Short Height Pistons
Turbo Mini Forums

As regular readers will know I've knackered a piston from det and looking for a replacement set of 4 just incase any of the others have suffered too.

I can't afford to strecth to forged so going for some cast Omegas, 73.5mm, MED seem to be the cheapest at the moment at £300 however I have 2 choices and wondered what peoples thoughts on the short height ones were?

My current pistons are 13cc (11cc resized to 13cc), I was thinking of getting the 11cc short height ones.

Standard compression height of 1.498", short compression height of 1.426"

http://www.med-engineering.co.uk/catalogue...istons-die-cast

http://turbominis.co.uk/forums/index.php?p=vt&tid=170151

If you do anything I'd keep the pistons flush with the deck and dish them out a bit.

there's an engine sat on my garage floor that was running 20psi and just shy of 200 nana's on flat top short height pistons.

It wouldn't bother me running them down the bore to drop the compression.

i've seen them run almost a couple of mm down the bore with no issues.

one day boost will be mine!

Flush all the way

Just because it runs with no apparent issues doesn't mean it's right.

In the 13's at last!.. Just

squish?

Speeding is like masturbating, everyone does it, but not all of us film it and put it on the internet

[quote=fab,8th Oct, 2010]fuck off

Short height pistons are for use with long stroke engines. Dont use them to lower compression, its not the right way of doing it

I seriously doubt it!

You're already on 7.9:1 anyway!

In the 13's at last!.. Just

yep as said squish its nice to keep this tight as it is supposed to help stop ringland damage. The n/a engines are standard approx .020" down the bore but with the turbo engine they were flush (or as close as) and i was to trust this was the reason why.

My small bore turbo project http://www.turbominis.co.uk/forums/index.php?p=vt&tid=473901

imo if its set up right you should not have ring land problems in the first place, and i dont mind trading squish for lower cr right or wrong, i think grinding out the beak in the combustion chamber alot worse than running 20thou down the bore its only the same as a thicker head gasket

Edited by Brett on 29th Apr, 2012.

Yes i moved to the darkside

Instagram @jdm_brett

i see, so this is one of those controversial subjects then!

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Ask a question about decompression plates

Speeding is like masturbating, everyone does it, but not all of us film it and put it on the internet

[quote=fab,8th Oct, 2010]fuck off

My personal thoughts on the matter is to run the pistons flush with the deck and sort your CR out with the piston dish/head chamber.

If something is worth doing, it's worth doing half of.

Can someone help me out with my new CR calc.

Previously it was 7.9:1
1380 cc
26cc Head
13 cc Piston dish
3.8 cc Head Gas
7.2 cc Ring Lands


Now:
1380 cc
26cc Head
11 cc Piston dish
3.8 cc Head Gas
? cc Ring Lands

Pistons are 0.5mm (0.0196") down the bore. (cast omegas)

http://turbominis.co.uk/forums/index.php?p=vt&tid=170151

I make it 8.87:1

7.72cc for the ring lands?? it should be under 1cc

Edited by Joe C on 29th Apr, 2012.

thats what Benross calc'ed for me previously when my pistons were further down the bores "heres my new calc which the cc in the ringlands is 4cc and the piston down the bores is 3.2cc making combined 7.2cc"

http://turbominis.co.uk/forums/index.php?p=vt&tid=170151

Even 1cc for the ring land volume is excessive.

Measure the bore diameter. Measure the piston crown diameter. Measure the crown height from top of top ring to top of piston. work out the bore area. Work out the piston area. Multiply Bore area but the crown height. Multiply piston area by the crown height. Subtract piston crown volume from bore crown volume. The result is the ring land volume, and I have not yet worked out a ring land volume greater than 0.5cc

Now:
1380 cc
26cc Head
11 cc Piston dish
3.8 cc Head Gas
? cc Ring Lands

Pistons are 0.5mm (0.0196") down the bore. (cast omegas)

Assuming 0.5cc for the ringland I get 8.6-1.

WWW.TURBO-MINI.COM

ignoring the the piston for a second, bore area X 0.5mm (piston top down the bore) = 2.12cc's on its own, correct?

thats where the 3.2cc came from previously as my pistons were further down the bores.

so i need to know the omega measurements to get that cc calc'd and added to the 2.1cc, which typically i didnt measure before fitting.

http://turbominis.co.uk/forums/index.php?p=vt&tid=170151

Bore area minus piston area multiplied by the hight of the piston crown from the top ring

Ring land volume is the volume between the piston and the bore above the top ring.

Deck volume is the volume above the piston. (piston down the bore)

Dish volume is the volume in the piston

Gasket volume is the volume in the gasket itself

head chamber volume is the volume in the cylinder head

Total chamber volume is the sum of all the above.

As Phil mentions assume 3.4cc for the head gasket and 0.5cc for the ring lands. If you are saying your pistons sit 0.5mm down the bore, this is not ring land volume, it is deck volume.

Edited by Sprocket on 29th Apr, 2012.

you mean 3.8 head gasket ?

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

erm yep thats the one

as for squish, if you taper the head to where it meets the piston in the squish area, wont this eliminate squish?

for a very over exaggerated example, if the head chambers were round and not kidney shaped.