Compression Numbers on a 79 - 308 (155 to 170 psi) Good?

Discussion in 'Technical Q&A' started by tomoshea, Mar 19, 2005.

  1. tomoshea

    tomoshea Formula Junior

    Dec 29, 2003
    Full Name:
    Tom O'Shea
    As prep for lifting an engine for a top end rebuild (sodium valves) and sycnchro's on 1st and 2nd I did a compression test - these are the results I got.

    Note this is a 79 engine in a 308gt4 with 73,000 miles on it, engine has never been out, heads have never been off, nothing done to the bottom end. It has just had annual maintenance and oil changes on schedule.

    Cylinder 1 158 psi Cylinder 8 165 psi
    Cylinder 2 160 psi Cylinder 7 165 psi
    Cylinder 3 165 psi Cylinder 6 170 psi
    Cylinder 4 170 psi Cylinder 5 165 psi

    These seem exceptionally good to me?

    Am I right for a 308 that old with the indicated mileage?

    Anyone know what would you should get with a new engine with standard pistons ?

    I was going to strip everything out and measure cylinders etc for wear, but given these results I think I would just be wasting my time?

    If it aint broken don't fix it!

    Anyone got any supporting comments or suggestions or am I completely wrong in assuming everything is OK!


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  3. LightGuy

    LightGuy Three Time F1 World Champ
    Silver Subscribed

    Oct 4, 2004
    Full Name:
    If you are taking the engine out and the heads off, why not change the rings and bearings too? If it were me and the compression was this good , only a synchro problem and maintenance on the valves, leave it alone and drive it; unless you like to tinker.
  4. matteo

    matteo F1 World Champ

    Aug 1, 2002
    On a plane somewhere
    Full Name:
    Heir Butt
    I agree with lightguy. I would pull the motor only if there were other issues like head leaks or oil pressure issues that could be related to a blocked passage. Cyl 1 looks lower the then others so that might be reason enough for you to pull it. It'd check the compression every 6 months and it it drops, pull it then.
  5. tomoshea

    tomoshea Formula Junior

    Dec 29, 2003
    Full Name:
    Tom O'Shea

    the main reason I am pulling the engine is to replace the exhaust valves 73K miles is entering danger territory for Sodiums, and I would much prefer not to have to replace a piston and a head or worse! I look on the valve replacement as Preventative Maintenance (of a very large bill!)

    The engine is also high in HC's (believe me I have done everything with the carbs and checked the timing) and given these compression ratings, I think this means I am leaking oil through some of the valve stems, which would be indicative of some valve wear!!/ movement in the guides - not good in the long term! - helps the buggers to crack right at the bottom of the stem and fall into the cylinder causing much grinding and gnashing of teeth and lots of rude words!

    I thought while I have it out I would look at the main bearings and replace if necessary (not a big job 10 hrs) relatively cheap - $300 for a set

    Replace the synchros on 1st and 2nd (not a big job10- 15 hrs) relatively cheap $400 for the pair

    And I will be sending the heads off to have the exhaust valves replaced/ guides as necessary.

    Thoughts - am I mad?, and by the way I do like tinkering
  6. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    Full Name:
    From the readings you present, my guess is that what you are planning is optional. If you want to do it, go for it and add some wild cams, porting and pistons at the same time. On the other hand, you could just drive it.

    I think the sodiums tend to be fine as along as the car is driven. You get into the problem with garage queens.
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  8. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
    Full Name:
    At the mileage you show, and given the time in service, along with the cars over the years that have dropped a valve, I think you are on the correct course. If the engine drops a valve the damage can be almost fatal, depending on how wound up it is when it drops.
    If your doing the work yourself, you should just consider bearings as a matter of course, along with rings, and if worn, pistons and boring as well. You wont regret the peice of mind, and the value of the car with a documented overhaul will significantly increase.
  9. Verell

    Verell F1 Veteran
    Consultant Owner

    May 5, 2001
    Groton, MA
    Full Name:
    Verell Boaen
    Given the car's age, I suggest first investigating valve stem seals as the primary cause of HC emissions. They're replacable w/o pulling the heads. The original ones have got to be hard as a rock & pretty worn at this point.

    Can be replaced with Viton seals that hold up better than the OEM teflon ones. BMW & Harley seals fit, also available from NAPA see:

    Doing the rest is your call & $$$.

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