question about heads and compression

[JoshKeller]

i know OMC used low compression bathtub heads on some motors, and higher compression heads on others. I have a 1989 110 laying around in the shop that uses the low compression heads and makes 105 psi per cylinder. I also have a set of high compression heads off a 77 140 that made in the high 120s, and low 130s psi. Is there anything to gain simply by swapping out the heads? any need to run higher octane gas? bump up the timing? Sorry for so many questions lately, just got the performance bug and trying to soak in and learn everything i can.

[SS Minnow]

https://www.screamandfly.com/showthread.php?28837-Shaving-Heads-For-Omc-V4-looper
https://www.screamandfly.com/showthread.php?69981-77-140-v4-with-85-87-looper-heads&highlight=looper+heads
https://www.screamandfly.com/showthr...rude-120-heads

[SS Minnow]

https://www.screamandfly.com/showthr...t=looper+heads

Somehow i messed up the links. this was supposed to be in there.
This was what i came up with by running an advanced search of thread titles only for "looper heads"

[JoshKeller]

i read that, however this is a crossflow

[racer]

Yes the tighter heads will help with low end power. You will need to run high test gas and when adding compression you decrease timing not increase. You will also need to rejet as the engine will need more fuel.

[JoshKeller]

biggest question - is it worth the effort and expense, especially on a jet that doesnt load up like a prop?

[JoshKeller]

i guess my next question is why did omc use the higher compression heads on the 140s, but not the 110s and 115s after the prop rating conversion? and why run the 88s, 90s, and 100s with regular geads, but not the 110 in 1989 models? if you put on a set of the 90 heads and boosted compression from 105 - 110 psi, to 120, what would happen? i had a set of heads off a 90 and put them on the 110 just to see how muvh it'd increase, and they came right up to 120. which is exactly what the 90hp ran at. would you need to run higher octane, reset timing, rejet if the 90 hp had the same timing specs? was the low compression heads simply to get the hp down to a true 110 hp after the rating change?

[Motv18]

Omc basically is like a small block Chevy (if you like cars) heads (caps) and carbs and you could make about any v4 you wanted. Trick was knowing which went together. Port timing and bore would poke their nose up every once in a while as well. Iirc even some of the 2 cylinder heads (caps) fit with tighter chambers. Boosting compression is one of the best ways to clear up issues 91 e free causes in low compression.

[JoshKeller]

so if i ran these 90 hp heads at 120 psi, vs the low compressiom heads at 105 - 110 psi, do i need to change anything? i guess my biggest area of confusion is why were the tighter heads ok for the 140 crank rated blocks, but not for the 110 prop rated?

[powerabout]

so if i ran these 90 hp heads at 120 psi, vs the low compressiom heads at 105 - 110 psi, do i need to change anything? i guess my biggest area of confusion is why were the tighter heads ok for the 140 crank rated blocks, but not for the 110 prop rated?

tight heads on 77 140 were jetted/timing to suit and the fuel used was probably better at the time.
I'm sure jetting and timing changes would make them ok for a 110 but you will need good fuel
There were SB's on them to use thicker head gaskets in later years to get 135's to live, 235's had the timing knocked back

[JoshKeller]

i definitely understand the higher compression heads needing compensation, but what was the reasoning for them changing rhe heads to the bathtub style and lowering the compression after the rating change? the 1984 140 used the higher compression, but then the 1986 110 used lower. yet why did the others (88, 90, 100) use the regular heads that n produced 120ish psi? i guess my biggest question is can i use the hwads on my 110hp that produce 120 psi without any ill effects? i can understand needing to rejet etc at 135+ psi, but at 120 when most omcs seem to run that regularly? sorry guys, just a bit confused as to why they changed JUST that motor in that year