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There is more we could talk about on this subject so I am starting new thread to deeper dive on this subject.

My understanding is that this is one of the advantages of the more modern pent roof combustion chamber? Smaller, tighter and faster burning combustion chamber allowing more power with less timing and an overall more efficient combustion. This is why I have been looking at mounting the 24V heads to our 3.0 12V block. I believe overall efficiency will improve and allow more boost and timing to be run at a higher compression ratio for better off boost efficiency and performance.

I am far from an expert on this, but the stock 12-valve head has a fairly small (modern) combustion chamber in it (and the piston.) My initial guess is that the 4v/cyl would increase the size (at least cross-sectionally where the piston and head meet.)

The main comparison is between old style V8 heads and newer modern ones. (how else is Chevy getting so much hp out of the LS2 (or whatever the latest 2v/cyl engine is.) I don't believe the valve size has changed as hp has shot up. )

4v/cyl have the advantage of more curtain area (more CFM.) However, to truely take advantage of this several other engine parameters would have to be changed (ie. max-rpm, displacement/stroke, cam lift, etc.)

I think there is a lot of potental still in the stock 2-v/cyl heads depending on the goals.

I would love to hear more about 24vshadow's tuning efforts are going.
 

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how else is Chevy getting so much hp out of the LS2 (or whatever the latest 2v/cyl engine is.
You're a few years behind. The LS2's came out in 2005 with 400 HP with 6.0L, where the 7.0L LS7 is in the new Z06 making 505 HP, and the ZR1 is powered by an 6.2L LS9 making ~620 HP (supercharged)


[/hijack]

Yes there is potential in 2 valves, but you can have an overall valve area larger with 4 valves/cylinder.
 

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Thanks for the interest Ed, things are going well so far :) got idle, cold starts, and an OK tune on the car. on a hard pull i can chirp 2nd gear right now, never got it to do that on the 12v before...

but here goes, heres a picture of the 24v compustion chamber. (not my OWN heads but the same type) total advance runs up to 32.5* and idle is around 22*



and heres my current config, you can see (i hope) the spark table on the right
 

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Your VR table looks a bit funky. Seems like your REQ fuel is too low. VE values shouldn't go much over 100% - only to richen the mixture under boost.

Probably a good idea to start a new thread for this discussion then irritate Odonti in his thread :)

I think 2 valve heads have great potential but 4valve is (typically) better out of the box. Every modern hi-po car (except for a few exceptions ie Corvette) is 4 or 5 valve per cylinder now. I'm going to buy a pair of heads and an intake / exhaust off a 2.5 SOHC 24V 6G73 and start my attempt to bolt those to a 12V 6G72 and see what happens. I will likely end up using factory DOHC TT pistons. Thoughts? I am after a motor that will have N/A fuel economy and off boost performance while surviving up to 10PSI of boost likely from an HX35. Megasquirt controlled water injection will be utilized so that I wont have to pull much if any timing when under boost compared to WOT N/A.

DOHC N/A pistons are another option. I have been tempted by the idea of a water injected 10:1 compression boosted 3.0
 

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and you want to run LESS timing with the 24v heads?

whats the max timing on the 12v motor (modified or normal) 12-20* at most? the Hi-Po spark map i have off of a known good boosted 24v (i only got the N/A part) runs up to 32.5* advance on a 9:1 CR motor

somethings not adding up in my head there lol

This is the theory that I understand to be true. I haven't been able to actually test under real world conditions. It's something I would like to discuss though.
 

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*shrug* 228K mile motor on an OEM clutch before the swap.

yeah, i have a bunch of things to iron out on the tune, been working on it lately, its running better but not perfect. it has some issues: hot idle surges etc, if you want to give me some pointers, go for it :) im hoping to take it to a dyno when i get home for the summer and see what i got. also going to the dragstrip in a week

back on topic -
heres an OEM high octane map for the 24v motor, a bit more conservative than the one im running
 

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a bunch of engines, 90's 2.5 avengers, 2000+ 3.0 eclipses, 3.0/3.5 Galants diamante's (just SOHC ones, DOHCs were the 3000GT/stealth and select other cars)
 

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The 2 valve heads have the advantage of being a crossflow semi hemi head which gives them a lot more possible potencial then a bathtub, but there area lot of good heads available for our motors.

I think Mitsubishi spent very little time on the older 6g72's figuring out what was "best"

They have the exact same 24valve sohc and dohc heads on 2.5L 3.0L 3.5L and 3.8L engines. (i believe 2.5L galant vr4's at one time had dohc 24v heads).

The only heads which truly match the engine on purpose (not accidentally) might be the Sohc Mivec heads on the 3.8L 6g75 found in the 4th generation Eclipses. These heads are made for the larger bore of the 3.8L unlike the dohc and sohc 24v heads which structurally dont have enough meat at the edges of the combustion chamber to seal the huge bore with lots of power (without modification). I believe both Sohc and DOHC 24v heads were used on the 3.8L in the montero (maybe just one) but you can see that mitsubishi wasnt very careful in the past about putting the proper heads on their motors.

They were too lazy to even change port sizes or combustion chamber sizes.
 

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someone said "6000rpm head on a 9000rpm bottom end" i believe the quote was, right brent? :)

a good set of cams would do WONDERS for the 24v motor. up the redline to 7-8000 rpms and this thing will scream. my 24v feels like it wants to rev quite a bit more but its hampered by the cams and intake/port work
 

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arnt the 2.s sohc24 valve cams a bit hotter then the 3.0 version,
i need to get picks of my buddies super charged 3.0 avenger soon

that timming map does that mean high octane as in rce fule or premium i might try it in on my 12 valve,i have a few maps also
 

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its too bad we cant mke some nice maps from the stock ones to use in ms,
speaking of ms havent seen m90daytona around in some time
 

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that ignition map was for use with 93/94 octane fuel.

as for the 2.5 cams, they dont have as much lift/duration as the 3.0s. swapping a 3.0 cam to a 2.5 is a gain, not vice versa
 

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Not only that, but the 24V SOHC engines have one cam phased a bit different from the other. From what I remember reading the front bank is usually the one to get fried due to this.

Anyways...the topic was about igition timing. :) Yes, the theory is that you want the maximum cylinder pressure to happen about 12-15* ATDC. Becuase of piston speed, flame front speed, and cam timing it is necessary to advance the timing of the ignition spark to some value BTDC so that this happens. A well designed combustion chamber(and this INCLUDES the piston top!!) will be able to propogate the flame front fast and control the burn so that ignition timing can be delayed as long as possible so the spark happens as close to TDC as it can. This will yeild maximum torque(hp is nothing more than a mathematical function of torque).

This is where quench design comes in to play. A normal open type chamber in the head with a dished piston makes it so there is a LOT of area for the flame front to cover. This means that ignition timing has to be advanced to some period BTDC so that complete burn can happen and max cylinder pressure happens at the correct time. Now, take an modify this chamber and piston to utilize quench. Make it so the piston top directs the mixture toward the exhaust valves and ANY other area is squeezed as much as possible to get the mixture to that condensed area. This is also where the spark plug is(on purpose!!!) on nearly ALL engines. If you can get the mixture in that compact area, the flame front has less area to cover and therefor ignition timing can be retarded back towards TDC...more power and more efficiency will result. Also, cleaner burn, and lower EGT's due to complete combustion of all of the mixture before the exahust valve opens!!(this depends on the cam desing, I admit).

This type of design also has aother advantage. The term "mechanical octane" I beleive was coined by TOO(of Endyn fame). What happens is becuase all of the mixture is forced in to the one area, there is a far less chance of stray mixture getting ingnited by hot spots, or self igniting.

This is an idea I REALLY want to try with the 2.2/2.5 engines. I also think it could bennefit the 3.0(as did Endyn as they DO offer pistons for the 24V 3.0 turbo applications).

I remember TOO saying that he typically didn't design an engine around a particular compression ratio. Rather, he designed the combustion chamber to work correctly, let the compression land where it wanted to and then tuned it from there. Before the TOO forums were shut down, ther was a guy that was playing with an old Olds or Pontiac V8(I can't remember exactly). He was colaberating with TOO on his design. It was to be a FI engine, so the guy was aiming for a lower compression number. I remember TOO saying to make the dome so it would take up as much of the intake side of the chamber as he could while achieving the ratio he wanted. It actually ended up that the piston had a very slight dish on the exhaust side and a mid-sized dome on the intake side utilizing as much quench as he could. I don't know if he ever got them made, but it looked to me to be almost somethign we could use!

If anybody gets a wild hair, I was told that most piston manufactuers will make anything you want. If you send them a test peice they will copy it for you...of course at a price. However, the price wasn't as bad as you'd think...not much more than if you were to get any other custom set of forged pistons made....
 

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I cant agree with doming pistons making for "much" better combustion.

A creative dome (like one that is biased away from the plug) is simply a superior way to increase compression, which does increase HP even though domes hurt flame front propagation.
The theoretical "best" situation of a combustion chamber that ignites almost all of the fuel right after TDC isn't applicable to any cylinder head I have ever heard of either. Doing the most with what you have and doing what is actually possible is what matters.

When adding a "good" dome, the hp gains are 95% due to compression increase, and 5% due to better design as far as I am concerned. Poorly designed domes hurt HP and cause detonation that properly designed domes do not have such problems with.

The idea of changing the combustion chamber really is not so helpful unless you have a custom head where you can make the valves closer together, angle them differently, and do everything to get a compact combustion chamber without having the compression too high.
And if you sacrifice port shape/design to do that, you wont be making any gains.

3.0 piston dishes actually mirror the 12v head combustion chamber, unlike 2.x chryslers. That puts them way ahead of the game, and I dont see how you can improve without changing the compression ratio. The moment you change the compression ratio, I am going to throw your results out because they cant be differentiated from compression increase gains.

If you want a weird dish that biases towards the spark plug, your piston is going to be very heavy and questionable on strength because of the way pistons are made. Also, since you cant really change the combustion chamber in the head very much, making a narrower area away from the spark plugs will just give more chances for hot spots to start detonation. Effectively ruining the whole point of quench areas. Quench areas actually cause detonation if they are not narrow enough. You would have a very hard time modifying the piston to fill the chamber yet not hit valves etc etc etc.
I don't think you can do much better then a flattop piston. Any sort of dome is going to make compression very very high. I believe I have seen an 11:1 sohc forged piston and there is a simple slight dome that mirrors the stock dish. The problem with this dome is that it requires goofy looking valve reliefs and I would probably rather have a flatop piston and run more ignition advance. That is what you run up against. It is often easier and more effective to run the best piston that will easily work, and throw a little more ignition advance on it.

And as far as I am concerned, combustion pressures before the piston reaches TDC are not slowing the crank much but they are relieving the upward forces that are being placed on the rod which is busy stretching.

A 20-30 degrees BTDC, the piston is pulling away from the crankshaft because piston speed is slowing down at the top of the stroke. So the only real gains I see are those which the weight of the pistons moving upwards for 20 degrees would pull on the crankshaft (adding what....0.005hp?). At that point you have decided to sacrifice rod service length for supposed hp gains. Not igniting the spark BTDC will cut rod life in half as far as tensile loads go..which are pretty much what kills almost all rods.

The best thing for a 3.0 head would be modifying the head to have a slightly different spark plug entry that is more focused on the center of the chamber (rather then tucked into the side of the chamber). It would make changing plugs a chore and probably require modifying the valve covers etc etc but it would actually help combustion.

We break pistons because the flame front starts far away from the thin parts of the ringland where the intake and exhaust valve reliefs are. This gives hotspots the opportunity to ignite fuel at the edges of the chamber, those hotspots collide and boom, no more intake ringlands. A centralized spark plug would allow gobs more boost/nitrous/HP before massive detonation breaks pistons.

Also, most 4 valve motors dont have pistons that match their combustion chambers. They just have a lazy circle shaped dish even though the combustion chamber is shaped more like a square.
So there is easily space to work on improving those pistons and also 2.x chrysler pistons.
 

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The chamber designs were part of an entire package. It really starts in the intake manifold, then the port in the head, ect, ect.

Domes by themselves won't make power. The entire chamber really has to be looked at and desinged as a system(just like everything else in the engine). Higher compression by itself will not yeild that much power. I just recently read that 3% is usually an accurate guess. However, if you design it in such a way that combustion is made more efficient, you WILL make more power.

If the piston top and head chamber are designed to work together to sqeeze the mixture in to a more confined, controlled area, then the flame front will propogate evenly throughout the mixture and fast. If you just stick a dome in there jsut becuase, sure it's not goign to do any good. The purpose of the dome in this case is NOT to purposly raise the compression, but to more effectively control the mixture and guide it to the area it needs to be in for complete combustion to accur.

I agree with what you said about breaking pistons...now utilize true quench areas and this will become less of an issue.

I also agree with what you've said about the spark plug placement, however this is out of the reach of pretty much everybody here. So, we work with what we have. That being said, we can still design a piston that guides the mixture to the spark plug and biased toward the exhaust side of the chamber without raising the compression much, if at all!

TOO argued these points over and over again. He's been doing it for decades. For those that have taken notice and listen to what he has to say, and put in to practice his methods, we will reap the bennefits. For those that choose to argue...*shrugs*. I'm not going to try to fight for everybody to beleive it. For me, seeing the dyno graphs and the BSFC numbers with the power being produced...that's all I needed to see to convince me that he knows his stuff. To top it off, he could answer almost ANY question having to do with engines and give you proof to back it up. That's something VERY hard to come by...
 

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i know there are a couple issues with the 24v. the quench area isnt the best (one reason the stock pistons die) and the pistons dont like high intake temps. the main reason that pistons die on the 24v F/I applications is the cams an people push the PSI too high and dont cool the intake temps enough causing preignition/detonation. usual result is broken ringlands (rings are gapped tight for N/A spec) Ian Rezlo did a bunch of work on boosting the 2.5/3.0 and had some interesting things to say about the motor.

see this post by ian for his finds
Rezlo's finds
 
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