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REVERSION WHAT IS IT ?                                                                                                                         

Reversion is simply the intake and exhaust gases momentarily flowing backwards during the overlap phase of the camshaft at low cycling rates.  Overlap means just what the word implies.  The intake event overlaps the exhaust event and the exhaust event overlaps the intake event.  The overlap event starts with the piston on the exhaust stroke.  As the piston is pushing out the last of the exhaust gases and about 30 or so degrees before the piston reaches top dead center, the intake valve begins to open.  At low cycling rates the intake charge and the exiting exhaust pulse have not yet created any momentum.  Thus, the piston pushes some of the spent exhaust gas into the intake manifold.  That is why engines with big camshafts have such a radical sounding idle.  The exhaust pulses shoot up into the intake manifold, causing a major disturbance.  The cylinders receive an uneven mixture of air, fuel, and spent exhaust gas.

The piston then reaches top dead center and begins the intake stroke.  At this point both valves are still open, and in fact the exhaust valve in some cases may not shut for another 50 degrees of crank rotation.  During this 50 degrees of crank rotation, the piston literally draws from both the intake and exhaust valves, causing the exhaust gases to momentarily reverse.  Enter water into the exhaust via a wet header and water will be pulled into the cylinder.  At high cycling rates the inertia of the incoming intake charge and the out going exhaust pulse keep the gases flowing in the proper direction.  But at low cycling rates, reversion can be severe enough to add water to the oil (milky oil), rust valve seats, and even stall the engine.  This effect only happens at idle, but engines encounter their greatest reversion pulse at shut down.

THE ONLY TRUE TEST FOR REVERSION IS TO IDLE THE ENGINE WITH THE HEADERS ATTACHED AND WATER GOING THROUGH THEM.  SHUT THE ENGINE DOWN.  REMOVE THE HEADERS AND IF YOU HAVE WATER RESIDUE LAYING IN THE EXHAUST PORTS OF THE HEADER, YOU HAVE REVERSION  .

Imagine this:  You're blasting across the lake at wide open throttle showing your buddies on the shoreline how cool you are.  Inside your engine, your exhaust valves are red hot.  You finally decided that you are the most bad ass out there and pull the throttle back to idle.  Because you didn't check for reversion, the water from your headers comes back and quenches the exhaust valves.  The head of one of the valves cracks and falls into the cylinder, and stops your engine.  Think it can't happen?  Think again.

When comes to reversion, camshaft selection is very important.  If a automotive style camshaft is used, the following guide lines apply:

Based on a 454 C.I. CHEVY engine with a Mercury replacement header (P/N 540304)and 8" collector.  The camshaft should be no larger than 230 deg. duration @ .050 lift.  Lobe separation angle 112 degrees.  Installed @ 108.  These figures are just guide lines.  Cubic inch displacement, valve size, connecting rod length, valve timing, etc. all can effect reversion.

ANTI REVERSION TIPS

CAMSHAFT SELECTON        We have spend countless hours developing camshaft and lifter combinations for our headers.  We have cams and lifter combos that are as big as 270 duration @ .050 and .900 lift that can be used with a standard set of wet headers.  

COLLECTOR SELECTION     Add collector length whenever possible.  The farther away the water is introduced into the exhaust stream, the better.  If noise is not an issue, use a standard JACKETED TAIL PIPE . Or, if noise IS an issue, use a  MUFFLER JACKETED TAIL. Both tail pipes extend through the transom and introduce water outside the boat.  Normally the tail pipes extend 6" past the transom and introduce water in the last 1/2" of their length.

CUBIC INCH DISPLACEMENT    The tip here is simple- the bigger they are the harder they suck back.  Either reduce cam duration, add a collapsible lifter, increase valve lash, advance cam, etc.    

CONNECTING RODS    Marine engine builders rarely consider connecting rod ratios effecting reversion, but it does.  A longer than stock connecting rod will make the piston dwell at the top during the overlap cycle, thus less piston movement with regards to crank rotation.

 

WATER VAPOR MIGRATION

Water migration?  What the hell is that ?  Yes boating fans, there is another gremlin trying to ruin your engine.  We are calling it "water vapor migration".  We now and then get phone calls from customers telling us that they checked for reversion by pulling a header off after idle and no water was present in the exhaust ports.  So why are the valve seats rusty?  Answer: water vapor migration. 

Lets assume that you are running  up the river and you see a good place to beach your boat.  You idle up to the beach and shut the engine down.  There is a good chance that a least 2 cylinders are somewhere around their overlap cycle.  Remember on the overlap cycle, the intake and exhaust valves are both open.  Steamy water vapor now has an avenue to float up the exhaust through the exhaust across the cylinder up the intake and out the carburetor.  Over time surface rust will form on the valve seats.  It is an inherent negative with wet exhaust.

Water vapor migration doesn't seam to do much damage.  Most of the time when your start up the engine after resting the valves just beat the surface rust off.  To winterize it'd be beneficial to wet the valves with some Marvelous Mystery oil or WD 40. 

The only fix is a JACKETED TAIL PIPE and never mix water with the exhaust.