Lobe seperation!!!!
 

I have cmi headers what
lobe seperation do recommend?



THANKS

Lobe separation isn't what you need to worry about.

It's overlap that you need to worry about.

With my CMI's I ran a hydraulic roller with 16* overlap @.050 and through hull exhaust with no problem.
If you have captains call or some sort of diverter system that may change the amount of overlap you can run.

DAVE

Lobe Separation
 

To make it a little easier for you. For a big block Chevy you would want a lobe separation angle of no less than 110 on a camshaft that has up to 224 degrees of intake duration and 230-234 degrees of exhaust duration. Anything with more duration would require a 112 degree LSA. The tighter the LSA (no tighter than 110) the better the engine will perform in a marine application (at the cost of some water reversion). This is pretty much agreed upon by camshaft manufacturers and a general rule of thumb.
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE

Dennis or anybody,

Do you know what the cam specs are on a stock 350 sbc from merc.

I am trying to figure what the next step up is from stock.

Thanks

FF, Go to Crane's website. It takes a while to find, but they have a lot of marine cams listed and showed the #'s for replacement merc cams. I only looked at BBC's though, don't know about SBC's.

What about a blower motor?

I think a blower motor application is supposed to work better with wider lobe seps.....like a 114* -------right?

Most blower motor builders do like the wider lobe sep at most any duration because the blower has to (for all intensive purposes) work less. The cylinder pressure will be higher, therefore creating "more boost" in effect.

BDS however has a different take on this. Most of their cam recommendations are on a 110 lsa. Except for their marine cams.
Remember that BDS is strictly a roots blower company. It goes back to the heat thing.

It has been argued by many engine builders which is better. The tighter the lsa the less the cylinder heats up but in turn you must spin the blower faster.
Wider lsa spin blower slower to get same amount of boost.

It all comes down to what performs best in your application. Sometimes that means you have to experiment.

Getting back to the original question however, VERY GENERALLY SPEAKING since cam selection should be based primarily on the rpm range it seems as though even the lower cost CMI's (small E-tops) can handle a large enough cam to stay within the normal operating range of a marine engine that is normally between 4500 - 6000r's with little to no reversion problems.


DAVE

Specs for GM small block marine engines camshafts
 

The flat based lifter (hydraulic) camshaft is 200 intake/212 exhaust duration @ .050 with .400 intake/.420 exhaust valve lift, 110 lobe separation angle.
The roller based lifter (hydraulic) camshaft is 197 intake/207 exhaust duration @ .050 with .430 intake/.450 exhaust valve lift, 109 lobe separation angle.
These are the GM camshaft specs, for my book SMALL BLOCK CHEVY MARINE PERFORMANCE, I pulled cams out of marine engines and sent them to Comp Cams to measure and they measured slightly larger on the duration (but within 1 or 2 degrees).
Hope this helps someone!
Sincerely
Dennis Moore

Thanks Dennis,

I really appreciate your help.