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verifying compression ratio of an already built engine?
Is there an accurate way to check compression ratio of an assembled engine?
How accurate would it be and could someone here offer a step by step process? Thanks in advance,
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-Jay '74 Mexico Blue 911 3.0 EFI (Fast and Loud) '70 914/6 Race Car (Faster and Louder) '71 73RSR tribute vintage race car 3.0 '68 SWB 911T "RENNRAT" 2.8 twin plug/915 gearbox '81 Magenta IROC clone in progress 3.6 varioram/G50 |
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abit off center
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In the car or on a stand?
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______________________ Craig G2Performance Twinplug, head work, case savers, rockers arms, etc. |
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Either and/or both. I've got one 2.8 litre engine on a stand that I'd like to verify and another in a car.
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-Jay '74 Mexico Blue 911 3.0 EFI (Fast and Loud) '70 914/6 Race Car (Faster and Louder) '71 73RSR tribute vintage race car 3.0 '68 SWB 911T "RENNRAT" 2.8 twin plug/915 gearbox '81 Magenta IROC clone in progress 3.6 varioram/G50 |
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Puny Bird
Join Date: Nov 2002
Location: Port Hope (near Toronto) On, Canada
Posts: 4,566
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Tilt it up fill with measured fluid. Oil will work but attacks rubber used in syringes.
When done tilt the engine upside down to drain the oil. Then do the math putting the cc measurement where it says deck height. don't know about in car.
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'74 Porsche 914, 3.0/6 '72 Porsche 914, 1.7, wife's summer DD '67 Bug, 2600cc T4,'67 Bus, 2.0 T1 Not putting miles on your car is like not having sex with your girlfriend, so she'll be more desirable to her next boyfriend. |
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Registered
Join Date: Jul 2012
Location: Simi Valley, Ca.
Posts: 265
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compression ratio
Tilt the engine until the spark plug is pointing straight up. Put the cylinder at top dead center. Fill the cylinder with a burette until the fluid is at the bottom of the spark plug hole. Calculate displacement of one cylinder and add the volume of the combustion chamber that you just measured. Divide this total by the same combustion chamber number and you will have the compression ratio.
Bob B |
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Registered
Join Date: May 2004
Location: Boulder, Colorado
Posts: 7,275
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I made a fitting topped by a clear plastic hose which screwed into the spark plug hole, and filled through that. That way I could fill to a mark I could clearly see, and not guess when I was just at the bottom of the plug hole. And I could rotate the engine a bit to persuade myself that I hadn't trapped any air. I had measured what it took to fill the fitting up to the mark, so that got subtracted.
There is also a nifty instrument which will give accurate results, even with an engine in the car. It is called the Whistler, and it works by blowing air into the spark plug hole at TDC firing (both valves closed). It reads the audio frequency produced (remember pipe organs from high school physics). That gives you the same result as the oil method for what the fully compressed volume is. The instrument is adjusted to what the displacement is, so it reads out as a compression ratio. Over $1,000, if still available. A fair number of shops have this, as it works on 400 cubic inch V8s as well as on much smaller cylinders. Maybe Ernie J has one? Or knows someone who does up there? |
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Registered User
Join Date: Feb 2013
Posts: 8
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divide compression reading by 1 atmosphere or 14.7 psi. should put you in the ballpark
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