mercruiser 4 cylinder 3.0 head vs 2.5 head

[77 gt]

Hi all,

Could anyone tell me if i could use a 3.0 head on a 2.5 block? The only difference I can tell is the valve size, since im going bigger head to smaller block displacement it should be ok correct?

 

[fishrdan]

Re: mercruiser 4 cylinder 3.0 head vs 2.5 head

Depends on the year, but most 2.5L heads are different than 3.0L heads, "port configuration" wise. Some Mercruiser 3.0L engines used a 2.5L head (why, I don't know...), so depending on the year, a 3.0L head could be a direct swap for a 2.5L engine. Clear as mud, right? Mercruiser used 4-5 (6?) different heads on the 4 cyl engines over the years, so it's best to match up the ports,,, and valve sizes.

Larger valves and large ports are good for high RPM power and but bad for low RPM power, the exact opposite of what you want in a marine engine.

 

[achris]

Re: mercruiser 4 cylinder 3.0 head vs 2.5 head

Depends on the year, but most 2.5L heads are different than 3.0L heads, "port configuration" wise. Some Mercruiser 3.0L engines used a 2.5L head (why, I don't know...), so depending on the year, a 3.0L head could be a direct swap for a 2.5L engine. Clear as mud, right? Mercruiser used 4-5 (6?) different heads on the 4 cyl engines over the years, so it's best to match up the ports,,, and valve sizes.

Larger valves and large ports are good for high RPM power and but bad for low RPM power, the exact opposite of what you want in a marine engine.

That series of engines was know as 3.0L and were rated at 115hp, as opposed to the 3.0LX which was rated at 135hp. It sort of filled the space left when the 2.5L/120hp engines were discontinued in 1989.

Yes, a 3 litre head will bolt up to a 2.5 litre block, but there are a lot of other issues, like has already been said, valve and port size, but also compression ratio... Which would then required a timing change... Just too messy.. Get a 2.5 litre head, or change the whole engine to a 3 litre (it will bolt straight in, and uses the same drive ratio )....

HTH,

Chris.

 

[77 gt]

Re: mercruiser 4 cylinder 3.0 head vs 2.5 head

thanks for the info. the 3.0 I have is a 1977 140 hp (which is the head i want to use) and the 2.5 is a 1971 120 hp bored .30 over. The 3.0 block is cracked and I have a rebuilt 2.5 ready to go in. I plan on using the 3.0 head and exhaust to bolt on to the 2.5 so exhaust ports shouldn't be a problem. the head gaskets and ports all line up to the block. I just wasnt sure how the larger valves would effect the engine. The main reason im using the 3.0 head and exhaust is because I dont have the throttle linkage that connects to the 2.5 exhaust as it is different than the 3.0.

 

[bobwinkel]

I take it from post #3 above that a 3.0 will directly replace a 2.5. That it will bolt in and fit the out drive etc. I too have a 3.0 long block and bad 2.5. I have heard there will be problems putting the 3.0 in place of the 2.5 unless you swap everything "from the pulley to the prop nut". Any advice you can offer will be appreciated.

 

[bobwinkel]

I take it from post #3 above that a 3.0 will directly replace a 2.5. That it will bolt in and fit the out drive etc. I too have a 3.0 long block and bad 2.5. I have heard there will be problems putting the 3.0 in place of the 2.5 unless you swap everything "from the pulley to the prop nut". Any advice you can offer will be appreciated.