I know it has been done, but I'm concerned about the efficiency of doing this. The Benefit: Considering you can't adjust boost level for a typical blower by doing anything but swapping the pulley. By adding a wastegate to the intake manifold instead, you can increase or decrease boost. It seems like a good idea. Instead of how a wastegate acts on the exhaust of a turbo application, can't it do the same thing for intake manifold pressures? For instance: If I only want 8 psi on pump gas, can't I install a wastegate to "purge" the pressure and keep it at 8 psi NO MATTER what the RPM's of the blower are? Then, when I run race gas I can adjust the wastegate to keep it at 12 or so? The only downside I see is possibly introducing more parasitic loss. If the blower is spinning at 45,000rpm's (motor 4000rpms) and is creating 8psi, and then the wastegate kicks in and keeps it at 8psi at 65,000 blower rpm's (motor 6000rpms) would this take more HP to spin the blower? Afterall, my blower is still only creating 8 psi so should it still be harder on the blower or take more HP to keep it spinning that fast? What are your opinions?

you're pretty much suggesting using a low pressure blow off valve or even something similiar to the bypasses used on GTP's and lightnings.

I was kind of thinking of the same idea for when I install my 3.4" pulley, I want the more boost to come on earlier but I don't want to run +16 psi as I go up in rev's. I'am not to sure how a wastegate works but I thinking maybe I can make unit up on the lathe with a plunger and some springs so as the boost comes on and over powers the spring the plunger will lift and bleed of what I don't want, just a idea.

Do you know who makes this low pressure blow
off valve?

I could be wrong, but I reeeeally don't think a BOV or even a low pressure BOV can flow enough to bleed off a constant 4 + psi of manifold pressure under high RPM and CFM boost.

EXACTLY! This is the prime benefit also, especially with a centrifugal s/c with linear boost.

I could swap out for the smallest possible pulley and start seeing 8 psi at 3500rpm instead of 5800rpm.

modern positive displacement superchargers like eatons already have this built in.

its called a bypass valve.

in OEM apps, it opens under any condition where you do not want boost. lets say, slow part throttle....
when its open, the blower is bypassed... its not forcing any air, and it draws almost zero horsepower.. its nothing more then a big idlerpulley.

this valve doesnt have to be wide open.. it could work, either electronicly controlled to maintain a specific pressure, or thru a simple vac/pressure feedback loop....

Yes!

It takes more HP to make 8 psi if you compress more air than you need.

Yes, it is harder on the supercharger.

Again, I really don't think a bypass valve can hold a specified boot level effiently. It's either all or nothing.

Of course, I could be wrong, but until someone gives me a real world example, I'm thinking a wastegate is really the only device that can do this accurately.

But you will NOT be making more than 8psi of compressed air with the example I gave. Yes, you will be moving more CFM and the blower will be spinning faster, but you will NOT be creating more resistance with boost because you will cap off the manifold pressure at a specified amount (i.e. 8 psi in my example)

more air means more hp if the boost is constant.

I've searched the web and asked a few people and so far I haven't found anything that would work like how I want, the only thing I know of that controls boost is a wastegate for a turbo application.

I'am sure it cost a little more HP to turn but not as bad as trying to use that full boost. To me it would be like comparing riding a bike up a hill to riding the bike on the flat using the same gear and at the same speed.

umm.

a bypass valve IS a wastegate. only more efficent.

if you hold it open, then you get zero boost.
so its effective at losing boost...

and its just a butterfly valve. like your throttle. your throttle isnt all or nothing, and niether is a bypass valve.

heck on a OEM supercharger app like a cobra, it opens progressively the more the throttle opens.
as the throttle opens, less vac is seen by the intake side, and as theres less vacuum, the valve slowly closes. this is why it rolls in boost when you roll in the throttle...


with a wastegate , you're spinning 16psi worth of air, and that energy is wasted by compressing 16psi worth of energy, the energy is absorbed by the springs and it holds it open.

with a bypass valve, you never make the additional boost, so you dont overheat the air more, and you save engery.

its more efficent because instead of making excess boost, then releaseing the stored boost, your limiting how much boost you make.

O.k. so in theory it should work right?

Then tell me how I can make it work? Tell me how I can control my max intake manifold pressure using a bypass valve to a constant 8 psi and nothing more no matter how fast I spin the blower. Then tell me how I can use a bypass valve to change that "ceiling" of boost at the push of a button or with a simple change.

If you can't tell me, then a wastegate/boost controller is still the only option I see.

But I'm all ears....seriously

One thing you mention however is overheating the air. I believe that since a centrifugal s/c operates by accellerating air it could in fact heat the air up too much to make 8 psi as efficient at a lower blower rpm 8 psi could. hmmm. That's interesting.

it would heat the air up alot more.


first thing that popped into my mind ofcourse is my car.
eaton M112 from a ford lightning on a SBC. my vacuum is made by a electric vac pump, so i constantly have a strong vac signal.


take the stock bypass valve, vac silinoid (opens when elec is applied) and a boost switch.

when the boost switch closes, it opens the vac 'noid. this lets vaccum pull on the bypass valve, closing it. that is, until it reaches its set level.. say 8 psi. then the switch will close.

yes it will open and close really fast, pulsing the valve to maintain constant boost. this is a easy electrical way.

another way is to tap the underside of the vacuum actuator and seal it off. the refrance to the actuator would be to the boost under the blower.
when the boost under the blower exceeds the spring rate (could be set by a screw) the plundger would lift, opening the valve. this is a simple mechanical way.

the prefered method in my mind would be to use some form of built in programmable electronic wastegate controller. since all it has to do is move a lever, exactly like a stock wastegate in a OEM app.

the two really arnt that diffrent in actuation. the diff is that one vents access boost, the other prevents access boost from occuring.

Cool. Thanks Mr_dude1. I have a cheapo bypass valve rigtht now, but I'm going to do some research on new ones. I'll have to rereard your post a couple times too before it sinks in since my blower setup is different, but I think I understand the concept.

A wastegate is a valve that allows some exhaust gas past a turbine to control turbine speed. A bypass allows some of the pressurized air after a compressor back into the compressor inlet to relieve pressure when the throttle closes. A blowoff vents that same pressure to the atmosphere.

No, not unless it’s huge, and if you used a wastegate that big on most turbo setups you’d actually have trouble controlling boost. You can wire most wastgates open and the engine will still see boost, but it will happen slowly.

The bypasses used on the eaton blowers are there to remove the load of the compressor from the engine under light load (high vacuum) conditions, not to regulate boost. This is how eaton gets away with making the claims of minimal (typically 3-5hp) loads on the engine and very little effect on gas mileage under normal driving conditions. They do have some effect on regulating boost but only because most of them do not have a complete butterfly, it usually has a section cut out of it, so even if it’s completely closed there is some air bypassing. It never opens when the throttle is opened.

On my brother’s car we got rid of the actuator and replaced it with a mechanical (cable) linkage straight to the throttle, so basically, it only closes when at WOT, and is open the rest of the time.

Not a clue where you’re going here. “spinning 16psi of air?” huh?

I’m not sure I’m really following the whole point of this discussion… why not just put a restriction in the intake before the compressor. That will limit it’s ultimate flow (limit the flow at the higher rpms/airflows) without having much of an impact at lower rpms/airflows. The compressor will still maintain the same pressure ratio and airflow across the compressor, but now since the compressor is seeing a lower pressure at its inlet, that pressure * the pressure ratio will be a lower pressure.

If you insist on some kind of valve (don’t see the point in making something mechanical if something without any moving parts will do the job and will be much less likely to break), then what you want is a butterfly or some kind of sliding valve that has a reasonably long travel an a pressure actuator with a similar travel vs pressure. The longer travel will allow smother action, and when you get it “tuned” right you should be able to adjust things so that you don’t feel it at all.