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.