New piston born, GC D21 Integrale 8 valve

[tmvolumex]

Guy,
I was looking in the GRE Parts and Service section of this site and saw your post on your new 8 valve Integrale forged piston, your D21 design. It looks very nice indeed. Because of the symmetrical dish design will this piston also fit a standard (non-reversed) Fiat / Lancia head? I am also curious as to the reason for running the top of the piston slightly below the block surface. Can you elaborate on this? Is it only to get the compression ratio to the value that you intended? Why not a deeper dish instead? Could you also comment on the combustion chamber shape and effect on flow between a non reverse port chamber and the Integrale head in the pictures? The chambers are similar but the Intergrale head appears to be much less angular than the early 8 valve head and obviously does not have any piston dome cutouts (no dome needed on low compression engines).
Very nice looking pistons, Ill have to put a set on my gift list!
Thanks in advance,
Tom

[Guy Croft]

Tom, hi

Re: 8 valve Integrale forged piston, your D21 design. It looks very nice indeed. Because of the symmetrical dish design will this piston also fit a standard (non-reversed) Fiat / Lancia head?

Yes, same valve angles and potentially (given that Int has 43.5mm inlets) same valve size. No thrust offset so can be reversed. But of course the D21 pictured (though I can alter it) is designed to give low CR on the smaller chamber of the reversed port design.


I am also curious as to the reason for running the top of the piston slightly below the block surface. Can you elaborate on this? Is it only to get the compression ratio to the value that you intended? Why not a deeper dish instead?

That's a design trick. You get a lighter piston if you run it lower than block deck. You can end up with a real heavy piston running it flush with a big bowl..

Could you also comment on the combustion chamber shape and effect on flow between a non reverse port chamber and the Integrale head in the pictures?

That head, http://guy-croft.com/viewtopic.php?f=15&t=1657 has been significantly opened out on the inlet side to deshroud it. The flow of the std reve port head on that side is pretty poor. With my mod (and it takes a long time) we get the squish band mixing and flow comparable in every way to the non-reversed port head.


GC

[tmvolumex]

Guy,
Thanks for the quick reply. So the combustion chamber shape in the picture is not stock. I remember seeing on your site your tests of a non reverse port head that had been modified to a reverse port (bathtub shape) chamber by welding up the chamber. As I recall, it did not flow that well until after you deshrouded it quite a bit. The chambers in your recent pictures, with the D21 piston, look very nice; I did not realize that the chamber had been modified by GCRE. I like the generous squish bands that you have retained. Is the squish effect and subsequent increase in detonation resistance affected much by the fact the piston is run below the deck? Abarth ran the Lancia 037 pistons well below the deck, maybe to lighten the piston as you have mentioned?
Tom

[Guy Croft]

Hi Tom

is the detonation resistance affected by the reduced squish? Could be, but if the inlet temp is kept below 45 deg C and the engine temp around 75 deg C and the unit calibrated in the 0.8-0.9 Lambda range it won't detonate anyway. There is so much more turbulence in the inlet valve event with a boosted unit that I'm certainly not worried about generating slower combustion.

Yes, 037s,Integale 16v units run low pistons too.

G

[tmvolumex]

Guy,
Great information, thanks. I was not aware that Integrale's ran the top of the piston below the deck also. The Fiat and Lancia 16 valve engines don’t have much squish band area compared with the 8 valve engines anyway, so running below the deck is probably not an issue. In addition, as you pointed out, in forced induction configuration, the engines inherently have more turbulence because of the intake charge being forced into the combustion chamber above atmospheric pressure.
Thanks,
Tom