HP500 - what CR can it take with 088-heads?
11-04-2018 | 01:04 PM
#1
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HP500 - what CR can it take with 088-heads?
I’m rebuilding my 1999 merc HP500 (carb) and I’m thinking about changing the pistons to get higher CR. The pistons I have now are forged SRP (4,5”) and the CR is 8,9:1. I’m running 088-heads with new valves and springs so I don’t want to change heads. Can the 088-heads handle 10,0:1 CR? Or what CR is recommended? Cam is crane original roller HP500. My machine shop says 10:1 is fine but they usually build street engines. My friend says cast iron heads can’t handle 10:1 on pump gas. Thankful for any help
11-04-2018 | 02:12 PM
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From: Cape coral, FL
I would think it really depends on tune. What ignitions? If it’s a standard all in by 3000 or locked timing I wouldn’t. I would think if you used something like a Daytona setup where you can do a custom timing map it would be possible.
11-04-2018 | 05:39 PM
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From: ALTO, MI
IF you are trying to gain HP than the only place to spend your money is new aftermarket Aluminum heads.
Great compression, great cam, lousy heads = low HP.
The above with great heads = HP increase.
The other thing to spend money on AFTER the other mods is new header style exhaust.
I ran Dart Aluminum heads at 10 to 1 compression and was able to run 98 octane.
Great compression, great cam, lousy heads = low HP.
The above with great heads = HP increase.
The other thing to spend money on AFTER the other mods is new header style exhaust.
I ran Dart Aluminum heads at 10 to 1 compression and was able to run 98 octane.
11-04-2018 | 08:07 PM
#7
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From: NW Michigan
Just be certain your machine shop - engine builder sets bearing clearances etc to marine/hp500 specs. Post 5 good advice. If your piston to wall clearance checks out fine and pistons in good reusable condition go for the heads. Youl'll see much more of a gain than a half a point or so in CR.
11-04-2018 | 08:37 PM
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From: yorkville,il
IF you are trying to gain HP than the only place to spend your money is new aftermarket Aluminum heads.
Great compression, great cam, lousy heads = low HP.
The above with great heads = HP increase.
The other thing to spend money on AFTER the other mods is new header style exhaust.
I ran Dart Aluminum heads at 10 to 1 compression and was able to run 98 octane.
Great compression, great cam, lousy heads = low HP.
The above with great heads = HP increase.
The other thing to spend money on AFTER the other mods is new header style exhaust.
I ran Dart Aluminum heads at 10 to 1 compression and was able to run 98 octane.
Last edited by mike tkach; 11-04-2018 at 08:41 PM.
11-04-2018 | 08:50 PM
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From: SF Bay Area
Someone on here with twins mentioned they had two similar engines but one was a CR point higher and he couldn’t tell any difference between the two. All of the power gains are to be found in the head with compression and cam being a factor to support it, not the other way around. I’m a complete hack, but that’s my understanding of it.
