Compression education...please explain!

[Digdug]

For educational purposes can someone please explain compression importance and what it actually means when considering a used motor. I am told that amount of compression is not important....what is important is that all cylinders be close to same psi. For example: You have 2 v6 engines of same year and make. Engine #1 has 110 psi on all 6 cylinders. Engine #2 has 135 psi on 4 cylinders, 130 psi on 1 cylinder and 119 psi on one cylinder. I would think that Engine #2 would be the better motor because more compression = less wear. Which engine would actually be a better stronger motor? Any explaination or comment would be great!

 

[alcan]

Re: Compression education...please explain!

Hello Digdug<br />Grossly uneven compression readings can indecate cylender damage. Scoring,galled piston rings, things that could become a real problem. Also because the uneven compression will make it impossable to tune. Your senairo is not far enough out of wack in my opinion. But more even more better. Good luck.

 

[jegervais]

Re: Compression education...please explain!

I don't know what V6 engines you're looking at, but even compression is what you want - to a point. Some manufacturers specify a compression value in psi. Some say within 10 or 15% and one other says the difference between the highest and lowest should not exceed 15 psi.<br /><br />It's helpful if you know what a compression gauge reads on a healthy engine.<br /><br /> If you have a gauge normally reads low, then 85 psi across the board is probably ok. <br /><br /> But if you have a gauge that reads more accurately (say 110-115 psi), an engine that reads 85 psi on this gauge may be about worn out.<br /><br />-John

 

[rbu]

Re: Compression education...please explain!

You should have a repair manual for your particular engine so you know what to expect. Not all engines will give you even pressure for all cylinders. Due to piston design, my V6 Yamaha, has lower compression on the bottom 2 cylinders. I believe that some Yamahas have lower compression on the middle 2 cylinders. These lower readings should not be tremendous but should be evident. What you then want to look for is even compression between oposing cylinders.

 

[Digdug]

Re: Compression education...please explain!

Thank you for the responses. I have learned a lot from this web site. I like working on outboard motors (hobby). <br />Thanks again....Doug

 

[PurpleDragon]

Re: Compression education...please explain!

Since we're on the subject of compression readings, I have a question, too.<br /><br />My old motor, a late 50's era 12Hp sears (by West Bend) motor has less than 10 total hours on it. After getting it running late last year (impeller, fuel pump diaphragm, new fuel lines) I checked compression with a $20 compression checker from Harbor freight. Both cylinders register right at 60psi. Tha manual (if I recall correctly) calls for 80-85 psi.<br /><br />My quandry lies here... In other posts, I recall seeing where some people automatically add 20psi to whatever reading they get. Why? Are the compression testers that far off? Could it be that I actually have closer to 80psi, instead of 60psi?<br /><br />Thanks,<br />Rex S.

 

[petryshyn]

Re: Compression education...please explain!

Yo purple dragon<br /><br />Compression will vary with gauge used, ambient temperature, cranking RPM, and elevation.<br /><br />Look for even numbers. The max reading is too subjective. If your max compression is actually low, you will have rough idle and hard starting. If it starts and idles good....rock on.<br />

 

[PurpleDragon]

Re: Compression education...please explain!

Thanks Schematic...<br /><br />I'm Rockin' On! <br /><br /><br />Rex S.