heads/compression questions

[JoshKeller84]

hey guys, i have a 1989 110 hp on my bench with the low compression bath tub heads, and run approx 105 - 110 psi. is there any reason that omc went to the lower compression after rerating the 140s down to 110? just for giggles i slapped on a set of 90 hp heads that were stock from the same year and the compression came up to 120, which is where just about every other omc v4 motor ive owned has been at. is there any dangers or reasons not to run the 120 psi heads? just seems odd the lowered the compression on the 110s, but none of the other's, including the previous crank rated 140s. same jets as well.

 

[JoshKeller84]

also, forgot to mention, the 110 was running slightly rich as evidence by the plugs and the fact that it would run an extra 100 - 150 rpm with the cowling off. would the 120 psi heads work ok with 87 octane and starndard timing at 28, or should i back it off to 26 or so?

 

[Faztbullet]

If heads have same casting number should not made any difference on compression

 

[JoshKeller84]

they do not, same year, different numbers - the 110 hp calls for a different part number than the 88,90,100 hp

 

[flyingscott]

Your motor is a derated 140. Essentially they rebadged the 115 as a 110/112 motor. I think they did this because the same year they came out with the 120 looper motors. They really couldnt sell engines within 5 hp of each other.

 

[JoshKeller84]

so is there any harm in running the 120 psi compression heads over the 105 - 110 ones with 87 octane, and the same jetting (67c) and 28 degree timing as the previous crank rated 140s?

 

[interalian]

My motor is a crank-rated 140 and I run it on 87 with no apparent problems. Timed to OE spec. My heads are HC heads.

 

[JoshKeller84]

28 degrees btdc and 67c high speed jets? mine arent the high compressions heads like you have, however the difference in the 90 and 110 heads are the 110 heads seem to be machined down inside the combustiom chamber and extra 1/8" around the sides.

 

[jimmbo]

Same head gaskets? There were some thicker ones made for the V4s during the 80s, they were used to lower compression

 

[interalian]

28 degrees btdc and 67c high speed jets? mine arent the high compressions heads like you have, however the difference in the 90 and 110 heads are the 110 heads seem to be machined down inside the combustiom chamber and extra 1/8" around the sides.

Supposed to be 65C according to the fiche, but I don't recall reading them. My carbs are from a '79 - 1-3/8" throats. I see that later carbs used 67C.