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Supercharger 101


phr3121

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2007 Ford Shelby GT500 Powerhouse Under the Hood - Supercharging and Intercooling

 

A supercharger uses engine power to spin an impeller. The impeller compresses the air charge and “blows” it into an engine’s intake. Boost is created when the supercharger pushes enough air into the engine to overcome the vacuum naturally created by the engine’s air intake. The supercharger on the Shelby GT500 produces boost of 8.5 psi., and because air heats up as it is compressed, a supercharger is often coupled with an intercooler to remove heat from the compressed air before it enters the engine’s intake system to provide a cooler, denser intake charge. An intercooler works just like a radiator — it cools air as the air flows over fins and plates inside the intercooler. This generates a denser intake charge and increases the engine’s ability to produce horsepower and torque.

 

The roots type supercharger is two counter-rotating meshed lobed rotors. The two rotors trap air in the gaps between rotors and push it against the compressor housing as they rotate towards the outlet/discharge port. During each rotation, a specific fixed amount of air is trapped and moved to the outlet port where it is compressed, which is why the roots type supercharger falls under the broader catogory of fixed-displacement superchargers (like the twin screw supercharger).

 

supercharger_schematic.jpg

 

The roots type supercharger is known for its ability to produce large amounts of boost while spinning at very low speeds. On an automotive application, a roots type supercharger can often make it's full (peak) boost by 2000 engine rpm. This characteristic has contributed to its success and popularity on the top fuel racing circuit and has made it ideal for use on smaller engines that traditionally struggle in the lower half of the rpm range. Another advantageous characteristic of the roots type supercharger is its simplicity of design. The roots type supercharger has very few moving parts and spins at low rpms, making it one of the more reliable and durable supercharger designs.

 

The big disadvantage to the roots type supercharger is its thermal inefficiency - or its nature to produce high discharge temperatures - which robs power from the engine. With a roots type supercharger, an intercooler is almost always a necessity to bring the air charge temperatures down to an acceptable level. This poor thermal efficiency can be attributed to the fact that it has no internal compression (compression is done after the air leaves the discharge port). Additional heat is created by compressed (hot) air that leaks backwards past the rotors and heats up the temperature of the inlet charge.

 

EatonInternals.jpg

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A follow-up question to the experts on this topic:

I understand the inner workings of the SC at high RPMs, but how exactly does air enter this vehicle’s engine when the SC is off? The belt to the SC turns even when the engine is at idle but the SC does not kick in until 2000 RPMs or higher. :headscratch:

 

As mentioned on another thread:

"It has to create a lot of pressure in the supercharger before it will move the pressure gauge."

"...you need to romp these things pretty hard before you actually begin to see it produce any boost, don't be afraid, that’s what its made for!"

"Unlike a turbocharger, the supercharger is driven directly off of the crankshaft, the harder you mash your right foot the more demand for gas and air, as the RPM's rise the rotors in the supercharger spin faster and boost will build....this whole cycle happens very fast and the boost gauge will be very erratic if you do not keep your foot it long enough to look down at the gauge."

 

Also, with the amount of heat generated by the SC (has anyone touched their SC cooling hoses at the end of a drive?) why doesn’t the radiator fan remain on for a certain amount of time after the engine is turned off? A nice feature in other cars!

I’m sure glad the hood has the standard venting ports.

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This mod will let you turn the fan on when you want it on.

http://www.stangsunleashed.com/forums/inde...amp;#entry49973

 

The supercharger is never off the rotors are always spinning if you hook up a vac/boost gauge you'll see that when not in boost there is always a vac. present. Boost is only created when the blower displaces more air then the motor can use at that RPM. At say 1200 rmp the motor is sucking up all the air there for no boost is made. or Somthing like that.............

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Thanks Brands!

That was a great read on the link you posted.

The guy who made the modifications to the fan did a great job of explaining how to do it.

Also, great times on your 1/4 mile run!

What kind of ETs where you getting when the car was stock?

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phr, just a suggestion... it's always a good idea to state the source of a narrative that's pasted into a post from another website, not only so folks know the source, but also to protect yourself from copyright infringement issues.

 

Unfortunately, words can be interpreted in many ways. For example, the [article?] you quoted repeatedly mentions how slow the roots-type S/Cs spin. Likely they mean relative to turbos and centrifugals, since rootes and twin-screws can spin up to 14,000 rpm [or more] depending on engine rpm, pulley sizes and their engineering design limitations. Also, that article mentions that boost occurs when the S/C pushes enough air to "overcome" vacuum, as if vacuum is fighting boost. A better choice of words on the author's part might have been: when s/c flow surpasses naturally aspirated flow, because the S/C isn't overcoming vacuum -- vacuun is pulling in the same direction the S/C is pushing.. the S/C augments the flow that ambient barrometric pressure is already providing.

 

Another potential misunderstanding is their statement that a Roots makes maximum boost at very low RMP. Again, true, but don't confuse boost with flow. Fixed displacement S/C flow is highly linear with RPM (hence fixed-displacement), mitigated somewhat by it's traditionally looser tolerances -- and it's relatively higher backwash and thermal efficiency losses at high rpm and boost-levels respectively compared to a twin-screw - the new Eaton TVS roots not having that disparity. So while it might reach it's boost (max pressure differential) early, which is good, it's higher rpm that permit it to maintain that boost level as engine (and s/c) rpm rise to redline, an important concept, imo, the article did not touch on. This is why fixed-disp S/Cs produce such flat torque curves.

 

The other questions you posted [from other SU posts?] are also prone to word interpretation difficulties, imo... Rather than explain those individual sets of words, it may be more beneficial to think of how things work in a slightly different way... then all those quotes become more transparent, imo...

 

The boost guage will 'move' (actually cross the zero point on a continuous vacuum-boost guage) only after your right foot demands more power output than the engine can produce without the S/C (i.e. naturally aspirated) because that's the point at which you ask the S/C to augment what the ambient barometric pressure (14-15 psi) without the s/c could otherwise push into the engine. That's why in Denver (mile high) your foot will tend to ask for boost earlier and you'll have less overall available (less barometric pressure and less boost from the same fixed disp s/c). It may seem like an NA motor 'sucks' air in by the action of the intake stroke (and in a sense it does), but, net power-wise, any energy spent on 'sucking' air is energy stolen from another cylinder's power-stroke and is a zero-sum game at best, so I find it more usefull to think of boost occurring when you ask the engine to produce more power than it can naturally aspirated (i.e. without boost added). The reason why you have to use substantial accelerator pedal on the GT500 before the boost guage moves, is because it makes a lot of power naturally aspirated -- so you won't see boost on the guage until you ask for more than 350HP or so (whatever the number) -- the power output point where boost is required to get more air charge into the engine than it can otherwise naturally aspirate.

 

If you go back and read those quotes with the above 'mindset' it should make the intent behind them fairly transparent.

 

Dan

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Point taken Dan!

I didn't want to make it a very long post with the original article but most of what I was quoting came from here:

Link: http://www.superchargersonline.com/content.asp?id=22

The rest actually came from the supercharger section of the 50 page 2007 Ford Shelby GT500 manual. (How many folks actually received one of these?) As always, a great explanation (read) from you!

Thanks....

 

P.S. Also, do you know how Ford restricts the SC performance until the new vehicle is driven for 5 consecutive miles?

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Point taken Dan!

I didn't want to make it a very long post with the original article but most of what I was quoting came from here:

Link: http://www.superchargersonline.com/content.asp?id=22

The rest actually came from the supercharger section of the 50 page 2007 Ford Shelby GT500 manual. (How many folks actually received one of these?) As always, a great explanation (read) from you!

Thanks....

 

P.S. Also, do you know how Ford restricts the SC performance until the new vehicle is driven for 5 consecutive miles?

 

Likely ECU logic tho I don't know the specifics.

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+1, Dan. thanks again for another great post.

 

thanks, guys... you all rock!

 

--

 

Hey, Doc, did I read in another thread you're going to Bandimere for some fun? I didn't notice if the 'challenger' ever told you what car he drives -- just curious (you probably are too ;-) Hope you're havin' fun!

 

My wife, Jan, will be out in Denver later this week visiting kinder ...so if you can just leave the keys under the front mat she'll take good care of your baby :shades: ...she said she wants to take it up Pike's Peak :nonono: ;-)

 

Dan

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thanks, guys... you all rock!

 

--

 

Hey, Doc, did I read in another thread you're going to Bandimere for some fun? I didn't notice if the 'challenger' ever told you what car he drives -- just curious (you probably are too ;-) Hope you're havin' fun!

 

My wife, Jan, will be out in Denver later this week visiting kinder ...so if you can just leave the keys under the front mat she'll take good care of your baby :shades: ...she said she wants to take it up Pike's Peak :nonono: ;-)

 

Dan

 

The "challenger" has not responded yet...

 

I do plan to take the car out to test and tune night with some friends soon. I'll post results. I'm worried about what the altitude will do to my 1/4 mile numbers.... expecting maybe low 13's/high 12's if i drive it right. I still have stock tires and stock rear. You have seen what the altitude does to my boost! at least I'll get a baseline of times before I move to the next set of changes....... I'll keep you posted.

 

The nice thing is... the altitude effects every car here... so it levels the playing field.

 

Ron

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The "challenger" has not responded yet...

 

I do plan to take the car out to test and tune night with some friends soon. I'll post results. I'm worried about what the altitude will do to my 1/4 mile numbers.... expecting maybe low 13's/high 12's if i drive it right. I still have stock tires and stock rear. You have seen what the altitude does to my boost! at least I'll get a baseline of times before I move to the next set of changes....... I'll keep you posted.

 

The nice thing is... the altitude effects every car here... so it levels the playing field.

 

Ron

 

For sure, Ron ...even the Pros have difficulty at Bandimere's altitude. The blown fuelers cope ok with smaller pulleys and the oxygen content in the NitroMethane, but it really most everyone else.

 

:idea: there's always the new Whipple 3.3, etc. <lol> ;)

 

Dan

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