What Is Normal Compression for 1.7, Compression Ratio vs. Tester PSI Options

[7TPorsh]

Years back when I bought my car it was presented to me (the newbie 914 guy) that the car had a 2.0 rebuilt motor in it. He had a receipt and everything. I thought cool, a black, running 914 with a 2 liter.

A few years later after becoming "informed" here at the world I see that I have a 1.7 block from 1971 (per engine number; probably not the stock engine it came with) and i have the angled plug heads so assuming they are also 1.7. So viola, 1.7

Car has dual Weber 40IDFs and also assume I have a cam to match. Car runs good and gutsy but i don't have any comparison. I'm trying figure out if my engine is old and tired or is it healthy and it's maybe not tuned perfect.

I read about compression ratio here and see numbers like 165, 180. i did a test on my car and it's an even 106 on all 4. I don't see any burning oil.

So I read that the 1.7 was a high compression engine....what does that mean? How does compression ratio relate to the compression tester numbers of 106. ex: 8:1 CR vs. 106psi

[VaccaRabite]

106 means your compression is fine. The engine is not fresh and new, but its got a bunch of miles left before it needs overhauling.

Also, compression ratio is different from cylinder compression. One measures how much the air get compressed in PSI, and the other is a ratio of what the air is before compression and what it is after assuming zero loss.

Also, basic tuning may raise your number a bit.
Are your valves right? Was the engine warm? Was the throttle wide open when you did the test?

Zach

[Mark Henry]

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Compression tester and compression ratio have no relation to each other.

If you have 106 psi and it's even across the cylinders I'd say it's a well loved engine but good numbers.
If you had 106 on three but one cylinder is 85psi then you have an issue.

If it's running good, no smoke I'd keep it serviced and drive it. The 1.7 was never a powerhouse, if you can go up hills without losing speed (gear change OK) it's doing great.
1.7's do have a tendency to drop an exhaust valve/seat, but this is often due to overheating, clogged fins, mouse nests, etc.

[914_teener]

I don.t agree with your assesment that it is a 1.7

You can build a 2.0 from a 1.7 case.

Measure the cylinders. If they measure 94 mm them you have a 1911 or a 2.0 depending on the stroke.

You can also use 1.7 heads if you fly cut them to fit the bigger jugs so check the numbers on the heads.

You can.t tell from just the numbers on the case unless you left out other info on your first post

[bretth]

Don't forget that a 1.7 case can be opened up to 2 liters and the 1.7 or 1.8 heads can be machined and retained like this too. So it my still be a 2 liter. Also your compression test may not be accurate and you may get even higher compression numbers if you remove all of the spark plugs and then block the throttle plates completely open (more air flow). Give it a shot a bet you get at least another 20 psi if not more. Also make sure the engine rotates at least 4 or 5 times as the compression has to build over a few strokes to it's maximum.

[VaccaRabite]

Yup, these are good points. My 2056 was built on a 1.7 block and 1.7 heads.

BUT!
If the engine runs, its fun to drive, and isn't giving you issues DON'T DROP IT and start taking measurements. Drive it. Drive it like you stole it. Enjoy your car, and worry about taking measurements a few summers from now when its time to overhaul it.

Zach

[Mark Henry]

Don't forget that a 1.7 case can be opened up to 2 liters and the 1.7 or 1.8 heads can be machined and retained like this too. So it my still be a 2 liter. Also your compression test may not be accurate and you may get even higher compression numbers if you remove all of the spark plugs and then block the throttle plates completely open (more air flow). Give it a shot a bet you get at least another 20 psi if not more. Also make sure the engine rotates at least 4 or 5 times as the compression has to build over a few strokes to it's maximum.

You don't have to machine the case, just the 1.7 heads. Check the head numbers you may have 1.8 heads.

You're suppose to do a compression test with plugs out and TB open, you're also suppose to do it hot.

[brant]

also probably not related to this poster's question...
but the compression check numbers can vary also due to the altitude (air density) of the location.

I'm assuming your at or near sea level, so this may make no difference in your example. But high altitude compression tests usually result with slightly lower numbers too.

[Dave_Darling]

I should add that numbers much below 100 PSI (engine warm, battery charged, plugs out) may well indicate a very tired engine, even if the numbers are even across the cylinders.

--DD

[porschetub]

Dave makes a good point,its not so much total max more how close they are ,many get all concerned a about 5% but IMO up to 10%+ is still ok,I always do the valve clearance check first and make sure the engine is in a reasonable state of tune then do the compression test a few times over...put your numbers together and go from there.
And there the old trick of putting engine oil down the plug holes and doing a final test,to see if the compression moves up,never done it myself but tends to made sense.

[VaccaRabite]

I would not assume your car has a carb cam. Carbs are easy to add, cams are not. The only way to change the cam is to split the case. Most people who replaced their FI with carbs left the stock cam. Even if the PO said the cam was a carb cam - assume it probably isn't unless you have his build sheet or the receipt.

So, again, don't go get curious and look. Drive the car and enjoy it. In a few summers when its time to rebuild the engine, at that point investigate your cam.

Zach

[Kansas 914]

I would not assume your car has a carb cam. Carbs are easy to add, cams are not.
Zach

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[914_teener]

I would not assume your car has a carb cam. Carbs are easy to add, cams are not. The only way to change the cam is to split the case. Most people who replaced their FI with carbs left the stock cam. Even if the PO said the cam was a carb cam - assume it probably isn't unless you have his build sheet or the receipt.

So, again, don't go get curious and look. Drive the car and enjoy it. In a few summers when its time to rebuild the engine, at that point investigate your cam.

Zach

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.....and or learn about the stock FI system or aftermarket .....while you are driving and having fun and change it over later when you have nothing better to do.