Can anyone explain superchargers?

[dablk]

@Ricardos excellent supercharged PB thread has got me thinking. And not a in a cheap way

Complete so novice sorry,

Can anyone give me a crash course in supercharger basics. Am looking at flow charts. Am I right in thinking your SC RPM is dictated by the engine vs SC pulley diameter ratio? Or have I go that wrong? So if engine pulley is 2xmm and sc pulley is xmm. You Rev to 5000rpm so your sc is at 10,000 rpm?

Then as far as engine power expectations does it double on double air pressure? If 1 bar is atmos and 2 bar (1 bar of boost), let's say book spec 180 bhp rs182 would develop 360 bhp at 2bar?

So if you wanted to match Rich's excellent figures of approx an extra 90bhp. Your looking at 1.5 bar (0.5 bar boost)?

Or am I talking a load of 6 am horse poo

 

[dablk]

Just seen there is also a gearbox involved. Not sure if I am understanding the spec numbers but on a Rotrex 30 series its 1:9.49. So guessing adding that (10 times for simplicity)to my basic calculation

5000rpm engine = 100,000rpm at SC using a 1:2 ratio pulley

Yes?

 

[coverman]

Dont forget the power los for driving the supercharger.
One of my customers dragster engines develops 3000hp gross, but after supercharger losses it is 2500BHP, ie it takes 500BHP just to drive the charger

 

[Cub.]

Some info in here;

http://www.cliosport.net/threads/rotrex-restrictor-sizes.668774/

Mine is running 250 bhp / 220 lb ft. @Dan@SJM can confirm the restrictor size / boost on mine, but I have specifically gone with a setup to allow me to hack it around track all day, drive home and do the same a week later. I don't want to destroy the internals for the sake of 'another' 30bhp or so. Dan did some destructive testing on his with the supercharger, to see what the internals could put up with.

Trust me, 250bhp / 220 lb ft in a clio with a decent diff is plenty quick enough to trouble alot of more expensive machinery if in the hands of someone capable (not me).

 

[Goodj]

@Cub. speaks the truth.
Superchargers FTW, everytime.
The losses made to drive a blower are offset by the fact that you're making more power in the first place. So even if he's losing 30hp to drive his 'charger, he's still 70hp up on stock.

 

[jam.]

 

[Dan@SJM]

I can't remember what size yours was cub. Mine was mounted in the cooler and was about 24mm by the time I'd finished (started at 19mm) and I was seeing 10psi @ 7200rpm on an 85mm pulley. Which was doing ~220lb-ft and 278bhp

 

[dablk]

Thanks folks,
Going to start saving. And keeping my eyes out for bits. Just wanted to try and do things a bit cheaper as far as mechanics. I appear to be lucky that EFI or RS Tunning really are not that far away.

Will probably ask a few questions of a few people of the next 6 months.

I noticed the RS Tunning conversation from a few years ago appeared to use the biggest SC in the rotrex 30 range. now if your never intending on chasing 400bhp. Is there any benefit to this? Or like a smaller fast spooling turbo is there a better choice if your only ever wanting to nudge over 300 bhp?

Thinking about a long term 3 stage plane. This will not only help with the pennies but also always keep the car sellable at any stage.

Like you both suggested 250 bhp ish for a start on standard internals... The. Drive and enjoy
Then add diff and box rebuild with a more serious clutch... Then drive and enjoy a bit more
Finally a second engine build to suit a bit more and a restrictor change/ remap... Just for a hobby. I assume You could ask your mapper to save your map vs restrictor setup on file so it could be changed easily with restrictor changes without needing a full remap?

Oil cooler.. Self explanatory.
Intercooler... How much work does this need to do compared to say a turbo. I know in the turbo world the intercooler is king. RS500 intercooler on R56 Cooper S defo does the trick that's for sure.

Fuel. Seems to be a lot of chat about using phase one parts. Why?

Ecu.. Appears to be just get the check book out and visite Paul or Chris.

And the donkey question. AC delete. I assume without it the car will just work as a normal hot/cold blower? Just won't get super chilled.

So many questions... Still there is no rush.

Thank you Chris.

 

[NorthloopCup]

Don't rule out the gen90 ecu from Ktec. Utilises the original wiring loom with no cutting required, and just as good as the other ecu's out there. They come up on here for sale from time to time and more than capable of running a s/c engine.

 

[Cub.]

I run a gen90 on mine. Great bit of kit, even if it came from Ktec.

Regards AC delete, the blower just blows hot or cold. Just not ice cold. I did a cup AC delete before I charged it, so when I charged it, I didn't notice the loss of AC to be honest.

Personally, for track work, I'd do the diff before the charger. A gripper or Quaiffe. Gripper in mine and my personal choice but horses for courses to some degree on the diff choice. I'd then use it with diff and once you get 'bored' charge it.

 

[Ricardos]

Whereabouts is your restrictor fitted @Cub. pre supercharger or post?

 

[dablk]

ok so now for a more complicated question...

How do I use this . I believe this was roterx the prefered choice for tunners

Again please assume I am talking b0ll0cks..

Y axis pr.. Guessing this is boost yes? 1.5 being 0.5 bar boost/7psi?

SC Rpm.. Well I guess that's the engine speed x (ratio of crank pulley vs scpulley) x SD gear ratio

So how do I calculate the x-axis?

appreciate this is complicated stuff but interested eitherway.

 

[Steven103]

Pressure Ratio - ratio of intake to charged pressure, so yes a ratio of 3 would = 2bar of boost assuming 1bar intake (e.g. atmospheric pressure)
Airflow - Mass airflow, fairly self explanatory. This is relevant because mass airflow is proportional to torque.
SC RPM - Again self explanatory
% numbers - efficiency of the compressor

Mass airflow (theoretical) comes from: Cylinder volume * number of cylinders * engine speed * volumetric efficiency * air density / 2. You need to be using consistent units, so either do everything in "English" and do lots of conversions, or be sensible and put it all into SI units: m^3 for volume, revolutions per second for engine speed etc. The divide by 2 accounts for the fact that it's a 4-stroke engine. For every 2 revolutions only 1 intake stroke occurs per cylinder. Air density comes from intake pressure & intake temperature, and then

Pressure ratio (convert it to boost if you like, but that's not very useful for anything except bragging in the pub) is very handy for calculating theoretical power output of a supercharged engine IF you know the power output of the N/A version.


Remembering the definitions & little equations is the easy bit, actually working anything out involves making two estimates, about 10 steps of thermodynamic calculations for each, then drawing about 3 lines which all end up intersecting on the desired axis to give you your pressure ratio or mass flow rate.

 

[dablk]

Thanks mate,
Doing all kinds of digging and agree the conversations are "fun".

 

[Steven103]

Sorry that's a crap answer now that I read it again.

A compressor map has 4 variables as I said:
* PR on Y axis (calculates/calculated from boost pressure: (PR/intake P) - intake P = boost)
* SC speed (thick lines in the middle, I'd probably agree with your equation)
* Thermal efficiency of the supercharger (as a %, heat loss is used to work out power consumed to make the thing go round, thin lines in the middle)
* Mass Airflow (calculates/calculated from power, torque or engine geometry e.g. RPM, # of cylinders, Volumetric Efficiency etc).

The maps are really suited to OEMs. The better your assumptions, the more accurate your answer. Some assumptions are easy (intake temp & pressure for example) but other essential pieces of info such as volumetric efficiency are only accurate from very thorough simulation or testing i.e. OEMs only. If you have the background knowledge then it might be fun to compare two superchargers in % terms, but I wouldn't use the amount of data that anyone besides Renault themselves have to try and predict power/speed/boost etc.


Working out something in the middle (speed or efficiency) e.g. "I want X power with Y boost, what gear ratio do I need"
* You calculate the values on the axis by measuring stuff on a running engine or calculations (e.g. the mass airflow equation in my previous post, or a MAF sensor reading on an engine dyno)
* You can then fairly simply draw intersecting lines to read off the stuff in the middle of the map for your corresponding X and Y

Finding something on the Y axis is much harder e.g. "I have a ratio & I want X power, what boost do I need" (and hence is that boost feasible with regards to intercooling blah blah)
* You basically guess the Y axis, read off the corresponding speed & efficiency and use it to calculate the X axis.
* You then calculate/measure X and see how close your Y guess was.
* Adjust your Y guess until the corresponding X matches your calculated/measured-by-another-means X (e.g. calculating mass air flow using engine geometry or by reading a sensor on a test engine)

Alternatively, you can find something on the Y axis by:
* Having a guess at Y & marking the corresponding X on the graph
* Note down the compressor speed/efficiency as this is essentially fixed for your desired RPM that you want your power to occur in, i.e. 4000 - 6500 RPM not 273743637437.
* You then have a different guess at Y but stay on the same speed/efficiency line and mark the new X. You now have an X that is too high and another that is (hopefully) too low.
* Then draw a line from both X to the chosen speed/efficiency and in the middle of them both (as one is too high, the other too low), draw a line to Y.


The "small" bit in the middle of all this is:
1) Maybe 10 simple-ish equations and quite a lot of engine data (you often need a starting pressure or temp, engine geometry etc which is far from simple) for all the "calculate" bits.
2) And the much more complicated task of compensating for the temperature change that happens when you compress the gas which is another load of, IMO, more difficult equations.
3) Actually deciding/remembering what 8 things you need to work out in order to draw the right intersecting lines can be IMO challenging & confusing (speak for myself haha!)