Technical Way to calculate right compression range for an engine?

[Fiatfarmer]

Goodevening all,

I want to do a compression test on my 899 Sei to find out about how well she’s keeping up, but I don’t know what pressure is right for this specific engine, is there a way to calculate this? It’s the 1170A1.046

Thanks in advance!

 

[vexorg]

I find the calculated compression and measured values are often very different, for some reason (most likely heat) the measured value on a compression tests is far higher than expected.

However, to calculate the compression you need to know the cylinder volume when the piston is a TDC, then divide the displacement of that cylinder by the TDC volume. Usually awkward as the head is a weird shape and the only true measurement is to take it off and fill with a liquid (petrol is best). Then look at the piston top shape and see how far below the deck it sits since you need to add that to the head volume, plus gasket thickness.

Or just look up the manual.

 

[Fiatfarmer]

I find the calculated compression and measured values are often very different, for some reason (most likely heat) the measured value on a compression tests is far higher than expected.

However, to calculate the compression you need to know the cylinder volume when the piston is a TDC, then divide the displacement of that cylinder by the TDC volume. Usually awkward as the head is a weird shape and the only true measurement is to take it off and fill with a liquid (petrol is best). Then look at the piston top shape and see how far below the deck it sits since you need to add that to the head volume, plus gasket thickness.

Or just look up the manual.

All documentation I can find on the internet says the 1170a1.046 has a compression ratio of 8.8:1. But how do I know if my engine is in good condition when doing a compression test? Between which values should the compression be?

Thanks!

 

[dac69er]

You are probably best off just posting your results and see what other members think.

I would say as a rule of thumb, you want to be in the high 100s psi and all within 5 psi of each other.

Thanks

 

[vexorg]

At that compression, expect something around 12-13 bar on a tester. Usually it better to see even results than an absolute value across the cylinders (unless they are all too low!).

 

[Fiatfarmer]

Goodevening all,

First of all, sorry for being this late too reply on this tread! second of all, before I tell the compression values of my Seis engine, I need to tell that it’s still quite snappy on the pedal and it accelerates just as I would expect of an 800kg 39hp car. All the measurements are done with a closed throttle valve.

So… here they come: cylinder 1 gives me a compression of 80 psi, cylinder 2 gives me a compression of 100 psi, cylinder 3 gives me a compression of 95 psi and cylinder 4 also gives me a compression of 100 psi. I started counting cylinders from the side where crankshaft pulley is located.

 

[dac69er]

Cylinder 1 I would be a bit concerned about.

Did you do a dry then wet test to see if that made any difference?

Do the 899 engines have adjustable valve clearances?

 

[Fiatfarmer]

Cylinder 1 I would be a bit concerned about.

Did you do a dry then wet test to see if that made any difference?

Do the 899 engines have adjustable valve clearances?

Goodevening,

What kind of test??? No, 899 Seis got hydraulic tappets.

 

[dac69er]

A dry compression test is as normal like you did it (you should really have the throttle fully open though).

A wet compression test you put a bit of engine oil down the spark plug holes and try the test again.

If the readings improve, you most likely have ring/cylinder wear. If they stay pretty much the same, then it's likely a valve related issue.

I assume you have taken all the spark plugs out at the same time as you are cranking the car over to get your readings?

You could have a seized hydraulic tappet holding a valve open a bit too long???

 

[Fiatfarmer]

A dry compression test is as normal like you did it (you should really have the throttle fully open though).

A wet compression test you put a bit of engine oil down the spark plug holes and try the test again.

If the readings improve, you most likely have ring/cylinder wear. If they stay pretty much the same, then it's likely a valve related issue.

I assume you have taken all the spark plugs out at the same time as you are cranking the car over to get your readings?

You could have a seized hydraulic tappet holding a valve open a bit too long???

Ring/cylinder wear? Is that possible at not even 130.000km? And yeah, all the plugs were out. A seized tappet could be a thing too, since the oil hasn’t been changed for an unknown time, but now I think about it, it could be cylinder wear too, when I changed the spark plugs at 126.900 km I believe it was still running on the original plugs, it were Fiat Lancia plugs, so IF they were replaced, they were replaced by originals. But one of those old plugs had a way shorter electrode then the other three, so it could be that it broke of…

 

[vexorg]

100psi is only 7 bar, that's low, did you turn it over once or many times?
Was the throttle fully open?

 

[Fiatfarmer]

100psi is only 7 bar, that's low, did you turn it over once or many times?
Was the throttle fully open?

Goodevening all,

First of all, sorry for being this late too reply on this tread! second of all, before I tell the compression values of my Seis engine, I need to tell that it’s still quite snappy on the pedal and it accelerates just as I would expect of an 800kg 39hp car. All the measurements are done with a closed throttle valve.

So… here they come: cylinder 1 gives me a compression of 80 psi, cylinder 2 gives me a compression of 100 psi, cylinder 3 gives me a compression of 95 psi and cylinder 4 also gives me a compression of 100 psi. I started counting cylinders from the side where crankshaft pulley is located.

I know, but it’s an old car… can’t it be that these old engines have a lower compression? Like I said, it pulls and accelerates like I’d expect from a 805kg 39hp car. Your second question, second paragraph, last sentence… no, throttle was closed.

Kind regards.

 

[Dave lee]

See this old thread

https://www.fiatforum.com/threads/compression-test-899-cinq.149903/

 

[David C]

Run the compression test again with a wide open throttle.

 

[Fiatfarmer]

Goodday @Dave lee and @David C

I have done some research about converting the compression ratio to a number of psi, so I can get a good idea about what my engine should give. And do correct me if I’m wrong, since there’s no use in learning myself something new which is wrong! So, the compression ratio of my car is 8.8:1 and I’ve checked the internet and found a formula which goes (atmospheric pressure*first number of CR)/second number of CR. CR=compression ratio) So this would mean that it would be (14.7psi*8.8)/1, which would result in approximately 130 psi per cylinder? The one thing I can’t add up, is the high compression from the old tread above, this would mean that my car is not even half that! But I personally think it’s quite snappy?

Kind regards!

 

[dac69er]

Calculated and actual compression readings are often different. The figure you calculated is too low. Your should normally get in the region of 150psi +
The car won't be running and performing at its best with the readings you have.

Do the test again with the throttle fully open. Then try a wet test and go from there.

At the end of the day, if the engine is worn out you have 2 choices, rebuild or drive it until it's undriveable.
If you are happy with how it is, then just keep driving it. It may slowly get worse, it may even improve if the rings are a bit stuck.

 

[Fiatfarmer]

Goodevening dac69er,

I’ll try again with a warm engine with throttle opened soon, I’ll do a wet test to. If my engine is up for a rebuild, what’s it going too cost me when I’ll do it myself? So parts only. And I saw once on a video that the walls of the combustion chambers were being pressed out, is this possible on every engine?

And now we’re talking about rebuilding stuff anyway, what’s a transmission rebuild going to cost me in parts only?

Kind regards.

 

[vexorg]

It depends what the rebuild needs, your cylinder liners may be in excellent condition. Usually if they are ok, then it'll just need re-honing and new rings.
Most of the time you'll not know what it needs until you get in there, could be cracks between the cylinder walls for all you know, then it;s only fit for the scrap man.

 

[Fiatfarmer]

It depends what the rebuild needs, your cylinder liners may be in excellent condition. Usually if they are ok, then it'll just need re-honing and new rings.
Most of the time you'll not know what it needs until you get in there, could be cracks between the cylinder walls for all you know, then it;s only fit for the scrap man.

Thanks for your reply @vexorg ,

Alright, so that’s step 2, step one is still performing a proper compression test, like dac69er said, it should be least 150psi, so if my engine gives about 95 to 100psi on a cold engine with a closed throttle (let’s keep cyl 1 to the side, since this one is obviously bad), could cyl 2 to 4 give approximately 150psi if the engine is warm and throttle is opened?

Kind regards.

 

[vexorg]

Why not just repeat it hot and with open throttle and see what it is. An old engine, 8.8:1, i'd say you'll get around 130-140 ish if it's all good.