Cam duration and water REVERSION

[WHATA SQUIRT]

I'm building a new 496 carb motor for my 21 ' jet,and have been warned about cams and water reversion problems. This motor will have log manifolds exiting below the water line..is there a max duration or a set rule of thumb please ,any help would be greatly appreciated....

[centerhill condor]

welcome to the forum... as I understand it, you'll be okay as you have logs and snails! the reversion issue is when cooling water used in open or over the transom pipes is pulled into the cylinder during valve overlap.

[sleekcrafter]

"Lightning Header's" Theories and guidelines for REVERSION
Reversion is simply the exhaust gases momentarily flowing backwards during the overlap phase of the camshaft at low cycling rates. During the overlap phase the engine is on the exhaust stroke and the piston is pushing out the last of the exhaust gases. Prior to reaching top dead center the intake valve begins to open. At low cycling rates the intake charge and the exiting exhaust pulse have yet created any momentum. Thus the piston pushes some spent exhaust gas into the intake manifold. This is why engines with big camshafts idle and sound radical. The exhaust pulses shoot up into 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 open, 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 will momentarily reverse. 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. Not a problem until you add water into the exhaust stream. Concerning headers, reversion can be severe enough to add water to oil (milky oil), rust valve seats, even stall the engine. This effect only happens at idle, but engines encounter their greatest reversion pulse at shut down.
For this reason Lightning Performance Marine has developed guide lines to help you decide what options to add when considering an exhaust system.
Our guide lines are based on a 454 C.I. engine with a standard Mercury header and 8" long collector. The camshaft should be no larger than 240 deg. duration @ .050 lift. Lobe separation angle 112 degrees. Valve lift is not that much of a factor. These figures are just guide lines. Cubic inch displacement, valve size, connecting rod length, valve timing, etc. all have an effect on reversion.
THE ONLY TRUE TEST FOR REVERSION IS TO IDLE THE ENGINE WITH THE HEADERS ATTACHED AND WATER GOING THROUGH THEM, SHUT IT DOWN, REMOVE THE HEADERS AND IF YOU HAVE WATER RESIDUE LAYING IN THE EXHAUST PORTS, YOU HAVE REVERSION.
Lightning Performance Marine makes many options to reduce or totally eliminate reversion.
ANTI REVERSION TIPS
Header selection The tip to header selection is to choose a design that will introduce water into the exhaust stream as late as possible. For example our 40340 header is actually made to replace a Mercury manifold with a 3" riser block. This header is 4" taller than a standard Mercury header plus it rocks forward 2" thus were able to add a 2" longer collector. If the height is not a concern you just added 6" of dry length to the header.
Collector selection If you have room to add dry collector length do it. Collectors are available in a variety of lengths.
Cubic inch displacement The tip here is simple, the bigger they are the harder they suck back. Either reduce the duration of the cam or start add some anti reversion options.
Camshaft selection With regards to headers the only thing your concerned about is how much piston movement in volume takes place while the exhaust valve is open on the intake stroke. A wide lobe separation angle actually advances the exhaust valve timing event which will close the exhaust valve sooner, but the down side is it also moves the horsepower and torque curve up the RPM range. Rhoades style bleed down lifters will deduce the cam duration by as much as 20 degrees. Available only in standard hydraulic these lifters are a great choice.
Connecting rods Marine engine builders rarely think of 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.

[sleekcrafter]

All that said, some cams, you will still have reversion problems. If you keep the idle up, it will relieve some of the reversion, but not the cure. The problem when running logs is not, that they are only below the water line, but how far the water is discharged from the ports, as it occures during idle or low rpm operation, and tries to suck the water back into the cylinders. Reversion or hydro locking also can occurer during start up to, with log manifolds, good flappers are a must! I believe the guide lines were told to me, to stay between 108-112 for lobe seperation, 114 on logs would be a problem.
Sleek

[Moneypitt]

[QUOTE=sleekcrafter]"Lightning Header's" Theories and guidelines for REVERSION
At this point both valves are open, in fact the exhaust valve in some cases may not shut for another 50 degrees of crank rotation.
?????????Boy that is one hellofa camshaft. Lets see, EX closing 50* ATDC, so the IN would be opening, maybe 35-50* BTDC..........Thats possibly 100* of overlap!!!!!! I don't think it would idle below 2500 with, or without water..........If it ran at all.................(is that @ .050???)..........MP

[SmokinLowriderSS]

I don't know it there is much difference in the snails out there, but the ones I just replaced with Lightnings had the water injected after the snail riser/divider. That's a foot aft of the last cylinder and at the bottom of a nearly 6" high divider. I don't see how these could possibly have idle reversion draw water backwards that far over that kind of "terrain".
Now, the lake water level was about an inch or so up from the bottom of the port so IT had only about a 5" divider but I think that would be driven down by exhaust flow.

[Moneypitt]

This is what I posted over in gearheads to the same question....MP
[quote]
Log manifolds? There is no way you will have a water problem, the water doesn't enter the exhaust flow until after the snails and it won't be sucked/climb back to the port from there unless the motor runs backward for awhile, a long while............MP

[IMPATIENT 1]

This is what I posted over in gearheads to the same question....MP
[quote]
Log manifolds? There is no way you will have a water problem, the water doesn't enter the exhaust flow until after the snails and it won't be sucked/climb back to the port from there unless the motor runs backward for awhile, a long while............MP
i'm with m.pitt on this.if you put in a "big" cam with logs on,the cam isn't goin to do much for ya anyway.buy yourself a 500 lift cam with a small duration.big cams need to breath and if your exhaust and induction isn't to par, then don't put in a big cam.put it in your head too that once you go so big on a cam ,springs,valves,pushrods,retainers, and locks will need upgrading too to accept the added stress.

[Moneypitt]

Long slot rockers, at least...................MP

[WHATA SQUIRT]

This is a complete build up!Carb to pan w/ alum heads.just came up a little short for the headers...they will come soon though.its gotta be ready by memorial day.not lookin' to go big necessarily just trying to avoid a possible problem once again thanks for all the help...

[Moneypitt]

So, now you're asking about headers? That is a whole different situation. The "log manifold" part got you alot of useless information about water reversion. Now you have to get serious about cam choice if you plan on running water thru the headers. The area of overlap is where you must be careful. You also must devert some of the water flow away from the headers with additional "dumps" overboard. I also restricted the flow to the headers with "pills" in the fittings. Each set up is a little different. I have been told the "t" ball check valve is to be closed at idle and only allow water to flow a "givin" RPM, (water pressure), from the jet.........Good luck.........MP

[SmokinLowriderSS]

Over transom or thru-transom?
Injected headers or jacketed headers?
The injected will likely run you the greatest risks of reversion problems, and like MP said, everyone I have seen discussed had them dry at idle to a bit above. I suppose there is some risk of reversion with jacketed headers if the collector dumps aren't far enough back (or "downhill" on thru-transom ones) but it is less of a risk. My duration should be no problem for my jacketeds. My hydraulic cam is only running 280* seat/seat with .572" lift.

[WHATA SQUIRT]

smokin' the headers will be a through transom.water jacketed,most likely a lighting-most reasonably priced.

[Marty Gras]

If you are worried about reversion, have your exhaust valve close at ZERO! The longer the exhaust valve is open after zero, the more you have a 'reversion' problem. The problem between 'max power cams', and the cams that really work in a marine motor, are the 'reversion problems' and how to control them. You can advance a 'performance cam' to get the correct exhaust timing, or just have a cam made for your use.

[HotDogz]

Take a look at Paul Bassetts Tri-Y TT, SS water-jacketed headers. Replaced my logs with em because of reversion but picked up 300 rpm just puttin em on. With 244* @ .050, I was pulling water back into the logs, wouldnt idle.
Bassett Racing (http://www.bassettracing.com/newpage11.htm)

[SmokinLowriderSS]

I've seen a set of Bassett Tri-Y's @ HTP's shop, and drooled suficiently (not on the chrome tho). Well outta my price range at this time. I found a set of used Lightnings that still set me back $1,500 by the time I bought 4" outlets too (oh yea, that damned 6.9% sales tax).