RE: Skoda Octavia vRS Revo Technik: Driven
Discussion
SuperchargedVR6 said:
Playing Devil's advocate here but does it still make 370bhp after 15 dyno pulls on a 28 deg summer's day? VAG's component protection maps are pretty aggressive when IATs hit 50+ C and / or EGTs soar past 850 C. That chunky catalyst ~4" downstream of the turbine wheel is not EGT friendly, so that would need to be binned for starters. And VAG aren't one for over-engineering their intercoolers either, so that will heat soak quite nicely.
I'd be impressed if the Octavia could hold its claimed power over 15 dyno runs on a hot day, but then does it need to? Does the AMG A45 only do 381bhp on a rolling road in optimal conditions? I wouldn't be surprised if it'd do more.SuperchargedVR6 said:
That's why I threw the AMG A45 engine into the mix because it's two tuning companies attempting the same thing - 200hp / litre.
It is comparing a similar core objective, but the peripheral requirements are very very different. Revo's efforts don't have to be certified or homologated in the same manner (if at all!), nor do they have to meet OEM durability requirements driven by warranty. In theory, that allows Revo a little more flexibility.SuperchargedVR6 said:
I'm not saying either Revo or AER are 100% correct, but something just doesn't add up here. With AMG's expertise and (massive) financial backing from Mercedes, how did they 'only' arrive at 381bhp from 1.8 bar and Revo achieved 410bhp from 1.7 bar?
See my comment above - the constraints are tighter for AMG than Revo... I can't quote the detail from the regs because I don't have them to hand, but whereas Revo quote a power range (in this case 386PS-420PS and journalists obviously pick the headline figure...) the guys at AMG will have to certify a power that they're confident all engines/cars will meet - even Monday morning/Friday afternoon ones.Revo's list of supporting mods also includes a turbo-back exhaust (not clear whether it's a 'sports' cat or a decat in there) and a larger intercooler - it's unlikely that an OEM could get away with fitting such parts, either because it risks emissions non-compliance - or in the case of squeezing a bigger heat exchanger in there, packaging constraints/design rules.
SuperchargedVR6 said:
As AER was suggesting from his calcs, the VAG engine cannot, in theory, produce that much power from the air it's fed.
I think if he showed his working, and explained/justified his assumptions, then people may be more inclined to agree.Revo themselves are saying 386-420PS. 386PS = 381bhp and a production car (AMG A45) can do 376bhp from a similar capacity, so the lower end of their figures is surely believeable?
Is it a fallacy to think that the OEM couldn't extract 5-10% more power from their production engines if they weren't constrained by durability/warranty; emissions; noise requirements etc, and managed to cherry-pick a 'good' engine?
I think most turbocharged cars could be happily ran at 10-20% extra power and torque.
The issue is
a) making a car to suit all markets (perhaps running on poor quality fuel)
b) making a car to suit all environments (in terms of atmospheric pressure, etc)
c) having the components (turbo, etc) all <well> within safe limits for maximum durability
(idiots who get in them, rag them from cold, don't service them correctly, etc).
d) meeting emissions requirements
The issue is
a) making a car to suit all markets (perhaps running on poor quality fuel)
b) making a car to suit all environments (in terms of atmospheric pressure, etc)
c) having the components (turbo, etc) all <well> within safe limits for maximum durability
(idiots who get in them, rag them from cold, don't service them correctly, etc).
d) meeting emissions requirements
VW suggest 400PS from the EA888 is achievable, with on the surface minor hardware changes (marketing guff sized salt pinch):
"To achieve the increase in power, the Golf R 400 features a newly developed turbocharger with a higher maximum charge pressure and reinforced crankcase. A water-cooled exhaust gas feed to the turbocharger integrated in the cylinder head helps to reduce full-load fuel consumption, while variable valve control with dual camshaft adjustment improves economy and performance."
"To achieve the increase in power, the Golf R 400 features a newly developed turbocharger with a higher maximum charge pressure and reinforced crankcase. A water-cooled exhaust gas feed to the turbocharger integrated in the cylinder head helps to reduce full-load fuel consumption, while variable valve control with dual camshaft adjustment improves economy and performance."
amstrange1 said:
SuperchargedVR6 said:
As AER was suggesting from his calcs, the VAG engine cannot, in theory, produce that much power from the air it's fed.
I think if he showed his working, and explained/justified his assumptions, then people may be more inclined to agree.AER said:
You need air with that fuel. More fuel alone does not equal more power.
Take that Golf R engine as a higher performance baseline.
At peak torque (280lb-ft @ 4000rpm - say), boost pressure of 115kPa (your figures - that's 215kPa absolute), an inlet temperature of 40°C (typical), a BSAC of about 3.75kg/kWh (again, typical - gets worse the more you push things, generally), the engine will have a delivery ratio (volumetric efficiency, relative to manifold pressure) of around 105%. Total volumetric efficiency will be about 205%. BMEP here is a respectably germanic 24Bar.
If we hold everything constant and wind up the boost to achieve your claimed peak torque of 380lb-ft, the boost pressure needed (even assuming no intake air temperature increase!) will be 190kPa (290kPa absolute). At 10°C more intake air heating, you're cracking the 2Bar boost threshold. At this point, you're alleging a slightly stratospheric BMEP of 32.6Bar - achievable, but not in a garage on pump fuel.
The same calcs can be done at the peak power point. For reference, assuming the Golf R is running the same boost pressure, BMEP is still around 24.2Bar, delivery ratio is 100% and VE is 206%. At 410hp @ 5400rpm, the boost pressure needs to rise to 220kPa (320kPa absolute) for the same delivery ratio. VE is now 291%! Take it to a more realistic 50°C IAT - because you're now compressing a shed load more air to a much higher pressure - it's now 230kPa (330kPaA). In any case, we're at a laughable and highly improbable 34.2Bar BMEP.
All this is assuming combustion efficiency, thermal effiency and friction remain constant (constant BSAC). This is highly unlikely, especially at the power densities you're claiming, meaning the boost pressure would need to be even higher, or you're producing even less power than you're claiming.
Take that Golf R engine as a higher performance baseline.
At peak torque (280lb-ft @ 4000rpm - say), boost pressure of 115kPa (your figures - that's 215kPa absolute), an inlet temperature of 40°C (typical), a BSAC of about 3.75kg/kWh (again, typical - gets worse the more you push things, generally), the engine will have a delivery ratio (volumetric efficiency, relative to manifold pressure) of around 105%. Total volumetric efficiency will be about 205%. BMEP here is a respectably germanic 24Bar.
If we hold everything constant and wind up the boost to achieve your claimed peak torque of 380lb-ft, the boost pressure needed (even assuming no intake air temperature increase!) will be 190kPa (290kPa absolute). At 10°C more intake air heating, you're cracking the 2Bar boost threshold. At this point, you're alleging a slightly stratospheric BMEP of 32.6Bar - achievable, but not in a garage on pump fuel.
The same calcs can be done at the peak power point. For reference, assuming the Golf R is running the same boost pressure, BMEP is still around 24.2Bar, delivery ratio is 100% and VE is 206%. At 410hp @ 5400rpm, the boost pressure needs to rise to 220kPa (320kPa absolute) for the same delivery ratio. VE is now 291%! Take it to a more realistic 50°C IAT - because you're now compressing a shed load more air to a much higher pressure - it's now 230kPa (330kPaA). In any case, we're at a laughable and highly improbable 34.2Bar BMEP.
All this is assuming combustion efficiency, thermal effiency and friction remain constant (constant BSAC). This is highly unlikely, especially at the power densities you're claiming, meaning the boost pressure would need to be even higher, or you're producing even less power than you're claiming.
Edited by AER on Sunday 14th February 03:51
Interesting.
Couple of points.
I can't remember last time I saw 40 degrees IAT (I have inlet air temperature readout on my car among others) and I have a roughly the same sized turbo as a Golf R (in terms of inducer and exducer size) except I run a lot more boost... 33Psi actual / 43psi absolute. I too have an uprated intercooler like the Octavia (not the same brand however).
Maybe in the peak of summer at full chat but not normally.
Secondly. these calculations have a great deal of assumptions about the engine. software. fuel. timing. etc.
For example the original car power calculations not taking into account the advantages of running on 99 Octane (stock car is able to run on 95 and we have already discussed gains of up to 50bhp , when remapped and modified to take advantage).
Engine tuning isn't linear. You can't just say, well we have X now, so to get extra power we've just added extra boost (and the necessary fuel). The engine has VVT which can be controlled also and timing can be adjusted if necessary.
Not only that but boost pressure will vary... At 5500rpm the boost pressure will probably be starting to tail off on a Golf R turbo. I have not seen a graph with pressure/power of a Golf R so can't comment for sure.
Assuming your calculations are correct (and I freely admit I don't know (or care) enough to check if they are, you have not accounted for the above (which I CAN tell).
This is a dyno chart from a Edition 30 Mk5 (TFSI engine) with a TTE420 Hybrid...just a tad bigger than the turbo on the Mk7R
Here's another from a Leon (same engine as the Edition 30) but with the stock K04 Turbo (k04 capable of a decent 360-380..perhaps a tad more with an aggressive tune)... turbo being roughly the same size.
I still don't know what you're arguing to be honest - that the 400bhp + from the Octavia is not possible?
Spend 10 minutes researching TFSI and TSI tuning and you'll see that it is.
Again I will state that you can't calculate the engine power on the back of a fag packet using a default set of assumptions which is what you are doing.
Couple of points.
I can't remember last time I saw 40 degrees IAT (I have inlet air temperature readout on my car among others) and I have a roughly the same sized turbo as a Golf R (in terms of inducer and exducer size) except I run a lot more boost... 33Psi actual / 43psi absolute. I too have an uprated intercooler like the Octavia (not the same brand however).
Maybe in the peak of summer at full chat but not normally.
Secondly. these calculations have a great deal of assumptions about the engine. software. fuel. timing. etc.
For example the original car power calculations not taking into account the advantages of running on 99 Octane (stock car is able to run on 95 and we have already discussed gains of up to 50bhp , when remapped and modified to take advantage).
Engine tuning isn't linear. You can't just say, well we have X now, so to get extra power we've just added extra boost (and the necessary fuel). The engine has VVT which can be controlled also and timing can be adjusted if necessary.
Not only that but boost pressure will vary... At 5500rpm the boost pressure will probably be starting to tail off on a Golf R turbo. I have not seen a graph with pressure/power of a Golf R so can't comment for sure.
Assuming your calculations are correct (and I freely admit I don't know (or care) enough to check if they are, you have not accounted for the above (which I CAN tell).
This is a dyno chart from a Edition 30 Mk5 (TFSI engine) with a TTE420 Hybrid...just a tad bigger than the turbo on the Mk7R
Here's another from a Leon (same engine as the Edition 30) but with the stock K04 Turbo (k04 capable of a decent 360-380..perhaps a tad more with an aggressive tune)... turbo being roughly the same size.
I still don't know what you're arguing to be honest - that the 400bhp + from the Octavia is not possible?
Spend 10 minutes researching TFSI and TSI tuning and you'll see that it is.
Again I will state that you can't calculate the engine power on the back of a fag packet using a default set of assumptions which is what you are doing.
xjay, you seriously appear to know nothing about engine design it seems. Your comments demonstrate this clearly to anyone who knows engines.
You surely can estimate power from a clean sheet. VE, BSFC, BSAC and BMEP are very useful normalizing tools to compare completely different engines. When you take the same engine and just change boost pressure, you can calculate to a high degree of accuracy what an expected upper bound of power should be. I say upper bound because almost invariably you will get degradation in these parameters as you increase boost pressure.
And I'd seriously doubt you can hold much less than 20°C above ambient post intercooler air temperature under sustained conditions, (such as the Bruntingthorpe run posted earlier). Most likely it'll be a lot more and when you start increasing your compressor pressure ratio (as you must to increase boost pressure), the amount of heat you have to reject to contain IAT skyrockets.
As with engines, turbochargers and intercoolers are a mathematical known quantity and figuring out their performance parameters on paper is not hard either, if you know how.
You surely can estimate power from a clean sheet. VE, BSFC, BSAC and BMEP are very useful normalizing tools to compare completely different engines. When you take the same engine and just change boost pressure, you can calculate to a high degree of accuracy what an expected upper bound of power should be. I say upper bound because almost invariably you will get degradation in these parameters as you increase boost pressure.
And I'd seriously doubt you can hold much less than 20°C above ambient post intercooler air temperature under sustained conditions, (such as the Bruntingthorpe run posted earlier). Most likely it'll be a lot more and when you start increasing your compressor pressure ratio (as you must to increase boost pressure), the amount of heat you have to reject to contain IAT skyrockets.
As with engines, turbochargers and intercoolers are a mathematical known quantity and figuring out their performance parameters on paper is not hard either, if you know how.
xjay1337 said:
This is a dyno chart from a Edition 30 Mk5 (TFSI engine) with a TTE420 Hybrid...just a tad bigger than the turbo on the Mk7R
I don't believe any of these bullshït curves, but I'd just like to point at that this engine is only "making" just over 350hp at 5400rpm, where the RevoT Octavia is supposedly "making" 410...AER said:
amstrange1 said:
SuperchargedVR6 said:
As AER was suggesting from his calcs, the VAG engine cannot, in theory, produce that much power from the air it's fed.
I think if he showed his working, and explained/justified his assumptions, then people may be more inclined to agree.But then you knew this already I'm sure, perhaps avoiding it so that people didn't tweak the assumptions to give a different answer? Or maybe just CBA. Either way, it'd be nice if you did - it'd be a more positive contribution to the thread.
I'm sure you're also well versed in correlating models to validate assumptions before the model can be taken to be accurate. So a grown-up would be willing to concede the output of your calcs is primarily indicative and not definitive, potentially with similar margin for error to Revo's rolling road figures?
Of course it's always nice where the theory (hand calcs/models/simulation) matches the practice, but in cases where that's slightly different you need to be a fact holder on both sides to evaluate where the reality of the situation lies.
AER said:
I don't believe any of these bullshït curves, but I'd just like to point at that this engine is only "making" just over 350hp at 5400rpm, where the RevoT Octavia is supposedly "making" 410...
Why are they bulls
t? Can you please validate why. Many, many cars are able to run that powerStage 1 map is around 300. Stage 2 with intake and exhaust is around 330-340.
Stage 2+ with the hpfp upgrade allows for 360+ without adding much, if any boost.
Maybe phone up Niki at rtech who has been tuning these and many vag cars for overa decade and tell him theyre bulls
t! Or go along to a rolling road day and argue it out there. Also in relation to your previous post instead of being a little child and calling me names/insulting my knowledge, how about answering the post and especially my point regarding working out to assumptions (as mentioned also by another poster).
Im sorry but your calculations do not hold water for me and you can call me whatever names you want in your childish outbursts but I'm a results driven person as opposed to a theorist.
amstrange1 said:
As I said, if you showed your working and explained your assumptions...
,,.perhaps avoiding it so that people didn't tweak the assumptions to give a different answer?
All of my assumptions are clearly spelled out. If you knew anything about engines and physics you could check my calculations from the assumptions I have given and the manufacturers data. That you haven't and keep wanting me to "show my working" proves more that you're incapable of understanding it rather than any error on my part.,,.perhaps avoiding it so that people didn't tweak the assumptions to give a different answer?
Do your own sums and give me a scientific rebuttal or offer your own calculations as a response. I am not here to give you detailed engineering lessons.
xjay1337 said:
AER said:
I don't believe any of these bullshït curves, but I'd just like to point at that this engine is only "making" just over 350hp at 5400rpm, where the RevoT Octavia is supposedly "making" 410...
Why are they bullst? Can you please validate why. Many, many cars are able to run that powerStage 1 map is around 300. Stage 2 with intake and exhaust is around 330-340.
Stage 2+ with the hpfp upgrade allows for 360+ without adding much, if any boost.
Maybe phone up Niki at rtech who has been tuning these and many vag cars for overa decade and tell him theyre bulls
t! Or go along to a rolling road day and argue it out there. Also in relation to your previous post instead of being a little child and calling me names/insulting my knowledge, how about answering the post and especially my point regarding working out to assumptions (as mentioned also by another poster).
Im sorry but your calculations do not hold water for me and you can call me whatever names you want in your childish outbursts but I'm a results driven person as opposed to a theorist.
daniel1920 said:
Few more points of standard car being mapped for 95RON and I am assuming the Revo map is 98 minimum, plus different spark point to add to the math above 
The standard car will definitely correct for 98RON fuel, but in any case the increased boost pressure will drive knock margin away from MBT ignition degrading the BSAC from the standard boost level. My assumption (spelled out) is that BSAC remains unchanged, so will err on the optimistic side if anything.AER said:
All of my assumptions are clearly spelled out. If you knew anything about engines and physics you could check my calculations from the assumptions I have given and the manufacturers data. That you haven't and keep wanting me to "show my working" proves more that you're incapable of understanding it rather than any error on my part.
Do your own sums and give me a scientific rebuttal or offer your own calculations as a response. I am not here to give you detailed engineering lessons.
If you can't explain it simply, you don't understand it well enough.Do your own sums and give me a scientific rebuttal or offer your own calculations as a response. I am not here to give you detailed engineering lessons.
Crack on lad.
AER said:
amstrange1 said:
SuperchargedVR6 said:
As AER was suggesting from his calcs, the VAG engine cannot, in theory, produce that much power from the air it's fed.
I think if he showed his working, and explained/justified his assumptions, then people may be more inclined to agree.AER said:
You need air with that fuel. More fuel alone does not equal more power.
Take that Golf R engine as a higher performance baseline.
At peak torque (280lb-ft @ 4000rpm - say), boost pressure of 115kPa (your figures - that's 215kPa absolute), an inlet temperature of 40°C (typical), a BSAC of about 3.75kg/kWh (again, typical - gets worse the more you push things, generally), the engine will have a delivery ratio (volumetric efficiency, relative to manifold pressure) of around 105%. Total volumetric efficiency will be about 205%. BMEP here is a respectably germanic 24Bar.
If we hold everything constant and wind up the boost to achieve your claimed peak torque of 380lb-ft, the boost pressure needed (even assuming no intake air temperature increase!) will be 190kPa (290kPa absolute). At 10°C more intake air heating, you're cracking the 2Bar boost threshold. At this point, you're alleging a slightly stratospheric BMEP of 32.6Bar - achievable, but not in a garage on pump fuel.
The same calcs can be done at the peak power point. For reference, assuming the Golf R is running the same boost pressure, BMEP is still around 24.2Bar, delivery ratio is 100% and VE is 206%. At 410hp @ 5400rpm, the boost pressure needs to rise to 220kPa (320kPa absolute) for the same delivery ratio. VE is now 291%! Take it to a more realistic 50°C IAT - because you're now compressing a shed load more air to a much higher pressure - it's now 230kPa (330kPaA). In any case, we're at a laughable and highly improbable 34.2Bar BMEP.
All this is assuming combustion efficiency, thermal effiency and friction remain constant (constant BSAC). This is highly unlikely, especially at the power densities you're claiming, meaning the boost pressure would need to be even higher, or you're producing even less power than you're claiming.
Take that Golf R engine as a higher performance baseline.
At peak torque (280lb-ft @ 4000rpm - say), boost pressure of 115kPa (your figures - that's 215kPa absolute), an inlet temperature of 40°C (typical), a BSAC of about 3.75kg/kWh (again, typical - gets worse the more you push things, generally), the engine will have a delivery ratio (volumetric efficiency, relative to manifold pressure) of around 105%. Total volumetric efficiency will be about 205%. BMEP here is a respectably germanic 24Bar.
If we hold everything constant and wind up the boost to achieve your claimed peak torque of 380lb-ft, the boost pressure needed (even assuming no intake air temperature increase!) will be 190kPa (290kPa absolute). At 10°C more intake air heating, you're cracking the 2Bar boost threshold. At this point, you're alleging a slightly stratospheric BMEP of 32.6Bar - achievable, but not in a garage on pump fuel.
The same calcs can be done at the peak power point. For reference, assuming the Golf R is running the same boost pressure, BMEP is still around 24.2Bar, delivery ratio is 100% and VE is 206%. At 410hp @ 5400rpm, the boost pressure needs to rise to 220kPa (320kPa absolute) for the same delivery ratio. VE is now 291%! Take it to a more realistic 50°C IAT - because you're now compressing a shed load more air to a much higher pressure - it's now 230kPa (330kPaA). In any case, we're at a laughable and highly improbable 34.2Bar BMEP.
All this is assuming combustion efficiency, thermal effiency and friction remain constant (constant BSAC). This is highly unlikely, especially at the power densities you're claiming, meaning the boost pressure would need to be even higher, or you're producing even less power than you're claiming.
Edited by AER on Sunday 14th February 03:51
You've thrown some maths up but used words like 'typically', 'around', 'assuming'. Is that factual? Until you can actually take this engine out of the Octavia and measure it against you're own standards to back up your maths, nothing you can say is definitive.
SuperchargedVR6 said:
You may know a thing or two but your attitude stinks. Knowledge and manners don't need to be mutually exclusive. If it were Paul Roche or Keith Duckworth asking if we need spoon feeding, fair enough, but who exactly are you? You could be a 15 year old regurgitating the internet for all we know.
You've thrown some maths up but used words like 'typically', 'around', 'assuming'. Is that factual? Until you can actually take this engine out of the Octavia and measure it against you're own standards to back up your maths, nothing you can say is definitive.
It's not the smelliest thing that stinks around here. Nothing I have said do I claim to be definitive. It's all open to independent validation or rebuttal. So far it hasn't happened and all everyone seems able to do is call people names and shout me down. No-one has offered anything concrete to dispute my calcs or offered an alternative explanation beyond posting another useless rolling road graph - the very thing in dispute! I'd be quite happy for someone to poke holes in my maths because at least I'd be up against someone with a grasp of the concepts and reality. Feel free to have a go!You've thrown some maths up but used words like 'typically', 'around', 'assuming'. Is that factual? Until you can actually take this engine out of the Octavia and measure it against you're own standards to back up your maths, nothing you can say is definitive.
Yoof said:
If you can't explain it simply, you don't understand it well enough.
Crack on lad.
There is no simple explanation that doesn't cut corners on relevant details. Let's face it, much of the theory of engines isn't even covered in degree-level engineering thermodynamics classes, so expecting a simple and useful explanation on PH is unrealistic, However, I'll give it a go. You'll have to do your own research to fill in the details, I know I risk people knit-picking the explanation but I'll accept that for now. I will mostly ignore knit-picking, but if anyone raises a relevent point or an interesting question, I will respond.Crack on lad.
So, an engine is fundamentally an air pump. It generates mechanical energy by injecting chemical energy and converting it to heat energy at the top of compression. Some of the heat is then converted into useful work (mechanical energy) and the rest is wasted to the environment through the cooling system and the exhaust.
This puts some fundamental limits on the process which can be easily calculated. Primarily, the amount of energy available is a directly related to the amount of air pumped because air is one half of the chemical reaction that creates the heat. That is, a certain amount of air can only produce a certain amount of heat some of which in turn can be converted into mechanical work/energy. The proportion that is made useful relative to the heat that is wasted can be manipulated to some degree by compression ratio, bore/stroke ratio, coolant temperature, etc. For a specific engine design, however, this characteristic won't change much unless you specifically (and dramatically) modify something. Thus the amount of air you pump is closely correlated to the amount of power you make. This is known as (Brake) Specific Air Consumption - i.e. BSAC. Brake, because it's relative to brake power, that is power measured at the flywheel on a dynamometer brake.
Key lesson no. 1 - the amount power you can make is very closely related to the amount of air you pump.
In the case of turbochargers, we can recover some of the waste heat in a separate thermodyamic cycle (effectively a second engine as Keith Duckworth would say). As there isn't any mechanical connection between the turbocharger and the flywheel, we don't get much useful mechanical work out of it - at some points we lose - but we can use it to effectively raise the "environmental air density". That is, just as the air density increases as you drive down a mountain, so it can be made to increase further by mechanical means. To the piston engine, increasing boost pressure seems just like the barometric pressure is rising. This isn't strictly true because the exhaust pressure is rising much faster than the intake manifold pressure. For this reason, increasing boost pressure has an incresaingly deletarious effect on the conditions the piston engine "sees". Note I say piston engine to distinguish it from the compound cycle of piston engine+turbocharger. The piston engine is the only thing generating mechanical power for us.
Thus, if we know the manifold air pressure and temperature, we can calculate the air density that the piston engine is operating at. This brings us on to the second major point - how much gas the engine is pumping. The swept volume is the gas volume of one engine cycle (720° in a four-stroke). The speed at which cycles occur (crank speed) can be used to calculate the total volume "sweep" rate - that is, the volume of gas it would pump in a given time at 100% pumping efficiency. This is where the term volumetric effciency comes into the equation. Due to cam timing, intake and exhaust tuning and throttle position, this efficiency factor can vary. Relative to manifold pressure - known as delivery ratio - it is surprisingly constant, all other things being equal. Relative to atmospheric pressure, it is extremely linear. There is an exhaust pressure component to this characteristic too, which is a much smaller effect and is related to engine's clearance volume. This acts as an offset term to the linear VE characteristic. The higher the compression ratio, the smaller this back pressure effect is. This is not just theory, BTW, because it's commonly used in ECU software to calculate airflow - known in the industry as speed-density airflow measurement - the perceptive will now see where the term comes from. Note that unless you change cam timing, valve sizes, inlet lenghts etc, delivery ratio won't change all that much. 90-110% is about the range most engines operate at unless extreme hardware design features are involved. Most engines come out of the factory within a percent or two of optimum too so tweaking your VVT settings won't give you very much extra if anything.
So, now we can pretty accurately calculate the mass flow of air the engine is pumping - that is, the density of the air the engine "sees" multiplied by the effective volume of the air the engine is pumping. From here, it is a short step away from determining an engine power by assuming a typical BSAC value to convert air mass flow to power. If you already have a data point from an engine (i.e. published engine dyno power figures) you can be pretty sure the BSAC of the engine won't change very much. If you do claim a lower BSAC, you'd better have a reasonable explanation as to why it has improved, else I'm calling custard on your data!
Of course, some will want to know where ignition timing figures in all of this. It is actually bundled in the BSAC term. Air-fuel ratio will also affect BSAC but only negatively so as you move away from best power lambda of about 0.85-ish. Almost all SI engines are knock-limited at some point. This is because the compression ratio is chosen as a compromise between part-load efficiency, cylinder pressure structural limits and the resulting knock margin required - usually occuring to the greatest degree around the peak torque area. Thus, for a given engine power spec, the CR is a compromise chosen based on a bunch of factors, including ignition timing. If you raise the boost level without changing the compression ratio, you will almost certainly be more knock-limited than necessary. I make this point because knock limited ignition timing degrades the BSAC value describe above. Note that in all my earlier calculations, I assumed there would be no knock-limit degradation of BSAC - an optimistic assumption under any circumstances. Reality will almost certainly be worse than this assumption, meaning less power.
So, the more you retard the ignition to contain knock, the more energy you are throwing down the exhaust pipe. The thermal mass of exhaust gas is relatively low, so it dnesn't take much extra waste energy to increase exhaust temperatures alarmingly. Engine controllers will usually enrich the air-fuel ratio at this point to contain exhaust temperatures, especially when close to pre-turbine exhaust temperature limits - usually the first limit you reach in a turbocharged engine. This is a maximum temperature chosen based on the stress-rupture and fatigue limits of the turbine wheel material itself at elevated temperatures. You can ignore this to acheive more power, but you will dramatically shorten the life of your exhaust turbine.
The interaction between engine power, boost levels, exhaust turbine temperature, turbocharger speed limtis etc. is pretty complex and can be somewhat confusing, but the reality is that asking extra work from your turbocharger/engine combination comes at a price. Modern engine controllers have pretty sophisticated temperature models to estimate this compromise and to keep a check on the power loss associated with enrichment and retarded ignition timing. The reality is, manufacturers aren't giving away as much performance at maximum power as people seem to think.
Peak torque is another matter. It's easier to raise the boost levels because at this point the thermal limits are further away. More torque in the mid-range will feel to some like "more power" and will certainly feel more thrilling. It will also improve your acceleration times, but it won't represent a higher maximum power level.. This is why 0-100 times are not a very good measure of increased maximum engine power. Most manufacturers will calibrate their engines to deliver a flat torque curve - out of respect to the gearbox and driveshafts as much as anything. More mid-range torque is therefore obviously available for boost fiddlers, The ability to accurately measure it, less so...
I could go on and on as there are many other details missed but fundamentally, the calculation people are all shy of is this: Charge density * swept volume * engine speed * delivery ratio * BSAC will give you engine power. You will have to make an assumption on DR and BSAC, but they are easily validated by comparing with data from other engines or even data from the same engine, as I did to estimate the real power increase for increased boost. The equation works at any point in the speed-load range of the engien - peak torque included!
This is by no means the last word on the subject - I haven't even mentioned BMEP yet! - but hopefully it is enough for some of you to understand engines a little better. More complex models exist to estimate engine performance to much higher degrees of accuracy - even clean-sheet designs - but more often than not they are used for arguing over the last couple of percent or defining what the hardware should actually look like, compared with this broad-brush, pencil-and-paper-sized calculation.
That's all for now folks.
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