp does increase then after it enters the cylinder and expands it should cool off so it's basically null.

Bret

 

OK, let me put it so the masses can understand.
It has been proven over and over again that the rod/stroke ratio effects the performance of the engine, and that a 2:1 rod to stroke ratio is the most efficient for an engine turning over 4,500 ft/min piston speed.
If you take 2 engines where everything is the same except the rod length and the pin height, the longer rod engine will lose a little torque, gain a little HP, and hold the power for more RPM's.

If you take two 355ci SB Chevy's, one with a 5.7 rod, and one with a 6.125 rod. The one with the 6.125 rod can run a 4 degree shorter cam on a 2 degree tighter lobe center and still make more HP, and carry it further.

Remember, air is elastic and it flows toward the lowest pressure point. Changing the rod/stroke ratio changes the pressures in the cyl.
A shorter rod/stroke reaches max piston speed in fewer degrees then a longer rod/stroke does. A 2:1 rod/stroke ratio reaches max piston speed close to 90 from TDC. This causes a more even pull on the intake.