Joby,

I agree with all of your comments except this one.


Actually what happens is that you are limiting the CFM. So if you peak to 10psi at 4000rpm from 4k to 6k as rpm increases, cfm stays the same and boost will in turn drop. I believe this will hurt you much more than a decrease in blower efficiency.



If you use the pressure relief you can get to peak boost and hold peak boost to redline. The other nice thing is that if you do any kind of engine mods(rockers, cam, anything) you can get the benefit out of the change where as the intake limit will not help you in the top end at all.



Also it is nice to have it adjustablity for future boost changes and what not.


Just some things to think about.

 

The major differance in power comsumtion comes from the differance in air volume that is compressed in the supercharger. Even if you let some of the air out, all the air is still compressed by the supercharger.

In my example with data from the compressor map.

1) 1050 CFM and 65% efficiency to 10 psi boost.
2) 800 CFM and 70% efficiency to 10 psi boost.

1050/800 = 1.31 = 30% more power to drive the compressor and with the adiabatic efficiency it is even more differance.

The major diffarance from higher adiabatic efficiency is that you get colder air into the engine with a restrictor.

 

Wow, now I know why I'm in finance and not engineering.

Here's where my thoughts are, and most likely they are hugely inaccurate, so correct me where I'm wrong:

Observing the blower as a stand alone unit, irrespective of intake and engine restrictions (boost psi), the V-2 SQ that I'm running flows 1,000 cfm at 70% efficiency (allowing for 2% efficiency reduction for curved discharge) at max rpm of 50,000 (pulleyed to hit max flow at 5,700 rpm). Restricting inlet air flow would decrease the parasitic loss with respect to the amount of force used to move the decrease in air volume.

Installing an adjustable boost limiter does not affect the amount of air flow the compressor is internally moving from inlet to outlet. In essence, parasitic loss is unaffected, but air flow from the outlet to the engine is reduced, and boost pressure is regulated regardless of engine volume capacity.

However (and here's where my non-engineering brain ceases to work, and this is purely hypothetical as I have no numbers to back this up), if we limit blower inlet airflow to, say 850cfm, and the engine at 10psi turning 5,700 rpm is capable of flowing 950 cfm, are we not losing potential hp due to lack of airflow? I see the "wastegate" methodology as allowing maximum system airflow, if the max boost pressure allows that specific volume.

Wow, I need an aspirin....maybe a beer.

 

It all depends on the design of the restrictor. It has a flow where it starts to restrict flow, and the pressure drop increases with the flow. I belive the the pressure ratio curve of the restrictor can be matched to the pressure ratio curve of the supercharger so they more or less cancel out each other at higher rpm. At higher supercharger RPM a higher pressure drop is needed over the restrictor to keep the total pressure ratio (boost) constant. Air flow would still increase with rpm.


If you go back to the wastegate idea. Use a huge one as a restrictor before the supercharger. Now you have an adjustable boost control and you could even use a standard electronic boost controller with it.

The downside is that it would be much more expensive and complicated.

A fixed restricor would have to be matched to the engine and supercharger, but I belive in the idea.

Just some things to think about.[/QUOTE]

 

Yes,
When restricting airflow into the supercharger, the supercharger requires less hp to maintain that rpm.

You can make a simple experiment with a vaccum cleaner. Restrict the air intake with your hand and the electrical motor rpm will increase. The electrical motor will rev up because the compressor uses less power.

Installing an adjustable boost limiter valve after the compressor WILL increase the air-flow thru the compressor.
Look at the compressor map and follow one red line (= constant rpm)

If you use the same supercharger at the same rpm on a big and small engine.
Limit both to 10 psi using an boost limiter valve after the compressor.
Both superchargers will require the same parasitic hp and the same CFM will flow thru the compressor.
The boost limiter valve on the small engine will release a lot more air.


Well ... reducing boost will reduce power ... Wasn't that the idea in the first place?

 

LOL, its too early for a beer.

It really depends on the combination of the system. Lets say that we use a 350ci at 100% efficiency and want to spin the crap out of the blower so that it makes 15psi at max rpm(You'd probalby want more, but I don't think its possible with a 6 rib setup)


NORMAL
RPM___1k___2k___3k___4k___5k___6k
boost__0____0____4____7___10___15
CFM___101__203_386__598__851__1228

CFM Restrictor(850)
RPM___1k___2k___3k___4k___5k___6k
boost__0____0____4____7___10___6
CFM___101__203_386__598__850__850

Boost Limiter(10 psi)
RPM___1k___2k___3k___4k___5k___6k
boost__0____0____4____7___10___10
CFM___101__203_386__598__850__1020

CFM is CFM into engine


This just an example, I think I got the correct boost vs rpm of the supercharger on the first scenerio please correct me if I'm wrong. You can see the differences between the 6k points. By changing the supercharger pulley ratios you can get a larger difference earlier in the rpm range. I got the CFM calculations from a spreadsheet I created, if you would like to see it pm me with an email address and I'll send it to you.

Basically you can see that the boost limiting method gets you more air into the engine compared to the cfm limiting method at 6k. One does a good job of limiting the boost while the other does a good job of limiting the horsepower.

 

But, once the dump valve opens, you are not compressing all the air. The amount of air going through the valve is just being passed through the blower. You're letting out the same volume of air that you're pumping in so that the pressure remains constant.

 

That chart is representative of what I was trying to convey verbally.

Would the hp gain from reduced parasitic loss be greater than the potential power gains from increased airflow? Not an easy question to answer without a dyno, I would imagine.

Hey, it's always 12 o'clock somewhere.

 

Interesting discussion.

Since the OP only wants to achieve 9-10 lbs of boost, it would seem reasonable to pulley the blower down to spin around 36,000-38,000 rpm at the maximum desired engine rpm based upon the S-trim compressor map. This should result in a PR of about 1.7 or about 10 lbs of boost at the expected air flow (roughly 800 cfm).

Then simply limit the engine's rpm range to be within the maximum boost limit. Adjust driving style and shift points accordingly to fit.

A limit valve could also be used as an extra safeguard for over-rev situations.

Of course, the boost will be reduced at lower rpm, but that's the nature of centrifugal superchargers.

I agree it doesn't make too much sense to intentionally overspin the blower into a low efficiency region and bleed off the excess air to achieve the desired boost level, since the compressor efficiency decrease and increased power consumption of the supercharger are both adverse effects.

 

I'll be using the valve for over rev, or in my case over grip situations. I'm seeing quite a bit of slip with my setup.

A common scenario that I see wastegates used are on the big boy blowers that are used on the street. Guys running the ysi that want to tone them down on the street because it's so easy to make big power these days. They'll combine it with a 'street' program in their computer just so that the car is manageable and reliable on pump gas.

I ordered the valve from mcmaster carr this morning. I'll probably set it to 16 psi and leave it. I see 14 psi in 1st and 2nd and about 11 in 3rd. I just don't want to go over 16 because I don't have enough injector or pump.

Good points by all.

 

I ordered one just to check it out. It just got here (pretty fast for ground service, since that's what I paid for). The discharge tube is .5" wide by a maximum length (internal from inlet to diaghragm at lowest spring setting) 2 1/16" and minimum length (highest possible spring tension) of 1 1/2". It's seems to be a very simple design. But I am concerned about the cfm discharge capacity on a boosted automotive application.

Anyone know how to calc the CFM capacity for that volume?

 

Wrong.
You ARE compressing ALL the air that goes thru the supercharger.

Look at the compressor map again and try to see what it show.
I use Vortech because they publish their compressor maps.

The compressor map show the relation between boost rpm and airflow and how they affect each other. You can not change one without affecting at least one of the other.

Let's say that we want 10 psi boost.
Calculate pressure ratio. With atmospheric pressure at the compressor inlet the pressure ratio is: (14.7+10)/14.7 = 1.68.

To get 10 psi you have to be on the horisontal 1.7 line.
If the supercharger is spinning at 40.000 rpm (red line) then the compressor WILL flow about 1050 CFM. That is a fact.

If the compressor is spinning at 35.000 rpm then you will not reach 10 psi ever.

I estimate that you need about 37.000 rpm and then you reach 10 psi at about 600 CFM.

If you have an engine that consumes 600 CFM at 4000 rpm at 10 psi boost. If you have a dump valve that keeps the boost down to 10 psi and revs the compressor to 40.000 rpm, then 450 CFM will exit thru the dump valve and 600 will go into the engine. Total must be 1050 CFM at 40.000 rpm and 10 psi.

If you use a larger pulley so the compressor revs 37.000 rpm instead then no air will exit thru the dump valve and the engine will still consume 600 CFM at 10 psi boost.

The hp required to drive the supercharger will be 1050/600 = 1.75 = 75% higher at 40.000 rpm compared to 37.000 rpm. That is because you are compressing an extra 450 CFM that you are loosing thru the dump valve.

As you see on the map, 600 CFM at 40.000 rpm will result at pressure ratio of 1.83.

With atmospheric pressure at the compressor inlet you get
(14.7 * 1.83 ) - 14.7 = 12.2 psi boost.

With -1 psi at the compressor inlet you get
((14.7-1) * 1.83 ) - 14.7 = 10.4 psi boost.

With -2 psi at the compressor inlet you get
((14.7-2) * 1.83 ) - 14.7 = 8.5 psi boost.

 

How does that compare to the stock bypass valves for the superchargers (the bosch ones). Aren't those about 1/2" diameter as well?

 

I agree with what you are saying, but I think it is more to it than that.



To the right where the lines are horisontal you have HP-limit.
My idea is to match the restrictor so the left side of the graph is used.
* In that area pressure drop increses as flow increaes.
* At contant boost flow increses with engine rpm.
* On the supercharger pressure ratio increses with rpm.
With these last three statments combined the increased pressure drop over the restrictor will cancel out the pressure ratio increase of the supercharger.
CFM will still go up with RPM and boost will be fairly constant.

Short version:
Low rpm : Restrictor has no effect.
Medium rpm : Restrictor will cancel out the pressure ratio increase of the supercharger.
High rpm : Restrictor will be a 'hp-limit' and boost will drop as you said.

What I an trying to say is that the restricor must be matched to the supercharger and engine. The restrictor should should be big enough not get into the 'high rpm' mode. It shuld only cancel out the boost increase from the supercharger at the end of the engine rpm range.

 

I'm not sure I agree with this statement. With that train of though I could be spinning the compressor at 50,000rpm, but only moving 400cfm of air and it would take less power to get it to spin.

The power taken to spin a set rpm also does not take into account the fact that with a smaller pulley you need to accellerate the mass of the blower more than with a larger pulley. This takes alot of power. Also, there is alot of friction inside these things at such a high speed.

I do not know the exact formula but I know the movement of air is not the only factor, I'm not sure it would even be the majority factor, for the power needed to spin the blower.

 

It is a very simplified calculation, but it is not a lie.
It takes twice the power to compress twice the amount of air.
You see the adiabatic efficiency in the map, but I don't think that affects the power required much.
Then you have friction in gears and bearings. Considering there are superchargers with internal oil only and no cooling, that friction can not be too much.
Compressing air is the biggest power consumer, probably at least a factor 10 higher than all the others.

 

I have given this idea a lot of thought previously, and I belive that it is the best solution to reduce high rpm boost with a centrifugal supercharger.

If a fixed restrictor does not work good enough then a variable restricor is the next best thing. That would more or less be a boost actuated 'throttle' before the supercharger. Very similar to a wastegate, but closes to control boost boost instead of open like a turbo wastegate.

I never tried this when I had my Vortech because I wanted all the boost I could get at all RPMs.

I would be interested to hear the results if someone tried this idea.

 

a mag did this with a ford. they used a wastegate and rigged it up on the presure side of the blower. worked pretty will for them.

cant remeber the link though. sorry.

Chris.

 

I did some calculations based on your 'NORMAL' data.

http://www.flowmeterdirectory.com/fl...fice_calc.html

First I calculated the 6k RPM point.
A 2.95" orifice in a 4" pipe will give enough restriction to bring total boost down to 10 psi.

Then I calculated the effect that restrictor would have at lower RPM.

This is the results I got:

NORMAL
RPM___1k___2k___3k___4k___5k___6k
boost__0____0____4____7___10___15
CFM___101__203_386__598__851__1228

CFM Restrictor(2.95" orifice in a 4" pipe)
RPM___1k___2k___3k___4k___5k___6k
boost__0____0___3.7___6.3__8.6___10
CFM___101__203_380___580__802__1030

By reving the supercharger more and restricting more you could bring
the midrange up higher than 'NORMAL' and still restrict boost at high RPM.

This is the data I calculated for the restrictor
CFM Psi drop
1265 4.0
1126 3.0
1040 2.5
923 1.9
877 1.7
829 1.5
803 1.4
776 1.3
747 1.2
685 1
584 0.7
576 0.68
564 0.65
543 0.6
497 0.5
446 0.4
387 0.3
317 0.2

As you see it is a quite smooth transition.

 

Wow, this turned into a much better discussion than I had originally anticipated.

First things first. I have to address the belt slip-grab issue. I could do this by:

1. adding a larger pulley and larger idler. Of course, my boost levels suffer at all rpms via this route, but no overboosting potential.

2. add the larger pulley, idler, and larger crank pulley to to keep the same drive ratio. But that doesn't address my overboosting issue, which is what this discussion is all about.

3. keep the same pulley measurement, but switched to a groved unit, and install a larger idler for better belt wrap, but still the potential for overboosting.

In all of this, the overboosting is the main concern, as I want to get the engine into the prime cam operating range of 5200-5700. The way I'm thinking after reading all of this thread again is, that venting excess boost after blower, while addressing the overboost issue, brings in a downside of un-addressed parasitic loss due to blower efficiency and airflow capacity. This appears to be a lose-lose-win scenario: power loss via lower boost, power loss as the compressor is still moving maximum airflow at a lower efficiency rating, and a power gain via increased boost levels at an earlier rpm.

The restrictor method appears to be a lose-win-win scenario: power loss via lower boost, power gain as compressor is moving less than maximum airflow at a lower efficieny rating, and a power gain via increased boost levels at an earlier rpm.

So, what to do? I like the boost earlier, but I don't like the idea of blasting my bottom end in a China Syndrome meltdown. The best scenario is an attempt to restrict incoming air.

How to do that? Well, the easiest way I can think of (since that formula included in the link is WAY over my head), is to install a smaller or more restrictive inlet air filter, install a larger idler, and install (or modify my existing pulley) a grooved blower pulley of the same size.

How's that sound to you guys?