153 head on a 181

[matt167]

Can I put a 153 120hp head on a 181 140hp?

I have a 1971 120hp and a 1986 140hp. Want to bolt a Clifford intake up to it but figure the extra displacement would help. Going into my rat rod which is basically a model A frame with a custom built cab

 

[Scott Danforth]

you can. some of the 3.0's came with 2.5 heads. not much changed in the heads when they stroked it until after 1990

 

[Bondo]

Ayuh,...... Doing so is a double edged sword,.....
With the smaller combustion chamber, the compression ratio will be slightly higher, but the smaller valves flow less,.....

 

[matt167]

you can. some of the 3.0's came with 2.5 heads. not much changed in the heads when they stroked it until after 1990

So the ‘86 3.0 head has the same ports as a 2.5?

 

[Bondo]

So the ‘86 3.0 head has the same ports as a 2.5?

Ayuh,..... no way to know for certain, til you look,....

 

[Scott Danforth]

There are two valve sizes that I know of. Would have to spelunk around inliners for more information

 

[Scott06]

If I recall around 1990-91 the head changed. We had a 1991 sea ray ( engine was a 1990 build with one of the last gen one drives), it was offered with a couple different hp options of the 3.0. Ours was the 3.0LX with 135 hp. Also belive there was a 115 or 120 hp version . Thought I read somewhere the head was the difference in hp. Splitting hairs at that hp level…Barr makes an adapter so later heads can be used with older manifolds and vice versa.

 

[Scott Danforth]

1991 and later motor is a new design using a 4" bore of the 5.7, a 1-piece RMS, and the 168 tooth flywheel

The same base motor was sold by GM. The various HP ratings were simply tuning.