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12VEG 06-09-2010 03:58 AM

Beginners Thread - Read First!

This is meant to help all of those who are new to the 12 Valve Cummins engine and it's components by providing a one stop piece of literature from which you may learn the basics and educate yourself before asking questions out there in the open forum. Many of the same basic questions get asked every day, and while it provides opportunity for everyone to help each other, it also makes for quite a bit of repetitive redundancy. Also many will simply tell you to search for your answers rather than asking them aloud, so the more you search beforehand enabling yourself to present educated and well thought out questions the better the responses you will receive.

Before all else PLEASE DO NOT BLATANTLY ASK HOW TO "MAKE MY TRUCK SMOKE/ROLL COAL/ETC.. It is a question that will be met only with lots of this :spank:, this:doh:, and this:banghead:.

Diesel regulations in the USA are becoming more and more strict, intentionally making your truck smoke in public not only pisses people off and wastes tons of fuel, it can also get you in trouble with smog station referrals arriving in the mail as more and more states crack down. Keep it off road/on track.

If you ask something like , "How do I get more power out of my truck?" you will get a much better response (especially if you ask these questions with a goal in mind and having done your research, state this goal when asking your question). Smoke will accompany power, but only in small amounts in a properly tuned and well-mannered truck. Pulling trucks are a whole 'nother story.

Please fill out your signature! When you ask a question, we all need to know what kind of truck you have, without always having to ask. Click: User CP>Edit Signature.

-Or just go here:

First things first:


- What is a KDP, and what do I do with it?

- My truck has XXX,XXX miles on it, is it okay to do mods reliably?

In short, yes.

In slightly more detail, yes, but before making any enhancements to a stock truck you must first make sure that all stock components are in good shape to handle the increases. I'm talking fuel delivery system, charge air piping (no leaks), etc. These things are bullet proof, you'll hear it over and over again. As long as you do upgrades in a logical procedure there's almost no way to go wrong (KDP before gauges before tranny etc., all explained below). Of course at 400,000 a truck is more likely to blow a head gasket than a truck with 100,000 on the clock, for no other reason than age and wear, so you have to make those considerations for yourself, if you want to take the risk or not. On one hand though, having your HG go is not really that bad because you then get the piece of mind that (~$500 later) you have a new fresh HG, get to torque your head bolts/do studs, etc.

- What's wrong with my fuel system? Why is it running poorly/does the truck have no power?

It is my opinion that fuel system troubles/woes should be troubleshot in this order:

-Get a fuel pressure gauge! Virtually impossible to troubleshoot otherwise.

-Check ALL fuel line connections for leaks/cracking/air entry. Visual inspections do not always yield accurate results, if in doubt, replace.
-Cummins' fuel pressure specs are 17-22 psi at idle, 25-35psi at 2500 rpm no load. Pressure should never drop below 25 psi under load. The first thing to do would be to change your fuel filter, and clean your pre-filter screen (see Joe G's article below).
-Next thing to check is the OFV (overflow valve, see link below). If you're experiencing lower pressures than what is specified above, change your OFV. Tork Technology has come up with what seems to be a great OFV, and it is adjustable, read for yourself:

-Next course of action is lift pump replacement, If none of the above have turned out to be the problem.
-After that it's the injection pump, highly unlikely, but they are not entirely fail-safe, just darn near. (94-95 IP's do have a known problem that is due to a warping barrel/plunger, and symptoms include poor performance/sputtering at higher engine temps and RPM's)

- My idle is too low, how do I fix it?/Truck needs throttle to start/dies in gear while turning/in reverse?

- Why does my 4x4/HAVC/Cruise control not function?

Likely it is a vacuum leak, especially if all of those things are not functioning correctly. A leak in one part of the vacuum system will affect all of those components. Check all connections, check for melted connections especially just above the turbo. Check the vacuum pump under the drivers side battery tray, if acid leaks onto it (which it tends to do) it will corrode the heck out of the diaphragm, and needs to be replaced.

Tech Articles

<<<There are many useful articles in these databases and they should be consulted before all else. Get to know them. If there is something not yet in there that should be in there and it is a procedure that you are doing, by all means document it (with photos) and post it there!

94-98 Tech Articles - Dodge Cummins Diesel Forum

Diesel Database :: Browse Technical Data and Articles

Ram Diesel Problems


You've probably noticed that a stock 12 valve Cummins can hardly get out of it's own way. There can be reasons for this that actually are problems (i.e. clogged fuel/air filters, bad OFV's, bad LP's, air leaks in fuel system, etc.) but it is also the case that in their stock form they simply don't have lots of power. Excepting actual problems, there are simple (free) ways you can milk an exciting amount of power out of 'em, but there are investments that must be made prior to this milking to make sure you don't destroy engine components:


- The most useful investment you'll want to start with is a set of gauges to monitor certain parameters if you plan to turn up your fueling any. Boost, Pyrometer (Exhaust Gas Temperature - EGT), and Transmission Temp if you have an auto trans. Fuel Pressure is another very helpful one, as fuel related issues are all but impossible to diagnose without one, and everyone has fuel issues at some point.

Brands with the best reputations include: Isspro, and Autometer. There are others of course, but as with anything, you get what you pay for.

IMPORTANT: When installing a pyrometer the best place for probe installation is pre-turbo, in the back half of the exhaust manifold about an inch from the turbo inlet. Intallation post-turbo will give you inaccurate readings, with up to 200º-300º F variation.


- The next most important thing is to upgrade your stock tranny/clutch.

<Stock auto trannys in these trucks are great for stock power, but will not reliably handle ANY power increases. The minimum to do is an upgraded Valve Body/Torque Converter, but most opt to save and spring for a "Built" auto trans from one of the reputable suppliers. Goerend, ATS, Suncoast and a handful more are good names to look for, do your research when it comes to this step in particular, make calls, compare prices and products so you can make an educated decision.

<Similarly the stock clutch in the manual trannys (NV4500) will not stand up to the power increase for very long. Lucky for (us) this is a cheaper upgrade. Valair and Southbend are probably the two most widely sought out brands, and the best. When deciding on a clutch CALL THEM AND TALK TO THEM so they can help you decide which clutch will best fit your needs. Tell them your goals so that you don't end up buying a 600 hp clutch when you will only be making 350 hp, your driving experience will be less than satisfactory. But a well matched power setup, now that's good stuff. :thumbsup:

Pump Tuning

- You have a Bosch P7100 Injection Pump. Congratulations, it's one of the very best. Wanna learn how to tune it? Read this:
Diesel Database :: Turning Up A P7100 Pump

AFC (Aneroid Fuel Control) Modification:

Tuning the AFC is one of the most important steps to tuning your truck for the greatest and most efficient power, everyone's always talking about it so if you lurk enough you'll learn a lot about it. In essence this is the deal:

-On the back of the AFC there is a little cover (you'll know what I'm talking about when you read that AFC article up there) that covers the pre-boost fueling adjustment. (Note: Some refer to this as the "Smoke Screw.") Its proper function is to allow a set amount of fuel in to light up the turbo, and after that point the spring load on the "starwheel" is what determines how fast fuel will come in with relation to boost pressure. Both of these things are adjustable.

When adjusting these settings you should be going for (as far as pre-boost) the least amount of fuel it takes to light the turbo with minimal smoke, and enough tension on the spring to bring fuel in at a rate that will only lightly haze during full throttle.

These adjustments are very fine, as in 1/4 to 1/8 turns with the pre-boost adjustment. Going more than that at a time will bring you out of the sweet spot.

*º*º*º*ºThe most informative AFC tuning threads are these:*º*º*º*º



And this one:

As of right now there are 327 posts to read through, some of them are very informative, but if you follow Craig's (Algae Eater's) adventures through AFC tuning land, you will come out at the other end with more AFC tuning info than you can shake a stick at. Trust me, it's worth it...

TST sells an AFC spring kit that contains a lighter spring which, with any added power makes tuning a much more pleasurable experience giving you much more control.

Fuel Plates

- Fuel plates control the fueling curve after the governor lever clears the AFC arm. You'll hear lots of talk and debate about fuel plates (#10/#100/#0/#5/ etc.), which one to run in which application, whether to run one or not, whether damage can result from not running one, etc. They are completely application specific, and there is no one size fits all. They are not pointless, as some would like to say, but they are a tuning tool. Appropriately they are a "Full Load Throttle Stop." If you have, say, large injectors but no turbo yet, you may consider a fuel plate to keep from overspeeding your poor little stock turbo @ full throttle.

More and more are simply not running them, it's good for about a free 80-100 hp (*estimated*), when combined with the AFC mods and lots of flathead screwdriver tuning) depending on what year truck you have. Many now conclude that running with no plate will not harm your truck, but there are still those who say it will. I personally have never seen an instance where a catastrophic pump failure was documented to have occurred due to lack of fuel plate. Tech section has more info on this subject.

"Boost Elbow" or "Boost Fitting"

-Stock wastegates on the HX35 turbos are set to open anywhere from 18-23 psi depending on what year you have. The wastegate bleeds off excess drive pressure (exhaust pressure out of manifold and into turbo) by way of a pneumatic signal from the intake manifold pressure (boost pressure). When you add more fuel, you must also add more air to mix with that fuel for the most power and most efficient combustion (less smoke). Doing this modification you will see boost increase from the stock 18-23 psi up to 30-35 psi, as long as you have the fuel to get you there.

Now, many companies market what they call a boost elbow, fitting, etc. which is simply a little brass 90* fitting with a set screw to limit this pneumatic signal and thus control the pressure at which the wastegate (W/G) opens. You can do the same thing with about $10 worth of parts at the hardware store. You only need a 1/4" needle valve with an 1/8" MTP (Male Threaded Pipe) on one side, and a 1/4" barbed fitting on the other side to connect into your AFC-to-wastegate hose.
*You can also cut into the W/G hose and use a 1/4" barb fitting on both ends, as in the pic below*

There are two locations to put this fitting, and it depends on what year truck you have:

1) On the back of the AFC there is a tee. Coming out of the top of the tee (the trunk) will be a straight barbed fitting with a 1/4" hose running in between the valve covers (between 4/5 I think) and down to the W/G. To install your boost fitting replace the one that's in there with the one that you have.

Note: Don't confuse this with the line that reads intake manifold pressure to the AFC. If you have two lines coming off of the AFC then location #1 where you'll be putting in the boost fitting. If there's only one smaller line, and it runs into the intake manifold you have the second kind, see below.

2) There is a little brass fitting tapped into the compressor housing of the turbo for the pressure signal (basically same thing as being tapped into the manifold), and from this brass barbed fitting is a hose running to the W/G. Use the same concept there as above. Your boost fitting will replace the brass fitting tapped into the comp housing.

Here are pics of my set-up, but mind you, I have my W/G completely disabled so my fitting looks a little funky (i.e. no hose clamp on outlet of needle valve, but I have it fully closed).

Boost Fitting pictures by waribu - Photobucket


- Here are some of the things that you can do beyond fuel plate and AFC stuff, but it starts costin' $...


-Rather thank buying expensive intake kits many people do a BHAF (Big Honkin' Air Filter) Search for installations, there are many threads on this.


-If you do want to spring for a nice expensive intake then S&B and AFE (among others) are a couple of good names to search for.


- Goes hand-in-hand with an intake, allows for much better breathing of the engine and lowers EGT's. Options are generally 4" or 5", with or without a muffler, all kinds of tips, some do stacks, so on and so on. Here's a good price on a 4" muffled kit for example:

MBRP Performance Series Exhaust System 94-02 Dodge 5.9L Cummins Diesel S6100P

Governor Spring Kits (GSK)

- Stock governor springs de-fuel starting at about 2200 RPMS, it's quite noticeable. Governor spring kits allow your IP to fuel up to 3000, 4000, 5000 + RPMS depending on what kit you buy. The popular choice is to do a 4K GSK. It fuels nice and hard throughout the RPM range. If you plan to do a 4K GSK and REV ABOVE ~3250 RPMS, YOU MUST ALSO UPGRAGE THE EXHAUST VALVE SPRINGS TO 60LB SPRINGS! Many kits come with both sets:

4000RPM GSK Governor Spring Kit with Heavy Duty Valve Springs 5.9L 12 Valve Cummins

Bang for Buck value = High!


- Most stock engines' injection timing is to spray at 12.5 degrees Below Top Dead Center (BTDC). Many choose to bump their timing up to 15º on up to ~25º+. The main advantages of doing this are significantly increased MPG, and increased power in the upper RPM range. You do not however, want to go over about 17* on stock headgaskets and head studs for reliability issues. Bumping timing increases cylinder pressures which increases wear on these components. Couple that with the increased level of boost you're probably now running, and you begin a recipe for HG failure. This is all debatable stuff, but those numbers are pretty widely accepted as safe points. 16º-16.5º is a nice happy medium.

Note: The higher up you go with timing the harder it becomes to start the truck, especially in cold weather. At the 16º-17º level it's still pretty easy.

Another Note: Timing done in conjunction with a GSK (esp. 4K GSK) will yield the most noticeable difference since you are making more power available in the higher RPM range, but that's only helpful if you can actually use it!

Here are links to timing instructions, and a very helpful chart.:

Diesel Timing

And here's what you'll need to do it:

Cummins Timing Kit & Barring Tool Snap On

Fuel Injectors

-Many companies offer aftermarket fuel injectors, but I'm just gonna go ahead and say it: SDX (Southern Diesel Extreme) 12 Valve 94'-98' is a company that is rapidly becoming known for their very high-quality injectors at a very good price. Many companies use code words to sell their injectors (i.e. stage 1,2,3,4 or 370's, 435's, etc.) The only way to really know what kind of injector you are looking at is by 3 things: Hole count, hole size, and spray pattern.

Example: 90 HP Injector = Stage 2 = 370 = 5x.012

What no one EVER wants to tell you (unless they have nothing to hide) is the spray angle. Proper spray angle for our engine is 145*. Marine application is 155*, and some companies sell stock or modified marine style injectors marketed as performance injectors for a Cummins (not gonna point fingers here). What is generally accepted though is the fact that 155* is just fine unless you're running over 17* of timing, then that angle will begin to spray outside of the bowl at the top of the piston. Smokey and hot. It's a better idea to stay away from them altogether and put your hard earned money into a proven and quality product built by people who KNOW what they're doing.

Wanna know how injectors work? VCO vs SAC? etc.?

Read Weston Shupe's writeup:


-Stock HX35 turbos (WH1C's on early 94's) will handle ≈ 350 HP. They can do more but you're pushing it out of it's efficiency range (out of it's "map")

If you desire to know how to read a turbo compressor map, the I suggest you take a look at Clint's (Wet_Thud) post that he took a lot of time to write up on this subject:

How to Read a Compressor Map

An HX35 should NOT be pushed past 35 psi! If you continually push it further than that, you're literally pulling the pin on a grenade that will explode fragments of turbo wheel into delicate parts of an expensive engine. So, do you need to upgrade? You might be wondering:

What the heck do all of those numbers mean?

For example:
For a 62/65/12 turbo
62 is compressor wheel size (compressor wheel makes the boost by compressing intake air)
65 is the turbine wheel size (turbine wheel is spun by the exhaust backpressure)
12 (cm²) is the housing, often they are .70, or .80 if its a 14cm². These decimal numbers are the A/R or air ratio of the exhaust housing.

-It would not be helpful to go into extreme detail here in terms of what turbo works best for you, as choosing the correct aftermarket turbo depends ENTIRELY on all of the other supporting mods that make the bigger turbo necessary. The application is most important, in the words of jkidd, "...a 500HP towing rig is going to be set up differently than a track/strip truck at the same HP."

The difference is that a towing rig requires fast spoolup and good street manners (twins), where a drag truck set at the same HP level will likely have a big single turbo.

jkidd of Diesel Auto Power gave this breakdown of some often sought out performance levels and I am including it here to help answer those of you who want to know, "what do I need to do if I want ___ HP?"

There are countless ways to get set up, and here are a couple of suggestions:

For 450HP that tows and daily drives some:
5x.012 injectors
Modified fuel plate
62/65/12 or 14 turbo
Governor spring kit (3 or 4K)
60# valve springs are a good idea
AFC tuning!

550HP towing/daily driving
hx35/s400 twins (600HP+ go with s300/s400)
5x.014 injectors
Head Studs
60# valve springs are a good idea
AFC tuning!
3pc manifold

550-600HP hotrod
5x .014 injectors or 5x.016
Head Studs
60# valve springs are a good idea
AFC tuning!
3pc manifold

This here article contains the answers to many turbo questions:

This is only meant to get people started and familiar with the simple workings of this wonderful piece of machinery. If the information you need is not here, nor could you find it by searching, then by all means ask away, we all will be more than willing to help you out and soon you will be able to help others out. At a certain point we learn best by teaching, being placed once again in the beginners shoes and forced to think how it would all make the most sense.

If anyone has something helpful to contribute please feel free, and I'll edit it in. :thumbsup:

Bubba88 06-09-2010 09:29 AM

Here's a link to the 12v CTD section of There's a lot of useful install stuff and answers on there. Hope it helps!

Diesel Database :: Browse Technical Data and Articles

JaimeG 06-10-2010 02:09 PM

Great info! Thanks.

Algae Eater 06-12-2010 01:27 AM

Nice job Nick!
Let's all keep this thread perma-bumped to the TOP!!!

And by noon tomorrow the URL link to this thread will be my tiled as my desktop background and tattooed down my forearm. :rof

12VEG 06-12-2010 01:31 AM

Thanks Craig! It's perma-bumped for sure, got it stickied no problem. I'm counting on you for that AFC tuning article that I said would be up soon (hint hint)!

Still chippin away at it...

Algae Eater 06-12-2010 02:08 AM

Glad it's being stickied, because it will be more of an instant gratification (like Cliff Notes version) for someone who doesn't want to spend forever reading dozens of articles.
This will provide some solid foundation.

I'm in no way qualified to do an AFC tuning article. That would be like the blind leading the blind. :rof
Just cuz I've managed to get mine and a couple others tuned perfectly, doesn't mean I know what the heck I'm talking about, or if my experiences will translate to everyone's truck. But I appreciate the vote of confidence. :thumbsup:

At this point I'm convinced everything with the 12-valve is like pin the tail on the donkey.
But that's what I appreciate about it. It's a living, breathing, spring-loaded monster.
Once you learn to use finesse, it can be tamed.

rtc1184 06-14-2010 07:51 PM

i am somewhat a newb. let me ask you about the boost elbow? if i was to just cap it off would that raise the boost of my turbo? i know there are probably plenty of articles but would hope you would address this in the manner you did the rest. i learned a lot in 10 mins reading this than i have in the 5 days i have been on here. thank you so much for the insight. it enlighted me and made alot easier. i keep seeing all these damn abbreviations and im googling them and what not, lol. also if the location of the boost elbow could be photographed that would be awesome. thanks again

12VEG 06-15-2010 03:05 AM

I'll get to this in the morning, thanks for the reminder. I was trying to figure out what I was forgetting the other night when I was doing a revision.

12VEG 06-15-2010 11:27 AM

Boost elbow info complete!

Keep the feedback comin'...

rtc1184 06-15-2010 11:57 AM

ok so my turbo is set up a little different then yours. i have only 1 line coming into the back of my afc. i located the 90* fitting a few inches below the air intake hose of the turbo. so can i get a 1/4 needle valve, 1/4" 90, and a 1/4" barb fitting and do the same. come out of the housing with the 90* fitting, screw the needle valve to that, then the barb to the needle valve? also how does it work? by closing it does it raise the boost or opening it? thank you!

12VEG 06-15-2010 12:09 PM


Originally Posted by rtc1184 (Post 3075031)
ok so my turbo is set up a little different then yours. i have only 1 line coming into the back of my afc. i located the 90* fitting a few inches below the air intake hose of the turbo. so can i get a 1/4 needle valve, 1/4" 90, and a 1/4" barb fitting and do the same. come out of the housing with the 90* fitting, screw the needle valve to that, then the barb to the needle valve? also how does it work? by closing it does it raise the boost or opening it? thank you!

Precisely! Closing it will limit boost pressure signal to W/G (read more boost). Opening it up will cause lees boost, because the W/G will be allowed to open.

Close it off all of the way, see what you see boost wise, if it's 35 or below, just leave it closed off. If it's higher, open it up in VERY SMALL increments until at WOT you don't go over 35 psi.

rtc1184 06-15-2010 12:12 PM


Originally Posted by 12VEG (Post 3075054)
Precisely! Closing it will limit boost pressure signal to W/G (read more boost). Opening it up will cause lees boost, because the W/G will be allowed to open.

Close it off all of the way, see what you see boost wise, if it's 35 or below, just leave it closed off. If it's higher, open it up in VERY SMALL increments until at WOT you don't go over 35 psi.

this sucks cause i got all the parts for this, but my boost gauge isnt here yet, but i want to tinker with this, lol.

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