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madokie · 04:36 AM

this is a question that ive thought of,lots of guys have low compression engine,and want hi-po cam,well if they put a LSA cam of say 110-108 wouldnt that lead to lower compression,beacuse of more valve overlap?where as wider LSA of 112-116 would help keep up compression or maybe increase it .5 because of less vaLve overlap??also what of advancing & retarding cam ,how would that affect compression??anyone ever try running heads without gaskets to boost compression??the permatex people ran a race car in some stock car race back in the 60'swith no gaskets ,only thier stuff in the tube, to seal up motor. i think , i wonder what they used to seal up the heads to block?

screamingchief · 12-09-2009, 05:53 AM

Quote:

Originally Posted by madokie

this is a question that ive thought of

Well,not to come off as an jerk here or anything,but you got a lot more thinking to do.

Quote:

Originally Posted by madokie

well if they put a LSA cam of say 110-108 wouldnt that lead to lower compression,beacuse of more valve overlap?where as wider LSA of 112-116 would help keep up compression or maybe increase it .5 because of less vaLve overlap??

First,that comment is far too vague to have any real way to address in this sorta discussion.

Beside the fact that it's backwards by most folks POV on this.

So lets just clear one or two things up here.

Overlap does not "bleed off" compression,nor does an abundance of it somehow lower an engine's dynamic compression/cylinder pressure capabillities.

It's entirely possible for a cam with much more total overlap to generate a higher dynamic compression/cylinder pressure in a given engine.

Overlap happens around TDC @ the end of the exhaust stroke (which is also right at the begining of the intake stroke).

So the only way overlap can possibly affect the intake charge compression process is via it's influence on the very end of the exhaust gas purging process,and the very begining of the cylinder filling process.

That's what's called exhaust scavenging.

If overlap is ideal,it can actually help boost cylinder filling to the point of increasing the charge density (and thus the cylinder pressure) as well as increasing the VE of an engine.

If overlap is less than ideal,yeah then it can hurt the dynamic as well.

So that situation alone can negate much of the narrow vs. wide LSA debate in terms of which is better (or worse) in terms of that whole "compression" situation.

Overlap is far more than just a "number",it's multi-dimensional,it has shape,area & timing to consider as well just the "total" overlap number.

And no way can any of this be discussed without talking about the duration and installed centerline numbers either,as both those directly affect the valve timing as well.

Most folks will drop back and state that cylinder pressure is entirely based on the intake closing event timing,but that's not 100% right either.

Those folks base that POV on quotes from their favorite experts,or on the fact that most DCR math/calculation relies on that IC event to determine the engines DCR,but that's one of the primary reasons much of that DCR theory is inherently flawed,because in reality there is so much more to this than just the IC event alone can account for.

As is often the case with cams,it's the sum of the whole that determines how a given engine reacts.

LSA,overlap are both interdependant on duration (and overlap is also interdendant on lift to a degree as well),so you cant talk about one spec without talking about the others,and if you wanna consider the big picture,absolutely the installed centerlines need to be discussed as well.

Just saying "a wide LSA will drop the cylinder pressure" or the flip side of that,which would be "a tight LSA will boost cylinder pressure" is just too great a simplification of a very complicated dynamic,and often it can be dead wrong.

If both cams were exactly the same duration & lift,one might think that would hafta be true,but when both cams are installed on the very same intake centerline,the intake closing event will then be the same,and as such the DCR theory then would try to demand that they will be exactly the same in terms of cylinder pressure (DCR),as both would then have exactly the same IC event,but anybody who's BTDT a time or two will know that's just not true in all cases.

So there has to be more to this than that one valve event (intake closing).

So in closing:

It's not the LSA alone.

It's not the total overlap alone.

It's not the intake closing event alone either...

It's not even a combination of the above.

You can even throw in the installed centerlines and that still is'nt gonna account for all that.

There is still duration,lift,and a few more "detail" items to ponder.

Are there "trends" that seem to be true & accurate,sure enough are,but you still gotta watch all the details very closely,or you can easily get bit in the a$$ by all this.

So,to end this,I'll just say that one will never likely know everything about this subject,even the experts @ the highest levels dont have all the answers,so honestly in many cases the only answers that matter are those answers that come from good ol' trial and error testing in any given combo.

Bret P.