1977 250hp Mercruiser compression ratio

[boatnut74]

I have a 1977 Mercruiser 250hp engine and was curious what the compression ratio is. I have seen anything from 8.7:1 to 9.4:1 I have search all over to no avail. Serial number falls between 4768000_4908849

 

[Bondo]

Re: 1977 250hp Mercruiser compression ratio

Ayuh,.... Odd question,.... Why ya gotta know,..?? not much difference 'tween the 2....

 

[billbayliner]

Re: 1977 250hp Mercruiser compression ratio

I have a 1977 Mercruiser 250hp engine and was curious what the compression ratio is. I have seen anything from 8.7:1 to 9.4:1 I have search all over to no avail. Serial number falls between 4768000_4908849

Was Merc using f/t pistons in 1977 or full dished pistons?
Peek in with a bore scope.
Has anyone changed the cylinder heads to a different casting number?
If you knew the cylinder head casting number that will get you the combustion chamber volume.
With piston type and chamber volume, you could use an online static compression ratio calculator to get you close.
http://cleanandsober.741.com/CompressionCalculator.html

 

[boatnut74]

Re: 1977 250hp Mercruiser compression ratio

The heads are 882 casting with 76cc chambers, pistons are flat top. Im going to be putting this in a truck I have therefore I need to change the camshaft. I want it to have a lopey idle and knowing the compression ratio will help me pick the right cam. I'm having the heads freshened but the bottom end is in excellent shape so I'm not going to touch it. Certain profile camshafts require a certain amout of compression for them to work at peak efficiency. I was just curious. I will assume somewhere between 8.5:1 and 9:1

 

[Bondo]

Re: 1977 250hp Mercruiser compression ratio

Certain profile camshafts require a certain amout of compression for them to work at peak efficiency.

Ayuh,... That's correct,... So, leave the cam alone....

A Loppy cam, in an otherwise dead stock motor, will Never work anywhere near peak efficiency.....

 

[boatnut74]

Re: 1977 250hp Mercruiser compression ratio

Ayuh,... That's correct,... So, leave the cam alone....

A Loppy cam, in an otherwise dead stock motor, will Never work anywhere near peak efficiency.....

This motor is going in an old chevy truck, so the cam that's in there won't work. Yes the bottom end is stock but it already has flat top pistons, almost all of GM's stock motors had dished pistons, so I'm already at an advantage. The heads are being redone, Edelbrock intake and carb and headers. I was just curious on what the compression ratio might be so I can get a camshaft that best matches the motor.

 

[billbayliner]

Re: 1977 250hp Mercruiser compression ratio

Yes you do have an advantage with the flat tops and 76cc chambers in a 3.480" stroke engine. This will get you in the range of 9.17:1 or so static c/r depending on which head gasket was used.
F/T's give a squish area that makes more low end torque if the cam compliments this.
Call a good camshaft company and tell them what you have and they'll make a recommendation.

 

[Bondo]

Re: 1977 250hp Mercruiser compression ratio

This motor is going in an old chevy truck, so the cam that's in there won't work. Yes the bottom end is stock but it already has flat top pistons, almost all of GM's stock motors had dished pistons, so I'm already at an advantage. The heads are being redone, Edelbrock intake and carb and headers. I was just curious on what the compression ratio might be so I can get a camshaft that best matches the motor.

Ayuh,... Is this the original Cam,..?? Why do you think it won't work in a truck,..??

The stock Merc. cam will run a 'ell of alot Better than a loopy soundin' cam,...

Hands Down....

 

[boatnut74]

Re: 1977 250hp Mercruiser compression ratio

From what I can tell its the original cam. From the research I've done it seems people don't recommended a marine cam in auto applications and vise versa. Ill Mic the cam to see about how much lift it is

 

[Bondo]

Re: 1977 250hp Mercruiser compression ratio

From what I can tell its the original cam. From the research I've done it seems people don't recommended a marine cam in auto applications and vise versa. Ill Mic the cam to see about how much lift it is

Ayuh,... A Marine cam, 'n a Truck, or R/V cam is darn near Exactly the same....

It's the Loopy automotive cams that quite literally Suck, when run in a boat...

 

[boatnut74]

Re: 1977 250hp Mercruiser compression ratio

Ayuh,... A Marine cam, 'n a Truck, or R/V cam is darn near Exactly the same....

It's the Loopy automotive cams that quite literally Suck, when run in a boat...

The original cam that was in the motor really isn't in that great of shape. A couple of the lobes are pretty worn. I guess it makes sense an rv cam and marine cam are similiar, they both work well under heavy loads.

 

[dan t.]

Re: 1977 250hp Mercruiser compression ratio

Actually the 260 hp cam makes an awsome 4x4 cam,gobs of bottom end torque, I know, I tried it.

 

[billbayliner]

Re: 1977 250hp Mercruiser compression ratio

Ill Mic the cam to see about how much lift it is

You'll want to know more than just lift. You'll want to degree this and check the duration and separation also.
If the cam has a number on the end, it might cross reference to the numbers that you are looking for.

 

[boatnut74]

Re: 1977 250hp Mercruiser compression ratio

I spoke with comp cams and they recommended a cam for my application.