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Piston Therory and Myth
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endwrench  



Joined: 07 Dec 2002
Posts: 1629
Location: Victor, Montana

PostPosted: Tue Mar 07, 2006 2:50 pm    Post subject: Piston Therory and Myth Reply with quote

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
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-nick  



Joined: 16 Nov 2002
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Location: Cambridge, MA

PostPosted: Wed Mar 08, 2006 4:14 am    Post subject: Reply with quote

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]
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sequential  



Joined: 18 Nov 2005
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PostPosted: Wed Mar 08, 2006 5:37 am    Post subject: Reply with quote

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 .
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Raceboy  



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PostPosted: Wed Mar 08, 2006 5:53 am    Post subject: Reply with quote

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!
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-nick  



Joined: 16 Nov 2002
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PostPosted: Wed Mar 08, 2006 6:04 am    Post subject: Reply with quote

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
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sequential  



Joined: 18 Nov 2005
Posts: 500
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PostPosted: Wed Mar 08, 2006 12:20 pm    Post subject: Reply with quote

Raceboy wrote:
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!


Hello Raceboy ,

Please show me a factory racing gtr engine with flat top pistons
i would love to see it and for the record that engine makes in the excess of 430 bhp OK , no myth just facts. the 375 bhp number is for those who read magazines as you will easily exceed that number with your street engine . BUT WOULD LOVE TO SEE A FACTORY GTR ENGINE WITH FLAT TOP PISTONS , PLEASE SHOW ME and the forum.
less not forget that for the 3 yrs porsche took the 924 to lemans and Daytona , most ran on 3 cylinders more than they ran on 4. Without electronics porsche had no way of properly controlling timing in the different cylinders, so most of us today, would be able to, with Modern EFI match those numbers with little better than stock parts .

Again i do not want to turn this into an anti-flat top piston debate!
i'm not telling anyone to throw away their flat top pistons or that you cannot use them. We are just discussing pros. cons , facts and Myths . anyone can add technically if they wish.

!
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Last edited by sequential on Thu Mar 09, 2006 12:03 am; edited 4 times in total
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sequential  



Joined: 18 Nov 2005
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PostPosted: Wed Mar 08, 2006 12:26 pm    Post subject: Reply with quote

-nick wrote:
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


NICK I HAD OVERLOOKED SOMETHING INITIALLY with your situation , i would bet the JE pistons had a higher CR than thought , that is what led to the pre-ignition with your stock setup i would bet closer to 9:1 CR

Always do a CYL pressure test to see how much is in each cyl. after changing piston type ( unless you have the tools and the know how to measure the Cr during assembly ) we had a similar situatio of pistons supplied to us from a well known piston supplier for an EVO , this car runs in the Centro American Championship and guess what 9.8:1 instead of
8.2:1 CR you have to always check.

You cannot use a flat top piston to achieve Cr numbers above 10:1 or there's about , so if you need 12:1 . 13:1 Cr a flat top piston is out of the
loop.
Now the question , If flat top pistons are the deal and they are easier to make and design , WHY DID PORSCHE GO TO THE TROUBLE OF DESIGNING DISHED PISTONS WITH A SHAPED OFFSET FOR BETTER WAVE PROPAGATION ON A STREET , LOW BOOST ENGINE ? they could have used a flat top piston and save alot of cash and time.

As stated before i have never seen a factory GTR engine ( 2 todate ) with
flat top pistons. if i'm wrong and what i have seen in the past is not correct , and they did come with flat top pistons then i owe raceboy an apology !
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CBass  



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PostPosted: Wed Mar 08, 2006 3:46 pm    Post subject: Reply with quote

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.
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sequential  



Joined: 18 Nov 2005
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PostPosted: Thu Mar 09, 2006 12:00 am    Post subject: Reply with quote

CBass wrote:
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.


Hello Cbass,

Did you measure the deck height ? also what about the head volume did you cc the head ? how are you making clearance for the intake valve with 11.5:1 and a flat top piston? are you going to run the stock cam so as to not have any clearance issues ? seeing that you have decked the block, to get more. I would think a flat top piston with stock deck height would be close to 10:1 maybe more .
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bass gt  



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PostPosted: Thu Mar 09, 2006 1:06 am    Post subject: Reply with quote

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
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sequential  



Joined: 18 Nov 2005
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PostPosted: Thu Mar 09, 2006 3:33 am    Post subject: Reply with quote

bass gt wrote:
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


everything looks good steve , but one , i would go no smaller than 550 cc
on the injectors ! the rest of the package is good. watch the turbo upgrades as the stock s1 KKK will be out there at 300 bhp.
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bass gt  



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PostPosted: Thu Mar 09, 2006 7:58 am    Post subject: Reply with quote

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
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john h  



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PostPosted: Thu Mar 09, 2006 8:56 am    Post subject: Reply with quote

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)
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sequential  



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PostPosted: Thu Mar 09, 2006 10:51 am    Post subject: Reply with quote

bass gt wrote:
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


900 kilo , wow i have never seen a non - tube chassis 24 that light ! are you sure steve ? I believe I will see 1030 Kilo's on mine, min.

That formula for calculating injector sizeing is mis-leading

To correctly determine your injector size is not as simple as that and hence you will always end up with the wrong size injectors. for your application , in a nut shell other factors needed to determine injector size, along with the above formula.

1. Number of cylinders . ( i know )
2. Total Max RPM
3. BHP max at said RPM ( a guess of this will work , er on the big side)
4. Max % flow at available inj ontime at max revs.
5. Static Fuel pressure. ( this is more important on Boosted than on n/a )
6. Desired A/F
7. Desired EGT

for ie. an engine that makes 300 bhp peak at 5000 rpm and is 300 bhp at max revs of 6000 rpm can use smaller injectors than another engine making the same bhp but at 12000 rpm it has 50% less on time available .
Also on a turbo engine making 300 bhp but requires an A/F of 12.5 to live vs an N/A making 300 bhp but can survive at 13,7 A/f the sizing is different, as less fuel is needed.

My suggestion's get 550 cc min or any changes to your current setup will require another injector purchase. 550cc at 3 bar static pressure on your engine will be good for 400-420 bhp ( crank hp ) this means at your goal of 300 bhp you will be comfortable at 70 % duty cycle at peak . with 440 cc you will be constantly above 85 % - 90 % duty cycle, and this is if you only spin the engine to 6500 rpm, above this you will start to run lean say if the engine is spun to 7700 rpm. past experience is 440 cc is good for 375-380 at max duty cycle 7000 rpm A/F 12.2 on a 2 L engine.

Ok now the part not told , above 70 % duty cycle there is no more sequential operation , as there is not enough of an event cycle to control the injectors as such ( this is not as big a deal as it sounds , just that technically this is no longer considered sequential operation )

440cc will work , but you will need to purchase injectors again , my suggestion min 550cc , this way you can put the A/F where you need it without high inj duty cycles ( i went with 720 cc , for later upgrades )

GT28 /GT32 take your pick as simon is watching !
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sequential  



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PostPosted: Thu Mar 09, 2006 10:57 am    Post subject: Reply with quote

john h wrote:


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)



And you will need all 6 motors! Ha, HA , HA , man that is the last time
i will say anything negative about CIS , wow the number of Hate mail i have been recieving.

The next new forum will be Mega Squirt or CIS which and for whom !
Ha, Ha, Ha, !


would love to see the engine idle and run on the low end with all 8 injectors going John .
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