New Engine Blown Up At Tuners

[Rolls]

Absolutely. Chamber pressures are indeed affected by boost pressures, but boost is not the only, and not the major factor in resulting chamber pressures.

Overadvanced ignition timing and preignition (mixture ignited by hot combustion chamber parts prior to spark) can cause very high pressures without any audible pinging.

What was the compression ratio in this engine?

It is airflow + ignition which affects chamber pressures, not boost pressure, as you can push the same air with 18psi on a small turbo as you can on 6psi with a large turbo.

Higher boost for the same airflow results in higher temperatures which will cause the engine to be more prone to detonation, if you need higher boost for the same airflow usually means more restrictive exhaust side as well which increases backpressure + heat which compounds detonation.

[Simon-S14]

yes people forget that when the fuel is ignited in the chamber that there is something like 1200 PSI of combustion pressure... so 10-18 psi of boost pressure is f**k all in comparison

[Rolls]

Yeah but if you run 36psi it doesn't make it 1218psi, it will make it much higher! so whilst the two numbers are miles apart, it does make a big difference if there is large airflow.

I guess what I was trying to say is the main factor for combustion pressure is ignition and airflow, airflow is affected by boost, but big boost doesn't necessarily mean big combustion pressures.

[Sarumatix]

edit: Ah sorry missed the context but statement is still roughly true.

12psi x compression ratio = 120psi

Otherwise simon is right,

shamelessly copied,

Brake Mean Effective Pressure/ Peak Cylinder Pressures

BMEP is defined as the average effective combustion pressure occuring in a cycle. It can be calculated by using the formula:

792,000 X BHP divided by (engine displacement in cubic inches X RPM).

This figure is useful in comparing different engines operating on different fuels and the highest figure occurs at torque peak. The average range for atmo engines is 150 to 225 psi. Turbocharged race engines may exceed 1000 psi.

Peak cylinder pressure (PCP)is the maximum chamber pressure achieved during the combustion process. This figure would normally be in the 600 to 2000 psi range.

[wolverine]

Det would cause a sudden spike in excess of 3000psi (if I remember my keyboard warrior lessons correctly).

In the scheme of things, 18psi is like having a 2.54mm wang at a heavy hitters of porn convention.

[Legionnaire]

It is airflow + ignition which affects chamber pressures, not boost pressure, as you can push the same air with 18psi on a small turbo as you can on 6psi with a large turbo.

Higher boost for the same airflow results in higher temperatures which will cause the engine to be more prone to detonation, if you need higher boost for the same airflow usually means more restrictive exhaust side as well which increases backpressure + heat which compounds detonation.

You're confusing airflow vs. boost for power production talk with peak chamber pressures that have broken rings in the above engine.

Airflow, or you should really use more correct term airmass, of course affects power output, more air mass mixed in proper amounts with fuel, releases more heat and increases chamber pressure. To produce power you take average pressure per cycle, not peak pressure, read BMEP, iMEP, so not really relevant here.

It's really boost pressure that affects chamber pressures, because chamber pressures are reached (calculated/measured) with all valves shut, so there is obviously no flow present. Of all parameters that directly affect pressures, boost takes part only in initial pressure conditions prior to valve closing. After that only majorly important things are DCR, gas characteristics, amount of heat loss and gas leak and ign advance, as well as chamber geometry, cylinder bore and stroke, fuel used and proportion of its mixture with air.

Sarumatix,

12psi x compression ratio = 120psi

You have to take absolute pressure so it's more like (12 + 14.7) psi. Otherwise almost correct.

Edited September 5, 2011 by Legionnaire

[O2 Autosports]

looks like det to me.

[rob82]

looks like det to me.

Classic ringland brittle failure there - Looks like det to me too.

[Rolls]

You're confusing airflow vs. boost for power production talk with peak chamber pressures that have broken rings in the above engine.

Airflow, or you should really use more correct term airmass, of course affects power output, more air mass mixed in proper amounts with fuel, releases more heat and increases chamber pressure. To produce power you take average pressure per cycle, not peak pressure, read BMEP, iMEP, so not really relevant here.

I don't think I'm confusing them, I am explaining how it is airflow (or more correctly airmass as you said) that dictates the combustion pressure and hence torque (I meant torque not power when I said that earlier) that can be made (what you said basically)

What probably killed his motor was too high combustion pressure (once again what you also said), however this was reached I do not know, too much airflow or detonation, it isn't the boost persay as he could have reached the same combustions pressures with a much larger turbo running say 10psi.

It's really boost pressure that affects chamber pressures, because chamber pressures are reached (calculated/measured) with all valves shut, so there is obviously no flow present.

I don't really agree, sure the valves are shut but depending on how much air mass is flowing you are going to get more air in the cylinder per stoke. The boost pressure doesn't dictate how much air mass there will be as can be seen with a stock turbo running 12psi vs a TO4z running 12psi, one has shitloads more air mass than the other.

If you keep the turbo the same then yes increasing boost increases combustion pressures, but if you are comparing different turbo's then it does not necessarily do so, this is all I was trying to get at with my earlier post. Look at the peak torque figures, intake temp figures, and ignition figures, not the boost number, boost number is irrelevant if you know the others.

edit: Actually we were saying the same thing all along, I just misunderstood what you posted on the last page. My bad, I just get annoyed when people say you can't run more than x psi or you will pop your motor, or 'oh you cant run 20psi that will destroy it!' without knowing what turbo and hence what torque it is making, I know no one really said this, I was just ranting.

Continue on! lol

Edited September 5, 2011 by Rolls

[joe blo]

yeah might take the bits somewhere else to see what they say.

[MrStabby]

Yeah but if you run 36psi it doesn't make it 1218psi

+1 and thank you.

Many faceplams in this thread. The extra air that the turbo adds doesn't just go along for the ride. It is used to burn the extra fuel that the ECU has added, creating more cylinder pressure. That's why you go faster!

[Legionnaire]

I don't think I'm confusing them, I am explaining how it is airflow (or more correctly airmass as you said) that dictates the combustion pressure and hence torque (I meant torque not power when I said that earlier) that can be made (what you said basically)

What probably killed his motor was too high combustion pressure (once again what you also said), however this was reached I do not know, too much airflow or detonation, it isn't the boost persay as he could have reached the same combustions pressures with a much larger turbo running say 10psi.

I don't really agree, sure the valves are shut but depending on how much air mass is flowing you are going to get more air in the cylinder per stoke. The boost pressure doesn't dictate how much air mass there will be as can be seen with a stock turbo running 12psi vs a TO4z running 12psi, one has shitloads more air mass than the other.

If you keep the turbo the same then yes increasing boost increases combustion pressures, but if you are comparing different turbo's then it does not necessarily do so, this is all I was trying to get at with my earlier post. Look at the peak torque figures, intake temp figures, and ignition figures, not the boost number, boost number is irrelevant if you know the others.

edit: Actually we were saying the same thing all along, I just misunderstood what you posted on the last page. My bad, I just get annoyed when people say you can't run more than x psi or you will pop your motor, or 'oh you cant run 20psi that will destroy it!' without knowing what turbo and hence what torque it is making, I know no one really said this, I was just ranting.

Continue on! lol

Haha, nice, we really were trying to make roughly the same point in different words.

Likewise, I hate unspecific statements about boost pressures when no other parameters aren't being specified.

Truth is they both matter, but in different ways. There are basically two stages of pressure rise in cylinder - first starts at intake valve closing and lasts till ignition point (actual compression stroke). Pressure levels at the end of this stage are not significantly affected by the airmass, but seriously depend on initial boost, or absolute pressure before valve closing. Second starts with spark and reaches its peak at approximatly 15 degrees ATDC, but this is obviously affected by spark advance. Peak pressure, among other things, depends on initial (pre-ignition) pressure and mixture energy release rate (which in itself is affected by plenty of parameters). Denser mixtures with more mass of air/fuel per unit of volume certainly release energy faster, resulting in higher peak pressures for a given ign advance and initial pressure - it's actually what you said. So we're both correct.

Now if that peak happens too early, say near TDC, or is too high and piston isn't moving down fast enough to relieve it, all that pressure has nowhere to go (yet) and starts to break things - like ring lands.

[STATUS]

Yeah you'd think a company with a reputation like that would make a quality cast piston. But that being said a cast piston genuine or not is a gamble with reliability even if they are new.

Unfortunately for the owner yes.

[Rolls]

Truth is they both matter, but in different ways. There are basically two stages of pressure rise in cylinder - first starts at intake valve closing and lasts till ignition point (actual compression stroke). Pressure levels at the end of this stage are not significantly affected by the airmass, but seriously depend on initial boost, or absolute pressure before valve closing.

Still don't really think this is the case, imo boost level is fairly irrelevant. Eg with a bigger turbo that makes the same torque at say 10psi as a small turbo does at 20psi the combustion pressure is the same isn't it. Two of the reasons why is because the bigger turbo has less back pressure causing high VE as the air can fill the cylinder faster, also the compressor is operating at a higher efficiency level hence less heat (denser air).

There really are so many factors that I would go to say boost level really doesn't tell you much at all. Really need to look at the airmass that is being allowed into the cylinder. You fit more air into the same sized space and hence you get higher pressure > higher combustion pressure. Airmass is really the only factor that matters.

[Legionnaire]

Nah, I was talking about pre-ignition pressures only, there is a well established formula for that pressure: P = Pinit * CR^γ, where

Pinit = P ambient + P boost (pressure prior to valve closing),

CR - obviously dynamic compression ratio,

γ - adiabatic index of medium that is being pressurised.

As you can see, no signs of mass or time in this formula, which is quite logical, as in the shut cylinder PV relationships play major role, mass is (almost, ignoring small leaks past rings) constant.

In other words, when you compress mixture in the cylinder, it doesn't matter how exactly initial conditions were reached. 20 psi is still 20 psi, either with small or large turbo.

The amount of molecules, their composition, density, etc. start to play role when you ignite things.

[Rolls]

Pre ignition pressures don't necessarily directly correlate to combustion pressures though. As if you have really high back pressure you will have poor scavenging, more exhaust gas and less air in the cylinder when the valve closes, same pressure (I think?) but less air+fuel so lower combustion pressure.

20psi with a large turbo is more air than 20psi with a small turbo, yes the pressure is the same in the intake manifold, but the 20psi found with a larger turbo will be much higher combustion pressure as there would be much more air being trapped in the cylinder, hence bigger bang. The 20psi with a larger turbo will also be a much cooler intake temperature which as can be seen in your formula will also increase the combustion pressure.

Can you see why I'm saying the boost measured by your boost gauge might as well be irrelevant as far as knowing if it is the cause of your blown ringlands? As with a larger turbo you decrease back pressure (decreasing intake temps, increasing VE) you also run the compressor more efficiently also decreasing intake temps so for the same given boost pressure you are flowing more air, hence when the valve shuts more air (less exhaust gas) is trapped in the cylinder (more air mass) hence higher combustion pressures.

I get what you are saying but combustion pressures are what is important, not pre-ignition pressures. Hence what he needs to look at is ignition timing being run and how much airmass he is burning, eg has he reached the limit of what the pistons can handle combustion pressure wise (via det or just purely exceeding the torque levels that the pistons are designed for) not what boost the turbo is running.

So yeah what you said is correct but I don't really think it is relevant here, I guess I am just trying to show why what boost pressure he was running is not the issue, it is whether he was experiencing detonation or just simply exceeding the max torque the pistons/ring combo were designed for.

[Legionnaire]

we're really trying to say the same thing using different words.

for the same given boost pressure you are flowing more air, hence when the valve shuts more air (less exhaust gas) is trapped in the cylinder (more air mass) hence higher combustion pressures

Denser mixtures with more mass of air/fuel per unit of volume certainly release more energy faster, resulting in higher peak pressures for a given ign advance and initial pressure

I understand what you're saying, and I agree with you. I just think you slightly overestimate the role of compressor efficiency and underestimate contribution of initial pressure in resultant combustion pressure. Don't be so focused on turbo. Look how engineers of restricted turbo motors extract torque out of their engines.

[joe blo]

another twist on the story...

just went to the guy that tuned it and had a talk. i only took one piston in but. had the top ringland broken out and the bottom ringland the bottom broken but still in there abit

he talked me through his diagnosis. says its from insufficient ring gap. showed me where it had butted together and says it has grabbed in the bored and the motor had pulled it apart from holding the ring and pulling the rest of the piston down.

sound right? sounded right when he went through it with me.

[James_03]

So the next obvious question, did you check/set ring gaps?

I would have thought that the rings themselves would have failed before the ringlands but if that was the case.

[joe blo]

shop put the pistons in. not tuner shop but another engineering shop