2871r/enthalpy what PSI for 300whp?

[Kingtal0n]

Originally Posted by LoSt180

My new goal in life is it actually understand the math & theory in that post.

It might seem like 'alot' because I wrote alot to give a full picture,

but in truth the math is not even college level, its intermediate algebra only...

so the math isn't the issue, I am sure anybody can do it.

The theory is a little more involved but still pretty simple,

text wall
The engine is a air pump, and so is the turbo. both displace some exact amount of air per revolution in a perfect world. We simply find those numbers using classical formula and compare them, and because the manufacturer of the turbo provides a compressor map which correlates air volume flow rate to air temperature (the islands on the map are efficiency which is basically air temp, center is coolest) we can make safe assumptions about the total mass flow potential of the running engine at some compressor flow rate by making sure it appears on the map somewhere and hasn't run off the edge on either side.

*more info*
There is some minor fluid mechanics, but it is basic. You must understand the volume flow rate from a device, and how this is different from fluid mass flow rate. Air is a fluid. Air mass is derived from knowing volume flow rate AND air temperature. All intercooler abilities and ambient air temperatures and turbo efficiency will always vary, there will always be some unrealistic approaches to these sort of problems, which means we have to look at several distinct "worst and best case" scenarios when attempting this sort of calculations.

*VE*
The other necessary aspect is engine volumetric efficiency and how it compares to engine flow rate in volume flow and mass flow. A cylinder filled 100% with fresh incoming air is considered 100% VE, but this can happen at 0rpm, 2rpm, 2000rpm, 5000rpm, etc... which means an engine flow rate is independent from VE, which means power output has nothing to do with VE by itself, you MUST combine VE to some engine RPM or somehow derive a mass flow rate to get power output figures from an engine.

Another variable is the amount of airflow pulled into the exhaust system during overlap, which adds to mass flow rate overall (it will tax turbo flow rate a little bit) and may help achieve a high 100% VE by doing so. This additional flow rate must be accounted for because the compressor flow rate is sensitive to all losses, whether they are boost leaks or exhaust overlap leaks it will tax the compressor and raise the exhaust gas pressure and temperature. A large boost leak can destroy an engine by raising the EGT and EGP because of this simple fact and it happens all the time.

And yet another issue is debatable... can a cylinder fill more than 100% full? Can you achieve 101%+ VE? It is common to use VE values over 100% (when say, tuning an engine) When dealing with forced induction since we are technically filling cylinders beyond 100% full... and yet that doesn't quite make logical approach since nothing can be more than 100% of anything. This brings a slew of questions to the table... how much of the 100% of cylinder fill is exhaust gas, and does exhaust gas count as VE? There is always some exhaust gas present even if its just 0.01% so technically VE can never truly be 100% for any engine, as it is impossible to remove every single exhaust gas molecule between events. Next, Naturally aspirated engines can fill a cylinder beyond 100% VE by utilizing the kinetic energy of incoming air, timed to the overlap period perfectly with the right valve events, causing 102% or 105% VE and bumping torque slightly beyond what should be possible. But a cylinder can only be 100% full, right? So how do we account for this additional VE without using nonsense numbers beyond 100%. The answer is in the air density, and in order to compare VE numbers beyond or near 100% we must set that density in our equations to some standard or relative to a known value. It is commonly accepted that atmospheric pressure and temperature is the derived air density from which 100% VE can be derived. In other words, if the cylinder is full of ambient air temperature and density air to 100% full, then VE is 100%. But common sense should kick in and tell you there is no way a cylinder with a 1400*F piston surface and 650*F valves contains air that is merely 80*F at any time. What this boils down to is we must work with a 'theoretical VE' value, a value for VE which correlates to engine torque & cylinder fill but has nothing to do with actual VE of a real engine. This will keep calculations simple and allow us to account for increasing air density and high temperature air without knowing the exact temp or density of the air. This is possible because mass airflow rate correlates to power, NOT torque, so the number we see on the chassis dyno will generally match up with the compressor wheel speed shown by our compressor wheel speed monitor (or paper-based calculations) and we can skip all calculations involving the engine and not have to worry about what is realistic VE.

*more VE*
The concept of VE directly correlates with cylinder pressure, from which torque is derived, for example all OEM naturally aspirated 2L engines no matter who designed will generally produce about 130 to 150lb-ft of torque maximum to the tires, as seen by thousands of dyno graphs, at near 100% VE. This can happen at any rpm, 1000rpm or 8000rpm, depending on the engine combination. Doesn't matter how much you port the head, big camshaft, super exhaust, etc... the cap to VE is near 100% for natural aspiration.
The correlation between engines (some more torque than others) is not linear due to myriad variables such as temperature, compression ratio, rodxstroke length, chamber design, ignition timing, etc... In other words if I set out to make a bit more torque than that I could definitely do it under the right circumstances, but it might not be reliable or cheap to accomplish. So again nothing is going to be exact in our equations, therefore once again as proper engineering students we should account for best and worst case scenarios. That said, there are limits, practical limitations to the amount of cylinder pressure and torque one may achieve which helps us rule out impossible results. For example it would be extremely unlikely to ever see 200lb-ft of torque or more from an N/A 2.0L engine, no matter what you do to it, when using typical gasoline fuel. We already know that N/A engines max out near 100% VE (102 or 105% is possible but not 115%+ that is unrealistic) and a torque figure of 200lb-ft would indicate over 115% VE already, so it can never happen unless the engine had insane compression and was running superior fuel than gasoline. Or perhaps being run with sub zero air temperature or something like that. It would not be practical or typical, or inexpensive to achieve such results.

Another aspect of VE that might be difficult at first is the fact that almost every engine in the world ever produced can achieve near 100% VE at some rpm. In other words, every engine will display some peak torque at some point, and it is almost always near 100% VE or 100% cylinder fill. That is because all combustion engines basically will have at least 1 specific RPM point where they are ideally effective/efficient at filling their cylinders, and that is where we find peak torque. This should help explain why VE by itself has no direct correlation to power. IF every engine in the world can achieve 90% to 100% VE at some rpm, then why don't they all produce max power? It is because VE doesn't correlate to power and is only one small piece of the power puzzle, you must have good VE AND at a high RPM as possible. 100% VE at 10,000rpm would yield good power even if the engine was only making 200lb-ft of torque. (200*10,000/5252 = 380hp) so in theory any N/A 2L engine could approach 350 horsepower if it could spin near 10,000rpm.

*How VE is really useful*
The theory of VE is important, but not super useful by itself. How can we use VE to gain useful information?
When we run an engine on the dyno from idle to redline at some intake manifold pressure (can be 0, 2psi, 5psi, 40psi, -5psi, -10psi, pick a number and hold it there) we generate a VE curve for the engine. This can also be called a torque curve. Basically the torque curve is a VE curve, it shows you how the engine cylinder is filling at each individual RPM breakpoint from idle to redline. If you point to the peak torque you could say this is peak VE or peak cylinder fill.

It tells us ALOT of information about the engine. Does torque fall off rapidly at some point? Does it spike wildly, jagged curve? Does it look 'good' (nice and smooth and kind of flat shape, like an arch or hill)? Any perturbations of the torque curve are warning signs about the engine, which could be some distress. And any sudden changes or drops in the torque curve indicate an issue with cylinder pressure, which could be cylinder fill (VE) or combustion related (e.g. spark blow out).
I always dyno my engines at 0psi of boost before using boost because it will lay the ground work expectations (VE curve) for higher boost pressure and take some guess work out of the tuning. In other words if the engine VE curve maintains it's shape and form and smoothness all the way from 0psi to 3psi to 5psi to 15psi to 30psi, this is a great sign of a healthy engine and good combustion quality. Probably Nice and safe if the curve looks the same shape and smoothness at 30psi as it did at 15 and 0psi, just higher up on the graph. Also the amount of torque gained per psi of boost is very important, as any diminishing returns tell you that the turbo or engine is having some difficulty in flowing air mass, could be an exhaust restriction or turbo is out of air for example. It is very easy to damage an engine at high boost pressures if you miss any of these warning signs, and you won't even realize there is a problem unless you established a VE curve from the get go.


*Cliffs* Summary
-We compare engine volume flow rate to compressor volume flow rate, then combine with temperature to get air density and mass flow rate which correlates to total power potential, and make sure to line up the compressor map with the engine flow rate ability at best and worse case scenarios.

-We establish a VE curve for diagnostic purposes and understand what VE is (torque curve relationship)