Mercruiser 3.0 valve size and head spec questions...

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craze1cars

Lieutenant Commander
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Hello...

I have a 1992 3.0L in my cheap-o Rinker runabout, rated 115 HP. It's tuned well and runs well, just as designed. And as with anyone who runs a 3.0, more power would be nice to find. And I'm cheap and don't like to spend money (so please don't waste your time suggesting a repower or buying a different boat...I'm very well aware those options would provide more HP...so there's no need to bring Mr Obvious into this discussion)

I know that In 1992 (and several other years) there was also a 3.0LX engine available, rated at 135 HP. Same displacement, more HP. Studying part numbers I see the difference is primarily in the heads and carb, which would be expected, as better airflow always equals more HP. Bottom ends appear totally identical. And curiously the cams are identical. So here's my questions:

1. What are the intake and exhaust valve sizes in the head of the 115 HP 3.0L? Part numbers 85803 and 62421
2. What are the valve sizes in the head of the 135 HP 3.0LX? Different, I assume? Bigger? Part numbers 52367 and 97661
3. Does the LX head have different size or shaped domes compared to the L? With likely a higher compression ratio resulting?

Why am I asking? I am equipped and have some experience with high performance 3 angle valve jobs, porting heads, etc. I am considering doing something to this 115 HP at the end of the season. If I can smooth out the airflow with some minor head work and gain a few ponies, it won't cost me anything but a set of gaskets and a weekend of time. I even have some high performance GM roller rocker arms laying around here that might just fit this head...and they might give a slightly longer/larger opening ratio to the valves, as well as reducing the friction inherent in the stock ball type rocker arms . BUT...if I would be better off starting out with a 3.0LX head instead of my current 3.0L head & cam because it has bigger valves/bigger ports to start with and a higher compression ratio, then maybe I should Ebay myself a used head and a few other parts from a higher HP variant of this motor and start my improvements from there? I can always sell my current head and break almost even.

For example, I see 1.94 intake and 1.60 exhaust valves advertised on a 3.0 reman head for sale on Ebay. Are these the dimensions the same as the valves on my motor? Or are these dimensions bigger than what I'll find when I pull my 115 HP version apart?

Any links to such specs would be nice to find, or if someone here has access or knowledge of these specs, that would be great. Thanks in advance to anyone who can help!
 
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craze1cars

Lieutenant Commander
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Messages
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Re: Mercruiser 3.0 valve size and head spec questions...

Thank you, Don...that info is extremely valuable and I will definitely use it. However, the specific info I seek does not appear to be there. The best clue I found is a statement that said "3.0L uses 2.5 head, and 3.0LX uses the 3.0 head." This is anoter clue, but still a bit of mystery here to me, not having personally held these two heads in my hands to do a comparison.

So it would seem to me just swapping out my 2.5 head for a 3.0L head would gain me another 20 hp all by itself just going stock to stock, effectively converting my 3.0L to a 3.0LX. And then smoothing airflow within the engine a bit better than stock should get me even a bit further. So this looks like a real easy way for me to bump my motor from 115 to maybe 140 or 145 hp? That would be quite sweet...and cheap

However, that service manual did not tell me what the true difference between these two heads is. And the spec sheets provide dimensions and specs for valve stems for purposes of wear measurements (which are apparently the same between the two models), but the manual does not provide the diameters of the tops of the valves themselves, unless I'm overlooking.

So I guess more accurately, my original posted question SHOULD have said:

Does anyone know what the diameters of the valves of the 2.5L head are, compared to the 3.0L head? And is there a difference in compression ratios between the 3.0L and the 2.5L heads?

If the 3.0L head offers larger valve openings and maybe a higher compression ratio than the 2.5 head, then it would seem I should be starting my mods with a 3.0L head, instead of the 2.5L head which I apparently have on my boat right now. Then again, maybe with my machining equipment I can just buy a set of new 3.0L valves and grind out the openings of my 2.5L head, effectively converting it into a 3.0L head? But that still doesn't tell me if I'd end up with combustion chambers of the same size/shape, if the heads have different shaped domes for instance.

Thanks for any further insight you or anyone else might have on the differences between the 2.5L head and the 3.0L head...I find this rather fascinating and am looking forward to this winter project. Just trying to educate my self on how to most effectively (and cheaply!) tackle it.
 
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