Compression test results

[JG1966]

I just did a compression test on my 340 for the first time since getting it stroked to a 416 about three years ago. Engine runs great, but I was bored and figured I'd check it out before winter. I also plan on doing a leakdown test in the spring (need an air compressor)
Test was performed dry with engine warm. Here are the results
1 -- 180
2 -- 179
3 -- 168
4 -- 180
5 -- 175
6 -- 179
7 -- 177
8 -- 177
The No. 3 cylinder is the only outlier. It's 6.7% below the top number (I tested it twice). Is that acceptable or am I going to spend all winter being miserable? Thanks

 

[Bird 426]

Go out and drive it and recheck. It will probably change.

 

[Doorkicker]

I just did a compression test on my 340 for the first time since getting it stroked to a 416 about three years ago. Engine runs great, but I was bored and figured I'd check it out before winter. I also plan on doing a leakdown test in the spring (need an air compressor)
Test was performed dry with engine warm. Here are the results
1 -- 180
2 -- 179
3 -- 168
4 -- 180
5 -- 175
6 -- 179
7 -- 177
8 -- 177
The No. 3 cylinder is the only outlier. It's 6.7% below the top number (I tested it twice). Is that acceptable or am I going to spend all winter being miserable? Thanks

Stupid question, no insult intended... but... did you take all the sparkplugs out before testing, or removed one at a time?

 

[JG1966]

Go out and drive it and recheck. It will probably change.

Thanks Bird. The engine was warm but it wasn't operating temp. Removing the No. 5 and No. 7 plugs is tricky when the headers are scorching hot. I'll take her for a spin and get her nice and hot and then check No. 3 again.

 

[JG1966]

Stupid question, no insult intended... but... did you take all the sparkplugs out before testing, or removed one at a time?

All the plugs were out and the throttle was open

 

[icetech]

Just curious, did you check compression when the motor was first built? and i was always told 10% variation is fine from high to low.. but i'm sure someone else will chime in with more

 

[JG1966]

Just curious, did you check compression when the motor was first built? and i was always told 10% variation is fine from high to low.. but i'm sure someone else will chime in with more

No. I never did. It has always run great since getting it built. I only did it today because I was restless and bored and wanted to do something on what might be our last warm day for a while.

 

[Sam69sat]

I wouldn't give it another thought. Even if #3 was 180 you'd never notice.

 

[Strato58]

If it runs good that means it is good. Don't be miserable over this because chasing numbers don't mean much. IMHO.

 

[R413]

Does it have a solid lifter camshaft? How is the valve adjustment?
Next time go for a walk or something! LOL

 

[JG1966]

Does it have a solid lifter camshaft? How is the valve adjustment?
Next time go for a walk or something! LOL

It's hydraulic. I haven't adjusted the valves in a while, but I never really need to.

 

[icetech]

It's hydraulic. I haven't adjusted the valves in a while, but I never really need to.

If you are worried i would grab a $25 bore scope and just take a quick look at the cyl to make sure it's not getting eaten up or anything.. just for peace of mind.. I love my bore scope.. one of my favorite tools now

 

[Rebelrouser]

I just did a compression test on my 340 for the first time since getting it stroked to a 416 about three years ago. Engine runs great, but I was bored and figured I'd check it out before winter. I also plan on doing a leakdown test in the spring (need an air compressor)
Test was performed dry with engine warm. Here are the results
1 -- 180
2 -- 179
3 -- 168
4 -- 180
5 -- 175
6 -- 179
7 -- 177
8 -- 177
The No. 3 cylinder is the only outlier. It's 6.7% below the top number (I tested it twice). Is that acceptable or am I going to spend all winter being miserable? Thanks

The compression check is fine. You are correct that a leakdown test is much more accurate. And as suggested on a performance engine I always do a leak test and compression test when the engine is fresh as a way to track what is going on with it down the road.

 

[Runcharger]

I have 5 classic motorcycles. I always do a valve set on them and ensure the carbs are as good as they can be, they run good so I don't do compression checks on them for this very reason. If they run good, be happy, sometimes ignorance is bliss they say.

 

[JARRETT LYNCH]

Stupid question, no insult intended... but... did you take all the sparkplugs out before testing, or removed one at a time?

Why would that matter other than maybe when plugs out you get a higher crank rpm?