Supercharging: the forgotten form of FI

[madfast]

Seems like everybody is jumping on the turbo bandwagon but there are other options!

The purpose of this post is JUST to show people, that may not know, what a proper supercharged, low displacement engine can do:

http://www.flyinmiata.com/tech/dyno_...percharged.pdf

and mind you, this is with the older gen MP62. check out the latest gen of superchargers out there:

http://www.eaton.com/EatonCom/Produc...rchargers/TVS/

same SC used in the ZR1. it has efficiency that rivals a turbo, but all the benefits of a SC.

how is this relevant? well TRD has a short history of releasing fully warrantied superchargers. i would LOVE to see a TVS sc as an option for the FT. look at the dyno above. how much more do you really need than that? granted an OEM kit wont make as much power, but you get the point.

AGAIN, this is NOT a turbo vs SC thread, but simply FOOD FOR THOUGHT for those that previously did not know. if you already know this, good, move along, nothing new here...

[Type[R]+]

I posted this link before, but here it is again.
Rotrex Subaru 2L boxer kit.

http://www.bulletcars.com/supercharg...er-system.html

[Random_Art]

Originally Posted by Type[R]+

I posted this link before, but here it is again.
Rotrex Subaru 2L boxer kit.

http://www.bulletcars.com/supercharg...er-system.html

Yeah, I looked at that earlier, though that particular engine had to make some pitiful power to begin with to only get up to 160hp supercharged.

That being said, I feel like I'm one of the relatively few people here planning on a Rotrex build for mine. I was thinking of the C30-94 with a front mount intercooler. Though, I was curious, that kit you linked had a water based intercooler with a front mounted radiator. Do those work better than the standard Air-to-Air intercoolers that you generally find on turbo cars?

[madfast]

im personally in the roots type SC fan club, especially the new TVS units. rotrex units are a nice compromise between positive displacement SC and turbo. but a roots blower still makes more down low and still has a flatter tq curve. for a low displacement engine, that's exactly what it needs most. that's my take. if you like your power to the right of the rpm range, then yeah rotrex or turbo all the way...

[Random_Art]

Originally Posted by madfast

im personally in the roots type SC fan club, especially the new TVS units. rotrex units are a nice compromise between positive displacement SC and turbo. but a roots blower still makes more down low and still has a flatter tq curve. for a low displacement engine, that's exactly what it needs most. that's my take. if you like your power to the right of the rpm range, then yeah rotrex or turbo all the way...

it's also an efficiency thing. Everything I've ever read about Rotrex units say that they're much more efficient than roots blowers. but, we all have different goals when it comes to modifying the cars. If you want instant boost down low, go roots. If you want a nice comprimise, go rotrex. if you want gobs of power and don't care about lag, go turbo.

I had originally planned on trying to have a set up that could keep up with my father-in-law's E60 M5. In theory it might have been possible due to the incredible weight differences between the two cars (I had worked it out to needing to get my Celica up to around 350hp and drop a bit of weight to do it.) but, then he decided to take it to a tuning shop and have them put another 100 or so hp on the thing. So, there went that idea.

[Type[R]+]

Originally Posted by Random_Art

Yeah, I looked at that earlier, though that particular engine had to make some pitiful power to begin with to only get up to 160hp supercharged.

That being said, I feel like I'm one of the relatively few people here planning on a Rotrex build for mine. I was thinking of the C30-94 with a front mount intercooler. Though, I was curious, that kit you linked had a water based intercooler with a front mounted radiator. Do those work better than the standard Air-to-Air intercoolers that you generally find on turbo cars?

Which is better? That's quite a hard one to answer. From what I've seen, as long as the radiator is large enough to prevent heat soak, there is a few advantages air-water has over air-air.
The most obvious one is response. No 5 feet of extra piping and intercooler core volume.
Second one is you can get over 100% cooling efficiency if you chill the water (like they do at drags).
Third is fairly constant intake temps.
Fourth is great for tight spaces.
I'm sure there's more, as well as negatives.

Which would I pick? What ever came in the kit!

[madfast]

Originally Posted by Random_Art

it's also an efficiency thing. Everything I've ever read about Rotrex units say that they're much more efficient than roots blowers.

maybe the older ones. the newest TVS blowers supposedly have adiabatic efficiency "up to 76%". either way, it all depends on how you want the graph to look.

imo a rotrex unit would be much better served in a larger displacement engine, that may not necessarily need help down low, but could use the help up top.

[serialk11r]

Eh the reason people don't supercharge as much is since it has shit efficiency...
But anyways, a Roots supercharger is a positive displacement pump, so what you are getting is essentially increased displacement, increased dynamic compression, but DECREASED expansion ratio. So it's like a larger displacement motor with crippled expansion stroke.

A blower would be a dynamic compressor, which operates differently in principle, and is more efficient at cramming the same mass of air into the same volume for complex reasons. Aside from the size of the unit, a small turbo is probably better than a centrifugal supercharger since you can potentially have more boost at low rpm since the turbine doesn't depend on engine speed as much. A positive displacement supercharger should feel like a naturally aspirated engine in terms of response.

This is all theoretical, someone fill in if I'm missing stuff :O

[madfast]

Originally Posted by serialk11r

Eh the reason people don't supercharge as much is since it has shit efficiency...
But anyways, a Roots supercharger is a positive displacement pump, so what you are getting is essentially increased displacement, increased dynamic compression, but DECREASED expansion ratio. So it's like a larger displacement motor with crippled expansion stroke.

that's changing. both twin screw and tvs superchargers are quite efficient units. not nearly as much as a turbo, but the response you get makes up for all of that... something to think about: the Audi S4 went from twin turbo V6 to NA V8 to a TVS SC V6.

Originally Posted by serialk11r

A blower would be a dynamic compressor, which operates differently in principle, and is more efficient at cramming the same mass of air into the same volume for complex reasons. Aside from the size of the unit, a small turbo is probably better than a centrifugal supercharger since you can potentially have more boost at low rpm since the turbine doesn't depend on engine speed as much. A positive displacement supercharger should feel like a naturally aspirated engine in terms of response.

This is all theoretical, someone fill in if I'm missing stuff :O

this imo is paramount on a car like the FT. or the miata. on cars like these where handling is held in such high regard, why would you ruin it with poor power delivery and muted responses?

that's why i scratch my head at all these turbo people. its like they are enchanted with the peak power numbers, etc and care nothing about power DELIVERY or area under the curve.

[Type[R]+]

Originally Posted by madfast

that's why i scratch my head at all these turbo people. its like they are enchanted with the peak power numbers, etc and care nothing about power DELIVERY or area under the curve.

My thoughts - Why lug a high revving engine? Why not play to it's advantages/inherent design? It's the reason Honda's respond much better to turbo charging than supercharging.

[Matador]

Originally Posted by madfast

im personally in the roots type SC fan club, especially the new TVS units. rotrex units are a nice compromise between positive displacement SC and turbo. but a roots blower still makes more down low and still has a flatter tq curve. for a low displacement engine, that's exactly what it needs most. that's my take. if you like your power to the right of the rpm range, then yeah rotrex or turbo all the way...

This. I don't think there is one "right" answer though, but roots vs +ve displacement vs turbo will vary in relevance to they type and displacement of the engine.

[Dimman]

Positive displacement SC's are great for an upgraded DD situation, but they are usually limited to how much boost you can make efficiently.

I know a guy with a positive displacement kit on his Miata but no IC (integrated blower/manifold), and when he changed the pulley to get 10 (or 12, I don't remember) psi he needed water/meth to keep it from pinging.

He's making about 200 whp now though, and it sounds awesome. But if he wanted more power, he would be going turbo.

[serialk11r]

The efficiency is not the pump's fault, the pump does its job perfectly well. It compresses air as it should, and has some associated frictional loss. The issue with supercharging is as you increase charge density, the proportion of kinetic energy that you blow straight out the exhaust increases significantly. Turbo if done right recovers some significant proportion of this, instead of sapping power from the engine itself (it still saps energy from the exhaust stroke, but when the exhaust valves are closed the pressure in the exhaust manifold is still doing work on the turbine, so instead of 100% of the compressor power coming from the engine, it's more like 50%).

Anyhow the way Audi engines operate has less efficiency loss associated with any form of forced induction since they use stratified injection and no throttle plate, I believe on all their current engines. It would work even better on a continuously variable intake duration engine, but obviously sacrificing efficiency at full throttle still.

With positive displacement supercharging you will see a significant drop in fuel economy. If you run 7psi boost for example (about 0.5 atm), and this engine is 2.0L and say 12:1 compression, your pumping loss becomes that of a 3.0L engine at part throttle, and at full throttle, you are wasting more exhaust pressure than a 3.0L N/A engine with 12:1 compression. Plus you might have to reduce compression to be able to run the boost, and that has an efficiency loss associated with it as well.

If you turbo a 2.0L, and you are running at part throttle, the exhaust pressure is low, and only a small percentage of that exhaust energy makes it back into the intake in the form of compressed air, so pumping loss is not increased by as much.

The point is, if you don't have a fancy valve control system such as Valvetronic/Valvematic/VVEL, or a throttleless system such as FSI, any energy going into the compressor at part load is completely wasted. A turbo has less energy going in at part load than any kind of supercharger. You could replace the dynamic compressor with a positive displacement pump and this would still be true.

When you do have a fancy valve control system it gets interesting In that case, ANY dynamic compressor will give a part load efficiency increase, because the intake/compression stroke is essentially a positive displacement pump, and a dynamic compressor can put charge in at a lower temperature even without intercooling. So basically your compressor is doing some of the compression work for the piston. If you use a positive displacement supercharger, you do not lose any efficiency up to the point where the charge is at equivalent density to what it would be if the engine were N/A, running full throttle. If you use a turbo, you gain efficiency at any point where exhaust pressure is greater than atmospheric.

[Neutral_Eyes]

If I were to put FI on the FT-86, or my Miata, I'd use the Rotrex supercharger. I like that its oil and cooling is separate from the engine's. I like the lower under hood temperatures. I like the linear power delivery. It just makes sense. My problem is I'd want enough power to start compromising my Miata's bottom end. That, or finding, paying for, and installing lower compression DI pistons for the FT-86.