Zero deck or not?
Turbo Mini Forums

Hi,

I searched, lots on decking but nothing that answers this particular question.

My engine will be +040 1310cc running 15psi boost, 18cc forged pistons, fully worked 81.3mm crank, con rods etc with a K series head.

It seems the vast majority are running zero deck (or very close to) but I think I may have to have the pistons finish short by 0.5-0.7mm. I can enlarge the cylinder head volume to 21cc to achieve a CR of 8.3:1 with zero deck, however is there any need to do this? On condition that you achieve the same CR, whats wrong with having the pistons finishing short of the block?

I seem to remember someone saying that having the pistons finish short of the deck would aid detonation...but why if you achieve the same CR?

Im confused basically because if engine A and B are running 8.3:1 CR, but engine A has zero deck and enlarges cylinder head chamber, however cylinder B leaves 0.5mm short of deck, with standard cylinder head chamber which engine is better???

Surely both the same as swept volume and CR are equal in both (given all others factors are equal)?

Sorry to ramble on.

Please someone enlighten me

Chris.

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the squish effect will suffer if they are down the bore, that said 0.5 mm isnt much.

But why do you want that low a CR with a K head. The chamber in that type of head is more effective, and you will probably run a programable ECU. this mean that you can gain driveability and power due to a higher CR.

Edited by miniminor63 on 3rd Dec, 2008.

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around the outside of the piston there is what is known as the "squish band" at tdc this approaches the face of the head (unless the head has been cut away to lower cr) and squishes the mixture toward the middle of the cylinder, this agitation of the mixture causes a more compete burn.

If for example you choped 1/4" off the top of the piston you could end up with a situation where the plug starts the burn which works its way across the cylinder, by the time it gets to the other side you could get an unurnt "pocket" of fuel that wil spotaniously ignite due to pressure.

thats what i've read anyway.

i know its abit 6 of one half a dozen of the other (and im no expert)

but whats to say the 0.5mm off the deck will effect the squish more or less that possibly altering the size/shape of the combustion chamber?

What about flat top pistons - are these rubbish at squish then?

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Is the squish area that important with a pent-head and forced induction ?
I'm sure i've read somewhere that some of the yank dragsters run with the pistons 0.090" down the bore at tdc , supposedly helps head gasket life ?
Still havn't finalised the spec for my 1380 turbo KAD engine , this is one of the things i'm still not 100% sure of, or decided on.

Some more discussion/knowledge on this would be great
Carl

dont know Carl, but looking at the standard BMW K engine it has a large squish band on the head itself. (there is a large flat area around the chamber but inside the bore), I guess its there for a reason.

Ah I see what your saing 63, JohnK said the same thing. Not questioning the logic just interested in learning more about it.

Il have to dig up a picture of the bmw head to appreicate what your saying.

I am running it with the ECU - so this helps also?

Chris.

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carl,

yes swings and roundabouts to an extent.... suposedly about 3-40 thou is about right for most automotive stuff, but as always there are many exeptions.

also remember that the headgasket is what .8mm thick? about 30 thou?
so .5mm dow the bore is going to give 50-60 thou ish squish.

Mr mini,

depends how close they come to the head, if the com close enough then you will still get the squish effect but it will sor of travel anong the surface of the piston a bit, many belive the piston dish should be a mirror image of the head chamber.

Carl, with the dragsters you need to consider what fuel they are running, if they are alcohol/ nitromethane burners then the mixtures are massivly rich and they get near to hydralic lock, so the squish would need to be opened up to prevent bent rods and smashed pistons.

also i read a bit on 5 valve heads that said it was very dificult to get enoiugh squish area in comparison to a 4 valve head.

A strange question but...

Like CR, is there a way of measuring the squish efficiency of an engine?

Regards,
Chris

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now there's a thought!

it would be very difficult eh?
Ive often wondered this myself, as i suspect, has anyone who have cut a head.
This "Squisshy Fissshy" test would have to test compression ramp as such, not a total compression calculation which is just based on cc. But this would not evaluate the eveness, or the localisation of the mixture which leads to det.
Any ideas any of you bright sparks out there

Regards
Colin

not a very good picture, but you can see the squish band on the K head chambers. A matching squish band on the piston will generate the turbulance in the charge that the squish theory says. I'm going with it

Iv been doing a fair bit of reading on the quench and the squish. I know its sad but I was facinated by it!

Iv read that quench and squish arn't as important in gas pumped engines as the air is already turbulent. And there is an ideal amount of quench, to develop a shockwave type of effect, creating more turbulence in the squish, and also acting as a cooling effect, reducing the chances of detonation.

I was a little worried about my pistons not being flush with the block - apparently an ideal quench is a min 0.035' and max 0.045' for turbo/supercharged engines. This is equates to 0.89 and 1.14mm.

So hopefully my engine will be fine as I will have a 1mm head gasket (uncompressed thickness) and pistons 0.5-0.7mm down the bore.

Sprocket, was just wondering, with the pic you posted, the cylinder head chambers appear to be perfectly aligned above of the bores? How did you accomplish this - is it a 1380?

Also in your post, do you say its best to grind the head so its forms a mirror image of the dished pistons? i.e. where the dish begins, the head chamber starts to deepen - does this maximise squish?

Shall shut up now

Chris.

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Infact 1mm + 0.5 = 1.5mm Doh.
So infact I have far to much quench

Chris.

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http://www.turbominis.co.uk/forums/index.php?p=vt&tid=229847

Is that offset 1380? (does offset only apply to 1380..?)

Did you keep the original front 3 stud fixing holes in the block, or are all 10 in new positions?

Tidy job on the block.

Chris.

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Edited by Sprocket on 4th Dec, 2008.

Oh pap. I have my pistons about 70 thou down the cylinder. Plus 25 thou HG. 1062cc engine with k100 8 valve head. Will it be ok?

miniboo im not expert! but 95thou sounds quite a lot. Saying that it might be fine iv heard people run big quench distances with no det problems.

Sprocket, cant tell from the picture but how much flat area of the head is visible within the cylinder? I.e. Does the head chamber volume take up most of the bore? Just wondering whether its worthwhile in the future having custom made pistons with a mirrored dish and flat portion for quench- but if not much flat area then pointless really?

Just a thought as the dished pistons out there atm are useless for quench?

Thanks, Chris.

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Would the extra effort be worth the cost, for the results. I dont think so.

You are welcome to have custom pistons made with a dish as you describe, but those pistons will be specific to your engine. The problem with these K head engines is how the head is positioned on the block. If you use the three original head stud locations, those themselves can vary by up to 1mm between blocks, hence the size of the cut outs on the MED pistons.

Don't forget the cut outs in the MED pistons are designed to clear oversize valves as well,wink,wink

JK

If Carling made Mini engines
it would probably be like this one!