Hi. I'm rebuilding my 2L type 4 engine (CJ). So far I've assembled a short block with a new standard cam. I'm now at the point of fitting the pistons I have a new pair of AMC heads and a set of 96mm AA pistons. According to the specs, the head combustion chamber is 55cc and the piston "valve pocket" is 7cc Feeding in the following values: Stroke - 71mm Bore - 96mm Combustion chamber cc - 62 Compression ratio - 8:1 ... gives a deck height of 1.58mm Do those values look sensible?
... according to the spec here - https://aapistons.com/collections/v...che-type-4-96mm-piston-and-biral-cylinder-set
As Geoff will agree ideal deck height is 1-1.5mm any bigger & you loose the squish effectiveness. I would say a cr of up to 8:5 to 1 would be fine.
Don’t believe what you read on the internet; measure the valve pocket cc. And the combustion chamber volume.
This^ Ideal deck height is 1-1.5mm but up to 1.6mm is okay. 8:5:1 CR is a bit high for a standard cam but if it pinks use higher octane fuel.
Valve pocket sanity check. A 7cc pocket would be 10mm wide, 10mm deep and 70mm long which it blatantly is not. I suspect 3cc is over estimating too. Measure the width, deepest depth, length, multiply it out and halve it - you'll be a lot closer than AA. AA could make a massive improvement to their site by accurately quoting the ccs of heads and pistons.
8.4:1...if the combustion chamber is really 55cc. Just about ok. You could lower it a bit by unshrouding the inlet valves. Even 1cc gets you down to 8.3:1, 2cc down to 8.2:1. I think I'd be doing this rather than a higher deck or 8.5-8.4:1.
Definitely increase the combustion chamber volume and not the deck - if you have to. But until you measure the valve pockets and chamber you don’t know what the CR will be.
I measured as carefully as I could. The combustion chamber itself is 55cc. However the additional volume of the flat area up to the "step" of the barrel mating surface is 3cc Do I use 58cc as the combustion chamber total, or take it as 55cc and add the 0.8mm "step" height to the deck height?
Playing with the calculator, to use 1.5mm deck height with a compression ratio of 8:1 would need a combustion chamber of 63cc Do I really need to start modifying brand new heads to make the numbers work?
Is that step in the head included in your deck height ... and yes step volume included in calcs..all volume left at the end of the compression stroke is your compressed volume.. add the swept volume to it to get starting volume Edit.. Just re read I'd go take the 55 combustion chamber and add the 0.8 to your deck height (top of cylinder to top of barrel)..make sure it's then included in your deck height volume.. gives a more realistic total deck height number. Sent from my SM-G930F using Tapatalk
What cam was it again.. can't find it.. And don't forget standard shop barrel shims step in .25mm's Sent from my SM-G930F using Tapatalk
"Mechanical camshaft 1700-2000cc Standard" from Heritage https://www.heritagepartscentre.com/uk/021109021cb-mechanical-camshaft-1700-2000cc-standard.html