Don Terrill’s Speed-Talk

I’m not looking at any specific combo, just asking a general question here...

Why do most cam companies recommend more compression for cams with tighter lsa’s but are otherwise identical? For example, cam A is 272@.0045, 216@.050, 110 lsa 105 ica, and cam B is all the same except 108 lsa, 104 ica. The manufacturer recommends 8.75 to 10-1 compression for cam A, and 9.25 to 10.5-1 for cam B. Same exact lobes, just different lsa and ica. I’ve seen this on many otherwise identical cams, and it seems counterintuitive to me. If you close the intake valve earlier, you build more cylinder pressure. What am I missing?

Why does a tighter lsa need more compression?
It doesn't.

you hear people say nonsense such as overlap bleeds off compression. when in fact the overlap usually helps VE at "mid" and " low" rpm (assuming it is starting from sub optimal) when used with a proper exhaust etc so it does the opposite to bleed off compression so probably should be doing the opposite assuming the engine was on the knock limit for pressure and temp before the change.....

My understanding is that higher compression has less combustion area to scavenge so it would need less overlap which could be achieved with a wider LSA or less duration.

A tighter LSA, does lessen the dynamic compression ratio.

frnkeore wrote: ↑Thu Feb 06, 2020 4:19 pm A tighter LSA, does lessen the dynamic compression ratio.

are you sure about that?

GARY C wrote: ↑Thu Feb 06, 2020 4:05 pm My understanding is that higher compression has less combustion area to scavenge so it would need less overlap which could be achieved with a wider LSA or less duration.

so more overlap less compression?

digger wrote: ↑Thu Feb 06, 2020 4:29 pm

frnkeore wrote: ↑Thu Feb 06, 2020 4:19 pm A tighter LSA, does lessen the dynamic compression ratio.

are you sure about that?

My limited experience a tighter LSA usually raises cranking compression.

digger wrote: ↑Thu Feb 06, 2020 4:30 pm

GARY C wrote: ↑Thu Feb 06, 2020 4:05 pm My understanding is that higher compression has less combustion area to scavenge so it would need less overlap which could be achieved with a wider LSA or less duration.

so more overlap less compression?

In theory, but of course RPM and other things would play into what is best.

For lower comp engines I tighten the lsa to build more dynamic comp.

digger wrote: ↑Thu Feb 06, 2020 4:30 pm

GARY C wrote: ↑Thu Feb 06, 2020 4:05 pm My understanding is that higher compression has less combustion area to scavenge so it would need less overlap which could be achieved with a wider LSA or less duration.

so more overlap less compression?

With EARLIER intake valve closing it will mean MORE dynamic compression and with more overlap, also more internal EGR at low speed.

So the mixture is more diluted at lower speeds with a tighter lsa...is the increased compression recommended to help offset the bad effects of this?

Crane cams is one company in particular that seems to recommend higher compression for their tighter lsa cams.

Most guys don't run enough compression, if you're no where near the det threshold then more comp on tighter lsa will make it behave better at part throttle & lower rpm.

If you're approaching the limit then tightening the lsa can push it into det at peak tq.

GARY C wrote: ↑Thu Feb 06, 2020 4:39 pm

digger wrote: ↑Thu Feb 06, 2020 4:29 pm

frnkeore wrote: ↑Thu Feb 06, 2020 4:19 pm A tighter LSA, does lessen the dynamic compression ratio.

are you sure about that?

My limited experience a tighter LSA usually raises cranking compression.

i also agree with that due to the IVC being earlier, the overlap doesnt matter so much when there is little exhaust activity (the engine is not being fired)

a digress on the topic at hand, Knight is right, a lot of guys don't have the comp they think they have, i have dealt with customers that have a combination of parts that they assure me makes 10.5 or whatever the claimed comp is for the combo, and i have suggested a wet out to confirm and have then been shown the ends of packets as confirmation of the claimed comp, i would say that nearly every engine i have wet out over many years has not made claimed comp, nearly always under rarely as claimed ,very very rarely over.Its very hard to get a customer to go shorter on a cam.

A conversation with a cam supplier here some years back came to the conclusion that cam lists were printed upside down, so if big is good, bigger is better and logically the biggest is the best, so go to the bottom of the page for the cam you think you need!

Cheers.