[screamingchief]

Originally Posted by Brent Flynn

So...is it longer duration, wider lobe separation, lobe profile or amount of overlap that enables higher compression engine to burn 89 or 93 octane?

Well,it sure aint overlap by itself,that happens @ the end of the exhaust stroke & the begining of the intake stroke,so nowhere near when the cylinder actually begins compressing the intake charge,the only way overlap can affect that is by it's influence on clearing the exhaust charge and initiating the intake charge,but that mostly affects charge density and such,so that mostly just affect the power output more so than it affects the cylider pressure and such.

Lotta guys like to focus on the intake closing event,that's mostly because of it's prominence in DCR theory and such,but it aint as simple as that either.

Wider LSA's tend to have later IC events,but not exclusively,that can also be affected by seat duration and installed centerlines as well.

Remember that later intake events from a wider LSA generally also means earlier exhaust events too (this can be very important,and largely misunderstood).

IMO,there is no one single feature of a cam that makes it "high compression" friendly,it takes a number of those features working together to make a cam like that,lots of seat duration,wide LSA for a later IC event (and thus earlier exhaust events too!),a decent I/E split to boost the lost overlap due to the wide LSA as that help exhaust scavenging initiate the intake charge,plenty of exhaust duration to help completely purge the exhaust charge before the intake valve actually opens,yadda,yadda,yadda.

But cams can only do so much on their own,there are a lotta other details that go into making a successful high CR street combo.

FWIW

Bret P.