Supercharger/Turbo Selection Hints for n00bs. - Mustang Boards


Power Adders Technical discussions for forced induction with nitrous and blowers.

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Old 12-17-2007, 09:36 AM
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Default Supercharger/Turbo Selection Hints for n00bs.

I'll do this as a series of posts so it's easier to digest. I don't like blower cars so you can bet your butt I don't have a dog in this race. Info here is based on facts and observations. The little opinion I'll give is just my opinion. You'll want to notice it and sort it out as needed.

This is going to be basics. No making complex decisions without knowing what you're selecting. We'll dive into more detail as time allows.

Boost types. We'll focus on Centri(fugal), roots, twin screw and turbo. Each will be described and the good, bad and ugly disclosed.

Boost delivery: Pick a place. Meaning pick how you want the boost to come on. Turbo's can be (but don't have to be) kinda sudden about delivering boost in the middle of the RPM band and holding steady on it to redline which can be problematic if put in the wrong car. Centrifugals develop boost very smoothly as RPM increases, roots give it all to you right away but lose the muscle up top as efficiency drops and twin screws are like roots blowers with more top end and better overall efficiency. For most people a roots or twin screw makes the most "performance" sense. Power is delivered right off the bat, holds strong and it's mega low maintenance. Twin screw is an evolved form of roots and delivers the highest output of any positive displacement (looks like a roots) blower. Higher HP numbers but lower average power come from centrifugals. Centris also spool up more like a turbo but build to peak boost steadily as RPM increases. They're a joy to tune but I hate them on the street. Most people buy them because they're inexpensive and compact. Turbos are wildly expensive to install but turn out great power when properly designed. Most systems you see on the street have too big a compressor and mighty turbo-lag. Correctly selecting your turbo will fix most of that. Turbos are HOT items. They increase underhood heat and require intercoolers for the intake charge. For most people the roots or twin screw is #1, then a centri then a turbo....simply for cost concerns.

Efficiency: This is basically the percentage of air that a blower actually moves compared to what it's trying to move. It's important to stay inside the efficient range of your compressor. Going outside that range promises to drop your average power like a used rubber and could lead to excess heat buildup and subsequent engine damage. Ever seen a massive turbo on a civic... car can make 900hp yet can't get out of its own way. Because the compressor is too big. So yeah, it can make all sorts of boost but only up really high in the RPM's and it's less fast than it was before at any RPM that's useful. Same goes with blower size... mostly.
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Old 12-17-2007, 09:39 AM
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Boyles Law:

STOP READING NOW. Go to http://en.wikipedia.org/wiki/Boyle%27s_law and read the whole thing. Then come back.



What that means is that as you pressurize a gas, you heat it. Boost = heat. Never forget that. Heat = BAD.
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Old 12-17-2007, 09:47 AM
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Centrifugal:
These are crankshaft driven, snail shaped dealies that are glorified fans. They throw air at high speed against the walls of the compressor housing and it piles up, creating pressure/boost. A centri effectively trades air velocity for air pressure. With the trade of speed for pressure the air also gets hot and should be cooled down. You can compress air 8psi and end up with over 200 degree air, that's bad since cool air is great for making power and hot air isn't.

Centrifugals are light, cheap and easy to install. They usually require a tap into the engine oil supply for lubircation and cooling. Do not fail to follow directions about plumbing that. Some centri's even come with intercoolers standard (ATI).

As far as brands.. there's a bunch but I limit myself to recommending those of sufficient quality and support prowess as to keep my reputation intact:

Vortech - Perfect in every way.
Paxton - Just about perfect and a better choice for bigger power cars
ATI/Procharger - Bargain basement. My third and last option.

If it ain't in that list... probably shouldn't be on yours either.
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Old 12-17-2007, 10:13 AM
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Positive displacement:

PD/Roots/Twin-Screw blowers are my favorite street car blower. They're moron simple to install and deliver boost as soon as you open the throttle blades far enough for that much air to get in. Twin screws and classic roots blowers work in similar but different exact ways but, they're close enough to lump together.

An Eaton M90 or M112 is what most people think of when they hear roots blower and they're right but those are TINY examples. My old 1970 chebbie stump puller pickup had a BDS 8-71 huffer sitting on top of it. That's about 3x the physical size of an 03 cobra's M112 blower.

Autorotor is a leading manufacturer of twin screw type PD blowers. Twin screws are more efficient than pretty much any other crank driven blower but they're also more expensive than anything but a turbo. They're idiot easy to install and produce buckets of power. In some tests we've seen within a couple % of the same power as a turbo motor would produce from a TS blower equipped engine. TS compressors deliver cooler inlet temps and use less engine power to turn them. They also have that cool looking machined aluminum case. Note: use a flat black coated blower. Chrome makes for unnecessary heat dissipation problems.

Eaton is about the #1 maker of PD blowers. The only real downside to them is that they run out of steam kinda suddenly and have a poor efficiency rating when compared to the other types. What they do have in their favor is price and simplicity. I've installed a PD blower in a car in less than an hour. You get monster low end grunt, good base tq and a nice consistent climb as RPM's increase. Make sure you don't put too small a unit on your engine. 90cid is not enough for a v8. M112's are as small as I'd go on a 4.6. Get bigger for 5.4's.

Last edited by r3dn3ck; 12-17-2007 at 10:28 AM.
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Old 12-17-2007, 10:22 AM
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Turbo:

Turbos are killer for places where they're appropriate. They make bucket loads of power, don't rely on the crankshaft to run them and they're not that complicated. Problems would be in the areas of heat generation/dissipation, cost and maintenance.

Turbos are run off the exhaust, so they're using wasted power to make more power. That's pretty cool and oddly environmentally friendly. Downside is that running them off the exhaust introduces a lot of heat into the metal bits so they require excellent cooling for both the compressor and the air. Most units will be oil cooled and lubricated, some oil lubricated and water cooled. It varies. The one thing that's certain is that you had better know your stuff when selecting a turbo. The compressor map should see the bulk of your CFM needs in the very core of the peak zone for the pressure you want. Bigger is rarely better with turbos so actually go out and get some help from a pro. Supra guys can probably be helpful there.

Turbo's always cost more primarily because there's a pantload of piping to get them hooked up to the exhaust, get the exhaust back out of the car, and cool the boost and deliver it to the engine.

Used turbo casings are great as you can just have them built with the wheels you need and have a custom turbo for your application. Remember... if the unit is too big you won't see boost till like 3000rpm or higher which sucks pretty hard, especially if you pass 3000 rpm in the middle of a curve with the pedal floored. Boost kicks in all sudden like and you notice the back end swapping places with the front...

Yanking a turbo out of a diesel pickup or tractor may seem like a good idea but it usually isn't.

Turbo's are usually a little harder to tune than a PD or centri blower and cam selection is critical. Seek professional turbo help if you think you might want a turbo.
 
 
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