Author Topic: compression test.  (Read 238 times)

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Re: compression test.
« Reply #15 on: May 23, 2020, 11:09:09 AM »
One for the scientists:

If compression ratio is 9.4:1 and sea level air pressure is 14.7psi

Why do they specify 170psi?

14.7x9.4 = 138


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Thought it's because compression ratio is calculated from swept piston volume compared to combustion chamber size. With the psi data having to add the compression ratio X the remaining combustion chamber volume + the "138" psi to give you a theoretical total psi reading above 170..........to which you have to subtract any mechanical sealing losses.

Making CR the figure that is used by the designer to burn the fuel as they want it to.

The psi figure is effectively the dynamic total achieved within the components you've built.  Hence not hitting target on psi illustrates something is not working as effectively as it should.
That could be from a race type cam timing and duration for which you may have a designated CR but a different psi from standard equipment  because of  valve overlap etc.

In short you'd probably need a different psi reading for a hot cam, which could also vary based on how you time it too.

 

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