So I've been pondering, as engineers often do, on how to increase the boost window on my CTSC without creating an extremely peaky engine which will destroy my clutch and be unable to fuel with my stock injectors.

The idea of using a wastegate has been thrown around, and seems to be successful. Thinking about this, I think this would be a huge waste, since we are still compressing the air and bleeding it off, forcing the supercharger to pressurize additional air that we would never use.

So here's the novel idea: you know how when you partially block the hose of a vacuum cleaner, the compressor revs up (noise gets higher)? This is because you have created a restriction on the intake side of the compressor, before the compressor. This creates a slight vacuum before the compressor, which actually makes it easier for it to move the air. My idea is to create a restriction before the supercharger, essentially limiting the max CFM the supercharger is allowed to flow.

Here's the data: I would use a pulley capable of producing 13 PSI (a pressure ratio of 1.88). Using a compressor map I hope is correct http://www.superchargerhelp.com/showthread.php?t=8215, I come up with 621 CFM at 8500 RPM, which should translate to a safe impeller speed of 48k RPM.

Of course, this would be far too much power for the stock clutch and injectors (approaching 400 HP), so I would need to limit the CFM the engine is capable of flowing.

The 3.5 inch intake provides no restriction for the CFM values this supercharger is capable of. Using a goal of having 8 psi at 8500, we can solve for the correct intake restriction in order to have this much pressure reaching the engine.

We know we in order to have 8 psi at 8500 RPM, we would need to flow 509 CFM. Using this calculator http://www.flowmeterdirectory.com/flowmete...ifice_calc.html we are able to plug in the proper values. To find out the desired intake restriction.

The pulley isn't changed, so the pressure ratio stays at 1.88. In order to have 8 psi instead of 13 psi at the engine, we must create a vacuum within the inlet of the supercharger. How much vacuum? Enough to see a 5 psi drop after the supercharger, which would be equal to 5 / 1.88 (desired pressure loss/pressure ratio of SC). This comes out to a restriction to cause a vacuum of 2.66 PSI (P1 - P2 in the calculator).

Values are:
D1 = 3.5 inch
D2 = ? (This is the restrictor diameter we need to find)
P1 = 14.7 psi (Atmospheric Pressure)
P2 = 12.04 psi (Needed for correct pressure drop after SC)
P1 - P2 = 2.66 psi
Desired CFM = 509

The calculator is set up in such a way that we need to plug in different values for D2 in order to arrive at a value close to 509 CFM. After a bit of trial and error, a restrictor diameter of 2.12" is about as close as we need to get, providing 510 CFM and achieving the desired pressure drop post SC to achieve 8 psi at 8500 RPM.

Why is this better than simply running a wastegate? Simplicity, for one. Adding a restrictor orifice to the intake would be extremely easy (and easy to tune, make hole bigger to increase boost). Likelihood of failure is also much less (wastegate stuck closed equals overboost - restrictor plate physically cannot overboost as long as it's in place). Finally, we are no longer compressing air that we would dump anyway. By tuning the intake orifice size, we create a pressure drop before the SC that allows it to pump more easily (pumping against a vacuum is easier) and no longer require the SC to pump over 510 CFM, which would all be wasted energy at a higher adiabatic cost on the compressor chart.

Any thoughts? Of course I simplified quite a few calculations, assumed 100% volumetric efficiency and air temperatures are 60 degrees F.

 

a dude literally just went through all f this by trying to use a restriction valve. search, you will find the thread.

 

So he did!

https://www.s2ki.com/forums/index.php?showtopic=820097

I wonder if he succeeded....

 

So the net result would be more boost at lower rpm... creating a similar effect as say a roots charger yes? Since you are running engine management/E-manage... you could likely make this feasible... since you will need to retard timing appropriately for the amount of boost you make at lower rpms... and increase fuel duty... which the comptech fpr will not be able to achieve alone. I am still on the fence as to whether the stock injectors with the E-mange will be able to provide you with enough duty for the boost you will make at low rpm, you may find that you need those larger injectors. Just have to try and see. For anyone else running just a stock Comptech system without engine management, this is a pipe dream. Of course then always comes the next question, if you have engine management... why not just let the blower fly?

I am always thinking for reliable ways to get the most for the least ...and so far my system reflects what I have found to be had. I am still thinking...

I appreciate the fact that your looking for cool new ways to up performance, though some have been thought up before, maybe someone will think of something new that will work well. The process is part of the fun for me. Of course good results are fun too. Keep up with it

 

Essentially, yes. More boost at a lower RPM. My knowledge of fluid dynamics can only take me so far, but I can do some more BS theoretical calculations

As you know, duty cycle is a function of how much air is being ingested. As horsepower increases, so does duty cycle. So I do not think I will need new injectors unless I make over what is 8 psi at 8500 RPM (510 CFM). From some rough calculations, I imagine boost would look something like this:

RPM PSI
1000 0
1500 0
2000 .5
2500 1
3000 2
3500 2.5
4000 3
4500 4
5000 4.5
5500 5.5
6000 7
6500 8
7000 9
7500 10.5
8000 9.5
8500 8

Compared to estimated boost with a regular 8 psi pulley:
1000 0
1500 0
2000 0
2500 .5
3000 1
3500 1.5
4000 2
4500 2.5
5000 3
5500 3.5
6000 4
6500 4.5
7000 5.5
7500 6
8000 7
8500 8

For a difference of:
1000 0
1500 0
2000 .5
2500 .5
3000 1
3500 1
4000 1
4500 1.5
5000 1.5
5500 2
6000 3
6500 3.5
7000 3.5
7500 3.5
8000 4
8500 0

As you can see, it's no roots blower, but it definitely blows the standard centrifugal boost curve out of the water. Take note that the boost actually is above 8 psi for part of the range. That's due to the fact that the restrictor is a CFM/HP limiter, not a boost limiter.

 

I had thought of something like this before too. Would be great as a broader powerband would be great!!