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Pre combustion chamber design research

Rockabillyrat

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Unfortunately pushing fuel to the wall of the prechamber is something we have to live with in our style IDI. That's the characteristic of a single orifice precup so both square and diamonds will do the same thing. Both suffer from rich areas at the walls during the start of combustion. And the volume of available oxygen is the same in both cups. The only difference is the velocity of air entering and leaving the prechamber due to difference in throat diameter.

You do bring up a good point about moving air in such a short period of time. Especially at high RPMs. That is why I want to measure the pressure duration. If we see any changes in the duration of pressure in the prechamber after TDC that could indicate if the cup is becoming a restriction. I would expect to see some change. But is it truly enough to make a huge difference in performance.

When it comes to the extra fuel volume. The prechamber isn't getting the full volume of fuel instantly. Its stretched over several crank degrees. And with our delay style injector nozzles. The fuel volume is limited in the start of the injection event. Small amount a fuel to start the combustion event and a larger volume at, and after TDC for combustion in the main cylinder. I think the smaller cups might have an advantage here because of the increase in air velocity leaving the cup should atomize the fuel better.

It's hard to argue with the real world experience WarWagon and Will L. Have. But even I feel that NA 6.2 cups may have been too small. I dont necessary think that the smallest cups possible are the answer. Nor do I think going with the biggest ones is too. I think the cup need to be set up for the power levels of the engine. Much like a carburetor. At the end of the day I'm just trying to figure out why GM really made changes to the cup design. The addition of a turbo makes sense. But on the other hand the introduction of the diamond precups line up perfectly with huge emissions changes in the mid to late 90s. Its completely possible the diamond cup was a last effort to keep the outdated 6.5 on the road till the duramax came out. Ford had ditched the IDI in 94 so they never had to deal with reworking the 7.3idi to meet the new emission standards. Who knows maybe if Ford stuck with the IDI as long as GM did we would see a more dramatic change in their turbo precups.

Maybe the matching of the pistons and cups make no difference. Or maybe it didnt make enough of a difference for GM to justify developing new pistons for a platform they knew was going to be phased out. They dont have a good track record of going the extra mile on this platform. Its completely possible that Gm once again took the cheap approach. What may have been "good enough " for GM engineers at stock power levels might not be ideal for us as we increase the engines power output. Or maybe its 15 years as a mechanic causing my hatred and untrustworthy view of engineers LOL.

At this point NONE of us have the answers. And we are all speculating. There are good points on each side of the argument. But no solid proof either way. I hope to put and end to that by Investing my own time and money to get some sort of answer and move this platform in the right direction. Precups are only a small part of the big picture though. So much still to learn about these old oil burners.
 

Kaulin C

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I definitely understand your view of automotive engineers. Seems like nothing is designed to be worked on these days. Case in point: the majority of my colleagues in the mechanical engineering department have never turned a wrench, nor will they ever be required to.
 

Will L.

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Other possible factors-

6.2 cylinder volume vs 6.5 so larger just to handle the increase in mass. Then as turbos changed (n/a to gm3 to gm4 etc) they altered the cup throat to match that?

also could be for starting ease. The old glowplugs sucked. Smaller hole to trap more heat? Really low probability on that but may have factored in.
 

Rockabillyrat

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@Kaulin C I understand engineers have their hands tied to find solutions for modern vehicle strandards while keeping production cost low. But these newer vehicles are something else... I can see why the younger generation doesn't want get into this career.

@Will L. I agree that GM probably changed the cup design as displacement and air density increased over the evolution of the 6.2/6.5. That makes complete sense. But what was the real reason? Especially the jump from the T cups to the Diamonds. Did they have to for emissions or was it a engine performance issue? Thats the million dollar question
 

Will L.

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All the military stuff doesn’t need emissions. They wanted reliability then power also never focused on mpg.
They went with diamonds. From optimizer to the p400. Even the bumped power hmmwvs that banks did. He had them lower compression, built db2 with aneroid, turbo s300/400 hybrid, different valve springs, balanced lower end, 4000 rpm and still kept diamonds.
Again no emissions looked at.

1994 gm had the emissions in place with ds4. All the specs were met with db2 except being able to monitor fuel called and fuel delivered. They hammered that in the class at stanandyne. In the class the instructor showed new db2 burned cleaner than ds4. We swapped the ip on the dyno engine as part of class and saw results.
Thats why I just don’t buy the diamonds are for emissions.
 

Rockabillyrat

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The military using diamond cups is a very good point. I dug heavy into that a while back. And what I found had me a little confused. I downloaded every military service manual I could find. Every pre 01 manual I looked at only list one precup part number. For both NA and Turbo 6.5 engines. All the post 01 service manuals listed a separate NA and Turbo cups for the 6.5 engines. So why did the military wait till 01 to start using diamonds in the turbo engines? Can anyone confirm this to be the case? I found that to be very odd that the service manual didn't mention the use of different cups till GEP took over production of the 6.5.

It's hard to argue with anything Gale Banks does. After all the guy is the grandfather of diesel performance. Did he figure out that diamond cups really are the best choice. Or did he discover that the cups didn't make enough of a difference to justify changing them? If they truly make that big of a difference then why didn't the sidewinder kits not come with different cups? It made nearly the same power as a 6.5 while using the smaller cups.
 

Rockabillyrat

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Maybe there are two benefits to the diamond cups. Emissions control, which GM needed in the mid 90s. And maybe they also helped increase reliability, which was good for GM and the military. If the military truly didn't use diamond cups till 01. Maybe its because they had a high failure rate on the turbo engines. And switched to diamond cups to help correct that issue.
 

Will L.

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Really the military is just running the engine as it came in the vehicle. AM General was using the engine as it came from gm until the overheating lawsuit Then GEP was formed and they built what they wanted out of it. So pre GEP is really just GM regardless of if military was the customer.

The banks kit didn't get inside any of the engines, just bolt on. even if there were an extra 10hp and 10 torque from precups- not worth it for dealer add on accessories.
 

WarWagon

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Mr. Banks worked with what was out there. Yes he showed the cheap OEM's that a turbo could help power and cut the smoke down.

You ever see the black cloud a 5.7 Olds diesel would leave behind climbing a grade to altitude? CA CARB did and banned it's new in CA sales for like 1985. Ever see a 6.2 NA engine blowing smoke out of both pipes in the stock configuration on a grade in The Rockies? Then blow blue with high altitude timing set by a shop? (Yes the '88 gas 350's of the era smelled bad on high sulfer gasoline and were slowly pulling away... The TBI 454's wern't exactally anything but dogs either.) OMG what GUTLESS Wonders!

FWIW Test driving a "classic" Olds 5.7 diesel in a Cadillac Seville was the inspiration for the high stall Yank converter I did. There was a 3 PSI turbo out there for the Olds 5.7 Diesel. One survivor, in more ways than one, El Camino with the Olds 5.7 Diesel and 3 PSI turbo went on ebay a while back. Who made it, IDK. Other IDI diesels have had turbo's forever like on Yellow Iron...

So the Banks turbo kit that came out... WHEN... That's important. Ford 7.3 and GM both had aftermarket Banks kit Options available. There wasn't a GM "turbo" precup available "when" his kits came out. Turbo precups were first available in 1992 for GM 6.5TD "High Output" option. I am not aware of any 6.2 Turbo precups official or otherwise. It wasn't clear if 6.2 heads have different size precups holes as I recall some discussion on this. The later one head p/n fits both 6.2 and 6.5's is clearly just that.

So Banks worked with what was available. Even the 6.2 "Small plunger pump" was a limitation. 6.5's DB2's got bigger plungers. This limited Banks to a GM3 ballpark size turbo because fuel wasn't available out of a 6.2 IP. Even if it was the 6.2 NA precups would have belched out black smoke without any power gain.

Easy bolt on turbo kit... No, even if precups were available we are not going to pull the heads off a NEW engine to change 8 EXPENSIVE precups. Not going to pay for it (Dealer sales) or dad and I ain't gonna do it. Ya Feel me here on a complex performance "kit" that involves pulling the heads off?

One more thing on the 6.2/6.5 diesels: The R/V series. GM's largest 6.2 customer being the military did not care the NBS came out in 1988. So again Diamonds were on Uncle Sam's dime and request.
 

Rockabillyrat

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So again Diamonds were on Uncle Sam's dime and request.
Then why did the military wait 4 years after GM developed the diamond cups to start using them? If it was at their request and with their money wouldn't they have started using them in the 97 hummer. Instead they used the circle cups till 01 in turbo engines.

I know Banks worked with what he had. And a precup change is not a selling point to a turbo kit. My point to the whole thing is he made 190hp with less fuel and small cups. Gm redesigned the injectors, cups and added a bigger pump and only made 180hp. To say the cups where opened up due the addition of a turbo make no sense based on that information. To me it sounds like GM made all those changes for emissions purposes.
 

Will L.

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Am General didnt set any engine specs- precup or otherwise for the internal engine. The only thing altered was the starter. Even the crappy thermostat crossover that went on the hummer engine which GM showed as “improvement” for the overheating issues- which was another point of failure where AM General spanked GM.

The components in hummers are a few years behind the pickups- always were. DS4 and turbo didn’t hit hummer until 1996. My 95 has the a/c control system from 92 pickup. The transmission is 2-3 years behind in the changes as well.

As to alterations in the military line of hmmwvs-there is still a list of hmmwvs waiting to get a/c. So as for them waiting for minor interior engine changes- haha they are lucky it isn’t still horse drawn.

The only problem I see with you testing these is if I get my engine together first then you prove me I need something different- which wouldn’t surprise me too much.
 

Rockabillyrat

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Nothing from me. I have been focusing my time on injection pumps.

But I've confirmed that Tyler with the buckwild drag truck is running square cups on his 500hp 6.5. He is having great success with that truck and is clearing up that custom DB4 with a 72mm turbo with no problems.
 

dixiepc

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Well that's good news. I've got several sets of heads with square cups.

What's going on with the injection pumps?
 

Rockabillyrat

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I was excited to hear he was running squares. Its some good real world testing that supports my theory. Now lets hope I'm correct about the marine injectors too.

I put together a fuel shop in my basement to rebuild DB2 pumps and 6.5 fuel injectors.
 
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