Vintage Mustang Forums banner
  • Hey everyone! Enter your ride HERE to be a part MAY's Ride of the Month Challenge!

1 - 11 of 11 Posts
G

·
Discussion Starter · #1 ·
Calling all engine experts! I’d very much appreciate your input on this: My history: A month ago, I just did a dry compression test (cold engine) on my 37k mile boss 351. The results: (1) 180: (2)95 (RED FLAG): (3) 175: (4) 175: (5) 170: (6) 175: (7) 100 (RED FLAG): (8) 160 (YELLOW FLAG):

I liberally squirted oil in the sparkplug holes on #2/7 (my low cylinders), and they pumped up to around 130 lbs (that is with me cranking away until she stopped).

--- --- ---

Yesterday I finished a (dry/cold engine) leakdown test with a dual gauge leakdown tester. I did the test twice. The first time, I guessed at finding TDC on each cylinder by watching the rotor in relation to the distributor cap, watching to make sure both intake/exhaust valves were closed and listening for the woosh of air through the hose attached to the spark plug hole. (in other words, chances are I was not scientifically exactly on TDC). Also, I readjusted the valves in each cylinder before and after the initial test. I did not record the reading differences, but I’ll say that I was able to improve my leakdown percentages on each cylinder by readjusting the valves to spec. The results on the first test:

#1 cyl: 19% (leakdown)
#2 cyl: 27%
#3 cyl: 20%
#4 cyl: 24%
#5 cyl: 27%
#6 cyl: 28%
#7 cyl: 2% (RED FLAG!)
#8 cyl: 35% (RED FLAG!)

Now is where it gets interesting. During the first go-round, I was guessing at finding TDC. My question…DOES THE PISTION HAVE TO BE EXACTLY AT TDC TO DO A LEAKDOWN TEST? Can you be advanced or retarded a hair? Here’s why I ask. Using the screwdriver method on my second go round on at least half the cylinders, I determined that (per my engine) I was at EXACT TDC with the rotor pointing halfway between the cylinder being tested and the next cylinder in the firing order. Makes sense – advanced timing. BUT, what I don’t understand is that when I would eeeek the engine over (by hand with a ratchet/breakerbar) while under pressure anywhere around TDC, my readings would shoot way up.

In other words, when turning the engine by hand anywhere just before TDC to after TDC, with the intake/exhaust valves still closed, I would find a SWEET SPOT, I mean the TINYEST sweet spot that would give me a huge boost in percentage. On some cylinders this would be before TDC, some after TDC, some on TDC – It never seemed to be consistent. I recorded these highs below.

Did I do this process right? Here are my results from the second test:

#1 cyl: 19% (second test) 7-12%
#2 cyl: 27% (second test) 7-12% (GREEN LIGHT)
#3 cyl: 20% (second test) 9-40%
#4 cyl: 24% (second test) 10%
#5 cyl: 27% (second test) 12%
#6 cyl: 28% (second test) 7-12%
#7 cyl: 2% (second test) 2% (RED FLAG)
#8 cyl: 35% (second test) 30% (RED FLAG)

Some of the (second test) readings have a range because for these cylinders, for some reason (probably because I could reach!), I found TDC exactly with the screwdriver in the spark plug hole method. The higher-in-number reading (for instance, #3 had a high of 40%) was at TDC exactly. The lower-in-number reading (#3 had a low of 9%) was me eeeeking the engine around hair by hair to find the sweet spot which would boost my % reading – in this case, 9%. Of course, as soon as I hit that reading, the % readings would drop dramatically, as the exhaust valve would begin to open.

Observations:
(1) #2 cylinder’s low compression reading was from faulty valve adjustment. Leakdown testers are the coolest – as it helped me determine that I was losing compression through my intake valve.
(2) #7 has problems, and hopefully has something to do with #8 (head gasket?). I was losing air through the crankcase.

Thanks a million for getting this far. I would really appreciate any comments. Especially if I did the test correctly. Very best regards.
 

·
Registered
Joined
·
5,368 Posts
That was qiute the read, interesting. Now, pull your heads off and get on with it. /forums/images/icons/wink.gif
 

·
Registered
Joined
·
6,911 Posts
I dont care for those leakdown tests......a standard compression tester tests ring/valve seal AND the rest of the valvetrain.....any wide variation=rebuild

Just looking at those numbers, I'd be surprised if you didnt need a rebuild; but I think leakdown tests are more vague
 

·
Registered
Joined
·
10,588 Posts
…DOES THE PISTION HAVE TO BE EXACTLY AT TDC TO DO A LEAKDOWN TEST? Can you be advanced or retarded a hair?

Get the valves closed and the piston near TDC and you'll be fine...

Remember, at any point the piston is compressing the air/fuel charge and sealing the resultant combustion, efficiency is important.

Your results, if accurate, suggest bad rings and/or cylinder wall condition anomalies..

As an example, after a full season of racing, the W in the race car had a 2% average leakdown, with the worst cylinder being 2.5%...a typical new racing engine (after break-in for an hour or so) leaks down in the 1 to 1.5% range.

You can test leakdown throughout the piston cycle by removing the rocker arms and rotating the crank though an entire revolution...I've done this before to determine cylinder wall damage.

Leakdown testing isn't the be-all and end-all of diagnostics but is an important tool to use in assessing an engine's health. I prefer to do them on a warm engine so things have expanded to their normal operating tolerances. Combined with a stethoscope to track down the leaks, that air usually won't lie *G*...

If you have more information and/or questions, post...
 

·
Registered
Joined
·
7,671 Posts
When the motor is out, always mark the balancer at TDC, 90, 180, and 270 degrees. You could do this with the motor in the car by using a degree wheel.

On an engine with hydraulic lifters, you could back the rockers off, counting the number of turns, then return them to the same spot after the cylinder is tested.
 
G

·
Discussion Starter · #6 ·
Thanks for the insight. One thing to note (typing error on my part): #7 cyl: 2% (second test) 2% (RED FLAG)

should say:

#7 cyl: 98% (second test) 98% (RED FLAG)

So Pat, if I'm reading you right, as long as I'm anwhere near TDC with both valves closed, I'll get an accurate reading. And because I'm getting such variences when moving any given cylinder around TDC, then, as you said:

"...suggest bad rings and/or cylinder wall condition anomalies.."

Looks like a rebuild is in order. Thanks again for your expert advice. I truly appreciate it. And good tip on removing the rocker arms to check pressure throughout the entire stroke.
 

·
Registered
Joined
·
7,671 Posts
TDC will be where the most wear will be, and filling the cylinder with 100PSI of compressed air when the piston is a few degrees off TDC will push the piston to BDC, and very quickly too!
 

·
Registered
Joined
·
10,588 Posts
filling the cylinder with 100PSI of compressed air when the piston is a few degrees off TDC will push the piston to BDC, and very quickly too!

Try it sometime with all the rockers backed off and the spark plugs in...you might be surprised.

I've never had any trouble doing it that way on racing engines, possibly because of the high CR
 

·
Registered
Joined
·
7,671 Posts
Checking leakdown percentages with the spark plugs in, that's a new one on me.

I guess we all have our own different methods and ways of doing things... /forums/images/icons/laugh.gif
 

·
Registered
Joined
·
10,588 Posts
hehehe...you should try it with the engine running...

I should have said all the spark plugs except the one with the adapter in it...

When I've done leakdowns on the race car, I've just popped the rockers loose after shutting the engine down warm and bumped the engine close to TDC with the remote starter (within 10-15 degrees) and run the numbers....never even gave it a second thought. I've never worried enough and had problems with my street engines where I thought a leakdown was worth the effort, so I've never done any on low CR engines.
I am old enough to remember hand cranking engines to start them so I do have respect for the force that air pressure exerts *G*....I work with pressures 20-30 times that amount every day.....

I've done some pretty impressive launches of hydraulic cylinder parts out the shop door over the years....65' in the air is the current record...hehehe...just imagine, from the picture in my sig line, nearly hitting the little office building from the shop...

My hat's off to our poster for making the effort...great example of diagnostics at work...
 

·
Registered
Joined
·
3,960 Posts
I prefer to do them on a warm engine so things have expanded to their normal operating tolerances.
-------------------------------
I agree with Camachinist 100% this is perhaps why the manufacturers tell us to properly warm our engines up before ANY operational loading......so that tolerances mimic a real environment.

Aside from its cooling the cylinder walls and rings, Oil also helps compression hence thicker oils on higher mileage engines and thinner slicker oils on newer engines.

Take a compression test (warm and wet) and see what you get

My opinion....you still have plenty of life in it. Just keep the oil clean.
I sometimes don't wait for 3000 miles to change it....everytime I fill
up I check the oil level and color....

Be like Jawstang rack up 300k and then get back to us!!!

:)
 
1 - 11 of 11 Posts
Top