Chipped precups
- Thread starter bullfisher
- Start date
Hydro-idi
Full Access Member
If you have chipped and/or cracked pre-cups, I would definately replace those. First thing that comes to mind when I see that is excessive heat (egt's) may have been a contributing factor. Maybe even a bad injector or overly advanced timing may have caused that damage as well. More members will chime in on what they think may have caused this.
"Hey man, my IDI's chipped!!!"
Seriously, IIRC some cracking on the precups is normal, and there is a tolerance limit...per Gary in this thread, as long as the cracks aren't crossing the fire ring, you should be okay. WIth that said, if you can practically find a replacement for the one with the chips missing, I think I would...
Seriously, IIRC some cracking on the precups is normal, and there is a tolerance limit...per Gary in this thread, as long as the cracks aren't crossing the fire ring, you should be okay. WIth that said, if you can practically find a replacement for the one with the chips missing, I think I would...
bullfisher
Full Access Member
yeah... its worth a fortune now!Has anyone ever played with the precups to get them more durable (ceramic coated), or altered the openings? I mean are they at there optimal design? If im going to replace them I might be just ballzy enough to try something different.
PwrSmoke
Full Access Member
The 7.3L pre cups are different but I don't know exactly what effect it has on anything.
I did fool around with different pre cups on the 6.2L GM, which also use the Ricardo Comet V combustion chamber. There were a few different types on the GM, including one used only on the turbocharged engine. I don''t know the details on why it was different, but it had a larger orifice and when using it, the static compression dropped about 0.45 of the ratio. I know that because when I installed them, I cc'ed both the NA and the turbo combustion chambers. I didn't do an objective power testing because I installed them when I was rebuilding the engine. There were many other changes down to the engine. It was a strong 6.2L!
I did fool around with different pre cups on the 6.2L GM, which also use the Ricardo Comet V combustion chamber. There were a few different types on the GM, including one used only on the turbocharged engine. I don''t know the details on why it was different, but it had a larger orifice and when using it, the static compression dropped about 0.45 of the ratio. I know that because when I installed them, I cc'ed both the NA and the turbo combustion chambers. I didn't do an objective power testing because I installed them when I was rebuilding the engine. There were many other changes down to the engine. It was a strong 6.2L!
I've never actually done anything with a precup, but I think that those who are looking to lower the IDI's compression ratio (to stuff more boost into the engine) would do well to look at taking material out of the inside of the precup instead of shaving piston tops. You have to be careful with airflow and fuel spraypatterns (certainly, that's beyond me), but I contend that, if you can use the precups to get the compression ratio down to 18:1 or so, you'll get the same end goal as shaving piston tops, but you'll have more air to be heated by the glow plugs, which SHOULD lead to easier cold-starting, which in turn will make a more streetable truck (most lowered-compression IDI's take entire cans of ether and a wing and a prayer to get lit off; note this .wav file). With that having been said, unless you want to go that route, I don't think there's much that needs to be done to optimize the prcups any better...
Ceramic coating might not be a bad idea, especially if you're also doing the piston tops...
bullfisher
Full Access Member
Well I see two of them are chipped. And apparently precups are extinct from the planet! I did find an obscure set on ebay but the seller does not specify their application. If they are different from mine I will have to buy a whole set. Bummer!
icanfixall
Official GMM hand model
Those appear to be the first type precup with the smaller ports. The revision 6.9 cups were different as are the replacements. I think only 7.3 cups are now available. Not sure really. I did open up the new set I got in the new offshore heads. I made a mistake removing the lip in the cups. I opened up the throats because I had a comparator to work on. you have to realize any changes in the port sizes from cylinder to cylinder affects the power to that cylinder. then you have a small vibration because one cylinder gets more or less from the cup port. What I also did was fabricate a custom ball end mill to clean up the ruff cut precup in the cast iron head. Because the injectors were pulled out of the precup chambers 60 thousands I cut them that much deeper in the heads. So they are now the same depth they should have been. they got raised 60 thou because they burned better for epa reasons. Not for power reasons. We got that fixed right away. Russ is the only other member that have used my ball end mill and he liked it too.
bullfisher
Full Access Member
8 new 7.3 cups are fine by me as long as all my cups are the same, I just can't find them anywhere.
