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Old 03-25-2012, 12:25 PM
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Mercruiser/Chevy 230/250 questions

Hi all,

Seems to me like info for these old I6 motors is much harder to find out there than for the SBCs that I'm used to working with, so I'm hoping I can get some of the technical info from somebody here.

The story goes, my dad picked up a 1966 Pennyan with a Merc 230 for super cheap last summer. The stingers look good so we are moving forward on the next phase and deciding what to do with the engine. Knowing this WILL end up being a can of worms type operation, I want to get as well informed as possible before cracking the lid.

I *could* just run as-is and hope for the best, but I would really *HATE* to get 15 miles out onto Lake Erie when I notice the rod knock begin or whatever. That's why I want to just give it an overhaul now, but I need to figure out what to do.

Step 1 is going to be take it all to the shop to assure that the block/rods/head/crank are all happy. If the crank needs any work I think I will just get a 250 crank to stroke it while I'm at it. Cost of the rebuild is the same, otherwise, so why not?

Regardless of whether I stroke it or not, I would like to get the CR up to 9.5ish vs the 8.5 that these apparently run. Obviously the increase in stroke would accomplish much of that for me, but I like to do the math first so I know exactly what all needs to happen. That being said, does anybody here know all of the specs on these motors, in terms of determining CR?

I've found so far the bore/stroke, compression heights, rod lengths and CR, but I cannot find information on the deck heights for these blocks or the CCs of the heads. From what I can tell I have little doubt those two things change between the 230 and 250s, seems like the only diffs between them is the stroke and piston design. I know how to do the math, I just need these last two variables to plugin.

Can anybody confirm/deny what I have figured out so far, as well as to help me with the DH and CCs?

Lastly, this is a pretty old motor and I'm wondering if the seats will require the use of lead or not. I found a site with info on Mercruiser engines and it told me NO, but I am hoping somebody can confirm/deny from experience.

Thanks a bunch guys!

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Old 03-25-2012, 01:39 PM
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To get more compression the old trick was to use a 194 head.
Here's a link for a book you'll need. Lots of tech in it.
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Old 03-25-2012, 06:53 PM
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I would be very careful about upping compression on a marine motor, constant heavy load means complete destruction if it detonated...and you won't hear it on a boat!

9:1 is plenty!
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”

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Old 03-25-2012, 11:10 PM
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There are basically 2 heads for these engines, the later style 230,250 and 292 head which had anywhere from 74-76cc's in uncut condition. The early 194 head was in the 64-66cc range. The deck height on most of the 230's and 250's was around .020" to .025", but you should have checked it when you tore it down to get a baseline. The 230 had flat top pistons and the 250 has dished, so swapping cranks wont change your compression for a stock rebuild in a 250 unless you buy custom flat top pistons. To get the most bang for your buck, put 1.94" and 1.600" valves in the large chamber head and put in the bolt-in lumps in the intake ports. If you find a 194 head, you can use it as an extra anchor . A group of us on Inliners did many dyno tests with both a 250 and 292 engine a few years ago, over 200 dyno pulls combined, and found that the 194 head never made as much power as an equally prepped big chamber head, it actually consistently made over 10 HP less or more on every combo we used them on.
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Old 03-25-2012, 11:40 PM
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The only thing I will say about compression is that the I6 motors come with very un-inspired chamber designs that barely avoided detonation on 1966 Leaded gasoline, but in a boat with endless supplies of cool lake water, you should be able to bump your compression to 9.5 as long as you have a steady supply of premium fuel.... which most marinas don't have

Once you factor in that a 1-point increase in compression is usually good for 10% more power.... then consider that increasing your hp by ten will probably only net you about 1-2 mph, it may not be worth the headache.

You also can't really add much more cam for two reasons: 1) shifting the the torque curve up in the RPMs is a recipe for sluggish planing, or (in extreme examples) no planing at all. It can also superheat exhaust valves if you are cruising below the torque peak's RPM. 2) with a wet exhaust, additional valve overlap can cause water reversion which can cause some bad stuff.

My suggestion is to go with more cubes. The term "cubes are king" is never more applicable than on the water.
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