Alternat way to check compression.

[onesubdrvr]

We often talk on here about checking compression to diagnose certain problems. I was doing a little reading the other day, and ran across this. I figured I'd share it, although it's nothing new, still helpful.

From "How to hotrod small block Chevy's"

"How to use your tachometer in tuneups

Let's say you're in a situation where the engine suddenly starts running rough or there is a noticable loss of power. A compression gage is not handy, but you have a tachometer. Short each spark plug wire to ground one at a time and note how much RPM drops on the tach . . . or pull off the plug wires one at a time. A leak in a cylinder caused by rings, valve seats or cracks in the cylinder wall or combustion chamber will allow pressure to drop within the cylinder. That cylilnder produces less power, affecting RPM.

A cylinder doing its share of work without serious leakage will drop 50 to 100 RPM when that plug wire is pulled off or grounded. If the tach shows a decrease in RPM of only 10 to 20 RPM, you've found a sick cylinder."

Keep in mind, this isn't the best way to do this, but a quick check to see how much further you need to go.

A note of caution to, BE CAREFUL IN HANDLING PLUG WIRES ON A RUNNING ENGINE!! :lightning :shocking: :lightning

Hope someone finds this useful,
Wayne

[Cuda]

I've never seen a set of rings bad enough to cause a low compression reading. It might with a leak down, but not a spinning reading. I had a car once that I had to put twice the dollars in oil everyday that I did in gas, and it ran fine everytime I cleaned the plugs. :)

Every single time I've had low compression, it has ALWAYS been a valve problem.

[Cuda]

It is usually a burned exhaust valve but in 20 years of wrenching I have seen broken rings, rings stuck from carbon buildup and even holes burned through pistons:eek!:
I agree with broken rings and holes in the piston, what I meant was I've never seen rings worn bad enough to affect the compression.

[boatnut]

If you have been twisting wrenches for 40+ years you would have witnessed quite a change in the wear and life of these motors we discuss so frequently. Some observations:
1) back through about the 50's, metal in motors and parts was not nearly as good as now, it was not uncommon to need valve work in 30K to 40K miles and overhauls (rings etc.) prior to 100K miles
2) valves did not solve all the compression problems, what typically happened is the block bores wore in a tapered fashion (the bore would become larger near the top of the stroke), the rings would also wear but they were harder, however old rings couldn't flex enough to compensate for the changing bore diameter and the cylinder couldn't build sufficient compression even with new valves --- it was time for "honing" the cylinders and new rings or if the taper was bad enough, reboring the cylinder
3) todays designs and metalurgy seems to prevent all or most of these faults during the total use time of most engines
4) those of us that restore old boats/cars and try to stay with original power still fight these issues

So, if you think only valves cause low compression, you are just too young.
I sure wish I had the same problem. Cheers, Ed :)

[mrfixxall]

try it with msd ignition,,,,,,,,65000 volts on water ouch..............

[Moody Blu']

try it with msd ignition,,,,,,,,65000 volts on water ouch..............

its a great thing:cool:

[joseph m. hahnl]

:
1) back through about the 50's, metal in motors and parts was not nearly as good as now, it was not uncommon to need valve work in 30K to 40K miles and overhauls (rings etc.) prior to 100K miles

:)



Back then valve seats weren't hardened. That is why they used leaded gasoline to cusion the valve seats and prevent the seat from becoming peaned. I would agree with you that Todays alloys are superior to the past. But I"m not sure if cast iron has changed much over the years.
which brings up a point about inconel valves.On any metal to metal contact one of the components is the wear out part. usually made of softer material and designed to give out before the other and is generally the easier to replace item .So when you put in a Valve that is harder than the seat the seat will give before the valve. Depending on how Badley damaged it is regrinding the seat may not work and the seat whould have to be replaced.
"very Expensive " They would also wear out the valve guide .

Just food for thought!!!!!!!!!!!!!!


joe

[Cuda]

So, if you think only valves cause low compression, you are just too young.
I sure wish I had the same problem. Cheers, Ed :)
I wish I was too young. I'm sure if you do it for a living you will see more of what you are describing, but I've been wrenching when I've had to for 35+ years, and mine have invariably been a valve problem when low compression is found.

I agree that todays engines are far superior to the one's 30 years ago. I remember my dad bragging about how our 1964 390ci Ford had 130,000 mile without ever having to take off the heads. That isn't remarkable at all now days. It used to be if a car had 100,000 miles, it was at the end of the life expectancy of the engine without a major overhaul.

[MOP]

Wayne never pull plug wires at the plug end, always pull them at the cap. Pulling them at the cat removes them from the power source! Leaves you holding a dead wire not a 50,000 volt live one.

Phil

[onesubdrvr]

Wayne never pull plug wires at the plug end, always pull them at the cap. Pulling them at the cat removes them from the power source! Leaves you holding a dead wire not a 50,000 volt live one.

Phil
Great note Phil, absolutely, should have thought about that!!

Wayne

[Dave_N]

Another thing to consider about a cylinder contribution test (no matter how you do it), is that if the engine in question is fuel injected, when you induce a misfire the computer will instantly work to bring the idle back to where it wants it making getting any meaningful results difficult, if not impossible.


A good alternate way to check compression and help identify a problem cylinder is to get a secondary ignition KV tester (I use a MAC tools ET3142 that I got on ebay for $25) and check your secondary ignition firing voltages. If one cylinder has considerable lower firing voltage than the rest, it is probably due to low compression. Any cylinders with bad plugs/plug wires or lean condition will show high firing voltages. This does not apply to Capacitive Discharge systems (like MSD).

Dave