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1st Generation Dodge Cummins 89-93 Discussion of 12 Valve 5.9 Liter Dodge Cummins Diesels with Rotary Injection Pumps

I have a 91 Cummins non intercooled. I was wondering can you just turn the smoke screw and fuel pin? and wil this hurt the motor?... JOIN NOW TO REMOVE TRACER

91 cummins smoke

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Old 01-05-2013, 04:52 AM

Default 91 cummins smoke

I have a 91 Cummins non intercooled. I was wondering can you just turn the smoke screw and fuel pin? and wil this hurt the motor?
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Old 01-05-2013, 10:47 AM
geddy's uncle's Avatar
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Need a pyrometer so you can monitor the heat. The way they roll out of the factory, they're not fast enough to "run", but, they'll walk forever. More fuel increases the heat/power (obviously), but, the heat can get real intense, real fast. Without a pyrometer, you won't be able to really tell when you're at the danger zone. The truck will pull like hell, smoke like hell, run like hell, and then promptly GO to hell. Not such a danger with factory injectors, and running unloaded, because the speed comes up and then you get your foot out. Running loaded, or uphill, however, there's more resistance, and your pistons don't descend down the cylinder quite as quickly. So, they stay in the heat-zone for a few milliseconds longer on each stroke, absorbing more heat from your combustion flames. Your aluminum piston crowns can withstand some short bursts of power where the flame temps exceed 1800-2000*F, but, if the aluminum itself attains about 1300*F, you create a see-thru piston.

geddy's uncle's Sig:My truck: "Tiny" '98 QC LB 2500 12-valve, G56, hybridized t-case (long story).
Son's truck: "Da Beast" '90 RamCharger, 12-valve from a '92, NV4500/NP205, matched D-60s, RhinoCoat inside and out.
Daughter's baby beast: '06 Liberty CRD, nearly bone-stock except for an oil-mist trap between the "hockey puck" and the turbine inlet's airhorn

Last edited by geddy's uncle; 01-05-2013 at 10:51 AM.
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Old 01-06-2013, 07:56 AM


Ok well I'm prob just going to stay stock. I pull heavy loads and live in the mountains . Thanks for the help
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Old 01-06-2013, 09:59 AM
geddy's uncle's Avatar
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If you want to, you can skip the big paragraph in the middle, and go right to the short one at the end, but, I put the big one in to illustrate why the short one might be useful to you.

When my son went from sea-level (Corpus Christi) to considerably higher (Salt Lake City) he had EGT issues that were a bit scary. .....background..... Engine was from a '92, so intended to be intercooled, used the whole cooling sandwich from a '96 (a/c, i/c, radiator), bigger injectors (don't know the specs...just know they send more fuel), and pump timing advanced a bit........ Stock turbo... those early years had BIG turbine housings, so, your first thought is, "GOOD! The back-pressure is low, so the heat gets out fast!!! YEEHAH!"... Well, that's not as straightforward as you'd expect. Big housings cause lazy spoolups, which keep boost low. (And you say...hmmmm not fun, but very safe....and you'd be correct). But, those housings were just too big, and were one the wrong side of the practicality/safety curve. SO, we swapped the big housing in favor of a 14cm, non-wastegated housing. Spool-up is MUCH more prompt, now, which means MORE AIR INTO THE CYLINDER AT LOWER RPMs....and you say, "Well, damn! How's THAT going to be a heat advantage? Smaller housing on the outbound side makes more backpressure, and more air on the inbound side makes a higher pre-spray cylinder temp!".... And, intuitively, you'd not be on a wrong train of thought, but, once again, we're working with CURVES, and not STRAIGHT-LINE relationships. Remember, that other housing was TOO BIG, and a 16 or a 14 is closer to Goldilocks's "Just right" zone. And, even though the pre-spray air temp inside the combustion chamber is hotter (higher pressure/same volume) now, you get a BETTER BURN on your fuel, so you don't need QUITE so much of your foot in it..... Little bit less fuel, better burn at an overall COOLER temperature. (yes, it's counter-intuitive and, no, it just doesn't feel right).... Well, we did that, and the temps came down, but he was still banging up against 1600*F on a hard acceleration. So, we called a local (Salt Lake City, again) Diesel shop and the guy said, "Thin air up here, need a bigger compressor wheel on the front side to grab and deliver more air".... So, we swapped in whatever more aggressive compressor would fit inside that front housing. Now, I think he said his EGT maxes out at about 1300, and cruises somewhere in the 600-700 area.... That's MUCH nicer, but, if he were towing something heavy, and climbing a hill, he'd still have to slow the truck down, stay out of 5th, and keep the rpms up in order to keep a steady reading of 1000 or lower. (If you want the engineer's blackboard full of numbers, there's probably guys here who can do that, but, now that you have somewhat of an overview, that blackboard might make better sense if you ever see it.) -------as a temperature reference, there's two areas for a pyrometer's thermocouple probe to be installed, 'pre-turbo' in the exhaust manifold, or 'post-turbo' in the cast iron elbow behind the turbine housing.... His is pre-turbo, so he's able to measure fresh-past-the-exhaust-valve. Many are post-turbo, so the numbers from one guy or another, even with identical trucks, might vary by as much as 300*F on a hard pull... Idling or easy-cruising they'll be very similar, since the turbo won't be doing much.

Bottom line, after all that, high altitude needs more air. Turbos make engines nicer at high altitudes, compared to a naturally-aspirated counterpart, but, if the engineer who designed it was at sea level, you still would benefit from a high-altitude adjustment. Smaller turbine/bigger compressor------->>>>faster spoolup/grab more of those thinly-distrubuted oxygen molecules

---AutoMerged DoublePost---

----Here's a piece of fuel-pin fun you can have for about $3. Just rotate the OEM fuel pin 90 or 180 degrees and re-install. When you pull it out, you'll see that it's an asymmetric cone-shaped piece of hardened steel. There's a steep side, and a not-so-steep side. If the previous owner(s) never moved it, the rub-mark where it contacts that little rod will be on the 'not-so-steep' side of the cone.........rotate and re-install in such a manner that the more-steep-side is working its magic on that little valve-rod. -----sounds like should be able to do that for free....why 3$? When you do, those 4 little slotted-head screws need to be ditched, in favor of some nicer-looking stainless TORX-heads. Then you can post a picture of some 1st-gen "bling" under the hood. -----I don't know if you've ever heard the analogy, but, the shape of that fuel pin, or the shape of a P-pump's cam plate have been likened to a 2x4 underneath your accelerator pedal. Different shapes/grinds/profiles of aftermarket pins or plates get compared to removing the 2x4 in favor of a 1x4, or a piece of 1/2" plywood, OR CUTTING A HOLE THRU THE FLOOR for more pedal travel...... except your pedal stays the same, just the little fuel-limitation parts inside the injection pump getting more movement.....

Last edited by geddy's uncle; 01-06-2013 at 09:59 AM. Reason: Automerged Doublepost
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