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Registered
Join Date: Feb 2002
Location: Chalfont Pa
Posts: 1,548
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How to measure compression?
I've got some aris pistons which look to have high domes and large pockets for clearance. How would you go about measuring for compression ratio? I will have the engine apart soon and will have to do this. How high can I go with compression before it will blow up? The engine will be twin plugged. At what compression do you need to run race gas? If I decide to run race gas how high can I go with the compression? How much better will the cooling system have to be in an engine like this?
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11-12-2010, 04:57 PM
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I would rather be driving
Join Date: Apr 2000
Location: Austin, TX
Posts: 9,108
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here is my method for measuring the dome volume on a piston.
Install top two rings on piston. smear liberal coat of grease around top ring. insert piston into cylinder invert cylinder on top of a flat hard surface (I use a granite tile) push piston through cylinder until top of crown touches said hard flat surface flip piston/cylinder combo upright Measure the depth of the piston outer edge from lip of cylinder sealing surface Make plastic cover that is larger than the diameter of the piston sealing surface smear grease on the perimeter of the cylinder sealing surface place plastic cover with two holes drilled at edges of cylinder bore on top of cylinder use a buret or graduated cylinder to fill the void area under the cover record volume of liquid used to fill void area (Be ACCURATE) Calculate the swept volume of the piston dome (use piston depth and cylinder diameter) subtract measured volume (using the liquid) from the swept volume to determine the piston some volume Repeat volume measuring process for cylinder heads. Using crank stroke, piston diameter, dome volume, deck height (installed) and cylinder head volume calculate the compression ratio. Too many variables to answer your questions regarding maximum compression ratio. Type of cam, temperatures during driving, street or track, etc must be factored in. I would err on the low CR value for longevity and peace of mind. Yes, you will need more cooling. however, modern technology for controlling mixture (EFI) and internal engine coatings can help reduce the thermal dissipation requirements of the engine.
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Jamie - I can explain it to you. But I can not understand it for you. 71 911T SWT - Sun and Fun Mobile 72 911T project car. "Minne" - A tangy version of tangerine #projectminne classicautowerks.com - EFI conversion parts and suspension setups. IG Classicautowerks |
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11-12-2010, 05:46 PM
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Registered
Join Date: Feb 2002
Location: Chalfont Pa
Posts: 1,548
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Could you repeat that? Only kidding. Wow not so easy.
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11-13-2010, 03:32 AM
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Registered
Join Date: Feb 2002
Location: Chalfont Pa
Posts: 1,548
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Will be running either ge60 cams expensive. Or will use 911s cams cheap. So which ones would you run for Drivers Education and autocross events?
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11-13-2010, 03:35 AM
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Registered
Join Date: Feb 2002
Location: Chalfont Pa
Posts: 1,548
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How will they effect compression?
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11-13-2010, 03:35 AM
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Registered
Join Date: Oct 1999
Location: Marietta, GA
Posts: 809
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The cam does not effect static compression. It is simply an equation of volumes. You do have to consider the cam when you get to checking piston to valve clearance. Check out this post for more info.
http://forums.pelicanparts.com/911-engine-rebuilding-forum/555996-piston-valve-clearance-solex-cams.html |
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11-13-2010, 05:05 AM
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Registered
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Porsche 356 Registry: Calculating Compression Ratios
How to check engine compression ratio Calculating an Engines Compression Ratio RSR Static Compression Ratio Calculator United Engine & Machine Co. Incorporated
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78’ SC 911 Targa - 3.2SS, PMO 46, M&K 2/2 1 5/8” HEADERS, 123 DIST, PORTERFIELD R4-S PADS, KR75 CAMS, REBEL RACING BUSHINGS, KONI CLASSICS |
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11-13-2010, 10:13 AM
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Stella
