BT

Overlap is a function of both IVO and EVC angles. The EVC point is a good reference number, but not the single determining factor regarding reversion. That is directly related to its placement in relation to the other events. There are limited parameters in which you can place these individual events and still have the engine run efficiently. Closing the exhaust valve sooner and the resulting decrease in duration will reduce overlap and the potential for exhaust reversion, however when you alter one valve event in most cases other factors (centerlines) are compromised also. Any resulting decrease in duration will also reduce your ability to make power, and alter the rpm range you generate that power. Roller cams are more flexible than flats in that respect because they generate more in/deg of lift with less duration. By moving the ICL & ECL either toward or away from TDC (essentially widening or narrowing LSA) you alter overlap. The camshaft as a whole is a series of interactive compromises and marine engines add one more variable (exhaust reversion) to the equation.
As for advancing or retarding the camshaft, moving the crossover point +/- 5* can be used as a hedge against, but not a cure for reversion. The trade off there is altering the power curve.
Bob

Great info RM, thanks. Sounds like my thinking is right on concerning cams and reversion.

I would guess 450hp pimcotrans. More important than max hp though is where the max hp is made, rpm wise.

BT :cool:

Cool! Learn somthing new every day. I guess I will put the motors in and test for reversion. Any suggestions on testing for reversion? I have had the boat for seven years, bought it cheap (So I thought), been throwing money at it ever since. I need to put it in the water to soothe the wasted cash. 90 degrees today. Should be on the water.

One additional point. The way to reduce overlap is to:
A) reduce duration
B) increase LSA
C) combination of the two
These are ground into the cam and cannot be changed. When refering to EVC you can move the angle, in your case, from 36* ATDC to 32*ATDC by advancing the cam 4*. This will not alter your overlap or potential for reversion. It's sooner but not better.
Bob

Bob, any feedback from your customer that ran this cam?

pimco - I put your numbers into my desktop dyno and this is what comes out: Used rect port heads

rpm hp tq

3000 224 392
3500 275 413
4000 328 431
4500 376 439
5000 407 427
5500 420 400
6000 410 359
6500 383 310

Good luck,

BT :cool: