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Triumph stag poor compression

  • 06-05-2018 3:00pm
    #1
    Registered Users Posts: 846 ✭✭✭ Wavey


    Afternoon all.

    I did a dry and wet compression test on my 71 Stag which gave 130psi dry and 180psi wet so I am looking at pretty much a complete engine rebuild and cylinder re-bore.

    Before I take the plunge on this, does anybody know what effect such a drop in compression would have on bhp?
    The stag is rated at 150bhp.

    Also, could anybody recommend a mechanic that would do this work in the Dublin area?

    Many thanks,
    Wavey.


Comments

  • Closed Accounts Posts: 8,600 jca


    Are all 8 cylinders reading the same?


  • Registered Users Posts: 846 ✭✭✭ Wavey


    Range between 130 and 140 psi so pretty close.


  • Registered Users Posts: 4,358 ✭✭✭ kev1.3s


    Wavey wrote: »
    Range between 130 and 140 psi so pretty close.

    I think that should be ok.


  • Registered Users Posts: 846 ✭✭✭ Wavey


    kev1.3s wrote: »
    I think that should be ok.

    Hi Kev, Do you think the level of pressure drop is ok between wet and dry test?


  • Registered Users Posts: 4,358 ✭✭✭ kev1.3s


    Wavey wrote: »
    Hi Kev, Do you think the level of pressure drop is ok between wet and dry test?

    Well it's not ideal but it is an old engine, I has a 20-25 psi difference in my fulvia but the dry results were much lower, 105 psi on cylinder no.3. You're readings are also closer together than mine and mine runs just fine.


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  • Registered Users Posts: 4,900 ✭✭✭ Bigus


    Define wet ?
    I can understand doing a compression test hot and cold , but not wet. Hot will show up burnt valves broken rings etc

    130 doesn't sound too bad , cold .


  • Registered Users Posts: 914 okistag


    As long as there is not a drop of more than 10% between cylinders its perfectly fine.
    when you say wet, what do you mean. maybe putting oil in a cylinder then you get an increase ? 
    personally, if all cylinders read close in compression, don't worry


  • Closed Accounts Posts: 8,600 jca


    Bigus wrote: »
    Define wet ?
    I can understand doing a compression test hot and cold , but not wet. Hot will show up burnt valves broken rings etc

    130 doesn't sound too bad , cold .

    Wet means squirting a drop of oil into the bore to help the rings to seal better. I can't see hot or cold making any difference, a burnt valve is a burnt valve it will leak regardless of temperature, the same with a broken ring.


  • Closed Accounts Posts: 8,600 jca


    Wavey wrote: »
    Afternoon all.

    I did a dry and wet compression test on my 71 Stag which gave 130psi dry and 180psi wet so I am looking at pretty much a complete engine rebuild and cylinder re-bore.

    Before I take the plunge on this, does anybody know what effect such a drop in compression would have on bhp?
    The stag is rated at 150bhp.

    Also, could anybody recommend a mechanic that would do this work in the Dublin area?

    Many thanks,
    Wavey.

    I think they're respectable figures tbh I certainly wouldn't be considering an engine rebuild. My aul Cortina struggled along on less than 80 psi on all 4 until No.4 cylinder got so bad it kept oiling the plug at anything over 55 mph. I'd soldier on for another while.


  • Registered Users Posts: 1,188 ✭✭✭ JabbaTheHut


    jca wrote: »
    I think they're respectable figures tbh I certainly wouldn't be considering an engine rebuild. My aul Cortina struggled along on less than 80 psi on all 4 until No.4 cylinder got so bad it kept oiling the plug at anything over 55 mpg. I'd soldier on for another while.

    Yeah, I'd tend to agree. It really depends on what your long term intentions are. If you see yourself putting up a lot of mileage in a short timeframe, then I'd do the work you suggest. But on a classic, I assume you won't be putting up the kind of mileage that warrants a rebuild.

    Having said that, if you inted to hold onto the car for 20 years or more, or are after perfection, then do the rebuild.

    But I'd be leaning towards leaving things as is. The figures you give aren't terrible, by a nice bit.


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  • Registered Users Posts: 4,900 ✭✭✭ Bigus


    jca wrote: »
    Wet means squirting a drop of oil into the bore to help the rings to seal better. I can't see hot or cold making any difference, a burnt valve is a burnt valve it will leak regardless of temperature, the same with a broken ring.


    I didn't ask what was wet ,

    I asked the op "define wet"


    Absolutely not re hot and cold, a hairline crack in a valve won't show up cold , or for that matter a compression leak in a head gasket mightn't cold, also rings and bores can act completely differently hot or cold FFS, especially in a Stag prone to overheating from original design flaws.

    Anyway unless you were very accurate with a pipette you're could be effectively raising the compression ratio with oil , proof being ultimately if enough oil is injected hydraulic lock will occur, which could bend rods and lower compression, if you didn't know what you were doing .


  • Closed Accounts Posts: 8,600 jca


    Bigus wrote: »
    I didn't ask what was wet ,

    I asked the op "define wet"


    Absolutely not re hot and cold, a hairline crack in a valve won't show up cold , or for that matter a compression leak in a head gasket mightn't cold, also rings and bores can act completely differently hot or cold FFS, especially in a Stag prone to overheating from original design flaws.

    Anyway unless you were very accurate with a pipette you're could be effectively raising the compression ratio with oil , proof being ultimately if enough oil is injected hydraulic lock will occur, which could bend rods and lower compression, if you didn't know what you were doing .

    There's no need for the FFS bit. All the cylinders are equal in value, so I can't see anything sinister like a cracked valve causing trouble unless all 16 are cracked. One squirt of oil from an oil can per cylinder would be accurate enough for me especially with an engine of that era. The cooling problems were mainly due to poor maintenance of the cooling system rather than the engine itself. Anyway I'm happy that I'm giving the op correct advice without over analysing 1960s technology.


  • Registered Users Posts: 4,900 ✭✭✭ Bigus


    Well I've had many a car with great compression cold and falling to as low 40 psi hot , caused by valves with cracks or not seated properly when hot, only for compression to go back up to near normal when engine gets cold again . So much so the car would start fine cold , but needed a tow to start hot.
    I've had hairline cracks in bore sides invisible to the naked eye , but detecacble by a hot compression test. Almost all Tolerances change when the block and heads heat up therefore Most manufacturers, recommend setting tappets hot for this very reason , so a cold compression test would only be indicative of a very serious problems , whereas a hot compression test will tell all, even slight issues.
    As for a wet test , that's artificial, regarding normal engine operating parameters.


  • Closed Accounts Posts: 8,600 jca


    Bigus wrote: »
    Well I've had many a car with great compression cold and falling to as low 40 psi hot , caused by valves with cracks or not seated properly when hot, only for compression to go back up to near normal when engine gets cold again . So much so the car would start fine cold , but needed a tow to start hot.
    I've had hairline cracks in bore sides invisible to the naked eye , but detecacble by a hot compression test. Almost all Tolerances change when the block and heads heat up therefore Most manufacturers, recommend setting tappets hot for this very reason , so a cold compression test would only be indicative of a very serious problems , whereas a hot compression test will tell all, even slight issues.
    As for a wet test , that's artificial, regarding normal engine operating parameters.

    None of this is relevant to the op, you're just scaremongering or looking for business. His engine seems to be running ok just getting a bit soft(aren't they all?) It will be fine for many miles yet, just keep a good eye on the coolant.. I'm sure you know that already.


  • Registered Users Posts: 846 ✭✭✭ Wavey


    Thanks for all the replies lads.

    I'm happy to drive it on and not drop a few thousand rebuilding the engine for sure!

    I would really only take it on if I knew there was going to be a marked increase in power afterwards.

    Would a 30% loss in compression equate to a 30% loss in BHP? Perhaps not.

    Thanks again,

    Wavey.


  • Registered Users Posts: 4,900 ✭✭✭ Bigus


    Enjoy it til winter anyway , once it starts and drives is the main thing, during summer season.


  • Registered Users Posts: 4,358 ✭✭✭ kev1.3s


    I don't think a 30% drop in compression would in any way equate to 30% drop in power. I still get plenty of oomph from my fulvia.


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