Compression Pressures - SU/Hitachi and running on. | The Z Club of Great Britain
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Compression Pressures - SU/Hitachi and running on.

Discussion in 'S30 (240Z,260Z,280Z) Engine' started by Rob Gaskin, Feb 25, 2019.

  1. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator

    For those of you who measure your compressions, what is your highest figure?

    I had trouble with my old track car 'running on' and that was 200psi.

    My blue car runs on too and I'm going to measure the pressures tomorrow.
     
  2. Paul_S

    Paul_S
    Z Club Member

    When I had mine tuned a little while back I was told my lowest was 180 and the highest 190.
     
  3. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator

    Bump!

    Anyone running 210 psi
     
  4. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator


    Thanks Paul.
     
  5. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Rob, last I measured mine it was 205. Running on was only due to lean mix causing hot spots in the chambers. Once RR tuned and needles re-profiled it was fine and never ran on again. (It's now running too rich but that is a different story and my fault for tinkering - soon to be RR-ed again when Sean's exhaust arrives).

    I was told by several engine builders that under no condition is dieseling / running on a good thing. Even if it's able to pull fuel through the carbs when switched off, there should be nothing hot enough in the chamber to ignite it. Otherwise that is also happening when you are driving and it'll be causing knock that you can't hear / detect.

    What do I know as a weekend warrior, I'm just relaying the advice. It does make logical sense though.
     
  6. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator

    Ali, I think it gets the heat by highly compressing the gas like a diesel.

    I do know what you have written and I have adjusted the ignition timing. It doesn't 'pink'.

    My red car did the same for at least 10 years. It is common with modified engines and SU type carbs.

    I have heard of someone fitting a diesel solenoid on the intake balance bar which can be operated on 'switch-off' to open and cause the engine to stop.
     
    AliK likes this.
  7. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    I hear you Rob - been reading about it on other forums too and your experience of the old car running on and being healthy says a lot.

    The thing that nags me is this: if the compression alone is able to ignite it, then during the combustion part of the cycle there are two combustion events 1. The bit when the compression kicks it off and 2. The bit where the timing / spark plug says go. So there must surely be uncontrolled ignition happening at all times no?

    Do your plugs look hot at all? I reckon mine must have been glowing when I switched off ...

    [​IMG]

    Like I said, my knowledge / experience of Zs is dwarfed by your decades of knowing these cars so I'm keen to discuss and learn.
     
  8. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator

    Ali, your plugs are whiter than mine. My plug colour is fine.
     
  9. franky

    franky
    Z Club Member

    Why is it common to su carbs?
     
  10. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator

    I think with this type of carb the action of the suction on the intake stroke lifts the damper and needle just enough to supply an ignitable mixture to the cylinder. Yes I know the throttle butterfly will be almost closed. I'm guessing here.

    Just did a search to try to find out (and didn't) but found this video. This chap hopes to fix the problem a different way - I wonder if it worked?

     
    AliK likes this.
  11. jonbills

    jonbills
    ZClub Administrator
    Staff Member

    If its running on through dieseling, just as a function of the engines compression, surely it will never stop.
    Does it run on and then stop after a few seconds Rob? I think that must be what Ali first described - something glowing red-hot and igniting the mixture.
     
    AliK likes this.
  12. johnymd

    johnymd
    Z Club Member

    My view on this.

    I think the mixture will always get ignited before any pre-ignition event under load as spark occurs before maximum compression. If you turn the engine off then ignition may occur from dieseling on a carbed car.
     
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  13. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator

    Jon I don't think it's a hot spot. When I want to switch off I put the car in gear, turn off and then stall it. Did the same with my old car. It will keep running which is not desirable.

    I have discussed this with the engine builders of each car and both say it's common for modified engines running SU type carbs.

    The car runs well by the way.

    To get back on topic what compression pressures have you got?
     
  14. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    Makes a lot of sense Johnny. So could there be a scenario where the flame front from the spark doesn't cover the whole charge and the flame front from the Dieseling meets it after the timing point and raises cylinder pressure above where it needs to be at that point in the cycle?

    I know this slightly off the original question posed by Rob (apologies).

    Franky, from what I've read, the theory is that Hitachi SUs don't have a separate idle jet, so as Rob says, the action of the suction is pulling in mixture even through idle level closed butterflies (which are slightly open). Do the triples shut off the fuel on power off?
     
  15. jonbills

    jonbills
    ZClub Administrator
    Staff Member

    No triples are like SUs in this regard to my mind. They will flow as much air as they did before sparks were removed for a given engine vacuum and they add fuel metered to match that air flow.

    I'll check my compression this morning.
     
    Last edited: Mar 18, 2019
    AliK likes this.
  16. jonbills

    jonbills
    ZClub Administrator
    Staff Member

    I have 180 psi.
     
  17. Rob Gaskin

    Rob Gaskin Treasurer in training Staff Member Moderator

    Jon, it's a known issue. My Sam on DCOEs is fine, stops dead.

    180 psi - interesting. Mine are way higher than that. My old car was 200 psi, this blue car is 210+.
     
  18. jonbills

    jonbills
    ZClub Administrator
    Staff Member

    My CR is only about 11.2:1, with 39cc Chambers and 5cc cut outs. I didn't get round to skimming it again last year when I had the head off. I'll bump it up to 12 next time. What CR do you have Rob? I imagine the cam is making a difference.
     
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  19. AliK

    AliK
    Z Club Member
    Staff Member Moderator

    OK two last thoughts Rob and then I will get back in my box:

    1. Are you running the BP6ES plugs or the BP5ES - when I was researching mine running on, a lot of people had cured theirs with the 6ES. Dead cheap here if you want to experiment.

    2. Do you use RON 95 unleaded or Super Unleaded? Again I was told with high compression to use the highest grade fuel. I watched a (fifth gear I think) review of various super unleaded fuels compared on a dyno and the winner turned out to be Shell's. If I find the review again, I will link to it.

    Edit: found it ....
     
  20. atomman

    atomman Active Forum User

    Interesting you say BP6ES to BP5ES , I have swoped to a hotter BP7ES plug in mine because the plugs were getting to hot.

    Never had a problem with mine running on, it was about 210 when we tested it on the dyno a while back and I run it on shell V power or the tesco 99 octane
     
    Last edited: Mar 20, 2019 at 8:18 PM
    franky likes this.

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