Static Compression VS Dynamic compression

[CamKing]

during the compression stroke mechanical energy is added to the air and fuel in the cylinder this causes the temperature of the air fuel mixture to rise.

Yes, and that starts at BDC, and is not effected by IVC.

[Enigma]

Flat earthers. Seems contradictory to use that nonsensical line of thinking , and "today's modern engine design" in the same post. May as well say "don't bother calculating that silly airflow requirement thing, just buy my cams, I'm always right". You can have my account back. This'll be my last post on "the blue H.A.M.B".

[CamKing]

Flat earthers. Seems contradictory to use that nonsensical line of thinking , and "today's modern engine design" in the same post. May as well say "don't bother calculating that silly airflow requirement thing, just buy my cams, I'm always right". You can have my account back. This'll be my last post on "the blue H.A.M.B".

"A" divided by Cheese = Root Canal

If you find this useful, I can't help you.

[CamKing]

5 pages, and no one can explain what the equation is actually calculating.

Just look at the Wallace Racing DCR calculator:
Here's the description:
Use this calculator to see what the effect of bore, stroke, rod length, cam timing, compression ratio, boost pressure and altitude is on your dynamic compression ratio. Of the variables, the most important is cam timing which has a dramatic effect on your "dynamic" as opposed to your static compression ratio. The more "overlap" your cam has, the lower your "actual" as opposed to your static compression ratio will be.

More overlap lowers actual compression ????
in what world ???

[Stan Weiss]

5 pages, and no one can explain what the equation is actually calculating.

Just look at the Wallace Racing DCR calculator:
Here's the description:
Use this calculator to see what the effect of bore, stroke, rod length, cam timing, compression ratio, boost pressure and altitude is on your dynamic compression ratio. Of the variables, the most important is cam timing which has a dramatic effect on your "dynamic" as opposed to your static compression ratio. The more "overlap" your cam has, the lower your "actual" as opposed to your static compression ratio will be.

More overlap lowers actual compression ????
in what world ???

Mike,
I have seen that before elsewhere, and have never understand what they are talking either. Also "boost pressure and altitude is on your dynamic compression ratio." Do not understand this either.

Stan

[CamKing]

Mike,
I have seen that before elsewhere, and have never understand what they are talking either. Also "boost pressure and altitude is on your dynamic compression ratio." Do not understand this either.

Yet, since people can plug their specs into the on-line program, and it spits out a number, they think it's correct.

[johnef]

Looks like we've been full circle. So if static compression ratio and dynamic compression ratio is not really relevant. What do you use to determine the limits of a fuel such Q16? Consider my mechanical compression ratio is 18:1.

[CamKing]

Mechanical CR is relevant, but it's just part of the equation. You have to be able to calculate how much mass you will be trapping, thru-out the RPM band.
This requires calculating for not only the valve lift curve, but valve size, port size, temp, fuel type, piston speed, and a few other things I'm forgetting.

Since the actual shape of the valve lift curve effects the flow characteristics into the port, I don't know how the average engine builder would calculate acceptable CR for a given fuel.

[plovett]

5 pages, and no one can explain what the equation is actually calculating.

I thought it was simple? Just a ratio of the volume in the cylinder when the intake valve closes compared to the volume at TDC?

If DCR is meaningless then static compression ratio is meaningless, too?

[plovett]

The more "overlap" your cam has, the lower your "actual" as opposed to your static compression ratio will be.

More overlap lowers actual compression ????
in what world ???

I agree that the above statement does not make sense.

[Tom Walker]

The "more overlap" statement must assume that more overlap equates to more duration overall that bleeds off some mass at low speeds. (Later I.V.C.) That is the only thing that makes it make sense to me.

[cjperformance]

5 pages, and no one can explain what the equation is actually calculating.

Just look at the Wallace Racing DCR calculator:
Here's the description:
Use this calculator to see what the effect of bore, stroke, rod length, cam timing, compression ratio, boost pressure and altitude is on your dynamic compression ratio. Of the variables, the most important is cam timing which has a dramatic effect on your "dynamic" as opposed to your static compression ratio. The more "overlap" your cam has, the lower your "actual" as opposed to your static compression ratio will be.

More overlap lowers actual compression ????
in what world ???

It would be more accurate or understandable if it was written as " more overlap reduces your low rpm running cylinder pressure due to the exhaust contaminated intake charge containing less combustible air/fuel ".

[digger]

The problem is the name and people trying to use it inappropriately. It's a rule of thumb

About the only use I see is along the lines of compression (SHOULD BE PRESSURE) testing etc and being able to ballpark whether a street type engine will have good torque on the bottom end but even then you can still screw things up with "good" DCR e.g say 8.5 by making poor choices elsewhere. Equally at 6:1 DCR you aren't getting good bottom end (relative to the cube and something in the 8's ) naturally aspirated. It also tells you have half a chance of running it decently on good pump fuel

This is exactly right. Except that you just made the same mistake that we all do. That's what starts the confusion.

what exactly is wrong? compression testing just gives you a pressure while cranking the engine which even on the internet most people are going to know what that means.

[digger]

Flat earthers. Seems contradictory to use that nonsensical line of thinking , and "today's modern engine design" in the same post. May as well say "don't bother calculating that silly airflow requirement thing, just buy my cams, I'm always right". You can have my account back. This'll be my last post on "the blue H.A.M.B".

"A" divided by Cheese = Root Canal

If you find this useful, I can't help you.

why take things out of context.

DCR is fairly well defined except for the wallace one lol. Its also fairly well understood that it's limited in what it can tell you, but most basic calculators are just as useless. i'd rather know it than not but really only if i want to drive on the street where the rpm are going to be low. it is what it is though.

[Tom Walker]

Static compression ratio, dynamic or whatever ratio you can have information about is good in my opinion. The more information I have about the engine I am concerned with is a plus for me. I believe most people on this forum are intelligent and experienced enough to process and use whatever information they have at their disposal to best serve the purpose at hand.