Compression test

Discussion in 'Trucks and Trailers' started by MTCK, Apr 13, 2001.

  1. MTCK

    MTCK LawnSite Senior Member
    from Idaho
    Posts: 254

    Alright guys, this shouldn't be too hard. I rented a compression gauge so I could check my engine again, and decide if it's time to get a new 350. So, I got all the plugs out, engine is warm, throttle wide open, and ignition disconnected. Do I crank the engine for just 2 rotations, or until the pressure doesn't climb any more. The reason I ask is because I have between 185 and 200 psi on all 8, and I know the motor is tired. It is a 305 with a shade over 125,000 miles, that seems worn out and uses a lot of oil. My highschool shop teacher told me to crank till it stopped climbning. It just seems too good to be true. Thanks for the prompt responses guys, I only got the gauge for 12 hours. Appreciate it.

    MT
     
  2. mike reeh

    mike reeh LawnSite Member
    Posts: 229

    definately crank until it stops climbing.

    mike
     
  3. MTCK

    MTCK LawnSite Senior Member
    from Idaho
    Posts: 254

    The compression ration is 9.1 to 1 I believe. Does 185 to 200 psi sound a little high per cylinder? I guess this is a good thing. Now I just have to figure out why I'm using so much oil.
     
  4. mike reeh

    mike reeh LawnSite Member
    Posts: 229

    compression test PSI is not an indication of compression ratio, but i was thinking that 200 did sound a bit high.. I think the last time I tested my '77 400sb I got about 165 across all eight.. the most imporant thing is not necessarily the #'s, but that they are all reasonably close to each other, i think 10-15% would be good. maybe carbon buildup in the combustion chamber?? not 100% sure on that one.. my engine uses a fair amount of oil too, not sure if I would call it excessive or normal, though.. I drive it a lot and drive it hard and I might add a quart a week, give or take. piston rings, valve guides/seals and even intake gaskets are a few things that might affect oil consumption.. maybe it just needs a major overhaul? 125,000 miles isnt really that much then again it may have lead a hard life.

    mike
     
  5. MTCK

    MTCK LawnSite Senior Member
    from Idaho
    Posts: 254

    Yeah, it's had a fairly hard life, I'd say. Hauled a pretty big boat for most of it, along with fire wood in the fall, and rock and other gardening supplies every summer. And that was all before I turned 16. It's what I learned to drive in, so it served as this sixteen year olds race car and off road toy. I'll go through the gaskets, and also adjust the mixture and timing, to see if that doesn't help the tired feeling she's been having. Thanks for the help.

    MT
     
  6. 66Construction

    66Construction LawnSite Member
    Posts: 125

    How much oil are you using. I wondered about my 350 once and a mechanic thold me it wasn't out of spec as long as it wasn't consuming more then a quart every 1000 miles. Seems like a lot to me but that's what he told me.
    Casey
     
  7. plowjockey

    plowjockey LawnSite Senior Member
    Posts: 332

    MT

    You want to run the engine over at least four revolutions due to the fact that it is a four stroke engine. This makes sure that you have actually had a compression stroke in there. As for poundage like Mike posted what you are looking for are any abnormally high or low cylinders indicating a very bad or very good cylinder. If they are all just about even and you are not using too much oil you should be ok.
    Another thing to remember is that if you rebuild just the upper end. Such as doing the valves you put more strain on an already weaker bottom end. If you do just the lower end the opposite is true. If you freshen it up you should do the whole thing for the best results.

    Just my $.02 worth

    Bruce
     
  8. MTCK

    MTCK LawnSite Senior Member
    from Idaho
    Posts: 254

    Thanks guys, appreciate the input.
     
  9. Garet

    Garet LawnSite Member
    Posts: 157

    Chevy 350's were supposed to have around 150?
     
  10. plowjockey

    plowjockey LawnSite Senior Member
    Posts: 332

    Wasn't trying to mislead you. I'm sure there is a criteria although I'm not sure what that is. I just meant that if you are in the range you should be then you are looking for any abnormallities.

    Bruce
     

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