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Cylinder Pressure and uncorrected vs corrected HP

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Currently live and tune at 8000-9000 DA levels at times so our uncorrected numbers are usually much lower than corrected numbers and cars tend to run almost a full second slower in the 1/4 mile here than at sea level.

Because of this, it typically takes much more for our cars to make power at this altitude. My question is, does 600whp UNCORRECTED at this altitude create more cylinder pressure and engine stress then 600whp would at sea level? Since we generally have to add more boost and timing to compensate for our altitude to create more power, does this equal more stress on components? Or, since the air is less dense here, would 600whp uncorrected still have similar cylinder pressures to 600whp at sea level all other variables being equal dispute it running more boost and timing?

hope that makes sense.


You've definitely got your work cut out for you at a density altitude of 8-9000'! Ultimately the engine doesn't really care what altitude it's operating at, and instead it's the cylinder pressure that defines how much power and torque it will produce. This means that on face value 600 whp is the same level of stress on the engine whatever altitude you make it at. The caveat here though is that you're going to need to work a turbocharger much harder in order to generate the same cylinder pressure. What you need to keep in mind is that for a given turbocharger, this is going to result in a lot more turbine inlet pressure at altitude than what you'd see at sea level so you will usually end up with the engine also working harder in order to maintain the same power level. For engines developed specifically for operating at altitude (Pikes Peak for example), you'd usually have different considerations with the turbo selection in order to level the playing field somewhat but then you tend to give away some response so there's no silver bullet unfortunately.

hi regarding cylinder pressure ,if i have lower torque it means im making less cylinder pressure

so a car with 500 tq at 40 psi is less likely to blow a headgasket than a 650 tq car running 30 psi of boost?

That is only true if the ignition timing is optimized, and there is no knock / detonation, on identical engines (same components / geometry) under the same conditions (Engine Speed, barometric pressure, air & coolant temps).

There are a bunch of things to consider, but in short it's as Andre said, down to mean cylinder pressure, which comes down to absolute* manifold pressure and, to an extent, exhaust manifold pressure.

David is also dead on - for mechanically identical engine assemblies -inlet to exhaust outlet - less torque normally means less mean cylinder pressure. Exceptions may be over-advanced ignition or too rich/lean fuelling.

Yes, you are going to have to work the engine, as an assembly, harder.

You don't mention if it's a NA or a forced induction engine?

If the former, the starting point will be to add as much compression as practical, and to carefully assess the selection of components used, as you may need to think about the build as a "low compression", or "restrictor", engine and how some of those principles may apply. Get cold air to the engine, use coatings to keep heat out of the pistons, valves and head if they're options during the build, size the exhaust pipe diameter and length to suit, not forgetting the collector which can have a big affect on power output.

IMPORTANT, not a "turbo'" guy, and trying to recall fluid dynamics for nearly 30 years ago, with 'some' turbo' basics - take them as discussion points rather than definitive, as I certainly may have incorrect assumptions.

If forced induction, certainly the turbo-charger will be stressed a lot more - to try and compensate for ambient air density almost 1/3 less than sea level, which will mean also optimising everything you have, no 'soft' tunes or timing, etc. Minimising pumping losses - making sure all the intake and exhaust is free flowing, heads and camshts, etc are optimised for the rpm range, your charge cooling is adequate - if air-air the cooler will be much less efficient as there is less air mass to transfer the heat to, so if not doing so, consider an ice/water box.

Note I said "absolute" manifold pressure - this is ambient plus 'boost', so get that the same you will need to add boost to compensate. NOTE, at this point, I'm considering the temperature, and hence net density, the same.

In simple terms you may think this just means, say, something like a 4psi loss of ambient means 4psi more boost, which is true, all else being the same, but turbo-chargers work on a pressure ratio and that 14.7 ambient to 54.7 manifold (40psi boost) ratio of 3.72 will become 4.68, which means it may be operating further outside it's optimum range. This could be further compromised by the available exhaust gas to drive the turbine being lower, although it's discharging against a lower atmospheric pressure.

Running a larger cold side to get a greater mass into the engine won't work well, because the pressure gradient, which the turbine has to over-come will be significantly greater - it may not be too much of an issue, but it will increase exhaust manifold pressure.

All in all, I would be looking at running series turbo's, to keep the boost ratios closer to their optimum efficiencies and improve the overall efficiency, with air-water charge coolers after each stage. however, there are $$$s and installation problems with that, and some single 'chargers do work well at those high pressure ratios, and you could do a lot worse than talk to their tech' guys about your specific application, as 600hp is "common" nowadays.

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