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General Minitrail Talk
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'72 CT70 winter restore project
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<blockquote data-quote="69ST" data-source="post: 184804" data-attributes="member: 5"><p>To paraphrase fatcaaat, TB stroker cranks all use 12v/"long rod" compression height pistons. Go with that, then match the piston dome to your head..."big/6v" "small/12v". </p><p></p><p>I'll add one side note. On a stock-bore stroker motor, static compression ratio tends to be high, since the combustion chamber volume remains the same, but displacement increases. All else the same, adding a 51mm crank to a stock 6v motor will increase static CR from 8.8 to 10.75:1. That's a healthy increase and pairs well with a "hotter" cam that has more overlap (which bleeds-off more cylinder pressure). Each 1-point increase in CR can be expected to increase efficiency by 5%. It's all about balance and approaching an engine as a balanced system makes things easier to understand.</p><p></p><p>Pairing compression ratio and cam profile is a longer discussion than we need to have, at least for now and in practical terms. The point I was going for is that a small dome piston, would drop CR back <em>toward</em> the stock 8.8 and be perfectly livable. Stock combustion chamber volume, at TDC is 8.18cc (nominal). With 88cc displacement and the "6v/big" dome piston, you'd have 10.75:1. If one is "stuck" for a piston choice, it very well may be possible simply use a "12V/small dome" piston with the 51mm stroker crank and 6v head. The small dome piston has a nice "squish band" that the big dome piston lacks so, while CR would be lower, that squish band makes the combustion chamber more efficient...a nice tradeoff, if there must be a compromise.</p></blockquote><p></p>
[QUOTE="69ST, post: 184804, member: 5"] To paraphrase fatcaaat, TB stroker cranks all use 12v/"long rod" compression height pistons. Go with that, then match the piston dome to your head..."big/6v" "small/12v". I'll add one side note. On a stock-bore stroker motor, static compression ratio tends to be high, since the combustion chamber volume remains the same, but displacement increases. All else the same, adding a 51mm crank to a stock 6v motor will increase static CR from 8.8 to 10.75:1. That's a healthy increase and pairs well with a "hotter" cam that has more overlap (which bleeds-off more cylinder pressure). Each 1-point increase in CR can be expected to increase efficiency by 5%. It's all about balance and approaching an engine as a balanced system makes things easier to understand. Pairing compression ratio and cam profile is a longer discussion than we need to have, at least for now and in practical terms. The point I was going for is that a small dome piston, would drop CR back [I]toward[/I] the stock 8.8 and be perfectly livable. Stock combustion chamber volume, at TDC is 8.18cc (nominal). With 88cc displacement and the "6v/big" dome piston, you'd have 10.75:1. If one is "stuck" for a piston choice, it very well may be possible simply use a "12V/small dome" piston with the 51mm stroker crank and 6v head. The small dome piston has a nice "squish band" that the big dome piston lacks so, while CR would be lower, that squish band makes the combustion chamber more efficient...a nice tradeoff, if there must be a compromise. [/QUOTE]
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