[screamingchief]

Originally Posted by pmd400

Just a question from a complete novice. Assuming we have a 455 with iron heads and close to 9.5:1 compression. Which of the following intake lobes would be considered larger?
Xe274-274 advertised, 230 at .050, 0.488 lift
Crane- 288 advertised, 226 at .050, 0.458 lift
I would have assumed the xe274. But does the crane make up for the lose of lift and .050 duration with the extra seat timing?

"Larger" is a relative term.

The one with more advertised duration is typically considered larger due to how that is gonna skew the total overlap numbers higher.

But put one on a tight LSA/early ICL and the other on a wide LSA/later ICL and they can "act" much differently than the specs might suggest,so honestly,it's the sum of the whole cam that determines which is "larger" and which is "smaller",and it's not always that easy to tell which is which,that's largely where problems with cam selection stem from.

And FYI: Any lobe with more lobe lift is going to tend to have slightly more .050" duration than another "similar" advertised duration lobe with less lobe lift,that's just a quirk of lobe design.

It's largely the same dynamic that higher ratio rocker arms exploit.

Originally Posted by llwta76

So ( correct me if I'm wrong) a lot of overlap sends a lot of fuel in to the combustion chamber not only creating a stinky smell( and burning eyes) but washing the cylinder walls (washing oil away and accelerating crosshatch wear) ?

IMO no.

Being too rich puts too much fuel into the combustion chambers,it's the root cause,not the effect of a copious amount of overlap.

Overlap can indeed let some portion of the existing intake charge out the pipes due to over-scavenging,but that typically only happens @ higher RPM when the charge flow is high enough for that condition to rear it's ugly head,that inturn actually creates a lean condition as a good portion of the intake charge is going right out the exhaust pipes and not going towards proper combustion like it should be.

This is often evidenced by the color of the exhaust ports (bone white coating in exhaust ports).

What typically happens with large overlap cams at lower RPM is actually an intake charge dilution situation,where-in the slow moving incoming intake charge gets diluted by the residual exhaust gasses backing up into the intake ports in a defacto "EGR effect" which inturn displaces some amount of said intake charge actually creating a (more or less) lean condition.

This is often evidenced by the color of the intake ports (black sticky goo in intake ports).

And FYI: one can have one/the other/both happening in the same engine.

Originally Posted by tpssonic

That being said, you can also have "stinky" exhaust with a LEAN A/F mixture.

Same is true for a four stroke engine,see ^^^^ above.

HTH

Bret P.