Twin charged bp

[bhearts]

i have a brilliant supercharger on intake manifold dreamed up for the 1.6, and i plan on purchasing one for rebuild this weekend. i just need to get a 5+ tooth serpenttine belt system lined up for that engine. got lots of other projects to build first before i can start doing me things.

[5.0zone]

So all this talk about how much crank HP it takes to turn a supercharger.
Does anyone know how much crank HP it takes to spool up a turbo?

[sketchman]

Nobody thinks about that. They only think about it using waste energy from exhaust that's already there, not about the fact that it's a restriction too.

[Advancedynamix]

That all depends on the turbosystem. A well designed turbo system, with the correct wastegate placement and size, and a ball bearing turbo, is practically free HP, because the turbine and wastegate only become a restriction at flow rates the unboosted engine would never generate. The turbo can also supply beneficial back pressure at low rpm, and reduce that pressure as the turbo spools and the wastegate opens.
The laggy, difficult to drive turbosystems of the old days have been obsolete for nearly 2 decades. With today's turbine technology, we can have a wide and useable TQ curve on practically any engine from 800cc on up.

[bhearts]

Nobody thinks about that. They only think about it using waste energy from exhaust that's already there, not about the fact that it's a restriction too.

Low a/r turbos can be very restrictive at peak rpm, but they do spool faster than a higher a/r turbo. The goal here is to choose a higher a/r turbo in conjunction with the supercharger to not worry so much about turbo choke at the top. Zephthechef was cramming numbers for his m122 3800 setup, and he calculated 3x NA exhaust gas volume with a supercharger pumping in 20psi of air under 1500 engine rpm. That's like trying to spool a turbo with a 10.6L engine, sort of, disclaimer (not responsible for precise plagiarism of numbers that I haven't crunched)

[bhearts]

Think about a minimum of 10psi regardless of engine rpm or gear. Not ideal fir launches, but I think it'd be a blast on the interstate passing folks. Wouldn't really be much different than having a v6t in these cars,

[ZephTheChef]

So all this talk about how much crank HP it takes to turn a supercharger.
Does anyone know how much crank HP it takes to spool up a turbo?

So all this talk about how much crank HP it takes to turn a supercharger.
Does anyone know how much crank HP it takes to spool up a turbo?

It is not as "free" as people tend to think. I have looked at backpressure studies with dyno comparisons. I have come to the conclusion that turbos in general are going to require approximately 30% of the hp that it would take to spin a supercharger at the same boost on the same engine. That's a huge generalization because there are a million tiny variables. It does illustrate the large savings over a supercharger, but to call it free power is super misleading. A turbo that is not horrendously oversized (therefore laggy) for your power goals may still be robbing 10% of your crank hp due to backpressure...But it causes the engine to make so much more that it seems more insignificant than a supercharger that may take 3x that much energy to power.

The best way to see it represented would be to look at true crank hp vs wheel hp at a set intake pressure, through the same intercooler. Tune to a set AFR and look at your injector duty cycle at peak power. Do the same for turbo and for super, and compare the fuel flows (as a measure of total engine up before any losses) to the wheel horsepower. Since we have a formula to fairly accurately calculate the power required for a supercharger, we can also calculate our "other drivetrain losses" for an accurate apples to apples comparison to the turbo numbers. In theory, I will be able to test this out on my car once it is finished up...and since I am curious myself to see the car's performance with supercharger only vs turbo only vs compound vs compound then bypass supercharger, I am sure that I will do just that.

[5.0zone]

I find it hard to believe that it takes no HP to compress air to say ... 15lbs and sustain that same compression ratio (apx 2 to 1).
Really? No HP, or negligible? Has anyone ever ACTUALLY measured the power it takes?
It isn't to difficult to measure for superchargers but not so easy for turbo.
"Turbos are free power" it just goes against everything I learned in physics.
But that is what Turbo guys always tell me.

[ZephTheChef]

That all depends on the turbosystem. A well designed turbo system, with the correct wastegate placement and size, and a ball bearing turbo, is practically free HP, because the turbine and wastegate only become a restriction at flow rates the unboosted engine would never generate. The turbo can also supply beneficial back pressure at low rpm, and reduce that pressure as the turbo spools and the wastegate opens.
The laggy, difficult to drive turbosystems of the old days have been obsolete for nearly 2 decades. With today's turbine technology, we can have a wide and useable TQ curve on practically any engine from 800cc on up.

Again, I agree with most of what you say except that I am dumbfounded by the phrase "beneficial back-pressure". Can you explain how back pressure can be beneficial? I have always heard that thrown around, but every time I have researched it, I have not been able to find a reason why back pressure would ever be beneficial...in fact you mostly find a bunch of articles and posts debunking that myth if you Google it. Note that scavenging is not the same thing, and the two are not related. I would love to hear the explanation if there is one because it's such a persistent piece of automotive lore that I am sure there is some kind of basis in fact.

[ZephTheChef]

I find it hard to believe that it takes no HP to compress air to say ... 15lbs and sustain that same compression ratio (apx 2 to 1).
Really? No HP, or negligible? Has anyone ever ACTUALLY measured the power it takes?
It isn't to difficult to measure for superchargers but not so easy for turbo.
"Turbos are free power" it just goes against everything I learned in physics.
But that is what Turbo guys always tell me.

See my post above, lol.

[ZephTheChef]

Also, Borg Warner's matchbot calculator spits out all sorts of interesting figures such as turbo shaft up. Now, I am assuming they have some pretty darn accurate formulas for calculating that, but I think it is a total and does not necessarily represent how much power it takes from the engine to spin it, as some of that does come purely from waste heat and not back pressure.

http://www.turbos.bwauto.com/aftermarket/matchbot.aspx

[sketchman]

Yeah, there is no free lunch. You cannot move something without expending energy. And surely the more pressure the comp is pushing through, the more restriction it's also causing on the turbine side.

[Advancedynamix]

Maybe back pressure is a poor way to explain it, but it's such a commonly used word that most can relate. Dynamic compression ratio is affected by the flow rate of the exhaust and intake. At low intake velocity, say partial throttle or low rpm, it's beneficial to have less air exiting the engine. This does affect scavenging, it also affects the tuned pulse and it affects the dynamic compression ratio of the engine. This is part of the theory behind variable valve control and variable exhaust control. Also, it is notable to mention that with increased exhaust velocity and heat the turbine will require less pressure to operate. This is why wastgate placement is very critical in a well designed system. As the turbocharger accelerates, it becomes more efficient and requires less pressure. If timed correctly, this works together with the engines requirements for exhaust flow. Matching the correct turbocharger to the engine is of utmost importance for may reasons.
A turbocharger takes pressure, velocity and heat to do it's job. However, the exhaust energy that is being expelled is being wasted on a naturally aspirated engine. The turbocharger is simply using wasted energy. This is why it is considered free HP. A properly designed turbo system will not make less HP than an N/A engine when not under boost, even when the accelerated air charge is being diverted under partial throttle opening (properly built systems use a diverter valve, not a blow off valve). The Turbochargers compressor also works differently than a screw type supercharger. A turbo is a centrifugal supercharger. This type of charger uses kinetic energy to build boost. This is a much more indirect load than a typical screw pump. The Turbo only becomes a restriction after it has already started increasing the airflow through the engine. There is very little parasitic loss, but Sketch, you are right, there is no transfer of energy that is 100 percent efficient according to the most basic laws of Physics. However, using energy that is otherwise wasted to increase the engines dynamic displacement is as close to free as you can get on this planet, unless you could figure out a way to utilize a solarthermal powered turbine on your engine. Heck, anything is possible.

[Advancedynamix]

Yeah, there is no free lunch. You cannot move something without expending energy. And surely the more pressure the comp is pushing through, the more restriction it's also causing on the turbine side.

Here is a twin turbo system I designed and built about a decade ago. The wastage is located between the turbochargers and the header is designed to maintain exhaust velocity, while having very low exhaust pressure. Building the system to utilize velocity and heat rather than pressure allowed me to run much more valve overlap and also tune the engines leaner than what would have been possible. As you can see in this picture, the header is designed to flow out the wastegate as a tuned portion of the exhaust, rather than just a couple of holes drilled into the side of a pipe. This was a 3.6L air cooled engine that made over 1000ft.lbs of TQ.

[ZephTheChef]

I love that wastegate setup! Also, that makes a LOT more sense if you are looking at a valve overlap scenario.