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  #1  
Old 12-19-2009, 01:33 AM
scottmcphee's Avatar
1987 w124 300D
 
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Does length matter?

Injector line length, that is.

I measured the stock lines, they are about 52cm tip to tip.

I have read threads about people using lines off other models, so long as the nuts fit. Sometimes in an emergency fix situation. Bending them to shape. Older lines were notably fatter, newer ones (on my w124) are thinner.

Is there a reason to keep all injector lines *same* length? Would having one shorter cause mis-match in timing?

And do they have to be a specific length for a giving timing setting? Or just consistent length so a timing advance setting applies equally?

I notice Benz goes to great lengths (pun intended) to keep all the lines on my car the same length, they are all bent an swooped in shapes that obviously are there just to take up the slack. The runs are not "shortest distance" or even the optimal routing for simplicity.

Does the volume of diesel held in the line matter? Does a large diameter pipe transmit the pop pulse just as well as a thinner one?

I was considering custom lines, a little more robust that the thin stock ones. And picking routing that better accommodates some things I've added to the car.

Just wondering what my design parameters should be, and where I've got freedom in the design.

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  #2  
Old 12-19-2009, 02:29 AM
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In a word Scott yes it does. dont mess with it. The fuel delivery is designed to take into account the amount of fuel in the line and allow for compression of it. If you change the line, you risk having inconsistent injector timing. Its one of the reasons why most recent engine designs have common rail injection, also the old GM 2 strokes had it as well. It gets rid of the lines as well as the IP.
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  #3  
Old 12-19-2009, 04:08 PM
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Liquids are incompressible, that is to say, volume does not change in response to pressure. There may be some "flex" in the lines, but it is inconsequential. Pressure at one end of a line is the same as all points in a line and pressure transmits instantaneously. A difference of a few inches one way or another will not make one whit of difference in timing. Rate of flow is affected by distance as a result of resistance to flow at the walls of the tubes, but I'd seriously doubt that it would be affected at these short distances.

My guess for the reason the lengths are the same is more for cost - only one part to manufacture, track, and maintain in supply rather than five. No matter the length all would have to be formed so that cost is fixed.
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  #4  
Old 12-19-2009, 05:12 PM
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Quote:
Originally Posted by micalk View Post
Liquids are incompressible, that is to say, volume does not change in response to pressure. There may be some "flex" in the lines, but it is inconsequential. Pressure at one end of a line is the same as all points in a line and pressure transmits instantaneously. A difference of a few inches one way or another will not make one whit of difference in timing. Rate of flow is affected by distance as a result of resistance to flow at the walls of the tubes, but I'd seriously doubt that it would be affected at these short distances.

My guess for the reason the lengths are the same is more for cost - only one part to manufacture, track, and maintain in supply rather than five. No matter the length all would have to be formed so that cost is fixed.

Pressure would transmit at the speed of sound in the medium, faster than you're driving, but not instantaneously. When you wind an engine out, keep in mind that an injector is firing 1200 times / minute or 20 times a second.

Back of envelope calculation based on speed of sound in oil = 1000 m/sec
length deviation of 5 cm gives delta T of 50 microseconds, or .1% of a 720 degree timing cycle at 4000 RPM. I may have dropped or added a factor of 2 someplace, but it looks like the injector timing would advance or retard .4 degrees at 4000 RPM. So, borderline significant, but when trying to design an engine correctly, I would match the injector line lengths.

I believe the design of the lines also takes reflection and interference of pressure waves in the lines.
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  #5  
Old 12-19-2009, 07:47 PM
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micalk,
'Liquids are incompressible, that is to say, volume does not change in response to pressure.'
May I refer you to either 'Chemical Engineers Handbook' Perry etal or The Handbook of Chemistry and Physics' CRC . They will both give you compressibility factors for various liquids. Junior High School may teach what you are suggesting, however more advanced studies in fluid mechanics will show another story.

The design engineers at M.B. have designed the length of the injector lines for the best operation. If it was possible to have the lines shorter, given the number made, they would have. Every one of the 5 lines is different in shape and so common parts is not an issue.
Moon161 has started to apply some engineering thought to the question, but I am sure he will agree, unless we know all of the parameters that went into the original design, best not to change things. Not all diesels have the same length injector lines. I have several other motors here in other applications with different length lines. There may be many reasons for this.
The diameter of the tube changes the volume of fuel in the delivery line. Again the M.B. design engineers will have had there reasons for the change from the 61x to the 60x motors.
The 61x motor has been around for a long time, and been very reliable. Best not mess with it !
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1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
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1968 Ford F750 truck. 6-354 diesel conversion.
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  #6  
Old 12-19-2009, 11:22 PM
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In Trade School I was told that the they try to make the Fuel Injection Lines all the same length and as short as possible.
The issue is not so much timing as it is the pressure waves inside of the Lines due to the sudden stopping and starting of fuel injection. The whole Fuel Injection set-up is a System with all of the parts designed specific to that system.

The long skinny Fuel Injection Lines have more problems than the short ones; and, not just on Mercedes.

On my Volvo Diesel the Fuel Injection Pump is at the rear of the Engine and I do not think any of my Fuel Injection lines are the same length and I guess the #1 is 6-8 inches longer than the shortest one.
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  #7  
Old 12-20-2009, 02:19 PM
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Quote:
Originally Posted by moon161 View Post
Pressure would transmit at the speed of sound in the medium, faster than you're driving, but not instantaneously. When you wind an engine out, keep in mind that an injector is firing 1200 times / minute or 20 times a second.
You're conflating sound waves and compression waves. The difference can be related this way. Think of a metal bar. Sound travels at a certain speed through the bar, but if you push one end of the bar the other end moves at the same time. However, the compressibility of the solid means that one end doesn't move at _exactly_ the same time as the other end, but rather moves as the result of a compression wave that travels through the bar pretty darn fast. Junior high physics notwithstanding. Confined liquids will basically behave the same way because of their relative incompressibility, and the speed of the compression wave is magnitudes of order faster than the speed of sound.

Diesel911 points out the most significant factor in these injection lines. The hydraulic transients of starting/stopping flow would have a much greater impact on the injection system than practically anything else. And the expansion of the injector lines under pressure has a greater affect on the speed of the compression wave and the resultant hydraulic transients than the compressibility of the fluid. The shape of the lines probably has more to do with changing the direction of the mass of fluid to counteract these effects and introducing structural rigidity by connecting them all together than on using a single established length of injection line. The ideal would be, as he rightly points out, to have short injection lines of all the same length. The fact that MB chose lines that are all the same length could be for many reasons, we don't know.

However, all that being said, the OP had a question: does the length matter? Volvo doesn't seem to think so. Or would you posit that the individual valve timing in a Volvo pump takes into account the differences in length of the injector lines? But as has been said, the system is designed as a whole. True. But every design has its compromises. The OP also mentioned that people have used different length and diameter of pipes, as long as the fittings are correct. Short term I'd say no problem. Would it have a long-term affect? Perhaps, but I'd guess no more than a rogue injector with a different pop pressure would. Is it optimum? Probably not. Would I leave it that way? As a practical matter I wouldn't be concerned, but I'd correct it more for aesthetic reasons than anything else.

And a 4-stroke engine fires all the injectors once during a two revolution period. Translates to 40 times per second or 2400 times per minute at 4800rpm.
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Last edited by micalk; 12-20-2009 at 03:11 PM. Reason: removed some snark
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  #8  
Old 12-20-2009, 06:50 PM
scottmcphee's Avatar
1987 w124 300D
 
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I like the sturdiness of 61x lines as compared to 60x lines.

I'm thinking of using something like that in heavier gauge, keeping them all relatively the same length, and routing to my pleasure. I know these lines can flex and crack if not secured firmly to something solid to damp the vibration. The OE clamp points seem pretty well organized. And probably re-usable in some fashion with heavier lines.

It's interesting to note that clamps are plastic snaps pressing the line against rubber pad, which is then bolted to the car. So it's not a metal to metal strapping, but something with some flex. Maybe if the clamps were metal-to-metal, that would just become another cracking point?
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  #9  
Old 12-20-2009, 07:04 PM
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Last Premise correct

Metal Strapping (Or Brackets) would create a "Hard Point" upon which the lines
would self destruct.

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