Cam Question?

[john108]

This is a basic concept question related to a targeted 10.5 compression ratio and a cam such as Comp XS290S or XS282S with a 110 LSA.

Now if we used a 440 Source 512 stroker kit (505 cu in), the piston dish is 17 cc, the compression ratio calculated out to be 10.33.

I understand that it is dynamic pressure in the chamber that is a prime factor associated with pinging. 

The question is: If the cam were ground on a 112 LSA, there is less valve overlap.  With less valve overlap, I would think that the dynamic pressure would increase.  Therefore would you expect the dynamic pressure to be equivalent, better or worse for the case of 10.50 CR with an LSA of 110 to that if the 10.33 CR and 112 LSA.  I understand that the cam installed centerline may also change??--------Please comments, as I expect that the answer is more complex than my question. 

[john108]

Sorry for the dumb question but I was trying to get a feel for the parameters.

[firefighter3931]

Sorry for the dumb question but I was trying to get a feel for the parameters.

Not a dumb question John....just a complicated answer. 

Several factors influence how an engine will behave with different cam profiles. The general rule of thumb is that wide lobe centers bleed off cylinder pressure and spread the powerband over a broader range. In contrast, a tight lobed profile will tighten the powerband and it will hit harder in the sweet spot. Of course duration comes into play along with LSA.

The key factor is the intake valve closing point. The earlier the int valve closes....the more cylinder pressure it will make. High cylinder pressures translate into torque. The trick to making a street engine run strong is to develop just enough cylinder pressure to make power without creating an envirionment that will create detonation. With a long stroke build you don't need to push the envelope because the long arm crank is doing the work for you.

That being said, most street/strip pump gas compression strokers work well with 110 + lsa cam profiles. If you tighten the lobe seperation too much...you risk too much cylinder pressure and consequently the dreaded detonation. 

The added benefit of a wide lobe profile is increased idle vacuum, better street manners and easier tuning. 



Ron

[john108]

Ron
I think I may have too much wood in my head because my thinking is obviously wrong.
When you say "wide lobe centers (larger LSA) bleed off cylinder pressure, I see the opposite. 
When I visualize the LSA increasing (without changing the shape of each lobe) I see the overlap
decreasing and thought that less time is available to bleed of pressure, resulting in higher Chamber pressure.
What am I missing?  Is it just because (as you said) the inlet valve closes later that the pressure bleeds of during the compression stroke, and has nothing (or little) to do with overlap??
If that is the case, would, could, or should you go with a higher CR to account for the added bleed off?
John

[firefighter3931]

John, here is a quick tutorial from the Comp Cams website. This might help you understand the dynamics of cam profile selection and how it affects performance. 

http://www.compcams.com/technical/FAQ/LSAproperties.asp

I wouldn't try to overcompensate for cylinder bleed off with higher static compression in a street type build. Stick with the basics and you will be much happier in the long run....trust me on this. 



Ron