Machine bigger dish in piston
Turbo Mini Forums

What is the best shape to machine the dish like?
I have 6cc MiniSpares Mega +0.040 pistons id like to machine to 11-12cc.
Should I machine the dish to a flat bottom like Morspeed etc.. or should I follow the existing curve and just make it deeper (like the picture?
If doing like in the picture the dish will be about 4,5mm deeper in the middle, compared to the original 6cc dish.

/Jonas

Didnt manage to attach the picture, but maybe you know what I mean without it.

I find it easier to machine normal or tangential to the chuck axis on a lathe. I'm sure this is the opinion shared by others, and hence why you don't often find curved piston dishes...

If you have a copy lathe, or CNC (or lots of patience) then a curve would yeild more CC results for sure...

In the past I have mounted pistons via a chuck to a rotary table on a mill and cut them using a bullnose cutter with success.

It is easier to cut them on a lathe though and you should end up with a better finish when using a single point tool.

Every day is a school day ...........

How fast and how expensive ...... the same question...

It will be done in a CNC so I can do whatever shape I want.
What shape would be best regarding strenght?

Well, the mega piston is a replica of the AE turbo piston, so if you're after 12cc, then the turbo piston bowl is a good starting point.

Somewhere, there is a post for estimating piston crown volume that I created many moons ago. Might be of interest also?

Good idea Dave,
I will measure thickness of the crown in the middle of the dish on the original piston and compare it with the new calculated Mega dish.
Calculating the volume isnt a problem since I have a friend at work draw a model in Inventor

Here's the link. Looking to do a bit of mahcining myself soon, so this all helps! :)

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=4605

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1/4mile in 13.2sec @ 111 terminal on 15psi

I have done some new calculations. If I do a dish 8mm deep and 59mm in diameter I will get 11,3cc. And the crown thickness in the middle of the dish will be 6,19mm compared to 6,83mm on the original turbo piston.
Any comment on this? Sure the crown cant be much weaker than original piston because the 0,64mm!?

well, don't forget that the 'mega' piston is shorter (pin to crown) than the turbo AE piston.

I'd have no concerns going even a shade under 6mm thickness on a 70mm piston.

This will be the spec of the engine:

Mini Spares Mega +0.040 11,3cc piston: 25,1(head)+2,8(gasket)+11,3(piston)+0,78(ringland)+3,7(deck hight)=43,68
327,44+43,68/43,68=8.496CR

Std cam (with 1.5 lifters)
Std T3 (relocated to above clutch) @ 11PSI
No IC

Any comments on the CR/Boost?
Will this CR affect the off boost peformance (was running 8.8 before, and was happy with that)?
Will it be safe to rise the boost to 15PSI with 8.5 CR?

I dont want a engine with super performace but would like it do be reliable.

/Jonas

When you say "25,1 head", I guess you mean the chamber volume is 25cc?

If this is the case you should be able to increase the volume to 28cc's without removing the beak and keep the chamber very close to the original shape overall.

It is possible to increase the chamber size to 30cc but I think that would mean taking some metal out of the beak (I stand to be corrected on that though).

Anything between 8:1 to 8.5:1 is a good compromise on/ off boost.

Just noticed "no IC", would definitely run an intercooler without a doubt, especially at 15psi!

Edited by matty on 23rd Mar, 2007.

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1/4mile in 13.2sec @ 111 terminal on 15psi

Yes its the chamber volume.
I want the keep the current chamber volume. If I want to lower the CR later I dont need to take out the pistons, only machine the head.

I'd just go straight to 15cc on the pistons personally...

Whats the max cc you can get out of an AE +0.020 pistons then?

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1/4mile in 13.2sec @ 111 terminal on 15psi

15cc piston will give me 7.9 CR. Wouldnt off boost power suffer with CR that low?
I will probably not go as high as 15PSI, just wondered if its possible with 8.5 CR.

Edited by Jonas on 23rd Mar, 2007.

I know of people running 15psi on that CR, I plan to run 15psi maybe a bit more if my engine allows it, running 8.3CR.

One thing to consider is the use, mines mainly used throughout the summer on the road and hopefully the odd visit to the strip.

From what ive heard, going any lower than 8.3 ish makes the car quite undrivable off boost e.g not good for road use, but for the strip its fine. All depends what you want out of it I suppose.

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1/4mile in 13.2sec @ 111 terminal on 15psi

I only use the car in the summers. I want it to be quite reliable, I want it to make it to the IMMs etc It seems like 8.5 is a good compromise for the boost/use of my car.
Thanks for helping!

Hi,
The lower the compression the less economical it will be. If your going to be doing long trips it's worth thinking about. 10 psi can be done on 9.3:1 comp ratio with MJ and an IC.
Cheers,
Gavin :)

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