Piston Dome Shape CIS vs. Carb

[crummasel]

Hi there,

in an other thread the question came up if the CIS really needs the "bowled" piston shape. My understanding was that the turbulence induced by the special shape was used to create a reasonable mixture inside the cumbustion chamber since the CIS injects fuel partly onto the closed valve so a bad mixture occurs when the valve opens and fuel is aspirated together with air before the components are mixed and the fuel was carburetted.
Maybe i am wrong or there are contradicting theories?
Even if i did an uncounted number of carbed engines i did not excessively work on CIS engines especieally i did not experience CIS on domed pistons.
Only indirect experience by a customer who was reporting a very bad performance of a 2.4 CIS engine which another engine builder has equipped with domed pistons.

Can someone enlighten me on this question?

Or: why did Porsche change to bowled when introducing the 2.4 cis, back to domed on the 2.7 rs and back to bowled on the 2.7 cis?

[Martinlassmann]

That,s easy2.7 es is Not cis
Martin

[crummasel]

Good answer. I would agree

Back to the question: What was the reason Porsche used bowled pistons for CIS and domed pistons for non- CIS (even with similar compression) ?

[tom1394racing]

All CIS 911 engines 2.4-3.0 used the CIS type domed piston. The 3.2 DME engines also used the CIS piston design.

The CIS system is sensitive to inflect tract pulsations that are more pronounced when using a high lift cam and domed pistons.

[crummasel]

OK, to make it more clear,
the first bowled piston came with the 2.4T CIS:


vs. the previous domed ones used in carb or MFI applications:


continued in the same way "bowled" for for 2.7 CIS


vs. domed in 2.7 MFI


Question again: Why did they do the bowled ones for CIS (i offered a theory in the initial post)
And: Does CIS really need the bowled pistons or is a CIS application capable of creating a reasonable air/fuel mix in the combustion chamber with domed pistons?

[Flat6pac]

If nothing else the CIS applies the fuel to the spark plug better than the normal RS style would and with the more radical suction of the CIS intaking.
Bruce

[Speedy Squirrel]

Intake tract pulsations and radical suction? Hmm...

Carburetors are kind of like CIS in that they are spraying all the time, but they are a long way from the intake valve.

CIS injects all the time, very near the intake valve. In fact the whole point of it is to keep the fuel off of the port and intake manifold walls, which it does.

MFI also injects near the intake valve, but has a higher pressure than CIS, and is not continuous. It always sprays at a closed intake, until the time required for injecting the desired amount of fuel exceeds the time that the intake valve is closed.

As a consequence of the way it operates, CIS is spraying into an open cylinder most of the time. Some of the fuel vaporizes, but some does not. With a domed piston, it would just collect at the edges of the combustion space, far from the spark plug, which is not good. If it has a little bathtub in the crown, it will collect there and evaporate near the spark plug.

Carbs, being furthest from the intake valve, have more time to evaporate fuel, and some collects on the hot intake runners, which also helps the fuel evaporate.

MFI is sequential, so it sprays mainly on a closed intake, and also runs at a higher pressure, which makes the spray droplets smaller to begin with.

[Trackrash]

It's my understanding that the CIS bowled pistons aid combustion efficiency. They cause a swirl to aid atomozation and confine the AF mixture near the spark plug. Both are intended to minimize emissions by increasing combustion efficiency.

Question is who is running domed pistons with CIS. Any difference that you can tell.?

[crummasel]

Exactly my point and interesting discussion.
Since the CIS sprays permanently onto the (mostly) closed intake valve i would expect the fuel to enter the combustion chamber in a more liquid state and consequently some aided turbulence would be needed to help atomising it to create a reasonable mixture. Carbs and MFI are are doing atomisation more or less synchronised to the opened valve and atomisation starts inside the inlet duct (or is helped by higher pressure spraying into te opened valve). At least this was my understanding and probably a quite static view onto a perfect world. That would imply that CIS would not reasonably work on semi heads and domed pistons.
Who would disagree to that having used domed pistons with a good result in terms of performance and fuel consumption?
If i try to doublecheck and compare with - say - BMW M10 and M30 engines i see almost no bowled pistons but more or less domed ones which would contradict to above statement. But looking closer we see that the BMW semi heads are not symetric but the valves are shifted sidewise so probably they create some sidewise swirl to help inner mixture building...
More ideas on that subject?