High cylinder compression readings - causes?

[Pagan]

Hi all, I've been trying to do a cylinder compression test recently on my 2.0i, partially to check the health of the engine and also just to eliminate some possible causes of slightly uneven idle. The car's generally running well.

When I've done this test, both with the engine hot and when cold, I'm getting very high readings. After cranking four times, I got the following PSI: 215, 200, 195, 200. I've tried cranking a little longer, but once up to 225PSI, I got concerned about blowing something.

As far as I can tell, normal should be about 150PSI. From what I've read, high readings seem to either indicate a problem with the gauge (it's new from US Pro) or possibly carbon build up. Any thoughts? Is there a simple way of testing the gauge?

Thanks.

 

[enuff_zed]

Not sure how carbon build up could improve on what the engine is designed to do really. Otherwise it implies that without carbon there is pressure escaping.
I'd go with the gauge. Try to find a local company who also use pressure gauges and ask if they can calibrate it.

Or take it to a garage and ask them to do the tests then compare their readings with yours.

 

[Rockhopper]

Consistency between cylinders is more important than the actual reading. Yoiu want less than 10% difference.

Did you pull the injection pump fuse out and had all the spark plugs out?

 

[Pagan]

Thanks for the thoughts.

Try to find a local company who also use pressure gauges and ask if they can calibrate it.

Good thinking. I wonder if typical garages would have calibration equipment. I see you're in Norfolk--me too.

Did you pull the injection pump fuse out and had all the spark plugs out?

Yeah, I ran the engine, then pulled out fuse 19, the engine soon stopped, I took out all the spark plugs and tested each cylinder.

 

[Spriteracer]

I don't know if this helps, but my BMC "A" series race engine runs a compression ratio of around 11:1. 1950's technology so no engine management but I use Torco fuel accelerator to increase the octane for safety. My gauge shows around 220psi +/- hot on a fresh rebuild. The N46 seems to be around 10.5:1 so not very different. As Rockhopper says the balance between cylinders is important.

 

[enuff_zed]

I don't know if this helps, but my BMC "A" series race engine runs a compression ratio of around 11:1. 1950's technology so no engine management but I use Torco fuel accelerator to increase the octane for safety. My gauge shows around 220psi +/- hot on a fresh rebuild. The N46 seems to be around 10.5:1 so not very different. As Rockhopper says the balance between cylinders is important.

Talking compression ratios made me think: has this engine ever had the head off? Maybe it's been skimmed?

 

[Pagan]

Thanks for the further thoughts. No, it's never had any modifications--and I've had it pretty much since new. It's interesting that your 11:1 engine is also reading high psi, Sprite.

I hooked up a tyre pump to the compression gauge, and it seemed to be fairly accurate, at least for the first few bar (couldn't hold the two nozzles together after that). Next, I think I might pick up a cheap borescope just to check it's all okay inside.

 

[Spriteracer]

The actual calc of what PSI should be is beyond my knowledge but I think there is more to it than just atmospheric pressure x CR. The psi figures I get are in the right range according to my engine builder. As long as they are all close he is happy. Years ago I knew of engines running as much as 13.5-14:1 CR on 5 star petrol and octane boost. Not so much now as they are very sensitive to timing and cooling and UK motorsport regs for most forms of the sport state pump fuel only. Bear in mind also I'm referring to 1950s and 60s technology with plugs, points, carburettors, timing set by strobe and no sensors ! I will ask my engine builder if he can explain.

 

[Spriteracer]

Just to go back to basics, just about everything that governs the compression is set by the design of the engine. i.e. cylinder bore, crank stroke, piston design, head design, valve timing etc. If anything is damaged or failed - rings, valves, gaskets etc. the result is likely to be lowered compression due to pressure leaks. i.e. what you find with a leakdown test. You might also see pressurisation of the cooling system, the oil system or the sump. I am no expert but I can't think of anything that would increase the compression that wouldn't also cause severe bad running. As an extreme example - I knew of one racer years ago who tried to increase the compression of a Healey Silverstone Engine by glueing shaped slivers of aluminium to the top of the pistons. They vanished without serious damage to the rest of the engine as far as I heard.

 

[Spriteracer]

OK I have a response from my engine builder. My question was - is there away of calculating expected PSI. His response is-

The quick answer is - there isn't one. There are way too many variables to make accurate calculations. Let alone the speed the engine is spun over to take the readings and the engine temp it is done at. All that can really be done is compare the results with the same type of engine with different compression ratios. 220psi on an A-series would indicate a healthy 11.5 to 12.0 CR. But that can be achieved with a compression ratio on the high side for a mild cam as low speed trapping efficiency is much higher than with a race cam with a big over-lap.

So, back to OP as has been suggested I think, all you can really do is to compare with other similar engines of similar mileage etc. Hope that helps.

 

[tug]

To go to all this trouble is obviously because you really care for your zed and to give you peace of mind, which is excellent. But if it is really just to get to the bottom of a slightly uneven idle then i wouldn't worry, the N46 is renowned for this. I found many years ago (i have had mine over 10 years now) that it needs a regular good thrashing, you will find after a while the idle becomes smooth and steady.

Cheers, tug

 

[Pagan]

Thanks again for all your thoughts.

I put a borescope in cylinder 1. Please see attached pictures. Healthy enough? The car has low mileage.

All that can really be done is compare the results with the same type of engine with different compression ratios.

I could be wrong, but from what I've read I believe 150psi is the around the norm for these engines. Please do chime in if anyone can confirm this.

But if it is really just to get to the bottom of a slightly uneven idle then i wouldn't worry, the N46 is renowned for this. I found many years ago (i have had mine over 10 years now) that it needs a regular good thrashing, you will find after a while the idle becomes smooth and steady.

The main reason I did the compression test in the first place was to check whether it's worth continuing to invest in the car, as it needs new suspension. The wavering idle unfortunately doesn't seem to be resolved by driving hard--doing this has sorted out other minor issues though! But as long everything else is healthy, the idle isn't a major issue.

 

[Rockhopper]

My thoughts - the valve seats seem very pitted. Tons of cross hatching left on the bores though. 150psi seems low to me but it may well be correct for that engine. I'd expect 170-200. But also as I've said before consistency between cylinders is more important, no more than 10% variance.

 

[enuff_zed]

NewTIS states:


15 Bar + 220.5psi

Therefore it seems your readings are about correct, allowing for a little wear over the years.

 

[Spriteracer]

I agree with Rockhopper, the valve seat looks pretty crusty. Could the engine have done a lot of short trips and not got properly hot often enough to burn off the carbon build up ? Ideally, you could take head off and grind the valves and or replace the valves and seat inserts. However, if the engine is running ok generally it can't be too bad and I would leave well alone. It might just be slightly down on power if the valves are not sealing as well as they could and with the carbon build up on the pistons and on the bottom of the head and valves giving the higher PSI. Enuff's figures show your achieved PSI is pretty close to correct anyway. As has been mentioned, driving it harder may help but it might take a while. You could dry driving in a gear lower than normal to keep the revs up. Not good for the economy though.

 

[enuff_zed]

To me, there isn't any problem with compression, factoring in a slight loss for wear over the years.
The 4-pots I've had have had slightly fluctuating idles as well. I just assumed it was 'a thing'.
Are you sure you're not just overthinking this?

 

[Pagan]

Yeah, it's driving fine and no error codes, so as you said Sprite, it hopefully means it's not TOO bad. Presumably the valves slightly not sealing could be the cause of the slightly uneven idle, bringing a slightly inconsistent amount of air in.

And yes, a family member had it before who did short trips. I'll keep it driving it hard, use some petrol system cleaner sometimes, maybe spray some water into the intake (something I've read about), and possibly it'll slowly clear over time. And then when/if I change the timing chain at some point, I could then clean the valves, etc.