Twin 260/350 upgrade
01-19-2012, 09:37 PM
#21
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The engine builder I talked to today, said that the valve springs could be tired, and would certainly cause this issue. Not sure on the hours, no hour meter.
But what has killed me is that he said the cam in the motors is worthless and that swapping out the heads will not net a good power gain. I was thinking I should easily pick up 50 HP between the heads and intake and roller rockers, plus shedding a considerable amount of weight.
But what has killed me is that he said the cam in the motors is worthless and that swapping out the heads will not net a good power gain. I was thinking I should easily pick up 50 HP between the heads and intake and roller rockers, plus shedding a considerable amount of weight.
01-19-2012, 10:10 PM
#22
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Haha yeah, that was the first thing I checked. The primaries open 90degrees. this has been a thorn in my side for a while.
Today I talked to an engine builder in the area that said the 1986 vintage mercruisers had probably the worst cam for power, basically a longevity grind. He said that swapping out the whole top end would more then likely get rid of the issue, but I would not be happy with the results as the cam would seriously limit HP/TQ gains. I was always under the impression you could not run an agressive cam on a marine engine because of reversion once you start changing overlap duration.
Ahh sooo many opinions! Maybe I will just get all new motors and be done, now where is the nearest ATM I can knock over??
Today I talked to an engine builder in the area that said the 1986 vintage mercruisers had probably the worst cam for power, basically a longevity grind. He said that swapping out the whole top end would more then likely get rid of the issue, but I would not be happy with the results as the cam would seriously limit HP/TQ gains. I was always under the impression you could not run an agressive cam on a marine engine because of reversion once you start changing overlap duration.
Ahh sooo many opinions! Maybe I will just get all new motors and be done, now where is the nearest ATM I can knock over??
You are correct you need to be conservative with the duration on marine engines for reversion, and there is only one guy to call for that -- Bob Madera from Marine Kinetics.
I don't know why he would say 86's are weak though, I had an 87' Liberator 350/260 and I hung out with a couple guys that had 86' and another with an 84' and they were all damn near identical as far as performance. We all turned 21" props right around 4800rpm's
If it were me, I'd pop off the front cover and try to identify the cams, or pop a valve cover and check the lift (you should be able to pull a # of the end of the cam and identify it). I mean, does it look like they have ever been apart? Obviously don't do this until you check the timing etc. This is where it can get REALLY expensive if you are not careful....Without knowing what you have it's tough to decide what to do -- your engine guy is right, bolting all those high dollar parts on are worthless without the right cam, especially if someone put towing/RV/truck cams in.....
