gofastpilot

When they say water reversion with HP500's are we talking about the exhaust manifold(GIL) cooling water getting back into the engine or is it water from outside the exhaust pipe that makes its way into the engine? It is said that the cam lobe centerline has alot to do with this reversion...true? If this subject has been already been beatened to death sorry I've been busy............gofast

Vinny P

Reversion means that water is being sucked into the motor from the exhaust. This can be caused by too much cam overlap, too much duration, too short of a riser, etc...

FloridianSon

A lobe separation angle between 104-108 will allow it to happen. A LSA between 109-112 might allow some water so mercruiser has suggested on their high per. engines that after extended periods of idling, the engine should be revved up a few times before it is shut off to clear any water.It is generally accepted that 109 degrees is the best LSA in a marine engine, but may have slightly too rough an idle for some boaters.

Dave F

Water reversion has to do with overlap. Period.

A combination of duration and lobe separation contributes to overlap.

The closer the lobe separation the sooner hp/torque will peak and drop off. A wider separation will allow hp/torque to spread over a broader portion of the rpm range of the cam.

It depends on your engine and what you want to accomplish for a recommended lsa.

The amount of overlap you can handle depends on the exhaust you have.

Most engine builders like to stay with a 112-114 lsa for marine use. The tighter the lsa the rougher the idle creating the need for a higher idle speed in turn creating more of problem maneuvering in the marina and clutch wear.
I have read that a 109 lsa is used for optimum hp for a high performance engines but to my knowlege even the Merc 575's don't use that tight of an angle.

DAVE

blackhawk

Dave F,

So the cam overlap allows water to that is in THE EXHAUST to go back in the motor?

How does a taller riser prevent this?

I know basically what reversion is but I am unclear to exactly HOW IT HAPPENS.

Thanks

Vinny P

Cam overlap means that for a short while, both valves are open at the same time. This happens when the piston is beginning its' intake stroke. For a small amount of time ( cam overlap ) the exhaust valve is still open. This causes a suction or a backwards pulse in the exhaust. It is that pulse or suction that can cause water to revert back into the cylinders. The longer the riser, thus the further back water mixes with exhaust, the less chance you have of getting reversion. The larger the overlap, which means the more duration and/or the tighter the lobe seperation, the more reversion will occur.

blackhawk

That makes sense. So, when the say "longer" riser they really mean "taller" meaning the water is mixed farther away from the cylinder.

This must be why the don't recommend using silent choice with big cams? Because silent choise usually has a short riser?

Thanks

stevejohns

you have it right BH... You can still use fairly big cams but the overlap and duration are restricted as is the resulting HP. According to the guys at nickersons, they are using a roller with over .585 lift with silent choice.

Cord

iTrader: (2)

The carbed HP500's had a problem with reversion and the gil exhaust. That's proably why they are now using the cmi's. The factory fix was to raise the idle to a higher level to reduce the chances of reversion. The next step was to go to a higher riser or dry tails. If you do have reversion problems, then check your valve springs. Apparently these springs had a tendancy to rust and break. The service replacement is the new HP500efi spring. I believe that it's being made by crane. Either way, if you have more than 250 hours on the stock springs, they should be swapped out.

Dave F

Simply put, the further towards the transom you can get your water to enter the tail pipe the better.

I ran a Crane roller 236/244 112 .610/.632 with CMI's that are the same as what's on the 500 efi's. The water entered the tail pipes approx. 3-4 inches from the transom. No problems.

The reason they still got reversion in the Gils is because they're just a slightly improved Merc design. They have dividers inside that help separate exhaust pulses but not much more than that.
It is also true that they had problems with the valve springs. I do believe that they changed to the Comp cams double spring.

The best set up would be a "dry" setup. The pipes are still water jacketed but the water never mixes with the exhaust gas. The water is expelled through it's own pipe.
With that set up you could run any cam you wanted to.

DAVE