Thoughts on science vs. realities of high boost and EGT relationships

[markrinker]

Many of us that have experimented with MAX boost pressures in the 15# and higher territories have noted a marked decrease in EGTs while also enjoying peak power from our modified 6.5s. We thought we had found 6.5 power nirvana...

However, I believe there are valid reasons to question that we are giving up in piston and ring life for these seemingly 'optimal' boost settings - even though they keep our two favorite gauges in their happy zones. (What if we all installed cylinder head temp guages??? How about piston crown temp gauges???)

Despite all the 'correct science', we are seeing a fair amount of melted pistons on these hot rodded, over-boosted engines. Many times the blame is cast on pre-gauge situations, but hey how long will a motor hang together with a melted piston crown? Hours, months, or years?

Personally, I have backed off my nearly 14# nominal (with off the guage MAXes...) setting to something closer to 7# or 8# nominal and 10-11# MAXes in hopes of preserving the engine life of truck #2. This is after observing increased oil consumption and moderate blow-by develop in only 30K of heavy towing under high boost settings.

The post turbo EGTs can now reach 950-1000 for a few moments under WOT loaded acceleration, but I realized that I had trained myself to use this mileage robbing driving style when my EGTs said "Go ahead, stomp on it...I am okay!!!"

I am now learning to take a few more seconds to get to cruising speed, and saving fuel along the way.

I plan to slowly edge the boost back up with a target of 12# MAX, 9-10# nominal after resetting my power expectations accordingly. Basically I am trying to learn to use WOT very sparingly.

Real world vs. science. I am on track?

[ 04-25-2005, 10:09 AM: Message edited by: Mark Rinker ]

[Andy Chesek]

I'd take a gander that extra boost (cold, not hot) and extra fuel would aid the engine. I think the recent rash of engine failures were caused by conditions that existed before certain mods were added. Take RonnieJoe's situation for example. He's pretty sure he knows exactly when and where he hurt his engine. I'm sure that extra boost puts more stress on components and will exacerbate any underlying problems.

Intercool, intercool, intercool.

[BUZZ]

Tell us about your intercooler?
Thanks
Buzz

[gmctd]

A lot of the failures are no charge-air cooler, or were initiated pre-cooler, and usually no or pre-gages.
If gages were on-board, post-turbo EGT probe seems to be the norm.

Factors are pre-'97 cooling upgrades, 195deg t-stats, 215deg fan clutches, blocked air-flow thru radiator, etc.
Some, pre exhaust upgrades - the Boost controller upgrade is cheaper.

Not even to mention 22:1 compression ratios.

Any of the above is detrimental to a one-owner truck upgraded for more power.
What about the pre-owned truck, where the current owner has absolutely no idea of previous useage - overheated and how often, extended oil-change intervals, poor over-all maintenance, etc.

When talking to Bill Heath, several figures are mentioned with his Boost controller upgrades, most of which, it can be noticed, are usually immediately ignored.
Get the PCM upgrade, and it's 'Katy, bar the door!'

Gages first, exhaust, cooling upgrades, Boost control, PCM upgrades, keep Boost below 15psi, even with charge-air cooler, particularly under loaded\towing conditions - that's the law.

Even with the 18:1cr upgrade, caution is still required, with this indirect injected Diesel.

Who cares about laws, or can remember science, when that computer between the ears says "Holy cow - gimme more!"?

EGT? IAT? ECT? Boost? Whadda they know, I can handle it.

Short between the headphones is the killer.........

IMO :cool:

[ 04-26-2005, 08:14 AM: Message edited by: gmctd ]

[eracers999]

I have seriously been considering a intercooler off e bay. 24x12x3 core, 31in long, 3in inlets. Does this sound like the one others have installed ? They are cheap and look of good quality, can anyone help me out here.

Thanks / Kent

[Andy Chesek]

Buzz,

Did you see the article Duramax Hot-Rod Intercooler Part 2 (http://www.thedieselpage.com/members/duramax/project2-ic2.htm) ?

I got a Ram intercooler off eBay with plans of having it installed behind the grille just as More Power did with his Hot-Rod. I think that setup is great, because I will be lifting my truck and I won't put an intercooler in what I feel is a precarious place... but I digress.

Not knowing how to weld aluminum, a friend recommended someone who had done good custom work on said friend's car. I even provided him with the articles More Power created so he could set my truck up just as shown in the pictures... see where I'm headed? *grumble* He went his own way, and cut every corner he could.


So, my intercooler is stuffed between the A/C condensor and the radiator, but the A/C condensor was never repositioned, and it was a fight to get enough clearance between the radiator and the engine fan. I'm disappointed in how it came out. It doesn't look like it came from the factory as it would have if the guy copied More Power's lead. Does the intercooler work? Yes, definitely. This all transpired during March. My friend is upset this happened and wants to do what he can to create the setup I wanted.

My radiator core support was chopped to smithereens and the intercooler was installed upside down. Because of the location of the end tanks, the batteries had to be relocated. They now sit underneath my bed in a tray the "fabricator/hacker" created. The tray is about the only nice thing to come out of it. Well, the powdercoat on the IC piping is nice, but there's more pipe now than there should be because of how it's all routed.

I recently discovered a guy I know can weld aluminum (go figure). So, I'm going to get another radiator core support and have this other guy modify the tanks of the intercooler the way it should have been done first time around. Call this an expensive lesson. But once again it proves the ageless adage, if you want it done right you have to do it yourself.

Now after all that I bet you want to see pictures, eh? tongue.gif

Like I said... at least the intercooler actually works.

[markrinker]

LOL - I definately suffer from that short between the headphones from time to time!

Was talking to a tech at Pennisular Diesel yesterday about their intercooler under development. Sounds like they'll have a shipping product in ~60-90 days.

He also agreed that stock pistons and rings in a stock compression engine can fail under high boost conditions without EGTs alerting you. In his words (loosely quoted) "...at that point, you are shoving so much air through the motor that the EGTs begin to drop, but the cylinder pressure problems are getting worse."

[GMC Hauler]

One thing I have thought about doing but havent done yet is making the mechanical boost controller have two easy adjustments, one for driving and one for towing. I know the guys running the electronic can do this easy.

I have thought about backing the boost control nut down to a "crusing" level. Then, install a spacer the right length in between the nut and the spring perch. This would have the same effect of tightening the nut. I would use a spacer that was not threaded inside, and would slip directly over the threaded rod. A dremel would be used to slot the spacer lengthwise, so the nut doesn't have to be removed to install the spacer. The spacer would be installed to raise boost levels for "Tow". A zip tie could hold it in place if necessary. You could even grove it to the spring perch to eliminate the zip ties.

This could be used to reduce boost levels, to reduce stress on the cylinders and pistons.

Thoughts?????? tongue.gif

[ 04-26-2005, 09:08 PM: Message edited by: GMC Hauler ]

[rjschoolcraft]

The Mean Effective Pressure (mep) in the cylinder is the average pressure that, if applied to the piston through the crank arm, would produce the torque measured when the engine is running. At the beginning of the power stroke, the cylinder pressure is maximum (p3). As the piston moves down, volume is increasing, so pressure is dropping. At BDC, the cylinder pressure is minimum (p4). mep is an average of the pressure from TDC to BDC.

Looking at the thermodynamic cycle for the diesel, a quantity of interest is mep/p3. p3 is the peak cylinder pressure. This quantity (mep/p3) falls dramatically as compression ratio increases. This means that for a particular power output (think mep), the peak cylinder pressure increases dramatically as compression ratio increases.

For two engines of equal power output (consider 6.5's, since that's what we're all primarily interested in), but one is 18:1 compression ratio and the other is 22:1. For these two engines, mep will be equal (power output is equal) but the 22:1 engine will see higher stresses because p3 (maximum, or peak, cylinder pressure) is higher for it than for the 18:1 engine.

This means that stresses on pistons and other components are higher in the 22:1 engine than the 18:1 engine.

Now, I probably am a little short between the headphones, but I still believe that a stock 6.5 can survive up to 18 psi if no thermal damage has been done previously. The advantages of lower egt and better efficiency are difficult to overlook.

For maximum durability, though, 18:1 is the preferred configuration. Here, I plan to run over 20 psi boost.

[99gmccrew]

Kent, I think I have the same intercooler your looking at on ebay. My towing performance is absolutely superior to pre intercooler. My egt's don't even go over 900F pre turbo, while towing up 6% grades WOT. You can check out the install on my profile. It's a very good intercooler. Quality craftsmanship. Great performance.

99gmccrew

[gmctd]

In direct-injected engines, the combustion pressure front is applied across the entire piston crown, against the entire head surface which seals the upper cylinder.

The 6.5 indirect injected engine, in 22.5 or 18:1cr, has a combustion chamber which consists of a pre-cup, inserted into the head, with a small oval opening, and a small mouse-ear shaped depression off-center in the piston crown, just below that opening.

Almost the entire pressure front is initially 'jetted' into and against that small area of the piston, off-center to the outside of the piston rod.

This is the main reason for applying low Boost pressures in I-I engines, vs the 45-60psi Boost levels hot-rodded in some noted D-I engines.

Some 18:1cr upgrade pistons reduce the crown-heighth, allowing the pressure 'jet' to more easily expand across the crown, but, never-the-less, the main pressure-front is still applied off-center to the piston rod.

Even tho 18:1 allows about 125% more Boost - ~16psi - without exceeding 22.5 mep with factory 7psi Boost level, the I-I design would still be the limiting factor, where exceeding factory mep levels would require caution, particularly in long-term loading, as in towing.

IMO, of course..........

[Billman]

More Power explained in another thread that the main reason for lower CR pistons was durability. Then he says the ability to make more power through boost and fuel is present. Don't know if I'm buyin' it.

Then you(RJ) say basically the same thing. Durability is increased with lower CR because of cylinder pressures. But you say that is compared to 2 similar output engines. 1 22-1, 1 18-1.

Now you're going for 20+ psi. Where is your durability now? Your cylinder pressures have to be closer now, don't they?

What about backpressure in the exhaust?

If you're gonna go 18-1 and keep boost at 15, then I can understand the benefits(I think).

Fire away...

[rjschoolcraft]

Again, I'm pretty short between the headphones, but the concepts that I spoke of previously are well established. If you have access to it, you should check out The Internal-Combustion Engine In Theory and Practice by Charles Taylor, a former professor at MIT.

I'm trying to move us beyond superstition and into the beutiful world of the physical sciences. I knew that when my engine grenaded, it would be fuel for every one, but that's ok. It's funny, though, that when Buffalo Guy's engine grenaded, no one questioned his use of WMI (except me) or the aptitude of the supplier of his engine. It seems that I've read of several problems from that supplier recently that have gone unquestioned. Maybe a spate of bad luck. Anyway, back to the subject.

mep is an indicator of power output, but not the mechanical stress that engine components must withstand. Tough to grasp maybe, but true. Peak cylinder pressure (p3) is a function of several parameters, including mass of air in the cylinder (boost), heat added (in the form of combustion) and compression ratio (I'm sure I missed something). The most significant driver is compression ratio (cr). As I stated previously, for a particular mep, p3 increases rapidly as cr increases.

As More Power stated previously, efficiency in reciprocating engines is related directly to compression ratio. The higher the cr the higher the efficiency. However, as compression ratio approaches infinity, the efficiency approaches a limit value. This is a fancy way of saying that there are diminishing returns. This is one reason why diesel engines are more efficient than gas engines.

This presents us with a dilemma. On the one hand, we want to increase cr to gain efficiency, but on the other, we want to decrease cr to maximize the ratio of mep/p3. Finding the best trade off is the goal.

Say that an engine's mechanical components can handle X amount of stress which is produced by Y amount of peak cylinder pressure (p3). For a high cr, that p3 of Y psi will be reached with a lower value of mep (think power output) than for a low cr engine. Therefore, you can either claim the difference as increased durability or as room to make more power (or some of both).

Another thing to realize is that these engines are not operating on a constant volume cycle (as most gas engines do). Part of the heat is added at constant volume, but part is added at constant pressure which further limits the peak cylinder pressure (p3). What this means is that fuel is still being injected well after TDC. This is one reason why the Duramax can make so much power without breaking up.

One of the goals that More Power had when building the Project Engine was to dispel some of the myths regarding IDI engines and power capability. While there are some differences that do affect power output, they are not nearly as severe as is being touted. There is nothing magic about 15 psi as a limit for boost. The back pressure argument isn't all that sound for diesels with low overlap. Aside from the piston failure, my engine responded well to 17 and 18 psi boost pressures. Egt was down, power was up, ect was down, life was happy. If back pressure was causing me a problem, it would have borne itself out in higher egt.

Even though I'm short between the headphones, I do understand the impact of thermal stress on mechanical parts. I likely suffered a low cycle fatigue failure of the piston as a result of repeated excessive thermal excursions. Those excursions all occurred when the engine was stock. Remember, I failed a #8 piston in this thing two months after I bought it when it was bone stock. Can't blame that on boost.

[DA BIG ONE]

Well, How about the 99+ motors having 19.0:1 CR or is that just more smoke being blown up our rears?

After digging into the cracked cylinder issue, I conclude it is more a hummer issue than any other, but this boost pressure string is making me think dropping boost some, perhaps for good reason.

I now wonder how many 6.5 TD motors actually last a long time after mods, or?

[rjschoolcraft]

The Project Truck has over 156,000 miles in it's modded configuration.

My engine went over 63,000 miles in its modified configuration. Those were hard, trailer towing, heavy duty miles. The engine had over 158,000 miles total when it failed. We're talking roughly 40% of the total miles on the engine in high output form...after several known near calamitous thermal events occurred. That 63,000 miles were accumulated in just over a year and a half. This engine was worked very hard.

If you look at the cracking poll I have posted at the top of this forum, you'll see that most of the block cracking incidents occur on stock engines. This problem is not related to power output at all.

I would venture to guess that nearly every 6.5 used in a 2500HD application and worked has had internal damage caused by high egt. The control system and lack of an intercooler practically guarantee that. Therefore, when starting modifications with a used, unknown engine, who knows what the condition really is? For me, my first engine failed while towing to Yellowstone in 2000 two months after I bought the truck. The warranty engine that was installed in Bismarck, ND is what just recently failed. I know the history of this engine and know exactly what it endured before I began to modify it. In fact, I wouldn't have pursued the modifications if I didn't have severe egt and ect issues. It nearly melted down three times on the way home from Yellowstone. Shortly after that, I found The Diesel Page and installed gages. I was shocked at what I learned then.

I'm quite confident that the engine I'm building will be durable...I'm also pretty sure that some new power thresholds may be achieved. We'll see.

Superstition is a dangerous thing.

[gmctd]

Well, folks. I was assuming that a degree in quantum mechanics would not be required to descern the direction of my original post, so, in order to mend fences, I offer this -

ronniejoe : you really need to get a second opinion, son - yer analyst ain't hittin' on squat! tongue.gif
;) :D :D

That being said, I stick by my statement concerning the Ricardo Comet combustion chamber configuration - it was never intended for continuous hi-Boost, hi-power output.

As in hauling, towing.

We're talking cracked and broken\melted pistons, here - no mention of cracked blocks or mains or cranks was posted

Bill Heath did some amazing 1/4 mile things with his twin-turbo 6.2, and JK did some amazing loaded hill climbs with his 6.5, but I do not recall the DP Project Truck running 18-20psi Boost levels over any of its 150k miles.

IMO categories should not be mixed or crossed, to establish a reference.

edited just because i wanted to...........

[ 04-27-2005, 05:37 PM: Message edited by: gmctd ]

[99gmccrew]

Humor mixed with wit is a beautiful thing. I just wish I had more wit. Got plenty of humor :D

[rjschoolcraft]

Originally posted by gmctd:
Well, folks. I was assuming that a degree in quantum mechanics would not be required to descern the direction of my original post, so, in order to mend fences, I offer this -

Let's don't bring him into it! :D

It's good to stick to your line of thought...until proven otherwise. 63,000 miles is a long time...for an engine that was hurt previously. Time will tell.

[gmctd]

Right - did enjoy reading your engineering treatise, tho.....

[chickenhunterbob]

If you look at the cracking poll I have posted at the top of this forum, you'll see that most of the block cracking incidents occur on stock engines. This problem is not related to power output at all. I don't know how many responses it takes to make a poll even somewhat legitimate, but to me this is far too few.

Just my thoughts, not intended to offend, or discredit the poll, but with the number of members on TDP, the numbers are miniscule.

[rjschoolcraft]

Originally posted by gmctd:
Right - did enjoy reading your engineering treatise, tho..... Cool, but I'm not so sure you understood it...what with the "analyst aint hittin on squat" comment among others.

What does the pressure front look like in a SBC wedge shaped combustion chamber? Hmmm...

I didn't mix categories to establish a reference as you stated. But, if that's what you think, OK.

I have a question about the Bill Heath drag truck picutres that has nagged at me ever since I first saw them... Where's the smoke?

[nvmtnlion]

Okay, after reading all of this I have a question along this train of thought.

Since the precups are removable, could they be replaced with different ones that would eject (erupt) onto a different or more distributed part of the piston and reduce that force that can kill the pistons?

[rjschoolcraft]

This "jetting" or "errupting" idea as a cause is really missing the mark. Look at where the cracks are occurring. Consider the stress distribution in the piston. I have yet to see a piston show any signs of errosion.

The whole reason for the "Comet Ricardo mouse ears" is to provide increased volume in the chamber directly under the opening in the pre cup. This was done intentionally to provide a balanced pressure distribution across the top of the piston when near TDC...during the short period of time that it is there.

What this involves is cyclic fatigue on a thermally weakened part.

Quantum Mechanics have nothing to do with this discussion. We're solidly in the Newtonian sphere here.

[markrinker]

RJ - Did the kids on the playground pick on you?

Just wondering. :D

[moondoggie]

Good Day!

RJ goes about 6' 4" & 240# - if they did, they sure don't anymore. :eek:

Blessings!

[rjschoolcraft]

Originally posted by Mark Rinker:
RJ - Did the kids on the playground pick on you?

Just wondering. :D :D No, not today. They did when I was little, though. I used to get beat up by gangs of three or four kids at once. It taught me mental toughness, but probably did make me sensitive to criticism.

I just re-read that thread and did get a little irritated with some of the remarks.

In the realm of this BB, there is very little scientific verification of the positions taken. All seem to be accepted with equal weight whether thay are provable or not.

I hate the fact that my engine failure has lent credence to this mythical 15 psi max boost "law". I also hate the fact that extenuating circumstances have delayed my engine project. I'm dying to get it back together and start proving my point.

In the end, I have nothing against JD or anyone else. I just don't like being scolded or labled as "short between the headphones" when the basic theory is not understood by the scolder.

JD does a good job of putting some of these concepts into humorous, understandable terminology. However, there are many areas where he still needs to listen and learn. I learn stuff here almost every day.

I do not expect anyone to take anything that I say as absolute truth. Read or hear me, but check it out to see if it is true or right. I usually provide quite a bit of supporting information with statements that I make, so they're not off-the-cuff remarks. You can dig into them and other reference material and draw a conclusion.

[nvmtnlion]

RonnieJoe,

Of course we don't take your comments as gospel, you blew up your truck! :D While this forum is full of great knowledge, following opinion up with fact is good advice no matter what. ;)

[Andy Chesek]

6'4" and 240 pounds? Well, that describes me too. I always tell people I'm average and they never seem to believe me...

[rjschoolcraft]

Originally posted by nvmtnlion:
RonnieJoe,

Of course we don't take your comments as gospel, you blew up your truck! :D Ouch! :cool: :D

[Marty Lau]

RJ;

How is the engine coming? You going to have it up and running all shook down to come to MT????

If your have time to hang out in Central Montana center of the Lewis & Clark activies there is a RV spot at The Muddy Dog Ranch, (our little 260 acres of MT) with your name on it. Let me know?
We are just 10 miles north of Great Falls.

[Mark Bajus]

hey ronniejoe, I would agree with the 15lb myth. My rebuild has been running for the better part of 4 years now with NO problems to speak of. Boost since rebuild was 18lbs NON intercooled with GM-4, then changed to Penninsular mid size whateveritis, now running 20lbs unloaded with spikes to 25 loaded NON-intercooled. Love to intercool it, just not in the budget now, and it would also mean I would just crank the pump up to use the new capacity anyway!!

Once they are built right they run well IMHO, if you are building one its not ALL that much more $$$ to build it right.

On the size front Im a stretched version of you: 6' 7" 240.

Cheers all!

[rjschoolcraft]

Originally posted by 84 Convert:
I have a 6.2 N/A that had carcked some pistons, 6 of them to be exact. I'm sure it was from high EGT's only having a single 3" exhaust, heavy towing and bad head gaskets ( water temp over 220*)! I now have a new pyrometer to keep an eye on things but I'll pobably have the turbo installed before I tow with it again.

HEAT KILLS

Gregg This was posted in the "Cracked piston" thread. Tough to blame this on "sustained high boost".

[gmctd]

Yeh - but quantum mechanic don't always mean a Volkswagen repairman.................... tongue.gif
;) :cool: