Question about reversion and superchargers
 

Ok, after reading a bit about reversion on pb's thread, you've got me wondering about whether having a supercharger changes the chance of reversion. Does running a blower decrease the chance of reversion? I realize this is probably not an going to be easy to answer, but as I'm a newbie at supercharged engines I need all the help I can get.

My guess it NO. This is a problem that only happens at idle or just off idle. If the motor has a blower or NA the motor is in vacuum when at idle. Camshaft design & exhaust type are the only items to worry about.

Indirectly, yes. A correctly cammed supercharged engine will have a camshaft with less overlap so the supercharger doesn't blow the air/fuel mixture straight through the cylinder and out of the exhaust when both valves are open at the same time (valve overlap). Most supercharger camshafts have either a wide LSA or less exhaust duration (some have less exhaust duration than intake duration).

The stock GM marine camshafts used in the 454/502 Mercruiser Magnum engines make fine supercharger camshafts. They have a wide LSA at 115.5 degrees and 224/224 degrees of intake/exhaust duration. Very little valve overlap!

Dennis Moore
www.mooreperformance.org

I won't know exactly what cams are in these engines until I disassemble them this fall. The engine builder (the previous owner's engine builder) isn't inclined to give out the cam specs to me, and I can't blame him for that. He did pass a some spring information to me when the previous owner asked him what I should buy to replace them. Isky 8005A, which is a fairly light spring, 130 lbs. on the seat I think, and .600 lift cam max.
I"m going to have one of the engines dyno'd before disassembly to se what kind of numbers it's making.

By the way, the engines are 509 cid, J&E flat-top pistons, Merlin rectangle-port heads, custom-ground hydraulic flat-tappet cams, 1050 Holleys on B&M 250 blowers. I had the heads freshened right after we bought the boat this past spring and the engines have performed very well. But gee, we all want just a few more horses, right?

2 kinds of reversion.

Exhaust reversion (that pulls water into the cylinders)
Intake reversion (that causes air to oscillate back and forth thru the carb, double-fueling it and making it slobbery rich at low engine speeds).

A blower will reduce or eliminate Intake reversion (draw-thru will eliminate it - blow thru will reduce it).

A blower will have no effect on exhaust reversion (versus the same engine with no blower).

Doesnt it seem like blowers smooth out the idle a bit? Or is it the mixture that does that? Greg

Yeah, they seem to idle much better. I'm assuming that's because of the reduced intake reversion, probably partly due to the decreased overlap too. I could be wrong though. I'm new to blower motors.

Exhaust reversion occurrs when the exhaust valve is not closed before the piston begins the down stroke causeing a vacumn in the cylinder and air/gas to be drawn in through the intake valve and, exhaust/water to be drawn in through the still open exhaust valve. A blower with 5psi of boost will charge the cylinder with 5psi of positive preasure when the intake valve opens and so the piston will have to travel a little further on the down stroke before a vacumn is created until which ttime exhaust/water will not be drawn in through any still open exaust valve. The effect is minimal the same as a few degrees less exhaust overlap, the more boost the more pronounced the effect will be.

"A blower with 5psi of boost will charge the cylinder with 5psi of positive preasure"

This all well & good except when you are at idle. Intake maniford is in vacuum, thus air is not being forced into the cylinders