Warp,

When doing calculations on power required to do x work, mass flow and pressure ratio is used in conjunction with efficiency. So for a compressor (Paxton in your case) you have to look at what impact reduced mass flow will have at x RPM (actually tip speed in ft/sec) on efficiency.

Now the Novi 2K appears to have a very wide stability and operating range. This helps when used in conjunction with restrictor disk. You do not want to push compressor stage too far out of efficiency island.

First thing we may want to look at is turndown, a centrifugal supercharger can easily have 10:1 or operate from 6500 down to 650 engine RPM. Centrifugal compressors are not known for their flow capabilities but their high pressure ratio per stage. Or they generate pressure better then flow. Axial compressors flow much better but cannot generate good pressure per stage. Now look at most compressor maps, low pressures (pressure ratios) are simply not plotted. Turbo-machinery (centrifugal supercharger) when running down in the mud (very low RPM/tip speed) simply do not flow that well. On low boost application or when overall speed is low it takes longer to get compressor wheel to flow well. By adding a restrictor disk and over-spinning a bit the compressor gets into more efficient range faster, but on high boost disk is not necessary. What I’m trying to get at is flow is increased more so then boost and this is supported by people reporting better performance at lower RPM. With flow you also get boost but think flow is dominant variable when discussing part speed performance.

If any of the above made sense then I’ll continue…

You are going to take an efficiency hit running with a restrictor, although mostly up top. The gains in part speed operation (daily driving) will outweigh losses up top. Now if you race allot or believe life begins at 4500 RPM then skip disk.

Losses I can run the numbers but will take some time. I’d have to do a quick study on say a 4 inch tube with 3.75, 3.5 and 3.25 disk as an example and use them to generate appropriate boost curve that ends at 10 PSI or some number. Off the top of my head your losses are likely very reasonable at maybe 5 or so HP additional parasitic load up top (~6000 engine RPM). Losses are mostly additional drag do to higher RPM and compressor wheel eye (inlet face) and passage being a little off design.

Hope some of this made sense,
Mike
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Some more info:

Pipe = 4 inch ID and Disk = 3.75 inch opening

Pressure drop in PSI
10 lbs/min = 0.00, 26.53 ft/sec
20 lbs/min = 0.02, 53.05 ft/sec
30 lbs/min = 0.04, 79.58 ft/sec
40 lbs/min = 0.07, 106.10 ft/sec
50 lbs/min = 0.11, 132.63 ft/sec
60 lbs/min = 0.16, 159.16 ft/sec
70 lbs/min = 0.22, 185.68 ft/sec
80 lbs/min = 0.29, 212.21 ft/sec

Pipe = 4 inch ID and Disk = 3.5 inch opening

Pressure drop in PSI
10 lbs/min = 0.01
20 lbs/min = 0.04
30 lbs/min = 0.10
40 lbs/min = 0.18
50 lbs/min = 0.28
60 lbs/min = 0.41
70 lbs/min = 0.57
80 lbs/min = 0.76

Pipe = 4 inch ID and Disk = 3.25 inch opening (almost like stock TB)

Pressure drop in PSI
10 lbs/min = 0.02
20 lbs/min = 0.08
30 lbs/min = 0.18
40 lbs/min = 0.32
50 lbs/min = 0.51
60 lbs/min = 0.75
70 lbs/min = 1.05
80 lbs/min = 1.41

Pipe = 4 inch ID and Disk = 3.00 inch opening

Pressure drop in PSI
10 lbs/min = 0.03
20 lbs/min = 0.13
30 lbs/min = 0.29
40 lbs/min = 0.53
50 lbs/min = 0.85
60 lbs/min = 1.27
70 lbs/min = 1.79
80 lbs/min = 2.47

80 lbs/min is approximately 800 FWHP…

So a stock LS1 pushing around 10 PSI will consume around 60 lbs/min with a pressure ratio of 1.68:1. To look at effect of restrictor disk, boost= PR*Actual_Inlet-14.7.

3 inch example;
1.68*(14.7-1.27)-14.7 or 7.86 PSI of boost

3.25 inch example;
1.68*(14.7-0.75)-14.7 or 8.74 PSI of boost

3.5 inch example;
1.68*(14.7-0.41)-14.7 or 9.31 PSI of boost

As can be observed from above restrictor disks are not that bad down in lower flow region and not much of a hit. It’s up top the drop becomes noticeable. With say a 3.5 inch disk blower can be increased a few 1000 RPMs and still make same boost, but part speed or cruising will improve.

Mike

 

Part 3 of tech question…

As mentioned above restrictor disks allow user to over-spin supercharger and put it in a more efficient operating range earlier, especially at lower boost levels (stock engine).

Below are calculations on a “theoretical” supercharger with constant efficiency (not so in real life) and that does not run out of steam up top. First boost column is with no disk, then other 4 are with varying disk sizes from 3.25 to 2.5 inch ID or passage.

The 2.75 inch disk at 4000 RPM showed a loss or dip in boost and did double check. Optimal values would take some testing but likely around 3 inch perhaps a bit less assuming 4 inch tube ID.

Engine RPM, Boost no disk, Boost 3.25, Boost 3.0, Boost 2.75, Boost 2.5
1000, 0.26 PSI, 0.26 PSI, 0.27 PSI, 0.30 PSI, 0.33 PSI
2000, 0.95 PSI, 0.97 PSI, 0.99 PSI, 1.11 PSI, 1.25 PSI
3000, 2.18 PSI, 2.23 PSI, 2.23 PSI, 2.55 PSI, 2.86 PSI
4000, 4.05 PSI, 4.14 PSI, 4.94 PSI, 4.66 PSI, 5.15 PSI
5000, 6.67 PSI, 6.75 PSI, 6.85 PSI, 7.43 PSI, 7.81 PSI
6000, 10.13 PSI, 10.13 PSI, 10.13 PSI. 10.13 PSI, 10.13 PSI

As can be seen from above at part speed (daily driving) flow is going to be increased more so then boost. Also in real life boost will taper off more up top then what my numbers indicate.

ATI/Procharger will act a little differently do to very little diffusion, but know Vortech and suspect Paxton will be somewhat close. Oh overdrive was about 29% from no disk to 2.5 inch restrictor and overdrive to 3 inch restrictor disk was about 10% again on paper.

I hate leaving stuff half answered…

Mike

--- EDIT---
Formating such as tabs and spaces does not work, sorry.

 

Geez Mike, I didn't mean for you to blow a couple of evenings answering my question.

Save your energy. I'll seriously need your help in an upcoming thread on alternative alchy injection.

 

Warp, I didn’t blow that much time, actually about an hour excluding beer breaks. I’ve been doing parametric studies for last two weeks so it was niece to think about different application (sometimes it helps me to change what I’m thinking about when I get a brain fart). Besides other members have asked about restrictor disks and wanted to provide a semi detailed answer.

Trust me if I spent a couple nights on it, I would have factored in low RPM impeller slip and provided detailed info at points of interest, such as HP, efficiency, point of diminishing returns… curves. At times I do stuff like this for a living.

Oh, alky thing bring it on...

Mike

 

oh my brain.... what do you work for NASA

 

Did the supercharger have the same pulley on it with the different size restrictor plates?