[Cliff R]

Not sure how we got off on "stinky exhaust", but it is a by-product of incomplete combustion, or an overly rich or lean mixture.

Overlap has been mentioned several times. Overlap basically describes the amount of time in crankshaft degrees that both valves are open at the same time. This bleeds off cylinder pressure and lowers dynamic compression, and combustion efficiency at idle and low rpm's. It makes up the lost ground later on, but a lot of overlap can produce a pretty "rough" idle, and stinky exhaust as a by-product.

For he most part cams with tighter LSA's produce more "lope" at idle speed, which is from pulling the lobes together and increasing overlap, but the story doesn't simply end there. Smaller cams on tight LSA's also tend to close the intake pretty early, so you can get what we call "reversion" here. I see this a LOT with short seat timing cams on tight LSA's, especially in the longer stroke engines. They have what I call a "quirky" idle quality, for lack of better words to describe it. Sort of a combination of having a fair amount of overlap and having the events happened sooner in the cycle. When we observe these symptoms those engines NEVER make big power, as the cam is simply too small, too far advanced, and intake closing to early to ever have a fighting change to breath well at high rpm's.

For bigger cams we can yank them down on tight LSA's and still enjoy good power, and that "rumpity-rumpity" sound everyone loves. Pound for pound they don't typically outrun big cams on wider LSA's. It seems like the wider we go with LSA, the better the long strong engines respond to it. We still yank down the LSA when we go out beyond about 260 @ .050", but we've still seen some very good performing engines, and not just Pontiac's, use "big" cams on wide LSA's in street/strip and full race applications and make great power with them.

I think Dave eluded to why in several of his responses, as the reasons some combos work well is more about the total combination of parts, and not just the camshaft, although the camshaft does play a big role with these things.

I started this thread to basically educate those who are venturing into a pump gas 455 or stroker 400 build, and are looking to make good power on pump fuel, and run a moderate to high compression ratio as well. There are cams out there that just leave WAY too much power on the table, and they can get you into trouble quickly with octane requirements, even at lower compression ratios.

I've known about this for years, but just about every time I've mentioned it, someone quickly takes the thread off course and calls me the Comp XE "cam basher", then the whole thing turns to poop.

The dyno sheets tell the story of what's going on here, and expect even WORSE results if you have a similar build and are using the smaller XE262 and XE268 flat cams in them......Cliff