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Discussion Starter · #1 ·
OK I had my annoying part load misfire return so whilst I have the plugs out to change them I thought meh ill check my compression whilst Im at it ...

Bearing in mind the engine runs sweet as a nut here and apart from this occasional misfire (which is the plugs) it has been faultless and has very low mileage on it - circa 6000-7000 miles.

All fairly even however quite low Id say !

1 -130
2- 129
3 -122
4 -125
5 -129
6 -130

Thoughts ??? whys this as low as that ? seems like someones dropped the compression a fair bit but how did they drop it that much ? Surely thats not just from the hks HG ?

I know its had extensive headwork too, could it be due to that ?

P.S thats on a warm engine and test was run dry.
 

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you should look more at whether the compressions are around the same rather than what they are as a general rule (unless they are all very low )
 

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Discussion Starter · #6 ·
Yeh I realise that which is why I'm fairl happy with them, just curious as to what compression drop can be gained via a metal hg and some headwork that's all :) I never realised how much it would drop it !
 

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apparently, head gasket thickness does make a difference - you've got 1.2mm, 1.5mm for lower compression and high-boost applications, and 1.0mm if you want to push things a bit. 0.2mm doesn't seem like a lot to me, but when my engine was built and my builder stressed out over parts being out 0.005mm, I guess it does matter quite a bit in the engine fit department!
 

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Oil down bores and see what happens Steve?

Plugs out of course.

Hate to say this.... is it using any oil? part load = vacuum = oil past rings= Mis-fire?

Oil control rings on way out?
 

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Discussion Starter · #9 ·
Its all good Gordon, had it confirmed that those are normal figures for an engine running a thick hg with head machined out.

But to anser your question, nope never uses a drop of oil and never has. It hardly even breaths anything either, shes a sweety :)

The misfire it turns out is due to overfuelling at low rpm which is caused by 2 duff o2 sensors :( I checked them about 30 mins ago after changing the plugs didnt solve my problem.

I think one has been out for a while and the other has been intermittant but failed last night after I got stuck in heavy traffic probably because she runs so rich at low rpm. That all would tie in to be honest because the mis only ever happened after being stuck in traffic or driving slowly. Usually though if you booted her a bit she would clear and start runnning fine again but this time i think there totally dead.

I get no voltage from them at all even If I test them on the bench with a blowtorch.

Anyone got a couple of working o2 sensors lying about ??
 

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I have an R32 O2 sensor, unused, sitting in my drawer. It was to be a backup, but I switched to O2 feedback-off mapping over a year ago. Dang, come to think of it, that means my O2 sensors in my car are doing nothing as well, might as well pull them and bung the ports. PM me if you're interested!
 

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btw, what ECU are you running? Your symptoms aren't ones I'd attribute to faulty O2 sensors...
 

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Discussion Starter · #12 ·
It's a custom tuned one mate. It's deffo them as i've removed and tested everything that could cause the issue and only the o2s are showing up faulty :)

our of curiosity though what would you have said ?
 

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Discussion Starter · #16 ·
You run a compression check with all plugs out and the throttle held wide open.
Plugs were out as I have said in post 1 and it Makes no odds regarding open or closed throttle mate, I have tried both in the past ;)

Like I said up top anyways I AM NOT CONCEREND ABOUT THE FIGURES BEING THE VALUES POSTED !!! this post was about the drop gain ed based on the HG and headwork to see if others had found the same thing !

I know fitting a thicker hg WILL drop compression from the 170psi that nissan specify, I just didnt realise how much and thats what this post was about. Yet some folk seem to think its a question about why its down at 130 odd's.

Everyone knows what matters is the difference between cylinders and not the actual values unless there crazy low like 50 odds !
 

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presumably the throttle ought to be open to check for intake valve leaks, but even if they're shut, and a valve is leaking, you're going to know something is seriously wrong anyways - there's a fair bit of volume from the valve to the butterfly!

Just my take on things, but I view compression checks in the same light as setting the cam angle sensor - no matter how precise you think you are in determining the actual objective value, it's going to be off a bit and then what you go on are relative values (compression - all being near each other, CAS - you adjust the ignition map accordingly to where the engine wants to be)
 

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Discussion Starter · #18 ·
presumably the throttle ought to be open to check for intake valve leaks, but even if they're shut, and a valve is leaking, you're going to know something is seriously wrong anyways - there's a fair bit of volume from the valve to the butterfly!

Just my take on things, but I view compression checks in the same light as setting the cam angle sensor - no matter how precise you think you are in determining the actual objective value, it's going to be off a bit and then what you go on are relative values (compression - all being near each other, CAS - you adjust the ignition map accordingly to where the engine wants to be)
I agree :) :thumbsup:
 

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Three years ago my compression results were 122-126 between all six cylinders. Same results today. Lots of 2+ bar boosting, no oil consumption or blowby.:thumbsup:
 
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