Don Terrill’s Speed-Talk

What an insightful bunch we were back in the day!

David Vizard wrote:
Forget RPM in a traditional sense, it's all about piston speed.Actually it is more about the rate of change of volume above the piston with respect to valve and port sizes.
DV

Probably the most important statement in the debate of relating piston speed to intake flow! The flow is related to the piston speed times the cross sectional cylinder area.

Vannik

Mid to high rpm engines depend on aggressive piston action, exhaust action, cam, port size, runner length etc. etc. etc. to generate the intake pressure magnitudes that they do.

At VERY low engine speeds it is possible to generate intake pressures of even greater magnitude than what is found on high speed engines. If you were to rely on the techniques used for mid to high rpm engines then of course the intake magnitude will be practically nothing on a VERY low speed engine, but there is another way to do it for VERY low speed engines.

Regular low engine speeds such as 2000 rpm fall in between the two extremes, so it takes a little of both techniques to make it happen. You can't paint all types of engines with the same brush.

hoffman900 wrote:Thanks, David. It all makes the discussion better. Hopefully others are learning as I've learned a lot from discussions like this.

Why, yes.

Yes we have

nitro2 wrote: ↑Tue Sep 26, 2017 10:14 pm Mid to high rpm engines depend on aggressive piston action, exhaust action, cam, port size, runner length etc. etc. etc. to generate the intake pressure magnitudes that they do.

At VERY low engine speeds it is possible to generate intake pressures of even greater magnitude than what is found on high speed engines. If you were to rely on the techniques used for mid to high rpm engines then of course the intake magnitude will be practically nothing on a VERY low speed engine, but there is another way to do it for VERY low speed engines.

Regular low engine speeds such as 2000 rpm fall in between the two extremes, so it takes a little of both techniques to make it happen. You can't paint all types of engines with the same brush.

Now here is a guy well worth listening too.
Keep it going Clint. This forum is all the better for your presence.
DV

nitro2 wrote: ↑Tue Sep 26, 2017 10:14 pm At VERY low engine speeds it is possible to generate intake pressures of even greater magnitude than what is found on high speed engines. If you were to rely on the techniques used for mid to high rpm engines then of course the intake magnitude will be practically nothing on a VERY low speed engine, but there is another way to do it for VERY low speed engines.

care to elaborate on what methods work best?

A project I am helping out a bit with Olds 500 inch alki motor. This thing has been a bear to get cfm increases. Stan upgraded the program for higher lifts. This is by far the highest port enery I have seen to date. That maybe one reason the gains were hard to find. I would like to see some more port volume but these castings are pushed now. Thanks, Charlie

DaveMcLain wrote: ↑Sun Sep 24, 2017 3:57 pm

RevTheory wrote:Thanks, Bob. Yeah, I followed that thread a couple of years ago when Darrin was talking about simply not having the piston speed to do anything with the intake tract. That isn't something I'd considered back then so it was a real eye-opener for me.

It would be nice to have at least some frame of reference, though. Like "3,600 avg. fps isn't going to cut it. 4,500 avg fps will give you something you can work with..." or whatever the numbers end up being.

I say that looking at piston speed in absolutes is nonsense, it makes no difference. Two cylinders of the same displacement, one with a large bore, short stroke and the other the opposite, with the same rod/stroke ratio pull on the port EXACTLY the same. They both sweep the cylinder and change the volume at EXACTLY the same rate. Any changes you see are all secondary effects but not caused by the speed of the piston itself.

I'm so confused... piston velocity times the diameter of the bore is literally the change in the cylinder's volume per second. How could piston speed and bore diameter not change the pressure differential vs time in the cylinder?

`

Zmechanic wrote: ↑Mon Oct 30, 2017 11:07 pm

DaveMcLain wrote: ↑Sun Sep 24, 2017 3:57 pm

RevTheory wrote:Thanks, Bob. Yeah, I followed that thread a couple of years ago when Darrin was talking about simply not having the piston speed to do anything with the intake tract. That isn't something I'd considered back then so it was a real eye-opener for me.

It would be nice to have at least some frame of reference, though. Like "3,600 avg. fps isn't going to cut it. 4,500 avg fps will give you something you can work with..." or whatever the numbers end up being.

I say that looking at piston speed in absolutes is nonsense, it makes no difference. Two cylinders of the same displacement, one with a large bore, short stroke and the other the opposite, with the same rod/stroke ratio pull on the port EXACTLY the same. They both sweep the cylinder and change the volume at EXACTLY the same rate. Any changes you see are all secondary effects but not caused by the speed of the piston itself.

I'm so confused... piston velocity times the diameter of the bore is literally the change in the cylinder's volume per second. How could piston speed and bore diameter not change the pressure differential vs time in the cylinder?

The reason why is because the pressure differential is produced by the change in the volume inside of the cylinder and that happens as the result of the displacement not the speed of the piston. If you have a larger piston that's moving slower or a smaller piston that's moving faster at the same engine RPM with the same rod/stroke ratio the results are the same as far as the induction is concerned.

DaveMcLain wrote: ↑Tue Oct 31, 2017 4:31 pm

Zmechanic wrote: ↑Mon Oct 30, 2017 11:07 pm

DaveMcLain wrote: ↑Sun Sep 24, 2017 3:57 pm

I say that looking at piston speed in absolutes is nonsense, it makes no difference. Two cylinders of the same displacement, one with a large bore, short stroke and the other the opposite, with the same rod/stroke ratio pull on the port EXACTLY the same. They both sweep the cylinder and change the volume at EXACTLY the same rate. Any changes you see are all secondary effects but not caused by the speed of the piston itself.

I'm so confused... piston velocity times the diameter of the bore is literally the change in the cylinder's volume per second. How could piston speed and bore diameter not change the pressure differential vs time in the cylinder?

The reason why is because the pressure differential is produced by the change in the volume inside of the cylinder and that happens as the result of the displacement not the speed of the piston. If you have a larger piston that's moving slower or a smaller piston that's moving faster at the same engine RPM with the same rod/stroke ratio the results are the same as far as the induction is concerned.

Ah yes, okay. I get it. My brain was NOT following for a second.

High engine VE has nothing to do with piston speed or rpm it is all about port velocity and head flow rates. The maximum rpm you can achieve for peak VE is limited by the intake valve flow rate and intake velocity and or restriction.
For example I flow benched a number of different ITB from ATPower with ram pipes of varying length. They were 36, 38, 40, 42 & 45 mm also did a BMW ITB from one of their cars it was 50 mm,
The numbers that surprised me were that the ATPower shaftless 42 mm flowed the same amount as the 50 mm BMW. The BMW air speed worked out at 83 msec and the ATpower shaftless achieved a airspeed of 116 msec. This was all done at 28 inches of flow on the bench. Provided the head port can handle it the ATPower will give a higher VE then the BMW assuming there are no other restriction in the engine somewhere.

I personally believe that given the knowledge that David Vizard has discovered and shared over the years and the advances in ability to measure data with pressure transducers etc we are more able to solve engine performance issues on paper then we could in the past. I am no convinced that for the average engine guy there are any simulators that go close to giving you accurate data. From what I have seen of the one's around such as the Lotus version it comes up with a answer but with out any real understanding of why. Equally its results is very questionable.
Tony

Charlie,

Got the dimensions and flow data for that olds so I can better see what's happening?? Send it on my personal email.

Thanks
DV

David Vizard wrote: ↑Wed Nov 01, 2017 2:33 pm Charlie,

Got the dimensions and flow data for that olds so I can better see what's happening?? Send it on my personal email.

Thanks
DV

Permission to share with you obtained and multiple emails sent. Info has not been released about these heads to the public so please do not post info that may be deemed "sensitive". Looking forward to your input! Thanks, Charlie

DaveMcLain wrote: ↑Tue Oct 31, 2017 4:31 pm

Zmechanic wrote: ↑Mon Oct 30, 2017 11:07 pm

DaveMcLain wrote: ↑Sun Sep 24, 2017 3:57 pm

I say that looking at piston speed in absolutes is nonsense, it makes no difference. Two cylinders of the same displacement, one with a large bore, short stroke and the other the opposite, with the same rod/stroke ratio pull on the port EXACTLY the same. They both sweep the cylinder and change the volume at EXACTLY the same rate. Any changes you see are all secondary effects but not caused by the speed of the piston itself.

I'm so confused... piston velocity times the diameter of the bore is literally the change in the cylinder's volume per second. How could piston speed and bore diameter not change the pressure differential vs time in the cylinder?

The reason why is because the pressure differential is produced by the change in the volume inside of the cylinder and that happens as the result of the displacement not the speed of the piston. If you have a larger piston that's moving slower or a smaller piston that's moving faster at the same engine RPM with the same rod/stroke ratio the results are the same as far as the induction is concerned.

Old topic, but interesting stuff.

Where the velocity comes maybe better into equation is, when higher piston speed changes exhaust and further affects intake via valve overlap.

Piston speed matters, just like bigger exhaust works better on higher rpm right?



-juhana