How much is too much?

[mciaio]

I know many boost posts have been started but I don't understand one thing. Some people say more boost with intercooler. Some people say more boost without intercooler. I agree with the later. I have a 99 2500 sub with a reflash, exhasut, gauges and homemade boost control. When I mash the pedal in OD on the highway I can get 18+ psi. Now since I am not intercooled, the air is superheated and not very dense so it should be something equal to about 12 psi if it were cool air. Does this make sense? I see people saying not to run more than 12 psi. My truck does that with out the boost controll in it. Am I harming the engine by running a max of 18 psi for about 3 seconds. By the way the truck still blows black smoke on hard accel. so I think it needs more boost.

[pannhead]

no polite way of saying it.....YOU ARE CRAZY :D seriously though, that's way too much boost without intercooler...do you have a EGT gauge ?

[britannic]

Think of it this way, a compression test can have values that differ as much 18+ psi and still be OK. Boost pressure that's downright damaging to a gasser, is nothing in a diesel.

When combustion commences, peak cylinder pressure is measured in thousands of psi, so adding 18psi is a drop in the ocean!

The real enemy is heat from over fueling, such that the piston surface temperature rises to melting point and combustion pressures from the additional fuel exceed the engine's design limits, so that head gaskets fail, cylinder head bolts break and pistons melt/crack.

For example, one guy I read about has a specially modified Detroit diesel, that puts out (if I remember correctly) about 1400HP with 68psi of boost.

in summary, the boost pressure isn't the issue (until you start exceeding 40psi with 21:1 pistons possibly), it's the excessive heat and combustion pressures generated by over fueling that are.

[ 12-07-2003, 06:18 AM: Message edited by: britannic ]

[mciaio]

Originally posted by pannhead:
no polite way of saying it.....YOU ARE CRAZY :D seriously though, that's way too much boost without intercooler...do you have a EGT gauge ? Post Turbo EGT, I have to really try to get it to 1250. Why is it crazy with out an IC, It would be crazy WITH an IC because the air is dense and cool. With out an IC the air is super heated 200+ degrees and there is alot more room to compress the air by the pistons. So is it crazy because of temps or because of excess cylinder pressure?

[mciaio]

Originally posted by britannic:
in summary, the boost pressure isn't the issue (until you start exceeding 50psi with 21:1 pistons possibly), it's the excessive heat and combustion pressures generated by over fueling that are. So in other words, Keep it below 1250 psi post turbo and all should be OK.

[whatnot]

1250 post turbo? Isn't that awfull high? I thought that was the limit for pre turbo. I don't like getting mine past 850.
It sounds like you have the same problem as me. You shouldn't get any smoke if you have at least 6 pounds of boost (if you have stock fueling) but I get some with 15 PSI. I havn't been able to figure it out.

[britannic]

Originally posted by whatnot:
1250 post turbo? Isn't that awfull high? I thought that was the limit for pre turbo. I don't like getting mine past 850.
It sounds like you have the same problem as me. You shouldn't get any smoke if you have at least 6 pounds of boost (if you have stock fueling) but I get some with 15 PSI. I havn't been able to figure it out. What's interesting about the GMx turbos, is that they're designed to provide relatively low boost levels at lower rpms without heating the compressed air excessively. Once they're pushed above their efficiency and backpressure becomes a factor, the air density decreases (and the oxygen with it) as it heats up, which may account for more smoke with stock fueling (assuming the fuel system and engine are in good order).

An intercooler and/or water injection will help with increasing the density of the air charge and allow the fuel to be fully oxidized.

You may also notice your engine will pull more strongly on cold damp days for the same reasons.

Pensinsular's 300HP racing Hummer 6.5LTD uses low boost pressure with high volume to ensure good intake air density (here's a quote from their Hummer page):

http://www.peninsulardiesel.com/hummer.htm

Finally, Peninsular offers a racing engine for the Hummer*, the same engine used in The Worlds Fastest Hummer* (see above). This engine will not fit in the standard Hummer* motor box, it requires some modification. It features stainless headers and a high volume low boost turbo, and runs at 300hp.

[mciaio]

Ooops, I have a pre-turbo in the crossover pipe, It's late here (1:30 am) and I am not thinking very well. Screwing around in our so called "blizzard" all night was fun. It's amazing how 12"-18" of snow will stop much of the NY metro area. Other cities deal with that on a weekly basis. evything was closed tonight except for a couple of 7-11's and a 1 diner. I passed 3 other diners that were closed.

[britannic]

Post Turbo EGT, I have to really try to get it to 1250. Why is it crazy with out an IC, It would be crazy WITH an IC because the air is dense and cool. With out an IC the air is super heated 200+ degrees and there is alot more room to compress the air by the pistons. So is it crazy because of temps or because of excess cylinder pressure? Temperature is one part of the story, the other is IC chilled air allows more fuel to be burnt with the increased oxygen, resulting in higher cylinder pressures. This combination of temperature and pressure can easily be increased to the point something will break, unless the engine is engineered to handle it. "More Power" has written some excellent information on the MP Project that considers the design limits that cannot be exceeded when searching for extra power in the 6.5LTD.

FYI: Unless you have ceramic coated pistons and Duramax style piston cooling along with lower compression, 1250F post-turbo is more like 1450F pre-turbo, due to the amount of energy the gases transfer to the turbine wheel.

[pannhead]

GMC...now you're saying 50 psi w/ 21:1 pistons would be ok but its just the heat that's the problem.......now you're just insane (friendly banter) ;)

[patrick m.]

As Brittanic said, the carge air becomes very hot, less dense, so the oxygen content is diluted, then, the hot air is detected by the "intake air temp sensor", and the computer starts pull back the fuel.
boost all you want with out an intercooler, you are doing no good.

[mciaio]

Originally posted by Patrick m.:
the computer starts pull back the fuel.
boost all you want with out an intercooler, you are doing no good. Unless you have a reflashed computer, like I do. My intake charge temp gets to about 300 degrees at 18 psi but the computer never pulls back the fuel becuase of the reflash. I can stay in it until 1250 pre-turbo and never lose fuel and itt's not easy to hit 1250 unloaded. I just want to know if I am doing damage with 18+ psi or I am just wasting energy try to build that much boost? Like I said, I have a trace of black smoke all the time. I think she needs more O2, but at what point does feeding boost become pontless because of the heat issue or pre-turbo exhaust back pressure build up?

[britannic]

Originally posted by mciaio:
</font><blockquote>quote:</font><hr />Originally posted by Patrick m.:
the computer starts pull back the fuel.
boost all you want with out an intercooler, you are doing no good. Unless you have a reflashed computer, like I do. My intake charge temp gets to about 300 degrees at 18 psi but the computer never pulls back the fuel becuase of the reflash. I can stay in it until 1250 pre-turbo and never lose fuel and itt's not easy to hit 1250 unloaded. I just want to know if I am doing damage with 18+ psi or I am just wasting energy try to build that much boost? Like I said, I have a trace of black smoke all the time. I think she needs more O2, but at what point does feeding boost become pontless because of the heat issue or pre-turbo exhaust back pressure build up? </font>[/QUOTE]The point at which boosted air density becomes inefficient due to overheating, is dependent upon many factors including the turbo's compressor/turbine trim and A/R ratio along with back pressure, ambient inlet temps, etc.

In general with the GMx turbos, once boost gets above 10psi, diniminishing returns can be expected as charge heat lowers oxygen content. With a different turbo, the boost pressure may be correspondingly higher without lowering density, due to a more efficient compressor/turbine trim etc.

For example, the Garrett T04B n/wg does very well to around 12-14psi without an intercooler - although many other factors (some already mentioned) will have an impact on this.

My MHI TE06H turbo does very well until around 13psi, when the charge air starts getting too hot to keep the oxygen levels up. I'm going to fit an IC to see if this improves matters, since I regularly see 15-17psi under WOT up hills. The next step after fitting my IC and playing with the IP tune, will be working with a turbo shop on the turbine and compressor trim wheels to better match my 4911 timing and fuel map.

FYI: Bank's pressure target for the TE06H was about 7-8psi for the 6.9/7.3 powerstroke, but it does produce a lot of volume for lower back pressure at those levels.

[ 12-07-2003, 12:27 PM: Message edited by: britannic ]

[mciaio]

So once again more is not always better, I am going to lower my boost to about 12 psi and see what happens. Maybe I'll lose all my smoke. Thank you all for your input.

[grape]

does a compressor map exist in public hands for any of the gm-x turbo's? if it does it's easy to see where the point of no return is, which is what sounds like is happening with that much boost and still black smoke........time to move more air not just compress it.

[ChevyDooley]

I run 18+ lbs of boost on mine with an IC and I get Zero smoke and lots more power than I had with 12 lbs so I guess thats ok?

[rjschoolcraft]

Chevy Dooly,

That's interesting to hear. How long have you run that way how does your truck perform? Is your egt lower than when you ran with 12 psi?

[britannic]

Originally posted by ChevyDooley:
I run 18+ lbs of boost on mine with an IC and I get Zero smoke and lots more power than I had with 12 lbs so I guess thats ok? Perfectly, the IC completes the recipe for more power. What EGTs are you seeing with this setup and boost level?