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Join Date: Sep 2005
Location: Winnipeg, MB Canada
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Help with supercharger calculations

Hi,

I am trying to setup a used SoK supercharger and I need some help trying to figure out some of the calculations involved.

Almost everything I read talks about calculations based on how many PSI of boost you are running. However I am missing something fundamental. How do I figure out how many PSI of boost I will have?

I have the compressor map which relates pressure ratio to air flow, and the basic specs of the supercharger.

I would like to know how many PSI of boost I will be running if I spin the SC at:

1.685 * engine rpm
1.764 * engine rpm
1.973 * engine rpm
2.142 * engine rpm

Thanx,
Kirk


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1989 911 Carrera 4 (964)

Last edited by kirkf; 03-09-2008 at 04:05 PM..
Old 03-09-2008, 10:21 AM
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Kirk,

It is very difficult to predict exactly because the actual amount of boost developed in the intake manifold depends on the flow characteristics of your motor. For example, at a given RPM, if you put on a smaller throttle body, sometimes you see the amount of boost measured in the intake go up. In this example, counter-intuitively, the horsepower delivered is less even though you show more boost than if you were using a larger throttle body that was more closely matched to the rest of your engine (valve size, lift, header restrictiveness, muffler restrictiveness, etc).

Since the supercharger manufacturers don't really know how much backpressure their blowers will encounter, they can't easily show you how much boost you'll see. Their maps usually give you a pretty good sense of how much air the blower will flow at an RPM and some sense of how much heat it will add at a particular RPM and boost level.

A faster way to estimate the best pulley size to use is to ask what others with a similar application are seeing. This doesn't have to be an exact match, but you can use it to get a reasonable start. I've seen boost levels vary by 2-3 psi between identically sized engines depending on the throttle body, valve, header, muffler, and inlet air flow (to the blower) characteristics.

Just remember, it is all about air flow. Your goal is to flow the most amount of air at the LOWEST boost you can because boost = heat (and heat = a less dense air charge and a higher likelihood of engine knock).

Sorry if my answer isn't very helpful in this particular case (pretty much - ask someone who's running your blower what kind of boost they're seeing). Sometimes it helps to understand the interactions at play. I'd give some guess from my own experience, but it has all been with Eaton blowers and on Miata engines.

Have fun. Blowers give you the feel of an instantly larger engine!
Mike
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Last edited by mppickett; 03-09-2008 at 03:46 PM..
Old 03-09-2008, 03:42 PM
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I've done the exact thing that your describing. What type of supercharger are you running? It really makes a big difference. I am running a SOK Paxton that puts out about 6 lbs of boost. The Whipple's will out out 8-10, depending on the pulley size. The main difference between the two, is that the Whipple is more on and off, making full boost right off the line. This translates into more low end power and torque, but much harder on the 915. Whereas the Paxton is more linear, building boost with rpm. Kind of like a turbo with zero lag. Much easier on the drive train and it wont effect the driveability of your car in any way shape or form. Your car will run, idle and drive like a normal 911, only when you put the pedal down you have around 350 hp on tap. I feel that the Paxton still allows you to retain that "Porsche" driveability and feel, only with much more power. FWIT I run a 3" pulley on the Paxton and it puts out 6lbs of boost at 6000 rpm on my 3.2. You are also going to have to play with your fuel pressure until you get it right. Make sure you have an airflow meter installed before you start.
Old 03-10-2008, 09:19 PM
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Thanx everyone,

I see exactly what you are saying about the boost PSI being hard to calculate because you dont know what the restrictions are. If I had a cam with more overlap, it would change the flow characteristics of the engine, and hence the back pressure at the supercharger.

I know of one other person with a SoK supercharged 3.6l porsche. He is running about 6 psi of boost.

The supercharger that SoK put in the 3.6s (964) is the whipple type.

Sadly since I posted this message I have run into another problem.
Although SoK is no longer in business they agreed to machine the pistons down to 9:1 for me. (This was about 6 months ago) On the weekend I CC'ed the heads / dome to calculate the compression ratio, and I came back with ~10.5:1.

So obviously SoK didnt take anywhere near enough off the pistons to get down to the target CR of 9:1. After the pistons were machined I had thermal barrier coating put on the crowns and dry film lube on the side skirts.

Ask me how happy I am today?

I am still trying to decide what to buy for measuring A/F ratio, boost, etc. Maybe something from Innovate. Any suggestions?

Thanx,
Kirk
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Old 03-11-2008, 06:48 AM
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To reduce the compression ration one trick I have tried successfully is to use the turbo cylinder barrel shims under the cylinder jugs where they meet the case. Normal shims are copper and 0.020 thick, I think the turbo shims are 0.050. This will effect your deck height and lower compression, or figure out how many shims you will need and possibly stack them. You cannot go too far here because all this will effect your timing chain housing sealing with the camshafts....but it may be enuff.
Fred
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Old 03-11-2008, 07:44 AM
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Hello. I'll try to help you. What you have in front of you is compressor efficiency map. It's farely useful as it indicates how much air supercharger can flow w/o being inefficient. Then it's a matter of calculating how much air your engine will swallow at certain boost/RPM and plotting it into the map.

Typical 300hp ROW 3.3 engine running stock boost will gulp around 0.26kg of air per second at pressure ratio 1.7.

Your graph is in m^3 but we can assume that 1 m^3 of air weights 1.29kg at zero deg. C and 1 bar athmospheric pressure. It means that 0.26kg air is .26/1.29=0.201 m^3. Muliply it with 60 sec and you get 12.1 m^3 per min.

If we assume same VE for 3.6L engine and decide to take out 411hp at 6000 RPM and 0.9 bar, it crunches down to roughly 0.35kg/sec and 16.28 m^3 air per min.

I plotted red dot in the graph for 300hp and green dot for 411hp.

Red:
3.3L, 300hp @ 5500 RPM
17kW (23hp) to drive the blower
65% supercharger efficiency
93 deg. C discharge temperature
roughly 8000 RPM blower revs (numbers are missing in graph, i assumed this by counting the lines)
345cc injectors are needed assuming 80% duty cycle
total fuel flow 100L/h at target fuel pressure (presumably 3 bar+boost).


Green:
3.6L, 411hp @ 6000 RPM
27kw (37hp) to drive the blower
64% supercharger efficiency
110 deg. C discharge temperature
roughly 11000 RPM blower revs (numbers are missing in graph -||- )
460cc injectors are needed assuming 80% duty cycle
total fuel flow 133L/h at target fuel pressure (presumably 3 bar+boost)

These numbers are calculated by using apprximated VE for stock 930 engine. YMMV.



So your supercharger is good for roughly 500hp but if you use it in normal fashion (300-400hp, 0.7-0.9 bar) it will be dead center on it's efficiency map and use between 25 to 40hp of power (to be substracted from engine).

So now it's just a matter of using the pulley that gives you boost you choose.

I used VE that is comparable to 3.3 930. Yours might be little higher, due to bigger ports etc.

When it comes to WBO2 equipment, I recomend Innovative LM1 and LMA-3 (AuxBox). Just buying a box that logs AFR's only won't help you much. You need something that will log AFR, RPM and MAP to begin with. LMA-3 can do this. Reasonable compression ratio to use with 0.7-0.9 bar of boost would be around 7.5:1 to 8.5:1, depending on fuel used and ignition curve.

P.S. Good turbocharger will have best case efficiency of 70-75% in best case and will rob less power from the engine. Now you understand why we don't see that many superchargers in OEM cars But if supercharger is what you want, Lysholm is probably the best supercharger around!
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Thank you for your time,

Last edited by beepbeep; 03-11-2008 at 12:07 PM..
Old 03-11-2008, 11:46 AM
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Old 03-11-2008, 05:37 PM
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