Originally Posted by NISFAN
If your goal is to reduce boost threshold to very low revs, without compromising top end power, then yes a tri turbo like M550d is a valid solution. However, the additional plumbing adds restriction to overall system, which adds in-efficiencies.
BMW's object was obviously to design a engine concept that creates boost through the entire rev range. So a very efficient design for what it is supposed to do. It would not be suited for a F1 engine, but neither was it intended to be... Additional plumbing don't necessarily mean added restrictions, and remember that not only volume but also exhaust speed and continuity are vital factors in the efficiency of a turbo system. Which part of the additional plumbing on the M550d do you suggest adds in-efficiencies?
Regarding the NA being smitten with lag all the way to redline is not difficult to understand. I'll explain....
Can't find any explanation of what you call lag on a NA engine below... Unless you are saying that a NA engine's lag is defined as the difference between it's NA power delivery, and it's theoretical power deliver IF it was torbocharged??? By that definition, any engine has lag, even a turbocharged engine, as it's, theoretically, allways possible to have more boost. But I have never heard ANYONE calling that "lag".
Lets take an S65 type engine, and add a turbo charger system to it. If we do it right, even if we disconnect the wastegates so no boost is ever achieved, it still runs like an NA s65, all the way through the redline. Anyone driving this car not knowing there are turbo's attached to the engine, would comment on it feeling exactly like an NA s65.
Now we connect up the waste gates allowing the engine to build boost. All of a sudden there is 'lag' right? Well not comparing it the the NA s65, the turbo doesn't make things any worse than before, the torque curve compared to an NA version is identical.....but yes there is a delay before an even greater hit of torque takes place.
You have just descibed what is known as "turbo-lag". Yes, a turbo engine produces more power (no one is arguing against that), but that extra power isn't instantly available.
Comparing a small displacement turbo engine with a similar power but larger NA engine is when you notice the lower off boost torque.
You do correctly paint the picture of lag, but lets just visit some factors....
A turbo is a small device capable of accelerating very quickly in the right conditions. It is an air pump. When boost pressure in the inlet tract is low, you are powering a device with no load on it, this allows it to spool ridiculously fast.
Yes, it spools up fast, but once there is pressure build up it meets resistance. And regardless, it takes time to build pressure. Atmospheric pressure is instantly available...
Secondly, common myth is that a turbo has to be at 140,000+ rpm before it makes any boost. Totally incorrect, it depends on the Turbo airflow charts, compressor sizes, etc, however, more commonly the truth is they can start producing positive boost at half those revs. Now you might think that 70-80k revs is still high enough to take time to reach? Not really, the turbo is never stationary, even at IC engine idle it rotates at a few thousand rpm (no load on the pump remember). When WOT is reached, the energy going through the turbo increases very rapidly. Think 335i producing full torque from 1200rpm. This indicates that full controlled boost is reached by 1,200 engine revs.
that doesn't leave much delay for lag, and in fact I would be surprised if the turbo was even below boost threshold at idle. The higher the IC engine is above boost threshold, the lower the lag due to the high energy in the exhaust. A 335i will require waste gate opening from as little as 1200rpm to vent excess exhaust energy.
In summary, yes lag exists, but is grossly over exaggerated in modern systems. You might get a 0.25 second delay before full torque is achieved for the revs, compensated for by increased torque shortly after.
I have never heard about anyone (with basic knowledge) that believes a turbo has to spin at 140.000+ rpm to make boost. I drove a 1 year old Turbo Diesel engined car to work today. From low revs (1000rpm), 1st gear, entering a roundabout, I counted between 1-2 seconds before there was noticeable boost. At higher revs, there was something like 0,5sec delay before full boost/power was delivered. That same engine, without a turbo, would obviously allways feel as lethargic as this engine did before boost. That's why turbocharging makes sense, especially on smaller displacement engines. Without at turbo, the car I drove would have needed a significantly larger volume engine. Power delivery would have been better, but fuel consumption would be worse.
Lag is a inherent flaw on a turbo engine, but in most conditions it's not a major problem. After all, most European cars are now sold with a Turbo Diesel and Joe Public have no issues with driving them. It's not like it's a minute's delay, but in some conditions it's really noticeable. But you must not confuse added power from a turbo and turbo-lag. Yes, a turbo helps the engine make more power, but by design there is a delay.
Coming back to F1, in the 80's turbo lag was massive, lasting seconds. But remember this, they realised that pumping enough air volume at the mandatory blow off valve, overloaded it in such a way that you could boost the engine to higher pressures than IIRC the 2 bar blow off valve allowed. This means the turbo was grossly oversized for the application. They used to run at something like 5 bar. That will not be the case for modern F1.
Next years F1 engines will typically run at 3,5 bar boost, but boost is unlimited. Fuel flow is limited though, so excessive boost levels will probably not happen.
Last little point on competition, there isn't a single race class where (when allowed) a turbo charged engine is not the fastest type. i.e. there are no race series where NA rules over Turbo.
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