Supercharger boost calculations?
 

Although I'm out of the country on business until early December, I decided my winter project will be to add a supercharger, SSI's, and a better 2-in-1-out muffler to my '83 SC. I'll be running about 4-5 psi and 93 octane gas. I've got a simple supercharging system put together. I'm waiting until I get home to start the work.

So here's my question: When calculating the theoretical HP gains from both improved exhaust and boost pressure, how does the equation work? Is it like this:

Stock HP (about 170-ish?) * (boost * .06802) + exhaust improvements = total theoretical HP gain

Or is it written this way:

Stock HP + exhaust improvements * (boost * .06802) = total theoretical HP gain

Make sense? Any thoughts?

I don't know the answer to your question, but I have worked many years on aircraft turbo chargers and super chargers. I prefer superchargers[not the first option for most porsche owners] and I am not sure why? Please us posted with your progress. I am intersted in supercharging my 87 3.2L. Terry

I looked at turbos first, probably because of the 930. Once I looked at the supercharger option, I found out it was easier and cheaper to implement. At 5psi of boost, it's an optimum setup because there's no need to intercool and the HP returns are more optimal that those with greater boost (you get diminished return over 5psi).

I'll keep everyone posted with my results. I have access to a crude dyno setup so I'll post those results as well.

I guess I'll have to answer my own question since I'm not getting any replies here or in the 930 forum. Maybe it'll be my contribution to this forum.

would you mind posting the details of your "simple" system, including filling us in on parts and cost? I'm pretty curious as I'm not that well versed in how such a system would work or be implemented.

Thanks! Looking forward to seeing how the project unfolds.

I have a C32 mercedes supercharged and it has a water intercooler with a separate pump. Anytime you pressurize air the temperature goes up wether you use a turbo or a supercharger. The main differences are a supercharger operates at lower revs, it eats up horsepower whereas a turbo doesn't as it runs off the exhaust. I wouldn't add any type of compressor to any engine that was not built to withstand the added compression.

Certainly. Let me get the project together once I get back to Chicago. If it works, I'll post full details including pictures, dyno stuff, etc. If it doesn't, consider me the guinea pig that saved everyone the trouble.

How much boost does your C32 run? I'll bet it's more than 5 PSI. Every supercharger expert I've spoken with and every single bit of documentation I've read so far suggests 5 PSI and below with no intercooler is perfectly acceptable and functional. The Superchargers of Knoxville system designed for the SC didn't use an intercooler. The BAE turbos for SC's didn't use intercoolers either. Or the early 930's.

I'm sure I'll get lots of couldn't and shouldn't comments. That's something forums are definitely good for.

Maybe not that technical but isn't one lb of boost worth around 15 hp? I think a more critical piece of information is determining what pulley ratios you will need to create the desired level of boost while keeping within the blowers max rpm. I would suggest finding a supercharger unit that puts out around 7 psi at its max rpm so that you can step it down a bit with pulley size. You will need to find the step up ratio in the blower. Example: setup up ratio is 4.4 to 1 times that by the crankshaft pulley size 5.25 divided by blower pulley size 3.50. Take this amount and times that by your rpm shift point (lets say 6200 rpm) This is your impeller speed. Hope this helps. :)

It is a myth that turbos don't require hp to run. Both turbos and superchargers take hp. Read "maximum boost" or other books like it.

Also, SoK (superchargers of knoxville) made a kit for the 3 liter that put out about 300 reliable hp without internal mods to the engine. They don't sell it anymore but I have a lot of info from them. PM me if you want it.

Also check out these threads...

http://forums.pelicanparts.com/911-engine-rebuilding-forum/451191-rebuilding-supercharging-3-0l.html

http://forums.pelicanparts.com/porsche-911-technical-forum/444521-supercharged-race-car-build.html

Also keep in mind that the 915 tranny can only take about 350 ft-lbs of torque and that's with all the wevo mods so supercharged motors will destroy these trannys faster (more low end torque.

Thanks. Yes, it does.

I was under the impression that the amount of HP increase is dependent upon existing horsepower; in other words, it's always a percentage of the engine's existing HP. On a 170 HP engine (the specs I've seen put the '83 SC engine's peak at 204 HP around 5K RPM's, and I haven't seen any indication of whether that's RWHP or FWHP) you'd see a theoretical boost of 6.802%, or 11HP. On a 3.2 that may work out to about 15HP? Not sure how much HP the 3.2 has.

SuperchargersOnline.com :: Supercharger Horsepower Calculator

How's your set-up? At 10 PSI I'd assume you're running an intercooler?

That would be helpful. I'd like to see how they did the plumbing and pressure relief stuff. PM sent.
Yes, I'm aware. It's another reason to keep boost low for this application. This summer I happened across a rebuilt 3.3 turbo motor for a steal of a price (desperate seller). I turned it down because of the massive torque the thing put out. It was just too much for the 915 to handle. I'm not looking for something to outrun my S8. :D

I am running a modified centrifugal Paxton unit which puts out 10-11 lbs of boost at its max rpm. I also have a small intercooler which keeps inlet temps safe. You are correct that the boost is a percentage and it works out to be around 15 on these engines. I try not to put the cart before the horse and simply set up a system to deliver the needed boost at a safe level. In term of hp gains, the chips will land where they land. Additionally the 915 is a fairly robust gearbox provided it is well sorted and you avoid too many drag races. The type of supercharger you choose also plays a big factor. A roots type will deliver tons of instant boost, translating into more low end torque. (much harder on the 915) This is why I chose a centrifugal unit that builds boost progressively. Four years so far and my 915 is still going strong.

Turbos require energy but they basically run on the waste energy in the exhaust (heat and sound). There is some hit from the back pressure but that is mostly countered by the increased intake pressure and it isn't anywhere near the hit you get from driving a supercharger.

Some of the considerations for going supercharger...
- The crank pulley was designed to run the fan and alternator. You are now going to pull a few times that off of it. (I haven't heard of issues relating to the crank snout so it is probably great, especially at the conservative boost being discussed.)
- An engine is a heat pump and the heat efficiency of the gasoline internal combustion motor resides in a narrow range. In other words: Double the power and you need to double the cooling.
- A naturally aspirated motor runs with an intake pressure of 15psi (sea level). Increasing the intake pressure by 5 psi will tend to increase the power to 133% of the naturally aspirated level minus the power required to run the supercharger.
- Boost can tend to gloss over intake and exhaust port inadequacies. In other words: The percentage of power improvement for a fixed level of boost will tend be higher for a motor with badly designed (restrictive) intake and exhaust. You can still get improvements from intake and exhaust but the percentage of difference will be different.- Fuel delivery can become critical. The intake tract is generally larger and acceleration enrichment becomes important. With a positive displacement supercharger (like a Roots) the intake tract is sealed and a backfire can be more like a bomb.
- Compression can be an issue and changing compression ratios is expensive in a Porsche...
- Superchargers can be heavy depending on type.
- Acceleration is instantaneous. (nothing but smiles:D)

Remember that the majority of those experts didn't factor air-cooled engine and the effects of cylinder head temperatures.

Thresholds of detonation vary with cylinder head temps, ambient temps, charge air temps, AFR's, ignition timing and a few others so factor that into the equation.

I've seen my share of broken parts in non-intercooled boosted motors so Caveat Emptor on that one.

I don't know about any formula, but a typical supercharger option seems to add about 25% to 30% to the horsepower rating according to manufacturers. So an SC might be in the 225hp range?

Like the other posters, I hope you post some photos and details. A before and after dyno would be great.

I wouldn't run any level of forced induction on these (aircooled) cars without:
a) an intercooler
b) enhanced cooling [smaller fan pulley, water spray, etc]

The back pressure is what I was talking about, as this restricts flow and essentially the torque to make hp.

Also cam selection is important with forced induction. If I remember the SC cam has a lot of overlap (I could be wrong with this) which means bad for turbos but better for superchargers. Small overlap cams with turbos can have some small restrictions too. It's been a while since i've read up on this stuff so be easy on me.

Also, if I remember correctly, the m62 eaton supercharger only takes something like 15 to 20 hp at max rpm to drive. I think this is 8k rpms with a 2:1 pulley system, so at lower rpms the hp hit is less.

I remember reading that porsche is using a supercharger on their hybrid cayenne now. Kinda interesting. I like turbos for racecars but SCs for street applications, unless you have the money for variable vane turbos like on the 2005 and newer porsche turbos. now THOSE are cool.

If all your going to get with 5psi and all the expense your looking at, to acheive 225hp. net. There are cheaper and more reliable ways to get that than supercharging.
With a properly modified 3 or 3.2 you could get close to 240 reliable HP

So you are saying that for the same cost outlay to add a supercharger you can modify a 3.0 or 3.2 and accomplish a greater gain?

Packy lists links to my build above. Here's my response from the other forum...

It is not cheap nor easy but a satisfying engineering exercise.

If you are trying to get 260hp you can do that through extensive but cheaper mods than turbo or supercharging.

I'm just one experience but I have actually done this...

a cheap easy source of water to air intercooling is these guys...

Air to Water Intercooler

they have tons of stuff at great costs.