quench theory discussion.

[94awdcoupe]

I haven't seen the insides of evo 4-7 engines so I don't know when the change was made. But when the evo 8 was introduced to USA market in 2003 I couldn't wait to get a look inside to see what was tweaked. One of the first things I noticed was the pistons. They were substantially different from earlier designs. They are strutted design which makes them both stronger and lighter. They have anti friction coating on skirts. they have hollow piston cavity to improve cooling, and last they are nearly flat tops with no quench zone design. It is this last feature I would like to discuss.

Why did Mitsubishi abandon the quench zone theory?
the quench zone on early 4g63 is .065=.015 deck height +.050 gasket
evo 8 is at .105 = .070 deck + .035 gasket
evo 9 is .120??? gasket is .049 I dont have deck value

I think it would be ignorant to infer that Mitsubishi engineers don't know what quench zone does. Any half way decent engine builder knows the theory. its been around for ages. I don't want to discuss the quench zone theory. I know all about it. What I don't really know is why an engine might be better off with out it? Mitsu clearly does not care about quench zone on the evo 8. It could even be argued they didn't care about it on early engines either. an effective quench zone is around .030-.035. The early piston doesn't have much of lip on edge of piston and the zone is pretty big at .065. maybe they were thinking of the serviceability of head? after you shave .035 off head the quench zone would be .030.

So the question is why is the evo 8 piston designed the way it is? I started this discussion on evo forums back in 2004. I called Wiesco, Ross and others to discuss why the 8 piston had so little deck height with no effective quench zone. I didn't find anyone who had an answer.

My own possible conclusions:
quench zone effective with large pistons, large chamber, large timing lead. (some v8 needs 45 degree lead) small engines with fast burn chamber don't need it. It likely hurts more than it helps.

the squish takes energy= slows piston down?
the squish causes too much turbulence? turbulence is good but too much would be bad?
less squish = better burn pattern? If you are pushing mixture to center of chamber wont it be lean on the edges?

burn pattern very important. lean pockets cause detonation.

fast burn chamber is good thing. engine with 10 degree lead has less pumping losses than engine with 20 degree lead.

 

[dsm-onster]

I agree that Mitsu doesn't really care much about quenching. The 1g pistons and 2g pistons are terrible designs when it comes to quenching. I have no good idea why Mitsu would abandon quench tech completely. It apparently is OK. But quench is proven to work VERY well. 8.5 Wiseco pistons knock less with a bit more aggressive tunes than stock pistons, from what I've seen.

--The quench zone increases the burn rate. So it helps a combustion chamber be a "fast burn" chamber. Less timing is needed to see MBT. Less pumping loss. Because of the faster burn rate, there's less chance of detonation because the fuel/air mix burns before detonation can occur. There's a faster burn rate in part because the purpose of the quench zones is to reduce the mean distance of the fuel to the spark. I think you're not really pushing the mixture into the center as much as you're preventing it from being farther away from the kernal. As you know: so that you can reduce the time it takes for the flame to get to those parts of the mixture. I don't think there could be lean pockets that could develop on the outside (close to the quench pads). I would believe the opposite. The initial state of the mixture is expanding out from the spark and the quench zones are pushing it back in. The fuel could bunch up along the rim of the quench zones*.

--A lean pocket can create detonation as it burns simply faster and cause a pressure wave to strike the initial pressure wave. But also, a re-formed gas droplet (100% rich) ignites with the edges of the initial flame front (tips of a candle flame are hottest). And starts it's own flame, then the pressure from that flame collides with the major pressure front of the initial flame front. The quench zones are there to churn up AS MUCH AS POSSIBLE the mixture to insure uniformity. I don't see how more turbulance would be detrimental. I don't think tuerbulance could ever re-organize the fuel to lean and rich pockets. The more the turbulance the more evenly the fuel is mixed. And the pumping loss from the energy to highly mix the air/fuel charge? I don't know. I don't see that being any measurable loss. No more loss than raising the compression. Much more losses come from advancing the timing due to a slow burn rate.

*Maybe this is the reason for mitsu ignoring quench zones. There's not going to be enough turbulance because it's good enough for stock power levels; and then there's going to be very much more boost and added pressure from higher compression in an aftermarket setting, which mitsu does target that the fuel begins to reform droplets easier along the quench pad rim if it were there. But that still doesn't explain why Ross and Wiseco pistons detonate less. A good comparison test would be evo8 pistons and Wiseco pistons.​

 

[94awdcoupe]

normal combustion is a controlled burn. the fuel air charge ignites at spark plug. the flame front expands from center at a controlled rate of burn. pushing down on the piston for an extended time. longer push=more torque=more power.

detonation is a spontaneous ignition of charge outside the normal flame font. the mixture doesnt burn slowly, it explodes. causing instantaneous spike of cylinder pressure. head lifting, blown gaskets, broken pistons are the results. detonation is started by pressure and heat compressing lean spots in the chamber.

detonation on the 4g63 always starts at the leanest point in the chamber. above the ring land on the intake side of piston. this is always the hardest place for fuel to reach. fuel has mass and doesnt make 180 degree turn very well.

an ideal combustion chamber would distribute an a/f charge equally everywhere through out the chamber. this is hard to achieve in reality because air and fuel has mass and doesnt want to make turns.

 

[94awdcoupe]

dsm-onster

Dave Buschur has stated several times he sees no performance gain from dropping in forged pistons in the evos. He has done many installs. He has never commented he was able to make more power because detonation thresholds were raised. 99% of the time people build their motor they are making more changes than one. This makes tuning observations with new pistons completely useless.

Its all theory if you ask me.

Yes I have seen some good articles where quench zone was raised from .060 to .035 (no other changes) and they were able to raise boost from 9 - 11psi. But that test was on V8 with 2valves and lazy combustion chamber. I dont think its a good idea to assume it would work same in 4g63 chamber and mounds of boost.

 

[dsm-onster]

I have to concede that other changes could and commonly are made when one swaps from standard to forged wisecos or ross. . .

Every single piston I've cracked has been on the exhaust side (two fractures right under the exhaust valve reliefs). I've scuffed up the intake side: bouncing against the cylinder wall.

. . . As far as quench zones and our motors. Perhaps there's plenty of turbulance in the first place. Is that what you're infering with "lazy combustion chamber"? I can see that. And most of my experience with quench pads does come from CARBURETED motors. Which have poor atomization in the first place.

 

[Defiant]

Quenching also raises NOx levels. Smog and mileage figures are worth more these days than just power output.

 

[groundPork]

I come across this issue a lot in the aftermarket. You have lot's of guys looking to make the most power and do it the most efficient way. I have to say, i think it's an excellent step in the right direction.

I'm not trying to shoot anything down, so i don't want anyone to take this the wrong way.

Quench does work. It has two effective jobs, that have been possible to document. one it can, but is not always very effective at, forcing the fuel air charge towards the center of the chamber, or the plug.

It does actually reduce the amount of free fuel droplets at the extremities of the combustion chamber, which thusly will reduce the possibility of fuel lighting off another flame front.

Does a piston being forged make more power in and of itself? Absolutely not. I know that's not what the previous poster was trying to say. The nice thing about forged pistons, is they are often made by people looking to make the absolute best part possible to do the job, and that means most efficient, and strongest most of the time.

Quench doesn't really give great benefit in already well designed chambers like we have in the Dsms. It will certainly help emissions, but it's certainly not the difference of 10 or 20 horse power in a turbo charged engine. There is already so much turbulence generated by just the pressure of air rushing in to the chamber when making boost.

Mitsubishi is paralleling something i've seen in the drag racing seen for a little bit now. I believe they are content for the most part with the burn formation in the chambers. I think they may be intentionally trying to slow the flame front down, or at least reduce instability in the chamber by allowing a larger reservoir for pressure to travel through. in a sense, dampen the pressure front itself, in the name of higher power potential at lower engine speeds. The less sensitive the chamber is, the more immediate pressure rise it can withstand without detonation occurring.

I think with use of MIVEC they can manipulate cylinder pressure to deal with any other issues brought on by the earlier rising cylinder pressure of the newer generation 4G's and even the 4B11.

i could be totally off the path, but i'm just going on common sense and experience.

 

[toofast82]

the quench zone on early 4g63 is .065=.015 deck height +.050 gasket
evo 8 is at .105 = .070 deck + .035 gasket

I'm assuming this thread was started per the discussion of this thread
http://www.dsmtuners.com/forums/newbie-forum/311850-any-2gs-evo-pistons.html

Now it was said there, that if evo pistons were put in a 6bolt, there would be .070" difference between piston and block at tdc. I also read in there i believe the evo head is 2cc smaller. My question is if you put the evo pistons in the 6bolt, would it effectively have the same compression as evo8 minus the differences with the 2cc of combustion chamber and thicker head gasket? Also if you know what difference that would calculate, that would be nice to know?

 

[delta448]

Disclaimer: I have not studied thermodynamics.

Mitsubishi is paralleling something I've seen in the drag racing scene for a little bit now. I believe they are content for the most part with the burn formation in the chambers. I think they may be intentionally trying to slow the flame front down, or at least reduce instability in the chamber by allowing a larger reservoir for pressure to travel through. In a sense, dampen the pressure front itself, in the name of higher power potential at lower engine speeds. The less sensitive the chamber is, the more immediate pressure rise it can withstand without detonation occurring.

I think with use of MIVEC they can manipulate cylinder pressure to deal with any other issues brought on by the earlier rising cylinder pressure of the newer generation 4G's and even the 4B11.

I believe this is exactly what is happening. When the burn occurs too quickly, as it can with the high pre-spark cylinder pressures of positive boost in a relatively small combustion chamber, you end up with detonation limited power unless the fuel burns slowly enough (read: has a high enough octane value). The pressure rises so fast that the unburnt fuel near the edges of the chamber explodes before it can burn completely. When you have an optimum ignition advance that is barely positive or even ATDC, you're seeing some very fast burn, and also probably quite a high level of NOx emissions.

I'm assuming this thread was started per the discussion of this thread
http://www.dsmtuners.com/forums/newbie-forum/311850-any-2gs-evo-pistons.html

Now it was said there, that if evo pistons were put in a 6bolt, there would be .070" difference between piston and block at tdc. I also read in there i believe the evo head is 2cc smaller. My question is if you put the evo pistons in the 6bolt, would it effectively have the same compression as evo8 minus the differences with the 2cc of combustion chamber and thicker head gasket? Also if you know what difference that would calculate, that would be nice to know?

You'd have to know the dish volume of both sets of pistons, or the difference in dish volume, to figure it out that way. Remember; dish volume also includes valve reliefs.

You'd take the volume between deck and piston, which includes piston dish volume and the 0.070" difference in height of the bore size to get up to the deck. Then you'd add that volume to the size of whatever your head cc's out to in order to get the combustion chamber volume. Once you have that, just divide the cylinder displacement plus combustion chamber volume by combustion chamber volume to get the CR.

 

[94awdcoupe]

Quench doesn't really give great benefit in already well designed chambers like we have in the Dsms. It will certainly help emissions, but it's certainly not the difference of 10 or 20 horse power in a turbo charged engine. There is already so much turbulence generated by just the pressure of air rushing in to the chamber when making boost.

Mitsubishi is paralleling something i've seen in the drag racing seen for a little bit now. I believe they are content for the most part with the burn formation in the chambers. I think they may be intentionally trying to slow the flame front down, or at least reduce instability in the chamber by allowing a larger reservoir for pressure to travel through. in a sense, dampen the pressure front itself, in the name of higher power potential at lower engine speeds. The less sensitive the chamber is, the more immediate pressure rise it can withstand without detonation occurring.

This sounds like really good reasoning. I would think Mitsu engineers would have some of the best engine simulation programs available. There has to be a reason. best I have heard so far.

 

[aspekt9]

This sounds like really good reasoning. I would think Mitsu engineers would have some of the best engine simulation programs available. There has to be a reason. best I have heard so far.

Sorry to bring up an old thread but it made sense instead of starting a new one... If this is the case, would it be more beneficial for us engine builders to strive for a larger than normal quench height? ~0.060 - 0.080?