Dished pistons and Squish band

[NipsuGoesRace]

Hi guys. I'm interesting your opinion about to dishening my MTC 81mm turbo Pistons, before I deliver them to he machinging shop. I have used these pistons as it  is, flat tops 1.9mm squish band and 8.8:1 compression ratio, they work great, doesn't knock even with the higher boost levels, but now I have fried my piston rings from 4th cylinder, so I was hoping if dished pistons and thinner squish band 1.0mm would be easier for the piston rings. So any thoungs about this design? I have only change Gsxr1100 78mm stock piston dome design to backwards to dish. Dish Volume is 4.2cc more than flat tops. so compression ratio little raises, with that 1.0mm squish. Any recommends for squish band thickness? Is that 1.0mm too thin?

[Gixer1460]

Don't do it! Something is wrong with your set-up if you've killed rings on a MTC piston - they are pretty bullet proof but nothing is immune from detonation! Not sure how you are achieving 8.8:1 CR with flat top pistons as MTC usually sell them as achieving 10:1 CR! MTC Turbo pistons have bowls machined in factory & are rated at 8:1.

The squish band does two things - 1st - forces the mixture away from the cylinder walls into the centre of chamber to minimise chances of detonation and 2nd - provides strength with material placement of the rings. IMO, a reduced squish would be completely the wrong way to go and would likely exacerbate the problem.

Stating the obvious (if I understand what you are planning to do) you can't substitute a 78mm piston for a 81mm dia. one, no matter how you machine it!

[NipsuGoesRace]

Yes these are 10cr Mtc pistons 8.8CR, only needed 1mm base gasket under the cylinder block Mtc piston valve pockets 0.7cc

Head chamber 28.5cc 

1.9mm squish 9.8cc

Cylinder Volume 304cc

343/39= 8.8cr

That 1.9mm squish band probably isn't optimal by the book and thats why I think that have something to do with that piston ring damage. Thats why I would like to have that bowl machined to the piston. And I hope that also prevents piston rings to get burnt. Hayabusa JE turbo Pistons have that round shaped bowl and it's about 4mm deep. But if that same 69mm diameter bowl machined to these gsxr1100 Mtc Pistons it only can be 1.25mm deep because the head chamber volume is so large. And I want that CR won't change too much.  So is that round shape only option or can that bowl be more like a Square, same shape like the head chamber? Thats why ask if that stock piston dome shape, can also be shape that bowl? And if not why? I haven't found any picture those MTC 8:1 CR Pistons with the bowl, if some one have, i'm interested to see that design.

[NipsuGoesRace]

[Arttu]

Ok, now your question starts to make more sense

As rule of thumb I would aim for 1.0-1.5mm squish band gap. Below that risk for piston-head contact starts to grow. For sure you can go lower but then you have to do your homework carefully. And above 1.5mm is the area where the squish may not work properly anymore and it may even increase risk for detonation. Like said this is just a rough rule of thumb and there are multiple factors affecting to the result. But in any case your 1.9mm squish doesn't sound optimal.

Piston dish doesn't need to be round. It's just the easiest shape to machine. Ideally the dish should follow the shape of the combustion chamber on the head. So if you can get that machined it should be good. Just be careful with the piston crown thickness. There seems to be plenty of variation between different pistons on how much "extra" material there is.

[NipsuGoesRace]

Mtc crown thickness is 5.0mm

Stock gen1 Hayabusa also 5.0mm

Je piston 5.6mm

I little suprissed how thin that mtc crown area is, because they are pretty massive design and they also are 55 grams heavier than Je and 60 grams heavier than Hayabusa stock piston.

Round type is the easiest and chepest to get machined, but i'm worried about these orange section. If the bowl is 1.25mm deep and squish is 1.00mm there is 2.25mm between head and piston on those areas. Is there increased risk of detonating on those areas?

[opalmanta]

Please consider the last piston pictured being mated to your combustion chamber that has had a taper cut starting at about 2-3mm in from cly wall dia. with a 3*-5* taper to the center of the combustion chamber. In the old days it was called a hemi cut now they call it softening the chamber.  Gurls blouse has written several papers on the  subject . Darin Morgan has done much work  on this.  It works on both 2V and 4V apps.  This mod really helps tamp down det. on large dose N2O and high boost engines.  All edges must have radius a run the Piston to head clearance at the edges close to touching at high rpm.  By reducing the amount of tight parallel areas between the piston and the head via the taper cut pumping losses are also reduced . Try pulling two perfectly flat plates apart keeping them flat to each other.  Similar effect shows a jetting effect when process is reversed. However the end goal is not to attain the highest jet effect only enough for movement of end gasses to center of combustion chamber for burn.  The taper cut on the combustion chamber still promotes the jetting effect but allows the flame front to initiate burn sooner in the peripheral areas tamping down detonation.   cutting this taper on the piston does not give the same effect as cutting the chamber. Lots of info on this , start with searching the Speedtalk forums started by Terrell.

[opalmanta]

What is with the Gurls Blouse ?  Gurls blouse is what I typed strange things these smart computers.

[opalmanta]

Did it again.  Big motorcycle company from japan starting with H

[Captain Chaos]

10 hours ago, opalmanta said:

What is with the Gurls Blouse ?  Gurls blouse is what I typed strange things these smart computers.

You should have read the rules. Especially number 9. 

[opalmanta]

I have read the rules but the word not to use was not specified. I won't use the H word again.