# Compression test

**URL:** <https://community.cartalk.com/t/compression-test/155105>\
**Category:** Power/Fuel\
**Created:** [December 21, 2019, 3:10pm UTC](https://community.cartalk.com/t/compression-test/155105 "2019-12-21T15:10:12Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![blackbelt99\_154374](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@blackbelt99\_154374](https://community.cartalk.com/u/blackbelt99_154374)\
**Post date:** [December 21, 2019, 3:10pm UTC](https://community.cartalk.com/t/compression-test/155105/1 "2019-12-21T15:10:12Z")

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Was discussing compression testing on another forum and nobody could answer this question.  
Let’s say a car has a compression ratio of 10:1 using 14 psi as ambient air pressure, how can a car produce over 140 on a compression test?  
Even if the exhaust valve closes early, creating pressure, wouldn’t that pressure be lost when the intake valve opens?

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**Author:** ![circuitsmith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/circuitsmith/32/40376_2.png) [@circuitsmith](https://community.cartalk.com/u/circuitsmith)\
**Post date:** [December 21, 2019, 3:29pm UTC](https://community.cartalk.com/t/compression-test/155105/2 "2019-12-21T15:29:09Z")

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> [@blackbelt99\_154374](#):
>
> how can a car produce over 140 on a compression test?

Adiabatic compression.  
When a gas is compressed its temperature rises, which leads to more pressure.  
When the compression is done quickly, little heat is lost to the chamber walls.

If the compression were done slowly and provision were made to remove heat and keep the gas at the same temperature then it’s called isothermal compression.  
In that case 10:1 compression of 14 psia would result in 140 psia (psi absolute.)

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**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\
**Post date:** [December 21, 2019, 4:11pm UTC](https://community.cartalk.com/t/compression-test/155105/3 "2019-12-21T16:11:28Z")

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+1 to @circuitsmith Add one more… air has weight, the intake runners are configured to use that weight and the inertia to increase the charge pressure.

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**Author:** ![blackbelt99\_154374](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@blackbelt99\_154374](https://community.cartalk.com/u/blackbelt99_154374)\
**Post date:** [December 21, 2019, 4:30pm UTC](https://community.cartalk.com/t/compression-test/155105/4 "2019-12-21T16:30:05Z")

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Technically, I believe the runners are there to eliminate turbulence. Which would reduce air flow. So you’re not really “ gaining” any charge.

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**Author:** ![oldtimer-11](https://avatars.discourse-cdn.com/v4/letter/o/e79b87/32.png) [@oldtimer-11](https://community.cartalk.com/u/oldtimer-11)\
**Post date:** [December 21, 2019, 4:33pm UTC](https://community.cartalk.com/t/compression-test/155105/5 "2019-12-21T16:33:50Z")

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Wnd the length of the intake runners is calculated to decide at what rpm the cylinder is most filled. The long intake runners of the Mopar slant six and cross ram V8s put the peak power at the low and midrange rpms where most street driving is done.

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**Author:** ![circuitsmith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/circuitsmith/32/40376_2.png) [@circuitsmith](https://community.cartalk.com/u/circuitsmith)\
**Post date:** [December 21, 2019, 11:23pm UTC](https://community.cartalk.com/t/compression-test/155105/6 "2019-12-21T23:23:06Z")

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@Mustangman has it right.  
However, the inertia effect will be small at the engine cranking speed a compression test is done.

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**Author:** ![Rod-Knox](https://avatars.discourse-cdn.com/v4/letter/r/a183cd/32.png) [@Rod-Knox](https://community.cartalk.com/u/Rod-Knox)\
**Post date:** [December 22, 2019, 12:31am UTC](https://community.cartalk.com/t/compression-test/155105/7 "2019-12-22T00:31:38Z")

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Cam timing takes advantage of intake inertia

![image](https://global.discourse-cdn.com/cartalk/original/3X/e/9/e9b453ff2b7e9b4d3a930468037121f1f2223d4d.png)

the intake valve remains open well past Bottom Dead Center (51\* here) and even though the exhaust valve is open air continues to move into the cylinder. But definitively the effect is directly correlated to increasing rpms.