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Performing A Compression Test

salemf

By salemf,
in General Automotive Discussion

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**RB2530** Proficient

**RB2530**

Posted
Posted
i can make up abbreviations and pretend i know stuff too... LOL.

with the throttle closed while you perform a compression test your pistons will try draw air in while the inlet valves are open and cause resistance against the rings (vacuum) while the motor turns. so if the motor begins its compression stroke with -10psi your compression test will show up 10psi lower. pretty much what Paul said but less technical.

NTP is a term they use in physics, here we'd just use "0 vacuum" as the air does not need to be at a 'normal' temperature and the air entering the combustion chamber isnt actually pressurised. it is the chamber that is causing the pressure.

Zero pressure (compared to atmospheric) is still a pressure and everything has a temperature...On earth anyway...

Also....awww shit, dont have time and I can't be bothered....

**RB2530** Proficient

**RB2530**

Posted
Posted
right.. so NTP is needed for a compression test? and having your throttle closed varies the temp? and WOT fixes this?

No...NTP is not required...But having a standard process such as WOT is and in the event that an engine manufacturer wants to test for and publish target comp test data they would do it in an environment close to NTP or STP.

Having throttle closed does mean air is hotter during comp test (if comp test done with engine warmed up). Simple reason is with TB closed not as much additional air is drawn into the intake. So majority of air drawn into cylinder is air that has been sitting in the plenum and come up to engine temp.

With WOT ambient air is drawn in engine easily...

Also by way of energy balance, some of the energy consumed in producing the vacuum in the closed TB case would dissipate as heat in the air drawn into the cylinder.

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