Pre combustion chamber design research

[dixiepc]

Unfortunately I've never heard of any dynos in my area. Wish I had the time and money to do that kind of testing though.

 

[Will L.]

If you get the chance, call around. Every once in a while, there is a race team or performance shop that does a few on the side low price just to help pay for their test equipment.

 

[Rockabillyrat]

HP numbers should not be the only factors when choosing precups in my opinion. I don't think there is going to be a huge difference between them when it comes to HP anyway. The thing I'm interested in is seeing the difference in precombustion chamber pressures throughout the RPM range. And compare that to crank degrees to determine its duration. As we increase the air density/pressure and fuel volume for performance what effects will it have in the prechamber? I have no doubt that either cup can handle the extra fuel volume. Even with a 300cc pump is only .3cc per injection event. And we should obviously see a rise in chamber pressures. But what about the duration of that pressure? At high RPMs will the square cups bottle neck the chamber too much causing a longer than desired duration of the flame front? These are the questions I want to answer. I think if we can figure out how to speed up the combustion event as much as possible while keeping cylinder pressure low. That should help keep the bottom end together and make good power.

But precups are just one part of the equation. Monitoring injection pressure and duration is another important thing to look at. For one thing it will be awesome for tuning. But more importantly we can see what effect the increased fuel volume has on injection duration. That will help us figure out if our injectors need to be modified for a more desirable injection duration. Or even possibly increasing the cross sectional area of the discharge ports in the pump it's self. The largest fuel volume at the shortest duration is going to make the most power. But will also increase cylinder pressures. So be being able to monitor as much as possible on the engine we can hopefully find the best combination of parts and calibrations to make reliable power and finally have answers for this platform.

As I typed this out prechamber temperature might also be another thing worth looking at.

 

[Will L.]

I understand how monitoring pressure changes can be helpful. But in the end, how much fuel is burned & what power it creates are the 2 factors that are the most relevant.
If someone has 18:1 compression with a huge turbo and someone else has 21:1 compression with a small turbo, does it matter that one has 200 more psi in the cylinder?
If they both burn same fuel amount and one produces 10% more power- that is better results. Or if they have same power but one burns 10% less fuel- that is better results.

temperature variation I don’t see as a controllable factor. Drive with empty rig on flat land vs uphill towing. Egt amount varies dramatically. Ect is mainly cylinder head temperature dependent, and we know there is huge swings in ect for a variety of reasons.

As to which will last longer, it is all speculative because you would have to have multiple of each and actually wear them out then learn at the end. That is a lot of what we did in fleet testing- 2 trucks set up one way, 2 with only a single changed item, 4 driver sets (multiple shifts) that rotated he trucks on weekly basis. Then saw end results. Having so many trucks that crushed 100,000 miles a year made it easy to see results soon. Some things people assumed would be worse actually helped.

 

[dixiepc]

Let's here some examples of those results Will L.

 

[Rockabillyrat]

Monitoring cylinder pressures is something the OEM has done for a while. And the aftermarket diesel performance industry has caught on. It's an important factor when guys are tuning these monster common rails these days. Now I know comparing a 6.5 to a 3000+HP billet cummins is insane. But with the history our platform has. I think its very important to take a look at cylinder pressure as we go for 400 maybe 500 HP. I'm just tring to do everything I can to keep a 6.5 together. And high cylinder pressure is a killer of any engine.

Power output and fuel milage are definitely what matter when it comes to results. But if an engine is reaching unsafe cylinder pressures to do it so. It's like driving a ticking time bomb. By monitoring pressures we can find the limits of the 6.5. I might be wiling to compromise on HP and/or fuel milage depending on what I'm setting the engine up for. Racing, hell yeah let's see what it can take. But my street truck I would dial back to keep it from scattering the bottom end.

 

[Kaulin C]

I guess I'm of the opinion that any 6.5 pushing upwards of 400 hp is a time bomb by nature. Sure they can make that much power, but they were never designed to, seeing as they already start cracking apart at OE power levels. The IDI, at best, is outdated tractor level technology. I'm content to chug along with great fuel economy, decent power and torque, and a great sounding engine.

That being said, I think it's awesome to read about the limits of this platform being pushed. It's impressive what an IDI can be made to do with the right touch of expertise in tuning. I just don't expect to get a lot of miles out of a build making that much power, no matter how much time and effort goes into keeping it alive.

 

[Will L.]

@dixiepc
No results on things like different precups. Just saying comparisons on identical trucks with only a single difference. Things like oil filter care vs none or controlled rpm ranges or db pumps vs ds pump operated trucks.

@Rockabillyrat
On the cylinder pressure- omega.com makes some of the best sensors in the world at pretty descent prices.
Just run sensor in adapter from a compression tester?

Guessing you’ve studied into this some, what is peak pressure you would expect to see? Let us all know and maybe someone can come up with a sensor on the cheap, possibly free.

Destroying some test engines that are cracked web engine to learn spec of head gasket damage range maybe?

@Kaulin C 400 is an obtainable long term number, depending how you build it. Strong bottom end, well balanced, flowed heads, big turbo with dropped compression, and keeping temps very balanced. How smoothly power is pulled and how often would of course come into play.

 

[Rockabillyrat]

@Will L. I would expect peak cylinder pressures to be in the 2-3k PSI range. But I really want to test a few stock engines to get a baseline and go from there. Then monitor pressure changes as modifications are done to the engine.

I want to upgrade to a PICO scope. My snap on scanner has a nice scope built in. But the PICO is a much better scope and is PC based. I've looked into some of the 10k PSI sensors that should work with an adapter in the GP hole.

@Kaulin C 400hp should be no problem for a GEP 6.5 to handle. The GM blocks I wouldn't trust though. N8in8or has hit the 350hp mark with a hot .330" pump. With his next upgrades he should be really close to 400hp. Of course once he tears it down for inspection we will see if the bottom end has any issues. For the 400+hp range a DB4 can be set up to push that and still be street driven. The 500hp range will take extreme modifications and really only be good for racing or pulling.

In my opinion the 300- 350hp range is perfect for a daily driver. A .330 db2 or cam ring swapped DS4 can supply the fuel needed and not break the bank.

 

[Kaulin C]

@Rockabillyrat yeah much better metallurgy in those blocks. Especially if you even out the coolant flow I'm sure they hold up virtually indefinitely within reasonable power levels. For more longevity I'll stay in the 300-350hp range when I finally get around to building my rig up.

 

[Will L.]

Obviously getting your hands on a p400 would be best in the upper ranges.

 

[Rockabillyrat]

Obviously getting your hands on a p400 would be best in the upper ranges.

Even with a p400 your not bullet proof. Two guys reported broken cranks this year. One
guy in Australia, the other here in the states. But I never got the full story or build specs on either one so who knows what really happen.

 

[Will L.]

True, and they dont come anymore balanced than any other engine. I would love to see the broken crank p400 if you have any pics or links to read about.

 

[n8in8or]

In my opinion, the 6.5 community will be best served to see how hard an Optimizer can be pushed and successfully survive. A properly prepped P400 should be able to live at a high power level, but most of us are never going to be able to get our hands on one now that GEP stopped producing them. Meanwhile, GEP is still making new Optimizers every day, and there are some used ones available I the market as well (though just buying a used Optimizer is not a guarantee of having a good, buildable engine). I want to focus efforts on an engine that is readily available to the masses for some years to come so any testing that is done can be readily replicated and the outcomes of which can be expected to be achieved.

 

[Rockabillyrat]

True, and they dont come anymore balanced than any other engine. I would love to see the broken crank p400 if you have any pics or links to read about.

All I have is this picture. Both post are somewhere on the 6.5 Facebook pages.

@n8in8or nailed it! My thoughts exactly!

 

[WarWagon]

Come spring I'm gonna pull my engine for a refresh, head studs, timing gearset, and a new Bull Moose pump. While I'm into this I'll have to make a decision on squares or diamonds. I have diamonds on now but I have 3 sets of Optimizer heads with squares. Want to use the best ones.

Run the biggest precups you have in your hands. That BIG Moose pump will use it. Assuming you want to build it once and not experiment...

You are building for power. Small econo precups are for economy and say 100 HP. Compromises in between the extremes.

The swirl aka high speed centrifugal force separating the fuel air charge into black belching sunblock smoke continues to be overlooked with some strange love of small precups in this thread. Running small NA Precups, again, they reach a point with turned up stock pumps where more fuel is turned only into smoke and high EGT. I LOVE to HATE the 6.2 NA precups I had to run due to budget. Maybe if I loved the coal rolling the small precups were capable of, but, I don't.

The efficiency of this IDI design is from swirl to finish completely burning fuel above the piston. It doesn't all burn in the precup. Small precup with 15-24 PSI of boost (depending on turbo I used) and lots of fuel has the air moving way too fast... It's moving in two circles... Just like a centrifuge separates soot out of engine oil... Too high of a speed swirl can separate the fuel from the air. For low power econo engines small precups were tuned for 100 HP and enough swirl to burn all the fuel at 100 HP to eeek out every cent in MPG a delivery truck could get. Add fuel and the precup is a choke point as well as spinning the air too fast.

@6.2 turbo has a point about the low end power with NA precups that would apply to the slightly smaller than diamond precups you ask about vs. diamonds. It's a trade off when the NA precups choke out say when you want to see what that Moose Pump's worth all throughout the RPM range. MT's, like I believe 6.2 turbo is running, make a difference here because I'll be dammed to see our automatics let you lug an engine to where you can really feel "low end TQ". They tend to want to rev to redline (downshift) with any heavy throttle. This downshift-happy auto is really a gutless wonder with a worthless above 2200 RPM GMx turbo. No, the stock TC clutch won't hold the engine back to lug it either: tried that and smoked the TC Clutch. I ran the 6.2 NA precups and ran clean, but, only with a 6.5TD stock setting of fuel. That gets about 12 PSI of boost on an ATT capable of 15-18 PSI. I agree with the point made about low end and smaller precups and suggest it applies to your choice. I believe some larger than diamond precups had a flat spot in the low end that Dennis experimented with.

No offense but the others can relearn this small precup lesson the hard way. Cracking one's neck to avoid looking at the unpopular black smoke cloud - Sorry but SMOKE difference is very important along with the HP and TQ numbers. Yeah, it's going to be hard to show the low end power differences and do you want to limit yourself to better low end power at the expense of top end power? If so why are you putting on a Moose pump again?

 

[Rockabillyrat]

I don't believe the super swirl theory.... But you are right about one thing. The air velocity on a single orifice precup like the ones found in the 6.5 do effect the combustion in the prechamber. The high velocity of air pushes the injected fuel to the chamber walls. Unlike a double orifice precup that does a better job of keeping the combustion centered in the prechamber. You can see proof of that in these thermal images.



But there is no way the increased air velocity from the n/a 6.5 cups are going to "put the flame out". Fuel is vaporized on the hot chamber walls then gets more heat from cylinder pressure to start the combustion event. So your theory of the air velocity pushing the fuel out of suspension is correct. But the flame isn't being put out. It's just been moved to the walls of the chamber. Fuel injection starts close to a TDC and ends slightly after. So the fuel injected after top dead center is being vaporized by the pressure differential at the cup throat. Turing it into a focus flame front positioned right at the piston channel. I believe either cup can handle any amount of fuel a DB4 can push out. After all the most I've seen is 300cc which is only .3cc per injection event. But what makes me like the square cups is it matches the piston channel better in my opinion. It should also increase the flame front velocity that might have good effects on the dual swirl in the main chamber. More pictures (Sorry about the quality, i had to zoom in)




The only thing that I can see being an issue with the square cups is if it can't exhaust the combustion charge fast enough when we increase air density and fuel volume at higher rpms. But then I look at what R&D had done with those small 7.3 precups and I believe I'm headed in the right direction.

 

[Kaulin C]

But the flame isn't being put out. It's just been moved to the walls of the chamber.

Pushing the fuel to the sides so it doesn't have good exposure to oxygen to efficiently combust is essentially putting the flame out (at least part of it). Too small of precups cause localized regions of rich fuel condition in the chamber, causing high egts, excessive smoking, and poorer efficiency for high fueling applications.

After all the most I've seen is 300cc which is only .3cc per injection event.

Yeah it's only .3cc, and that doesn't sound like much. But a fair portion of the fuel will have been vaporized by the time it's moving through the throat (thus taking up more volume). And beyond 3,000 rpm, that amount of fuel has to move through the precup throat very fast (at 3,000 rpm, the injection event happens 25 times per second. taking into account the 4 stroke cycle, 25 injections of fuel has to move through the precup throat in about a quarter of a second total). Add on top of that high boost from a big ole turbo, and there is a lot of mass that has to move through that little hole in a fraction of a second.

But the whole point of this effort is to do a side by side comparison to see which ones actually will perform better. And that may require tweaking things like the fuel flow curve of an injection event, the compression, the pop pressure, piston to head clearance, precup throat angle, etc. for a small precup throat to work with lots of fuel. There are a lot of variables. I just tend to lean to the side of previous experience, and @WarWagon has more firsthand experience with these engines than most of us ever will.

I think that under the right conditions, a small precup will make more power than a large one. Those conditions are smaller amounts of fuel in NA or low boost applications, where the flow speeds through the throat are in an ideal range for complete combustion of the fuel. Throw too much fuel and boost at it, and a small throat is a choke point, causing more separation of the fuel than vaporization.

The fact that the diamond precups don't line up with the channel in the piston is an interesting point though. My theory is to reach a desired power level, they just needed a little larger throat, and didn't want to bother retooling to change the shape of the piston as well for a minimal effect on power at stock levels. Perhaps reshaping the channel in the piston to match the diamond precup would be something to investigate as well (or reshape a square cup to have the same width but a larger throat area to match more with the channel in the piston). When reaching the upper limits of horsepower, there may be a perceivable difference if the precup and piston shape are matched.

 

[WarWagon]

I am more theorizing about the combustion occurring above the piston rather than in the prechamber. It could be both with a bottleneck. The very bottleneck makes a difference on overall combustion and economy. It's pretty clear the precups have a range and adding a turbo and more fuel changes it.

Matching the precups up to the piston relief is an eye opener. I would bet the new piston tooling cost guess is correct.

 

[Will L.]

If matching the exact width were that crucial, they would have have altered the piston tops too match the sizes over the years.