Compression testing

[ShaunW]

I want to do a compression test on my 2500 (no issues, just curious) so I thought I'd have a word with you guys first.

I know about disconnecting the fuel pump, removing the rotor arm and holding the throttle open(?) but....

Questions:
1. Hot or cold? It will take me a while so will probably cool down anyway?
2. How long to crank for?  I've seen anything from 5 turns to 10s which seems a lot.
3. With and without a little oil in the bore?  5ml? 10?
4. I know it's about relative readings, but what's considered poor/normal/good?

No rush to reply if you're all busy enjoying the weekend,  it's a job for soon rather than now.

Thanks
 

 

[Pete Lewis]

No need to discon the pump just 

All plugs out , warm or bot engine turns better

With a good battery  and Open throttles 

Crank till gauge reaches its max aboit 4 or 5 turns 

Expect old rule was atmosheric psi x comp ratio    eg 14.7 x 8.5 = 125  

Many things will vary this approximation  all should be within 10psi of each over

[dougbgt6]

As to adding oil in the bores, this is referred to as dry or wet testing. Do it dry first, all within 10? Hunky dory. If some are low try the oil, if this makes a big difference then something more seriously wrong, rings? 

db

[DanMi]

To add to Dougs comment, If adding oil to the bores increases the compression this indicates a piston ring issue as the oil helps seal them, if it makes no difference then valves are more likely the problem. But of all within 10% then happy days

[ShaunW]

6 hours ago, Pete Lewis said:

Expect old rule was atmosheric psi x comp ratio    eg 14.7 x 8.5 = 125  

That's the sort of thing that's really useful to know. So much more useful than just quoting a number.

Again many many thx to everyone for the feedback. 

(gripe of the day....new carpet arrived today, hooray, it's the wrong *&!@#**%ing colour, booooo!!! I've got such a love hate relationship with online shopping.)