Ramos, if you are running stock exhaust, you may be on the edge of reversion if you are running duration in the 230's with a 112* LSA, but there are a lot of factors that come into play here. First of all, you really need the full cam opening and timing event figures to determine the true total overlap to really make a statement on this. For example, a flat tappet cam may have nearly the same specs as a roller in regards to duration at .050 and LSA, the two figures that most people use to compare cams, but the total overlap (the time when intake and exhaust valves are both open) could vary greatly. A roller may be able to get that valve up off the seat much more quickly than the flat tappet, so the overlap below .050 lift may be greater. This is why you have to be careful.

In addition to this, other factors like the cubic inch displacement, type of intake manifold, and even the angle that the exhaust leaves the riser and heads out the transom can affect the tendency for a given motor to revert.

FWIW, I ran a Comp Cams flat tappet cam that had specs in the neighborhood of .540 / .550, 226* / 236* @ .050, 112* LSA on a 454 with stock Merc exhaust and Silent Choice with no signs of reversion, but your mileage may vary. I am now running a hyd roller cam with what would appear to be much milder specs (114* LSA) in a 489 with Lightning headers that have the silent choice collectors, and I have been put on notice by the cam grinder to be vigilant about possible reversion with this setup. If you looked at the specs side by side, you would think that the second cam is much safer in regards to reversion, but you have to check all the factors. Now I don't know if I will have reversion with the new setup - don't have it running yet, but you can guarantee that I am going to be careful with it, just in case.

Good luck with your build.

Thanks Budman. I will be running merc 496 exhaust aluminum manifolds with stainless risers. I could extend the risers but only a little as my exhaust isn't straight out the back.

Stock Gen VI block that I was using for my built came with the main caps stamped from GM. I used aftermarket rods in my build so I don't know if the GM rods were stamped from the factory. Of course, it may have been done differently back when they were building the Mk IV's.

I think the 496 manifolds used a longer, taller, less restrictive riser than the old cast iron manifolds, so that should help. Running without the Silent Choice will also help with reversion issues. Bob would be the guy to talk to about avoiding reversion issues with your setup. One thing that can be said about Bob - he will not give you advice that will lead you into trouble when it comes to durability - he is all about that!

One more thing - if you are still thinking about going with boost, this tends to work in your favor for avoiding reversion, as the positive intake pressure tends to prevent reversion.

I'm now planning this build for present time better performance than stock and also with future potential. If next season I decide that I will not be running boost with these engines then I can mill the heads for a little extra HP. Yes, I believe my exhaust choice is much better than the stock cast iron's. I ran this set up with Ultradyne 230/238 112LSA and was ok in the past. Here is a pic of the motors in bilge for reference of exhaut. No silent choice but there are some bends.

[ATTACH=CONFIG]511631[/ATTACH]

That's an unusual exhaust setup. Is there something on the outside of the transom that prevents you from sending those center exhaust pipes straight out the back instead of making that 90* turn and combining with the outboard exhausts?

The picture appears to me to show stock cast iron Merc manifolds and risers.

That is my stock set up before pulling the engines a few weeks ago. The bottom of the molded swim platform might be in the way of 4" cut outs if wanting to go straight back but the bigger issue is closing the current 5" hole at either end of transom. And, that would require removing the swim platform. Really don't want to remove taht thing yet. When I do it will be to replace it with an aftermarket aluminum one but that's definitely later in the future. I will be removing that diamondplate covering you see along the transom and bulkheads. It looks good but hides the origins of any leaks and I know I have one somewhere. Fixing that too this offseason.

What's going back in will have the 496 exhaust.

Wow- I've been gone for a while and wow has this forum as changed, and not for the better. Call Bob Madara and be done with all this garbage, there is a reason why most of the top marine engine builders use his cams. His number is 585-654-8583. He will tell you exactly what will work and what won't, and you won't be told to change anything that doesn't need to. This includes every aspect of the engine, in addition to that Bob is one of the friendliest guys you will work with. I've consulted with Bob on every engine build I do, and I've yet to regret one suggestion he has given me, that includes what engine builders/machinists to use.

Everyones gotta do a shot!!