Well I pulled off the upper intake off of our 6.5LTD and it has the HD intake with no cast webbing thankfully, so I just installed the boost gauge.

From a standing start, stomping the gas pedal, the boost will only go up to 5 or 6 psi.

I can make the thing spike to about 11psi if Im coasting at 60mph then gun the engine, but within a 1/2 second it drops back to 5psi.

Is that normal? I thought 8psi was normal not 5...

That's the vac system for you. Put a stiff spring on the wg arm and see if it doesn't hit 15 psi and stay there.

hmmm what boost level will the ECU kick the truck into limp mode or is that IAT dependant?

above 10 psi but that's what resistors are for.

hehehehe indeed!

im going to do some searching after i get off work at 10pm tonight, but is there a way to make the truck use the VAC system to build up to ~15psi without a heavy spring and not set a code?

Yes, the same resistance on the MAP will let the WGDC go to 99.97% from the stock 68%.

Spring's better in the long run, it won't fail and one less pulley and a shorter belt helps.

Quantum STOP giving advise that will cause harm you are a moderator for Christ sake give ALL of the information not just some of it. There is more to this folks I don't have time right now to put out the rest of it myself, I'm locking this thread until then.

Bumpin 'yota has done some research on his engine(looking for the cast web and the snorkel) and modified his exhaust. He's ready to turn up the boost. a weak vac system making only 5 psi is going to blow his head gasket faster if he pulls something than 15 psi will.

You should put a pyro guage in if you havn't already.

Quantum you are wholly wrong on this one, Yota is modding a USED truck with limited known history. What DATA to you have to support your OPINION 5 psi boost is worse than 15psi There is a reason GM selected the levels they have programmed into the PCM, and that reason is longevity, the 6.5 will support power enhancements but they MUST be added in a way that the engine can safely take the enhancements. Just adding boost and fooling PCM sensors is wrong way about it.

Adding boost for the sake of adding boost is wrong; levels above 10 PSI without post turbo cooling is risky high IATs will add heat, hot/ high level boost will crack pistons. I advise anyone contemplating adding fuel or boost do do so in an educated manner, vist Gale Banks web site I've linked in the FAQs or welcome new member thread I don't recall which it was now (builder of the worlds fastest Diesel powered truck).

I also want to add don't take my word or anyone elses word as an absolute, moderator status or number of posts does should not imply expertise. I am still learning everyday and try to do due dilligence & make sure what I say will not cause harm. The net has a great wealth of knowledge, but remember to also do your homework you can't be lazy make yourself as knowledgeable as you can BEFORE following ANY recommendations you read on it, mine included, some of what you read is wrong or almost correct, and can get you into trouble.

For those that have been around a while may remember what other long time members have cautioned QM on some of his adventures into the real of the unknown, follow his lead with caution. When he 1st showed on the scene he had no need for gauges, started adding boost, timing, optical bumps, and a host of experimental stuff to his truck.

He has had overheat problems, truck left him stranded more than once, latest adventure here http://www.dieselplace.com/forum/showthread.php?t=15378 is classic what happens when you add too much boost to an older truck. He repaired only 1 piston adjacent one is long in the tooth and IMO next one to give way.

I'm not trying to bash a fellow moderator or Place member but feel important to use this particular thread as an example of dangers of internet information not fully explained in detail.

The 6.5 by it's indirect combustion design is power limited in what you can safely do to it in bolt on modifcations, and they must be done in proper sequence or early engine failure is a certainty, just a matter of time. You have got to lower temps and improve airflow before adding any fuel or major boost to this engine. GMs program limits power as this is a light duty Diesel even when installed on the heavy duty frame trucks.

To get into the medium duty Diesel power world you need to go into the block strengthen the bottom, for sustained high level boost post turbo cooling and lowered compression ratio are also required. The 6.5 block is prone to cracks, some years handle it better than others, but all are at risk when pushing the envelope, 15 psi on a un strengthened block without lowered CR is pushing the envelope.

Gauges are a must not an option; the second you start altering any fuel or boost curves either with altering PCM sensors input or reflashing or blind fuel additions with HO injectors, you become the safety factor as soon as you start modding, your brain is the PCM use it. More to follow I'll open this thread back up for comment.

I would simply like slightly peppier response from 8 to maybe 10lbs at most, and no more boost than that.









Sorry about the edit, I couldn't undo it.

right or wrong is subjective. That is, what's right for me may seem wrong to you.

I have provided information and it is bumpin 'yotas choice what to do with his truck.
All caveats are valid and should be acknowledged. 15 psi is a lot of hot air but it takes alot of pedal to get it that wound up so the right foot's still in control.

I have to fix my trucks all the time but three ten year old trucks require many parts to keep rolling. Three modifed pulling trucks require even more.

If I blow my engine, I'll tell what I did to cause it, and then fix it with pictures. I am still in the learning phase here and open to fallibility, so I am looking for caveats, occasionally.

Subjective ???,

Possibly, lets review some; the 6.5 is known for it's weaknesses in block, precups, overheating and several other maladies so much so that GM had to make special policies to address these issues extended warranty for the block & IP, HO waterpumps with dual thermostats, high nickle content blocks in later years, piston oil cooling.

Aftermarket vendors galore glean used blocks looking for crack free ones, or hi nickle content ones, splayed mains and stud girdles are recommended by most reman engine builders.

My apologies to you QM in my wording that you are wrong, I should have worded it differently so it wasn't an attack on you but a disagreement to what you recommended, you have been successful at building power for your truck, (my opinion and just that); you could have gotten the same power gains with less distress to your engine.

For facts to back up that opinion I would need access to GMs design notes, been part of the design team to review risk assessments this margin vs. longevity & cost to own & manufacture, maybe instrument the engine with strain gauges at critical stress points, and a period of study in metalurgy.

GM has done this already and decided that the cost to get more power from it's Diesel offerings to meet the competition's offerings it would be better to start with a clean sheet of paper and the DMAX was their solution.

Prior to spending so much time here I used to hang out at the Diesel Place, where they built a 300 Hp project truck, Peninsular & Avant Diesel aslo have made higher Hp than stock offerings of the 6.5. Critical to these builds longevity is lowering CR to 18:1, and beefing the bottom as well as improved cooling flow, longevity is the key and what I was trying to address with my earlier comments, that is where I say there is a right and wrong way.

Yes you can make it go faster/stronger than stock your way but for how long, most folks that I have encountered at this and other sites, want enhanced performance without risk to the engine. Ronnie Joe just had a fail similar to yours, running 17psi boost with IC and sensible monitor of gages, but he also had pre-existing overheat incidents so the higher boost most probably wasn't root cause but a contributor to ultimate failure.

With the history of weaknesses of this engine design, and what the reman builders do, leads me to put out the words of concern, I'm not even certain I'm not headed for a failure at some point myself, but am saving for a 18:1 rebuild, I do limit my boost to < 14 when towing, have spectral analysis of the oil run every oil change, and with my scanner monitor for downward trends. For my daily driving needs never use more than 10 lbs of boost, still have a vac system and capability to go to 17 if I want to, but know that every run at that power is pushing design envelope of this engine to it's extreme limits.

An important fact that has gotten lost in this the 6.5 was never intended to be a race machine or HD hauler, you can make it better at those things but you must must do something to overcome the weakness of design built into these engines, or its not a matter of if it will fail but when.

I'm looking to have the hottest LD diesel I can, same as most of us. I'm just at the point where I'm going to lower the compression on all three of my engines myself and put in headstuds, and weigh the pistons/rods compared to the counter weights looking for imbalances and look for main web cracks. Then I'll be able to push a bigger turbo into the 25-30 psi range. LOL

spikes to 22 lbs...seems to run sweeter every day! Not recommended for faint of heart/wallet!!!

IMHO 22 with my turbo is a totally different story than stock turbo at 22lbs. Runs cooler, way less back pressure. Also builds boost slower, less at lower RPM, so torque down low has probably been killed somewhat, further reducing stress I would think.

You have stiffened up the bottom end with splayed, & 18:1 I'd have no problem then, what I was cautioning against too much boost on a stock bottom end and stock CR

While in Detroit in 1985, as a member of GM's Master Technician Advisory Council, it was explained in a meeting with GM brass in this manner. "The Olds 5.7 and the GM 6.2 diesel engines were designed as "30% engines", The 8.2 Detroit was described as a "70% engine and the H.D., Class 8 truck engines (i.e. Cat, Cummins, Detroit 71 and 92 series, etc.) were called "100% engines." This was said to describe useage. A 30% engine was designed to operate at full throttle 30% of it's lifespan or at a 30% throttle setting for 100% of its life. Same scenerio for the others The only engines designed to run full throttle (against the governor) all the time were the H.D. "big boys". Mods are great fun as long as we remember from "where we came".

While in Detroit in 1985, as a member of GM's Master Technician Advisory Council, it was explained in a meeting with GM brass in this manner. "The Olds 5.7 and the GM 6.2 diesel engines were designed as "30% engines", The 8.2 Detroit was described as a "70% engine and the H.D., Class 8 truck engines (i.e. Cat, Cummins, Detroit 71 and 92 series, etc.) were called "100% engines." This was said to describe useage. A 30% engine was designed to operate at full throttle 30% of it's lifespan or at a 30% throttle setting for 100% of its life. Same scenerio for the others The only engines designed to run full throttle (against the governor) all the time were the H.D. "big boys". Mods are great fun as long as we remember from "where we came".
So then a '94 6.5 would also be considered at 30%'er?

You got it Yota these were designed as LD engines with infrequent medium & HD use, to push the envelope involves more than just bolting on stuff & reprogramming it's brain

Not much difference engine wise in S & F engine, big difference is 1500, 2500, & 3500 truck is physical ability to handle more weight, & stop it once rolling.

Crank bearings, rod bearings, were designed to meet a certain loading , when we exceed those design margins by bolting on doo-dads we put the engine and it's innards at risk, all is doable with right ammount of $$$, but at some point you hit physical limit of the design and either have to live with what you have, or get a bigger truck/engine.