PDA

View Full Version : blowoff valves and superchargers. worthwhile potential?



Swade
11-23-2005, 08:57 PM
I know simular questions have been asked before, but would there be any worthwhile advantage to using a blow-off valve or pop-off valve on our cars?
I am not talking about the "Cool Guy" sound effect, but more about being able to regulate boost pressure.

For example could you run a smaller pulley for more boost down low and then bleed excess boost back into the intake before the sc. On the surface this should allow you to manipulate or extend a superchargers "sweet spot".

I read in an old post about a couple of you who were trying or considering something simular. Obviously its not as simple as I proposed and there are alot of variables to consider but what are some of your opinions? Is there potential in trying it?

Mike8675309
11-23-2005, 09:02 PM
You'd have to fab something up rerouting the air after the super charger back into the inlet of the charger just like the current bypass valve. It's just that the blow off valve would be metered rather than just on and off.

All it takes is someone with some tig welding and time. Oh and money.

As far as benefits? If you could engineer it right it may be nice.

91ScDrIvEr
11-24-2005, 01:29 AM
Most people in this forum will tell you that it cant be done and there is no sence..... I seen a artical in the 5.0 mag that there was this dude he had a 302 stroked with a blower similar to ours i think it was made my "kenny belle". he had a "blow off valve" on his car.. When you let off the gas it releases all the left over forced air stuck in your engine there for puts pressure on your gaskets.... It allows the motor to breath properly and less risk of granading your motor...why do you think the turbos have them......Tubos are almost the same sinario "forced air" but only threw exhaust instead of belt does the same thing. but only after a certain rpm. If i had welding exp. and i knew some one cheap and had spare parts i would def. do it. it wouldnt hurt your motor anyways even if you did try it. Dont let any one tell you it isnt gonna work... do some recerch and if it works out to your benifit more power to you in many diffrent ways lol............"IM NOT KNOCKING ANY ONE IN THIS FORUM"....... but i was told its impossible and it wont work at all....so if thats the case why are there cars in mags that are doing it.... in fact ill try to find the article and post it if i can.

91ScDrIvEr
11-24-2005, 01:41 AM
1679216791 This is a super charged car with blow off valves 2 on the same pipe... not the same supercharger as ours but still the same sinario proven fact it does work! lol im doing it when i have the money just to see if there is a power diffrence

Roadhawg
11-24-2005, 01:56 AM
So would the Maf have to be moved after the supercharger?
With the Maf in a blow-thru configuration and the blowoff valve would be inbetween the supercharger and Maf?

91ScDrIvEr
11-24-2005, 02:07 AM
That you would have to look into I dont know alot about blowers im use to natual inpired motors this is only my second sc i ever owned but the thought also crossed my mind. I dont want it for the swisssshhhhh sound i want it to release the air thats trapped "better for racing" i beleave. Sorry i cant help you on that one. but if you get it done please let me know how it ends up. If you made more or less power. so i can get crackin on it

JimmyDean
11-24-2005, 06:08 AM
Jeez understanding that was a workout :o . Our cars already have a bypass valve built into the supercharger... Once the intake reaches atmospheric pressure all of the boost generated by the blower is going into the intake. Once a vacuum is drawn, it starts venting boost (I believe).

Roadhawg
11-24-2005, 09:49 AM
Jeez understanding that was a workout :o . Our cars already have a bypass valve built into the supercharger... Once the intake reaches atmospheric pressure all of the boost generated by the blower is going into the intake. Once a vacuum is drawn, it starts venting boost (I believe).


The bypass valve recirculates the air back into the intake plenum. In the M90 setup the air in the system cannot be vent to the atmosphere because the Maf has already measured it. If you want to use a BoV I'm fairly sure you would have to disable the bypass system and relocate the Maf after the supercharger and place the BoV in between the supercharger and the Maf. So that the BoV could vent excess pressure to the atmosphere before the Maf meters it.

drummerzrok24
11-24-2005, 10:43 AM
Ok, I have been thinking about doing something like this for a long time now. My friend, same guy who made my lambo door kit, is a HP Welder. He TIG weld justa bout damn near anything you want. What would be required to do this?

Swade
11-24-2005, 12:26 PM
Ok, the excess boost pressure would have to be rerouted back to the intake like the current bypass valve otherwise the car is going to run rich. This would happen because the maf is foing to tell the computer there is more air going into the motor than actualy is. The idea for a blow through MAF is an intriguing idea that probrably needs a thread of its own.

Maybe I was unclear on what I was asking. I am probrably getting the name wrong for the part I am talking about, basicly somthing to regulate/maintain boost (BOV, pop off valve, boost controller ect.). I envison being able to set boost for a desired safe level say 15 psi but build a system capable of puting out 20psi. Would this allow you to build and maintain 15psi faster than a system designed for just for 15?

More importantly I am wondering if this would allow us to extend the efficiancy ranges (drive power, thermal efficiancy, abdiabatic ect.) of a supercharger or at least allow us to exploit them more.

Blown 91 Bird
11-24-2005, 01:43 PM
a bov/bypass is used on boosted motors to stop surging when the throttle blade closes (and to also prevent bending the throttle blade)

Mike8675309
11-24-2005, 02:03 PM
Turbo's have blow off valves for specific reasons that simply don't apply to crankshaft driven superchargers.

I don't have the engineering background that's needed to figure out the specifics. But the benefit of a bleeding down boost pressure from a Positive displacement supercharger (Which is what we have) would require some research. Unlike a turbo or a cetrifigual supercharger, the plot of HP load vs Boost pressure is much more direct. Thus pumping efficency of developing 20psi at 2500rpm would need to be investigated compared to developing 20psi at 4800rpm.

To do a "blow off valve" with our intake design and maintaining the pull through MAF, the ideal solution would be to tap after blower pressure, and based on the pressure limit bleed pressurized air back into the inlet of the supercharger. Our supercharger doesn't compress air as the air moves through it, it compresses the air by forcing more air into the intake than the engine can consume. Squishing it between the outlet of the supercharger and the back of the vavles in the head. So feeding air back into the supercharger that's already been compressed relieves pressure on the outlet of the supercharger thus decreasing boost. But since the SC maintains being fed (by that redirected air) it's able to keep running without cavitating.

That's how the bypass valve works, but it works either all the way on, all the way off, based on manifold vacuum.

Note later model M90 and other eaton blowers now have a bypass valve integrated into the housing of the blower itself.

xThunderbirdSCx
11-24-2005, 05:27 PM
heck....i would want the cool psssshhhhhhsssttt sound. Dont lie, you all do too! :D

Hock
11-24-2005, 06:54 PM
Turbo's have blow off valves for specific reasons that simply don't apply to crankshaft driven superchargers.

Thats not exactly correct. For our setup a BOV is not required besause the boost is made after the throttle plate. Now on the "typical" centrifical setup, which blows through the throttle, you can run a BOV to vent the excess boost when you let off the gas. If I'm not mistaken, there are also bypass kits for those as well, made more for the daily drivers.

Now as far as ours, unless you change the whole design of the induction, I don't believe that a BOV would be worth the time or the money to look into.

But thats just my opinion, I could be wrong.

Toms-SC
11-24-2005, 06:57 PM
Pretty useless IMO

Blown 91 Bird
11-24-2005, 10:27 PM
Thats not exactly correct. For our setup a BOV is not required besause the boost is made after the throttle plate. Now on the "typical" centrifical setup, which blows through the throttle, you can run a BOV to vent the excess boost when you let off the gas. If I'm not mistaken, there are also bypass kits for those as well, made more for the daily drivers.

Now as far as ours, unless you change the whole design of the induction, I don't believe that a BOV would be worth the time or the money to look into.

But thats just my opinion, I could be wrong.

you are correct. Standard Procharger/Vortech/Paxton kits come with regular bypass valves like found stock on DSM's/Supras/etc.... which work great up to 15psi. Works the same exact way a Vortech mini-race bypass, or any other "BOV" works.. stays somewhat open while not under boost, then closes when boost is being made and releases the pressure as the throttle blade closes to like i said before, prevent surging (when the throttle blade closes, the pressure has nowhere to go but back to the blower/turbo.. causing problems) and prevent it from bending the throttle blade.

you "could" put a BOV on one of the intercooler pipes on our car if you just wanted the "sound" effect... but it would be a waste of time and money.

Mike8675309
11-24-2005, 10:53 PM
Clarification. The original poster is discussing "blow off valve" but in reference to allowing a high level of boost at low rpm, yet limiting the boost increase as RPM's increase.

In the context of a "turbo" the blow off valve is ahead of the throttle plates and is used to control pressure that builds when the throttle snaps shut. Without a way to relieve the pressure the air flow would stall possibly stalling the turbo charger and creating a surge condition.

Turbochargers also have a Waste Gate which is their primary method used for controlling actual "boost" generated.

What we should be discussing here would more comonly be considered a waste gate on a turbo. Our Bypass valve already acts as a blow off valve in how it allows a place for the boost to go when the demand cuts out.

One point to raise on this whole topic. Simply increasing boost down low does nothing to increase max horsepower or torque. You're just moving the curve lower in the power band. And to be honest, our cars already have too much torque down low for most uses. Maybe if you were going to make a drifting car, bolting on a crazy low rpm high boost pulley would make sense. But for autocross, roadracing, or drag racing, more boost down low simply is a recipe for a tire destrution system.

drummerzrok24
11-24-2005, 11:48 PM
you "could" put a BOV on one of the intercooler pipes on our car if you just wanted the "sound" effect... but it would be a waste of time and money.

How would that work?? Please explain...

SK_BlownThunder
11-25-2005, 12:13 AM
Just signing in on the thread but, I do think a blowout valve may be a possibility. Remember, the MAF reads the amount of air coming into the entire engine and intake system. If you bipassed the air from the blower through a blowout, you would need to install a way to read how much air is not being used so the computer knows how much fuel to deliver to compensate for the extra air still in the system. By letting the air out of the system like on a normal SC inlet, the computer does not have to worry about this "extra air" still trapped in the system. The whole reason behind the blowout such as on a turbo is because when you let off the gas, the exhaust gases are still at a high enough rate that the turbo will continue to spin until the exhaust gases slow down. A turbo also usually turns faster than a blower which allows for much higher boost capabilities. This high boost needs to escape, and if it doesn't by the time the turbo kicks in again, well use your imagination.

Now I said the blowout is a possibility, but because the plenum on these cars is already eqipped with the bypass, there really is no reason for a blowout valve. Basically with the blower, the drop in boost should be close or near to the same rate as the rise in boost, which is usually pretty instant. Where the same is for a turbo. The rise in boost is governed by how fast the exhaust gases can reach the turbine at a high enough velocity (turbo lag). At the same time, it needs that same amount of time to drop its boost, with which the momentum of the turbine and exhaust "cut off" lag, the turbo will continue to spin quickly until the exhaust rate lowers when letting off the gas. With the blower, this is instant, so unless you plan to run at over 20-25 psi of boost, I don't think a blowout is necessary.

Seriously though, I too agree the ssswwwissshhhh sound is kind of cool. Just as long as you keep the SC from a rice wagon, go hard and hope it works out for you.

Swade
11-25-2005, 06:33 PM
Okay so creating an elaborte or expensive system to simply replace and do the same job as the bypass valve would be pointless and silly. And Isee your point on making too much low end torque.

Does anybody have any speculation as to what would happen to a supercharges performance curves (drive power, discharge temperature, efficiency, ect. ) after a given pressure ratio is reached and then maintained by venting boost while the sc's rpm's would continue to increase. I have a hunch that drive power would level off and probrably temperature would also. Efficiency may decrease as the supercharger would not be able to produce its full potential.

I imagined this working in a system where the sc has a higher flow potential than a motor can use. Then using a combination of pulley size and regulating boost, keep the sc close to its most efficient operating range. Therefore using less drive power and producing less heat than a smaller blower straining to make the same boost.

Does that make sense or am I making the question more confusing?

Mike8675309
11-25-2005, 08:41 PM
It's all math. In general the stock M90 blower simply can't flow sufficient air to make further optimization of it's input rpm more than a questionable value. If ya have the spare skill and tools to give it a shot. Go for it.

David Neibert
11-25-2005, 10:07 PM
Swade,

I understand what your talking about. I researched doing the same thing a few years ago and even located a progressive relief valve that looked like it would do the job of regulating the excessive boost.

There is a rather lengthy thread on the topic in the members forums. Basicly the idea was to spin the supercharger fast enough to create around 18or 19 pounds of boost, mainly for the bottom end torque then set the bypass valve to bleed off excessive air to maintain only around 15 or 16 pounds at high rpms. Doing that would give you more bottom end snap without pulling excessive power off the crank at high rpms.

The valve I was planning to use was a safety device intended to prevent overboosting on a turbo. It could handle temps avove 400 degress F and had a soft open and seat function that would allow the smooth release of only the excessive boost and it was also adjustable, which would make it easy to experiment with various boost levels. As I recall the valve was made by Greedy and cost about $160.

I was planning to install it on the upper IC tube and plumb a 1" hose from the valve back over to the intake pipe after the MAF and before the TB. That way any vented air would not cause the a rich condition. The major concern is excessive heat from spinning the blower faster that it was designed for.

In hindsight, you could probably get the same effects from overdriving the blower about 20% and just letting the belt slip to regulate top end boost.

David

Swade
11-26-2005, 02:58 AM
I guess without a way to test the benefits, it would just be interesting speculation. I did find something that might be a cheap way to expierment with the idea.
http://www.xmission.com/~dempsey/perform/grainger.htm
It looks like this could be slaved in to our system and let the bypass valve work as normal. That website also had an interesting link to a two stage boost controller using two Grainger valve's and a 3 barb wastegate solenoid.

Unfortunatly I really dont have the setup right now to see any benefit anyway. Once I open the exhaust and overdrive the blower I may look into testing the concept.

91ScDrIvEr
11-27-2005, 03:30 AM
You know what its to damb dificult to think about even trying it to much involved if i want that swoosh sound they sell adapters for your tail pipes that make that sound lol

bubbaflaat
04-20-2006, 12:55 AM
there isn't enought pressure buildup in the intercooler piping to even make a BOV release pressure. Our cars have a bypass valve already. There is NO reason to put a BOV in our cars other than to waste money for no results.... Not even a neat sound from it.