db2797

I know that sc's have a parasitic loss due to needing that HP to power the sc. So my question is this. Vs. a turbo, are you leaving some power on the floor because of this? So let's say that with a turbo you determine you can safely put 450HP to the wheels and just rounding for the sake of this example, that becomes 500HP at the crank.

Now with a sc, let's say you want to put the same stress on the engine and let's also say that the sc requires 50HP to power it. So now, do you actually lose that HP putting the same stress on the engine? So in this example, does this mean you're losing 50HP at the crank, and again just for rounding, 40HP at the wheels?

So all things being equal, and putting the same stress on the engine, will the sc net 40HP less to the wheels in my above rounding example?

AlsCobra

I went cross eyed reading that. Rwhp on a dyno is gonna be true hp. That's after whatever it takes to spin the charger. This is a constant battle charger vs' turbo. The turbo is most of the time able to produce more hp because the turbine is not restricted to a certain speed so it flows more air. But it is also harder to tune for and has a better history of blowing motors.
Supercharger is a little more consistent and easier to tune IMO. Also doesn't require exhaust modification. The parasitic loss to spin a charger is less than it takes to spin your a/c compressor so it's really only a couple horsepower loss anyways. A built and tuned correctly turbo has usually the best hp results but if your goal is around 500hp or so, no real point in a turbo system.

db2797

I understand that RWHP is going to be true HP. I'm not referring to strictly the power. My question is the stress on the engine. RWHP doesn't take into account drivetrain loss or in the case of a sc, the loss to power the sc. So my question is really if you're comparing pure stress on the engine, then would you always put less HP to the wheels with the sc with equal stress on the motor?

Also I think I've already answered my question. I'm sure the answer is yes. But then again the higher heat that a turbo produces might negate that as well. So you might still in reality be putting more stress on the engine. hmmm...

I'm also not following why if you were planning on 500hp why there wouldn't still be an advantage to a turbo? Especially if it is true that you could put the same power to the wheels with less stress on the engine with a turbo (again, this is because you aren't losing HP at the crank for powering the sc).

db2797

ahh...missed this part of your post before. So it is no longer around 50HP loss for a sc?

AlsCobra

The turbos are almost a must for really high hp gains. Pro drag cars are just about all going turbo. 500hp is not that far out of reach. Supercharger kits are easy and proven to hold up. Easy and reliable is always good.

Bigjohns97

Not true

http://en.wikipedia.org/wiki/Supercharger

Positive-displacement superchargers may absorb as much as a third of the total crankshaft power of the engine, and, in many applications, are less efficient than turbochargers.

AlsCobra

That's not the blower were talking about. That big old school blower has about 80lbs of screws to spin. Nowadays the twin screw and centrifugal blowers don't require that much force to push them. Lot more efficient than those monsters of the past. Realistically your probably looking at about 25 or so hp max. But yes turbos are more efficient and also blow more motors up.

Ltngdrvr

The OP never specified which supercharger he was asking about.

80 pounds of screws to spin??? Whatcha' talkin' 'bout Willis?

Centri doesn't take as much crank HP to spin as a twin screw and a twin screw doesn't take as much to spin as a roots.

But a centri doesn't make boost as low as a twin screw or a roots so it doesn't make as much low rpm torque either.

Anyway, HP for HP, a turbo uses the least crank HP but does represent a exhaust restriction which does absorb some potential HP.

It's all a matter of compromise, pros and cons to each system.

AlsCobra

this compared to this.

SlowRiderr

The 2013 Shelby gt500 supercharger requires 150hp to turn at peak horsepower output. Less than an a/c compressor? I think not.

Ltngdrvr

Didn't answer the question.

What 80 pounds of screws?

And nobody was even thinking about a big 6/8/14-71 blower so that's a mute point.

AlsCobra

This. ^^^
The wiki reference was using the 671 type as an example. Hence 80lbs of screws. That's where it came from. Relax.

Ltngdrvr

I still don't know what you are talking about, what 80 pounds of screws?

AlsCobra

Damm dude your impossible. Pull an old 671 down and weigh the screws. Hell they may only weigh 50lbs. Maybe 100. All I know is that they are really big and solid steel. Guestimate and possibly exaggeration. Bastards are BIG.

AlsCobra

Methodology behind my madness. It's pretty much impossible to see the parasitic hp drain on a roots blower. Just taking the belt off won't work because its a positive displacement blower so no real airflow when it's not spinning. If its spinning it's forcing air in the intake. Just a near impossible test to perform on a dyno.

With a centrifugal blower you first need a dyno baseline without the blower installed. Next you needs to bolt and belt it in, cap off the discharge of the blower and dyno again. I feel pretty safe to say it does not rob 100hp from the motor even if that's what the turbo forums say. It may in fact cost you some noticeable horsepower but no way 100+. Would love to see the test and maybe even be proven wrong.

whoah

AlsCobra, have you ever aired up a bicycle tire with a hand/foot pump? Notice how when the hose isnt hooked up to the schrader valve, it's pretty easy to move the pump, right? When you hook it up, and start putting air into something, you now have a bit of a restriction, and it gets harder to pump.

Now, with the pump hooked up to your tire, notice the difference between pushing down on the handle slowly and smoothly, and pushing it down very quickly. It's quite a bit harder, isnt it?

Think of how many CFMs that bicycle pump is putting out, and if you're anything like me, and have to use one to air up a car tire, by the time you're done you're quite winded.

just becuase a centri. or a twin screw is easy to turn when it isnt installed(and it should be!) doesnt mean that when it's all hooked up, and there's nowhere for the air to go, except into the engine, it won't cause a signifigant parasitic drag. *ALL* pumps do this, whether it's the supercharger sitting on top of your GT500, a water pump, or an air compressor.

But because an S/C is forcing more air into the engine, the net gain is greater than your parasitic losses

And yes, AlsCobra, I know you have an automotive background, and I am fully aware of your credentials. I'm not calling you out man, just sharing a little mental excercise with you

AlsCobra

Why I don't care for turbos:
1: What energy you force into an engine must also be expelled from it. So under boost conditions, you need to not restrict the exhaust system. Back pressure between the cylinders and the turbine means more pressure on the crankshaft trying to force the exhaust back out of the cylinder. So theoretically you are adding more load to the crank on the exhaust stroke. Ignition forces the piston down and the opposite piston up. Exhaust back pressure has now made that even more difficult.
2: tuning. Boost PSI is only one small aspect of FI. Scfm is the volume of actual air being forced into the intake. Which is usually much more than a S/C because rotation speed of the turbine is not limited. Getting the MAF to read and compensate for this volume of air is really important. If the application is using a BOV or a Pressure regulator, it may interfere with the actual airflow after the MAF. A really spot on tune that factors all of this is a must.
3: Heat. Nuff said.

whoah

quoted for truth. Not to mention the biggest detractor of all, turbo lag.

AlsCobra

I hear you man. You're talking about 20psi usually max. Centrifugal blowers will get to a max pressure and that's it. It's designed to deadhead the discharge. You can't do that to a roots system (why it is after the throttle) so it doesn't hit a wall of pressure. Like I said this is mostly theory about parasitic hp loss and how much actual HP it robs an engine of to spin a deadheaded centri at say 5k rpm. But I would really love to know that answer through the test I described. Could be wrong. Not sure. New knowledge never hurts unless it's about your wife and best friend hooking up.

Ltngdrvr

You've obviously never had one apart. The rotors are ALUMINUM not steel and don't weigh anywhere near that much.

And that's not why they take so much power to turn DUDE!

Next time maybe you should know what you are talking about before posting.