100 PSI seems low but not per chart

[Aaronhl]

Riddle me this why is there a compression difference between the two Merc block sizes and if you get 100 psi on a 3.0 that seems like it is ok per the chart, roughly 20 psi less than the smaller block, seems like each would want to be near 125 if that makes sense,,,, would 3.0 horsepower truly be at 225hp if psi is in the middle of the range

2.0/2.5L







3.0L

[David - WI]

Compression alone doesn't really have a lot to do with horsepower... I went from 110psi to 225psi and picked up about 5hp on a 100hp motor.

"one full point of compression can add between 3 to 4 percent power. So, if an engine is making 50 horsepower and we add a full point of compression (from 11 to 12:1 for example), this could potentially push the power to 51.5 horsepower."

[XstreamVking]

Low leak down number is more important than a hi compression #.
A 225 EFI 3.0 is more like 245 hp when healthy even in stock form.

[Aaronhl]

OK got the charts uploaded in the first post

Thanks for the explanations

[Glastron1987]

Not sure how the 3.0 blocks are, but when you have exhaust relief notches it can make it lower. At least I think.

[Aaronhl]

I asked the question because I have 100 psi exactly on each of the six on my 3.0 250hp
Runs great hitting the rev limiter all day long so i need to get more pitch
Guess 100psi is okay for my sport boat

[OnPad]

Your doing better than my 96 225efi, it has more like 90 psi, and fairly low leak down numbers.

[David - WI]

Compression numbers depend on a bunch of things... cranking speed for one, which depends on battery condition, cable length & condition, starter condition, engine temperature, ect (none of which indicate anything about engine health).
Some people say the engine will crank faster with the other 5 plugs "in" than "out"... and if you have a bunch of fuel & oil in the cylinder from trying to start it, compression will be higher. Not all compression gauges are the same, the "good" ones screw in and have the Schrader valve right in the spark plug hole.

There's a lot to it, but if you have 6 that read the same, you can be pretty sure you're in pretty good shape.

[Asstracker]

consistency is the biggest factor when checking compression. as long as your numbers are all within a couple psi youre in good shape. could also be that your gauge just reads low and you have more compression than it reads.

[Jim Speros]

Most modern engines have lower compression for the poor fuel they are designed to run on. If it designed to run on 87 octane it will be lower then an engine requiring 91.

I agree the point is that they are even. I never got hung up on what the exact number is. Leak down tells you more about the condition of the engine anyway.