everything i've been reading suggests that longer duration camshafts don't agree with stock style manifolds. i need to pull both motors this winter/spring and was wanting to install my bigger cams. can i get away with .544/.547 adv. lift and 270/286 adv. duration camshafts and aluminum manifolds until next winter(headers) or am i going to be sucking water or cracking water jackets? i've already sanded the casting marks out and polished them if it helps. thanks, Eric

What manifolds, how long are the risers, and what are the complete cam specs???

stock mercruiser manifolds and risers right now. replaced and smoothed runners late this season when i rebuilt the port motor. was thinking of picking up no-name cast aluminum stock style until next winter. rebuilding starboard motor before this season. 10.25 c/r,,, edelbrock rpm marine heads,,, specs@seat to seat are: .544int/.547ex lift,,, 270int/286ex dur,,, duration @.050 lift is 226int/236ex,,, 112 degree lobe center
thanks Griff, Eric (got a little more if ya need it)

Eric not to change subject but sounds like you are runnin cc XM270 I have my 502s apart right now and this cam is on my list, could I ask how youy liked them? I am going to Gills and think I will have to dry them some because of short riser reversion issues thanks Randy

randy, wish i could help but i've got a pair still in the box. i was wanting to install them this spring if i could so i wouldn't have to pull engines again next winter. you're right on on the cams though. i crunched all the numbers into desktop dyno and it showed a small increase in upper rpm horses with their bigger cams but was not worth the drastic loss of torque and power down around the 1500 to 2000 rpm mark. the 270's looked like they would work better for me. i'm just worried about jumping the gun on installing them w/o the headers. i'll let ya know, Eric

With stock merc center rise manifolds, I'm pretty certain that you will get reversion at idle with that cam. An aftermarket aluminum manifold won't help unless the riser dumps water farther back.

Randy, I don't think you will have any problems with Gils. I had almost the same spec Crane cam with Imco exhaust and no issues.

Thanks Griff for your help with my project. Erci if you are going to get new alum replacements get on the suppliers forum teague custom marine selling take-off 496 manifolds alum man w/ssriser cheaper than the alum replacements ask for Adrian

FYI Guys,

.230 @ .050 is kind of the magic number duration wise. Obviously there are many parameters that can affect this. Here's what happened to Mercury Racing they used a Crane cam that is .232 @.050 and had to do a huge recall on the Gil/Mercury risers on the first HP500 Carb engines and build new risers that come up higher and drop faster so that the water runs away from the top of the riser with more speed. The advantage the stock manifolds have is that there outlet hole is smaller creating more back pressure.

Just a thought you may want to look at a camshaft with some more lift and slightly less duration. I believe it will perform better than the cam you are looking at.

Take a look at these http://www.cpperformance.com/detail.aspx?ID=2362

http://www.cpperformance.com/detail.aspx?ID=2352

I used the merc 420 cam, 228/236 duration with old style 3" log manifolds with no issues. Its the same cam as the 400HP mercs that had 3" log manifolds from the factory. My buddy runs those cams with the stock center rise merc manifolds, no reversion issues in many years.

With that being said, you would be better off with some good aftermarket manifolds with long risers for both reversion and performance. You may not see as much of a gain from the engine upgrades using the merc exhaust.