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I recently bought a 67 427 Vette with the 400HP engine. The motor was recently rebuilt by a previous owner. There is no documentation on the rebuild specs. I did a compression test on all cylinders and got readings between 120-125PSI. How can I calculate the engines current compression ratio assuming the rebuild was to factory specs?
You cant really calculate CR from a compression test. There are other variables that affect compression test pressure. 125 is low. Should be closer to 150 to 170 on that motor.
Robert - Curious what made you decide to do a compression test on your new Corvette? Just wondering what it might show or are there drivability issues going on?
Also, since it looks like you just signed on to the Corvette Forum...Welcome.
Mike T - Prescott AZ
Keep in mind that the carburetor has to be open for the air to pass thru and get good readings. In addition, I believe the manual calls for the engine to be at or near normal operating temperature. A leak down test should give you a more meaningful idea of the engine condition.
Thank you for your feedback! The engine was not at normal operating temp. There is no smoking and the driving seems okay to me. I just recently bought the car. It has been completely restored but I have no documentation about what was done to the engine internally, so I thought I would have the compression checked. All the numbers are closely grouped, but as you can see others have replied to my post that the compression pressure should be between 150-175PSI. I'll have it checked again at operating temp and inquire about a leak down test.
If it is running good and not using oil, I don't know that I would worry about it. Did you put a little oil in the cylinders to determine if the rings were sealing? If it is a new rebuild, it could need some miles on it to seat the rings, especially cast iron rings.
I did a compression check on an engine and it came out 190 - 200 psi which I couldn't believe, but then found I had a bad gauge. A leak down test will give you an idea if the compression is leaking by the rings or valves. Compression is affected by various factors as others have stated.
To have any meaningful results; warm up the engine, wire the carb wide open, remove all spark plugs, hook up a battery charger. Crank engine over for a count of 3 sec. Let battery recover on charger while changing tester to next cylinder.
Last edited by ghostrider20; Jun 17, 2021 at 04:28 AM.
To have any meaningful results; warm up the engine, wire the carb wide open, remove all spark plugs, hook up a battery charger. Crank engine over for a count of 3 sec. Let battery recover on charger while changing tester to next cylinder.
I agree with the the process above except to the 3 second crank. That may not be long enough to get get maximum compression. Watch the gauge as you crank and you will see the compression build. Crank it until it no longer goes up. I do 8 compression strokes for each cylinder. You can hear them while it cranks.
Keep in mind that the carburetor has to be open for the air to pass thru and get good readings. In addition, I believe the manual calls for the engine to be at or near normal operating temperature. A leak down test should give you a more meaningful idea of the engine condition.
Correct. The factory service manuals specify that a reasonably healthy engine should show no more than 20 psi spread between the highest and lowest cylinder. So consistency is actually more important than the absolute numbers, which can vary widely considerably from engine to engine.
Given that the OP did not run the test properly - engine at operating temperature and the throttle blocked wide open the data is worthless, and the test should be rerun properly.
Keep in mind that the carburetor has to be open for the air to pass thru and get good readings. In addition, I believe the manual calls for the engine to be at or near normal operating temperature. A leak down test should give you a more meaningful idea of the engine condition.
Don't get hung up on your results. If it pulls hard, runs well, does not smoke or dirty its oil quickly close the hood and drive your car.
A boroscope inserted into the spark plug holes will show you what type and shape piston you have. This will also help to estimate compression ratio. Low top pistons will likely be around 9-10 CR. Domed pistons should be 10.5-11 CR.
To expand on what Larry said, if someone used cast replacement pistons they have a very small dome which would result in low compression. You could easily see this with a scope. The number you are asking about to calculate compression is the number shown on the compression tester/14.7=your compression. But that isn't entirely or really even close. Because you are measuring dynamic compression and not static it will be lower than actual. Actual compression is calculated by the total volume of the cylinder divided by the amount of volume with the piston at tdc. When checking with a compression tester you are measuring dynamic which is measuring after the intake valve closes. The piston isn't at bottom dead center at this point so effectively you are measuring a cylinder with a shorter stroke. If everything in the engine is in good condition 125/14.7=8.5:1. That would point towards the very small dome cast pistons or an open chamber head. This is not exact you have to know what's inside.
In my experience testing compression hot or cold, throttle plates open or closed does not make a big difference. Less than 15 psi per cylinder.
Many times I check compression cold and with the plates closed just looking for a problem or to judge engine health, I know it is not a perfect read but it is close and reliable.
That said doing another test on this engine with it warm and plates open he is not going to see 150-175 compression, maybe 130-135 is my guess.
Like others already stated you have to crank it over 6-8 revolutions to see max compression.
If a cam with lots of overlap was installed at rebuild that could bleed a lot of compression off. A large cam would be evident by a lumpy idle.
Many of these engines were rebuilt with lower compression so the owner could run pump gas, that would not be unusual at all.
My scan tool can do a compression exam now. Does not give numbers but reads the amp draw on the starter as you crank the engine and reports back if all 8 have the same amp draw. Very quick and clean test but it can detect a problem cylinder.
In my experience testing compression hot or cold, throttle plates open or closed does not make a big difference. Less than 15 psi per cylinder.
Many times I check compression cold and with the plates closed just looking for a problem or to judge engine health, I know it is not a perfect read but it is close and reliable.
That said doing another test on this engine with it warm and plates open he is not going to see 150-175 compression, maybe 130-135 is my guess.
Like others already stated you have to crank it over 6-8 revolutions to see max compression.
If a cam with lots of overlap was installed at rebuild that could bleed a lot of compression off. A large cam would be evident by a lumpy idle.
Many of these engines were rebuilt with lower compression so the owner could run pump gas, that would not be unusual at all.
My scan tool can do a compression exam now. Does not give numbers but reads the amp draw on the starter as you crank the engine and reports back if all 8 have the same amp draw. Very quick and clean test but it can detect a problem cylinder.
If it runs good drive it.
I agree completely.
Determining engine specs after the fact is somewhat pointless and the point of my earlier remark. It is one thing to want to understand engine health and another to stand in for the machine shop report and invoice with boroscopes and hot/cold open/shut compression tests. The best determinant of engine health is performance.
Dan
Last edited by dplotkin; Jun 17, 2021 at 02:27 PM.
In my experience testing compression hot or cold, throttle plates open or closed does not make a big difference. Less than 15 psi per cylinder.
If a cam with lots of overlap was installed at rebuild that could bleed a lot of compression off. A large cam would be evident by a lumpy idle.
Many of these engines were rebuilt with lower compression so the owner could run pump gas, that would not be unusual at all.
It's not overlap that bleeds off compression, it's a late closing inlet valve. This is common misunderstanding, maybe because late closing inlet valves are usually associated with long duration and high overlap in vintage designs. Modern engines typically have much shorter duration and overlap, but the inlet POMLs are much later than vintage designs, which means a relatively late closing valve. Examples of modern and vintage valve timing designs are in my "Tale or Two Camshafts" article.
At cranking speed (and low engine running speed) a late closing inlet valve causes flow to reverse and push the inlet charge back into the manifold once the piston passes BDC and heads back up. This is called reversion, so all other things equal two identical engines with different inlet closing events will show different cranking compression at the same cranking RPM and the one with the later closing inlet valve will measure lower cranking compression pressure.
At high engine speeds a late closing inlet valve improves VE torque and power. The inlet charge column may be moving at up to 500 ft/sec, so even with the relative small mass of mixture there's enough kinetic energy to keep it flowing into the cylinder even after the piston passes BDC and heads back up. In fact, of the four major valve events IVC has the greatest impact on power. Up to a point the later the inlet valve closes the more the torque curve will move up the rev scale meaning more top end power, but on the downside, low end power is reduced. There's no free lunch.
Compression calculation
