Type 4 rebuild - deck height and compression ratio

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?

 

Yes, but the valve pocket is probably less than 7cc and therefore the CR will be higher.

 

... 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.

 

I will be checking

 

Going by the specs, a 1.5mm deck height would give 8.1:1 compression ratio

 

.. the AA ones I had were specced at 7cc.. 3 is closer..

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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.

 

2.7cc ...give or take 0.1

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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?

 

The piston valve pocket is 2.4cc

 

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.

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55cc plus your 0.8mm added into your deck height if you haven't already.

 

What cam was it again.. can't find it..

And don't forget standard shop barrel shims step in .25mm's

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"Mechanical camshaft 1700-2000cc Standard" from Heritage

https://www.heritagepartscentre.com/uk/021109021cb-mechanical-camshaft-1700-2000cc-standard.html