The Ford Barn

rotorwrench · 11-11-2014, 07:29 PM

We ues a differential pressure test set up to check compressions on aircraft engines. It uses a test rig that has two pressure gauges on a little manifold with a calibrated orifice between the two of them and a pressure regulator to bring the psi up in the cylinder. You have to have the piston for each cylinder up on TDC compression stroke to get a good reading. A perfect cylinder would be 80 psi test pressure with 80 psi applied to the cylinder but that is rare. Anything above 65/80 is acceptable with no more than a 15 psi split. It's difficult to get them to stay on TDC sometimes and a person keeps having to turn it back to TDC if it slips one way or the other. Once you get it in the sweet spot, it will stay. Leakage by the rings will exit the crank case vent pipe. Leakage by the valves can be heard either in the exhaust manifold or in the intake manifold depending on which one is leaking. A good test is if all are above 70/80.

JSeery · 11-11-2014, 07:39 PM

Quote:

Originally Posted by rotorwrench

We ues a differential pressure test set up to check compressions on aircraft engines. It uses a test rig that has two pressure gauges on a little manifold with a calibrated orifice between the two of them and a pressure regulator to bring the psi up in the cylinder. You have to have the piston for each cylinder up on TDC compression stroke to get a good reading. A perfect cylinder would be 80 psi test pressure with 80 psi applied to the cylinder but that is rare. Anything above 65/80 is acceptable with no more than a 15 psi split. It's difficult to get them to stay on TDC sometimes and a person keeps having to turn it back to TDC if it slips one way or the other. Once you get it in the sweet spot, it will stay. Leakage by the rings will exit the crank case vent pipe. Leakage by the valves can be heard either in the exhaust manifold or in the intake manifold depending on which one is leaking. A good test is if all are above 70/80.

That sounds like the same set-up I use on car engines. Only difference is I set the pressure at 100 (but can use a lower pressure) and check the differential between the gauges.

Walt Dupont--Me. · 11-11-2014, 07:44 PM

billfish. the air pressure check is a good check for finding a compression leak, your 8BA engine is an easy one to find TDC, find the spark plug wire in the cap that goes to the bad cyl, make a chalk mark on the side of the dist wear that wire is, remove the cap, crank the engine over until the rotor point to that chalk mark, now the piston is on TDC, It's nice to take an old spark plug, drive the poscelin out of the plug, then make a thread of brass a fitting on top of the plug for an air hose to clamp onto, It's nice to have regulator in the air hose so you can start with low pressure. If you put 125# air into the cyl your probaley blow the piston down the hole and then the exh valve open, so start with low pressure and work up, take the air filter off and block the throttle open, if you hear in the carb the int valve is not seating, if air in the tail pipe the exh valve, you'll hear some air going past the ring, but it's got be pretty bad before it caused a problem. Walt

billfish · 11-11-2014, 09:57 PM

Thanks for the information on a leak down test. Since I'm retired and have to much time on my hands I was thinking as a quick test if I put 100 lbs pressure in the problem cylinder and rotated the engine by hand while someone was listening for air noise at the exhaust, carb and breather I should be able to here it blow on and off as the piston and valves go through their normal cycle. If the air noise is continuous through the entire rotation from one of these sources that would point to the culprit that is leaking. Does this make sense or should I stop thinking so much and go back to being retired.

JSeery · 11-11-2014, 10:23 PM

Quote:

Originally Posted by billfish

Thanks for the information on a leak down test. Since I'm retired and have to much time on my hands I was thinking as a quick test if I put 100 lbs pressure in the problem cylinder and rotated the engine by hand while someone was listening for air noise at the exhaust, carb and breather I should be able to here it blow on and off as the piston and valves go through their normal cycle. If the air noise is continuous through the entire rotation from one of these sources that would point to the culprit that is leaking. Does this make sense or should I stop thinking so much and go back to being retired.

Good luck with attempting to rotate the engine against 100psi. When both valves close you are going to be trying to compress that 100psi! Nothing wrong with your plan, just be careful, that is enough pressure to really cause the engine to turn with force. As was pointed out above start out with low pressure and find the place where the valves are closed and then you can increase the pressure if you need to. But still be careful because it can turn at anytime with higher pressure.

flatjack9 · 11-11-2014, 10:59 PM

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Get the engine up on TDC firing and put the car in gear and nothing will turn. When you pressurize the cylinder, you listen at the carb to see if the valves are leaking and listen at the dipstick to see if the rings are leaking.

rotorwrench · 11-12-2014, 12:22 PM

Our helicopter engines can turn either way if they slip and not cause any concern but airplanes have a big propeller and that will hit you pretty hard if you turn loose of it. If you put the car in gear it could push the car forward or in reverse unless you have it tied down or on blocks. A 100 psi of air pressure has a lot of energy in it. We only use 80 psi and you can't hold that with a breaker bar once it starts turning.

V8COOPMAN · 11-12-2014, 12:31 PM

Quote:

Originally Posted by rotorwrench

A 100 psi of air pressure has a lot of energy in it.

And with a single piston's exposed area being in the 7.97 sq. in. neighborhood, that equals something near 800 lbs. of force pushing down on that piston crown.......careful! DD

Mike51Merc · 11-12-2014, 01:54 PM

Quote:

Originally Posted by V8COOPMAN

And with a single piston's exposed area being in the 7.97 sq. in. neighborhood, that equals something near 800 lbs. of force pushing down on that piston crown.......careful! DD

Guys:
100lbs of air pressure is a lot, and you certainly can perform the test with less pressure, but on a V8 engine pushing one piston from TDC to BTC will only move the crank 22.5 degrees. It isn't exactly enough to thrust the car forward (or backward) anywhere.

flatjack9 · 11-12-2014, 01:54 PM

Also, think hand brake.

V8COOPMAN · 11-12-2014, 02:10 PM

Quote:

Originally Posted by Mike51Merc

Guys:
100lbs of air pressure is a lot, and you certainly can perform the test with less pressure, but on a V8 engine pushing one piston from TDC to BTC will only move the crank 22.5 degrees. It isn't exactly enough to thrust the car forward (or backward) anywhere.

Mike......Maybe you haven't had coffee yet, but TDC to BDC usually equates to 180 degrees of crank rotation.

The only REAL reason I bring this up is to remind folks of the forces involved with an operation so seemingly simple. For instance, a guy has his hand wrapped around a wrench on the crank pulley nut, to "hold it" at TDC. Anyone applying those kinds of pressures....stuff CAN go south in a hurry. DD

cmbrucew · 11-12-2014, 03:16 PM

Always safest to err on the side of safety. DD is saying just that.
Bruce

Mike51Merc · 11-12-2014, 04:04 PM

Quote:

Originally Posted by V8COOPMAN

Mike......Maybe you haven't had coffee yet, but TDC to BDC usually equates to 180 degrees of crank rotation.

The only REAL reason I bring this up is to remind folks of the forces involved with an operation so seemingly simple. For instance, a guy has his hand wrapped around a wrench on the crank pulley nut, to "hold it" at TDC. Anyone applying those kinds of pressures....stuff CAN go south in a hurry. DD

OOPS, pour me a double espresso. I was thinking about ignition timing degrees for some reason.

V8COOPMAN · 11-12-2014, 04:17 PM

Quote:

Originally Posted by Mike51Merc

OOPS, pour me a double espresso. I was thinking about ignition timing degrees for some reason.

I knew a cup o' "Joe" would get you back in the mood again. DD

billfish · 11-12-2014, 04:26 PM

Thanks to all for very good advice, especially the safety issues. I made an adapter from an old spark plug as suggested, applied 35 lbs pressure to the problem cylinder after marking the plug wire location on the distributor as suggested and cranking the rotor to that position. I had air flow at the tail pipe, I could hear some air noise at the vent and carb but detected no air movement there. I also tested other cylinders the same way, There was a distinct on and off of air flow in the other cylinders indicating proper operation. With pressure on the problem cylinder air came out the exhaust pipe no mater what cylinder was in position to fire. It seems that the exhaust valve is not closing in that cylinder causing the low compression there. Does this all make sense, I have not fixed anything but think I learned a lot. I'll try the MMO and just run the car and see what happens. I notice a miss at idle but it runs fine otherwise. Any other advice would be appreciated.

Mike51Merc · 11-12-2014, 05:09 PM

If a valve wasn't closing, you wouldn't have 70-80 lbs compression in the "bad" cylinder--- you'd have nearly a total loss. I think the valve is closing but you have leakage either from a burned or carbonized valve or a potential hairline crack in the valve seat.

By the way, there are lots of flatheads running around with less than 80 lbs compression overall.

11-11-2014, 07:29 PM	#21
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,425	Re: compression question? We ues a differential pressure test set up to check compressions on aircraft engines. It uses a test rig that has two pressure gauges on a little manifold with a calibrated orifice between the two of them and a pressure regulator to bring the psi up in the cylinder. You have to have the piston for each cylinder up on TDC compression stroke to get a good reading. A perfect cylinder would be 80 psi test pressure with 80 psi applied to the cylinder but that is rare. Anything above 65/80 is acceptable with no more than a 15 psi split. It's difficult to get them to stay on TDC sometimes and a person keeps having to turn it back to TDC if it slips one way or the other. Once you get it in the sweet spot, it will stay. Leakage by the rings will exit the crank case vent pipe. Leakage by the valves can be heard either in the exhaust manifold or in the intake manifold depending on which one is leaking. A good test is if all are above 70/80.

11-11-2014, 07:44 PM	#23
Walt Dupont--Me. Senior Member Join Date: May 2010 Location: Gardiner Me. Posts: 4,200	Re: compression question? billfish. the air pressure check is a good check for finding a compression leak, your 8BA engine is an easy one to find TDC, find the spark plug wire in the cap that goes to the bad cyl, make a chalk mark on the side of the dist wear that wire is, remove the cap, crank the engine over until the rotor point to that chalk mark, now the piston is on TDC, It's nice to take an old spark plug, drive the poscelin out of the plug, then make a thread of brass a fitting on top of the plug for an air hose to clamp onto, It's nice to have regulator in the air hose so you can start with low pressure. If you put 125# air into the cyl your probaley blow the piston down the hole and then the exh valve open, so start with low pressure and work up, take the air filter off and block the throttle open, if you hear in the carb the int valve is not seating, if air in the tail pipe the exh valve, you'll hear some air going past the ring, but it's got be pretty bad before it caused a problem. Walt

11-11-2014, 09:57 PM	#24
billfish Senior Member Join Date: Jan 2014 Location: egg harbor twp nj Posts: 303	Re: compression question? Thanks for the information on a leak down test. Since I'm retired and have to much time on my hands I was thinking as a quick test if I put 100 lbs pressure in the problem cylinder and rotated the engine by hand while someone was listening for air noise at the exhaust, carb and breather I should be able to here it blow on and off as the piston and valves go through their normal cycle. If the air noise is continuous through the entire rotation from one of these sources that would point to the culprit that is leaking. Does this make sense or should I stop thinking so much and go back to being retired.

11-11-2014, 10:59 PM	#26
flatjack9 Senior Member Join Date: May 2010 Location: Oshkosh, Wi Posts: 4,526	Re: compression question? Sponsored Links (Register now to hide all advertisements) Get the engine up on TDC firing and put the car in gear and nothing will turn. When you pressurize the cylinder, you listen at the carb to see if the valves are leaking and listen at the dipstick to see if the rings are leaking.

11-12-2014, 12:22 PM	#27
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,425	Re: compression question? Our helicopter engines can turn either way if they slip and not cause any concern but airplanes have a big propeller and that will hit you pretty hard if you turn loose of it. If you put the car in gear it could push the car forward or in reverse unless you have it tied down or on blocks. A 100 psi of air pressure has a lot of energy in it. We only use 80 psi and you can't hold that with a breaker bar once it starts turning.

11-12-2014, 01:54 PM	#30
flatjack9 Senior Member Join Date: May 2010 Location: Oshkosh, Wi Posts: 4,526	Re: compression question? Also, think hand brake.

11-12-2014, 03:16 PM	#32
cmbrucew Senior Member Join Date: Dec 2010 Location: North of sandy ago, CA. Posts: 2,064	Re: compression question? Always safest to err on the side of safety. DD is saying just that. Bruce __________________ Works good Lasts long time

11-12-2014, 04:26 PM	#35
billfish Senior Member Join Date: Jan 2014 Location: egg harbor twp nj Posts: 303	Re: compression question? Thanks to all for very good advice, especially the safety issues. I made an adapter from an old spark plug as suggested, applied 35 lbs pressure to the problem cylinder after marking the plug wire location on the distributor as suggested and cranking the rotor to that position. I had air flow at the tail pipe, I could hear some air noise at the vent and carb but detected no air movement there. I also tested other cylinders the same way, There was a distinct on and off of air flow in the other cylinders indicating proper operation. With pressure on the problem cylinder air came out the exhaust pipe no mater what cylinder was in position to fire. It seems that the exhaust valve is not closing in that cylinder causing the low compression there. Does this all make sense, I have not fixed anything but think I learned a lot. I'll try the MMO and just run the car and see what happens. I notice a miss at idle but it runs fine otherwise. Any other advice would be appreciated.

11-12-2014, 05:09 PM	#36
Mike51Merc Senior Member Join Date: May 2010 Location: Northeast Pennsylvania Posts: 1,582	Re: compression question? If a valve wasn't closing, you wouldn't have 70-80 lbs compression in the "bad" cylinder--- you'd have nearly a total loss. I think the valve is closing but you have leakage either from a burned or carbonized valve or a potential hairline crack in the valve seat. By the way, there are lots of flatheads running around with less than 80 lbs compression overall.