Reversion, cam limits for common setups.
 

So where would the cut-off point be with any of the common power packages and exhausts? And power limits with the standard exhausts? Everybody is always asking what is too much with my 330 or my 365 mag or the L29 with 496 Gils, etc...Maybe we can get most of the answers in one place.

First off I am not knowledgeable on exact numbers for that but I can tell you it can be very boat dependent. ESPECIALLY with molded in swim platforms.

Chris

I have a Merc 525sc. I am running EMI thunder Exhaust. The cam I will be using is Crane 134561. Theres a few on here with that cam and no reversion. Ill know for sure in a month or so.

A lot of factors goes into a reversion issue what are you trying to do? how high get you get your tailpipes .lenght of pipes are a few factors

Watching with interest. Thinking about next build and wondering where that line is for Stainless Marine manifolds. How much more would going dry close to tip give you?

if you look at the cam,s that merc uses with the black engines you will see that the duration,s are short and the lsa is wide.the mix of water&exhaust gas in the tailpipe[usually a 4in rubber hose]is the problem.the best way to avoid reversion is to remove the water from the exhaust flow.that is why aftermarket exhaust manifolds&dry tailpipes exist.any time you put a cam with a tighter lsa or much more duration with wet exhaust you increase the chance of reversion,period.i hope this helps people to understand reversion.edit in,when you add duration and tighten the lsa the overlap period is longer,now it is trying to suck some of the exhaust[and water]back in.this is reversion.to much and it can ruin an engine quickly.

One notable exception is the cam most utilized Merc SBC, it has a 109 LSA, but very short duration (GM 395 cam).

From my reading, ignition timing plays a role in reversion too. A motor running 11* of timing at idle will have less vacuum than a motor with 28* of timing at idle, increased vacuum increases the reversion. I assume if you have a cam that's on the verge of reversion, running less timing at idle will help.

if you have a reversion problem changing the ignition timing won,t fix it but a dual plane manifold can help.

because the duration is so small the 109 lsa is ok.

Just curious, what are the specs roughly on say a 741 or 651 cam, they seem to be talked about a lot, and how do they fare reversion wise? Never really hear what exhaust folks are using with them.

Edit:looked up the specs,still curious about reversion in 651 or 691 cam. Looking at something similar except in a solid roller

both of those camshafts need non stock exhaust.they need dry or very close to it or they will revert.

I concur totally. It's just an oddball configuration that will throw people unless they look at the entire cam profile. I think the .050" duration is like 196 on the intake for this cam.

that engine must idle as smooth as a piece of glass.

What matters is how long the exhaust valve is open ATDC during overlap. If you do a search, I have many cams, including the popular blue motors, showing specs including overlap #'s.

Even if not shown, overlap is easy to calculate - it's the # of degrees from when the intake opens to when the exhaust closes. ie: IVO to EVC.

Way back when, Crane said it's easy to get into reversion issues when overlap at .050" specs exceeds 6 degrees or more.
Again, if not stated, most cam companies with show events at .050" also. Take how many degrees from IVO at .050" to EVC at .050"

Yes, it's 196/206 at .050" with 109LSA. Overlap at .050" is a negative number. It's a truck cam...literally.

sb,i was going to include that info but i am way to lazy tonight.

LOL.

Tomorrow, being Monday, I had to release some info from my limited grey matter so I'd have a few brain cells free'd up for work . hah !

Oh, I know and they used them in droves for nearly every EFI SBC marine motor from Merc to ski boat motors.

It does idle smooth. Pulls strong in the midrange then falls on it's face above 4000-4200 rpm.

that is good information,the overlap is what causes reversion, the lsa is just the placement of the cam lobes.

so essentially, flow past valves is essentially nonexistent below .050 opening? and I can see where a single plane could add big time to the problem. the idea behind the dual plane is to spread the intake pulses out so no 2 cyls in a row draw from the same primary venturi. so a valve still closing isn't subjected to the high vacuum of the one next door in full suck mode. no, that doesn't sound dirty at all...

so an exhaust valve still closing (ATDC) isn't subjected to the high vacuum of the one directly across the street in full suck mode. Next door can influence, but directly across the street is a straight path.

Figured I'd do a little correction to your wording, but yes, you are correct. I forget which cyl#'s it is, but there is one port 'directly across the street' inhaling while the other is ATDC (after top dead center) in it's exhaust valve closing. Since the throttle blades are closed, the exhaust valve still open has less resistance for air to come in backwards.

I have been told that modern solid and hyd roller profiles get the valve up off the seat much quicker than the flat tappet profiles, which have longer, more gradual clearance ramps. This means that a roller profile may have more "area under the curve". Couple this with the more efficient, better flowing ports of a lot of aftermarket heads, and you could see reversion occur with a roller that has identical specs to a flat tappet at 0.050" lift. I am running a hyd roller with very mild specs - 226* / 230* @ 0.050 and 114* LSA - that on paper should not revert, but with the much better flowing raised exhaust port heads and a single plane, it was just enough to cause it. Longer stroke can also be a contributing factor.

My cams are close to a 731 in duration, and you will see my exhaust needs the tall risers to prevent reversion. You need to decide if you want to choose your cams based on your exhaust, or choose your cams and then get an exhaust to fit them

I would think the flat tappet, with slower open/close ramps would be worse culprits. Cuz you can get more effective duration with less real duration with rollers. valves flow enough to cause reversion at .050, but they don't flow enough to make power. with the slower ramps on a flatty, you would have start opening sooner and hold open longer to get the same .200 to .600 duration...

My opinion is you can reduce the duration at .050 by six degrees to match the performance of a flat tappet camshaft. Example a flat tappet cam 230 I / 236 E at .050 will perform similar to a hydraulic roller that is 224 I / 230 E at .050. When ordering a custom cam I check the overlap at .050 and keep it below 5 degrees for stock center riser exhaust.

Guys, You are making my day. I stole an idea from a respected member on here and bought a high lift for sbc roller which is 218/224 at .050. It's in a box in my garage.
Going in a 383 for my slow fat Formula to pick up some MPH.
My good running 350 mag won't push that tub over 45. It might not even be a mag. It has a sticker on the flame arrestor cover but who knows. I can tell you it's a 97 engine & has un vortec truck heads. Smooth but no rpms.
I got nervous about reversion and got 3" extra height stainless long tube exhaust risers from Eckert. That should keep my engine dry of reversion.
Boy do we all need July!

I don't know BB. but one other thing small block folks can do is to get a 2 piece front timing cover. If you can get to the front of the engine you can change the cam in the boat that way. If you have to dial back your cam and don't have the cake for stainless exhaust, it's an option.

thats what happened to me a couple of times,,had an edelbrock rpm perf cam(flat tappet) w/ wet tips(old gil trs set up) and no problem at 240,248,went all the way up into the 250's range,no problem.decide on new engines to run isky 228,238 roller,could see the water getting sucked back up.ended up welding the tips,totally dry.Isky cam did get 1.80 rocker arms too so cam was a little bigger than mentioned.