I am new to FI and I did a search but did not find anything. I did see something about intake temp being a factor but I would like any other input if possible!

 

I think it depends on the size of the turbo, and the size of the supercharger. I think my buddies 671 roots blower is going to make more power than my turbo at 8psi (this this is fuking huge its for a 355 Chev truck)

 

Maybe, maybe not. That sc will have larger rotatign parts, which means more parasitic drag. Also that supecharger is designed to flow alot more than it was designd for. Can it flow so little and still hold pressure efficiently? I kno super large urbos are inefficicient at low boost levels with a low flow rate.

 

drag and heat. a turbo might compress air at 70% efficiency, while some superchargers are as bad as 40%(or so i've read), also turbos usually have intercoolers which lowers temp even more.

 

They don't always but when they do it may be because the turbo is flowing more air (CFM vs. PSI) and/or there is a higher parsitic affect with superchargers.

A blower at 8lbs. is going to make more power than a turbo at 8lbs if it's pushing twice as much air through the engine.

 

you are incorrect. the engine would have to have 2x the displacement...

superchargers subtract power right off the crank. turbo's reclaim some of the energy that is typically lost as heat when it uses the expanding gases in the exhaust manifold to spin the compressor wheel.

in my opinion, some power is lost from the crank from a turbo also and this is how: the piston is forcing the exhaust gases into the exhaust manifold. there is typically 1.2 times the amount of pressure in the exhaust manifold than how much boost you're running. so at 8psi your exhaust manifold is theoretically at 9.6psi, which is higher than you would see with no turbo, so there is some loss in forcing these hot, expanding gases into a pressurized chamber. but nobody talks much about this when we're talking about power since its not significant (like the parasitic loss of a supercharger) since the hot expanding gases entering the exhaust will probly be much higher than 9.6psi briefly.

lets say a Roots blower needs 45hp to spin it. your supercharged B18C1 is making 220hp if you've bought every frickin bell and whistle needed to extract that out of a 8psi JRSC. You'd be at 265hp if you could spin the SC with another power source like a huge electric motor or something. (just an example, there is no practical way to do this).

In my opinion, I'd say there is a basic rule of thumb to determine whether or not to supercharge a motor. your B18C1 has 180 crank hp. 45hp to spin a blower is 25% of what the motor outputs at the crank. thats a lot. too much actually. run the same numbers on supercharging a 4.6L V8 out of a mustang and you'll see that supercharging it requires a much smaller amount of its available power (even if the SC requires more than 45hp), and the gains are even greater power since you're cramming more air into 4.6L vs 1.8L which is a 2.5x bigger motor.

those are my thoughts on why a SC doesnt make as much power as a turbo on our lil motors. plus turbos are plumbing nightmares on V8s so SC's look even more appealing.

 

Joe,

I think 45 hp to spin a roots blower is a bit of an exaggeration. I think there are definitely cases were a supercharger at x psi can make more than turbo at the same boost pressure.

Take two equal engines and put a turbo with a very small turbine and very small compressor on one engine (Like a Garrett T2 with a .36 a/r turbine housing and a 40 trim wheel).

Put the Eaton M90 (that's the big roots blower that was planned for the JRSC Prelude...it's 50% larger than the M62 used on the B/H series VTEC engines).

I would bet money that the supercharged engine would make more power.

You wouldn't have to turn that M90 very fast to make 8 psi and there would be so much backpressure from that turbo that I bet it could hardly breathe.

Extreme examples...but you get the point.

Am I off base?

Sonny

 

sorry sonny, i was looking at practical examples. theres a damn good reason the m90 wont be mated to a H series motor and sold in a kit and thats parasitic loss.

i dont thin 45hp is too far off for a JRSC with an upgraded pulley. but hey...its just a guess.

i dont know what your examples are all about. i dont see the point of considering improperly sized power adders.

 

Haha...just ribbin' you, Joe.

Actually, I think M90 was not ditched due to parasitic drag. It was ditched because of the size of the blower. They had to do mad trickery to get the M62 in that Prelude engine bay as is. Pulleys and jackshafts galore!

On a side note, I would be very interested in trying to calculate the exact amount of parasitic loss created by the blower. How could it be done?

Sonny

 

I think you could get a ballpark figure with a very rough model of the blower as a couple of paddlewheels and calculating the force required to compress the air against the surface of each paddle. once you convert the force to torque, at the blower rpm, then voila you have the parasitic hp curve for the supercharger.

there is a lot more going on than that and i dont know how close you'd get to actual power requirements for the thing (the air kinda sutddersteps at the exit of the blower since the pressure is higher in the manifold and when the rotor introduces more air at 0psi there is a brief inrush into the SC...how ineffecient!) ...it would be a neat engineering exercise but i am too lazy for that (and they pay me to do those when i am not post-whoring on here).

theres probly a better way to measure it directly. someone will probly figure it out

 

i dont know if this was said already, but you did not saying anything about cooling the charge down. just because the charged air is not directly connected to the exhaust does not mean it does not get hot. just the fact that the air is being compressed will make it really hot and cause you to loose hp

 

Not necessarily. Just because you're pushing 8lbs with each doesn't mean you're cramming all the air that can "fit" into a given engine. In any case we're on the same page really, turbos tend to make more power because they have lower parasitic losses (crank vs. exhaust driven). There are other factors, specifically air flow (CFM), that are just as relevent as pressure though, which is what I was trying to (unsuccessfully?) address.

 

What the hell does this mean?

Anyway... Look at the efficiency of the two units. The turbo generally runs between 72% and 78% efficiency. The supercharger? About 50- 60 %.

As for CFMs, it comes down to turbo size vs. supercharger size, and since both are variable, that's not a good gauge.

Parasitic drag vs. backpressure losses? This too, is variable and depends on the design, but generally backpressure losses are in the 5- 10 HP range, while parasitic losses are 25- 35 HP range. A little side note- I read somewhere that a Chevy 350 requires 60 HP at idle to keep itself spinning.

 

Look at a compressor map on a huge turbo. You will see that the turbo is inefficient at low flow, low pressure positions. Spin the turbo too fast or too slow and its out of its efficiency. A perfect example of this is the Vortech. It spins too slowly, so it doesnt make much power down low, and the closer you get the redline (the faster it spins), the more boost it builds. The power seems to increase exponentially (tho its not) with a linerar rev curve. Now put on a 15-20psi pully and the torque curve will start to look better because the sc is more in its efficiency range.

 

save your calculating efforts and click below

http://www.capa.com.au/eaton_m62data.htm

get out your reading glasses though. it says at 8000rpm (blower speed) and 5psi, the m62 requires 10hp. at 10 psi it requires 15hp. at 14000rpm & 5psi, it eats up 25hp and at 10psi it takes 35hp.
hope that was of help.

 

how much TORQUE is it robbing? I thought about using torque * rpm = hp, torque = hp/rpm, but that says its only taking 0.0019043991620643686916777756617787 ft/lbs, which doesnt sound right.

 

Torque = (HP*5252)/(RPM)

 

How come no one brought up the part about a turbo reaching full boost before a supercharger? I know this has nothing to do with peak hp and tq, but it is still important.

 

AIR DENSITY RING A BELL? It's also related to the air temps that are being inducted into the engine. It's basic thermo here.
The roots design really doesn't sap as much power as they say. It's just like adding displacement to the engine without actually "adding" any. Until you up the boost too far and then it's like an overheating engine.
Turbo's just increase the "VE" of the engine by "recycling" (for lack of better term) wasted energy, and increasing the pressure on the intake side which in turn just increases the amount of air going in over the amount coming out. That is until you reach and pass up the peak efficiency of that specific turbo. This is why sizing is so important with turbo's. You should size it to have your peak efficiency at your peak rpm so your temps are steady all the way up the rpm band.
I guess those college educations really paid of here, huh?
Conceptualize it. Not compute it. There aren't any significantly hard facts relating to the numbers here it's just all relative thinking in this case. Think Einstein - everything thing is relative to everything in it's energy fields. It's all connected in layman's term.


[Modified by tzsir, 2:54 AM 1/22/2003]

 

Duh

 

Thx, I cant believe I forgot to add that in

Anyways that m60 robs 6.5 ft/lbs of torque @ 5psi, which is pretty signigicant to me since 5psi isnt going to be adding a whole lot of torque in the first place.

 

You're kidding, right? The JRSC has INSTANT boost. There is no lag whatsoever.

Note: I am not a big fan of the JRSC setup. I owned one and I liked it, but it wasn't enough for me. It is however, a pretty good kit all things considered.

Sonny

 

Take a minute and ask yourself what psi is then go back to the original question and you should have your answer. Psi this psi that, it doesn't matter. Fi isn't about psi, but rather cfm. With that aside, why does turbo at 8psi make more than sc at 8psi? (although i hate comparing apples and oranges) your answers lies here: PV=nRT

-demitri

 

Dude, wtf? A turbo @ 6psi will flow the exact same volume or air as a sc assuming the flow characteristics of the 2 intake manifolds are the same. A turbo usually makes more power at the same psi because its usually more efficient, and there is less parasitic loss with a turbo. Has nothing todo with less flow.

 

haha okay