Roots Blower Vesus Centrifugal-Which Nets More HP?

I've looked everywhere, and cannot find the info I need.

I've read several places that centrifugal superchargers make more net horsepower than roots type superchargers, because of the HP needed to turn the roots blower.

Yes, I understand the First Law of Thermodynamics- that's why I question this!

First off, is this true?

I'm trying to figure out approximately how much more horsepower my Procharger D1SC could make, than my stripped 6-71, at 14lbs boost.

Anyone know where to find this info, or have dyno comparison numbers to share?

 

38 same motor 408 sbc f1 made 55 more hp than a new weind roots set up on dyno same carb set up also, did have a intercooler on f1 my thought cooler air charge better power .ps boost pressure was a little lower on f1 at 11 psi on 116 fuel

 

Good question! A blower is a blower, it takes x amount of power to make a given pressure, no matter how you do it. Centrifugals are supposedly more efficient than Rootes type, though I can't see why. The GM blowers have no built-in friction. With no boost they turn quite easily. A centrifugal, whether belt driven or turbo, have to spin like mad to do anything. The GM makes power from the get-go, which should work better on the street. Plus, there's nothing more impressive than a 6-71 sticking up there for all the world to see!

 

The Roots design is a positive displacement pump, while a centrifugal is a compressor. They work differently, the Roots will move air into the manifold, but when the pressure in the manifold gets kind of high (over 10 psi or so) you end up making more heat than pressure.

Also it's easier to add an intercooler to a centrifugal setup, so you can run more boost without heating the air too much.

The thing about centrifugal superchargers is that you can make a LOT of power if you run more than 14 psi boost.

 

Thanks guys!

Skull Orchard, did you have the intercooler on both applications, or is it possible the HP increase was due to the intercooler?

Squirrel, this is for my Caddy flathead, so I'm approaching boost numbers with some caution. For this year, 14 will be about max.

Next winter, I'm contemplating a switch from the roots to the centrifugal. Both applications will be intercooled. The only real difference, besides the huffer, will be fuel induction. The roots has mechanical injection, and the centrifugal will be carbed.

The application is for racing, so I'm only worried about WOT.

 

The only experience I have with Bville was hanging out with the guys from Hot Rod in 2008 when they ran the Camaro with and without a centrifugal blower. The year before, they had a different engine in the car that they ran with and without a Roots blower. I just reread the article about the 2007 thrash, where they set 5 records (feb 08 issue)...they had an intercooler under the 14-71, sitting on a 555 ci Chevy. They experimented on the dyno and settled on only 11 psi boost, as it seemed to make the most power there. And when they went to the salt, they ran faster without the blower! The aerodynamics of that car, and other things are probably are a bit different than your 38....but it is interesting how things like that happen.

In 08 they ran a 572 with a Vortech, and they went a bit slower, didn't set a record with the blower. They did set an unblown record.

Keep in mind that in the AA blown class, they are fighting air drag and lack of traction, and more power isn't always a good thing.

 

no sir only with f1. roots system intercooler you need to be totally commited to its style its tough to do and a bit pricey looks hard to package.

 

Thanks Squirrel!

I've got a BDS marine style 'chiller' intercooler.

 

Steve Morris did some testing on f1r centrifugals (2700 CFM which would compare to 8-71 at 80% over at 6000 RPM) and they were drawing 350 HP at 30 PSI. This is why everyone is going to cog drive, but you won't have that kind of clearance in front of engine on prewar car. If you go cog belt then you will need a Kurgan brace from pulley to pulley as that kind of load flexes the 'charger mount. Check parasitic loss 1&2 here http://www.stevemorrisengines.com/steve-morris-engines-store/research-and-development-1.html

 

damn 38 thats a nice set up how much does it cool the charge? does it use onboard water or lake water to cool or air only.

 

A tank of ice water is my guess...

 

Jim is correct- a water pumps ice water through the chiller intercooler. it looks similar to a radiator, and cools the air as it passes through the fins.

 

All I have is what I see, and that is in fastest street car shootouts drag racing, you only see 3 things:

Tons of Nitrous (Most Common)
Turbochargers (not common)
Procharger centrifigals (Common)

I don't remember ever seeing a roots style on one of these mega horsepower cars.
In Drag Racing anyway, for competitive drag racing you only see a (Helical) roots style blower anymore in Top Fuel and Top Alcohol, dragsters and funny cars.

**Edit, just remembered too reading an article on Top fuel blowers and how regulated they were in NHRA, they talked about the amount of twist on the rotors is limited, Which means in reality they are a helical supercharger, not a traditional roots, so I spoke out of place above calling them roots.**



Scot

 

You need to read the book "Maximum Boost" by Corky Bell.

 

OK.

...but until I read it, can you answer my question?

 

The mega high hp cars don't use roots blowers...but they aren't 70 year old flatheads, either.....

 

I've had them both. 408" Windsor with centrifugal 8lbs boost edelbrock heads. 665 horse on engine Dyno. 408" clevor chi heads 697 on Dyno. Both were too much for the street. Roots blower hits hard down low...if you are traction limited try the ATI pro charger....it makes boost as rpm increases...roots has it almost instantly. The AtI centrifugal will cost you less and is easier IMHO to tune....goes under hood too.the 6-71 does impress teenage boys and old gear heads. I am building a new ride....keeping it very simple, tired of the complexity of the blowers. Remember your ENTIRE driveline must be built to handle the torque, that's where it gets expensive! My new mill is a 292 y block 2bbl...life in the slow lane...ok for me! Get the Corky Bell book and the one by Pat Ganahl. Remember your boost gauge is only measuring the boost buildup in the plenum..you can actually have more power with a lower boost number if u have excellent heads and a free flowing exhaust. A roots blower will make the boost gauge peg with a single 2-1/2" exhaust pipe. BDS was great to me..good guys.

 

From what I've read a Centrifugal Supercharger has a better adiabatic efficiency than a Roots Blower does.

What I think that means is that the less efficient it is, the more power that gets put into it gets converted to heat.

(Compressing air will generate heat anyway, but less efficient systems will produce more off it.)

Again, going by memory from things I've read, Roots Blowers get less efficient with higher boost numbers
( thats why some pre WWII Silver Arrows had two stage blowers)

And why the V16 BRM which gave crazy horse power for its time had a centrifugal blower.
( trouble with that one was that the rising rate of boost delivery didnt match the engine's RPM Vs charge intake very well.
Which made the power curve very peaky and the car almost undrivable )

Best compromise for efficiency and power delivery seems to be a two stage system with a roots blower and a turbo charger,
With a intercooler after each stage.

 

So if you are racing on the salt, where traction is a problem anyway, I'd keep the Roots.
But ad a second stage with a turbo ( or two, one on each bank ) if the car can use more Horse Power.

 

We have drag race cars with both.....Centrifugal makes more engine power due to two reasons.....doesn't generate as much heat (air temp vs boost pressure) as a roots blower, doesn't require as much parasitic HP to turn it. And a third reason towards the car set up is they are much more aerodynamic under the hood and it weighs less than a roots.

The centrifugal is much more boost pressure managable and tunable for small displacement engines and less effective on large cubic inch engines.

Both have their places in the racing world.....

 

Per what others have posted, a centrifugal blower has the POTENTIAL to be more efficient than a Rootes blower. For the reasons others have already detailed, higher efficiency equals more power.

The biggest down side centrifugal blowers is; the output of a Rootes blower changes roughly in proportion the the engine's needs at various RPMs. At low RPMs a centrifugal produces essentially nothing, but increases geometrically with increasing engine/blower speed. The more efficient the blower, the more that is so. Excess boost can be limited by a blow-off valve. Running the blower faster is a way of increasing low RPM performance, the over-boosting that results at higher RPMs is controlled by a blow-of valve. Unfortunately it takes horsepower to produce the wasted supercharging released by a blow-off , so controlling excess boost with a blow-of valve is a trade-off. With efficient compressor designs a blow-off valve can result in surge, which can cause a number of problems.

Some of the Roots hybrids, like the ones with screw shaped rotors, are nearly as efficient as a centrifugal can be. Because of good efficiency and relatively linear air delivery, that's what road car manufacturers now use on supercharged cars.

 

How about twin turbo's Buddy, less loss and would make the Flat cad sound completly different, humm twin turbo's and 8" exhaust...................

 

The non linearity of centrifugal blower output was used to advantage by early exponents of supercharging like Auburn and Graham.

Engines of the time were optimized for low and mid RPM power. Additional boost would have resulted in knocking and roughness. But their mild cams and restricted intake and exhaust limited power at higher speeds.

Their superchargers boost came in where it was needed to improve highway passing hill climbing and high altitude performance. The superchargers themselves were very simple and trouble free, consisting of one moving part (the impeller) and the accompanying drive mechanism with a belt a shaft and 2 gears.

It is interesting that the supercharged Auburn straight eight replaced a V12 of the same power and the supercharged Graham six had the same power as the straight eight it replaced. The supercharger allowed these manufacturers to offer a more powerful model at much lower expense than previously.

 

Yeah, that's what you'd want to do. But you also have to make sure you don't drop HP too much when you shift into high, if you get the rpm down to where the motor won't pull the car at that speed....

 

I ran a Procharger D1-SC in an off topic late model Mustang. It was the very sad 98 GT 2 valve motor. When I sold the car it was making 641 rwhp with 18#'s of boost on 91 octane. I should've stepped up to an F1 but the car was to the point of barely passing smog here in California. I've driven some 03-04 Cobras with either a Kenne Bell or Whipple and they pull hard but there is something about a centrifugal that is coming on hard that makes you smile. I ran a 3 core air to air intercooler, you could lay your hand on the head unit and it wouldnt burn you. I love the self contained Prochargers, want to run one on the 302 in my 55. I say run the ATI and if you need it I still have their race by-pass valve that vents to atmoshpere that I would let go of cheap.

 

I am thinking of something like a camera shutter that would close down to limit intake air as boost increased with RPM. Spring to keep it open, cylinder attached to air tube to bleed pressure to the cylinder piston. Set it up using shop air. That way you could get on the boost sooner with out going over the top. It would be fun to try the compressed air deal on a small (K) sized engine that wouldn't need as much air and would suffer the most driving a blower.

 

Yes.

Centrifugal compressors are more efficient at some combinations of boost/flow than others. Although a mismatch can work, for best efficiency you want a compressor that best suits the flow and pressure you will want/need. The more efficient the compressor is, the more critical it is to match it properly.

 

Creative, but long story short it's not a great idea. And worst case, with an efficient compressor restricting the output could lead to surge

 

Well since you need an answer immediately and I like what you are doing ill try to answer your question J<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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A roots blower gives you instant boost as soon as your foot hits the throttle and continues to do so until you let off. Throttle control is on the suction side of the blower. Most of the time they are mounted to a special manifold for a given engine and that kind of makes them more difficult to adapt fancy intercoolers and EFI.<o></o>
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A centrifugal blower you get a little lag time until it really starts making boost but not as bad as a turbo since its connected to the crankshaft. The thing that makes centrifugal blowers “hot” these days is the ease of putting on any engine and EFI. Blowing through a carburetor works but takes quite a bit of fooling with to get it to operate correctly. <o></o>
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The question of to how much HP one makes over another brings up the question of “how is the fuel managed with each type of blower?”. Keeping the fuel vaporized so it can all be burned is what will give you the most power.

With a 671 and 2 Holleys on top the fuel gets sucked through and splashed all over the inside of the blower and manifold, the fuel that is lucky to still be vaporized goes into the cylinder while the stuff dripping down the blower and manifold runs into the cylinder and doesn’t get burned all the way. A little heat after the blower helps keep the fuel vaporized, but as always too much heat is not helpful. Liquid fuel does not burn and sending it through a intercooler causes it to cling the to the cooler walls, condense and turn back into a liquid. Lost power with liquid fuel. <o></o>
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A centrifugal blower most of the time is used with EFI and here is where the big power comes from… You can blow cold air in the engine all the way to the intake valve before it mixes with the fuel. The fuel is vaporized when the injector sprays it on the hot intake valve with cool air right behind it filling the cylinder. What we have here is vaporized fuel and cold air under boost. What blower is going to make the most power here? A centrifugal blower blowing through a carburetor can still make good power but depends on a lot of tuning. With a intercooler you run into vaporization problems once the air passes the through the carburetor and tries mixing with the fuel in the manifold. You now have vaporized fuel trying not to condense on the manifold walls with cool air inside a ice cold manifold with the only possibility for total re-vaporization being the hot intake port and valve in the head. Without the inter cooler, you have warm/hot air going through the carb and in the manifold helping keep the fuel vaporized, but once again too much hot air is not helpful. Tuning your intercooler to deliver the perfect temperature for vaporization and cool dense air is a fine line. When combined with proper carburetor jetting you can have a potent package. Also watch your manifold port length and size to keep the velocity up and once again, fuel vaporized.