Compression Testing - '71 350 Engine
Thread Starter
Cruising
Joined: Apr 2024
Posts: 12
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Compression Testing - '71 350 Engine
After purchasing a compression tester some time ago, I decided to test my 1971 350 engine on my C3. It was my first time performing such a test, and now I have a few questions. The results were:
1. 145
2. 140
3. 155
4. 150
5. 150
6. 140
7. 155
8. 145
After reading several articles online, I ensured that I did the same number of compression strokes (4) for each cylinder. Was this correct, or should I have continued until I reached the maximum possible compression for each cylinder?
I also read that the results should be at least 120 psi and within a 10% tolerance of each other. My results show some variation that stretches this rule. Should I be concerned about needing a new engine or a rebuild soon?
Thanks for any help!
1. 145
2. 140
3. 155
4. 150
5. 150
6. 140
7. 155
8. 145
After reading several articles online, I ensured that I did the same number of compression strokes (4) for each cylinder. Was this correct, or should I have continued until I reached the maximum possible compression for each cylinder?
I also read that the results should be at least 120 psi and within a 10% tolerance of each other. My results show some variation that stretches this rule. Should I be concerned about needing a new engine or a rebuild soon?
Thanks for any help!
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Your numbers are very respectable and all within the average range of each other.
Usually, the last two cylinders tested have the lowest readings. Reason being, the battery is getting tuckered out.
Always keep a charger on it during the test.
Pick two of the easiest cylinders to install your compression tester in and squirt a little oil in there.
That will give you a rough idea about the condition of the rings for all the cylinders.
Unless there is a drastic change in results, there is no need in testing the rest w/ oil.
Usually, the last two cylinders tested have the lowest readings. Reason being, the battery is getting tuckered out.
Always keep a charger on it during the test.
Pick two of the easiest cylinders to install your compression tester in and squirt a little oil in there.
That will give you a rough idea about the condition of the rings for all the cylinders.
Unless there is a drastic change in results, there is no need in testing the rest w/ oil.
Race Director
Joined: Nov 2006
Posts: 13,151
Likes: 7,765
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
I always spin em with the throttle wide open and until they are done going up. How I was taught. And Harley Davidson still teaches to check with throttle wide open and until they stop going up. Generally within about 6 pump's.
Thread Starter
Cruising
Joined: Apr 2024
Posts: 12
Likes: 0
Thanks for all the input!
The reason for the compression test was due to some misfiring issues.
Since I’m already in the process of changing the spark plugs, which are likely the cause of the misfiring, I thought it would be a good idea to also perform a compression test.
The reason for the compression test was due to some misfiring issues.
Since I’m already in the process of changing the spark plugs, which are likely the cause of the misfiring, I thought it would be a good idea to also perform a compression test.
Burning Brakes
Joined: Jan 2003
Posts: 961
Likes: 275
From: Frederick MD
Thanks for all the input!
The reason for the compression test was due to some misfiring issues.
Since I’m already in the process of changing the spark plugs, which are likely the cause of the misfiring, I thought it would be a good idea to also perform a compression test.
The reason for the compression test was due to some misfiring issues.
Since I’m already in the process of changing the spark plugs, which are likely the cause of the misfiring, I thought it would be a good idea to also perform a compression test.
Safety Car
Joined: May 2001
Posts: 4,741
Likes: 1,639
From: Fairfax Virginia
If you are experiencing a Mis-fire then try opening the hood after dark and look for any shorts. I have seen a noticeable decline in quality of the Spark plug Wires. Ignition coils are another great source of mis-firing as they age.
The Leak Down test is a better diagnostic test but takes more time. They perform a leak down on piston powered aircraft and record the measurements in a log book. This allows you to know when the engine needs attention.
The Leak Down test is a better diagnostic test but takes more time. They perform a leak down on piston powered aircraft and record the measurements in a log book. This allows you to know when the engine needs attention.
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Melting Slicks
Joined: Dec 2009
Posts: 3,224
Likes: 652
From: Thunder Bay
I spin the engine until the numbers stop increasing. FWIW I would be happy with your numbers.
As for leakdown tests, yes we do them in the aviation industry but only because it's a legal requirement. They are notoriously unreliable and are rapidly being replaced by a combination of borescope examination and oil analysis.
If the leakdown test appeals to you, do a search on YouTube. There are a gazillion videos on the subject. The tool is not terribly expensive.
As for leakdown tests, yes we do them in the aviation industry but only because it's a legal requirement. They are notoriously unreliable and are rapidly being replaced by a combination of borescope examination and oil analysis.
If the leakdown test appeals to you, do a search on YouTube. There are a gazillion videos on the subject. The tool is not terribly expensive.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,008
Likes: 4,166
From: US-PNW
Resist the temptation to buy a Harbor Freight leak-down tester. A video-er on YouTube discovered his HF tool was giving significantly low false readings and then someone here on the forum had the same experience. I got the OTC brand and it seems like others have had good experience with it (I've yet to take readings on my engine.)
Race Director
Joined: Nov 2006
Posts: 13,151
Likes: 7,765
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
I work for Harley Davidson. When ever there is a engine warranty issue. They require us to submit both compression and leak down test results.
Generally, one just supports the other. The main difference is a leak down test will point you in the right direction as to where the problem is. Leaking exhaust valves will create noise in the exhaust for instance.
If a compression test shows all is good. I can't really see a good reason to run a leak down test. Unless of course the engine still has a known problem. Like excessive oil consumption.
Generally, one just supports the other. The main difference is a leak down test will point you in the right direction as to where the problem is. Leaking exhaust valves will create noise in the exhaust for instance.
If a compression test shows all is good. I can't really see a good reason to run a leak down test. Unless of course the engine still has a known problem. Like excessive oil consumption.
