CM871 Engine cycle...

[Rawze]

(12-24-2016 )Rig Wrench Wrote:  What does the higher boost (40+psi) do that causes the damage to the engine? Particularly the Isx. What is the few extra pounds of air pressure doing to cause damage. I understand overspeeding the turbo, but other brands of engines survive at higher pressures, and competition engines often much higher. I've heard of cm870, 871 and 2250s running upwards of 46 psi for hundreds of thousands of miles, although most of them have fixed vane turbos. I'm in no way endorsing it, just tryin to understand.

Boost "pressure" is only HALF AN EQUATION! --- YOU ARE LOOKING AT ONLY HALF A PICTURE, OR REFERRING TO HALF A PICTURE IF THAT IS ALL YOU ARE CONCERNED WITH.

CHARGE FLOW (Boost and flow combined) is what needs to be considered,.. NOT boost levels alone.


That being first understood,...


Boost "pressure" level itself should always be based on the compression ratio of the engine. Sure you got a bunch of ABSOLUTE MORONS!, especially in big-name tuning companies who are so ignorant to think more boost = more power,.. or more efficiency, etc. but they are complete idiots!. -- There is a point, based on compression ratio and displacement, where more boost DOES NOT mean more power to the flywheel any more. - Sad part of it is that THERE ARE MORE IDIOTS THAN PEOPLE WHO KNOW BETTER out there by a factor of 9,000 to 1. This leads to a LOT of bad information and BAD opinion,.. and BAD tuning/methods.


ANY ISX engines running 46+ PSI are way over-boosting and in extreme danger of engine damage. Just because people sometimes get away with it for a while, does NOT mean it is OK by ANY means. Don't let their extreme ignorance fool you,.. NOT ONE BIT!.

I guarantee you without any doubt or question, because I have experimented with it heavily,.. that you LOWER THE BOOST PRESSURE on those engines and they will have a LOT more power end efficiency for the same exact fuel and other settings. - Those morons are only fooling themselves and making a LOT of excess heat that goes nowhere but the radiator and oil pan.

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As Simply put as possible here, crude explanation --

Last I checked, the purpose of an engine was to convert fuel to mechanical energy. Burning fuel in a cylinder of that engine creates BOTH heat AND pressure based on oxygen (and other gas/moisture, etc) content and fuel.


Mechanically, the cylinder size/diameter, and compression ratio of the engine will dictate how much power it is capable of producing. Bigger pistons and cylinders, more power capacity. Smaller = less power. Turbo-charging an engine "cheats" this by not just "stuffing more in" the cylinders, but by providing increased overall air flow into the engine but it comes at a cost. --

That cost is the way in which that extra air flow (intake boost and pressure) is produced. For a turbocharger to make more boost pressure/flow at the intake, it must RESTRICT the air-flow at the exhaust to spin the turbo faster, or to move a higher volume of air. The energy to make pressure and flow has to come from somewhere.

Restricting air-flow at the exhaust of an engine has many negative effects. The biggest is trapped heat. Right along side this is the exhaust flow restriction itself that takes AWAY from the flywheel output, because the engine has to labor harder to push exhaust gases out against/past this restriction.

Basically, you have one side, producing "stuffing more into" the engine, producing gains above its mechanical/physical limits, but on the other side, it is costing you in flow restriction, heat retention, and internal engine load.

To put this in the most simplest terms possible --

Adding more pressure/flow to the intake simulates bigger mechanical components a bit and adds more to the flywheel, but only to a point. Above that point, you start burning the fuel where it produces more rapid "pure heat" energy than "heat expansion"/mechanical energy. This is because the ratio of heat to expansion ratio in the cylinder diminishes as temps go higher and higher. -- I.E. stuffing more in, produces diminishing returns. I.E. -- more, more,. more starts producing less and less gains.

Making more, more, and more,.. requires you to choke off the exhaust flow more, more, more to get it. As the intake gains become less and less per added pressure,... The exhaust restrictions, heat buildup, and laboring of the engine to push exhaust gas out starts overcoming the gains. -- Assuming for a moment, you are under engine load and fueling, etc. is correct -- It is said that in a 15-litre engine, roughly 16 PSI pressure buildup at the exhaust manifold between the exhaust ports and the turbo, can be as much as 50-HP taken away from the flywheel. - A decent turbo will make about 10 psi at the intake when there is 16 at the exhaust. That is great if the added air makes 100 more overall HP,.. but as you increase this, the effect becomes less and less.

At 46-PSI boost pressure, even WITH the fueling etc. to go with it,.. AND with a really efficient turbo, your looking at upwards of 80-90 PSI of pressure at the exhaust manifold (almost the same pressure in your tires!), possibly much more if the turbo was not specifically designed for it. - That then becomes, easily, 200+ HP of internal engine load just to produce the pressure at the intake. -- heat buildup and damage aside, the gains would be so diminished,.. that you would be producing LESS POWER at the flywheel than when you only had 36-38 lbs of boost.

That competition dyno report gearhead and I did while back for the ISX at 800+ HP reveals this quickly. it is a classic example of what I have seen several times.

You will notice power FALL OFF as boost pressure starts to exceed 39 psi. This is because as the engine labors harder to produce this pressure, the gains are so low, that the power at the flywheel suffers. ALL that extra effort is now starting to go into the radiator and oil pan instead of the flywheel.
http://rawze.com/forums/showthread.php?t...03#pid6803

I have seen and done this many times with DIFFERENT turbochargers and modifications to air flow over the last several years with people. The mechanical compression ratio dictates where this break-over point will be every time. That break-over point is also the point where engine damage will start to set in, even if HP levels are much lower due to cylinder heat being absorbed int the metal too rapidly, combined with a "snap/flash" firing of the cylinder due to excess oxygen, hammering the internal components like the crank, pistons, and liners.

Even the extreme high HP ISX-CM871 engine that Unilevers helped to build and program of more than 1500+ HP output can only use about 34-35 PSI of boost. There is a much bigger volume of air-flow into and out of this engine because of a bigger turbocharger displacement and a different valve cam that holds valves open longer. This easily allows for a higher volume of overall charge flow, overcoming the boost pressure limitations.

In the end, to get more power from an engine,.. you need more charge flow (boost and pressure combined). You also cannot exceed "boost/pressure" levels above the mechanical displacement/compression ratio of the engine without causing problems and possible damage. Increasing boost pressure alone to get more charge flow will get you so far, but has its set limit. Once you start to get closer and closer to this pressure limit,.. to keep increasing charge flow, you need to trade that away and start considering ways to achieve a higher volume of flow into and out of the engine instead.

* Cap for a CM870/871, that is about 38-39 PSI max pressure.
* Cap for a CM2250, that is about 34-36 PSI max pressure.
* Cap for a CM2350, that is about 30-33 PSI max pressure.

This is based on the design considerations and testing of boost pressure levels with these engines that has been done, and follows the compression ratios/displacement. NOT what turbo or other components these engines have in them, nor what someone has slapped onto them.

Even knowing these caps, does not mean these engine SHOULD be running these levels of boost pressures. An example of this is the comparison between the CM870 and CM871. Even after both are de-mandated and have no more soot problems, and even though they have roughly the same components throughout the engine,... A LOT of CM870's make it well past 1.1-1.3 million miles, yet it seems that CM871 engines struggle to make it to 900k before an in-frame. -- One difference in how they run? -- Boost response and pressure.

Although slight, the cm871 engine tends to have higher boost levels, and it comes in quicker all the time compared to CM870 engines. Exhaust manifold temps show this too, as they tend to be slightly LOWER for the Cm871. - This is a telling sign that lowering boost levels down to 34-36 psi in a CM871 could actually extend engine life long term, allowing for higher exhaust manifold temps (less cylinder absorbed heat) not raising it. - Just my own theory though, but I think Hedly, here on the forum has figured some of this out with them by tracking several trucks and their fuel mileage, wear, etc. vs company drivers too. He mentioned a while back that he lowers boost levels and the trucks perform better at 34, instead of 36-38 with less problems against those abusive drivers for their given power.

This is what I have observed and are my own thoughts on it all. Take it or leave it.