New Engine High Compression Readings?

[Jeff B.]

In an earlier post I discussed my recently rebuilt 1964 327-300HP and the problems I was having with blue smoke in the first hour of operation. During the diagnostics of this problem I did a compression and leak down test. The compression was 195 in all but one cylinder that was 198. The leak down was between 16 and 20% and sounded like the leak was in the rings. My conclusion is that the rings did not seat yet. However, the 195 compression readings seems high. I have the stock replacement 10.5:1 pistons but the heads were surfaced. The shop manual shows 160 lbs. Anyone else seen high readings like this? I can find no way to reverse calculate the effective compression ratio based on the compression readings but I have seen rules of thumb saying 16-20 times the ratio = compression lbs. 10.5 x 19 = 199.5 so that would make some sense.

[Timothy B.]

Re: New Engine High Compression Readings?

Jeff, It all sounds good to me, I think that engine needs to be run and don't drive like grandma!!

[Bill B.]

Re: New Engine High Compression Readings?

Jeff, It all sounds good to me, I think that engine needs to be run and don't drive like grandma!!

Yes I agree, vowy, vowy important...

Bill


1961 Black/Silver/Red Int. 283/315 FI
power windows. Frame off 65% completed.
1989 White/hard top/Grey Int. daily driver 167,000 miles

"The problem with Liberalism is that eventually you run out of other people's money" Margaret Thatcher

[Jack H.]

Re: New Engine High Compression Readings?

The calculation is pretty straight forward...but there are quite a few variables! A common rule of thumb for barometric pressure is to presume it's 14.7 psi at sea level. Now, that figure WILL change with altitude, temperature, and humidity. You're essentially 'weighing' a 1 square inch column of air from your starting altitude to outer space...

So, for grins, presume you're dead nuts on the rule of thumbs' average. When the intake valve opens and air/fuel flows from the intake into the cylinder chamber, and absent any artificial aspiration, it's driven by atmospheric pressure. So, you start with 14.7 psi, then close the valves and pump the cylinder up via piston motion.

Now, the rote assumptions given are a 'lie' because you're NOT dealing with pure air rather you've got an air/fuel mixture to work with AND you're NOT at ambient temperature because the air/fuel mixuture has been heated by sitting in the intake AND there's a 'ram' effect of air/fuel in motion to consider as well. But, set those fine points aside for now...

14.7 psi x 10.5 CR = 154.35 psi. So, you see the logic involved with the 160 psi 'thumbnail' that's cited in the books... It's NOT rocket science! But, the other variables (exact altitude, ambient temperature, humidity, air vs. air/fuel, ram effect, Etc.) DO have their effect(s) on the precise pressure numbers you'll measure.

The big picture is: (1) are you in the 'ball park' with the readings you get? and (2) are the numbers consistent, cylinder to cylinder, across the banks?

[Duke W.]

Re: New Engine High Compression Readings?

The 195 psi cranking compression pressure is high, but the variation is virtually nil. The shop manual allowable variation is 20 psi, and this is what's really important, not the actual compression pressure numbers, however, the leak down numbers are huge and indicate a serious problem.

Depending on deck height and head gasket thickness, your likely actual CR is probably in the range of 9.5-10:1. The advertised CR of these vintage engines is as realistic as the highly inflated gross horsepower numbers.

There is not a direct correlation between cranking compression pressure and CR (and the adiabatic compression formula for an ideal gas is invalid) because the inlet valve closes after BDC, which causes reversion at cranking speed. The specified cranking compression pressure of SHP/FI engines is lower even though they they have higher advertised (and actual) CR. The reason is the very late closing inlet valve of the SHP/FI cams relative to the base engine cams, which allows even greater reversion at cranking speed.

A healthy engine should not show more than about 5 percent leakage on any cylinder and the variation should be in a narrow range across all cylinders. A cylinder with high compression pressure should show low leakdown.

The bottom line is that the test data - high compression pressure, high leakdown numbers - don't make any sense, which makes the test equipment and/or test procedures suspect. For example, cranking pressure and leakdown tests should be done with the engine at operating temperature, the throttle blocked wide open, and all spark plugs removed. Cranking speed should be about 200-300 RPM, and it will take 4-5 cycles for maximum readings on the gage.

Leak down numbers should be specified by stating the input pressure (usually 100 psi) and the pressure drop to the downstream side. For example, if input pressure is 100 psi and the downstream gage reads 95 the leakdown is 5 psi. This can also be expressed as "5 percent", but only if the leakdown tester has a "standard" size restrictor, and I'm not sure if all do.

Duke

[Jeff B.]

Re: New Engine High Compression Readings?

Here is a bit more background. The motor is in the chassis but the body is not on so I really can't get a load on the motor. I had initially installed 5-30 oil and run the motor about 30 minutes total, 20 of them at 2500 RPM. These readings were taken at that time because I was getting a quite a bit of blue smoke from both sides when you hit the throttle even after the initial break-in. I changed the itake manifold gasket but that did not help. Because of the high leak down numbers, I was thinking the rings had not seated yet. So yesterday I pulled out the 5-30 oil and put in straight 30 and ran the motor another 30 minutes. By the end of this run 90% of the smoke had cleared up and I was getting just a puff of blue from the right bank when opening the throttle - and not every time. I let it cool down and repeated the leak down test and got numbers between 7% and 11% with the highest being #6, which would make sense for the remaining smoke on the right side. At this point the compression test is showing 200 psi. What are your thoughts.

[Michael H.]

Re: New Engine High Compression Readings?

Here is a bit more background. The motor is in the chassis but the body is not on so I really can't get a load on the motor. I had initially installed 5-30 oil and run the motor about 30 minutes total, 20 of them at 2500 RPM. These readings were taken at that time because I was getting a quite a bit of blue smoke from both sides when you hit the throttle even after the initial break-in. I changed the itake manifold gasket but that did not help. Because of the high leak down numbers, I was thinking the rings had not seated yet. So yesterday I pulled out the 5-30 oil and put in straight 30 and ran the motor another 30 minutes. By the end of this run 90% of the smoke had cleared up and I was getting just a puff of blue from the right bank when opening the throttle - and not every time. I let it cool down and repeated the leak down test and got numbers between 7% and 11% with the highest being #6, which would make sense for the remaining smoke on the right side. At this point the compression test is showing 200 psi. What are your thoughts.

If the issue is with piston rings/cyl finish, it may be somewhat curing itself but I don't think it will ever be a happy motor. If you have that kind of (air/oil) leakage past piston rings, something is wrong and it isn't ever going to be 100% better.

A properly rebuilt engine shouldn't show any blue smoke. As I've mentioned in a previous post, piston ring break in is a term from 40 years ago.
If cyl's are PROPERLY honed using deck plates and an automatic feed precision hone, there is no break in.

Engine builders that don't have the expensive Sunnen CK-10 honing equipment will always tell you that it's not necessary.
If it wasn't necessary, the really good engine builders certainly wouldn't spend the $15,000 for one. (or what ever they cost today)

[Jeff B.]

Re: New Engine High Compression Readings?

Well Mr. Hansen, I agree with you. I think the motor is coming back out next week and this time I will do the assembly work myself and supervise the machine work closely. Easier now then when the body is back on!