924Board.org

[endwrench]

OK, Sequential suggested someone start another thread on piston theory and get it out of his KKK thread So here it is!

Judging from the previous discussion I think the first thing needed is a glossory of terms. These are my definitions so please feel free to help me edit them if needed:

Piston crown: Piston top usually referring to the outer circumfrence on a dished piston.

Piston dome: Protruding part of piston above outer edge of circumference.

Pop top piston: Usually a high compressin style piston with a protrusion that enters the combustion chamber.

Combustion chamber: The space left in the cylinder when the piston is at TDC.

Flat top piston: As it sounds. A piston with a flat top maybe with some valve relief eye-brows. Has nothing to do with were the piston is in the bore in relation to the combustion chamber or head.

Quench or squish: This is the area between any flat area of the piston and any flat area of the head when the piston is at TDC.

OK, I don't pretend to be some sort of expert on piston\combustion design. I am starting this thread because I would like to hear others theorys and applied science. I personally think combustion design is a bit of a black science based sometimes on a lot of luck-trial and error. What works well for one thing may not transfer to another.

If we start with the most basic of piston and combustion design it would be the Heron style with a flat-top piston. In some respects this is probably a fair design for flame travel in an ideal enviroment. There is nothing to get in the way but the cylinder wall itself. Flame travel probably sucks though because there is very little turbulance or swirl introduced except for the incoming charge. This would leave both lean and rich pockets thru-out the chamber.

Next would be the 924 design with pockets and a dish in the piston top. This probably introduces more turbulance but incomplete combustion due to the irregularities. These pockets can also setup detonation problems.

Next would be the 931 design wich introduces a crown and a quench ring around the top of the piston with a piston dish and a small combustion chamber in the head. This doesn't seem to be too bad of an idea as it does introduce both turbulance and swirl to the equation. It leaves a large portion of the piston above the wrist pin though limiting the amount of piston clearance due to piston rock but it protects the top ring well from detonation.

Next is the design I am most familiar with wich is a flat-top with a combustion chamber were the chamber accounts for the bulk of combustion space. This is most commen in American V-8's. In this design nothing beats a flat-top piston if you can control your compression thru chamber volume. It almost always works best with minimum quench usually around .040" or less. If you must use a dish it is best to have it machined into the shape of the combustion chamber. Pop-tops are generally not a good thing and are usually as smooth and round as possible to promote the flame front. Anything other than a flat-top will almost always call for more ignition timing and fuel.

There are a ton of others designs I know nothing about such as pentroof and Hemi. Also several multible valve designs. I want this discussion to be primarily about piston design though. Basically name a piston design and then tell us what combustion designs it works best with and "why" if you can.

Todd
_________________
'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

[-nick]

I can only comment on experience. I thought I had a reasonable idea why my dished pistons didn't work out very well, but it sounds like sequential has other experience? My expertise is pretty limited on this topic, but here's my experience:

JE 8.5:1 dished pistons with 10psi on a newly rebuilt 1980 k26/6 and 92/93 US octane fuel = broken piston crowns for me. I assumed detonation as there were only some "hammered" looking dents on the crowns of the two good pistons. These marks were on the part of the crown that is most shrouded by the head. The #3 turned a gold-ish color, which is apparently indicative of extreme heat for a forged piston.

Granted, I had some vacuum leaks here and there that were probably making the turbo work harder and pump out hotter air, but nothing huge. Otherwise, everything seemed to look good - the plugs had a nice looking color, the timing was correct and advanced correctly, and the car ran strong. At WOT I couldn't hear any pinging over the normal engine/exhaust noises. The engine began running oddly and I pulled the plugs to find #4 electrode mangled. By the cleanliness of the #2 piston, it looks like there was a headgasket break between that chamber and a nearby coolant passage.

The #4 piston was the worst. It had no crown left, the top compression ring was *gone*, and it looked like it had been hammered by a cymbal-smith. The #1 piston had a ~1inch chunk of crown missing with some hammer marks. Numbers 2 and 3 only suffered some hammer marks along the crown. The wrist pin bushing on the rod was also worn on #1 (noticeably sloppy). All in all, there were some _really_ nasty things going on inside that engine!



My current build has the VW flat tops. I smoothed out any sharp edges in the combustion chamber in the head, and polished it lightly. I'm still putting all the pieces of this new build together. Hoping to have it to the tuning stage within another two months.

Also to note: these VW pistons are "press forged" as opposed to "drop forged". I haven't found out much about the two different processes. Maybe somebody here can chime in?

The flat tops don't give any squish in the 931, whereas the stock dished pistons (and JE's) do. I've heard whispers that forced induction doesn't really need squish area, but who knows? The 924na has no squish at all. It's also pretty low-output for a 2.0L however, and not very high-strung.

I've looked for combustion chamber design info before, but never heard/saw/read anything definitive. Making me think there is a lot of trial and error involved too.

nick[/img]

[sequential]

the 931 exhaust manifold design and high cylinder head temps at 3, 4 means you will always lose the head gasket there. The damage to your engine was from pre-ignition , i would assume and you did not mention if your car was running hot or hotter than normal , the high cylinder head temp will allow this to happen on 93 octane fuel .
conditions that would lead to your damage ,

1. Too high cylinder head temps , ( water temp )
2. too much ignition advance
3. Sparks plugs to hot for running conditions ( most likely this one )
4. last but not least lean fuel mix.


Now high intake temps will lead to pre-ignition on 93 octane but a boost leak would mean the car would be richer than normal which would have combated the higher intake temps. also the fuel octane could have been lower than 93/92 .

THe problems encountered here by nick would have the same results with the flat top pistons, everything else being the same. Nothing would have been different because of no squish area. a little secret here , turbo engines respond to most of what makes a N/A engine work , 98% of the time.

FLat top pistons work best in Hemi type cylinder heads with Spark plug located centrally, so all you guys with hemi 924's should be alright !


The reason why all factory and big dollar race pistons are not flat top is to promote correct wave propagation or flame travel on the power stroke , a flat top piston cannot do this an hence will not have the same power stroke all the time , in the race business it is called Harmony. This is easily witnessed on the dyno doing power pulls. Now for the street not a problem wide open throttle 15 mins per week good ! but in a competitive racing environment that 20 -35 hp lost , plus reliabilty will cost you races and money.
So at this level a flat top piston will not hurt you , but it is in no way equal to or superior to one designed to control wave propagation etc.

In summary ( for those who read the first line than attack)

FLat top pistons pros:

1. Easier to make .
2. Lower cost ( no development work )
3. Can possible be beeffier

CONS:

1. Poor wave propagation
2. Heavy
3. Does not promote good cylinder scavenging
4. Inconsistent power stroke .
5. limited Compression options.
6. Requires Centrally located spark plug for optimum operation .

PLEASE DO NOT TAKE THIS TO MEAN THAT YOU HAVE TO THROW AWAY YOUR FLAT TOP PISTONS THEY WILL WORK , MAKE POWER BE HAPPY !
THIS IS JUST A DISCUSSION ON THE TOPIC , ADD WHAT YOU CAN.

So please no ! i'am anti flat top pistons and will be reported to the flat top piston society, yaddy , yadha !

WE ARE DEALING WITH THE PISTON TOP AND CROWN NOW , NEXT WE should MOVE TO PIN , SKIRT AND RING HEIGHTS .
_________________
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

[Raceboy]

Wayne, are you saying with your points that GTR for Le Mans was built having low-cost in mind? Or just to go the easy way? Sry, but I don't think so. That car made 300+ hp for 24 HOURS!!! And not one of them, but 3!
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[-nick]

Great info!

For the record

-water temps were always very good. Was using the lower temp fan switch and thermostat.

-ignition advance possibly a culprit. Idle timing was spot on and revving showed the timing to advance. Since there isn't a tick on the flywheel for total advance, I couldn't really check it. I would expect that if the advance mechanism had problems, it would give too little advance rather than too much?

-plugs were a step colder from what I remember, but I'd have to double check. I wouldn't have expected the plug heat range to make such a huge cylinder temp change? Maybe this is the culprit?

-a/f ratio - I'm just judging this on the plug color. It's possible that the fuel pressure regulator vacuum enrichment wasn't working properly making the mix on the lean side under boost. I wouldn't be surprised at this one.

Too bad I wasn't smart enough to install the wideband o2 and EGT probe with the JE + CIS build...

Anyway, enough of my little catastrophe. How inefficient are the flat tops compared to a dish/squish piston? 20-35hp is a huge when at or near the 200hp mark. Though not as big when at the 300+hp mark. Also where does the lessened reliability come from? If the power stroke is less consistent then I imagine the balance would be less than perfect and create more wear.

I didn't weight the flat tops, but I didn't notice them being substantially heavier than the JE's. If they're both forged, I would expect the dish pistons to be a touch heavier - it's just a flat top with an extra crown and different compression height, right?

Anyway, at $150 off the shelf, compared to $650 + 4-6 week wait, they're tough to argue with for us weekend warriors unless we're giving up substantial power.

ps- Raceboy, part of the factory's reasoning could just be the technology. It wasn't that long ago that carmakers realized that aerodynamics were important. We're talking 1980 after all for the GTR. Commodore computers hadn't even become a hit yet

[sequential]

[sequential]

[CBass]

Flat top pistons work great with quench chambers like the 931 chamber. Unfortunately, they work great with the proper quench distance, which is usually 1-2mm or so, not 5.8mm.

My new build is naturally aspirated using the 931 head and the VW pistons, deck is being brought down considerably to attain higher compression, thinking a ballpark figure of 11.5:1 in an 8000rpm build.
_________________
'81 931 in various states of assembly

[sequential]

[bass gt]

Guy's,

Well, the timing on this thread could not be better as i'm about to start on the new engine. So, I've been thinking of steel rods forged, dished pistons, probably JE or Woosner, with an 8:1 CR, DTA ECU, S1 KKK turbo, Integral stage 2 cam, solid tappets, heavily modified head,Jenvey ITB's 440cc/min injectors, 1.1/1.2 bar of boost, water injection. target is 280/300BHP.
Thoughts please,

cheers,
Steve

[sequential]

[bass gt]

Wayne,
Cheers for the reply. 550cc/m eh? Interesting. I though the rough calc for injector HP capacity was inj size/5xnumber of injectors. So 440/5=88*4=352 bhp
As i am only aiming for 300-320 BHP, i thought the 440's would give me some leeway.
As for the turbo, i am currently chatting to the UK Garrett importer, as to go for a GT28 or GT32 model. However, lets not all open that discussion up again eh???
The first iteration of the engine will be nthe new head, S1 turbo, DTA ecu cam ect, and peg it back to 250BHP, but with plently of low down grunt. Even 250bhp in a 900kg car should be fun!!

Steve

[john h]

I'm aiming for around the same hp as bass gt but going a slightly different route.
Using standard Euro pistons, turbo head, turbocharger off a Subaru (turbo model VF24 or VF28) mounted on left side of motor (same side as brake booster). Bosch CIS unit off Mercedes 500 using 931 injectors as primary injectors and 4 injector from the Merc in the inlet runners.

I know sequential doesn't like the CIS but at this point in time I can't be bothered with EFI and the Merc unit had been sitting around in a friends workshop for ages and as he's going to do all the engine work for free with me paying for parts (new rings and bearings only, as between us we have about 6 non tubo motors to destroy)
_________________
Remember a Porsche is not just for Christmas,

if you take it to pieces slowly it can provide anguish all year long!

[sequential]

[sequential]