rotarenvy

maybe you would like to define it then?

found it myself

wikipedia has a good explanation of lag
lag

now I see, I was refering to boost threshold.

MazdaManiac

iTrader: (3)

Yes.
Both boost threshold and "lag" can be managed to zero without taking a turbo out of its efficiency range for the desired application.
That is because there is a nearly infinite range of turbo configurations to match any desired boost level, efficiency and engine dynamic range.
There are essentially 1 or 2 different versions of the 3 or 4 viable supercharger designs.
Its mostly one-size-fits-all.

Brettus

iTrader: (2)

MM - one senses a slight bias towards turbos ........

rotarygod

I like both superchargers and turbochargers but that needs to be further quanitified by stating what the intent of the car is. I don't always like turbos and I don't always like superchargers. On top of that I don't like all types of superchargers.

Brettus

iTrader: (2)

The centrifugal from Rotormaster (in Aussie) has been available to US buyers for a few months now .
One of the things that stands out about this SC for me is that it seems to be such an easy install . Very much bolt it on an forget about it .
OK - it will be just like stock in lower RPM range but it just seems like a very low risk FI option .
You are not increasing torque by much so the stress on the drivetrain is lower & yet when you get on the gas it will be just as quick as any other FI alternative out there .
Maybe this is just my perception but it just seems like the way to go if you want reliability & can accept that low down power will not be improved.

Charles R. Hill

iTrader: (7)

One of the conversations I had with the N.E. group I visited last weekend was this turbo vs. supercharger debate and RG has directed this particular thread to the very thing I pointed out; application. Types of expected racing and driving aside, I tend to consider turbos better for cars with automatic transmissions and S/Cs better for manuals. It is easier to get the boost up in a car with a turbo and properly selected torque converter as opposed to a manual trans. Of course, a performance converter and tranny "PCM" for the RX-8 is as much a mere theory as is a S/C at this point. One thing that has always puzzled me, though, is why anybody wants boost down low (as it usuallly comes at the expense of optimum top-end power) when the RX-8 really isn't meant to be performance-driven in the 3K or low-to-mid range anyway? What I am getting at is that if you design your F/I system to best deliver from 6K to redline won't you get the power you need only when you need it and save money on unnecessary expenses such as fuel, wear and tear, and etc.? Plus the F/I system would be better executed by not having to deal with such a wide rpm range.

Brettus, you were editing your last post while I was posting mine but we seem to be saying the same thing......

mysql101

That's why a turbo is so great, you can modulate how much boost you get at any time. You don't have to get your full boost at 3k, but you can if you wanted. And if your turbo can flow enough, you'll keep making it to redline.

But you already knew all this, so maybe I am misreading your post.

Brettus

iTrader: (2)

Yes - although i'm sure mm will point out that a turbo can deliver at both ends of the power range . I just see a turbo as something that will give me more problems .

Charles R. Hill

iTrader: (7)

I think I have pretty good handle on the concept but I decided to stick my neck out and maybe learn something. Another difference I see is that since a S/C is belt-driven and a turbo pretty much freewheels it would be easier to control the boost with a S/C. I guess the simple view is that a S/C can have the boost controlled mostly through design and mechanical means but the turbo system requires a bit more than that. Could it be that turbos are only for the technologically sophisticated and S/C are for meatheads like me? One of the points I was proposing was really coinciding with that which MM alluded to; that people tend to place much more demand on their F/I systems, whichever they choose, by placing unrealistic expectations on them. The GReddy turbo issues gve a good example. People buy a kit that was intended for one set of tuning circumstances and then wonder why difficulties arise when they change those circumstances. The idea that GReddy issued that kit before it was fully developed is not germane in my asserton.

I don't gather that Mr. MM has ever asserted that a turbo can deliver on both ends. Unless I have misunderstood him for the past couple years, he and I have similar views on this particular topic of misapplications and expectations.

As for my own desires, suffice it to say that once I rebuild my engine (which will be soon) and have achieved the desired goal of a 150 shot of nitrous that is fairly reliable, I will look into some sort of F/I of my own. As of now I am leaning toward a setup that will really wake up around 5K and up. Maybe I'll end up being a hypocrite and getting a turbo. We'll see......

mysql101

brattus,

the turbo system is a lot less complex, and less moving parts.

you just have the turbo to worry about. the rest is just hollow pipes...

MazdaManiac

iTrader: (3)

Definitely. I've done both - even on the same car - and the turbo always results in more power, less issues and more options and flexibility.

Yes - a turbo can deliver at both ends if it is sized correctly. That is exactly what I am saying.
An SC will give you more problems - there are more moving parts and it is muce more sensitive to installations concerns an balance.

Brettus

iTrader: (2)

i'll ignore the name calling & get to the point .

What about heat ? you have a high compression N/A motor that gets its power through high rpm . Now you put a heat generator on it that also supplies boost .
I'm not sure of what the difference would be with a CF SC as far as this is concerned but I think it would be less likely to be a problem . That combined with what Charles was talking about (more contorolled boost) surely makes it less of a risk to your motor.

MadDog

Heat will come just from compressing the air. Whether you use a SC or turbo to compress it is absolutely immaterial. The temp change of the air is dependent on the efficiency of the compressor, not its name.

MazdaManiac

iTrader: (3)

How is a turbo more of a heat generator than an SC?

A properly sized turbo is no more a heat generator than an SC - both are dependent on the efficiency of the flow map and a turbo will have a wider efficiency range than an SC. A turbo will spend far more of its life producing power at its adiabatic peak than an SC.
As far as the AFSC goes, I have yet to see any numbers. How about you?
How about Roots?
Or Twin-screw (Lysholm)?
They all have their own distinct adiabatic peak, but since they are tied to the drive assembly, you have no way of holding them there.

As for under-hood temps, all of the same heat management strategies for headers (or OEM exhaust for that matter) work on a turbo.
My under-hood temps are lower than stock because I shielded everything.
Don't be somehow under the impression that being attached to the exhaust makes the the turbo any less efficient.

That said, the most telling part of your post is the "I think it would be less likely to be a problem" part - you don't know for sure and I do.

Its that simple.

You can take my word for it or not, but I've been there and done that and a turbo, when properly sized, installed and setup adds nothing to the system but power and a similarly constructed SC mostly adds complexity.

I'll remind you again of the concept of "dynamic range". A rotary motor has a LOT of it and an SC is trapped by this same factor that actually liberates a turbo.

Brettus

iTrader: (2)

Can you just clarify something MM - when a turbo is "spooled up" to its designated boost level - how much slippage is going on IE the turbo is able to supply more air than the motor actually requires & just ends up producing heat ? I imagine if the turbo is designed for optimum flow at peak rpm that there would be a lot of heat generated at lower rpms ?

Charles R. Hill

iTrader: (7)

Like I said, I stuck my neck out with the chance of maybe learning something. I also mentioned that I might end up being a hypocrite. Given MM's latest statement it seems as if both have/will happen. I am convinced that I can come up with a turbo system that will meet my desires if they are well thought-out, but that's because I have a street car. Would it be correct to say that turbos are better for road-racing and S/Cs are better for drag-racing, at least in a general way? My previous mention of transmission and F/I combos for drag racing remains the same as that is my personal experience much like MMs is F/I.

MadDog

Just spend some time learning how to calculate the flow demands of the engine and how to read compressor maps and all will be clear. The question you are asking is at the heart of properly sizing a turbo for a particular application (engine and pressure ratio). The max efficiency can occur for low or high engine RPM depending on the flow requirements of the engine and the PR you desire.

MazdaManiac

iTrader: (3)

That is a supercharger you are describing, not a turbo.

First, you need to divorce yourself from equating "boost level" to airflow.
A belt-driven supercharger can only produce one particular mass of air at any given RPM.
A turbo, on the other hand, will vary its speed (and therefore its output) based on engine requirements at that specific load level - it will produce no more air than is required, so it will produce no additional heat.
The trick is to have a firm understanding of the airflow through the motor and simply apply a compressor/turbine combination that delivers precisely that.

With an SC, you can only match the flow characteristics (which some will claim are "linear" - which may, in some cases, be essentially true, but is irrelevant since the airflow demands of a motor, especially a rotary motor, are not) of the compressing element since you have a fixed relationship to RPM on the drive side.
This means you must create an excess of air somewhere and that means an excess of heat.
Even if it were possible to attach an airpump to the motor that displaced exactly twice the displacement of the motor (for a theoretical PR of 2.0) at an exact relationship to the Renesis' peak VE (meaning 100% of the engine's displacement as intake volume), the motor itself wound not match it (because VE is dynamic and so is your right foot) and you would have a wildly varing adiabatic efficiency deficit.

mysql101

lol.

No name calling. Search my posts, not my style. I was posting while at work, can't look like I'm slacking, so my posts are done quickly and I don't review what I said... my apologies if it looked otherwise.




Charles, In some drag racing sports, you'll find no turbochargers. It's not because they aren't as good, but because turbos are banned. They want to make the sport "safe", and so they prevent turbochargers because they don't want people going over 300 mph. Given an apples to apples comparison, the turbo will always make more power. It's more efficient. I don't think anyone is even debating this...

People still think SC > Turbo due to low end torque, when it's really not the case anymore. Properly sized turbos spool up almost immediately, and can give you full boost at the low rpm all the way to redline. So if I floor the throttle from a standstill, I'll see the boost climb, and by 3,000 rpm I'll have 10 PSI and it will stay at 10 PSI till redline. If I decide to give it 60% throttle, I might see 6 PSI instead. If I don't give it any sudden throttle, I can remain out of boost all the way to redline, it'll drive like a normal NA car.

As MM said, a SC is linear in the sense that if you're matching peak pressure at redline, that means you're only getting HALF that pressure at 4000 rpm (This all varies based on what your SC setup is and what kind of SC you're using, but I'm trying to make it simple). It's because your SC compressor is directly tied to engine rpm. So if you're setup to hit 10 PSI at peak (lets say 8000 rpm), you'll generate around 5 psi at 4,000. So what's the SC advantage? It can generate a little bit of PSI at 1500 - 2500 RPM that I wouldn't have on my turbo, but uh... how long are you in that range? You can gear your SC to give you a huge amount of boost down low, but in the mid to upper range you're wasting a lot of power since the SC isn't going to meet your engine requirements... It's only a single speed pully. It can't adjust to the engine requirements.


I've never had a SC car, so I can't speak from personal experience, but I'd imagine you'd also get lower fuel economy with them since it's pulling on the engine.

rotarenvy

may be we can explore this in more detail and people can point out the deficiencies and design problems?

as the curent sc options are all centrifugal i'd like to start there. also as Charles points out performance driving on the renisis is between 6krpm and redline where a centrifugal charger will operate. rotrex give a lot of good information to get you started and give flow maps for their range.

I read in a magazine that knightsports uses a rotrex c38. they dont specify the trim so I'll start by posting the flow maps of the smallest one the c38-61.



from hymee's VE work and the rotrex formulas I gestimate the renesis needs.

0.34kg.s-1 at a pressure ratio of 1.79 for about 335hp at the crank. and spin the c38-61 at 70000 at 8500rpm with a 10cm crank pully and a 9.1 pully on the rotrex.



where would the other pressure ratio flow points lie on the graph? at ~7300rpm the chosen gearing has the sc on the 60000rpm line but at what pressure ratio and flow?

Rootski

iTrader: (1)

You know, the turbo vs super grenade-throwing is great and all, but like its been said, some of us really just want to learn about superchargers. I don't really care who thinks a turbo is better for what reasons, I want to learn which SC design is best. kthxbye.

kartweb

Thanks rootski, thats why I asked.

Some people prefer turbos and thats OK. I like the turbo on my CX7, but it's factory. All turbos have some lag in increments corresponding with the level of boost. Neither a Turbo or an SC is a perfect solution, they're only added hardware.

MazdaManiac

iTrader: (3)

Go to my Rotary Math thread, download the spreadsheet and plug in the values.
It computes in pounds per minute, so just divide by 60 and multiply by .454 to get kilos per second.
To compute boosted values, just use super-100% VE to input PR and correct the temps to something reasonable for the output of your supercharging system.

rotarenvy

thanks maniac,
the units are giving me a headache so it might take me awhile to figure it out.

super-VE% will just be a guesstimate?

after reading about your mx-3 supercharger install I can see why you think turbos are easyer I though it was very ingenious and well executed intall.

did you do the flow calc's before you chose the mx-3 sc or did you just go with what was available?

MazdaManiac

iTrader: (3)

No. Well, sort of.
Assume that while boosted, a PR of 1 (atmo with no boost) is 100% VE. So, 12 PSI would be a PR of 1.81 or an effective VE of 181%.

Thanks. That was a really tricky install. I've had easier. I don't really calculate the difficulty of that particular install into my typical rejection of supercharging as a viable method of power increase on a street motor. If I did, I'd be REALLY adamant!

Others did it for me. That particular SC was a popular choice for the Ford Probe people and mine had the same motor. I was limited by availability and packaging also.