The ZZ502 cam has a 110°LSA, the same as the Merc HP500 cam.
But that's OK if you have long risers. The standard Revolution riser is not going to work. Call them up and see if there is a way to extend the inner pipe 4-6".
I also agree that advancing the cam to like 108° will help.

GNJ, I would stay with the cam you have and focus on the exhaust like Pat said. Look for a good used set of headers or similar in which you can extend the inner tube out the tip. These tailpipes will be of the stainless tube variety, not a cast riser.

When comparing cams for reversion characteristics focus on the timing event EVC (exhaust valve close). You have a baseline with your current cams EVC degrees. Go from there if you are going to change cams. The lower the EVC number the less time the exhaust valve is open while the piston is moving down in the bore. This can be impacted by grind or by how the cam is installed. It is wise to install cams with the IC specified and select the proper cam up front.

263 and 271 @ 50. Installed @ 108 Deg.

Buizilla, not sure how it ended up st 104.5 centerline. I am using the stock Merc single roller timing set that came with the motor. There is only one keyway on the crank gear so no way to get this put on wrong. I verified timing marks line up.
Maybe machining tollerances?
Blue Thunder, motors have a PVC valve.
So my 104.5 centerlin should actually be helping the situation.... It seems to make sense about closing the exhaust valve sooner on down stroke. This means if I correct that mistake my other issue will get worse. Oh poops!
Does anybody think going to a 114* LSA cam would reduce reversion? I 'm thinking the farther apart the lobes are, the less time the intake and exhaust valves would be open together on the downstroke.
Pat: Moving the centerline of the cam to 108* - wouldn't that be moving 2* past ceterline the other way and retarding? I will try to call RM to see what they say. The extended risor idea is worth looking into. Does any body have the EVC of the HP500 cam and the ZZ502 cam to compare. I never received a cam card when I bought my cams.
Thanks to everyone for your thoughts and insight.

GNJ,

Part of the confusion is in the calculations. Overlap is a direct function of duration and lobe separation. The zz502 cam has a LSA of 110* and that is ground into the cam and cannot be altered without regrinding the cam. You cannot open or close your lobe separation. The lobe separation angle is the fixed amount of degrees between the intake and exhaust lobe. If a cam is ground with a 110* LSA and the intake centerline (ICL) is set at 110* ATDC and the exhaust centerline (ECL) is 110* BTDC, the cam is straight up. If you advance the ICL by 4* the ICL becomes 106* ATDC, the ECL moves a corresponding 4* and becomes 114* BTDC. No matter how far you alter your installed ICL, the lobe separation and duration remains the same, as does the overlap for this cam. Given 224*/234* @ .050” on a 110* LSA, the overlap for the zz502 cam will always be 9* @ .050”. When you advance or retard your cam all you are basically doing is moving your overlap crossover point in relation to TDC.
We always use seat-to-seat figures (adv duration) to calculate overlap because it will give a more accurate representation of the entire lobe.
The differences between adv duration and .050” figures give the indication of lobe intensity, which may alter overlap by inch/degree calculations but not by degrees of rotation.

If the current cam was installed @ 104.5º Intake centerline, that would result in the cam (per that cofiguration) being advanced 5.5º, which would be close to ideal for your application.

It has been my experience that altering the overlap crossover point +/- 4* in either direction will do little to eliminate the effects of exhaust reversion. There are many other variables that may initiate water intrusion on the exhaust side, I would not focus entirely on the cam profile.

Bob

The RM BBC marine exhaust manifold does not dump the water far enough back, even with their "water dam" design. IMHO
http://revolutionmarine.com/images/manifolds5.PDF

Or the Custom Marine exhaust. If Mercury uses them, that's a pretty good endorsement. (I didn't say great)
http://www.custommarine.com/pdf_docs...2007-03-20.pdf

Or for your application (NA 502), there is the Keith Eickert cast Aluminum marine exhaust. If you really want to, you could route the water from the header (plug the tailpipe water entrance tube), to a hull/transom dump fitting. Question is, even using a high temp exhaust hose, would the exhaust temps still be too hot for the exhaust hose coupler? The hose I was referring to is the light blue colored hose. Do not remember if this a Goodyear product. Hmmm, I get back to ya on this one.
http://www.keitheickert.com/products...ke-exhaust.htm

RM Builder: Thanks for the details you provided. If the cam was 5.5*advanced, it should close the exhaust valve sooner and help reduce reversion, correct? I'm trying to understand when the EVC happens in relation to TDC for this ZZ502 cam. I have been told that a HP500 cam closes the exhaust valve at TDC and I thought having a cam with this characteristic might greatly reduce my reversion issue. I was also condemplating using a long risor in conjunction, but I would have to sacrifice my silent choice exhaust.
I noticed Eddie Marine makes a little longer risor than what came with my RM manifolds. My 2 ideas at this point was:
1) Change to a cam with similiar lift and slightly shorter duration profile but ground on 114* LSA. Change stock risors to the slightly longer risor by Eddie Marine. (2" longer)
2) Lose the silent choice exhaust and install long risors
(16" long before water dump) which Eddie Marine also makes, change cam to similiar profile, but with 112* LSA.

I'm open to suggestions from anyone who knows more about this than I do or has had experience with this type of situation.

Thanks - Gary

You're assuming both the intake and exhaust pressures are the same. The only time that would happen is at full throttle, with a perfect exhaust system. As long as the intake pressure is less than the exhaust pressure (such as at part throttle), and cam overlap exists, there will be reversion.

Also it is too bad that so much emphasis is given to Lobe Separation Angle. That number really doesn't tell you much. What really matters is the number of degrees of overlap at 0.050 valve lift. You can have a low duration cam with a low LSA which will have less overlap than another cam with a higher LSA and more duration.

Michael