Help me determine my compression ratio |
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Help me determine my compression ratio |
Cevan |
Apr 14 2010, 11:45 AM
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#1
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Senior Member Group: Members Posts: 1,079 Joined: 11-December 06 From: Western Massachusetts Member No.: 7,351 |
So I've started my engine build and have the engine mocked up to measure the deck height. My preliminary measurements are around .030. I've got to do some juggling around of components to get them more equal to each other. Here's what I've got:
head volume: 58.2 CCs bore: 96mm stroke 71mm Based on the cam I'm using here's what I get for different deck heights: .040 static C/R 8.84:1 dynamic C/R 7.76:1 .050 8.63:1 7.57:1 .055 8.53:1 7.48:1 .060 8.43:1 7.4:1 The engine is going in a 914 (obviously) that will be used for spirited street driving. I have 93 octane gas available here. The car will be driven between sealevel and 1000'. I'm using webers with SSI H/Es and a Bursch muffler. |
Dave_Darling |
Apr 16 2010, 09:02 AM
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#2
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914 Idiot Group: Members Posts: 15,048 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
There is a formula that takes valve opening times into account for dynamic CR. It doesn't take into account chamber shape or any of a zillion other variables, but at least it's a number... (IMG:style_emoticons/default/wink.gif)
The conventional wisdom I've heard is to shoot for .040" clearance between the pistons and the heads. There are people who run less, but they really REALLY know what they're doing... The advice most of them have for the Rest Of Us seems to be .040". --DD |
Cevan |
Apr 16 2010, 10:15 AM
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#3
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Senior Member Group: Members Posts: 1,079 Joined: 11-December 06 From: Western Massachusetts Member No.: 7,351 |
There is a formula that takes valve opening times into account for dynamic CR. It doesn't take into account chamber shape or any of a zillion other variables, but at least it's a number... (IMG:style_emoticons/default/wink.gif) The conventional wisdom I've heard is to shoot for .040" clearance between the pistons and the heads. There are people who run less, but they really REALLY know what they're doing... The advice most of them have for the Rest Of Us seems to be .040". --DD If I use my lower number (.026), then I would need a .014 shim set. The best I could find were shims that come in sizes every .010, so a .020 shim gets me to .046. |
ME733 |
Apr 16 2010, 03:31 PM
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#4
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Senior Member Group: Members Posts: 842 Joined: 25-June 08 From: Atlanta Ga. Member No.: 9,209 Region Association: South East States |
There is a formula that takes valve opening times into account for dynamic CR. It doesn't take into account chamber shape or any of a zillion other variables, but at least it's a number... (IMG:style_emoticons/default/wink.gif) The conventional wisdom I've heard is to shoot for .040" clearance between the pistons and the heads. There are people who run less, but they really REALLY know what they're doing... The advice most of them have for the Rest Of Us seems to be .040". --DD If I use my lower number (.026), then I would need a .014 shim set. The best I could find were shims that come in sizes every .010, so a .020 shim gets me to .046. .....................well there you have it...Use .020ths of shimms...to achieve .046ths inch of P/H clearance....you will be safe even from a 7500rpm missed shift.(unless the valves float). |
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