WarNose
Full Access Member
I just learned that 6.9 injection lines are very slightly shorter than 7.3 lines. Are they different enough to cause any trouble if they are mixed?
6.9 vs 7.3 Injection Lines
- Thread starter WarNose
- Start date
IDIBRONCO
IDIBRONCO
The #1 line (or #4 on vans and Internationals) is definitely shorter on a 6.9. I'm pretty sure that someone said that the 6.9 lines are smaller diameter too. I don't know about the other lines actually being shorter. Either way, the small differences will be enough for your timing to be slightly different between the injectors with the 6.9 lines and the ones with the 7.3 lines. It's not the best way to run your engine although it would work in an emergency.
WarNose
Full Access Member
I just measured the OD of both lines and they are both 0.25". I heard that the 6.9 lines are a few hundredths or a few thousandths of an inch shorter because the injectors don't sit as deep in the head. But it's just about impossible to measure the length to verify that.
A while back, someone on here was talking about reflaring the end of one line. He argued that it didn't matter if it ended up shorter than the other 7 lines because of how the injection pump and injectors work. I can't remember what his reasoning was though.
Edit: I may be imagining that, being that these flares can't be made in the home garage.
A while back, someone on here was talking about reflaring the end of one line. He argued that it didn't matter if it ended up shorter than the other 7 lines because of how the injection pump and injectors work. I can't remember what his reasoning was though.
Edit: I may be imagining that, being that these flares can't be made in the home garage.
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IDIBRONCO
IDIBRONCO
Maybe it was the I.D. of the lines?
ifrythings
Full Access Member
@WarNose if you have small copper wire (like network cable) you could feed that threw the inside of each line then measure the wire instead.
WarNose
Full Access Member
I.D. seems to be the same at .007", as best as I can tell with my dial calipers.
Good idea. The only wire that I could fit in is very skinny so there's no way of keeping it perfectly centered in the tubes. Twisting two together was too fat. But it seem that the 7.3 lines are roughly .010" shorter than the 6.9 lines.
nelstomlinson
Full Access Member
Interesting. I think that folks like Mel and Wes try to get all the injectors within 50PSI or so of the same pop pressure? I wonder how +/-25PSI pop pressure compares to +/-0.010" line length?
The infallible internet tells me that the speed of sound in #1 diesel at 20MPa is 1400 m/s. Figuring 39"/m, that's 1400m/s*39"/m=54600"/s, so sound travels 0.010" in 0.010"/(54600"/s)=183 nanoseconds. 2000rpm means 360*2000 degrees/minute, 6*2000 degrees/second, so that hundredth of an inch translates to .002 degrees? When we're setting timing, we try to get it to the nearest degree, right? So that hundredth of an inch means nothing if I did the arithmetic right.
Benm2002
Registered User
If wire doesn’t work to measure it, blow a string through the line (dental floss would work).
WarNose
Full Access Member
Wow, I can't follow all of that math but I ran some tests today. The longer 6.9 line on my #4 cylinder showed 1.5 to 2 degrees more advance than the 7.3 line. It's opposite of what I expected. I thought the shorter line would advance the timing more.
Either way, I went from two 6.9 lines and six 7.3 lines to all 7.3 lines and the motor is much smoother now.
jetfly12
Registered User
This would prove the theory that the 6.9 line would have a smaller diameter, and achieving the injector pop pressure sooner but who knows. How were you able to test timing on a different cylinder other then #1 with a timing meter? I suspect that my #6 line is different then all the rest and probably should test it first before hunting down a #6 line. Thanks
WarNose
Full Access Member
Good point about the ID's of the lines. It looks to me that the flare tips are fused to the tubes? It that's the case, the ID's of the tubes could be different even if the flare tips have the same ID.
#1 and #4 reach TDC at the same time, so you can use either line for timing.
But I'm wondering how we can trust the accuracy of the timing marks. I have seen harmonic balancers on gassers that have slipped and the marks were completely inaccurate. For the life of me I can't remember if I checked the timing mark while I was assembling this motor. I hope I did
jetfly12
Registered User
I had a broken 6.9 and 7.3 #8 line laying around and comparing the two, the 6.9 is longer then the 7.3 but no difference in ID from both. I cut the flares off and confirmed with a dial caliper.
Now I need to figure out what lines on my engine are short and long because I mixed the lines up to replace broken ones and I assume the spare injection pump with lines I had was from a 6.9...
Now I need to figure out what lines on my engine are short and long because I mixed the lines up to replace broken ones and I assume the spare injection pump with lines I had was from a 6.9...
It is true that 6.9 and 7.3 lines are different for sure. Conventional wisdom says don't mix them, but if it runs OK, obviously whatever effect it causes is not the end of the world. Personally, I wouldn't do it because it would bug me. Effect or not. Now if you do something like delete the timing adapter on your #1 line on your 7.3 and bend the line to connect directly, that much of a difference does create a perceptible change.
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