Compression Readings.. V8 5.7 Need Input Please!!

J

jmw129

Petty Officer 1st Class
Joined
Jun 12, 2011
Messages
253
Hey Guys,

I need your opion on the following compression reading regarding the health of this engine that is for sale. Seller says it runs good, it's an older 1978 V8 5.7 Merc.

He told me he took the following reading when he ran the engine cold. Does it really make a difference with cold vs. wet compression reading?

1 - 155
2 -160
3 - 140
4 - 155
5 - 140
6 - 160
7 - 160
8 - 145

Is this engine healthy enough to drop into another boat?

Thx Team!
 
Oshkosh1

Oshkosh1

Ensign
Joined
Jun 8, 2009
Messages
968
Re: Compression Readings.. V8 5.7 Need Input Please!!

You need to take the compression when the engine is up to operating temp.

Cold comp numbers are not reliable.

Those numbers are a little far apart...10-12 lbs should be the most difference between any two IMO. 15 is borderline, 20 is time to price a rebuild.

However...as I stated, a cold compression test does not give a true picture of it's health at operating temps.

Also...do they change with a little oil squirted in?...Was the throttle blocked open? It's not JUST the high number...too many variables...not enough info.
 
D

dollarten

Petty Officer 2nd Class
Joined
Jul 28, 2011
Messages
136
Re: Compression Readings.. V8 5.7 Need Input Please!!

jmw129 said:
Hey Guys,

I need your opion on the following compression reading regarding the health of this engine that is for sale. Seller says it runs good, it's an older 1978 V8 5.7 Merc.

He told me he took the following reading when he ran the engine cold. Does it really make a difference with cold vs. wet compression reading?

1 - 155
2 -160
3 - 140
4 - 155
5 - 140
6 - 160
7 - 160
8 - 145

Is this engine healthy enough to drop into another boat?

Thx Team!
Click to expand...
If the engine has been sitting a while , sometimes all it will take a just a ver small piece of carbon ro get between the valve and the valve seat and give a lower reading . Best to have the engine warmed up
 
Bondo

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,513
Re: Compression Readings.. V8 5.7 Need Input Please!!

jmw129 said:
Hey Guys,

I need your opion on the following compression reading regarding the health of this engine that is for sale. Seller says it runs good, it's an older 1978 V8 5.7 Merc.

He told me he took the following reading when he ran the engine cold. Does it really make a difference with cold vs. wet compression reading?

1 - 155
2 -160
3 - 140
4 - 155
5 - 140
6 - 160
7 - 160
8 - 145

Is this engine healthy enough to drop into another boat?

Thx Team!
Click to expand...

Ayuh,... It depends on what motor was in the other boat, 'n it's Vintage...
 
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