I don't have oil thermostats and i'm running circulator pumps with 140 water stats. The engines run a little over 150. Since i owned this boat over 7 years i have never seen the oil temps lower than 210-215. One thing i did notice after running and then stopping the oil temps drop quickly under 200. I guess this how Mercruiser designed it. At least i won't have to worry about condensation

Engine oil / vapor is a very low octane "fuel" and does no good when introduced into the intake.

Couple that with the fact that the PCV valve is fully open (low vacuum) under full throttle / high cylinder pressure and you have a recipe for detonation. I'm betting that's what your carb guy was getting at.


PCV valves are great for keeping your bilge smelling nice and your valve covers nice and clean. Not so good for highly loaded / high cylinder psi endurance engines.

If you have a bunch of crap coming out of your breathers, I'm of the opinion that you need to look at the cause instead of the symptom. Excessive blowby, improper breather baffles, etc.

I dont like what pcv systems do to efi manifold plenums. Over time the plenum is like a oily sticky gooey mess inside there.

The part Merc used for the HP500 closes under low vacuum conditions (some where around 2" or so the piston is closed). Under high vacuum conditions, it also chokes down down the flow. All of them might close at high vacuum, but the HP500 is the only one I've ever tried to test. PCV valves are designed to provide a metered flow under all conditions, so "fully open" is the same at all vacuum conditions. IMO, the amount of low octane oil is not significant.

I agree - if you have a bunch of stuff coming out of your vavle covers (either breathers or PCV), you have another problem that should be fixed. On an engine in proper operating condition, you shouldn't see an oily mess - even in your intake from a PCV.

How do you keep the dipstick in the engine if the PVC closes under a full throttle/ low vacuum condition?

It has a PCV (back of carb to intake manifold - see 1st pic#1-#5) and two open crankcase vents from valve covers to flame arrestor, see 2nd pic#2 and #6:

What SB said (showed). The breathers are still there, tubes vent to flame arrestor just like a non-pcv Merc install. I still see some minor accumulation on the flame arrestor where the tubes are, but it is 1% of what it used to be. After all, how much time do you really spend at WOT?

Cool. Thanks for the pic SB.

I figured it had to be vented.

i wonder how they bolt a bbc valve cover on a sbc head?

Nice catch Mike.