Sittin' here wondering to myself:
"self, is there such a thing as too much boost bein' a good thing?"
Self doesn't know what to think. So, self says, let's involve other selves out there and come up with a better answer.

Here are some numbers that I came up with here:
CR CR
psig psia 21.3 18

0.3 15 319.5 270
1.3 16 340.8 288
2.3 17 362.1 306
3.3 18 383.4 324
4.3 19 404.7 342
5.3 20 426 360
6.3 21 447.3 378
7.3 22 468.6 396
8.3 23 489.9 414
9.3 24 511.2 432
10.3 25 532.5 450
11.3 26 553.8 468
12.3 27 575.1 486
13.3 28 596.4 504
14.3 29 617.7 522
15.3 30 639 540
16.3 31 660.3 558
17.3 32 681.6 576
18.3 33 702.9 594
19.3 34 724.2 612
20.3 35 745.5 630
21.3 36 766.8 648
22.3 37 788.1 666

Left is PSIG, (boost gauge) PSIA, (absolute pressure, dat's for us number crunchers.) Next is the amount of pressure at that is developed in the cylinder itself at 21.3 compression ratio. Last column is 18:1 compression ratio pistons.

Now, my disclaimer is that I'm not too sure 'bout my math on this one, but it is in the ball park maybe.

Question one: is there a point where your boost becomes counter-productive? Meaning, does it require more energy to "squish" that air and thereby reducing the amount of power that can be made at the flywheel?

Question two: Taking those numbers there, is that comparble to what you would see in combustion/post-combustion pressures?

Now the above numbers are pressures per square inch. Now, the piston face is not one inch square. Whereever that info is, (which is somewhere on the board here, just too lazy to search at the moment) what does that translate to on the crank/bearing surfaces?

I figure this may generate some interesting discussion.;)

I am not sure on the math part, but part of the equation of lower compression allows more fuel and more air so that the result is more power. So, I'm not really sure what to tell you, other than more boost = higher intake air temps and without an intercooler can become very detrimental. Also, for engine longivity I personally wouldn't run more than 14 psi on a 6.5 with stock compression. Weak link is still the head gasket.:eek:

Me thinks running less boost with more O2 in it would be much better, especially for me with the huge fuel rates and no O2 to burn it with, or as D.Cam says, intercooler!

Actually - lower compression ratio allows more Boost, to which more fuel can be injected.

Note how 10psi Boost at 22.1 gives 532psi cyl press - follow that down the 18:1 column, and you may note how same cyl press is obtained with 15psi Boost.

Add fuel to suit.

Repeat as necessary...........

Purpose of Boost is to pack more O2 into same volume, increasing effective flow rate.
Or - to increase the weight of the volume, which increases the mass air flow.

And remember - stock PCM with stock 63mm fuel will make 20psi Boost, easy, with IAT over 300deg if sustained.

Charge-air cooling required - cool air is denser, heavier air.

EGT gage required.:cool:

So what then is the mechanical limits of our engines, and what is the limit there for?

Is the limit of our engine the amount of pressure sustained on the piston and bearing surfaces? Or is the intake air temperature the real problem?

Which is worse?

IAT can be kept in check easily.

It's the lack of head bolts, junk block, & backpressure that are limiting...

Absolute worst, is the original 135hp design - porous, crumbly block, cast-iron crank, indirect injection, inadequate coolant configuration, etc.

Check out the construction - they got pics! - of a 360cuin 6-cylinder, over in the Cummins forum, here, for what we shoulda got.

Some have run up to 26psi Boost on short time\load trial runs, but the billowing black smoke - pics! - indicates that much Boost ain't working.

Indirect injection don't like high Boost levels.

Indirect injection was meant for ease of starting, smooth drive-away, economy, in a time of noturbo, where black smoke meant power.

I remember it well...........

Not fondly, but well.

Stanadyne\Detroit Diesel\Chevrolet gave us the first "smokeless" Diesel.

18:1cr, 16, maybe 18, psi continuous Boost with charge-air cooling, should be safe - if you have a good block and crank, and all the cooling upgrades.

But, not with the GM-X series turbo - you'll need a Holset, or Schwitzer, or equivalent to open up the exhaust for lower exhaust back pressure, so that 18psi input can output.

IMO, of course.

Guess we were posting at the same time, so yeh - like Billman sez.......

Okay, I was reading just last evening ye ole Haynes manual, and found the bore at 3.98" So 3.14*((3.98/2)squared)= 12.434714 (or there about).
So if I understand this correctly at 10 psi of boost, you've got about (532*12.43)=6612.76, man that's a bunch of pressure on the piston at stock CR.
18:1 at 10psi boost = (450*12.43)=5593.5
That's a difference of 1019.26 (a cool half ton of pressure)
Now, if we cruise at 5 psi of boost, which sounds like maybe it's kinda common, then at stock CR we are lookin' at ( 426*12.43)=5295.18

These numbers are a bit loose, but I think illustrates what's happening in the engine at least.

Is there a point where you've got so much boost that the engine is working harder on the next compression of the next clyinder that it's actually taking power rather than adding it?

So why then does indirect injection not like high boost levels?

Thought - Combustion pressures are around 3000psi..........

I am probably mentioning the blatantly obvious so please humor me. Does your example use the stock turbo? If so, there is a sweet spot the turbo likes to operate in. Beyond this point the turbo is better at heating the air than moving it. I have not seen a compressor map for our turbos, but it seems the turbo likes pressures around 12 - 15 at the most.

I've always thought a more efficient turbo like a properly sized Garrett would do a lot for the performance of our trucks. It would also address the exhaust restriction that Jim always talks about.

I had an interesting article on how to read a compressor map - I'll see if I can track it down.

Hmmm...overlooked that last bit, knk - it's about volumetric efficiency, combustion chamber location, shape and size, but I can get back with more later, if need be.................

Increase boost until engine blows, then back off 1/4 turn :D

Tim

Now, if we cruise at 5 psi of boost, which sounds like maybe it's kinda common, then at stock CR we are lookin' at ( 426*12.43)=5295.18

These numbers are a bit loose, but I think illustrates what's happening in the engine at least.


It's impressive, I never thought about it that way. But I'd think the pressure on the cylinder is getting way higher once combustion starts. If it did not, the engine won't have power to compress the air in the cylinders and at the same time yield the power to drive the truck. I wonder how high it gets --- maybe the torque gained on the crankshaft can give some numbers to guide us.

I'm running 15 psi max right now intercooled and it seems to be very happy. EGT is lower now and takes quite a bit of work before you have to get up to the danger zone. I am going to back it off to 14 though to see what kind of difference there is. I too am leary of putting over 14psi into the 6.5 even with my intercooler.

It's impressive, I never thought about it that way. But I'd think the pressure on the cylinder is getting way higher once combustion starts. If it did not, the engine won't have power to compress the air in the cylinders and at the same time yield the power to drive the truck. I wonder how high it gets --- maybe the torque gained on the crankshaft can give some numbers to guide us.

You are very correct there. If compression took more power than combustion, we'd never start the engine. I was just wondering if there may have been a "sweet spot" where when you've added so much boost, that you actually start going backwards...

Backpressure will set you back at some point --- if not from the exhaust, it will come from the charger. If you get over that point, the engine will generate more heat only, but not more power. I wonder if a decent downpipe with an unrestrictive exhaust actually gains you that much more than the stock downpipe with the same exhaust.

Lee, the stock downpipe is one of the biggest pieces of crap EVER. My truck runs so much better with the new exhaust over the factory one

Thought - Combustion pressures are around 3000psi..............

Lee

gmctd has said many times the backpressure issue is between the exhaust valve and the turbo. An aftermarket downpipe and exhaust system will only do so much for you.

You're repeating yourself again jd...

:) :cool:

The turbo functions best when exiting gasses are allowed to quickly expand and depart - restrictive exhaust plumbing will increase ebp in the turbo.

Replace the factory exhaust system for best results, even when using the GM-X series turbos.

Like the old saying, a system is only as good as it's worst component.:exactly:

So why don't IDI engines not like high boost levels?

I could be wrong, but aren't most IDI engines a bit higher in compresion? Higher compresion will not take as high a boost due to increased cyl pressure.

That, too, but we have access to lowered compression pistons, so here's the rest......

Purpose of Boost is to pack more O2 into a specific volume - if 1cuft of air at Baro - 15psi - makes X power, 1cuft of O2 at 15psi Baro + 15psiBoost should make twice X power.
And it can, depending on engine configuration.

Max power is achieved when each new power cycle is begun with a full, fresh, cool, dense air charge - no residuals from previous cycle.

Power cycle being intake, compression, injection\combustion\power, exhaust - repeat as necessary.

A direct injection Diesel has its large, open combustion chamber in the crown of the piston, with the head surface flat - no valve-shrouding, fapp.
To ensure full cylinder evacuation, intake valve and exhaust valve may be held open at the same time - overlap - where cool, fresh Boost pressure forces any previous combustion residuals out the exh valve.
This total flushing is complete because of the flat head surface and dished piston crown - no pockets, nooks, crannies, etc, for the residuals to hide in, under, behind, beneath, etc.

Enter the indirect injection system, where part of the combustion chamber is in a small cylindrical 'pre-cup', inserted into a pocket in the cylinder head, and another part in a small, mouse-ear shaped depression, offset to one side in the piston crown.

In this configuration, the flat-top piston crown is in very close proximity to the cylinder head, at Top Dead Center.
Very close, allowing no room for any valve overlap, required to flush the cylinder.

Next - injection method, location, etc.

Hmm, Makes sense! Thanks!

Okay, which brings back backpressure from before. Valve overlap doesn't work because exhaust pressure is actually higher than incoming boost pressure. Due to the fact that the GM-X turbo is bootlenecking and driving up the pressure. So, any instant that both valves (intake and exhaust) are open at the same split second, exhaust gas can go passing backward into the intake.

So, from what I gather here, your IDI has a greater "volume" to play with, as compared with DI engines. That volume, either full of fresh air or post-combustion air. That volume is counterproductive with Volumetric efficiency (VE) VE defined as the ability to completely rid itself of all post combustion products after the end of that cycle.

So throw on a GM-X turbo with high backpressure between exhaust port and exhaust turbine, and a lower VE, boy, looks like you start to go backwards with higher boost with our engines. So as backpressure rises with boost, your actually beginning to "pollute" your clyinder with post-combustion products? You drive up the intake air temp, bottleneck your exhaust, drive up the pressures inside your engine, head gaskets, bearing surfaces, etc, for the joy of the needle moving up to a higher number??

Did I understand that right?:idea:

Yes - but that's where having no valve overlap helps, by closing off the cylinder to the high ebp.
And the exhaust valve opens before the end of the power stroke, when cylinder pressure is still high, forcing the residuals out the exit.

Since there is little volume above the piston, the id-i engine is somewhat more efficient than a d-i engine with high ebp and no overlap - if such an engine existed.

This is where knowledgeable DIesel experts suggest 12-14psi as the limits for max efficiency with 21.5cr, altho some folks have seen EGT drop with Boost levels of 17-18psi

But, as I wasn't finished with the theme in the first post, yet, I'll continue this soon as I've had my pout.................;)

Lee, the stock downpipe is one of the biggest pieces of crap EVER. My truck runs so much better with the new exhaust over the factory one

It's just that I had been crawling under my truck and closely examined the downpipe to check out how I could install the pyro probe. My impression was that the downpipe is not as restrictive as I thought, and I got the idea that the rest of the stock exhaust is more restrictive than the downpipe.

The downpipe is about impossible to replace simply because I cannot get a replacement here ...

If your down pipe is like mine, it's only 2.5", not three (van). Those crimped 90° bends in the exhaust don't help much either.

If your down pipe is like mine, it's only 2.5", not three (van). Those crimped 90° bends in the exhaust don't help much either.

GMCTD, I keep checking back to see "the rest of the story" there Paul Harvey, I'm as anxious as a kid at Christmas.

Sorry, knk, and others - not pouting, just wore out.

Been moving my Customer Service department, and a larger one from Ohio, into new (to us) facilities, and setting up a Shipping\Receiving dept to handle the flow.

I start a response, and just doze off - hope to be done with the move, shortly.............

knkreb and others - note the mouse-eared depression off to one side of piston centerline, in Turbine Doc's 18:1 Rebuild post

That is part of the 6.5 indirect injection combustion chamber - note that initially, combustion pressure will not be applied equally across crown of the piston.

It now has less depth than oem, as the piston crown was shaved to reduce 21.2cr to 18:1cr.

Wrist pins are in oem location

Also, re-read QM's Feb 05 post on 599 rebuild, for additional info with pics.

There are other pics on here for the pistons, heads, pre-cups, and etc, but I am a computer illiterate, barely able to do post and search functions.:confused:

Next - injection method, location, etc.

I guess your still thinkin' or dozin . . .

Note also, for this exercise, the pre-cup port in QM's removed heads in his 599 rebuild post

That depression in the piston serves to create and focus tremendous swirl into the pre-cup on the compression stroke, ensuring exellent mixing as the fuel is injected.
This results in the exellent effiiency and fuel economy that indirect injection is noted for in the normally aspirated 6.2\6.5L Diesel engine.

Some of the injected fuel remains in the pre-cup, some exits into the swirling air in that depression.
The injected fuel ignites in the pre-cup, and the flame and pressure front 'jets', or erupts outward and into that depression, then spreads aross the piston crown as the piston begins to drop on the power stroke..

Comparatively, the direct injection combustion chamber is in the dished crown of the piston, the dimension of which - diameter and depth - is determined by the cylinder bore, crankshaft stroke, and required compression ratio.

The piston crown is flat around the outer diameter, and with the dished center, creates the required swirl necessary for good mixing.
Fuel is usually injected directly into the center of this area, such that the flame and pressure front spreads evenly across the piston crown, resulting in complete burn and max energy conversion, heat to force.

The id-i pre-cup 'jet' port, or throat, has required change thru the years, reshaped and enlarged as power increased -

1982 6.2na - 135hp - round hole, flattened on side toward the cylinder center

1993 6.5td - 190hp - larger, slightly-curved jellybean shape

1999 6.5td - 215hp(proposed) - diamond shaped

Non-emissions F and J engines had larger 'ports' than S and C engines, but that was the only mechanical difference.
Heads, block, crank, rods, pistons, cam - same.

The emissions -cups were sized and shaped for max swirl and economy
Non-emissions -cups were sized for power.

That id-i pre-cup jet-port and mouse-eared chamber is the limiting factor in producing power comparable to d-i's open-chamber configuration.

High Boost pressures allow high fuel rates creating high power - 5.9L Cummins with 45-60psi Boost @ 900hp\1200ftlbs torque, twin turbo, iirc

The 18:1cr 6.5 maxes out at around 500hp, twin-turbo - check Pen Diesel and others for specs.

Exhaust back pressure is caused by the turbo, sized for quick spool-up off-idle.
High ebp is a limiting factor for high Boost levels, as ebp inreases quicker than Boost in this configuration.

Change the turbo, and id-i becomes the limiting factor, where the flame\pressure-front cannot exit the pre-cup in a timely manner.
Note the size of the port opening, as compared to cylinder diameter.

Then, there's valve sizing, exhaust port sizing, crankshaft and block strength, and etc.

That's my story, an' I'm stickin' to it............

p.s. - pressures on compression stroke effectively cancel as they also equally force the piston back down on the power stroke, even if combustion has not occured.

The pre cup pictures are a few pages back.
http://dieselplace.com/forum/showthread.php?t=25433

p.s. - pressures on compression stroke effectively cancel as they also equally force the piston back down on the power stroke, even if combustion has not occured.

Interesting, how does it cancel if no power from combustion during the power stroke?

Intake air is compressed to ~400psi on the up stroke, which pressure then forces the piston back down on the return stroke.

Ever cycled a model airplane engine over? Prop is resistive to rotation on the compression stroke, then 'pops' forward on the 'de-compression' return stroke.

Same principle here, just more of it.

Thanks for the link, QM.................

Ever cycled a model airplane engine over? Prop is resistive to rotation on the compression stroke, then 'pops' forward on the 'de-compression' return stroke.


Naa, I just fish model rockets out of trees, no airplanes. Ah! maybe I'll stick a whole bunch of "C" engines on my bumper, and save my engine. That'll reduce my 0-60 time!:joke:

:D

That's the S equiv.

Try the J, which is F equiv.

More Power to ya!!!! :cool: