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GM marine 3.0L (Mercruiser)

General engine tech -- Drag Racing to Circle Track

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GM marine 3.0L (Mercruiser)

Post by rmyauck » Mon Aug 07, 2017 4:31 pm


Does anyone know if the 1987 head from my 3.0L (has dished D shaped pistons) will work on a similar year 3.0L rebuilt .060 short block (it also has a 2 piece rear seal and my flywheel will bolt up) I found that has flattop pistons (the owner said it had those) or will the compression ratio be too high? Maybe some companies did that for a performance increase.

Maybe also worth the possible HP increase as premium pump gas could be used with no problem.

I see a site showing 2 part # s for their replacement quicksilver heads, with one for 1963-94 and the other for 1990 and up having a different part #

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Re: GM marine 3.0L (Mercruiser)

Post by Truckedup » Wed Aug 09, 2017 6:19 am

You will have to find out the combustion chamber volumes on both heads and the volume of the dish in the piston....But likely if the dished piston compression allows 87 octane gas, then 91 octane should be ok with the flat top pistons...
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Re: GM marine 3.0L (Mercruiser)

Post by Art_H » Wed Nov 06, 2019 2:17 pm

My 3.0 Merc has 100% flat to pistons. No valve reliefs. They are essentially 327 pistons, with no reliefs.

I think it's a 1989.

The valve angle precludes the need for the valve reliefs.

Mercruiser wants $180 per piston though...so I was looking to replace with 327 with minimal valve reliefs.

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Re: GM marine 3.0L (Mercruiser)

Post by midnightbluS10 » Thu Nov 07, 2019 6:34 am

The 153/181 is based on the 4 cyl engine used in Chevy II's in the 60s, but not related the the Iron Duke. Gm did use a variant in passenger cars in Brazil.

You can find the info on Wikipedia.

https://en.m.wikipedia.org/wiki/Chevrol ... der_engine
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Re: GM marine 3.0L (Mercruiser)

Post by BillK » Thu Nov 07, 2019 1:20 pm

The dished piston is a 14cc dish. I am editing my reply because I thought the dished piston was 9.3 to one but that might be the flat top. Will get correct figure and let you know :)
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Re: GM marine 3.0L (Mercruiser)

Post by ProPower engines » Thu Nov 07, 2019 3:28 pm

9.5 is the outer limits for those engines to live.
Marine fuel also has no alcohol in it so using street fuels with the alky blended in it will hurt them.
There is several different cylinder head port designs and a couple different chamber sizes as well so do not
get the compression up or you will be doing it again. I am currently repairing an engine that had the dished piston smaller chamber because the shop did not know they were making it higher comp. and it melted a piston in about
5 hrs run time.
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