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Old 06-11-2011, 10:36 PM   #1 (permalink)
Glow Plug
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Pencil Vs. Poppet I.P.

As we 350 diesel fans have heard many times before, the pencil engines tend to be peppier. Many have ascribed that to the I.P. not necessarily the injector. However, the poppet engines use a mechanical light load pump, and the pencil engines use speed advance. The two pumps are known to not be interchangeable. Part of that I believe is the pressure equalizer circuit in the hydraulic head of the poppet I.P. What if the hydraulic head for a poppet pump were transplanted into the body of a speed advance pencil style pump? This would provide the correct hydraulic circuitry for the poppet injectors, while affording the superior performance of the pencil I.P., and the better performing speed advance setup. I came to this idea after reading my GM Product Service Training book "The 5.7 Liter Diesel Engine" yet again. I spent a great deal of time reading and re-reading the section on the DB2 pump, and suddenly the idea was there. What do you guys think?

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Old 06-12-2011, 09:14 PM   #2 (permalink)
Freeze Plug
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I have one of those books on the way. Looking forward to it myself.

I have some experience in this area, and you guys helped me get straightened out a while back. Since then I've learned a few things about what happened to my fuel system.

When I bought my 84 Cadillac, it had a GW reman, pencil injectors, D3A heads but it had a poppet pump on it, a DB2829 SH3914. At the time, not knowing any better and having to drive it back from CA, I took off for FL. I had no major issues on the way back, however I noticed that the MPG fell off the closer I got to home. It started at 23 MPG in CA and by LA I was down to 18. No real perceptible loss of power, but driving at night I noticed A LOT of smoke in the headlights of cars following me, especially at idle.

Long story short, when I got home I took that pump apart and it had substantial metal floating around in it. I couldn't get that pump to work right again. I think the pencil injectors with the higher pop off requirements may have beat that pump to death. If what I read was right, pencil injectors pop off at 1875 PSI and poppet at 1000-1250 PSI. Yes, I was using lubricity additive at each fillup.

I finally got a DB2825 PC3742, tore it down, cleaned and resealed it (it had EID) and the performance and MPG improved substantially. I get 26 MPG now at 70 MPH all day long. I do have some smoke at idle and when heavy into the AP. I do believe I might have mortally wounded a few of my injectors on the way home from CA with that wrong pump.

However, I found a running poppet engine that I'm in the process of a rebuild on...with it's own set of challenges, but that's for another post.

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