Mike00FRC

iTrader: (1)

See sig for mods...

I'm looking for any info on how Boost relates to RPM in centrifugal superchargers. The main reason is that I'm trying to decide between the 4.1 and 3.85 pulley. I dont really want to go above 7lbs, which is what is advertised for the 3.85 puley on theh vettes.

With the cam/springs that are currently installed, I plan to set the shift light at 6400. So, essentially, how much of a change is present between say 6000 and 6500rpms?

FLPs will be added over winter.

Thanks,

ChrisB

The CFM of air a supercharger flows is going to be essentially fixed with a given impeller speed. This impeller speed will be determined by your engine rpm, pulley combination, and blower internal step-up.

E.g. at 6000rpm with a 7" crank pulley and a 3.5" blower pulley and a 4.10 step-up you would have

6000 * 7/3.5 * 4.10 = 49,200 impeller RPM.

If you then get a compressor map for your blower you can look at that impeller speed and get an idea of what CFM you will flow (it will depend a little on restriction, but primarily on impeller speed).


The boost is then going to be a function of that CFM and the restriction of your motor. The same CFM may make 12 lbs on a 346 while it would make 4lbs on a 422 - yet they would both make similar power (the 422 would probably make a bit more, but not nearly as much as the NA differential - torque would be greatly increased though - and pulley sizing would be different because the rpm range would probably be lower - to maintain the same impeller speed).


The cam you have actually has a decent amount of overlap, though not tremendous - so you will be "blowing out" part of your intake charge during the overlap phase - this will effectively reduce your observed boost. The same camshaft on a 116lsa would probably give you a higher indicated boost with the same pulley combination.

Realistically, once you have altered from a stock configuration unless there is someone running the same setup as you then you will need to just try out the different pulleys and see where you are it.

Mike00FRC

iTrader: (1)

Chris,

Thanks for the quick repy. I'll check with ATI regarding the compressor map. By relating impeller speed at a given RPM to the compressor map, I'm assuming that the boost of a given application is not linear?

As far as the cam, I know its not the best suited for a FI application, but it shouldnt hurt performance too much... at least I hope.

Thanks again,

Black LS1 T/A

That boost thing is so convoluted.
One configuration you may see 8 PSI instantaneously, ramping to 10. Another, you may see 5 PSI instataneously, ramping to 10.

Stall speed, rearend ratio and tire height can vary how quickly your boost increases, cubes, exhaust and pulley size, how much you have in RPM range and how high it peaks.

So many variables... If your car is not stock, you just don't know until you try it.

Mike00FRC

iTrader: (1)

Ok,

Just checked a few numbers and am going to post an example.

First I'm assuming that the impeller speed will directly relate to observed boost levels for a specific application. I know that no two apps are the same, but if a single change is made to the same vehicle observed boost levels for that specific application should vary accordingly.

For example, using Chris' formula, lets assume that on a vehicle 7psi is observed with the following condidions...
RPM=6000
Step-up=4.1
SC Pulley=3.85"
Crank=7.0"
At 6000rpms with an impeller speed of 44,727 ~ 7psi is observed.
(6000*(7/3.85)*4.1)

Now, change out the SC pulley, replacing it with a 4.1" pulley and up the rpms to 6400...
You now have an impeller speed of 44,800.

Question is; would you now have an observed 7psi at 6400rpms given that the impeller speeds are essentially the same. The only difference is that with the smaller pulley, the same impeller speed is observed at a point 400rpms lower.

{Edited)
Now that I think about it, at a given impeller speed I believe that the SC is attempting to move a specific volume of air. By spinning the motor faster, after adding a larger SC pulley, the impeller speeds are the same, so the SC is attempting to move the same volume of air. However the motor, spinning 400rpms faster, is now able to pump a greater volume of air, so the observed boost at should be lower that the 7 which was observed in the initial test, even though the impeller speeds are the same. As there would be less back-pressure in the higher reving motor, this would show up on a gauge as less psi than would be observed in the initial test.

Any thoughts?

Now, lets assume that by pulley sellection, 7psi could be observed through two different combinations...
3.85" pulley @ 6000 rpms, or
4.10" pulley @ 6400 rpms

Which should make for the better street car?

ChrisB

Yep, in your second comment you got it right - boost should actually be lower at a higher rpm with the same impeller speed - since the motor can pump more air.


To answer your other question -

horsepower is going to be made pretty much based on the CFM you are flowing into the motor - in your original example with both situations flowing the same CFM horsepower would be pretty similar, even though you are revving the second one higher (the second would have a little more power assuming optimized cam timing, but not nearly as much of a delta percentwise as a NA situation).


In your second example I think you reversed your pulley choice - the second motor will make less bost since the motor is turning faster (exactly as you observed above). If you pulley it to make 7psi at the higher rpm that represents a greater airflow, so it should make more power.

The other factor here is camshaft though - that is going to effectively determine where your rpm range is (though the supercharger will extend it). Since you are sticking with that cam, and you have a fixed boost limit, there will really only be one choice for you.

Mike00FRC

iTrader: (1)

I know I should probably go with the 4.1 with my setup and limitations, to be safe. I was just tossing around the idea of using the larger pulley and short shifting it.

Guess I'll just keep it with the 4.10 and see how it reacts to spinning it up to 64-6600. If its not enough, I can always swap it out.

Thanks,

QuickSilver2002

Just one more thing to add. A smaller pulley will be more likely to slip (go as big as you can). The single IC that I see in your sig is also going to give you a bit more boost than what most get (probably worth 1 psi).

Just FYI, I have a somewhat similar setup to yours (single IC c5....), but I have the D1 blower. With a 4.65 pulley I see 8lbs of boost at 6k. D1 and P1 are pretty different though.

One word of wisdom, upgrade the fuel pump.

Mike00FRC

iTrader: (1)

QS,

Thanks, I'm getting the racetronics(sp?) and also picked up the MMS puley bracket, from A&A, so hopefully belt slip will not be an issue.

NoGo

iTrader: (2)

Here is an easy rule 'o' thumb boost calculation

1) Calculate your Engine CFM:

((Cubic Inch * RPM) / 3456)) Vol Eff = Engine CFM


2) Determine your Blower CFM:

((Crank Dia / Pulley Dia) * RPM * Blower Setup = Blower RPM

Blower RPM --> Look up on a chart


3) Determine Back Pressure Ratio

Blower CFM / Engine CFM = Ratio

4) Determine Boost - Absolute

Ratio * 14.7 * Setup Efficiency = Boost Absolute

5) Determine Boost

Booste Absolute - 14.7= Boost


So an example for your motor

Engine CFM:
((346 * 6500) / 3456) * .85 = 553 CFM

Blower CFM:
(7 / 3.85) * 6000 * 4.1 = 44727 RPM
44727 by linear estimating (I know its not perfect) = 1009 CFM

Back Pressure Ratio:
1009 / 553 = 1.826

Absolute Boost:
1.826 * 14.7 * .8 = 21.47 psia

Most blower setups have an ~80% efficiency as long as you are within the intercooler / ducting allowed HP.

Boost:
21.47 - 14.7 = 6.77 psi


Your motor should generate around 7 lbs of boost.

Good Luck,
Kevin

ChrisB

NoGo: Have you been able to get any compressor map's from ATI? I have talked to them and emailed them repeatedly, and all the answers are basically "We don't have any compressor maps (to give you)" - all I can get from them is "80%+ effeciency with our intercooled setup" - which doesn't help to much!

Vortech's are all on their website - but no such luck with ATI?


Thanks!

JimmyKash

ATI has never given out compressor maps

OSR 1

I don't have compressor maps from ATI. However, it is just a centrifugal pump and should have a flow map similar to most other centrifugal pumps.
Personally, I just use a linear guestimation to figure out what the blower is going to flow.
Most centrifugal flow maps are fairly linear until you get close to the stall speed or stall pressure. Most people operate their blowers well away from these areas anyways, so a linear estimate works fine.

Unless you know the exact efficiency of your system, the exact VE of your engine, the exact..... a guestimation is all you really need of the blower CFM.

JMO

ChrisB

OSR1: I agree with you that it's not a huge difference - I am just curious to actually get a compressor map and see what it looks like.

Regarding VE if you have some logged runs you can get this fairly accurately - you log MAP, Baro, RPM, and IAT - then using normal SD calculations (which solve for massflow) you instead know massflow from the map and solve for the VE. I have used this before to make a VE map on a car awaiting a FAST/DFI transplant and it has corresponded *very* well for the most part.