S/C Efficiency at 5psi...

Oh Jeffrey,

It is only useless if you already have it SPANKY!!!!!!!!!!!!!

 

Yes...it is. The density difference is about 8%. Therefore, you could get 8% more air MASS in the cylinder at 77F and 5lbs than you do at 120F and 5 lbs. If you tune to the same A/F, constant vol eff, and constant BSFC, then that 8% more air mass leads to 8% more power.

As a rule of thumb, all other things constant (as I've stated above), if you cool down the air 10F, you get 2% more air mass.

All of this also depends on the intercooler, and the compressor effeciency. If the compressor is matched properly, the engine lug line runs down the spine of the compressor map.

Good discussion guys...I'm contemplating getting the ProCharger down the road. Before I do, I will be asking them for the compressor map and doing some analysis on my own to see which one of their compressors is better for my application. Bigger isn't always better.

Later,
Keith

 

yep and this mass and temp help with the rule of every 10 degrees 1% power.

 

JMO on this subject.

Boost is nothing more then back pressure in the intake. Staked air waiting to get to the cylinders. How efficiently the engine uses the boost will dictate how much power it makes. For example when we went from the stock 5.7 to a build 392 the same blower setup with one size smaller pulley lost 4lbs of boost on the gauge.

There are 2 sides to the roots/screw blower vs centrifical.

The roots blower will be more fun on the street at lower boost levels because of the seat of the pants (instant power) feel. But take it to the higher 10lb+ level and traction becomes a real issue. For a street car (fun factor) twin screws are great.

The centrifical blower will feel dead at lower rpms with low boost levels - 10lbs. Once you pulley down and spin the centrifical up some. They produce boost right at the converter and build to peak. Which is more managable for traction (et)

For our set up on the truck. We have 6lbs of boost at the converter. It moves quick from there.

1.59 60ft @ 5140 race wt.

 

No. I was talking about percentage of gain.
Let's say you're making 100 hp (NA) at 77F
Then you should be making about 92 hp (NA) at 120F?

Now you put a blower on there than generates 5lbs boost.
Let's say you get a 10% increase in hp and make 110 hp at 77F.
Wouldn't you expect something around 100 hp at 120F?

So in the above example you would be getting roughly the same percentage gain in HP. If you're going to compare 5lbs boost at 77F and 120F then you have to make it relative to NA at the same don't you?

 

Quick...I think I understand now...I think we are on the same page.

What you are saying is lets say you get 150 hp from the centrifugal type with an intercooler...thus making your engine now producing 575 hp. With one less effecient (let's say at a penalty of 40F intake manifold temp), you are now at 575hp*0.92 = 529 hp. The gain is then only 104 hp from the NA. So, now if I look at the percentage gain it is 150/425hp = 35% gain with the better effecient one and 104/425 = 25% gain with the less efficient one...still a pretty good deal.

I could see where you could get a 40F cooling at the intake manifold location (if not a lot more) from a centrifugal supercharger with an intercooler in comparison to roots type with no intercooling.

FYI...the 0.92 came from the ~8% density change associated from the 40F temp...at a constant boost. This was derived from the Ideal Gas Law.

Later,
Keith

 

Thanks guys! I thought I was going to get by today without feeling stupid, because the wifey isn't around, then you guys go and post that!

 

It's not that bad. You got confused when Keith used the term "less efficient blowers" right?

That link explaining the differences between the different types of super chargers said that roots and twin screws were more efficient than centrifiguls. What they were talking about is moving/compressing air at a particular rpm. Air leakage. For example a centrifigul will have to spin faster to generate 5 lbs of boost than a roots or twin screw would.

When Keith is talking about efficient he is talking about energy (in this case parasitic) required to produce the boost. Energy not converted to work is wasted in the form of heat. That same link pointed out that the roots and twinscrews heated the air charge more than the centrifiguls. Using that measure, they're less efficient.

The heat gets transfered to the air charge. The gas law says that as temperature goes up the gas molecules have more energy and need more space between them to bounce around. The gas expands. Fix the size of the container (manifold), fix the pressure (5 lbs was the example), and the only thing left to change between hot air and not so hot air is the number of molecules in there (mass/density). Less molecules (mass) because it's at higher temperature means less oxygen means less power even though it's at the same pressure.

So we started with "5 lbs is 5 lbs" and Keith pointed out that roots and screws are going to deliver that at a higher temperature (leaving coolers out if it) so you got to take that into consideration. We were just talking about the degree of how bad that is.

See, you understood all that right? It was just that you got tripped up because "efficient" was applied to two different things.

Just don't make the mistake when your wife comes back of telling her "Heh, I'm not that stupid after all" 'cause she'll fix that. :grin:

 

Could air distribution be the slight difference?
I know you don't wnat to talk about lag due to piping.
A roots and/or twin screw is compressing at the manifold so the air is directly forced at all cylinders.
A centrifugal is sending pressured air (proportional to RPM) thru a tube to the intake manifold where it is distributed to the cylinders, which is not as efficient. This also explains the slight lag of a centrifugal.

Now for the rest of the story...

Also because the centrifugal is not positive displacement so it suffers from "slippage" of the fan reducing volume (Compressed Air slips around the blades to the low pressure side). Also the rpm will have to be a lot higher in a centrifugal for 5psi boost.

"The twin-screw supercharger is a positive displacement air mover, which means it moves a fixed amount of air per revolution, like the Roots-type blower. Unlike the Roots however, which is only an air delivery system, the twin-screw supercharger is also a compressor. The counter-rotating lobes and chambers of the twin-screw are designed for a screw-like tapering effect running intake air into a smaller space for output, thus compressing it. The rotors have very close tolerances yet never touch. Compressed air is delivered into the compression environment of the intake manifold with very little leakage or energy loss."

"The centrifugal supercharger is NOT a positive displacement/fixed displacement supercharger because it does not move a fixed volume of air per revolution. The centrifugal supercharger essentially operates like a high-speed fan propeller/impeller, sucking air into the center of the supercharger and pushing it to the outside of the rapidly spinning (40,000+ rpm) impeller blades."

More info and VERY good.... follow all the links on the page (near the top) and you get an in depth technical story of each supercharger. Forget Part 1, unles you want to know history.
http://www.superchargersonline.com/content.asp?ID=76

of interest
http://www.failsafe.cc/melting-brain/suckOrBlow_Page4.html

Once again this site:
http://www.superchevy.com/technical/engines_drivetrain/induction_poweradders/0510sc_theory/index.html

 

I don't think so. That's strictly going to be a manifold design issue. The centrifugal is using the stock manifold and the air is still coming in through the throttle body. We assume they did a decent job to get the same amount of air to all cylinders. The others would have a manifold designed for them.

They still have a manifold right? I don't see any difference there.

I'll buy the lag part. But that's just a matter of getting the rpms up. The lag in the tube is probably much shorter than the time it takes the engine to wind up. If you already have the rpms up then the boost is already there.

 

How much can you truely play with the dead area you all talk about regarding the C type blower? Can you tune it to where it will produce boost sooner thus acting similiar to the roots/twin screw blower? I'm not saying you can get the same hard launch in both types, but what I'm wondering is how close can you get the C type blower to acting like a roots/twin screw blower. I know a good tune can make a huge difference to any and all mods, but is there a middle ground to obtain this?

izza:izza:izza:izza:izza:izza:izza:izza:izza:izza:izza:

 

Not really. It's centrifigul. Think of it this way. It's like a merry-go-round. It's got to be spinning pretty fast before the kids fly off with much force. Roots and screws are more like garbage compactors.

SharaDon was talking about using a smaller pulley. It's going to spin fast enough to make boost at a lower rpm (but it's also going to spin faster at high rpms...). A higher stall TC would help a lot and you could tune for more torque down low. I'm not sure what the tradeoff of all that would be.

 

The SWAMI doesn't know! I know they work differently, but I figured you could get the C to boost sooner, not as soon or the same type of boost as a Roots, but sooner then the out of the box tune for the C..............................

 

Not really the same manifold.
A centrifugal is feeding our intake manifold thru a throttle body opening at the front (or rear) of the block.
A twin screw is forcing boost directly into the cylinders after the throttle body.
A twin screw is supplying uniform boost from front to rear due to its design, even if its sitting on an aftercooler.

 

Heh, I BS my way through all this stuff. But... I don't believe you have a pressure release valve to limit boost? So if you spin it faster at lower rpms (smaller pulley) then it's going to spin faster at higher rpms. That means that at the top of the rpm range you're liable to be overboosting it or you'll hit the upper limit of the blower and maybe get cavitation that Blown7 mentioned (doubtful, I think you got a good ways to go on the blower you got right?).

It really doesn't have that much to do with the tune. The tune needs to match the blower but the tune won't change the blower. I suppose you could tune for a little more torque down low but not much. You could do a lot more in that area with a cam right?