Cam overlap degrees for marine cam selection

[jeff swisher]

This does not pertain to your exact cam selection.
It may help some who are running too much overlap with through hull exhaust.

Buddy built a 350 for his Baja boat and I helped him port his 993 heads for it.
His cam choice was the Mutha Thumper and he was running the Mercury exhaust manifolds that fed water into them at the front.

That did not work (imagine that ).
I told him we could move the water from the front of the manifold to after the manifold and we installed a fitting to run the water a couple inches from the transom.
That put the water way down stream of the manifolds and no issues.
Ran a GPS 67 MPH with 3 adults in the boat with that 350" engine.

[CamaroMan]

not sure how you moved water after the manifolds? boats require cooling in the manifolds too- that would just have cooled the exhaust elbows and tubes..

impressive though- 67 on a 350.. i was initially trying to see if a: the calculator i referenced is an "ok" way to avoid reversion issues, and b where the danger zone (overlap?) starts creeping in..

Most motors I build are for fisherman who want torque not high speed, its not practical on the ocean unless you have the correct hull- the 383 HT cam does produce 440ft lbs of torque at 3000 rpm which is almost perfect cruising conditions on a good day.. the cam (14097395) fits on stock vortec heads, uses stock springs too and goes for $250 which makes it a great contender for a marine engine..

when u start adding beehives, machine work, porting, rockers, studs, pushrods its over a grand easy and not viable for my market here on the west coast..

I did throw together a budget 350 with 193 heads for my 22ft fishing boat and it performed incredibly, very torquey and quite fast considering its stock, just higher compression (mid 9s) - and a quadrajet.

[Tom68]

https://mgispeedware.com/camshaft-calculator (this one is great cos you can enter 4 profiles and compare!)

Watching thread with interest.

Geez mgi made a mess of this illustration.

camshaft-overlap.jpg

[Walter R. Malik]

You need to look at overlap at the seat duration, not the .050" duration.
If you look at seat duration, the Comp cam actually has a lot less overlap.

Yes ... most people don't realize that the water from the exhaust into the cylinder happens at slow cruising or even at idle sometimes when intake manifold vacuum is highest.

[David Redszus]

You need to look at overlap at the seat duration, not the .050" duration.

Mike is absolutely correct.

Setting aside the special case of marine applications, the general overlap analysis
consists of a comparisons of overlap areas.

Overlap area is a function of valve lift, duration, and valve size for both intake and exhaust.
Once the overlap area has been determined, it can be converted to an equivalent
overlap diameter. The quantity and direction of overlap air flow now becomes a
function of inlet and exhaust pressures.

This is especially important for turbo engines where pressure ratio (I/E) balance is critical.
If intake pressure is greater than exhaust, fresh air and fuel will be blown into the exhaust.
If exhaust pressure is greater than intake, flow reversion will push hot burned gases into
the chamber. Either case is a loser.