4playta

I was just wondering why I am seeing some procharger cars with a blow off valve? I know you need them for turbos but why would you need one on a supercharger?

KrisW

iTrader: (5)

Some guys use them so that they can run a larger supercharger but limit the boost externally.

Some people think it lets them run more boost in a faster time period, some people just think it's cool...

I wouldn't run one on a supercharger, I'd more likely change the pulley size to set the boost max where I wanted it mechanically. Now, turbo, that's another story...

4playta

On a turbo dont they help keep the turbo spooled up when shifting gears? Thats what I always thought anyway so thats why I was wondering why you would need one for a supercharged car? Anyway yeah it makes sense to help control your boost if you have a larger supercharger than you need.

Six_Shooter

iTrader: (2)

They are used on superchargers for the exact same reason as turbos.

the reson is to protect the turbo.

Whne you are at high RPM or high boost, with the throttle open or wide open, and then suddenly close the throttle, there is a shockwave created that bounces off the throttle plate, this creates a high pressure area, and so that shockwave travels the only way it can, through the path of least resistance, back towards the compressor. This shockwave and high pressure can stall the compressor, yes even on a supercharger, it can stall, if that pressure is high enough. So what happens when the compressor wheel stops, but the drive input doesn't? Things break, that's what happens. Shafts shear, compressor wheels break, compressor wheel and housing make contact, etc.

4playta

Now excuse me for my lack of knowledge in this department I am just doing my homework before I get a procharger. I have one more question about the blow off valve. I have heard of adjustable boost control, is that controlled through the blow off valve? And if so are they controlled by electronics or mechanically?

Thanks for that explanation SIX_SHOOTER that makes realy good sence.

xpndbl3

iTrader: (5)

turbos use a boost controller for the wastegates
BOV is for when you close the throttle plates as described earlier.

mattsv8_03

iTrader: (2)

they both use bov because at higher than 8 lbs boost the impeller is spinning at a rate and moving air that trys to spin it backwards when the throttle is shut. this is bad on a turbo becuase its a good way to tear up the bearings or bushing or break the shaft if bosst was highe enough. on a supercharger it wants to thow the belt becasue the motor is decelerating when you close the throttle but the boost is already built and needs go somewhere or you will be spinning against it or driving the charger backwards. thats why almost all supercharger come with bov or bypass valve. neither want to spin backward. thats bad mkay
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ill be using a turbonetics new gen wastegate and a godzilla bov.

383backinblack

let me inform you a little bit about how it works......

its technically NOT a blow-off valve.......when attached to a supercharged system its a bypass valve.....here's the difference:

a turbo BOV opens when the engine is under boost, then goes into vacuum (the throttle is closed) to vent the boost to atmosphere or to the intake of the turbo.......for 2 reasons......to prevent the pressure shockwave of the charge-air from backing up and trying to stop the compressor wheel....therefore damaging the turbo......and also in high boost applications to prevent the charge air from actually bending the throttle plates.

On a supercharger, it works a little differently......2nd reason i listed above is the same, but the 1st reason with a supercharger has another component to it. the bypass valve is ALWAYS open when the engine is in vacuum (the throttle is partially closed) because unlike a turbo the supercharger is ALWAYS making boost. So, by venting the unused charge air to atmosphere, it eliminates the old "surging" problem centrifugal blowers were known for. It also make a condition where you're not trying to boost the engine when it doesnt need to be......such as part throttle.........

So, the bypass ONLY closes when you go to WOT (its vacuum operated) and it snaps shut and then it goes right into boost.......I have video's of the one on my car functioning if you want more clarification on how it works.....you can see and hear the functioning.

ALSO, it DOES NOT adjust boost pressure in any way......its possible to build a setup with a waste gate/boost controller to control the boost with a centrifugal blower, but the bypass valve doesn't do that at all. The boost level is controlled by the pulley ratio between the crank and the supercharger.

the big red thing in the bottom right is the bypass valve

383backinblack

thats completely and totally false.....no offense, see above post for actual explanation

junkcltr

iTrader: (1)

Huh, I run a compressor bypass valve on my turbo setups. They are from stock OEM turbo cars. Most OEM turbo cars have a bypass valve. They operate the same way as your "superchager bypass".

A compressor is a compressor........the wheel doesn't like being stalled. A CBV is used on both turbo and supercharger setups.

383backinblack

They don't perform the same exact function..........being a compressor has nothing to do with it, they don't operate in the same way......

AS I ALREADY EXPLAINED, turbo's are not spooled up all the time, and the bypass (blow off valve) opens to release boost pressure when the throttle plates are closed.......superchargers ALWAYS MAKE BOOST BECAUSE THEY ARE MECHANICALLY DRIVEN.....the bypass is ALWAYS VENTING BOOST PRESSURE because the supercharger is always "spooled" it doesnt' have a waste gate to keep it from spinning. this caused a major part-throttle surging problem with centrifugal blowers back in the old days.....which was fixed by using the bypass valve.

and in NO WAY is the bypass used to regulate boost pressure.......i want to make that crystal clear, i dont know where people got that idea from, most likely from people who are confused about what a wastegate and a BOV do.

the difference is they are commonly referred to as a Blow-off valve on a turbo, and bypass valve on a supercharger because of how they fucntion in the individual applications.

the principal is the same.....the nomenclature and practical functioning is different

Six_Shooter

iTrader: (2)

I think you need to learn a bit more about this compressor side of it. "spooled" is a reletive term, and last time I check on my RUNNING turbo engines the turbos were spinning AKA "spooled" at idle, part throttle, etc.

"Boost" as read on a guage is load dependant, in both a turbo and super application. You can sit there in idle in both a turbo car and a super car reving and not read "boost", but in gear under load, the boost will be shown.

The bypass/bov work EXACTLY the same way in both a turbo and super application, end of story.

383backinblack

i beg to differ........i think you need to learn more about how a supercharged application works

they're called different things for a reason.....
my supercharge will put 2000cfm of air at 7500rpm......while the engine is under load, or not......that air won't see the engine because of the bypass.

a turbo WON'T do that....unless your wastegate isn't working or its non existant......a turbochargers output is independant of the engine speed as controlled by the waste gate/boost controller.......a superchargers is DIRECTLY tied to engine speed and needs to be bypassed at all times.

this much air doesn't need to be bypassed, nor a bypass be open this far at idle on a turbo My f1-r blown 383 small block starts for the 1st time

junkcltr

iTrader: (1)

I beg to differ...........you need to read up on how a turbo works.

And you don't think a BOV would help an SC at the point where the throttle is snapped shut and boost is present before the RPM starts to drop. Yes, an SC CBV is there to bleed off air. I wasn't arguing how the SC CBV works.

Call all the car manufacturers and let them know they made a mistake putting the CBV on the turbo engines. Yes, they put CBV on the turbo cars and not BOVs. When a turbo is under slight load it will push more air even though it is not under boost. The CBV bleeds off the air.....better fuel mileage. Taking off at light throttle you can hear the CBV bypassing air because of the vacuum port on it is pulling it open, if you go WOT it goes silent and the turbos whistle. Let of the throttle and the CBV opens because of boost pressure over-riding the spring pressure.

A turbo WILL do that. You are confusing WOT boost with light load non-boost in turbo applications.

383backinblack

no sh*t, thats why i listed that as one of the correlaries between the two systems.....its the biggest function of the valve on BOTH of them.....did you read what i said?

i think you're severly mis-reading or failing to read at all what i said once again, NO WHERE did i call it a mistake, or say that it wasn't important......my only contention was that it is NOT the same thing.....and you've made another error here, the boost does NOT override the spring pressure when when the throttle is closed to open the valve, the engine immediately goes into high-vaccuum when the throttle is closed, and the vacuum opens the valve....because thats how it works. If the boost pressure overrided the spring pressure the valve wasn't nearly strong enough for the application.......if thats how it worked, when you jumped off the throttle the boost would have to spike and actually cause the dangerous reversion through the compressor you're trying to avoid....in order to open the valve. It's a vacuum operated at all times

no, im not confusing anything......in fact i said EXACTLY THAT.........were you drunk or something when you read the last post? here it is again....

.

if you're turbo is putting out 2000cfm (it would be a HUGE turbo first off) when the car is at 7500rpm with NO LOAD on it......you're in trouble because you're waste gate is not working. a supercharger is linear, it makes the same boost at a given RPM no matter what.......a turbo does not. If you want to argue that, then you're voluntarily throwing your "turbo's are more efficient argument" out the window, because then they would work EXACTLY the same was as a supercharger

Blue-Beast

Idle surging was related to the use of blowers (a 6-71 is truely a blower) and fuel injection. That combo created rich/lean conditions at idle because of the desigh of the mechanical fuel injectors. My efi'd/blown Biscayne has a smoothe idle.

I agree with Back'black.
A supercharger ('chargers are not "blowers") is a gear driven turbo, compressing the air as a turbo does, but much different in the function. If it's rated at 2K cfm @ 7500rpm, then that's what it'll do. It's mechanically driven from the engine, giving it the same crank ratio whether it's loaded or not. If you're flying down the highway (hopefully the track) @ 7200 rpm at light throttle, the 'charger is trying to push 2K cfm. The engine won't consume that much air at part throttle, so it's bypassed back to the low side of the intake tract (between the charger and the air filter). When the throttle is opened and the intake goes boosted, the BPV closes and pressure builds.

Using a BOV on a bigger charger unit isn't uncommon. In a WOT/boosted-to-a closed-throttle situtation, sometimes the BPV can't flow the demand, so a secondary outlet is needed in the form of a BOV. A BOV isn't always needed but a BPV is, ther is a difference in the two.

Some turbo systems do incorporate a BPV, but only when people realize the function and take the extra effort to install one into the system. A charger is always pushing air, so low-end throttle response USUALLY isn't an issue. If a turbo is mis-matched, then there are lag issues to deal with. A BPV does just what it's called. It bypasses air from the air cleaner tube directly to throttle plates, bypassing the turbo all together and creating better low-speed throttle response. The instant boost comes on, the BPV closes, and the pressure side of the turbo takes over.

The pulley sizing on charger is what regulates WOT pressure, the waste gate is what regulates the boost of a turbo. When boost builds up enough to overcome the spring in a WG, it opens and dumps exhaust pressure away from the turbo. An electrically (or manually) controlled pressure valve can control the signal between the pressure tube and the WG, making up a boost controller.
Boost also depends on the demands of the engine. You could be running down the highway (down a steep hill) @ 7500rpm at part throttle and the turbo (turbos) will be pushing only a slight amount of air, if any. The engine needs only a small amount of air, so the turbos are pretty much free-wheeling.
Now change that and go up hill. Pulling a hill in high gear, high engine loads and low rpm, the turbos will be boosted up and running on the waste gates. If the throttle was now closed, the BOV would open, dump the boosted pressure to atmosphere, and cause less stress to the turbine wheels. It's always best to challange an up-hill race if you're running a turbo car vs a non turbo car.
The only time a turbo system doesn't need a BOV or a BPV is in a draw-through system. The inlet turbine is always on the vacuum side of the system, but it will still need a WG.

I own a blown car, and the power is always instant.

I built a twin turbo Camaro recently, and it's performance is amazingly brutal. Really snappy throttle response, quick spool-up and useless 1st and 2nd gears (sometimes 3rd too-Turbo 400).

I reengineered some issues on a supercharged Mustang. I can honestly say that I wasn't impressed with that one. It ran good for what it was and made decent power, but by the time it had to wind up and get going, the Camaro would have already been long gone.

Hope this helps clear things up a bit.

junkcltr

iTrader: (1)

Everything you wrote seems correct to me. The only thing 383backinblack was wrong about was how a CBV works on a turbo.

Like you said, a properly sized turbine and compressor will have great N/A and boost characteristics. Fuel and spark have a lot to do with spool time also.

EDIT: You are building a first gen, right? Did you pull the SC and going turbo with it?

Blue-Beast

There's a lot of truth and a lot of false ideas in this thread. I'm just not one to nit-pick and tell peeps when they're wrong. Though I feel that superchargers really don't make boost all the time, they have to spool up just like a turbo.

The turbo Camaro has the timing locked in at 36* and is retarded by an MSD BTM. So you are correct about timing (and the proper tune) to maximize potential. It has more throttle snappieness than most NA cars that I've driven.

Junkcltr, you've seen the turbo car and my 1st gen. project?
My Biscayne has the 6-71
and my 68 Camaro will have twin Master Powers. BDR has the white turbo Camaro.

What does CBV stand for? I looked back in the tread but I guess I missed it.

junkcltr

iTrader: (1)

I recognized the pic you posted in another thread. BDR posted it a while ago. There was a first gen. in the background and he said something about you liking how the third gen came out. Possibly going boost on the first gen.


CBV = Compressor Bypass Valve. It is the term used for the OEM turbo applications. I use them on my turbo rigs instead of a straight BOV. The Bosch units have two 1" diameter ports (air in/out) and one vacuum port.

junkcltr

iTrader: (1)

A little blurb on the CBV:

http://www.spdusa.com/blow-off_valves.htm

Blue-Beast

That makes sense. You have a part# for the Bosch units.

I read BDR's thread and saw what was said. I remember you asking about the 1st gen.
It'll have the same specs as Alex's car, and possibly an inter cooler (or AC).
I just finished the Vette IRS and my custom frame, click on this pic to enter one of the Camaro albums.


This is my blown Biscayne.


And BDR's turbo Camaro for those that haven't seen it.
Click on the pic for more:


Thanks for the interest and the kind words,
Gene.

junkcltr

iTrader: (1)

BOV / CBV: 2002 VW Jetta 1.8T (marked Bosch 710N)
Same as the Audi P/N. Audi wanted $60,
and VW wanted $40. Box has Porsche
Audi, VW logos on it. Took a while to find it cheap.

It came from this thread https://www.thirdgen.org/forums/powe...bo-camaro.html
as post #10. There is some info. there about MAP sensor part numbers too. The rest is just stuff about learning the Syclone ECM that I ditched later on. The whole thread isn't worth reading, but the part number stuff in post #10 is useful. It is a second gen. build. Nothing fancy & small turbos.

EDIT: complete part number: Bosch 06A145710N. With large compressors I would run two or maybe just one and a larger BOV because of the small 1" diameter in/out ports on the CBV. Use them more for just the bypass part and slight blow-off action. Have a large BOV take care of venting off excess air.

Blue-Beast

Thanks for the link.
I'll have to take a look at it some time.