Why no tighter than 112 lsa in marine engine?

[gmrocket]

The 350 mercruiser I just did was just a cam and carb swap..low dollar but best bang for the buck award.

Mid 90's stock mercruiser 350 , it cranked 145 psi with the lazy wide LSA marine cam. Can't remember exact specs, something like 114 LSA .400"/ .410" 204/210 @.050". And big .006" numbers which explains the low crank #'s

Went with a comp 260AH-8. 108 LSA in a couple advanced. 212/218 @.050" .444"/.444". With its short 260 adv it bumped cranking psi to over 165! It was a totally different boat... Way quicker to plane& mid range acceleration from 30mlh was fantastic. Never had condensation or water in combustion chamber/exhaust issues with this or others.

[DaveMcLain]

Don't automatically assume that the stock "marine" cam is anything unusual. Last year I rebuilt a 350 Chevy Mercury Marine engine from a ski boat. I dyno tested it with the water log manifolds etc and it made right at its 260 HP rating on my dyno. The marine manifolds hurt it about 15 horsepower and the stock flame arrestor about 9HP! vs the open carburetor. Here's a plot of the original stock cam:



When I plotted the cam I assumed about 114 lobe separation in the engine on about 110 intake centerline which is why it shows it as being retarded.

[dfarr67]

Just a FYI, I was searching high and low for a Volvo Penta vortec intake- $$$, Edelbrock makes the highrise with bronze inserts- I wanted the Q-jet spreadbore which limited the field.

[gmrocket]

Don't automatically assume that the stock "marine" cam is anything unusual. Last year I rebuilt a 350 Chevy Mercury Marine engine from a ski boat. I dyno tested it with the water log manifolds etc and it made right at its 260 HP rating on my dyno. The marine manifolds hurt it about 15 horsepower and the stock flame arrestor about 9HP! vs the open carburetor. Here's a plot of the original stock cam:



When I plotted the cam I assumed about 114 lobe separation in the engine on about 110 intake centerline which is why it shows it as being retarded.

at what tappet lift are your intake and ex adv dur of 271.6& 282 taken at?

[MT Jeff]

Just a FYI, I was searching high and low for a Volvo Penta vortec intake- $$$, Edelbrock makes the highrise with bronze inserts- I wanted the Q-jet spreadbore which limited the field.

There is one on EBAY starts at $250 no bids 4hrs left http://www.ebay.com/itm/Volvo-Penta-Int ... G5&vxp=mtr
lowest I've seen one, look at two barrel engines also. Like that one some were used with an adapter.

[MT Jeff]

That cam is what I've found listed. It was GM's one size fits all marine cam forever. For the most part they had two cams, that one and later a roller they used on everything unless it was a specialty motor like a Scorpion. They used Crane cams i believe.

[DaveMcLain]

Don't automatically assume that the stock "marine" cam is anything unusual. Last year I rebuilt a 350 Chevy Mercury Marine engine from a ski boat. I dyno tested it with the water log manifolds etc and it made right at its 260 HP rating on my dyno. The marine manifolds hurt it about 15 horsepower and the stock flame arrestor about 9HP! vs the open carburetor. Here's a plot of the original stock cam:



When I plotted the cam I assumed about 114 lobe separation in the engine on about 110 intake centerline which is why it shows it as being retarded.

at what tappet lift are your intake and ex adv dur of 271.6& 282 taken at?

.004 lift

At .006 they are 261 273 degrees

[wyrmrider]

.004 would be Crane standard or SAE = 2 (that's equal to) .006 at the valve with 1.5 rocker

[user-23911]

Closing the LSA moves the peak torque up in the rev range. Opening it moves the torque down.

Say what???

Spreading the separation generally widens the power band with more at the bottom AND the top at the expense of the mid range peaks. There are, of course, some exceptions especially when the overlap gets to be quite a lot.

No, you're back to front.

Spreading the separation decreases the overlap.

No overlap means no top end.


Going tighter in LSA increases overlap but kills the bottom end.
You'll get reversion at idle and suck exhaust back into the intake.

[digger]

More overlap doesn't kill the bottom end, too much overlap kills the bottom end. It depends on the reference point or baseline from which the comparison is made so it's hard to make generalisations and be correct 100% of time

[wyrmrider]

boat log wet exhaust is not going to like a lot of overlap in any case, no top end in any case build for what you have to work with and through the prop is even more problematic
we had to run through the prop in some of the offshore blown BBC's
Jones cam wet log exhaust through the prop- works great- ask him

[DaveMcLain]

I think most all of this wet exhaust/overlap stuff is nonsense. Right now I can think of a 434 Chevy build that I have in a jet boat application that's running a solid roller with 250 degrees at .050 intake and exhaust on a 108 lobe separation with water cooled manifolds and it has been used just about every weekend every summer for the last eight years with no problems. What am I missing? This is with Marine Power manifolds and exhaust that exits through the transom. The only time I've ever seen a boat engine suck in water was on a Ford 460 with a set of Bassett wet headers and a cam with a flat intake lobe or two...

[Walter R. Malik]

Spreading the separation generally widens the power band with more at the bottom AND the top at the expense of the mid range peaks. There are, of course, some exceptions especially when the overlap gets to be quite a lot.

No, you're back to front.

Spreading the separation decreases the overlap.

No overlap means no top end.

Going tighter in LSA increases overlap but kills the bottom end.
You'll get reversion at idle and suck exhaust back into the intake.

I am guessing that you never thought much about the intake closing and that it also has something to do with the "band" before and after the distinct peaks in the curve..

[gmrocket]

There has been a lot of guys saying narrow LSA will cause reversion and moisture to enter the exhaust port/combustion chamber.

That's only true "if" the cam duration is too much for the application. Over camming in a marine app may cause that.. But that's not what I'm saying, or suggesting in the discussion. Why would anyone go to big? A cam that improves on plane time, has better mid range and top end speed is the point! And I've found that a narrower LSA will do the trick.

A blanket statement like "narrow LSA causes water reversion" is just plain false! That's totally out of context when so many other factors that are not even mentioned.

The wide LSA marine cam "spec" can go right into the BS garbage bag.

[MT Jeff]

There has been a lot of guys saying narrow LSA will cause reversion and moisture to enter the exhaust port/combustion chamber.

That's only true "if" the cam duration is too much for the application. Over camming in a marine app may cause that.. But that's not what I'm saying, or suggesting in the discussion. Why would anyone go to big? A cam that improves on plane time, has better mid range and top end speed is the point! And I've found that a narrower LSA will do the trick.

A blanket statement like "narrow LSA causes water reversion" is just plain false! That's totally out of context when so many other factors that are not even mentioned.

The wide LSA marine cam "spec" can go right into the BS garbage bag.

That kinda what I was thinking. Most would want too much duration to run less than 5000 rpm and then a tighter lsa could cause a problem.

Do you have the timing card on the can you used? I went to Comp and done was available. Shows as "special purpose" cam. Card specs not available as least online.