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Old 02-05-2020, 11:10 PM   #1
Chevy Chase
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Question How to determine compression ratio?

Drove my avatar 57 Wgn to a friend's house. He came out to look at it for the first time. He is a machinist. He drives a '70 Challenger 340 bored and stroked to 416 c.i, TKO, FiTech. He listened to the '57 wgn's 350 engine with FiTech/lumpy cam/alum heads, headers. He asked what the compression ratio is. I said I don't know anything about this engine. How do I determine the c.r. without disassembling the engine? He pulled six books from his library but couldn't find the answer. Is their software that can determine c.r.? I assume bore, stroke, combustion chamber cc's, piston specs, will be required.
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Old 02-05-2020, 11:41 PM   #2
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You can't really determine compression ratio on an unknown engine without having it apart. The best effort would be by making an educated guess knowing the engine size and what heads are on the engine.

The compression ratio is algebraically CR = (V + CV)/CV where V is the displacement of one cylinder and CV is the clearance volume.

The clearance volume is the combustion chamber volume + the volume of the head gasket + the volume due to deck clearance + the volume from the piston deck down to the first ring +/- the volume of the valve reliefs and/or piston dome. Each of these can be measured in some way on a partially assembled engine.

For a running engine, you just use an estimate for the clearance volume based on what you know and making assumptions from there.
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Old 02-06-2020, 12:02 AM   #3
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Just a quick guess using Boyles law. P1 V1 = P2 V2
I think this would get an estimate without taking the engine apart. There are some good methods on Google but the engine head is off.
With opened spark plug, pressure at bottom of stroke (P1) is atmospheric 14.7 psia. moving to TDC get the pressure (assuming no ring leakage, or use the highest pressure).
Knowing stroke and bore use this volume for V1, then calculate V2. the ratio of V1 divided by V2 is a close approximate of cr.
or try this.
http://www.csgnetwork.com/compcalc.html
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Old 02-06-2020, 12:56 AM   #4
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Thank you for the response to my questions. The Compression Ratio (CR) Calculator more than met my expectations. Had a feeling their was a calculator where values are entered to determine compression ratio. Cheers
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Old 02-06-2020, 01:08 AM   #5
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Static compression ratio is bovine fodder for whizzing rights @ the local watering hole.
To run on pump gas without detonation you need a maximum dynamic compression ratio of 8 to 1 + a curved distributor or an FI system that incorporates a knock sensor if your close to the 8 to 1 number.

You engine does not start making compression until the number of crank degrees of rotation after bottom dead center (BDC) of the intake stroke that the intake valve is fully seated. Valve timing varies by cam manufacturer & grind = a difference in DCR for a given engine is dependent on camshaft grind.

An attempt to figure static compression without knowing the engine internal sizing is nothing more than a SWAG.

If it runs on pump gas without detonation your good to go.

DCR calculator
http://www.wallaceracing.com/dynamic-cr.php

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Old 02-06-2020, 06:15 PM   #6
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Churchkey, Thanks for your colorful reply. The verbiage made me chuckle. "A SWAG" scientific wild-ass guess.
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Old 02-06-2020, 06:41 PM   #7
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Quote:
Static compression ratio is bovine fodder for whizzing rights @ the local watering hole.
I have to disagree. For me, that's what "dynamic compression ratio" is.

What makes "pump gas" need to be part of the discussion? One could say that compression ratio (or "dynamic compression ratio") could help you pick the proper fuel, not the other way around.

And just how do you compute the "dynamic compression ratio" except to enter numbers in an internet "calculator".
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Old 02-08-2020, 09:48 PM   #8
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Checking DCR manually is accomplished during short block assembly.

Attach a degree wheel to the crank, find TDC using the positive stop method, zero the degree wheel.
Measure deck height, rotate the crank 180* + the number of degrees on your cam card
when the intake valve closes after bottom dead center (ABDC). If advancing or retarding the cam
adjust accordingly. Use a depth mic to determine where the piston is @ in the cylinder &
subtrack the deck height measurement from the depth mic number = the actual stroke that your
engine is operating with. Calculate the compression ratio using the new stroke # to determine
the actual compression ratio your engine is operating at = DCR.

An easy to understand example: If a 4" stroke & the intake valve were to close 90* after bottom
dead center the piston would be 1/2 way up the cylinder before compression begins.
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Old 02-09-2020, 02:07 AM   #9
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I would just do a cranking compression test to determine cylinder pressure and adjust the tuneup accordingly.
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