I have a fresh rebuild engine with about 1500 miles. I broke a nylon 7.5 adv timing gear and replaced it with a new gear. I did a compression test and only got 30 pound readings on all the clylinders I have double checked the timing gear settings. I have tried oil in the cylinders and the readings didn't change.
Mine is an older rebuild, and I got between 49 and 52 between all four cylinders....
I have a Z head on a year old rebuild and i get 76# on all four cylinders.
Since you double checked the timing gear marks to ensure they are correct I suspect the issue lies elsewhere. I would buy a 1/2 inch pipe bushing and install an air hose fitting to do a leak down test. Put your bushing / fitting in the hole in place of the plug with the piston at TDC of the compression stroke. Connect the hose from your air compressor and add air at about 80 PSI to the cylinder. Listen to where the air is leaking out. If its coming out the breather, it is getting past the rings. If its coming out the carb or exhaust then the valves are leaking.
I found 80# is way too much, and will blow the cyl down with force if it's just a bit off TDC. For me, 20# was enough in a T engine to detect leaks without risking spinning the engine unexpectedly.
And that was using a differential compression gauge that limits the air flow. It's just not that easy to put a T cyl at exact TDC.
I just checked mine (using the starter) with a good screw-in ratcheting gauge. I got 110, 98, 100, 98 psi in all cylinders. I have a high compression low style head on a '14 runabout. It runs good, and has no problems. If you are getting 30 psi, there is something drastically wrong. Maybe your gauge, maybe your engine? How you turn the engine over will have a BIG effect on your readings. Hand cranking will generally produce lower readings (unless you are a lineman in the NFL!). For example, my car runs in the 80's by hand cranking.
If you put oil in the cylinders and got no change, either the valves are leaking, you have a blown/leaking head gasket or lastly, some broken rings. Check the oil and see if it looks milky and see if the head bolts are tight. If that's good, I would suspect the valves are leaking.
The center third area of those gauges are the most accurate.
If your gauge goes from Zero to 300 psi, change it for a Zero to 100 psi gauge
for greater accuracy.
The formula is 14.5 psi (at sea level) times the Compression Ratio for determining the actual compression reading.
An engine with a standard stock head should read about 58 psi.
Most of those high compression heads seem to have about a 6:1 compression ratio, and that should produce 87 psi on an accurate gauge, plus a little more if the block top has been shaved some.
The last one I tested with a high compression head had 94 psi on all cylinders.
The new Kevin Prus 8:1 head should be around 115 psi.
Since the readings are same for all the cylinders, I would suspect the gauge or the method of turning the engine. You also don't state your altitude. The higher the altitude, the lower the reading will be.
you also stated that you broke a nylon advance timing gear. Is your new gear also advanced? Are the marks in alignment?
Have you started the engine? How does it run? Is it different from before you broke the gear?
Jim, in my correspondence with Kevin Prus he wrote the 8:1 claim came from his measurement of about 100 psi with his head compared to about 50 psi with a regular 4:1 head..
My measurement of actual compression chamber volume in my Prus head showed about 5:1 actual compression ratio. The Tulas T club measured 5.3:1 actual compression ratio in a Z head: http://mtfctulsa.com/Tech/head_design.htm
ouch, should have been "the Tulsa Model T club"
Before you analyze it to death how much effort does it take to "tip it over"?
And does it take about the same amount of effort on all four?
Roger, you didn't note what the actual compression number in psi was that you measured with the Prus head.
There are different methods to express that reading and compression ratio.
The article that you referenced notes that the piston shape also affects the reading.
Some block tops are shaved a significant amount, but evidently your block was stock. Were your pistons stock, as well?
When I talked to Kevin about buying one of those heads last year, while in Vermont and on the MTFCI Summer Tour, he told me that a fixed compression number was normally about 100 psi, but there were so many variables that an exact number could not be stated.
The two biggest variables were the type of piston (especially the piston top) and amount the block top was shaved.
The difference in the cylinder wall and piston diameter also affect the final reading, by allowing more leakage with the higher pressure.
One of our club members demonstrated that a high compression head is an excellent gauge for measuring a tired engine. He got almost five miles with the new head and more power, before all the rods started knocking rather loudly.
The Frontenac R, racing head is advertised at 100 lbs pressure, and has been listed somewhere as considerably less than 8:1, so Prus was way off base. My R head has been shaved so I cannot calculate as it was stock, but I know what it is now.
I'm sorry you got misled, Roger. Anybody who seemingly cannot calculate compression ratio has to be hopelessly lost with the thermodynamics of head alloys and heat transfer rates from the 1700 degree chamber to the 200 degree coolant.