924Board.org

[endwrench]

I think the true concensus here should be that pistons should be designed to work well with the particular head design.

I do not believe flat top pistons are only used for econemy of production and simplicity. It has been proven over and over it is much better to control compression by altering your combustion chamber size rather than your piston shape on an American V8 2V engine. Flattops rule in this arena. Of coarse, there is a point of diminishing returns when the chamber size starts to shroud the valves. This is why Alky guys usually run a poptop piston.

Piston shape has to be altered to control compression on your (sequential's) example because you obviously have little or no combustion chamber. If you flattopped that engine you would need to convert to diesel!

Believe me, if converting a SB Chevy to a Heron style head and dished pistons, tweeked even 5% more power, someone would be making money with the concept right now.

And the "hemi" design ain't all it is cracked up to be either. The new Dodge Hemi isn't even a real hemi beause they won't pass emmisions due to excesive unburnt fuel. Does this sound like great "flame propegation"? Personally I think this is why it worked so well in the 911 and other race applications. All the excess fuel keeps things cool. No quench = no hydraulics.

Todd

[sequential]

Hey Wrench , where is my bike pic. ?

Never seen a professionally built engine with flat top pistons and that includes $65,000 2 V Merlins making 2200 bhp. or 3 valve, 4V, and 5 V /cyl. engines. DTM v6 , f1 v8, cart v8 , man even cheapo 924 ,s don't come with a flat top piston.

Now as much as the new Hemi's not burning the fuel correctly. i can check into that in a heartbeat ( naw that's for the chevy guys ) ok ,ok i can check into that like a ram !

And rememeber the key to all of this like real estate , location, location , location , Spark plug that is.
_________________
928 gts prototype
baby blue engine block
steam in 1,2,3,4 sometimes
cold star issues while on stands
112 whp with new 4 valve head and MIS 2

[endwrench]

Hey your bike picture is stuck in limbo on my e-mail. It keeps coming up as a bitmap file and I can't save the sucker to host it! I'll reboot my computer later and see if it helps.

Anyway, I think we have gotten back to comparing apples and oranges again. Comparing a $65,000 2200HP Merlin (I don't even know what that is!) to a $2500 500HP Chevy. I think you are probably correct in assesing the fact a scratch built engine designed for an extended high performance, high revving application is better off with a purpose built and designed dished piston and small combustion chamber.

But this does not translate to an iron American V8 engine. There have been many thousands of these purpose built to race including endurance races with great success. You can bet all of these had flat tops or worse, pop top pistons. I cannot believe if a well developed dished piston\small combustion design were that much better someone would not have built it by now. I haven't seen it yet .

To my knowledge plug location is very important. Closer to center the better but not always possible. Sometimes even the best designs have to compromise.

Todd

[CBass]

Of course there's a little dish built into the dome of that 911 piston, there has to be some volume somewhere in the engine at TDC. Since the spark plug is centrally located, which is a great place for it, that's where the dish is.

The best built engines I have ever laid hands on or have seen all utilize a flat top piston with a dish. I'm going to assume as a specialist shop you don't work a great deal with what goes on in the domestic aftermarket, but these guys really know their 2 valve per cylinder engines. Very small chambers, lots of quench area and a dish in the piston to keep the compression down.

I have seen engines making as much as 90ftlbs/liter on pump gas. Regular 91 octane pump gas. Granted, this particular engine has everything made bespoke, but that's running with a carb and a distributor! The engine has 13:1 compression and 253/263@0.050 cam timing. Head volume is 65cc, and the rest of the volume is carried in the quench area and the piston dish. That's a very small dish, around 6cc. Granted, if the compression wasn't so high that dish would be of a greater volume, but that is a 100hp/liter and 90lbft/liter build running on pump gas, with what is pretty much a flat top piston.

Just because an engine is not a hemi head does not mean you cannot have central spark plug placement. Flame front propagation is excellent in a closed chamber head with a dished piston. This combined with it's excellent swirl and atomization characteristics that come with quench area make it THE way to go for building V8 engines these days.

I'm not saying a flat top is the absolute best, I'm saying that a flat top with a dish can net impressive quench area and that brings benefits which allow for very high cylinder pressures. If the head flow and the intake and exhaust tuning is there to take advantage of that, it makes for a very powerful and detonation resistant engine. Since no 924 has ever come with a hemi head, although they did come with a pretty good closed chamber head in the 931 head, this is what I am going to take advantage of. You have to treat this combustion chamber as you would a smallblock Chevy or Ford. Quench and dish, high compression and bob's your uncle.
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'81 931 in various states of assembly

[leadfoot]

Rolls royce merlin engines are v10's and sound fantastic.... normally find them in aeroplanes though
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[endwrench]

All v10's sound fantastic!

Here are some pictures from Sequential:





14.5 to 1 compression!!! I'll let him explain the rest.

Todd

[924RACR]

Real quick note on features of $hit-hot pistons; a coworker recently received a set of pistons, custom-made for his Audi 5-cyl turbo... very nice forged blanks, with teflon-coated skirts and ceramic coated tops... <drool>
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[endwrench]

After looking at sequential's pictures a little more it became clear to me why he posted them. To bolster my argument for flat top pistons!! . Thank You

Todd

[Khal]

[sequential]

I don't think so,
The last pston pic is not a flat top piston , look carefully , i will look for a different pic profile for the dimensionally challenged. and there is no way to achieve, 14:1 with a flat top piston .
All merlins where v12.
_________________
928 gts prototype
baby blue engine block
steam in 1,2,3,4 sometimes
cold star issues while on stands
112 whp with new 4 valve head and MIS 2

[endwrench]

Umm, 931 head + true flat top = 14.6 to 1

Todd, (the dimensionally challenged)

[sequential]

The 931 has a very small chamber hence the design actually splits and includes the piston so going to a flat top will increase the CR but the design will be poorly inefficient , Hence the poor HP/Liter numbers on a 931 /924 combination engine. The dish on a stock 931 piston serves a lot in the flame control area this would be defeated by going to a straight flat top piston. NOW, I'M NOT TRYING TO SAY YOU CANNOT MAKE POWER FROM A FLAT TOP PISTON , it is just not technically what one developing and engine would do from a manufacturing or racing position.

If you look at the top of a stock 931 piston the dish is also offset this piston was designed to control how the piston will react to the force applied on the piston head, preventing unnecessary side loading . quite possible necessary due to the angle of engine operation
_________________
928 gts prototype
baby blue engine block
steam in 1,2,3,4 sometimes
cold star issues while on stands
112 whp with new 4 valve head and MIS 2

[sequential]

[endwrench]

OK, this is a pretty good example of what I am talking about. What the heck should a 14.5:1 piston look like for a 931 head? Where would you add the metal back after creating a "flame propergating dish"? You can't really. I am not saying a properly designed and tested dished style piston is not superiour to a flat top design. I am saying, in some applications, the flat top or pop top are the only reasonable alternative making it superiour to any dished style piston.

The piston is forced to delegate the compression ratio in relation to the combustion chamber so it has to conform. This isn't always optimal to performance.

I think this whole dicussion got started while talking about putting Type1 VW pistons in a 931. I think you pointed out the stock 931 piston was a superiour design. I agree. But, in my mind, we are not comparing a dish top to a flat top. The VW piston does have a flat top but it does not take advantage of quench in any way. The 931 piston, on the other hand, does. I think the reason Porsche designed a combustion chamber into the 931 head was so it could take advantage of the quench. It allowed the piston design to travel further up the bore without creating a HUGE piston bowl or valve interference problems. It had little to do with creating a flame propegating dish and more to with creating a flat top style piston that happened to have a dish too

Todd

[CBass]

Assuming a quench distance of 1mm/0.040, which is optimum for a steel rod/6500rpm application with everything within wear tolerance, you cannot have any sort of dome on a piston with a 931 head. The compression ratio would be astronomical. Consider a 518cc chamber compressed into a 21.5cc chamber. Now start subtracting CCs from that chamber with a dome.

Yes, that pic you posted has a dish. It also has a chamber with much more volume than the 931 head does. The 924 head has no appreciable volume. Quench is impossible for the 924 head unless you have a massive dish in the piston.

The piston in your pic has a dome. It would have to have a dome to achieve that kind of compression with that kind of chamber. Now a meaningful tidbit of information might be fuel grade used in that build, and specific torque output. Details?
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'81 931 in various states of assembly