With flat top pistons and 8.1:1 CR your deck should be 1.93mm. You already have 0.33mm so the cylinder spacer needs to be 1.6mm (1.93 – 0.33). A 2mm spacer would be 7.8:1 CR with a 2.33mm deck. For good engine efficiency and squish you should keep the deck between 1mm and 1.6mm - you really need pistons with a shallower dish.
You also need to avoid the death zone deck which is 2-3mm... so they say. Smaller gets the squish, larger (like mine at 3.75mm) turns your heads into semi-hemi which allows combustion in the deck area. Mine is huge for many reasons but the primary causes are using 1700 heads with 104mm pistons = 2413cc and pretty well impossible to have the tight deck and a reasonable CR. If I had set my deck at 1.5mm my CR would have been 11:1, or to get it down I would have to increase the chambers by 20cc which just isn't possible. In hindsight building a 2.4l engine this way was daft, but it was also cheap. Life's a compromise. I just searched back and my CR is 8.7:1, technically too high for a stock cam but hasn't been a running problem even with cheapo supermarket fuel. Not recommended, but I thought you'd be interested in just how high you can get away with and might make you feel better if you end up a little over 8:1 to get the squish.
Hmm, Swish.....something else I know nothing of. Hi ho, more learning. I did see your original posting, and ideally would prefer to make this with AA/Mahle dished pistons (cos that's what I have) so your calculations would work for me. BUT how did you achieve deck height of 0.8? And how about your combustion chamber of 68?? Are you running stock heads with what seems to be 56 cc and then allowing 12cc for the dish?
Well this is getting more entertaining....there's a death zone! Genuinly getting quite into this, but appreciate it's a world of learning! Thanks for the insight.
There is - it's where enough unburnt fuel can be trapped around the perimeter to explode on compression and knock. Big deck like mine allows combustion, smaller deck squeezes it to the centre... or something like that. The smaller deck is the winner for several reasons - keeps heat away from the top of the cylinder, causes the fuel to tumble in the centre and mix better, keeps the burn centred near the spark plug which makes it faster. Having wide pistons and a big deck like mine is the worst for efficiency. Mine only has extra grunt due to the capacity. Mr Westy's will be much more powerful and more efficient at around the same cc's as he has smaller pistons, longer stroke and a tight deck. But I'm guessing it cost him at least twice as much to build. Sadly when I "designed" mine I had a short term cash shortage that was actually gone by the time it was built, but it was too late to start again by then, I wanted it finished and running.
Just fished out my notes as got me thinking about my calculations. I got 14cc in the piston dish & 54cc in the head.
Hmmm, so pretty well same as mine, but you just got much better deck height. Makes me wonder why my new ones are so bad, and my old ones are marginal! I get there are tolerances in the build, but these new ones have gone the wrong way! Hmm What size gasket/spacer do you have between your case and cylinder? Mine measure in at 0.17mm so my deck height without them in would actually be 0.16mm!!
The case comes into this too. If it's been decked (machined) to renew the sealing surface, the pistons will come further up the cylinders. IIRC the nominal distance from the centre of the crankshaft to the deck the cylinders sit on is 100mm. This kind of thing is why it's ALWAYS recommended to measure everything you can on your engine, ignore the "stock" numbers and do your own calculations.
Can’t remember what size base shims i used but I think they were slightly different side to side to obtain the 0.8mm piston head to the top of the cylinder measurement. Also remember a stock engine had a cr of 7.3:1 So anything more than that is an improvement. Nearer to 8 is better but if you’re calculation leaves you at 7.7 7.8:1 your not going to notice much of a difference. A reconditioned engine from such as Heritage would probably be built with a lower CR & probably not even measured during the build.
Using your volume numbers for combustion chamber (56cc) and piston dish (14.7cc) and a 0.8mm deck the CR would be 7.4:1. Not good but as good as you’ll get and you might worry about the piston getting intimate with the head. If you change the pistons to flat top, you’ll have the opposite problem that you can’t get the CR low enough with a reasonable deck height because the combustion chamber volume would be too small. Then you need to worry about the death zone. Your options are: Use shallow dish pistons. Like the AA with a 10cc dish. Use flat top pistons and open up the combustion chamber to increase the volume. Use the pistons you have with the tightest deck you’re comfortable with and live with the low CR. There are other solutions involving machining the pistons and cylinders but I’m guessing you don’t want to go there.
Yep AMC with the step which gives another 0.8mm to the total deck height. 1.6mm deck in total. It’s just when I did the calculation i used the extra volume measurement including the step, instead of adding the step to the deck height box. Calculation either way worked out at the same CR.
I recon i used a couple of thin shims on 1 bank and just sealer no shim or gasket on the other bank to get them all even at 0.8mm. So adding the head step at 0.8 gave me 1.6 total deck. Couldn’t have got it any tighter without case or head machining.
Yarp - I thought it should be spelled out as IIRC the OP has original non-stepped heads so we wouldn't want him thinking 0.8mm is a good deck to aim for in his case.
Yep, as best I know they are not stepped. By that, I presume you mean that there is more than just a minimal step between the face the cylinder sits on and the face of the heads. Just fyi this is an out of focus shot of the heads from when they got first dismantled. I mean, there is a step, but nothing very serious so I assume they are not what you would call stepped.
That looks like a head gasket to me, the one you're supposed to chuck and replace with spacers at the case end of the barrels. Are they VW heads? The step on AMC heads would look very like that size-wise. You say minimal, but it's not really, it makes a big difference to your calcs. Has the plug thread stripped or is it just the photo?
This is a picture of my AMC heads from my engine rebuild thead with the step shown. (edit) just seen @Gooders beat me to it...
