I built a Merc 496 w/Raylar’s 625HP Stroker kit. Initially I welded on 18” extensions to the stock manifolds to keep the exhaust dry much further back than the stock risers. Now that I am bolting on the CMI headers to squeeze the last extra ponies out, I am trying to fit the correct tailpipes to prevent reversion. See build and boat here: The Raylar Experience Mercury 496 mods


So there is plenty of reversion discussion and cam spec talk like LSA and valve overlap lending to reversion issues. But does anyone have any ballpark data on how much is enough? How dry is dry?

Meaning, how far distance-wise from the head or manifold downturn can you introduce water and still be safe? 12” 18” 24”? Or does it not matter and water will be drawn back up any distance over enough low rpm time?

I did contact CMI tech about the standard tailpipes which introduce water approximately 8-9 inches behind the collector, all they had to offer was that they are adequate for the stock motor/cam.

No one seems to know but rather most err on the side of caution and say run a full dry exhaust or dry to the tip?

I know there can be more factors to the specific build but how can you ballpark how much dry exhaust length you need before water is introduced?

I’m definitely going to run the boat at idle for awhile before pulling stock manifolds to see if the extensions were adequate. Hopefully they were!

Should I run full dry or just to the tip?

Cam specs:

Lobe separation: 116.1
Overlap @.050 12.7

Intake: centerline 112.0
valve lift .634
Duration .050 242.0

Exhaust: centerline 120.2
valve lift .647
Duration .050 249.0

You may want to call a cam company and ask their opinion. I can't imagine a reversion issue with that small of a cam on a 116 LSA., I could be wrong.

Yes, I felt relatively safe running that extension off the stock riser to the transom, but I’m now hesitant looking at the shorter CMI tailpipe. Heck, I’m starting to second guess that modified stock length now that everyone jumps right to “run it dry”.

I guess a cam expert is probably a good next step, thank you!

The noise Police don't care for "run it dry", if your cam was 10 or so degrees bigger cut on a 110 LSA I would be checking plugs for water.

I would err on the side of caution and have CMI make your tails dry to transom. I would be leary with your setup on the standard short tails. It's trial and error, but longer is always better!

Run your current setup at idle for a couple minutes. Pull off the risers and look down into manifold for any water.

Yeah those short tails worry me.

My next step, since I’m pulling my manifolds off anyway, I’ll idle and inspect then go from there. If the manifolds are dry now I will assume that distance is adequate.

I definitely have an inch or two I could probably extend further into the muffler and be right at the transom, not sure if the muffler will then help create a reversion issue.

The bump stick seems to be regarded as a “big cam”, but looking at the specs it doesn’t seem particularly big enough to be a reversion red flag to me, but what do I know?

Thanks for the input!

Are those rope seal tails to gasket seal headers?

Would there be enough water from the hose even at idle to create the in water conditions of reversion?

Got me, it’s what CMI provided.

valid question I guess, but reversion is only likely to happen at idle and the way my set up is the hose feeds directly into the seawater pump plumbing so I would assume it gets generous amounts of water, it comes dumping out of the exhaust on the hose so it’s definitely present to suck up. I can fill up a gallon of water from the hose and time it to determine GHP. The hose is large and water feed is generous.