2 vs 4 valve/cyl heads, their capabilities and the impact on tuning

[Reaper1]

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....

[Ondonti]

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.

[Reaper1]

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...

[87turbodance]

Might as well attach some pretty pictures

DOHC 24V N/A pistons:


12V 6G72 N/A pistons:

[Shadow24V]

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

[thejoker1290]

so if i wanted to put the 24v heads on my car i should change the pistons? right

[Shadow24V]

6G75 on the left, 24v SOHC on the right

[87turbodance]

I stole a little info from 3SI.org

Per stealth316.com: (Stealth 316 - 6G72 DOHC-Turbo Head Info)

================================================
Flow bench measurements, stock DOHC TT head, cfm
Valve Lift Intake Exhaust
0.100" 86-93 77-80
0.200" 165-179 151-164
0.300" 223-230 191-200
0.400" 246-262 195-216
0.500" 249-256 196-219
Values from Barry King (Team3S) and Pat (aka PWR on 3SI).
Max vol. flow at 8000 RPM, 100% VE each cyl. bank = 210 cfm.

Basic port job on stock heads.

INTAKE Exhaust
Lift Flowmeter Corrected CFM Flowmeter Corrected CFM
.150 52% 177.7 42% 123.0
.250 72% 246.0 65% 190.4
.350 82% 280.3 73% 213.8
.450 89% 304.2 79% 231.4

Peak Intake Flow 304.2
Peak Exhaust Flow 231.4
Intake/Exhaust Ratio 25%


I'm going to assume the SOHC 24V head wouldn't be far from these numbers with a little port clean up

[87turbodance]

compare those to Ondonti's heavily ported 12V heads:

87 cfm @ .100" lift This was very amazing The stock head only flowed 43.2 Crappy thing is I lost a few cfm .100"-.200" lift on exhaust
132 cfm @ .200" lift
184 cfm @ .300" lift
200 cfm @ .350" lift
210 cfm @ .400" lift
215 cfm @ .450" (cant measure the ported heads past there because valve guides are installed but you can see there would not be much gains lifting any higher).

Basically shows me that going much higher on cams (mine are .450" compared to the .350" stock valve lift) would not bring much gains and just cost a buttload since they would have to be custom billet cams.

Exhaust
137 cfm @ .350" lift
149 cfm @ .400" lift
156 cfm @ .450" lift
157 cfm @ .500" lift

[c2xejk]

This thread is really taking off. A couple notes. VEs over 100% are very possible. The heads on my Voyager had the VE peak at 108% @5200RPM. With the new set that are going on soon I hope to hit 120+% (won't know until I get them on and the vehicle tuned.)

To find the port flow at peak valve lift, that the theoretical valve lift and multiply by 87%. The 87% is "rule of thumb" for how much the lifter will squish down. For example, I am currently running a cam that will create a theoretical lift of .409". So my estimate is that the actual lift will be ~.356".

So what lift cam are the 24v guys using? Also one of the two sets of flow data numbers indicates some port flow backup issues (a problem even if the cam doesn't cover that lift.)

I believe there is a simple fix to increase max valve lift a little bit more. Shave a little off the underside of the valve spring retainer.

The 3L has a very big bore compared to stroke. So there is a lot of room for valves (even in the 2v/cyl). If Brent has to spin the engine to ~8K to max out the +2/+1 heads he has right now... I think money might be better spent on stroking the crankshaft than going to 24v heads... There is such a thing as too much flow...

[Shadow24V]

i emailed Russ at Tearstone.com about stock vs P&P 24v head flow numbers. alse on his site there are some cam specs, stock and different stages that RPW offers. ill post up the numbers if i get them

[Reaper1]

Lifter squishing down? Huh? They're filled with oil(or they're supposed to be). That's a fluid...fluids are inompressible last time I read my fluid dynamics book. Please expalin this to me...

[87turbodance]

Again from 3SI.org

DOHC TT

LH Inlet Camshaft MD150391
(MD175577)
MD175577
MD192011 1

1
1 9004.1-9201.3

9202.1-9205.3
9206.1-end Cam height (std):
up to 9205.3: 35.49
from 9206.1: 34.91
Journal diam.: 25.96
Lift (gross):
up to 9205.3: 0.375 inch
from 9206.1: 0.352 inch
Valve timing:
opens: 16º BTDC
closes: 55º ABDC
Duration: 251º (gross)
Duration: 208º @ 0.050"
Lobe separation angle:
108º cam
Overlap:
33º crank, 16.5º cam
Overall retardation: 1.5º











LH Exhaust Camshaft MD159598
MD192019 1
1 9004.1-9205.3
9206.1-end Cam height (std):
up to 9205.3: 35.20
from 9206.1: 34.91
Journal diam.: 25.96
Lift (gross):
up to 9205.3: 0.364 inch
from 9206.1: 0.352 inch
Valve timing:
opens: 50º BBDC
closes: 17º ATDC
Duration: 247º (gross)
Duration: 206º @ 0.050"
Lobe separation angle:
108º cam
Overlap:
33º crank, 16.5º cam
Overall retardation: 1.5º

[c2xejk]

Quote:

Originally Posted by Reaper1 Lifter squishing down? Huh? They're filled with oil(or they're supposed to be). That's a fluid...fluids are inompressible last time I read my fluid dynamics book. Please expalin this to me...

Ever taken lifter out and squeezed it? They do squish... Why I don't know, just what I have observed... This is why solid lifters can give you more hp than hydraulic lifters...

[mopar3.0]

on the piston topic that did the stock 10.1 mitsubishi diamante make hp wise, was it 175?