Cummins 4BT & Diesel Conversions Forums banner
1 - 20 of 24 Posts

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #1 ·
Maybe this has been covered, but on the CTD Dodge 6BT's the guys say that early 1st Gens, no aftercooler, had better & fatter injectors than those used on later aftercooled 6BTA. Anybody know the answer to the obvious question: is the same true for 4BT and 4BTA's??

Also my engine has 7mm injectors as opposed to the later 9mm. They're saying 7's in a 7mm head flow better than 9's in a 7mm head, or 9's in a 9mm head....?

For what it's worth my injectors are P/N 3912468 on a '90 CPL 0858 4BT.
 

·
Registered
Joined
·
645 Posts
Jimmie - I think the 9mm are the early injectors, then they went to the 7 mm tips because of the head cracking issue between the valve ports and the injector bore. The 7 mm's allowed more iron between the injector bore and the ports. Since the 9 mm's are bigger around I'd think they would flow more because the internal bore is also bigger? The 7 mm's can be used in a 9 mm head with an adapter ring but the 9 mm's can't be used in a 7 mm head. I have a CPL 858 Cummins recon 4BT, it has the 3912468 injectors also but they have a 9 mm tip. Confusing. Does your engine have a 7mm or 9 mm head?
 

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #3 ·
I've posted about my injectors on several forums, and on this forum in the past. I've had a very hard time getting any solid info. Cummins Customer Service tech rep said mine were 7mm injectors, and "...the wrong injectors for that engine..." He refused to give me any more info.

I don't know how or why but I've read on other forums that the 7mm injectors can flow more than 9's! The other thing I've read elsewhere several times is that non-aftercooled engines had better & fatter injectors than aftercooled [on 6BT's]. For me it's just a whole bunch of questions with few if any good answers.
 

·
Registered
Joined
·
645 Posts
I'm guessing the 7mm and 9 mm tips are interchangable between the same injector body. Maybe that's why your injector has the 7 mm tip and mine has the 9 mm tip but they both have the 3912468 stamped on the body?
Last time I was at the local Cummins shop I asked them how much they wanted for a set of 3912468 recon injectors, they said you can't get them anymore and gave me the replacement part number which I don't recall but it was the 7 mm injectors. About $100 each plus tax :eek: course then I'd have to get the adapter ring to use them in my 9 mm head, who knows how much they want for them. Think I'll stick with the ones I got and see how they work...
 

·
Registered
Joined
·
645 Posts
Paul - the diameter of the bore in the combustion chamber that the injector tip goes into will tell you if it is a 7mm or 9 mm head. I don't have the exact measurements since my engine is together but a 9 mm head will have the bore OD at slightly larger than 9mm. The injectors with the 7 mm tips can be used in these heads with an adapter ring that seals it to the 9 mm bore but the 9 mm injectors won't go into a 7 mm head.

Steve
 

·
Registered
Joined
·
808 Posts
Paul - the diameter of the bore in the combustion chamber that the injector tip goes into will tell you if it is a 7mm or 9 mm head. I don't have the exact measurements since my engine is together but a 9 mm head will have the bore OD at slightly larger than 9mm. The injectors with the 7 mm tips can be used in these heads with an adapter ring that seals it to the 9 mm bore but the 9 mm injectors won't go into a 7 mm head.

Steve
Thanks Steve, I understand

Paul
 

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #8 ·
To clarify, I have never pulled my injectors or checked the number on injector body. I'm getting my info off the CPL Data Tag, so it originally came from Cummins with these injectors. A couple of guys have said the 7's flow better than many 9mm, I have no idea. I found rebuilts from Oregon Fuel Injection but I'd have to double check their price.

I guess I'll have to pull one and see if they have an adapter ring, but supposedly they don't. Next step is to chase down the best 7mm injectors but that$ in the future ha ha!

Same subject: what do y'all think on the truck hitting 1,200-1,250 EGT on a steep grade under full throttle? About 5,500lbs truck, stock H1C, no mods, boost can hit 12-15 or more. From what I hear that's about 300* more than I should be seeing on a stocker, so that's a part of this injector question.
 

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #9 ·
At the link, about 3/4 of the way down the page, rebuilt 3912468 injectors at $58.50, $30 core. I talked to a guy there about a year ago but he couldn't give specific info as to specs. Maybe that second number 3802131 listed beside 3912468 is another clue as to what they are?

http://www.o-f-i.com/index.php?pid=32
 

·
Registered
Joined
·
645 Posts
Paul - I suppose the 9 mm injectors will go into a 7 mm head with a BIG hammer :) Jimmie - thanks, that price is much better than Cummins, I'm going to take a wait and see with mine, I'm not looking for crazy power, maybe 140 -150 so I think mine should be fine.
 

·
Registered
Joined
·
292 Posts
To clarify, I have never pulled my injectors or checked the number on injector body. I'm getting my info off the CPL Data Tag, so it originally came from Cummins with these injectors. A couple of guys have said the 7's flow better than many 9mm, I have no idea. I found rebuilts from Oregon Fuel Injection but I'd have to double check their price.

I guess I'll have to pull one and see if they have an adapter ring, but supposedly they don't. Next step is to chase down the best 7mm injectors but that$ in the future ha ha!

Same subject: what do y'all think on the truck hitting 1,200-1,250 EGT on a steep grade under full throttle? About 5,500lbs truck, stock H1C, no mods, boost can hit 12-15 or more. From what I hear that's about 300* more than I should be seeing on a stocker, so that's a part of this injector question.


Post turbo or Pre turbo pyrometer. preturbo pyrometer factory specs the engine should hit 1200-1250 at WOT
 

·
Registered
Joined
·
1,068 Posts
Paul - the diameter of the bore in the combustion chamber that the injector tip goes into will tell you if it is a 7mm or 9 mm head. I don't have the exact measurements since my engine is together but a 9 mm head will have the bore OD at slightly larger than 9mm. The injectors with the 7 mm tips can be used in these heads with an adapter ring that seals it to the 9 mm bore but the 9 mm injectors won't go into a 7 mm head.

Steve
This says it all. The 7mm or 9mm has nothing at all to do with the performance of the injector. Fatter has nothing to do with it. It was simply a head design change or design upgrade, due to head cracking issues.

I am no expert but from what I read the performance is in the number of holes, the size of the holes the placement of the holes, protrusion into the combustion chamber etc. And more I'm sure...

If you have the old style head and you want to upgrade injectors it is advisable to buy the newer 7mm style and use the adapters, the reason for this is simply the resale factor, you will have a broader resale buyer market with the new style.
 

·
Registered
Joined
·
278 Posts
Same subject: what do y'all think on the truck hitting 1,200-1,250 EGT on a steep grade under full throttle? About 5,500lbs truck, stock H1C, no mods, boost can hit 12-15 or more. From what I hear that's about 300* more than I should be seeing on a stocker, so that's a part of this injector question.
http://www.dieselinjection.net/BC/booklet.html

I highly recommend this website for modifying the B-series engine. I have no affiliation with them, but I like the way he thinks.
 

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #14 ·
Thanks for the link! Lots and lots of info there so I may tackle it in the morning when I have a brain.
 

·
Registered
Joined
·
225 Posts
(Quote from JimmieD): Same subject: what do y'all think on the truck hitting 1,200-1,250 EGT on a steep grade under full throttle? About 5,500lbs truck, stock H1C, no mods, boost can hit 12-15 or more. From what I hear that's about 300* more than I should be seeing on a stocker, so that's a part of this injector question.

I have read that the cummins can sustain 1250* egt and up to 5 minutes at 1350* pre turbo. This was verified by a local injector guru. My truck weighs 6000lbs, is aerodynamically equivalent to a brick, and for 8 months after installation 1250 was a common sight and 1350 was very easy if I didnt pay attn. Discovered a severe intake restriction:idea: , made timing adjustment, turbo mod, separate cooling system for JWAC. Now I have to push really hard uphill to get it to 1100*. Normal driving 65-70mph seldom exceeds 925*on those same hills. Back in the day I avoided the high temps with my right foot no matter how many dirty looks I got, but if 1250 is your highest you should be OK. At least I dont you will murder your engine any time soon.:)
Carl
 

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #16 ·
Thanks, Carl!

It's very easy for me to control, but your reply tells me I should do some poking around to make sure evrything's okay. From what I have read in posts it seems that it's unusual to hit 1200-1250 with a stocker so I thought maybe the pump had been turned up. The metal cap is still on the power screw so I doubt it. These are some very steep hills and long grades, if I can find a pic I'll post it. Probably normal considering the road conditions. Thanks!

Dusty: Pyrometer is tapped into the exhaust manifold, pre-turbo. I don't think I'd want to see 1200 post turbo hah! Thanks.

What all this boils down is that the two most difficult parts of diesel technology for me to grasp are injector tech [sizing, spray patterns, etc.] and of course turbo mapping-turbo A/F ratio calculations. Other than these two areas this diesel stuff is pretty simple once you get the hang. So I was trying to follow up on some info I read about 6BT and 6BTA injectors and to help whoever stumbles on this.
 

·
Registered
Joined
·
94 Posts
I've posted about my injectors on several forums, and on this forum in the past. I've had a very hard time getting any solid info. Cummins Customer Service tech rep said mine were 7mm injectors, and "...the wrong injectors for that engine..." He refused to give me any more info.

I don't know how or why but I've read on other forums that the 7mm injectors can flow more than 9's! The other thing I've read elsewhere several times is that non-aftercooled engines had better & fatter injectors than aftercooled [on 6BT's]. For me it's just a whole bunch of questions with few if any good answers.
Remember, "Its not what you know.....Its what you know that aint so" LOL
 

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #18 ·
Yeah, that's pretty close up to it! For instance, some professional sources say that the above P/N is for 7mm injectors, other professional sources say 9mm. Okay, well that sure clears up a few questions....
 

·
Registered
Joined
·
3,391 Posts
Discussion Starter · #20 ·
Thanks for the further tips, Carl! I've been thinking about re-registering my other truck so I can work on the Cummins. I was trying to save non-existent money but it turned out I was getting a multiple car discount, so when I dropped two trucks and and added a small economical motorcyle it only saved me about $12 month hah! Plus the other rig is 4WD and the season is coming....

So once I have a backup vehicle it's roll up the sleeves and DIVE IN! :)
 
1 - 20 of 24 Posts
Top