Supercharger - technical questions...

[Synack]

I love it when people talk about boost like this. Shows me there are some intelligent beings on this forum.

[serialk11r]

@madfast, what we're seeing with the 1900cc charger is actually EXTREMELY poor efficiency. The 1900cc charger only needs to turn half the engine speed to pump air in at atmospheric pressure. However these chargers have poor volumetric efficiency (they leak quite a bit of air backwards) at lower speeds, and if you notice the charts Eaton gives they don't even tell you the performance under 7000rpm or something because it sucks. That's why the boost is going up as the engine rpm is increasing, the charger is still in the zone where its volumetric efficiency is poor. An appropriately sized charger should have a much flatter looking torque curve.

Whoever tried to bolt an SC that big to a 2.0L motor is an idiot. The blower is supposed to operate at higher speeds than the engine, so you're not supposed to go much over 1/2 the engine's displacement for the blower rated displacement. In the 1900cc blower case, at 22psi (2.6 bar absolute pressure?) the blower is spinning at maybe 1.4 times engine speed, and so the maximum speed it hits is about 10000rpm, whereas the blower is designed to hit 17000-19000rpm, and be most efficient somewhere higher than 10000. That's the reason that Eaton has recommended maximum power and maximum displacement for each blower, you don't want to always be leaking a crapload of air out because your blower is too big. For anyone reading who doesn't understand why, in short a bigger blower has physically bigger gaps between the rotors so it leaks more air at the same pressure at any given time. For this application, an R1320 would already be kinda overkill, as you'd be running at 2x engine speed or so, which still isn't fully utilizing the blower's capabilities. The best would be an 1100cc or so which Eaton claims to be manufacturing or something, but no one's ever laid eyes on one.

[madfast]

Originally Posted by serialk11r

@madfast, what we're seeing with the 1900cc charger is actually EXTREMELY poor efficiency. The 1900cc charger only needs to turn half the engine speed to pump air in at atmospheric pressure. However these chargers have poor volumetric efficiency (they leak quite a bit of air backwards) at lower speeds, and if you notice the charts Eaton gives they don't even tell you the performance under 7000rpm or something because it sucks. That's why the boost is going up as the engine rpm is increasing, the charger is still in the zone where its volumetric efficiency is poor. An appropriately sized charger should have a much flatter looking torque curve.

Whoever tried to bolt an SC that big to a 2.0L motor is an idiot. The blower is supposed to operate at higher speeds than the engine, so you're not supposed to go much over 1/2 the engine's displacement for the blower rated displacement. In the 1900cc blower case, at 22psi (2.6 bar absolute pressure?) the blower is spinning at maybe 1.4 times engine speed, and so the maximum speed it hits is about 10000rpm, whereas the blower is designed to hit 17000-19000rpm, and be most efficient somewhere higher than 10000. That's the reason that Eaton has recommended maximum power and maximum displacement for each blower, you don't want to always be leaking a crapload of air out because your blower is too big. For anyone reading who doesn't understand why, in short a bigger blower has physically bigger gaps between the rotors so it leaks more air at the same pressure at any given time. For this application, an R1320 would already be kinda overkill, as you'd be running at 2x engine speed or so, which still isn't fully utilizing the blower's capabilities. The best would be an 1100cc or so which Eaton claims to be manufacturing or something, but no one's ever laid eyes on one.

yep. btw i never said the 1900 was a good fit for the engine or anything like that. it was merely an example used to illustrate how the amount of air pushed by the blower is too much for the engine.

[serialk11r]

Originally Posted by madfast

yep. btw i never said the 1900 was a good fit for the engine or anything like that. it was merely an example used to illustrate how the amount of air pushed by the blower is too much for the engine.

I don't think that's the right explanation...because blowers leak air, they never make max boost at idle, simple as that. If the blower could actually operate across a big pressure difference at low speed without leaking a large portion of the air back, then yes the boost would correspond "inversely" with the amount of air the engine naturally consumes, in some sense. At low engine speed the engine typically has the least ability to use a lot of air. I suppose the old wisdom may come from a time when engines had pathetically low rev limits and so the blower volumetric efficiency would look a lot better only because the idle speed corresponded to a decently high blower speed.

Of course increased manifold pressure changes the behavior a bit, but yea in a spot in the powerband where the engine has inherently low VE, the boost will go up because the blower is attempting to shove a proportionally larger amount of air into the cylinders.

[madfast]

Originally Posted by serialk11r

I don't think that's the right explanation...because blowers leak air, they never make max boost at idle, simple as that. If the blower could actually operate across a big pressure difference at low speed without leaking a large portion of the air back, then yes the boost would correspond "inversely" with the amount of air the engine naturally consumes, in some sense. At low engine speed the engine typically has the least ability to use a lot of air. I suppose the old wisdom may come from a time when engines had pathetically low rev limits and so the blower volumetric efficiency would look a lot better only because the idle speed corresponded to a decently high blower speed.

Of course increased manifold pressure changes the behavior a bit, but yea in a spot in the powerband where the engine has inherently low VE, the boost will go up because the blower is attempting to shove a proportionally larger amount of air into the cylinders.

yeah i think you get too technical at times. if the blower leaks at low rpm then it isnt pushing more air than the engine can use, and so boost doesnt go up. if the VE is low, then the engine isnt using as much air as it can and boost will go up. so in general, if the SC pushes more air than the engine can use, then boost goes up. is this not the case?

[serialk11r]

Originally Posted by madfast

yeah i think you get too technical at times. if the blower leaks at low rpm then it isnt pushing more air than the engine can use, and so boost doesnt go up. if the VE is low, then the engine isnt using as much air as it can and boost will go up. so in general, if the SC pushes more air than the engine can use, then boost goes up. is this not the case?

Right, and I think I did get too technical now that I reread that, but the point that I was attempting to make (but didn't lol) was that your graphs didn't really show that principle well because the blower was just so inappropriately sized. You'd want to find a chart where the stock engine's torque curve has a "dip" somewhere (and is shown at the same time, of course), and then the boost pressure would have a "spike" in the same spot, and the torque curve would be flattened.

[benster]

Originally Posted by diss7

Before I get into some examples, I should point out that I am aware that my difficulty in completely understanding these systems probably revolves around my understanding of pressure and volume. FYI, my understanding of this is that pressure/volume are directly related. If one container is compressed to 1 Bar, it effectively contains twice the volume of air than it did at atmosphere. I realise that my understanding here maybe flawed, as I have seen discussions on similar turbo systems running the same PSI but one flowing more air than the other. This has never made sense to me.

I'll clear this up since no one seems to have noticed it was a question, or bothered to answer it. The answer is simple and I'll try to make it easy to understand:
You have a 45mm turbo capable of pushing 15psi. That turbo will be moving, let's say, 300 cfm. You get tired of the 250 whp it gives you and want to step up. You then buy a 78mm turbo and set it at 15 psi. That turbo is pushing 800cfm since the intake path and the compressor housing is bigger, thus can take more air in and in the process will not drop as much pressure when the intake valves opens then the smaller one.

The difference between both at 15 psi is the larger turbo will create 15 psi in the same motor with less effort than the smaller one, creating less heat in the process. both turbos are pushing 15 psi, but since one is almost twice the size of the other, he is pushing a lot more air in than the other. Your engine will always take the same amount of air, but since the bigger turbo is able to fill your engine faster and with less heat, you get more hp.

If my understanding of it is flawed, by all means correct it, but this is what I learned talking to engine builders and ppl involved in turbo 4s and diesel engines.

I hope this helps.

[Coheed]

You guys are on the right track. A larger turbo compressor can move more air efficiently, with less heat. The corrected density is actually quite a bit more than the smaller turbo at 15psi.

But you have to look at VE as well. A larger compressor likely has a larger turbine if its matched correctly. The larger turbine flows more and restricts exhaust less. This promotes better scavenging and better VE.

A lot of superchargers don't actually compress the air, but rather move it into the manifold where the compression actually happens. This is different than a Cent charger because the compressor design compresses the air throughout the volute. The Cent blower is far more efficient and has a lot less heat generated. But they also tend to perform over the higher rpm rev range. Any cent system will give gains right off of idle if designed correctly. but it will make more power per psi compared to a twin screw/roots/eaton, mostly because of the compressor efficiency lending itself to better high rpm use.