vectorminds

So, in the quest for more power I have started thinking about supercharging, but i really dont want to spend the money to buy a 3000 dollar (when all is said and done) atkins setup, so i turned to ebay, and they have for usualy under 400 dollars lots of used superchargers off of things like 3.8 liter buick's and other things like that.

I was wondering if anyone has explored this in detail and has come up with a system that would work (or if anyone is actualy using one of these superchargers).

Also, a technical question, ive noticed that most off the times when i have seen a centrifugal blower being used it is before the throttle/injectors (paxton blower for example), and when a roots type blower is used it is after the throttle/injectors (atkins). What i want to know is is there a real reason for this, or is it possible to use a roots type blower before the throttle, because that would certainly be cheaper.

I would definately want to use a roots type blower because of its linear air output ratio, (unlike a centrifugal blower).

I know it wouldnt be exactly easy, but i dont think it would be all that hard, i would need the supercharger iself (about 200-400 bucks for a used one off ebay), some sort of custom made bracket to hold the supercharger, and a bunch of custom tubing, and some way to set up the belt drive.

Any comments would definately be apreciated.

DrifterFD3S

by the time your done customizing and fabricating and jury-rigging, itll have costed you more than buying a TII setup and swaping it. unless of course you own a fabrication shop.

digitalsolo

iTrader: (2)

Okay, as a new RX-7 person (should have my '86 FC in a week or so), I'm not sure how much help I can be...

BUT, I do have an extensively modded (read: LOW 13 SECOND) GTP, which has the aforementioned buick S/C on it. First piece of advice, don't use one. The roots chargers are easy to adapt to a lot of configurations, but they're horribly inefficient. Also they place the roots charger after the TB because of the way a roots charger compresses the air. Unlike a turbo or centrifugal S/C (which are VERY similiar) a roots charger compresses the air in the manifold, which is a large part of it's low adiabatic (damn I spelled that wrong...) efficiency. A CSC would be a better setup, but if you're going to go that far, you really might as well go turbo, and avoid the parasitic belt losses, and belt slippage issues that you'll hit with the CSC setups.

Also, it's a major PITA to intercool a roots charger (because of the way they compress the air). You generally end up with an "aftercooler" or a water-air setup. The added heat (beacuse of crappy cooling) is exaggerated on an N/A car because of it's higher compression design, and tendency to detonate (though not so much on a rotary... though I'm first to admit I'm a rotary NOVICE)

Anyway, hopefully that was SOMEWHAT helpful. Once again, I'm just a new guy, and all, so don't go hatin'

vectorminds

Well, one of the reasons i found the roots type more apealing was because of the fact that it has a linear air ratio, instead of the exponential one a centrifugal one does.... so i would be able to make full boost all the time instead of only at higher rpms

The reason thats important, is cuz we all know the rotary has next to no low end torque, so being able to make boost faster would definately be a plus.

And while a turbo would be more efficient the complexities of turboing a n/a engine are rediculous, and im looking for something cheaper and less labor intensive than doing a whole turbo2 engine swap

I mean i wouldnt be looking for too much boost, prolly like 4-5 psi (so with that little intercooling isnt the biggest issue, but of course it will still help)

And no i dont have access to a fabrication shop (that would be nice ), but i can get access to an arc welder, a cutting torch, various air grinders, and a bench press... so it shouldnt be too hard to make everything

I mean all and all its just some piping, which i could have made at an exaust shop if need be, and then a custom mounting plate which is just really a peice of 1/4 inch steel cut out to shape with some holes drilled in it, and sure it would be a PITA to make, but it would be a days work at most

Anyway, keep the responses coming, I know there are N/A people out there looking for a relitivly cheap to get some more power out of their engines

digitalsolo

iTrader: (2)

another point occured to me... the N/A trannies in the RXs aren't the strongest, correct? watch out for the roots, because you'll make max torque low in the powerband, which can be a real tranny killer (trust me on this one). Just one more thing to keep in mind.

Finally, roots chargers "spin-out" at fairly low RPM, causing the heat from increased boost to outweigh the performance increase of the boost itself. with the high revving rotaries, I'm thinking that could be a problem as well? Forgive my lack of knowledge on that subject, my redline is at 6000 RPM in the 'Prix

vectorminds

True, but if I blew my tranny because of too much low end torque I would be a happy man

What is the "redline" so to speak of that supercharger that you used?

88IntegraLS

I think about adapting a mercedes Kompressor S/C to my FC all the time. You can control boost with its electromagnetic clutch (I think). I would not use boost on an NA with the 9.4:1 compression ratio rotors without water injection. I would use an air / air aftercooler front mounted like an FMIC. I would use a TII upper and middle manifold fed by the aftercooler piping which would meet the roots S/C, mounted where the power steering / AC was. The throttle body would be mounted to the roots inlet and use a custom duct going to the stock airflow meter with a cold air intake. S-AFC would be mandatory as well as a wideband O2 sensor.

It would not be a small project! People are right when they say going turbo is easier. I would rather be different and invent my own setup with surplus parts though.

vectorminds

as far as the electromagnetic clutches go, im preety sure they are just on off devices, and not variable slip (which would be awsome and very useful), they are basicaly just like normal clutches, except the friction surfaces are held together with a magnet.... so if you let it slip by reducing the magnetisim the clutch will burn up real fast...

although that would be cool way to turn it on and off, although you would then get lag as it spooled up

Also, what is the difference between an intercooler and an aftercooler?

digitalsolo

iTrader: (2)

Okay, I'll do this Q&A style...

The "redline" on my blower...

Hmm, hard to say, I'm nearing the max boost it can do without being a hairdryer... it tops at around 12-13 lbs of usable boost... but once again, it's really hard to me to apply the speeds it turns vs. my engine speeds to a rotary application (yet... I'll get used to these engines before long, they're damn cool)

"Aftercooler"

Okay, well, an intercooler cools the air by pushing it through a big radiator with cool air flowing over it. An aftercooler cools the air by pushing it over a radiator with cooler air in it. basically the exact opposite of an intercooler, but it's a LOT more compact. Unfortunately it's also a lot less efficient They are working on some single pass/multi-path plate style aftercoolers that suck a little less. Still not turbo level stuff though. FWIW, I'd recommend a "screw-charger" if you like the roots style. You get the same linear air ratio, but it compresses the air in the housing, which is MUCH more efficient at higher boost levels (i.e. I can put a whipple or kenne bell S/C on my GTP and add 3-4 lbs of boost at the same or lower heat level). This also makes it possible to intercool the air (though plumbing it would still be rather un-fun) but hey as you'd be doing a custom fab anyway, you might as well go all out.

edit: I just remembered there are other styles of aftercoolers out there as well, but that was the example that came to mind-

ViperKillerWannabe

The roots blower would most likely run out of steam well before redline on nearly any RX-7. If you REALLY want to use a supercharger, I really think you should use a centrifugal type, because they can rev much higher and still be useful. If it were me (in fact, it is what I'm planning), I'd save a little bit more money and do the TII swap.

vectorminds

which brands of superchargers are "screw-chargers" and whats the difference between these and ordinary roots type superchargers?

Also, being that im not planning on taking it over 4-5 pounds of boost anyway is it really all that important about things like temp and high boost efficiency?

ViperKillerWannabe

I belive lysolm (not sure on the spelling) is the screw charger type. I'm not quite sure what makes them more efficient, but the rotors in are a different shape. One rotor is like a gear with widely spaced teeth that are narrow, and the other has notches cut into it to fit the teeth.

digitalsolo

iTrader: (2)

Whipple and Kenne Belle both make screw chargers. If you're planning low boost, it won't be AS important, but the problem is, you're going to have to have 0 boost on the low end to make only 4-5 lbs in the 7000-9000 RPM range. If you tried to make 4-5 lbs in the lowerend, say with a 3.9-4.0" pulley, you'd be making 18-19 at the top of the RPM range. Not good. Anyway, hope that all makes sense.

-Blake

vectorminds

Blake, I was under the impression that the roots type blowers produced a linear amount of air output (just like the engine draws in a linear amount of air) so the boost should stay the same across the rpm range... I thought it was the centrifugal blowers that would behave the way you described (with an exponential air output)

digitalsolo

iTrader: (2)

The faster you spin the blower, the more boost you get. i.e. if I make 4 lbs of boost at 2500 RPM, and 14 lbs of boost at 6000 RPM. If I run a smaller pulley I make 8 lbs at 2500 RPM, and 20 lbs at 6000 RPM. I think that the linear air ratio relates more to the way the boost increases vs. RPM, as a roots/screw blower increases in an almost straight line, where as CSC/turbos increase in more of an exponential curve. That's just a conjecture though. I am positive of the more RPM=more boost part though.

projekt

it's jerry rigging and you're right. get a swap, it's mo reliable and mo better

vectorminds

That doesnt seem to make sense to me, because the roots type blower (postitive displacement) is basicaly just a pump, assuming it is perfect, it would mean that for every revolution of the supercharger X amount of air would be pushed thru.

Therefor, if at a given speed the supercharger is putting out X amout of air, then at twice that speed it should be putting out 2X units of air. Since when its spinning at twice the speed the engine will be spinning at twice the speed as well it would imply that the amount of boost would not change.

The reason it would change with a centrifugal blower is because that type of blower uses a turbine to compress the air, which does not move a specific amount per revolution but rather moves an optimal amount over a very narrow rpm band (the whole point of the wastegate on a turbo is to hold the rpm of the turbine where it should be for optimal boost), thus to get max boost out of a centrifugal without hurting the engine, you need to tune it for redline, and get much less boost lower in the rpm range.

You said that you were using a roots type blower on your GTP... what are the boost levels at various rpms that you get... and what is the pully ratio between the supercharger and the engine?

(keep in mind that im confused because what ive read and what you are saying are conflicting... so im just trying to figure out whats goin on :p )

vectorminds

And well of course its jerry rigging... if it wasnt it wouldnt be nearly as much fun

Besides, if i can get more low end torque out of this than i could a turboswap this is more apealing for me

digitalsolo

iTrader: (2)

The problem with the theory you have is that it assumes that an engine needs twice as much air if it's moving twice as many RPMs. Better put, if you are moving 100 cfm at 2000 RPM, that doesn't mean you can move 200 cfm at 4000 RPM. Perhaps on a perfect engine, but definitely not on our engines. I do not, unfortunately know my pulley ratios on my engine... As I understand what you're saying, you think that you'd make 5 lbs of pressure all the way through the RPM band with a roots charger? Or something, like that, correct? I'd definitely re-check your sources on that, because I can assure you that's not how they work (at least on my car... I don't see any way they could, but hey, I've been wrong before... often actually... ) And, finally, I hope I'm not coming off all-smart *** or know-it-all or anything, just putting forth what I know about the blowers.

edited to make sense...

vectorminds

heres a great site that explains all (most) of the supercharger possibilies and their drawbacks and strengths

http://www.grapeaperacing.com/GrapeA...supertypes.cfm

vectorminds

Also, here is where I initialy found my numbers as far as postitive displacement vs centrifugal

http://ackthud.com/shawnfogg/cs_vs_pd.htm

ViperKillerWannabe

Last year sometime SCC had an article where they compared both types of superchargers to each other and to turbos. You may want to check it out as well.

rotarygod

iTrader: (1)

A roots or screw type of supercharger is a positive displacement device. You do have it right. It spins the exact same amount per revolution of the engine regardless of rpm. If it is setup with a 5 lb. pulley then it will run 5 lbs at 2000 or 10000 rpm. It doesn't matter. This is mostly true however. Due to tolerances within the supercharger, at lower rpms there is a small amount of leakage past the supercharger rotor tips which results in a little less boost. You will gain a little pressure as rpms increase since the blowby is reduced. Lowend will stay remain fantastic however and it is very nice on the street. The lowend torque should make you think you are driving a much bigger engine. These are typically installed after the throttlebodies or carbs since they are for the most part always providing boost. If they were before the throttleplate then they would need some sort of bypass valve so boost is not built up in front of the throttleplate. The Eaton style of superchargers do have a bypass valve which is open when not under power. This recirculates boosted air to before the screws and therefore is basically running like a naturally aspirated engine until the gas pedal is pushed down farther. These can probably be mad to work before the throttlebody. Unlike a clutched type of systems though it would still present itself as a strain on the engine.

A centrifugal supercharger such as the Paxton, Vortec, etc. are not constant displacement types. They are geared systems. The faster the engine rpm, the faster their speed rises exponentially to the engines. These only produce max boost at max rpm. They have little low end power benefit since they are making so little boost here yet they do present a parasitic drag on the engine. These are always used before the throttlebody since at idle they are not providing any boost. They should use a blowoff vavle of some sort although many kit manufacturers don't provide them.

You obviously are not shooting to obtain a mega horsepower supercar but more along the lines of a more streetable car with a little more pep. Positive displacement all the way. You'll love it. Just be sure to buy one according to your engines requirements. If one that is too small for the application is used then it will become very inneficient. A larger unit running the same amount of boost as a smaller unit will produce less intake air heating and spin slower therefore becoming more efficient. If you choose to integrate one, find a larger one.

digitalsolo

iTrader: (2)

rotary god,

I have a ? for you, (and I'm not arguing your point, as it seems valid). Why is it, that at WOT, I make an increasing amount of boost as RPM increases, peaking at redline? What am I misunderstanding here? It's a roots style blower...

Thanks-

Blake

edit: Question 2: If a smaller pulley (I run a 3.0", instead of the factory 3.8" increases boost by spinning it faster, how does the crank spinning it faster (i.e., higher RPM) not increase boost? My logic (albeit generally somewhat flawed) seems to dictate that spinning it faster HAS to increase boost, no?

vectorminds

The reason is because (simply speaking) the rotary engine itself is a postitive displacement pump... thefore the suction provided by the engine will rise equaly to the amount of air put out by a postitive displacement supercharger.

Of course thats all in theory.

I have no idea why you would be getting more and more boost as it keeps going up.... maybe the internal seals in the supercharger are so bad that it gets a tremendous amount of blowby....

But rotarygod should answer as it sounds like he knows more than i do

Edit:

and the numbers would work like this:

assume the supercharger can produce 0.05 cfm at 5psi per revolution

assume the engine draws 0.1 cfm per revolution


then at 1000 rpm the engine draws 100 cfm

then the supercharger will produce 100 cfm at 5 pounds of pressure at 2000 rpm

you would then run the supercharger at a 2:1 ratio to the engine

and at idle would be making 5 psi (less some due to compressor blowby)


if we bring this up now to 4000 rpm engine speed, then then the engine draws 400 cfm

the supercharger, thru the the 2:1 ratio is now spinning at 8000 rpm and will be producing 400 cfm at 5 psi


I think this is why the boost never changes... but will someone please confirm this?