I did a cold dry compression test on a stock 1970 340 T/A block. Engine hasn't been cranked in several weeks. Air cleaner in place, throttle plates closed. Ambient temp ~60 degrees. I still need to do a wet test and one after it’s warm and throttle plates open air cleaner off. Results were all consistent except one cylinder. Not sure what they are supposed to be. Any help with this that anyone offer would be greatly appreciated.

Right 78 80 83 85


Front-------------------->Rear


Left 82 80 85 110

I'd say you have one cylinder with good valves.

The rest doesn't look so good.

A leak down test is best to determine true condition of the motor.

Do you think it needs a head job or complete rebuild? Test was done in a way that would likely provide the lowest possible reading. Cold, throttle closed, dry, air cleaner on.

I'm not 100% sure, you should post in Unlawfuls Race and Engine tech section.

I'm used to seeing one really low cylinder if I've got a miss, but 7 down? Something must be really wrong there.......

How did it run when you last started it up? Really need to do it right, throttle wired wide open, charged battery, then follow up with about 3 or so squirts of oil in each cylinder as you do a wet test. Not really enough info to give you even an educated guess.

I think it runs pretty good but doesn’t really have the power it should. So I decided I would check compression. Should I warm it up first before checking again or is it okay to do it cold?

I would run it up to operating temperature, then shut it down and wait maybe an hour or so, then test. You can roll it through about 4 cycles as the first and second are still pumping up the gauge hose etc. Those numbers you have do not sound right.(Too low)

Your test results don't mean anything. Your procedure was incorrect. Nobody seems to have noticed that.

The correct procedure for a compression test is remove all spark plugs, block throttle wide open, then do compression test. The engine has to spin over at least three times to get a good reading.

Sorry I'm sounding so cranky. My wife didn't even make me a turkey sandwich yesterday.

R.

With the throttle wide open will I need to do anything to the fuel supply? I’m going to test it tomorrow. Also if the results tomorrow are consistent with these what is the likelihood the valves are out of adjustment?

No on the fuel supply--it's fine. Just be sure when you open the butterflies you get them open on the first attempt so you don't dump a bunch of gas into the motor by operating the accerator pump more than the one time.

Pull your coil wire so there is no arcing from plug wires laying around--that's a fire hazard if the motor spits any fuel out the plug holes.

Charge your battery! You want a fully charged batt.

Don't pull the coil wire because the cool will still make a spark.

Remove the wire from the ballast resistor, the side where the blue and brown wires that go into one connector at the resistor.

Remove the throttle return spring and the carb stays open a lot easier.

Do this with the engine warm-hot, don't let it cool for an hour.

Ya pull the coil wire out of the coil--no spark. Pulling the ballast wires will work too.

warm engine

wire throttle FULLY open, that'll shut off any fuel supply as the fuel won't flow unless there is air passing thru the carb, which it won't be at cranking speed, which is also how you start a flooded engine.

by the time you remove all the plugs, the engine will be cool enough

spin the motor over a FEW times, that'll spit out any fuel in the system -intake, carb passageways, head passageways, combustion chamber.

screw in the gauge and go for it.

crank a minimum of 3 revolutions

don't worry about any spark, as there won't be any fuel there, just don't touch any wires while doing the test, as in holding the gauge too close to a wire. If you really want to, then just pull the coil wire from the coil.

Oh, and there won't even be any sparks if you don't use the ignition key to crank over the engine, that is what a remote starter switch is for. Buy one, or make one using a push button momentary switch from the hardware store, been there, done both, have both.

How's the best way to do a hot test without burning myself or melting a plug wire or compression gauge hose or ??? on a manifold?

Just do the best you can. On my 440 Cuda access to the plugs is a beech. If the engine/headers are hot it's brutal.
There are other cars that you can perform the test on a much warmer engine.
The main thing is you want SOME warmth in the engine. It also ensures the oil rings are well lubed from the recent run, thereby ensuring some oil sealing of the piston/cylinder.

You are working on a small block here. A spark plug socket and a long extension are your friend. No manifolds to touch that way.

As already said use a battery charger when testing and repeat the first test with the throttle opened and then warm the motor up (get it HOT, not barely warm ) and do it again, you may be pleasantly surprised with the differences
What do you know about this motor? Was it rebuilt or is it is a stock motor with a lot of miles on it?
BTW,#7 is the leanest cylinder usually on most V8 with that have the firing order when #5 cylinder fires 90 degree before #7, especially on single plane single four barrel intake manifolds, not so much on stock Mopar intakes
Let us know your results

And a piece of rubber fuel line can be pushed over the spark plug ends, once they are loose, to remove them and not burn yourself. Also works great to reinstall the plugs and not cross thread them.

Yup, on the hose. A spark plug socket is best, it already has the rubber insert to hold the plug.

Here's what I got. Much more consistent, but not so great. Warmed engine up to full operating temperature then began removing things and by the time I got everything off it was not too hot to touch. I used two batteries in a series and a charger. Cranked each cylinder 7 times (wanted to be sure) did dry test and wet test on each cylinder as I went along. I can see tons of carbon on top of the pistons. As far as I know the engine is likely just a refresh and never completely rebuilt. I believe it got new bearings, seals, rings etc. Anyway here it is first number dry second number wet. I started at the back right when the engine was the hottest and it was 95/107. Went back after finishing and it was 90/95 dry/wet so I through the first one out.

Right 78/84 80/85 83/95 90/95


Front---------------------------->Rear


Left 83/83 80/93 82/98 85/94

looks good.

Now make sure that your gauge is accurate, mine went bad the other year and was showing too light a pressure.

Also make sure the timing chain isn't stretched.

Also do the leak down test to check the rings and valves, etc.

Now that you have spent all this time on a cheap compression test, all it will tell you is you have a problem. Buy a leak down gauge and it will tell you what the problem is.

I have a brand new gauge and will double check the pressure.

Not sure how to check whether the timing chain is stretched or not??

Also purchased a leak down tester but am unsure how to perform the test as I have never done it.

take the dist cap off

turn the crankshaft in one direction to top dead center mark

then turn the crankshaft in the other direction watching the rotor in the dist. when the rotor STARTS to turn, stop rotating the crankshaft

tell us how many degrees the crankshaft turns before the rotor starts to turn turns

Um, just to clarify, cause it's pretty frikken important, I am sure you meant two batteries in parallel....not SERIES! GASP!

My thinking was that the point of doing a compression test was to see if you have an issue and needed to do further testing and diagnostics to narrow down the issue.

Yes, parallel

leak down testing/tester;

put each piston at their tdc on the firing stroke and screw the fitting in the sprakplug hole with the piston at tdc

supply 5 psi or so, air pressure and listen at the carb, exhaust, radiator, oil fill or dipstick holes. listening for any air coming out and bubbles in the radiator. Also if the kit has a gauge then watch for pressure drop in a given time span, should be noted in the directions with the kit.

parallel is more amps = good

series is more volts = bad. will burn out the starter.

I'm sure it will be in the instructions but how do I know when each cylinder is at TDC?

Start at #1, and use the firing order. Then stick a plastic rod, straw, or something that won't damage anything and won't break off in the cylinder, or use the pressure gauge, and turn the crank over by hand, it'll work out that way.

Disclaimer: Please understand I have no experience of the process for a leak down test. To start the test I need to get #1 to TDC and do the leak down test. Next lets say I do #8. Question is do I just rotate the crank around to TDC again to get TDC on the next cylinder?

Bring the motor up to TDC on # 1 cylinder on the firing stroke and test it, rotate the crankshaft 90 degrees clockwise to test #8,and another 90 degrees to test #4 and test, then rotate another ninety degrees to test # 3 and go through the firing order in sequence to test the rest, one at a time (Imagine straight up as 0 degrees and 1/4 turn is 90 degrees
Most car motors in the car are hard to get 90 degrees rotation with one movement, if you stop more than 5 degrees before or after TDC the air pressure will probably blow the piston down to bottom dead center, make sure and remove the ratchet or breaker bar from the socket or crankshaft bolt before each test If you don't you may regret what it breaks or damages, like your arm or radiator shroud
The leak down meter will come with instruction, some are better than others, I own two, one old single gauge from Moroso that quit reading accurately years ago(it would stick ) and a newer one from Aircraft Spruce for testing my old airplane motor, it is a dual gauge tester and easier to use

Yes, and you won't have to rotate the crankshaft very many degrees before the next piston gets to it's tdc.

Those numbers look way more like running compression than cranking compression. Are you sure the gauge is accurate? Do you have another engine to try a couple holes on? I've seen plenty of cheap gauges read bad(think Craftsman) I've seen plenty of check valves(schrader valve in the end of the hose) cause inaccurate/inconsistent readings.I would be very surprised to see under 100psi on a decent running stockish engine...even cold with the throttle closed. Good luck!!

Brand new gauges different results. Dry Test.

Right 120 135 125 130


Front-----------------------Rear


Left 128 129 136 130

I'd be happy with that.

Oh heck yeah.

Those numbers look better. Do as Cab says on the leakdown test. I have a two gauge set. Set the in air to 100 lbs of presure and check the second guage. If it holds 90 lbs, you have 10% leakage. Listen in the oil filler for a ring leak, the carb for bad intake valves and the tail pipe for bad exhaust valves.

I’m ready to do a leak down test. I have the harmonic balancer timing mark lined up at 0 and can see that the #1 piston is as close as it could be to the bottom of the spark plug or underside of the head. Couple of questions. The instructions say to do on a warm engine. Everything else I’ve read and heard say cold engine. Firing order I believe is 1-8-4-3-6-5-7-2. How much do I rotate the crank to get TDC on the next cylinder to test?

90 degrees.

I'm going to do the leak down on Weds. Just to make sure. I have the harmonic balancer at the 0 mark and it appears that #1 is TDC. I rotate the crank clockwise 90 degrees and cylinder 8 will be TDC another 90 degrees and 4 will be TDC and so forth and so on?

as long as you're certain that it is set to fire #1 with timing mark lined up at top dead center, and not set to fire cyl #6 which also lines up the same way.

If you align the timing tab on zero with the timing mark on the balancer align with the tab on #1 it will not leak out of the valves very much, if it is align on #6 it won't hold any pressure at all due to it having both valves opened on #1 overlap stroke with #6 on the firing stroke
Either way if it isn't on #1 TDC on the firing stroke you can move the hose to the other cylinder at TDC on the firing stroke (#6) and start testing from there

Not sure of the circumference of the harmonic balancer and I don't have timing tape. I believe when I have done something like this in the past I was able to use a piece of painters tape cut to an exact length and put marks at 90 degrees intervals around the balancer. I need to know the circumference of the balancer or the correct length of the piece of tape that I need to make to do a template that will allow me to make marks all around the balancer at 90 degree intervals so I can do the test correctly and efficiently.

I just tried to do the test and I must be doing something wrong. I have #1 at TDC. Checked with a tool and harmonic balancer is lined up with zero. I tested #1 first and have 45% leakage. I hear a hissing in the opposite valve cover on the passenger side and can feel air coming out of the breather hole. So I moved over to #6 and same result 45% hiss and air on the same side. Thoughts?

Air coming out of the valve cover breather would be air coming up from the oil pan, which would be coming past the piston, and therefore would be bad rings.

Turn the crank over a couple times and try again.

And make sure the rotor in the dist, is pointing to #1 tower. I also like to put my finger in #1 hole while cranking it up to #1 tdc to be sure it's the compression stroke. You can tell by the 'lots more' pressure building up, the exhaust stoke won't build up that much pressure. When close I'll watch the timing marks and piston top to be sure it's at tdc. That's where the straw comes in, sometimes you can, sometimes you can't see the piston top thru the sparkplug hole. Or use a bore scope. Just don't insert it very far so it doesn't get stuck between the piston and head. Barely stick it in the plug hole.

I’m doing this on a cold engine and using low pressure (gauge says 7-100 psi) I’m using about 15-20 psi is this ok? Also if the rotor is pointing close to cylinder one on distributor does that mean it’s at TDC on compression stroke?

Yup.

Just checked distributor rotor and it’s pointing straight toward the back of the car so basically toward cylinder 6

Turn the crank until the rotor points at #1 tower. Should be just one revolution of the crank.

Leak Down Results


Right 45% 45% 40% 50%


Front-------------->Rear


Left 45% 47% 45% 34%

Disclaimer: The gauge was new, but I don't trust the gauge at all. It would give consistently inconsistent results and was hard to get regulated. Sometimes it would go straight to 100% loss then you would have to reset it and start over because once it hit 100% it somehow cuts off the air flow. Never could get it to start at exactly 0 so some of the results with this gauge could be off by 5-10% or more. Turning the regulator on the gauge would some times get it in the yellow start range, right in the middle, but once I finished it would be out of that range. Also turning the regulator was so inconsistent in what it did on the gauge. Sometimes a lot of turning resulted in little movement of the needle and other times a little would make it move a lot or it would keep moving after it was set. I believe if the engine was warm it would have been much better but not for sure. Either way regardless I could hear air in each valve cover, out the breather port and dipstick hole. Nothing in the coolant and nothing at the exhaust. One time I got 100% loss over and over again and I ended up moving to the next cylinder I had previously tested and gotten different results and it did the same 100% loss vs 40% I got before. I got it regulated on that one cylinder and then went back to the other and ended up getting 47% instead of 100%. Thing is to go back to that cylinder I rotated the crank backwards 90 degrees. Is this okay? That's the only time I went backwards.

You need to read what pressures that gauge instruction sheet wants you to use, I haven't seen any leak down gauge that will work below 50 lbs of pressure, but there are a lot of things in this world I haven't seen yet also
A single gauge unit probably wants between 80 and 100 lbs., my two gauge aircraft legal calls for 80 lbs static before starting and the using the regulator to bring the first gauge (left hand side looking down at them) back to 80 lbs. with the valve opened reading the leakage so I would see 80 Lbs. on the left gauge and say between 78 Lbs. to 0 lbs. on the right gauge with a hole in the piston or a broken valve

It's a dual gauge setup. The instructions literally say "NOTE: Set the air compressor's regulator to 7 to 100 PSI. Never opperate this tester with air pressure set higher than 100 PSI which can damage this tool."

Try testing it warm with the primary gauge pressure set at 80 and then divide your reading into each other, if it reads 78 on the low side and you divide 80 into that it equals 2.5%, correct
If you set the pressure at 100 Lbs. even and then read the low side it will read the percent leakage with no math, 100/98=2.0 %
Street motors that test from 3 % to 25 % aren't bad
If it leaks worst than 25 % then you have to decide to fix it or drive it until you can't stand the oil consumption and poor performance

I have never cut corners on tools. Mine is a two gauge Matco unit. Works real simple. Find tdc, input 100 lbs of air into the cly by first gauge, then read second gauge of the amount lbs it is maintaining. If it holds 80lbs, you have 20% leakage. Sounds like you have a worn out motor with bad rings to me.

I’ll retry this way some time next week. The thing is there’s no smoke or oil consumption. The gauge I’m using is from Harbor Freight. I don’t do a lot of these tests so I didn’t want to spend a small fortune on a gauge I may use once.

Wow, next they'll be telling you to get a borescope!

Those compression numbers from your first legitimate test are low. They are lower than low.

You said there is a lot of carbon in the engine. Last year Hot Rod magazine in one of their "we fix a problem car" articles worked on a mustang and it had low compression in some holes. The mechanic said lets take it out and "blow the carbon out." I always thought that was just a myth. He told the writer that sometimes engines get carbon deposits holding the valve a little open and losing compression.
Go out and get the car warmed up[, do a bunch of hard acceleration runs and some high speed runs. That may help. Get back home and aim the exhaust system away from buildings, people, etc. Remove air filter and with engine running about 2500rpm, trickle some water into one side of the carb. You should get a blast of white steam, maybe carrying with it little chunks of carbon. It's pretty dramatic and will stain a wall or fence. Repeat for the other side. Take it out and run it back up to operating temp and do it again. This may help.

It sure sounds to me like rings aren't working. Try to seat the rings with some sharp acceleration blasts followed by letting the engine pull the car down to speed with throttle closed.

Leakdown tests can tell you the condition of things. If you pressurize a cylinder and air leaks out past the piston the next step is surgery.

R.

After "blowing the carbon out" likely you will need to clean/replace the plugs. On reseating the rings if you do rering here is a good read on ring sealing: go to www.mototuneusa.com & scroll down to & click on "breakin secrets". its a motorcycle oriented site but the principle works. their main take is (1) to run it HARD (with their procedure) for ring breakin (2) & you get ONE shot to get an excellent breakin that gives good leakdown numbers (as opposed to a so so breakin with average numbers).

Not that id say do it, but every time I see a chevy guy float his valves, his carbon goes out the tail pipes as well.

Robert, most the time we would run them hard to blow the carbon out a good part of that was to clean the carboned up plugs as well.

Id check the plugs and see if they are burnt back and replace if needed.

Heck, you guys already got his rebuild done for him.

Hey we're here to help!

We don't mind spending HIS money.

The initial and second set of compression results were done with a rental gauge with unknown history from a local auto parts store. The results were questionable. The latest ones were done with the new gauge and were in within spec of > or = 110.

Can rings become unseated? If so how and why?

If I do the leakdown on a warm engine and get good results vs bad results on a cold engine what would that mean?

Did cold leak down today new leak down gauges and got 13.5-14.3% loss at 90PSI. First had to fix my compressor it wouldn’t build pressure and fix a few air hose leaks.

Are you going to do a warm or hot test? If not maybe do one cylinder and let us know what you find
I'm thinking your motor is good based on these results

Probably not based on the latest results.

What is the problem with this car? What are you trying to solve? Does it run correctly?

Not sure there is one just trying to figure that out by testing.