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porting precups

I wish I had an exact temperature, it should be around 2500°F though. They seem to crack not because of combustion pressures behind the precup, but rather because of the middle part of the cup heating up significantly faster than the outer part. When it cools, the inner part contracts more and pulls it away from the outer part, causing cracks. Basically, I crack them with heat.
 
2500 degrees is pretty hot,I would imagine you would have to seriously be beaten the hell out of your engine to reach those tempatures.Maybe not,but that's pretty damn hot
 
One other thing I read in my Harry Ricardo book (yes, I finally broke down and bought the reprint) is that the concept of the pre-cup design ALSO buys you a lot of tolerance for timing variances between injectors on a mechanical injection setup like ours. No matter how accurately the cam ring is machined, and how close the pop-pressures are set, there simply isn't any way our old-school injection systems can rival the accuracy of common-rail EFI on a D.I. setup (DS-4 included).

According to the Ricardo concept, the smaller the pre-cup volume, the more tolerance is afforded. That certainly doesn't mean opening up cups should be avoided, -simply because it is the key to unlocking power. As with anything, there is probably a point of diminishing return, -or a practical limit.

I should have never started reading that book, -it's a hard one to put down!
 
Very good stuff. It also affords a programmer with large variances in injection timing that work well. So there doesnt really need to be an optimization for each engine. And less than optimal timing still works well in most cases.

I firmly believe that the larger openings allows greater power near peak torque angles, allowing the flame to expand quicker.

Although, I would really like a set of older 6.2 precups, the smallest ones out there, because they have thicker walls, less internal volume for higher compression ratio. And then machine out the mouths of those. I want to preserve CR, because another mod is going to bring it down some. And then hopefully the precups are stronger with the thicker metal as well.
 
buddy said:
I firmly believe that the larger openings allows greater power near peak torque angles, allowing the flame to expand quicker.
Click to expand...

I agree 100%, -hence the larger mouths on the turbo cups. In all seriousness, the Ricardo comet pre-cup design was way ahead of its time. It solved so many other injection shortcomings at the time it was conceived. It allowed the use of simple and reliable "pintle" injectors (that during WW2 were really the only option considering clean fuel was more the exception than the rule). It also offered better fuel atomization, and allowed a wide tolerance of injectors and timing to be used which made things more reliable and easier to build and do field maintenance on. One other huge benefit was the fact that low injection pressures could be used, -which at the time meant stuff would be more reliable and last longer, -a big deal when you're being shot at. Kind of funny that after all these years, the engine of choice for the Humvee is still a precup IDI with mechanical injection, -apparently for damn good reasons.

Also mentioned are some drawbacks to the small-mouth cup design (in terms of volumetric efficiency suffering slightly). With that being said, it makes perfect sense that to begin making increased power with more air and more fuel, improved volumetric efficiency needs to be addressed by opening the mouths of the cups.

However, he also makes positive mention that the small mouth cups are more efficient due to the improved "mixing" and turbulence on the compression stroke (think 6.2 cups), -and greatly improved fuel atomization during the injection event (because the high velocity flame traveling through the small mouth actually assists in breaking up fuel droplets being sprayed into the cup as the injector continues to spray fuel).

This is a marvelous topic we're on, and I am subscribed, -or maybe a better word is addicted...

Dang, I can almost envision a day when you can order up a set of CNC ported 140HP precups from SS diesel for $950 and a $400 core charge....................-Just kiddin...... :hihi:
 
Porting the pre cups does a few things, and it depends on where you port the pre cups. Inside porting throat port or exit port. A little of both , mix and match and you get different combinations.

I have been running a set of extremely modified pre cups for pretty close to a year, needed to do some tweaking on the tune (thank you Buddy) and after a few tries and running a few of his tunes, we just about have the tune close. Mileage is up at higher rpms and down at lower, truck will smoke when ever I want it to and it isn't for everyone. What I do know is that the truck runs much smoother at idle and at higher rpm with the modified pre cups. Best combustion is going to be with the smallest pre cups you can get, As you go larger there are inherent trade offs, between fuel economy, smoke, and power in different rpm bands. Still trying to figure it out. But I know someone will benefit from the ones I have now, but that is all speculation, messing with the pre cups is not for the faint of heart.
 
ONE BIG NOTE ! the larger you go with the modification to the pre cup port and exit area you must have them machined so that they are consistent from pre cup to pre cup. I don't do mine, I have HANK do mine,he knows more about machining than most people forgot. I know HANK has done couple of sets for some here but they are machined only a little larger. After the Diamond cups anything larger seems to make EGTS go up even without fuel changes. But this in of itself is not a problem.EGTS are not really a problem once you open up the pre cups but that is also for further discussion after more data is collected.
 
I've got a set of these ported precups from Jason and they will be going in a motor here pretty quick. Has anyone heard from him lately? I had to put my whole build on hold for about a year+ due to military training. I'm waiting till next weekend to start assembling the bottom end as it just got done being balanced the other day and my machinist has to hang the rods and check everything out before giving me my crap back. I'm considering doing some ceramic coating though on maybe the precups. Gonna look into it...
 
Ok there here it is img_3481.jpg This image is a prototype to a set of pre cups I installed. They are not hand done as this pre cup was. But this was taken and measured,put on a machine and precision cut so that all eight were the same size. The floor of the pre cup was altered. THIS IS NOT A setup for use, it has inherent issues with some rpm ranges.

Without custom tuning the pre cups would not have even been usable, for that I need to thank member Buddy for providing the tweaks. The pre cup altering has some very interesting point that need to be explored, and I am not done yet. All tests were done using an A-Team Turbo not a gm.

There are big benefits to larger pre cups but I am not sure what size is best, We can definitely go larger but not without some trade offs. EGTS are one of them and some smoke at specific rpm ranges. This comes more from fuel air mixing than the pre cup itself. Injector pressure also plays a big role, Timing and fuel delivery. It has been almost a year and I have covered a lot of ground and feel there are benefits to this modification within reason. This is just my opinion, take it for what it is an opinion.
 
Going "too small" on pre cups gives you some interesting results. I have always wondered why 6.2's smoke so bad under load say going over mountain passes in Colorado. (1988 6.2 burb.)

Fast forward to now and I am using a 1986 military 6.2 with a an ATT on it. We pushed the tune and the engine to the extreme by putting a trailer on it and heading to Payson.

At 17 PSI of boost we could not generate anymore boost with timing or more fuel. All we did was generate smoke and excessive EGT's. It was just a "wall" caused by the 6.2 pre cups - they simply wouldn't burn more fuel and more timing wouldn't allow them to burn more fuel. We figured that the charge was still on fire when the exhaust valve opened. We hit extreme EGT's testing this wall and I still have a running engine. Conditions with the EGT matter more than a single number.

So it appears the pre cups being so small contribute to smoke of a NA 6.2 under load and higher RPM. Not able to move enough air out of the pre cup during the power stroke at high throttle.

Larger precups can take more fuel before they hit a wall with a trade off of MPG in NA applications. Turbo has other things affecting the MPG with GMx units. With an ATT below 2000 RPM you are running the engine in NA mode without boost.

However the 1988 4x4 suburban got 18 MPG. The 1995 I have now with the 1986 6.2 is getting 15 MPG in town. So I am not able to get all the performance and fueling possible out of an ATT, however, I appear to benefit with better MPG in town. Appear as I have not done a 'test' with other pre cups to verify this. As it is the MPG on the Payson run was 7 MPG with a GM3 turbo and 6.5 turbo or 6.5 NA precups. (tested both due to engine failure. And even tested the J code 6.2 pre cups with the same MPG on that run.) The ATT added 3 MPG to get 10.X MPG even while testing the tune to the extreme on 6.2 military (J code) pre cups. (I may have done better with larger pre cups and the ATT with this load.)

Comparing 6.5 NA pre cups to 1993 turbo 6.5 precups makes zero difference with a GM3 under load. Pulling a trailer to Payson had the same 7 MPG with both of them. The GM3 turbo being the choked POS it is was too big of a restriction to notice any pre cup difference. Swapping turbos with the same fueling went from 1250, GM3, in town to 1100 with an ATT. Sure I can turn the fuel up more with the ATT, but it was maxed out with the GM3. The ATT is also hitting 18 PSI of boost where the GM3 was limited to 12. (and this TM setting got us 14 PSI at altitude.)

In conclusion you need to get rid of other restrictions like exhaust and GMx turbo's before pre cups matter. Then you will see differences in MPG biased on cup size and how you use the engine. More constant load would dictate larger cups while a DD would want smaller ones. Smaller ones for MPG when you are not under boost. Again it only matters when you get rid of the restrictive turbo.

Note Military J code 6.2 NA pre cups are larger and emission exempt vs. the many civilian pre cups found in 6.2 NA engines. So you will have likely worse results on a civilian NA 6.2 going turbo. Small NA 6.2 or 6.5 econo pre cups are not a good choice for turbo's. If you have a 6.2 and are adding a turbo you should change the precups - it is easy to do and you want ARP head studs with the recommended 6.5 turbo head gasket update.
 
Differences in precups may also cause a little different spooling behavior of the larger turbos, at least my brain thinks so. So every truck will perform different based on several configuration factors, such as CR, displacement, precups, heads, timing, fuel system health, exhaust, intake.....

If rebuilding a 6.5TD, I would always at least use the diamonds to get the most from the build. When I spoke to Peninsular about a year ago, the technician there seemed to think there was an even larger GM precup for Marine heads.
 
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