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Discussion Starter #1
I have a mystery 2.0L in my '79 Spider. I say that because the car has had several previous owners and no records of what they might have done to it. The woman that I bought the car from told me that the guy that she bought the car from put several thousand dollars into the engine. He wanted an Alfa that would be competitive against Porsches. Not sure if he was racing against 914's or what. He sold the car and bought a Porsche.

I d know that the cams are stock but both have been advanced.

I did a wet compression test, all plugs out, and throttle wide open this morning and came up with these numbers:

Cyl 1 240

Cyl 2. 230

Cyl 3. 250

Cyl 4. 230

Repeated the test with a second gauge and got the same results. I'm going to do a leak down test this week. Any idea what could have been done to this engine to achieve this high compression?

Thx,

Drew
 

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I used to wonder about this too. Atmospheric pressure is about 15 psi. At 9:1 compression 1/9 the volume would give 9 times the pressure or 135 psi. Where does the extra pressure come from? Adiabatic heating. Adiabatic means no heat lost to the surroundings. In a very short time the work done compressing the gas in the cylinder heats the gas and this heat makes the pressure go up even more. If the gas were compressed very slowly then the heat generated would be absorbed into the cylinder walls and the pressure would be near the expected 135 psi.

Two interesting examples of adiabatic heating are Diesel engines where the adiabatic heating is enough to ignite the fuel. Another is Chinook winds where in the Midwest winds blowing east coming to lower altitude from the Rocky Mountains can be very warm. The air comes from high altitude (low pressure) to low altitude (high pressure). The compression of the air by higher atmospheric pressure heats the air. Compressed air exiting the cylinder of an air compressor is very hot because of adiabatic heating.

In an older engine leaks around valves and rings might reduce the pressure a little bit. A performance camshaft would also reduce the compression at cranking speeds.

The opposite, adiabatic cooling, takes place when a gas expands quickly, like releasing air from an inflated tire.

230 psi does seem high though. The engine probably is in good tight condition, the starter motor spins it fast, and it doesn't have a racing cam. Maybe an experienced mechanic who has done many compression tests will chime in with typical values.
 

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I was thinking that it must have been modified to run a much higher compression, but then the standard camshafts don't really make sense. Personally, I'd just be happy, or maybe your gauge reads high (test it on another engine, maybe).

But sounds like a good engine :)
Pete
 

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That sounds high, but Im no expert.

If you can get a borescope down through the spark plug hole it's relatively obvious if it has high compression pistons.

Does it have carbs? Or Spica?

It's possible it had better cams and someone removed them.

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Discussion Starter #5
That sounds high, but Im no expert.

If you can get a borescope down through the spark plug hole it's relatively obvious if it has high compression pistons.

Does it have carbs? Or Spica?

It's possible it had better cams and someone removed them.

Sent from my ONEPLUS A3000 using Tapatalk

Thanks folks. It's Spica. Wes rebuilt the pump a few years ago. My former mechanic could never get the car to run right. Just told me to drive it and enjoy it. Problem is that it is not enjoyable to drive. Idles at 2500. I don't know if the cams were swapped put it is possible.

Drew
 

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Thanks folks. It's Spica. Wes rebuilt the pump a few years ago. My former mechanic could never get the car to run right. Just told me to drive it and enjoy it. Problem is that it is not enjoyable to drive. Idles at 2500. I don't know if the cams were swapped put it is possible.



Drew
What ignition does it have? Marelli plex by any chance?

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The Marelli plex has a really short advance curve. So to get full advance at 4000 RPM the distributor is too far advanced at idle, and it will never idle properly. I ran into that problem on a Spider and finally ditched it for a stock distributor with a pertronix ignitor. That worked much better.

Swapping out your distributor for a stock unit or 123 would probably help. I agree having a car that doesn't idle properly quickly kills the joy

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Discussion Starter #9
The Marelli plex has a really short advance curve. So to get full advance at 4000 RPM the distributor is too far advanced at idle, and it will never idle ptroperly. I ran into that problem on a Spider and finally ditched it for a stock distributor with a pertronix ignitor. That worked much better.

Swapping out your distributor for a stock unit or 123 would probably help. I agree having a car that doesn't idle properly quickly kills the joy

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Thanks. Going to a 123 has always been on my plan of intent. I have a scope and will try to get it into cylinder 1 tomorrow. Will try to take some photos from the screen and post them.

Drew
 

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I've never seen compression numbers that high, but there's no reason why they aren't obtainable. It's simply a matter of buying ultra high compression pistons and shaving the head until you get the compression ratio you want. I'm surprised that the engine doesn't detonate (ping).
You should talk to Wes about the engine see if he remembers it.
I know that 1960's era drag racers used to run their big V8s at 15 to 1 compression ratio using 100LL (100 octane leaded gas)
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The Marelli plex has a really short advance curve. So to get full advance at 4000 RPM the distributor is too far advanced at idle, and it will never idle properly. I ran into that problem on a Spider and finally ditched it for a stock distributor with a pertronix ignitor. That worked much better.

Are you talking about distributor degrees or crankshaft degrees? I've been running 'Plex ignitions for years using a modified Plex distributor that gets it's max advance at about 3k. I have the max advance set at 36 crankshaft degrees (I never use the static timing marks, BTW). My much modified 2 liter in my Super idles smoothly at 700 rpm and runs equally as smoothly all the way to my imposed 7k redline. In multiple decades of Marelli Plex owning and driving, I've never encountered the problem you describe.

As far as compression ratio is concerned, the most important criterion is whether or not the engine can run happily on pump gas. If your engine runs happily, the actual compression ratio sorta becomes academic. If your engine was "built" at some point, my guess is that you have a combination of high compression pistons and a cut cylinder head. If you want to reduce your compression ratio you can fit a thicker copper-clad head gasket to get a few tenths lower compression. Or you can have an even thicker copper head gasket made.
 

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“I did a wet compression test”

Normally, this means you poured a small amount of light oil into each cylinder to seal the rings, then did your test to confirm you had no leaking valves.

If you pour a lot of oil into each cylinder you can artificially increase the CR to the numbers you quote. It’s a bad idea for a number of reasons.
 

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I would redo the test, this time dry, wide open throttle, all plugs removed and hot engine

(yikes!, yet another new BB to get used to.....)
 

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Timing of the intake valve events will influence static compression. Closing the valve earlier results in higher pressure. So a short duration cam running with an advanced LC will make the highest pressure.
 

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Discussion Starter #15
Thanks all. unfortunately my borescope camera is too big to get thru the spark plug holes. I was able to use it to look at the tops of the pistons. It looks like there are 3 numbers on each. Can't tell if they are 7's or 2's or some combination of both and then what looks like a /.

I use 93 octane gas and have never had a problem with pinging. When I did the wet test I only sprayed a small amount oil small amount oil into each cylinder and I will try the test again dry this time. I'm also going to do a leak down test this week. I'll post my results.

Drew
 

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Hi there, the numbers likely line up with borgo 10.4 pistons, as others have said you likely have a shaved head as well. Redoing your compression test dry would be a good move, it will tell you if you need to be concerned about having to run super high octane gas. On the bright side having high compression already makes getting the engine to run correctly easier. Ignition timing set and spica reset to nominal throughout would probably serve you well.

Cheers,
 

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numbers you are seeing are likely 7222/ C (or 7222/D) and below that would be 84.0

so yes seem to be 10.4:1 pistons.
 

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Discussion Starter #18
Unfortunately it's too cold to try to work in my unheated garage today. Temp is down in the 20's with winds out of the north at 10 mph. They're saying that the temps will be be back up to the 50's by Friday. I'll do another compression test and a leak down test then and post the numbers. I just want to make sure that the engine is in good shape before taking on the Spica issue. The spark plugs do indicate that the engine is running rich.

- Drew
 

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Discussion Starter #19
O.K. It warmed up today so I was able to work on my spider. I did the compression test. This time dry, the engine up to temp, plugs out, and throttle wide open.

This is what I got this time:

Cyl 1. 200

Cyl 2. 200

Cyl 3. 200

Cyl 4. 200

Did the test twice and got the same numbers both times. The consistency is incredible so I'm not going to bother doing a leak down test right now. The plugs confirm that the Spica is set too rich. I have found a mechanic that knows how to properly tune a pump that Wes has rebuilt. The car has summer tires on it so I'll have wait until spring to have the car flatbedded up to him.

- Drew
 

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sounds, to me, absolutely perfect:)
 
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