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Old 06-17-2012, 02:34 AM   #21
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Quote:
Originally Posted by Mobius View Post
Positive displacement superchargers are not the only ones available. I guarantee mine is cooler than a turbo. I have less area under the curve, I admit.
And yet you have less torque under curve than both discussed here. Ouch.
No offense, but I think the failtrex kits were the biggest let down ever. We (or at least me) expected greatness when it was being hyped up by Emilio and others.
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Old 06-17-2012, 02:35 AM   #22
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Roots blower through the hood or go home.
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Old 06-17-2012, 03:21 AM   #23
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I'll take the reliable car that drives like a C6Z. That means turbo > hand-dryer.
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Old 06-17-2012, 09:39 AM   #24
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Just to be fair, the biggest knocks on MOST sc'ed set ups are the lack of real engine management, the fact that they are bolted to stock motors and the fact that MOST of their owners are clueless. It's more than possible to have a M62 setup make as much or more power then most turbo setups and to have perfectly linear and transitional throttle response. BUT it does get hot without water, it does kick the belt from time to time, it does require C12 or E85, it is a purpose built motor and it does whine like a MF'er.

I still say it all comes down to what your plans are with the car, what your fab skills are and what your budget is. I love the way my car instantly pulls out of a 35mph tight sweeper in second gear(1,500rpm) after being at TPS=0 for 2 seconds all the way to 73.4 mph in second. In a properly sized turbo car that's going to be more difficult to accomplish. That being said there are a number of reasons my car wouldn't survive a 30 min session on a road course, one of the biggest is heat.
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Old 06-17-2012, 10:51 AM   #25
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Im about to build a twin charged BP4W. As a bit of a fabrication exercise.

I intend to run an SC14 bottom mount hotside, with a filthy big turbo high mount. Perhaps a t3/t4 hybrid.

I would NEVER run just a blower.

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Old 06-17-2012, 10:56 AM   #26
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^^^While a neat novelty the added weight and complexity of such a setup would not make it a viable option for just about any form of competitive driving^^^
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Old 06-17-2012, 11:44 AM   #27
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Quote:
Originally Posted by wannafbody View Post
Interesting, I assumed a supercharger would run cooler than a turbo.
This is a common perception, and it's certainly easy to understand why. We've all seen pictures of turbochargers glowing red on the dyno.

But what really matters for our purposes is not the temperature of the turbine housing, it's the temperature of the air being emitted from the compressor.

Centrifugal compressors have a higher efficiency than positive-displacement compressors. It doesn't matter whether the compressor is being powered by exhaust gas or a belt, it's all about the style of the pump. For our purposes, this mostly means that centrifugal pumps heat the air less as they compress it.

So with a positive-displacement supercharger, you are using more energy (from the engine) to turn the screws, and also creating more heat as you do so. BHP will always be lower for a given BMEP, owing to the power lost to turning the supercharger. And BMEP will always be lower for a given Mainfold Pressure, owing to the reduced density of the air.


Does this mean that supercharged engines can't make big power? Of course not. The dyno sheet that mgeoffriau posted is one of many showing big-number supercharged engines. But because of the lower efficiency, we can universally state that the same engine, if fitted with a turbocharger and otherwise in a comparable state of tune, would tend to make more power for less money and less fuel consumed. And we're not simply referring to peak power as is often stated, but total area under the curve.


So it's kind of a lose-lose proposition.


Now Mobius has pointed out that there are some superchargers which use centrifugal compressors. Within the Miata community, the Rotrex is probably the best known example in the present day. These devices are not without their charms, however they do suffer from one annoying weakness- with a centrifugal compressor, the mass of airflow through the pump is not linearly related to the speed of the pump as it is with a positive-displacement unit. At low speeds, centrifugal pumps hardly work at all, and the result is that boost with such a system rises with RPM. Low RPM = low boost, thus negating the usual (if highly exaggerated) selling feature of superchargers in general. Of course, this can to some extent be rectified by altering the pulley / gearing ratio of the unit to increase its speed, but you have another set of limitations at the top end- you can't exceed the compressor's own redline speed, and you have to figure out some way to cause it to stop making more boost beyond a certain point, usually by placing static restrictive orifices (like the restrictor plates used in NASCAR) at its intake.

So why don't turbochargers also suffer from these flaws? Because the shaft inside a turbocharger isn't mechanically coupled to the engine. It's free to spin up to a very high speed even at low engine RPM (provided that it's been sized properly), and then at the high end we can easily regulate its maximum speed by using a wastegate to bypass the exhaust around it.


Does this mean that centrifugally-supercharged engines can't make big power? Again, that'd be a no. But as with the first example, expect it to cost more, be more complex, and produce a less desirable torque curve.




In the end, it all boils down to a very simple question: what is the "best" way to achieve the goal of having a fast Miata, or a fast car in general?


Consider the following: If Lotus dropped the price of the Exige to $5,000 tomorrow morning, would it still be possible to build a Miata which was faster than a stock Exige? Of course- the science doesn't change. The only difference is that now, you'd have to be a damned fool to do so, and the Exige owners would be fully justified at laughing their asses off at anyone who tried. The Exige is a "better" platform to build from, and if it's also cheaper, then there's no justifiable reason for buying a Miata anymore.

Substitute "Turbocharger" for "Exige", and "MP62" for "Miata", and the basic logic remains the same.
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Old 06-17-2012, 11:53 AM   #28
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I'd wonder how different this thread would have gone on a supercharger forum.
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Old 06-17-2012, 12:26 PM   #29
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Quote:
Originally Posted by triple88a View Post
I'd wonder how different this thread would have gone on a supercharger forum.


http://www.mayer-johnson.com/power-c...me-and-autism/
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Old 06-17-2012, 12:26 PM   #30
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Quote:
Originally Posted by TNTUBA View Post
I love the way my car instantly pulls out of a 35mph tight sweeper in second gear(1,500rpm) after being at TPS=0 for 2 seconds all the way to 73.4 mph in second.
Umm, 35 mph @1500 RPM is more like 6th gear.
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Old 06-17-2012, 12:35 PM   #31
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Very fun and easier to drive autox than my 94 with a 2554r.

For street driving I definitely prefer the turbo. This is purely because of my driving style. With the turbo I get the nice rush of torque down low and shift at a leisurely rpm. I drive the centrifugal the same and I miss my turbo.

But driving to, and close to redline and the super feels wonderful.


I am still debating whether to keep the super (quieting the exhaust), or go through the effort of swapping in my turbo bits.

Btw, does not eat belts.
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Old 06-17-2012, 12:39 PM   #32
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Quote:
Originally Posted by 94mx5red View Post
<photo of Rotrex install>

Very fun and easier to drive autox than my 94 with a 2554r.
Kindly expound.

Do you have TPS controlled boost in your 94?

I have the opposite (not exactly) experience. My E36 M3 has a centrifugal supercharger, and my miata with the GT2554 is easier to throttle modulate at autox. Possibly due to TPS controlled boost (and low 5 psi wastegate can).
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Old 06-17-2012, 12:42 PM   #33
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Man thats one sharp *** elbow.
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Old 06-17-2012, 12:56 PM   #34
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Jason, not a rotrex, but a VF Engineering. I think the model is a V9, without looking. 10psi at redline.

Did not have TPS controlled boost on the 94, just a MBC running 12psi with 3" exhaust attached to the crappy begi S downpipe.

Never drove a TPS based, so not sure how that effects things. Maybe I could be faster in the 94 if I knew how to drive and predict the power onset better.


Quote:
Originally Posted by triple88a View Post
Man thats one sharp *** elbow.
Not sure if the compressor housing can be rotated.
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Old 06-17-2012, 01:11 PM   #35
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Its easier to drive because its slower. Slow cars are usually easier to drive.
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Old 06-17-2012, 01:13 PM   #36
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Quote:
Originally Posted by Joe Perez View Post
This is a common perception
...
...
...
...and the basic logic remains the same.
Joe, you should get the MT medal of honor for this post
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Quote:
Originally Posted by triple88a View Post
I'd wonder how different this thread would have gone on a supercharger forum.
There isn't a MIATASUPERCHARGER.NET.

That should tell you something.
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[IMG]https://www.miataturbo.net/attachment.php?attachmentid=46574&dateline=1339946 775[IMG]

http://www.mayer-johnson.com/power-c...me-and-autism/
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Old 06-17-2012, 01:32 PM   #37
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I has supercharger. It's a nice day driver and autocross car. Oddly enough, I post better times naturally aspirated than I do with the supercharger on.

If I went back 3 years, I may have decided differently on the system. I'm satisfied with the power I've got, although it is barely enough to outrun the new precedent of 250hp Korean crossover SUV's.

My biggest battle has been maintenance actually. It's been a PITA to pull the supercharger off to service a loose egr tube/cap or calibrate the 02 sensor or change the clutch. I've probably had the supercharger off or loosened in order to adjust the belt alignment 6-10 times since I've installed it. I don't think I'd be pulling off a turbo that much...
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Old 06-17-2012, 02:46 PM   #38
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Quote:
Originally Posted by elesjuan View Post
Here's an article on compound boost (Turbo pushing air into blower): http://www.musclemustangfastfords.co...a/viewall.html
The author there doesn't realize there's a difference between twincharging and compound boost. When I read compound boost, I think of this:

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Old 06-17-2012, 02:59 PM   #39
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Quote:
Originally Posted by kotomile View Post
The author there doesn't realize there's a difference between twincharging and compound boost. When I read compound boost, I think of this:

Interesting.. I think it's a matter of semantics really, I've always understood using a compressor to feed any compressor is compounding..

http://www.lsxtv.com/news/granatelli...early-1200-hp/

Quote:
Compounding is a very good thing. Earning compound interest on your money will make you rich faster, and if you compound boost on your LSX it’s a colossal amount of horsepower and torque that you’ll be earning. Compound boosting, sometimes referred to as “twin-charging” is simple; you use a turbo (or turbos as the case may be) to force air into a supercharger, which pushes the twice-compressed air right into the combustion chambers.
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Old 06-17-2012, 04:10 PM   #40
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Quote:
Originally Posted by cymx5 View Post
I've probably had the supercharger off or loosened in order to adjust the belt alignment 6-10 times since I've installed it. I don't think I'd be pulling off a turbo that much...
All the stuff i've done to my kit since i first installed it.. During my install i realised begi sent me the wrong bolts from turbo to downpipe (either that or the person that sold me the kit bought the wrong ones)... Anyhow during my install i installed them and then next day i bought new ones to replace the short ones. Then i had the 2 pieces of the downpipe welded together since i didnt like the shitty slip fit design (was leaking and coming apart everywhere) After that i did an intercooler install about 3 months after installing the turbo. 2 weeks ago i checked the bolts on the turbo.. everything is perfect. i've had 0 reason to do anything else to my turbo after the initial install. Obviously other than essentials for the motor. I've been turboed for about 2 years now.
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