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Old 08-06-2005, 04:23 PM
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1970 SB rebuild

I am rebuilding 1970 SB - reportedly 300HP engine when new. It has heads marked 186 - valves measure close to 2" but I haven't taken them out - and have "double bump" casting marks on end. The head's combustion chamber is smaller than standard due to "bump" in spark plug area - I believe they are 63 cc. Orginal pistons are flat top w/ valve reliefs.
Engine is planned for 65 Belair - just a daily driver.
My question:
I am trying to decide on pistons - should they be flat top or dished. From what I have been able to find on 1970 300HP 350 - the compression ratio was a littel over 10:1. The engine kits on summitracing show around 9.3:1 for a flat top piston. I have been told compression ratio (over 9:1) may be too high with flat top and I would need to run premium fuel. Does anyone have experience or opinion?
Also - any opinion on cam would be appreciated.

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Old 08-06-2005, 06:04 PM
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The 186 castings should be 64cc...or thereabout depending on whether they've been milled. They have the little/short humps. Good heads.

With flattop pistons, I thought the compression was a little higher, more like 9.7:1 static, using 64cc heads. That will put you in the premium gas range. If you are trying to stay with regular gas you probably need to either find a larger chamber head or go to a dished piston.
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Old 08-06-2005, 06:37 PM
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3927186....69-70...302/350......Camel hump,64cc chambers,accessory holes.
flat top pistons will give you slightly over 10 to 1 compression.

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Old 08-07-2005, 11:31 AM
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Thanks for feedback. I believe they are 64 cc heads, they haven't been machined and almost everything I read shows 186 heads as 64cc. Bracketeer - Can you point me towards a source for info on engine specs like you listed?
I understand that dynamic compression changes with cams - I'm thinking about a cam with a little more lift (I've heard them called torquers?) - will that help or hurt?
Also - I've read some on flat top pistions may be less likely to ping due to squish/turbulence in combustion chamber than dish pistons with lower compression ratio? Anyone have experience on this?
I appreciate the help!
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