Supercharger Project

I think I'm just not getting my point across very well. Here are the compressor maps for the MP90 roots blower I used with my first set-up. I ran a drive ratio of 2:1 so you can pull the number off the maps in a form that makes sense to you. It made about 9.5 psi at 6800 and 10.25 at 7700 with the inlet air temp 260+ F...260 is as high as my sensor would read. Remeber my engine is a QV.

The second key point is that the engine flows more air at 6800 than it does at 7700. Even if your number are right and it will flow 40 lb/min at 7700 and 10 psi, it's flowing 44-48 at 6800, dropping the boost to 6psi. That means the HP calc is wrong. The right yeild is about 288, well under the goal. It's works out better with the V1 compress becaue the flow lines are flatter than the V9 in that range.

I don't have the maps for the current crop of twin scew units, but they claim 98% VE and 80% AE, so better than the best centrifugals, not that it matters.
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Scott,
I still don't think you're getting it. The positive displacement units are rated in INLET cfm. That's CFM at ambient conditions, efficieny is not in the equaltion yet, it's the same for any compressor. To get 9.5psi manifold pressure at 6800 rpm, I needed about 620 inlet cfm, which is about 46 lb/min of air. Now because the efficiency is low on a roots, the temperature was about 260F. With a more efficient compressor that had a discharge of say 180F, the manifold pressure would drop to about 8.4 psi, but it would still be 46 lb/min and 620 inlet cfm of air.

I you want, Monday I can post the compessor map for the compressor that made the dyno graph I posted, so you can compare flow in any form you choose to measure it to boost.

 

Scott,
The outlet cfm is a very very hard number to work with any compressor. Let me try again.

HP is produced as a direct result of air/fuel mixture being burned. A bigger fire in the same engine must make more hp. The size of the fire is directly related to the mass flow of air and the mass flow of fuel is (or should be) directly related to the mass flow of air.

It does make some difference where the air comes from due to parasitic losses. A turbo eats something like 5%, a centrifugal or screw SC something like 10%, a roots maybe 12%, something like that with all delivering equal mass flows of air. Now here's the part I think you're getting all wrapped around, the boost pressures or PR will not be equal unless the compressors are identical efficiencies. The higher the efficiency of the compressor, the lower the temperatures rise and therefore the lower the boost pressure for a given mass flow.

What type of compressor is used to generate the mass flow doesn't come into the calculation anywhere when you are talking about a single data point, say 6800 rpm. The compressor type comes into the equation when you pick a second point, where that point is relative to the first point, and what the engine is doing at the second point relative to the first.

Efficiecy is efficieny, again it doesn't care what type compressor it is. A screw type will generate heat at low rpm, but because it is compressing, a centrifugal does not, because it is not compressing. At high rpm, the compressor with the higher AE rating will make the least amount of heat and require the lowest shaft power for any given mass flow of air. In order from low to high they go root at about 40-50%, centrifugal at about 70-75%, screw 70-80%.

There are 2 reasons my flow data don't match what you calculate. The first, which I just thought of is that I switched to EFI making my base hp # is about 260hp, so my boost is a bit lower at the specified mass flow rates then a stock engine would be. The second is blow-down.

Blow-down isn't in the calculations anywhere because it is assumed negligible, but on a 308 it is not negligible. About 30% of the air that goes into the engine ends up going straight out the exhaust pipe unburned. That happens because ferrari uses a very low lobe separation angle, long ramps on the cams for the lift they run and the exhaust flow rate is very high compared to the inlet flow rate. Together they add up to an engine that pumps very well, but still has a relatively low VE…because a lot of air gets pumped into the exhaust. The compressor needs to be big enough to deal with the blow-down or it just won’t make boost. I'm going to leave it at that, let you do what you think is best on your system, and not speak of this again.

Getting away from theory, it occurred to me that since you are going blow-thru there is going to be at least 1 psi, probably 2psi going from the blower to the manifold. That means that if the engine will tolerate 10 psi, in the manifold, you’ll need 11 or 12 at the blower to get it. The numbers I supplied are all manifold pressures.

On an engine that makes peak hp well below redline like a 308, and fitted with a centrifugal compressor, a pressure relief valve is about the only way to make good boost at the hp peak without obscene boost at redline. The other solution is a variable ration drive, and while elegant, it is a bit complicated and expensive.

 

Scott,
It looks like you are using a constant of 14 ft^3/lb and that is where the problem is coming in. If you change temp or pressure, then the ft^3/lb change. There are charts where it's all worked out, but just adding a P1/P2*T1/T2 to it should get you really close.

Burning a fixed amount of air/fuel MUST produce a fixed amount of power and tghe comverse is also true, to make a given hp, a fixed mass of air/fuel is required. A calculation that yeilds a solution other than that must contain an error. The energy must balance, Ein=Eout.

The second error is the engines NA air requirement, there is a term missing. It needs either a pumping term of a loss term to account for air that is pumped, but is not burned because it escapes before the exhaust valve closes. On most engines that number is very low and can be ignored, but on a 308, it's big and goes up as boost goes up.

 

It looks like you have the pressure accounted for, but not the temperature because i don't see it and as soon as I plug it is, all the mass flows are within 1 lb/min of each other like they should be.

Ein=Eout always, that is the 1st law of thermodynamics. The out isn't always what you want it to be (friction and such), but it's alway there. What's true is that the process is not reversible because of friction and such, that's the 2nd law if I recall. With equal mass flows, all the superchargers options will produce equal grossoutput hp from the engine, but the net hp will vary by the efficiency of the compressor with the lower efficiciecny unit yielding a bit less net hp due top higher shaft work required to turn them.

Blow-down is not included in VE. VE is the air/fuel mass sealed in the cylinder for combustion divided by the mass/fuel mass that should fit in the space at the give ambient conditions. Blow-down shows up in mechanical efficieny in that pumping air requires shaft work, but air that escapes unburned does no work. It would show up in a full Ein=Eout analysis (with is really really hard to do), but it is not in VE.

 

You're accounting for the temperature rise altering pressure to calc to allow you to calculate pressure correctly. So, when you calculate mass flow, you are using the correct pressure. But the mass flow calc requires 3 variable, CFM, Pres, Temp and you don't have the temp plugged in. At the end, you need to correct the cfm for the temp rise (you already have it corrected for press) or vary the cfm to lb/min conversion factor accordingly. Try it, you'll see the mass flows are the same for each compressor.


VE, like any efficiecny is what you got divided by what you expected. It is the amount of air you are getting into the cylinder divided by what should fit at ambient conditions. Air that goes out the exhasut pipe in by definition not in the cylinder and therefore not part of VE. Air that doen't get into the cylinder due to an intake restriction is basically acconted for in VE, because it need gets into the cylinder, in this case the VE and Pumping Efficiecny are reduced. However air that goes in and staight though, does lower the VE, but was pumped so the PE stays high, this is the case with a 308. It pumps more air than it's low VE would lead you to believe because it's PE is high.

To calculate the blow-down, you simply use the data point I gave you. The measured flow (I gave you) minus the theorectical flow (what you calculated) = the blow-down.


When you are looking for a mass flow, the only thing the compressor maps do at a given point is tell you what rpm to spin the compressor and what temperature to expect the discharge to be. It makes no difference where the mass flow number comes out on the map as long as it is in fact on the map showing the compressor is cabable or delivering it

The maps become important when you try to estamblish a curve for mass flow at every rpm based on the initail point you fixed on the map. The other place they are critical is to allow you to determine what mass flow you will get at conditions the engine will tolerate. So far we have been talking about hp at a given mass flow, but the truth is the engine very much cares about pressure and temperature to keep it from detonating. The maps let you pick a max mass flow to stay under the detonation theshold of the engine.

 

Scott,
Dissagreeing is for politics. You have a 1st princilple equation that isn't balanced, that makes it wrong IMO.

Everything you do is correct up until the last line where you calculate lb/min. You are dividing your CFM (which is correct as far as I can tell) by 14 to get lb/min. The error is that 14 applies at about 80F and you have calculated the Pr and therefore the CFM based on hot air, and you even specify that in your step 7, that means 14 doesn't apply until your correct the CFM back down to ambient temp.

I guess in the end it doesn't really matter, the correct lb/min is 32 vs your 34, which really needs to be more like 45 due to blow-down...so your error is in the right direction anyway. The only thing you can't do with the error in you calculation is use the data I provided, which really only means an extra iteration on pulleys size since the compressor you're looking at are big enough to flow the air you're going to need, once you get the right dirve ratio.

 

Here's another way to try the math...and it's a little easier than the way you're doing it.

You want 325hp, you haves 205, so you want 1.585 times more power.

To get it, you need 1.585 time more air. You calculated 280 scfm, so you need 443 scfm to make 325hp.

443/14= 31.7 lb/min, just like you get after all your calculations and the temp conversion you're missing.....but it's still wrong because of the blow down.

 

Or better yet, start right of with the mass flow.

280 cfm = 20 lb/min

20x1.58= 31.7 lb/min of air to make 325 hp

That's as simpe as it can get .

 

A Mondial owner asked me to pop onto this thread and see if I could help.

I have designed, developed and installed supercharger kits on every Porsche 928 from 1978 to 1995, many of which are CIS, K-Jet cars like your 308. Some have said I was the first to ever supercharge the CIS fuel system (I do not knwo about that) but I do know that we are the only ones currently supercharging CIS engines.

Do you need some help with this? I should be able to keep you from making some simple early mistakes in your design.

I'm located in Horicon, WI (about 1 hour North of Madison, WI) and if somebody would like to drop off there 308 for the winter, it'd pull out in Spring with a blower on it!

Check us out here: www.928motorsports.com

 

Welcome aboard, Carl!

Maybe you can help clear up the most debated question here so far... How many cfm does a 928 take to get about 325 hp at redline?

Mark is estimating around 58 for a 308 from the emperical data he has from prior supercharging projects, and Scott feels it's more like 40 from the calculations he's done using data from Vortech. My numbers are somewhere in between.
Thor
www.JavelinArt.com

 

I should say, I've no statements about 928 cfm, only the 308QV which I believe apply to 328 and 308 2V engine in proportion to base hp.

Carl,
I took a look at you sight and the graghs look really good, significantly better than other engines I've seen using centrifugal superchargers as far as far as a wide usable power band.

I have some question's, when you say 6 psi, is it manifold I'm guessing? Do you have a boost curve or boost at various rpm points? And last, do you have the compressor discharge pressures at the same points?

 

I think I got an answer to my power curve shape question after finding baseline 928 graphs....928s have torque curves that peak very early then drop progressively which is a very nice match to the centrifugal compressors nothjing down low then boost that builds progressively yielding a much better looking curve than would be expected and not at all similar to 308 base curves.

I'm still interested in the boost at rpm points at the manifold and at the compressor if you have it.

 

I'll try and answer the 2 questions you have posted so far.

I thought I would receive emails from the Forum when people had posted, I know now that I do not because I have not purchased a subscription. So if you need me or an answer more quickly - try us at 920-485-0488 or [email protected]

In order for me to answer "how much boost we will need" I need to know the Discplacement of the 308 CIS motor, and the Compression Ratio too.

A lot of novices with small V-8's reach for a Vortec V-1 in S or T trim right away, and at 1200 to 1400 CFM (Cubic Feet per Minute) they have over-blown their application, and end up chasing lean fuel conditions FOREVER and they will not likely ever catch up.

Selecting the right supercharger for the application is critical - we are looking for a match to about 120% of need, and no more.

Example: on the 4.5 Liter Porsche V8 (a mear 262 cunic inches) a blower that produces 850 CFM is plenty, and more is not better. This motor produces 220 HP at the crank stock, and we raise it to 300 HP with only 6 psi, and 340 HP with 8 psi.

On the bigger Porsche 9228 engines - (1986-1995) 5.0 liter - we use a 1200 CFM blower and routinely push 400+ HP.... but they are not equipped with CIS.

Anyway - hope that helps. I cannot really state what size blower you need until someone gives me that information. There are a lot of variables - camshaft lift and duration, valve size, combustion chamber shape, exhaust system restriction, the WUR and fuel distributer, size of the fuel lines from the tank to the fuel distributer, flow of the fuel pump in LPM, and more, but I should be able to get you close.

 

Scott,
Did you look at the V5-F? it's about the same dimensions as the v-9, but the specs say it's a bit lower flow and higher efficiency than the v-9, although I haven't seen the actual map....the v-9 might work out better.

I was just thinking that it might work out good if after 6800 rpm you get in to a range of decreasing efficiency where flow vs rpm starts to drop a bit as the unit approaches choke....maybe limiting the boost rise as the engines volumetric efficiency and flow drop. Just a thought. The specs say the v-5F flows about 700 scfm vs 800 for the v-9. 700ish scfm (that's at the inlet to the blower, so before the terrible efficiency heats up the air and expands to a lot more cfm) is what that first blower I tried and it had enough for the QV with efi.

You probably already look at the 5....just a though.

 

Makes sence

That's certainly the nicest way to do it. I kept the stock one and drilled and pinned 3 holes in the base of the hub to center and drive the pulley.