project 1980 Austin Princess

[chompy_snake]

Get the valve guides checked, worn ones can cause low compression readings and sometimes happens without burning oil. 

Something always worth checking on older engines 

[Dick Longbridge]

The black in the lower image looks like a massive tide like painted on to hide rot as it creeps up the side of the car. 

[Talbot]

On 10/24/2020 at 3:56 PM, vulgalour said:

Workshop manual tells me the compression ratio is 9:1 on this engine and it should be giving me a reading of 170-195psi.

That's a bit on the hopeful side.  100% volumetric efficiency would achieve just over 132psi.  Not quite as simple as that in the real world, as the valve timing can cock things about a bit, but 195psi would be over 145% volumetric efficiency on a 9:1 engine.  Which I very much doubt is possible.

Normally "over 100psi" is considered a good result for most petrol engines, which would be ~80% volumetric efficiency, which is far more like it.  195psi is getting into turbo-diesel levels of compression pressures.

[colino]

Did you do a wet compression test?   Cylinders 2,3 & 4 are within +/-10% of each other fine for an old engine. 

[vulgalour]

What I did was get the engine up to operating temperature, then pulled the plugs and put the compression tester on.  I assume that's a dry test since I didn't put anything extra in the cylinders.  Last time we did it this way when it didn't have a blown head gasket I'm sure it was getting around the 185psi range, give or take a few psi, across all four cylinders.

Current suspicions are a duff head gasket, worn/damaged valve guides, and worn piston rings.  We'll know better when the engine comes out (stand arrived today, crane will be arriving soon) and can actually have a look.

@Talbot I can only tell you what the book tells me.  I don't really understand the intricacies of ratios and numbers and whatnot, mostly because I have no passion for engine work so never bothered to get into it.

[vulgalour]

The car does use some oil, whether it's leaking out or burning it I'm not certain, but it's always done it since I got the car in February 2012.

Reasons to take the engine out:

• Leaking sump gasket
• I want to paint the block
• Suspected worn piston rings, I've new ones to go in
• I've got a new engine stand that needs breaking in
• Removing, inspecting, and rebuilding the engine would give peace of mind

I'm sure I can come up with more reasons if pushed.  Suffice to say it's happening and it's going to do no harm to do it.  What is a wet compression test going to tell me that a dry one hasn't?  Valve clearances aren't due to be checked yet, I've not finished running things in from when I last did them, and the shims for the Princess were proving so difficult to get in the sizes I needed that last time I ended up having to get a friend in the States to send me some in the sizes needed.

There'll be nothing doing until the engine crane arrives, it stops raining, and I'm motivated to dig into it.  For now, it's OMGHGF and we'll pull everything apart to find out what else is wrong later.

[Talbot]

A wet compression test is very much as it sounds:  You do a compression test, but have to do it in the wet.  As the weather has gotten a lot worse recently, it's a lot easier to do this, as you can time it for when the forecast says it's going to be wet.  At a push, you can get someone with a hosepipe and a sprayer on "mist" to go over you, the car, your tools and the whole working area, but the results aren't as accurate as trying to do a decent wet test in pouring rain.

What you're looking for is the time (usually in minutes) before you give up, shout "fuck this fucking car" and go inside to get dry and warm again.  You then use this number of minutes as a multiplier on the original compression test results to get an over-inflated view of the health of the engine.

Bonus points can be gained for doing a lumpy wet compression test (in hail) or a swirl check test (in a tornado).

[Talbot]

10 hours ago, vulgalour said:

What I did was get the engine up to operating temperature, then pulled the plugs and put the compression tester on.  I assume that's a dry test since I didn't put anything extra in the cylinders.

You're exactly correct on that.  A wet test (despite what I said above) is the same, but you introduce a squirt (maybe 25ml) of engine oil into the cylinder (through the spark-plug port) first.  That will then saturate the rings and any gap between the pistons and bores.  If the compression pressure then jumps up, you have worn rings/bores.  If it stays low, compression is being lost through non-seated valves.

Another good test to do is to make up an adaptor from an old spark plug that allows you to fit an airline to the cylinder.  This is called a blowdown test.  You then "inflate" the cylinder (just open it up to the compressor reservoir) and listen for where the air is coming out.  It will either come out of the oil filler cap (blow past the rings) out the exhaust (exhaust valve not seating) or out the inlet (inlet valve not seating.  It's also a way you can test for a very blown head gasket, as if you get bubbles in the coolant, OMGHGF.

As for the compression pressures.  I'm only commenting that it sounds high, as when you're doing a compression test, you're turning the engine into a compressor.  It draws in atmospheric air at 1bar, and then compresses it to 9 times that pressure (a 9:1 engine), so you'd expect the maximum it could achieve to be 9 bar (132 psi).  The bit about volumetric efficiency is to do with how well the cylinder is filled before it compresses the air in it.  if it's 100% full, that means it's at 1 bar before it starts compressing.  Most engines run at less than 100% volumetric efficiency, hence the cylinder is still at a slight vacuum before the air is compressed, meaning the final figure is a little less than the theoretical maximum.

I absolutely don't doubt your book says 195psi.  I just don't understand how.

[sloth]

odd that your saying 195psi is unacheivable - my Triumph 2500 is sitting at 170psi per pot.  and have seen many modern engines acheive 190psi. 

either way that engines borked. id yank it out and strip it.  no point stripping in situ

[sloth]

also combustion pressure can be affected by bore vs stroke, i.e smaller piston pushes less air but the same c/r 

[Noel Tidybeard]

dog dick pink on a princess- only on AS 🤣

[Talbot]

20 hours ago, sloth said:

also combustion pressure can be affected by bore vs stroke, i.e smaller piston pushes less air but the same c/r 

That makes no sense.  It shouldn't matter what the geometry of the piston is, all it's doing is compressing air.  Whilst an over-square engine will have very different performance to an under-square one, if the CR is 9:1, then the CR is 9:1, so it compresses the same volume of air into the same clearance volume.

the one thing that would make a big difference to the compression test pressure would be the valve timing.  If the inlet valve isn't closing until a good bit after BDC, then it's possible that even a healthy engine would show a lower test pressure, as the piston is already rising in the cylinder before the valve is closed, so that bit of compression is "lost".

 

[vulgalour]

Water stopped falling out of the sky when I wasn't at work!

Good enough.  So let's do the wet compression test.  I've also included the figures for the dry compression test below for ease of comparison.

DRY TEST
Cylinder - First reading - Second reading
1 - 85 - 80
2 - 158 - 158
3 - 145 - 145
4 - 150 - 150

WET TEST
Cylinder - First reading - Second reading
1 - 148 - 148
2 - 160 - 155
3 - 150 - 150
4 - 155 - 155

Cylinder 2 seems like a bit of an anomoly but perhaps that's something to do with 1 having failed so there's a bit of leak through or something.  3 and 4 are probably okay?  The jump in compression on 1 was a surprise but perhaps that suggests that the rings are actually good and doing their job.

I'll open this one to the floor, I don't really know what these figures are saying beyond confirming the issue with the HGF is on 1 and 2.

[Talbot]

Other way around.  The fact that the wet test shows a jump up in compression pressures suggests the bore or rings are boogered.  If the figure had stayed the same, that would then point elsewhere, but the fact that the rings being saturated with oil has improved the pressure means there's something amiss with them.

How much oil did you use (as in, just wondering if it got into anywhere else to improve the pressures)

[vulgalour]

I used the very technical quantity of "a good squirt", I didn't have anything to actually measure it out that would also allow me to pour it into the spark plug holes.

[colino]

That reads to me that No 1 has a ring/bore problem, but I'd take the plugs out and using the anti-seize of your choice, squirt plenty down the bores and leave for a week.  Then empty the bores by spinning it on the starter, putting the plugs in and going for a thrash.  If your compression tests are the same after that, you ponder a DIY rebuild or sticking in someone elses second hand problem.

[vulgalour]

Going for a thrash not really an option when the head gasket has failed.

[captain_70s]

Is the sump droppable in situ or is it burried in gearbox and subframe and shit?

[vulgalour]

Gearbox in sump means removing the engine is the easier than dropping the sump.  Which is okay, I got the tools to do just that, it's just a faff.  No subframe to worry about, so that's something.  Current plan is to unbolt the sump and clutch assembly, remove the bonnet, and lift the block and head as one lump out through the top.  Once it's all on the engine stand, I can then get to more dismantling and inspection.  Car isn't consuming vast quantities of oil so I'm not really expecting horrors, hopefully we get away with a light hone and new rings and haven't got worn out bearings or the like.

I do find it less and less likely that the 75k is genuine on this car.  I blame Quentin Wilson.

[colino]

The cleaned up version.  It's already broken, you can't break it any more. 

[Talbot]

"A good squirt" could well have also wetted the head gasket area, and possibly even a valve or two.  That said, it's noticeable that even with oil in the bore, the compression pressure for cylinder 1 is still slightly lower than for the others.

I suspect that your OMGHGF is probably betwixt cylinder 1 and the water jacket.   It would be very interesting to to a leakdown test (as described earlier) to see if bubbles appear in the coolant.

If you are able to do a leakdown, the test is carried out with the cylinder at TDC at ignition.  You can move the piston down the bore a bit on either compression or ignition to see if it changes anything, but the first thing is a TDC test.

Or just* heave the engine out, whip the head off and see what's up.

[vulgalour]

I haven't got an air compressor so I can't do a leakdown test.  I don't have the space to put one either so it's not something I'm going to buy (even if I could afford it, which I can't right now) so I reckon the best bet is to pull the engine and start having a look inside it to see what's what.  A rebuild is pretty much unavoidable now but I've got most, if not all, of the bits I'm going to need to do it.

[captain_70s]

22 hours ago, vulgalour said:

I do find it less and less likely that the 75k is genuine on this car.  I blame Quentin Wilson.

Enough BL engines shat themselves in less than 10k that I'd not be surprised that some would need a light rebuild at 70k...

 

[vulgalour]

I'm still blaming Quentin Wilson on principle.

[Talbot]

On 11/16/2020 at 8:03 PM, vulgalour said:

I haven't got an air compressor so I can't do a leakdown test. 

Have you got a spare old scrap spark plug?  Given that I need to bring this wing up to you soon, I could bring a compresser with me and we could do a leakdown test there and then if you like.

[vulgalour]

I might in the spare head, I'm not sure.  I'll try and find out tomorrow, new garage roof goes on then so we'll be emptying some of the garage out for that.

[catsinthewelder]

 

Happy Princess day Angyl

https://www.daysoftheyear.com/days/princess-day/

[Rocket88]

22 hours ago, vulgalour said:

I'm still blaming Quentin Wilson on principle.

He's responsible for most of the ills in the world.................!😡

[Talbot]

On 10/26/2020 at 10:21 PM, Talbot said:

As for the compression pressures.  I'm only commenting that it sounds high, as when you're doing a compression test, you're turning the engine into a compressor.  It draws in atmospheric air at 1bar, and then compresses it to 9 times that pressure (a 9:1 engine), so you'd expect the maximum it could achieve to be 9 bar (132 psi).  The bit about volumetric efficiency is to do with how well the cylinder is filled before it compresses the air in it.  if it's 100% full, that means it's at 1 bar before it starts compressing.  Most engines run at less than 100% volumetric efficiency, hence the cylinder is still at a slight vacuum before the air is compressed, meaning the final figure is a little less than the theoretical maximum.

I absolutely don't doubt your book says 195psi.  I just don't understand how.

Ok, I am a complete idiot.

It took me until now, while pondering this while driving one day to realise the fundamental error I made in the waffle above:  Compression in a car engine is Adiabatic, not Isothermal.  I was putting numbers together based on Isothermal compression.  The compression happens so quickly there is no time for any heat (or at least very little) to be conducted away by cold engine components, meaning I should have been using Adiabatic calculations.

On that basis, 195psi is more than reasonable.  In fact, it indicates that the valve timing isn't closing the inlet valve until quite significantly past BDC (which is generally correct)

So yes.  Ignore my previous bollocks please.

[High Jetter]

Thank you for another schoolday, I'd forgotten about those measurements.