Tim Murphy

MURF 928 Stage 3 kits run near max impeller speed at redline. It wasn't always that way but the system is now designed to do just that. For this reason I had to upgrade to a larger blowoff valve on the new systems do to the additional air being released when not under boost.

You mention blowing off boost which has already been generated as a good method of limiting max boost. I'm really surprised that someone is actually doing this and then wrote an article about it. Perhaps it is because Procharger has a patent on limiting boost via a variable inlet to the supercharger (only patented, never made). This in my opinion is the correct way to do it. Why not limit the air into the supercharger when you get to the desired boost level before you use a bunch of energy to create the boost and then just throw it away.

Just my opinion.

killav

Tim,

I was hoping you would catch this thread and give your much appreciated opinion. This pretty much settles the discussion in my book for use on the stage III Murph kit anyway. I'm still kicking around the stage 1 though in my mind.

Short story, my first year at Sharks in the Mountains in 2007, myself and John Curry (soontobered) were following a few of you guys down the mountain. All of you must have been boosted, cause John and I were driving the tires off our cars, and you guys were at the bottom having dinner before we could park.

hacker-pschorr

I understand the concept, I'm trying to understand the why.

I see no point in limiting the HP upstairs unless you are worried about "that much boost" damaging the engine. Which I'm trying to point out is a backwards way of thinking.

You are not "adding boost down low" you are taking away boost upstairs.

As killav already pointed out, this mod doesn't do anything but limit upper end HP on cars already spinning the blower at max speed. So this mod requires spinning up the supercharger which will (or at least should) add more HP and torque across the board. But for some reason beyond my comprehension, people want to cap off the power just to make the dyno chart look more like a turbo? The only rational reason to do this is if you are worried about the added power upstairs wrecking the engine. Which I'll point out again is a very dangerous way of thinking.

There is a Corvette on our race team (normally aspirated) with over 100hp tuned out of the engine for NASA power / weight rules. This is the only scenario where I can see this making sense (limiting HP).
That is, unless you are worried about wrecking the engine. The motor in this Vette would have no problems lasting under full power, but that would put the driver in a class he doesn't want to run in.

Capping off the power upstairs doesn't make the car any more drivable, just take your foot off the gas!!
I can see the mentality behind this "mod" coming from someone in the turbo world where they do cap off boost for driveability reasons since with turbo cars, the boost is less predictable since they are exhaust driven (versus RPM dependent with a blower) you have to cap it off somewhere.......
Otherwise you my not only power past where the car is tuned for, you can over drive the turbo. These scenarios simply don't happen with superchargers which IMO makes this mod a solution looking for a problem.

soupcan

I can see how this would be useful in the race gas example I gave and if you have reached the mechanical limits of your particular engine.

No way I could put a smaller pulley on and not use the additional boost, I have nowhere enough self control.

Tim Murphy

The current MURF 928 Stage 1 kits are also designed to run the supercharger at near max impeller speed. The boost limit is part of the intake design which not only limits max boost but provides more boost at lower rpm. Hope that makes sense. Again, in my opinion, it is never a good idea to generate more boost than you are going to use just to throw it out the window once you have spent the energy.

I'll add, a wastegate on a turbo limits the amount of boost before more boost is made. Same principle as closing off the air inlet to the supercharger once you have reached the desired boost level. MURF Stage 1 and Stage 3 systems are already close to the tire adhesion limits. Stage 3 just puts it into HYPER MODE upstairs when the car is well underway.

Cosmo Kramer

Tim is right, parasitic drag of the blower increases with both CFM and pressure. I found a neat chart created by a company that uses centrifugal blowers in industrial applications powered by electric motors.

http://www.processairsolutions.com/vor_sizing.htm

As you can see on chart 2 the power requirements increase with both pressure and CFM. At 4.7 psi and 500 CFM it needs 12.3 hp. At the same 4.7 psi and 1000 CFM it needs 23.1 hp. So blowing away CFM while maintaining a constant boost pressure wastes HP.

Limiting the size of the inlet achieves the same result, but the problem with that is the CFM will hit a plateau at a certain RPM and the boost will drop at higher rpm instead of it maintaining constant psi like the waste gate. The variable inlet Tim mentioned that would remove the restriction as RPM increases to maintain a constant boost pressure would be ideal.

Ketchmi

Well Hacker, for example when you live at 4400' and you are using a much larger supercharger to get the performance you want...what happens when you take the car down to a lower altitude? I think that 16psi is enough on a bone stock S4 (other than Cometic head gaskets) that produces more power than is logical for the street. The Vortech V7-Ysi is capable of well over 20psi @ sea level on a 5.0 32v spinning to 6400rpm but it is maxed out at this altitude. There are limits and I believe that on a stock 5.0 1.1 bar is pushing it with the factory rods and pistons.

killav

But you are adding boost down low. That's the whole reason it is done in the first place. Not to limit upper end HP, which remained identical in the article I read. The shop added 80 ft/lb down low in the meat of the power band without overboosting the engine at the top end. This will never be bad in my book. They did this by using a more aggresive pulley, and different compressor wheel. If they didn't "control vent" on the big end, the engine in question would not have survived at 20 psi with the stock compression ratio, no mater what intercooler they used.

I will try buy this magazine and scan in the article. It is not the same article I posted in the link. If memory serves, they even lined up two similar cars, one with this modification and one without-same max boost, and the wastegate modified car easly pulled away from the car with a normal set up.

killav

Thank you Tim, I had no idea both were maxed out. The Mustang and Chevy guys are using this mod, simply because they are buying kits that have not been designed to run at max impellar speed in the first place, unlike the Murph kits for our cars. What you have done is very clever. I think you already have the best boost curve possible designed in.

Ian928

What would be interesting is a system that created a much higher boost than you would want in the manifold, then got rid of the excess heat through the IC and then reduced the pressure again through a regulator to the wanted manifold pressure. With a system like this you could introduce air at considerable lower than ambient temperature into the chamber.

FredR

The concept is easy to understand- get more boost earlier in the rpm curve to improve low end torque response- the penalty being loss of some top end potential as the boost is now limited but the parasitic losses are not. Thus for a given [fixed] boost, some top end will be lost driving the s/c. If that is what you want all well and good but for sure it will not appeal to everyone.

This can only make sense if you have a compressor that is bigger than what you really need for your engine and gear it such that it maxxes out [speed wise] at max engine rpm. You then blow off the excess pressure your engine cannot handle be it due to detonation or mechanical destruction.

Is it worth it? Well I guess that depends. For a manual tranny owner - I doubt it very much but for an auto tranny owner maybe- but only if you can effect a nice boost in torque from 2k rpm upwards.

As an auto owner I think I would prefer a TS setup because of the "torque table". For outright power the centrif is more desirable. I have always liked torquey engines and that is probably why I like the 928 32V motor- but for sure I would like a torque curve with a nice incremental overlay over the full rpm range.

Torque is what accelerates you, power is what gives you top speed. An engine invariably feels at its best at max torque rpm's - that is because it is at maximum acceleration.

Given that a stock S4 will hit about 165 mph more power is "theoretically" a waste if you do not want to go faster than that. If you want to accelerate harder then you need more torque in the rpm range you intend to operate the engine at. Then the relationship between power and torque kicks in. This is a simple concept but it does seem to confuse a lot of folks.

Regards

Fred

Cosmo Kramer

I would have liked to see a dyno comparison in that article of that supercharger limited to 15 psi via larger SC pulley vs using the waste gate and compare the dyno charts down low to see the difference. The problem is the centrifugal blowers aren't very efficient at low rpms and they are internally geared to spin fast. Maxing out the impeller speed at redline has less of an effect at lower rpms.

For example, my V2 right now runs 44640 rpm at redline (6200 rpm) and 6-7 psi. A pulley swap could bring me to max impeller speed (50000 rpm) and give me about 11-12 psi of boost unless I decided to bleed it off.

At 2500 rpm my impeller runs at 18000. With a smaller pulley maxing out out the impeller at redline it would be spinning at 20000 at that same 2500 rpm. It would basically gives me the impeller speed I get at 2800 rpm with the larger pulley. At these low rpms the centrifugal is not very efficient so how much more power will this mod give me down low? Probably not that much.

Z

It goes faster.

I remember when some people said that 4psi (yes, that's FOUR psi) of boost was the limit. I really wish people wouldn't be so fixated on boost numbers. Boost level is not what will kill an engine, no matter how high it is, unless it's high enough to physically blow the intake manifold apart.

That would work if your goal was lower intake temperatures. It would mean creating higher temperatures and pressures in the first place, requiring more power to do that. Besides the additional power required to run the system, getting rid of the extra heat created would mean a much bigger intercooler, more weight, and probably more complexity. Would the car go faster? I'm guess probably not, otherwise you'd have been seeing setups like that on both street cars and race cars for a while already.

A lot of people seem to confuse boost and torque, and assume that more boost automatically means more torque. That's definitely not the case, for a couple of different reasons. Change something like heads, valves, cams, intake manifolds, exhaust, or even just the location of the air filter, and you might see changes in boost go one way, and torque the other. More boost just means more measured pressure in the intake manifold. More torque means more pressure pushing down on the pistons, assuming the internals of the engine are the same.

I really don't care at all what the pressure in the intake manifold is. Maybe there are some kind of boost gauge olympics, contests, or something like that which some people are interested in? I've never seen the model in the bikini give the trophy and giant check to somebody just because their boost gauge went the highest, or rose the fastest. I guess something like that could exist, but personally I have more of an interest in acceleration and speed.

anonymousagain

article: http://www.musclemustangfastfords.co...l/viewall.html

Vortech Centrifugal Supercharger Wastegate Install - A Gateway to Street Supremacy
This Graph represents Grundman's Saleen versus a recently dyno'd Whipple-blown '03 Cobra. The boost curve (lower graph) is intense as the twin-screw blower is all over the centrifugal blower in the low-rpm range. But once 4,100 rpm hits, the centrifugal power curve overcomes it, despite the Saleen making nearly 7 psi less than the Cobra. According to Spetter, this Saleen would outrun most Cobras if they ran from a roll.

Ian928

In a way I think there are cars around with somwhat this principle, the Mazda Millenia. In the Miller engine you are introducing compressed and cooled air into a engine with high static compression. The fact that some of the compression has alredy been done and the resulting heat has been removed before the air enters the engine makes it possible to run higher compression and get more power.

The Miller engine is already made, and you could probably convert any engine to do this by changing the camshaft. What you can not do is:

• Create a Miller engine with a turbo- or centrifugal supercharger (off-boost compression would be far too low)
• Create a bolt-on system for a othervise standard engine

My point is that when you have compressed the air and intercooled it, why introduce it at near ambient to a special engine like the Miller engine when you can expand it from say 2bar to 1bar in the intake and use a engine with a standard engine?

I am sorry, I understand that this was not the original topic of this thread...