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1991 OMC 5.7 upgrades

Hi,

I am the proud new owner of a 1991 Chris Craft 228 Concept cuddy with a OMC Cobra 5.7 motor.
Can,t find HP ratings, guessing 260, would like to know what compression readings are on a new 1991 engine.
Boat has a near perfect survay record and engine is all original with a 650 cfm Holley 4bbl carb.
I am presently rebuilding the drive unit and machine work being done on worn out obsolite parts.
My next step is rebuilding the 27 year old engine.
Any opinions on how the original power was or modifications that are worth while.
I am willing to go the route of intake manifold, heads, camshaft, carb, even EFI if anyone done it before.
Ignition upgrades ect...
I don't want a speed boat, just like to upgrade everything I touch when it is worth while.
Would like to have a 300hp boat that starts up easy, pulls strong with good mid range acceleration.

Thanks for any info
 
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Hi,

I am the proud new owner of a 1991 Chris Craft 228 Concept cuddy with a OMC Cobra 5.7 motor.
Can,t find HP ratings, guessing 260,
260 hp was very common for the Marine built SBC.

would like to know what compression readings are on a new 1991 engine.
For an accurate account, you would need to know the build specs.... i.e., Bore/Stroke, cylinder head chamber volume, piston dish volume, etc. and then use an on-line static compression ratio calculator.

As a general rule for a healthy engine, you will want to see no more than a 10% difference between your high cylinder pressure reading and your low cylinder pressure reading.


Boat has a near perfect survay record and engine is all original with a 650 cfm Holley 4bbl carb.
I am presently rebuilding the drive unit and machine work being done on worn out obsolite parts.
My next step is rebuilding the 27 year old engine.
Any opinions on how the original power was or modifications that are worth while.
Yes..... read this thread starting at post #7.
http://www.marineengine.com/boat-forum/showthread.php?394668-V6-to-V8-engine-swap
Other than a small additional cost for the quench style pistons, the build and labor cost will be very much the same.

I am willing to go the route of intake manifold, heads, camshaft, carb, even EFI if anyone done it before.
Ignition upgrades ect...
I don't want a speed boat, just like to upgrade everything I touch when it is worth while.
Would like to have a 300hp boat that starts up easy, pulls strong with good mid range acceleration.
Use caution if going to aluminum intake and/or aluminum cylinder heads IF the engine will be Raw Water Cooled!

Your OMC Cobra drive is at it's limit at around 300 hp. It is also a candidate for the Volvo Penta DP-S conversion.
Although the Q/E build is very important (and I would suggest doing that build), the Duo Prop will give you as much or more performance than the engine upgrade alone will.


Thanks for any info


.
 
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Do not rebuild your 27 year old block and heads. Buy a new engine, a complete drop in.

Get the SEI bell housing kit and convert that out dated crap drive to an Mercriser Alpha Gen2.
 
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Do not rebuild your 27 year old block and heads.
If Raw Water Cooled, I would agree.... too risky in that it may be rust compromised by now.
However, if it was an AQ200B or AQ225B, or an upgrade to a 5.7L, it will have been fitted with a Closed Cooling System.


Get the SEI bell housing kit and convert that out dated crap drive to an Mercriser Alpha Gen2.
That would be going backwards from a Cone Clutch drive to a Dog Clutch drive.
If you were to do that, you would now have to replace the Flywheel Cover, find a Merc gimbal housing and Merc inner transom plate, buy a Merc drive coupler, undergo annual engine/coupler alignment, have gimbal bearing issues, etc.

With the VP, you will have no Gimbal bearing, will have main suspension fork/pivot tube geometry, easy to work on 3 pc stern drive, engine located seawater pump,
will not need annual engine/coupler alignment (in lieu of fixed geometry), and so on.
 
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The Volvo Penta SX or Alpha gen 2 is the only drive conversion he should do to a cobra. to do the merc conversion all you need it the transom assembly bell hosing kit. To do the SX yo need to change the shift cable.

Going to the AQ set up is a bigger mistake than buying a OMC powered boat.
 
I need to make a correction!
In my last post I was confusing threads and was thinking that the OP’s boat was equipped with an AQ series Volvo Penta. My apologies.

So yes...... he could do the Merc / SEI conversion and stay with the dog clutch, or he could do the SX or DP-S conversion and have the cone clutch system...... any of which are gimbal suspension systems.

As for the AQ series, I’ll take a DP-C2 over any drive.
 
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