S3 Appia compression test

[John B]

I have just done a cylinder head compression test and don't know what to make of the results.
All 4 cylinders give a reading of around 90 psi which I believe to be quite low - 140 psi should be achieved
with a compression ratio of 8:1 according to the instructions that came with the tester.
I think the Appia engine is rated at 7.8 so I appear to be at least 40 or 50 psi down.

I intended to do a top end overhaul sometime soon but perhaps it needs more than that?

Hopefully someone will say 90 psi is fine, don't worry about it .......  

[simonandjuliet]

When I've got a few minutes I will pop a plug out and test mine

[lancianut666]

seems OK to me I had 0,0,0 and 50psi. problem was the rings sticking in the piston grooves and one broken top ring

[John B]

seems OK to me I had 0,0,0 and 50psi. problem was the rings sticking in the piston grooves and one broken top ring

Based on those results mine is positively brilliant.......

[lancianut666]

My engine man mumbled something about having the block rebored and the pistons on the conrods by Friday we will see..when it is all back together we will see what a new engine compression should be
Clarkey

[nistri]

90 psi is very good on the  Appia and should be uniform on all 4 cylinders, Andrea

[John B]

90 psi is very good on the  Appia and should be uniform on all 4 cylinders, Andrea

That's good news - many thanks for the info Andrea.

[simonandjuliet]

Checked mine yesterday - 105 psi

[John B]

Many thanks Simon.
Did you do much to the engine that would have improved the compression during the recent extensive restoration of your Appia?
I am now wondering if just a top end overhaul would make any significant difference.

Slight change of subject .......
Since buying my Appia I have been using a lead substitute additive in the fuel because the guy I bought it from said he always had, even though he was unsure if it needed it. So if I do remove the head should the valve seats be changed?

[simonandjuliet]

Re engine work, all I did was replace the rings - the bores and pistons had some wear, but within tolerances and I wanted to keep the cost down

The rest of the work was on the bottom end

However, the engine does smoke quite a lot due to valve guide wear, so that is next on my list - I have a new head with new guides and stem seals ready to fit. I did change the valve seats as well so I don't plan to use a lead substitute

I have not been using a substitute in my current engine, because I had always planned to change the head, so it will be interesting to see if there has been any seat damage in the 3,000 kms since it went back together

[Jaydub]

Hi Guys,
If I could add my two pennorth of years of engine building, a compression test alone is not conclusive, it only tells you how efficient the engine is acting as a pump but not what the problem is. You then need to carry out a cylinder leakage test. This will determine where the losses are, inlet or exhaust valves, piston rings or head gasket. Obviously not many people possess the tester, but worth borrowing/ hiring one before stripping the engine. At least you`ll know it`s possibly just in the head and not tear the bottom end apart unnecessarily. Also as you know, a cold engine will give lower readings due to oil drainage from rings. If the leakage test suggests worn rings, a few squirts of oil down the spark plug holes will temporarily seal the rings and a retest will maybe confirm a valve seating problem. I hope I`m not preaching to the converted.

[John B]

Thanks Jaydub, all info gratefully received.
My compression test was carried out on a warm engine and in a strange way I was hoping for No 1 cylinder to show
a lower reading than the other 3. The reason being that I suspected those valve stem guides were worn because of the fairly regular occurrence of a misfire and an oiled up plug.
Perhaps the loss of compression due to worn valve guides is minimal and would not show up with a basic (i.e. cheap)compression tester?

[ben]

I would suggest that valve guide wear leads to oil consumption and perhaps smoke but not loss of compression.

I think 90psi is fine especially if all cylinders are the same. If it was mine I would leave well alone,assuming your oil consumption is not excessive.

In simple terms 110psi is the theoretical max for 7.8 to 1 compression ratio although it will depend on the design valve overlap and the cranking speed I would imagine.

[John B]

I think 90psi is fine especially if all cylinders are the same. If it was mine I would leave well alone,assuming your oil consumption is not excessive.

Thanks Ben
Oil consumption is not at all excessive so for the time being I will monitor the situation and leave things as they are.
 

[lancialulu]

Some other tests you could do is determine if the oil is being burnt on the over run (worn inlet valve guides) or on acceleration (worn pistons/rings).