[gtofreek]

Had a long talk with Harold Brookshire about dynamic compression ratios. He doesn't believe in this approach and here's why.

Lets look at 2 cams. A hydraulic flat tappet and a solid roller cam. Lets say that both cams close the intake valve at the same point. Let's say at 55° ABDC. The roller cam is going to allow much more air/fuel into the cylinder and trap it there when the valve closes. If the roller cam lets in a lot more air and closes the valve at the same point, are both cams really going to have the same dynamic compression ratio? No, they will not. That's why cam manufacturers never base their compression ratio recommendations on dynamic numbers. Always static.

[johnta1]

What would the LSA be on them?

[gtofreek]

Originally Posted by johnta1

What would the LSA be on them?

If your asking me this, it is a hypothetical response pertaining to dynamic compression. LSA doesn't matter when you are talking only about the intake closing event.

The point was that a roller cam has a lot more area under the curve than a flat tappet cam so it can fill the cylinder a lot more. If your trapping a lot more a/f in the cylinder and compressing it for the same amount of degrees, you will have more dynamic compression.

Therefore, dynamic compression of 9:1 with a flat tappet cam, may be more like 11:1 with a roller. Even though the dynamic compression ratio calculators show the same dynamic compression between either type of cam, Harold's belief is that the roller cam will have more dynamic compression than the flat tappet cam simply because it is compressing more a/f. This is something that the calculators do not take into account, therefore, they cannot be deemed 100% accurate.

They are OK for generalizing things if you are comparing between two cams of the same type, but that's about it.

[BruceWilkie]

Originally Posted by johnta1

What would the LSA be on them?

Intake closing point determines start of compression or conversely amount of time in compression.

LSA and ICL are mere coincidences of chosen timing events. Best used for cam installation and inspection purposes.

To further follow Paul and Harolds statements. TRUE VALVE SEAT time is when full static compression starts. And here even a solid flat is usually far different than a hydraulic.

I recall Harold saying a solid flat can have as short if not shorter seat duration vs .050 than a roller.

Not so with a hydraulic as there is no clearance ramp.

.050 cam lift numbers are also best used for installation/inspection and secondly comparison purposes. Certainly shouldnt use .050" cam lift to guesstimate dynamic compression. Rocker ratio is part of reason why not. How fast and how many degrees sooner/later the intake shuts off is affected with rocker ratio.

Some of the biggest differences in cams from one brand to the other with seemingly similar/equal "specs" is in the numbers not advertised. That 1st .004"-.006" of lobe lift plays big roles in both valvetrain stability and staging of events. Sure not much is going to "flow" past a valve at .006-.009 lift but important pressure exchanges begin and end there.

One other point to consider.... as speed increases and ve improves... pumping losses decrease, plus any "boost" from exhaust "purge"/"scavenge" during overlap as well and you are going to trap more volume when the intake closes... Sooo...
In other words, dynamic cylinder pressures rise with VE and eventually VE (and torque/power fall off) drops and dynamic compression drops. Did you know compression also affects VE?

Dont crutch excess compression for the fuel being used...instead get the most out of the fuel being used by having the right compression and ignition timing for it. Yes cam timing does play a role but follow cam manufacturers compression suggestions. There is a reason!

[johnta1]

are both cams really going to have the same dynamic compression ratio? No, they will not.

Yes they will.



Now designing a cam for an engine just for the dcr, I probably wouldn't either.



Think of the piston motion/position of when the cam intake is closing.

If it's open longer, it's not building up pressure as much as if it closes sooner.