 Dynamic CR Opinions - WebCam 86b/86c |
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| yeahmag |
 Jul 1 2026, 03:20 PM
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#1
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Advanced Member  Group: Members Posts: 2,484 Joined: 18-April 05 From: Pasadena, CA Member No.: 3,946 Region Association: Southern California 
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My next autocross motor is coming together, but I'm a little hung up on the CR. For your calculating pleasure:
Bore: 96mm (3.77953") Stroke: 78mm (3.07087") Deck Height: 0.045" Cylinder CC: 50cc Connecting Rod Length: 5.325" IVC @ .050": 54 degrees (WebCam 86b/86c) Gas: 91 octane E10 CA gas I get: STATIC Compression Ratio:10.69:1 DYNAMIC Compression Ratio:8.2:1 To get it down to roughly 9.5:1 static and 7.5:1 dynamic CR I need to: * Find 6cc - 8cc in the head or dish the piston * Go to a HUGE DH of 0.085", which I find completely unacceptable Any opinions on 1) how hard it will be to find 8cc in the heads or 2) if putting a small dish in the piston? My calculations have the dish at something like: * Dish Diameter: 82mm * Dish Depth: 1.7mm Thanks! |
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| worn |
Jul 1 2026, 04:59 PM
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#2
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Winner of the Utah Twisted Joint Award Group: Members Posts: 3,616 Joined: 3-June 11 From: Madison, WI and North Bend WA Member No.: 13,152 Region Association: Upper MidWest
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I have built engines but have much to learn. Why is extending the length of the cylinder by the use of spacers or gaskets so bad?
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| emerygt350 |
Jul 1 2026, 05:44 PM
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#3
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Advanced Member Group: Members Posts: 3,663 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
Quench I believe is the term. You really want no more than .04 I think. I am not too good with the reasons myself but when I was building mine I asked people that know... They will respond.
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| friethmiller |
Jul 1 2026, 09:01 PM
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#4
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Senior Member Group: Members Posts: 1,352 Joined: 10-February 19 From: Austin, TX Member No.: 22,863 Region Association: Southwest Region
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Head cc’s seem low. My 1.8L P&P heads are 60ccs.
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