Camshaft lift

jasonnb

Petty Officer 2nd Class
Joined
Nov 8, 2001
Messages
171
In looking at rebuild kits for my Mercruiser 888, the kit I am looking at includes a new cam with the specs of 258/270 duration @ .381/.395 lift. My Clymer manual says the lobe lift should be .260-.278 when measured with a dial. What effect would this lift difference have? (No, i'm not doing the rebuild, but I'm curious. The rebuild will be done by someone who knows what they are doing)
 

Saperry

Petty Officer 3rd Class
Joined
Nov 13, 2001
Messages
99
Re: Camshaft lift

Jason<br />Lift and duration are not the concern with marine engines. It is valve overlap. The two are very closly related but not the same thing. Overlap is the amout of time from when the intake valve starts to open untill the exhaust valve fully closes. It is measured in degrees of rotation or degrees of duration since this time would vary at different rpms.<br /><br />In an engine whos exhaust is underwater, like a boat, too much overlap would cause the engine to try to suck water into the cyliners from the exhaust system. <br /><br />I really don't know how much is too much as I have never done any mod's to I/O.<br /><br />As far as the engine and cam lift and duration, you can increase the HP and shift the power band<br />(the range of RPM's where the most HP is produced) by changing the cam. 1/4 mile dragster's are better in one power band, short track oval cars annother, trucks annother, etc..<br />Boats annother....prob mid to high RPM?<br /><br />You will not realize the full benifits of a cam change unless you also ensure you have the intake manifold and carb to deliver more fuel and the exhaust system to carry away more waste gasses.<br />And the proper piston to compress the fuel/air mix in the cyl.. Some high lift cams require "dished" pistons that will allow them to clear the valves and/or fully adj. valves and aftermarket push rods. All of this can get very expensive.<br /><br />I would not make much if any change of the cam spec's unless you are sure all parts will work together.<br />Also you MUST USE A CAM DESIGNED FOR MARINE ENGINES. I cannot stress this enough. Someone who has hotrodded ford V-8's in boats for a while might be able to find an perfomance automotive cam that will work. Let them do it to thier boat. <br /><br />If you want a dependable boat that is low Maintenance, go back stock or real close to it. If you a all out racer that sucks gas and requires lots of TLC then go for a performance rebuild. <br /> ;) <br />Sean
 

jasonnb

Petty Officer 2nd Class
Joined
Nov 8, 2001
Messages
171
Re: Camshaft lift

Reliability is my primary goal with at least stock performance levels. The cam is part of this vendor's standard rebuild kit for Marine 302's.
 

Saperry

Petty Officer 3rd Class
Joined
Nov 13, 2001
Messages
99
Re: Camshaft lift

If it is a standard rebuild kit you should be ok. That seams like a lot of lift/duration but again I have not done much with marine 302's. Which vendor id it?<br /><br />Sean
 
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