[curtis73]

I'm trying to decide between two engines for a boat. Both are marine engines with marine cams and I want to know how different the idles will be in RPM. Both engines are chevy 350s, 9.5:1 compression and vortec heads.

Cam 1: flat hyd, 202/213 @50, .427/.433 lift, 110LSA, -12* overlap @50
Cam 2: roller hyd, 212/218 @50, .488/.495 lift, 112LSA, -9* overlap @50

Don't worry, I've already machined the vortecs for the extra lift. They both make nearly identical power peaks on desktop dyno, but of course the roller makes about 75 more lb ft at lower RPMs.

-How much higher will my idle have to be with the roller?
-does the type of lifter (and its corresponding ramp speed) have anything to do with idle speed, and if so, what?
-Does advancing or retarding the cam affect idle speed and if so, how?

 

[curtis73]

Here is a desktop dyno comparison if that helps things. The lines with dots on them are the roller.

 

[johnsongrass1]

I'm trying to decide between two engines for a boat. Both are marine engines with marine cams and I want to know how different the idles will be in RPM. Both engines are chevy 350s, 9.5:1 compression and vortec heads.

Cam 1: flat hyd, 202/213 @50, .427/.433 lift, 110LSA, -12* overlap @50
Cam 2: roller hyd, 212/218 @50, .488/.495 lift, 112LSA, -9* overlap @50

Don't worry, I've already machined the vortecs for the extra lift. They both make nearly identical power peaks on desktop dyno, but of course the roller makes about 75 more lb ft at lower RPMs.

-How much higher will my idle have to be with the roller?
-does the type of lifter (and its corresponding ramp speed) have anything to do with idle speed, and if so, what?
-Does advancing or retarding the cam affect idle speed and if so, how?

Idle speed won't be affected by types of cams. Long cams usaully need higher idle speeds to help keep the plugs clean. A lot of it is the carb setting though. Even my all out race engines idles at 1000 with a good carb. cams are in the 255 to [email protected] range.

 

[curtis73]

I'll have to respectfully disagree in my applications. At 1000 RPM, maybe, but I'm talking about the differences between like 450 and 600. Cams have a huge effect on idle speed.

Going from a stock cam on my 454 to a 218/224 cam made the idle go from 500 with 12* initial to 600 with 36* initial. So I should have been able to drop the idle much lower with that much initial, but the cam wouldn't idle that low. Sputters and dies.

 

[curtis73]

Sorry... misread your post... you said "TYPE" of cam.. .I thought you meant that duration won't affect idle. sorry

Ok, good to know that roller vs flat won't have any effect. Its a bit frustrating... when you look at the dyno charts, I would pick the chart with the lower RPM band as the one that would idle lower, but that's the one with more duration so I'm stumped.