Mike; I know almost nothing about marine applications. Just enough to mess you up,lol.
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But; hang on a sec; on this part I think I might be helpful;
With low compression I went (want?) single pattern camshaft to help cylinder pressure
After the engine is built, it is the Ica that controls the pressure. Not the cam being a single or split, pattern.
You can have two cams installed in identical engines, with the same Icas; one a single, and the other a split; and with the same Icas, the CCP will be the same.
What the Split pattern does, with the extra exhaust duration, is steal degrees from power extraction, usually carrying it into the overlap.
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Try this on;
Here is a FTH, single pattern suitable for 5500 in a car; (NOT a boat)
268/268, but for use at a fixed rpm could be ground on a 106 for marine use at WOT, giving you a 268/268/106.
In straight up, the events will be;
268/120/120/268/56 overlap, and Ica of 180 less 120=60*
Now lets put that on a dual pattern with a typical 8* split, but keeping the Ica to 60* again, and keeping the 106LSA. Here are the numbers;
268/120/116/276/60 overlap, and Ica of 180 less 120=60* again. Notice I took 4* from power extraction and gave it to the exhaust, which rotated the exhaust duration 4*into the overlap, increasing it from 56Effective to 60 effective.
So in the switch I concede that you may lose some torque at low-rpm, but with headers you will gain some power at peak, by virtue of the more overlap.
In a typical marine application you would have water-cooled log manifolds so overlap is not gonna account for much if anything, unless the exhaust "back-pressure" backs up into the intake.
Now, lets advance the single-pattern cams to Plus 4*.
The single becomes.
268/124/116/268/48 Effective overlap,
Ica of 180 less 124=56*now . compare that to the split pattern above;
268/120/116/276/60 overlap, in straight up.
notice the gain of 4* of compression. and
the loss of 12* of overlap! and
the same extraction now.
now, in an 8/1, 318 with an Ica of 56* the pressure is predicted to be 131.74 .
At Ica of 60*, the pressure is predicted to be 127.32; this is
a difference of 4.42 psi and maths to an increase of 3.47%...
Now, riddle me this; in a boat, what is your engine required to do?
That engine is doing one of three things; idling, cruising, or full speed ahead; am I right?
So when exactly is this 4.42psi pressure loss gonna be a problem? Certainly not at idle. Certainly not cruising; you just use a tiny bit more throttle. And at WOT, that 12* of overlap, with headers, will kill any thought of the missing 4.42 psi
I rest my case.
Now; if you do NOT have headers, as already said, the overlap won't mean much. In which case there is no good reason to run a split, nor to run a 106LSA. However, to run a 268 cam with a an early-closing intake, you sorta got to run a tight LSA anyway.
Like I said, IDK much about marine engines ............
BTW
at WOT,and at rpm;
your engine does not care much about any of the advertised numbers.
What is does care about, is the .050 numbers, and the lift, and being able to process massive amounts of cold air.
I used the 268 FTH numbers because this will get you about a [email protected] cam, which usually has about a 5000 rpm power-peak. If you really need a power peak of 5500, then you might want about 244@ .050 which is a stinking big cam, and it will about kill your, already stated to be, low cylinder pressure.
