Overlap and idle vacuum
Thread Starter
Joined: Jun 2005
Posts: 2,265
Likes: 671
From: Oskaloosa, Iowa
Overlap and idle vacuum I’m planning an LM7 based 347 build. My current cam is 228/228 112 Lsa. It idles and drives great. It pulls 14 inches of vacuum at idle and barely sounds “cammed”.
I’m wanting to be a little more aggressive on the cam as well as being more suited to my procharger.
It is my understanding that cams are designed around valve events opening closing points etc. while the duration and Lsa just end up where they are based upon those things. For my understanding of the characteristics it’s easier to look at duration and Lsa.
The cams im considering are the ccp jam cam at 230/242 115 and a cam motion grind 232/248 116 Lsa.
With that in mind my current cam has 4 degrees overlap while those have 6 and 8 degrees overlap. Is my understanding that camshaft overlap is the best indicator of a cams idle characteristics or is that way over simplifying and misleading?
Top end will be 243 heads, ls6 intake, stock throttle body looking to run 15 plus psi.
I’m wanting to be a little more aggressive on the cam as well as being more suited to my procharger.
It is my understanding that cams are designed around valve events opening closing points etc. while the duration and Lsa just end up where they are based upon those things. For my understanding of the characteristics it’s easier to look at duration and Lsa.
The cams im considering are the ccp jam cam at 230/242 115 and a cam motion grind 232/248 116 Lsa.
With that in mind my current cam has 4 degrees overlap while those have 6 and 8 degrees overlap. Is my understanding that camshaft overlap is the best indicator of a cams idle characteristics or is that way over simplifying and misleading?
Top end will be 243 heads, ls6 intake, stock throttle body looking to run 15 plus psi.
Overlap isn’t the only thing. How late the intake valve closes is also a major factor. Longer durations with a wide LSA will usually delay that intake closing further. This can really hurt manifold vacuum at idle and low engine speeds.
Thread Starter
Joined: Jun 2005
Posts: 2,265
Likes: 671
From: Oskaloosa, Iowa
Ok that makes sense and probably explains why those cams have advance ground into them. The cam motion cam I mentioned has a plus 5 advance so an icl of 111.
