On a stock pulley, will it show a vacuum most of the time? Unless I am on the throttle above say 20%, my boost gauge is reading on vacuum side. If I'm not producing pressure, does that not hurt mpg? Even at say 55 mph, on level ground just cruising, I'm not having any pressure. Is my car asking (begging) for a smaller pulley? Thanks. Brian

 

there is a valve on the car, that shuts as your throttle opens, even though the supercharger is spooling boost, it isn't forcing it in unless you are into the throttle, do to the bypass valve being open. it shouldn't hurt your mpg as long as you're not building boost. hope this helps! :D

 

Is it harmful to adjust or modify this? I feel there should be boost at all times. What's the point in having a s/c? If I install a smaller pulley, would I have to do something with this valve? I don't mean to sound flippant. I may not understand the principals behind the s/c Thanks. Brian

 

Do not adjust the bypass valve, Buy a pulley if you want to go faster/have more boost.
if you don't, don't worry about it.
your car is functioning as normal.

 

the point of a supercharger is to have boost without the lag, and you do. whenever you want it, its there. however, its not always being forced into your motor, only when the bypass valve is closed. a smaller pulley will give you more boost, but you still have to give it a decent amount of throttle to allow it to be forced into your motor.

for the most part. if you have a s/c mini with a 19% pulley pushing 28psi (don't thrash me for this, im not sure on the boost[but figuratively]) and a stock mini, and they are both using 1/4 throttle, you will accelerate the same. the power without boost will be the same in every mini unless you modify something that has nothing to do with the sc/turbo such as cams, head, ignition, **exhaust/intake**(does have something to do with forced induction) but will benefit the motor off of boost also.

 

I seem to also have some boost issue as well but im going to change out by bypass valve and see if that helps.

 

Making boost all the time will drop your MPG substantially. There will be more load on the motor dragging down cruising mileage. Also the fuel mixture needs to be richer to prevent knock under boost. Your car is working perfectly fine and doing exactly what it should.

 

My '03 MCS has a 15% pulley with an ATI 2%, a DT Bypass valve and RMW tune. The mods give me more boost than stock with a quicker responding Bypass valve however just like you most of the time it runs off boost.

My right foot is in control of the boost numbers and I wouldn't want it any other way. My car wouldn't be safe or comfortable to drive on boost the whole time I also installed a Sprint Booster which allows 3 modes of accelerator sensitivity.

Lots of mods available to " boost yur BOOST", but your right foot and structure above your neck needs to remain in control!!!!!

 

Eggburp you jogged my memory of my experience being in constant boost while cruising on the highway recently. I live right near the newly open Texas 130 with the 85mph speed limit. I went for drive down it and was cruising at 90mph. The RPM's and load on the car are high enough that I was in constant boost. Any touch of the gas made the car lurch forward, Not very comfortable at all. The cruise control was also useless. With my mods(which aren't that extensive) the cruise control couldn't compensate correctly. When it tried to maintain speed it would accelerate pretty hard and over shoot the cruise speed by 3-4mph then close the throttle and de-accelerate down to 3-4mph below and then repeat. This all was very herky jerky and would make you motion sick pretty quickly.

It wasn't as bad as an 84 RX-7 I had though. I swapped a later turbo motor in and was making 260hp at the wheels. I tried using the cruise at 60mph and it was exciting to say the least. The cruise ECU just had presets on how much throttle to give the car to speed up based off a 130hp engine. With the new motor it would rocket you up to 75mph and then close the throttle and repeat.

 

I thought the tune got rid of the need to have the booster controler or is it just a added push for you? I dont mind a minor lost in fuel not a big problem for me, but like eggburp said ur in control of the boosting.

 

detroit tuned bpv ftw! it has a pretty hefty spring on it and snaps shut immediately. also a good tune will get rid of your delay on the throttle response. power on demand is fierce.

 

Thanks. I do believe that is the answer I was wanting. I know how a s/c works. I needed to know more about the bpv. I appreciate the explanation of how they interact. Interesting stories on what it's like with constant boost. Brian.

 

Yeah just to add in...

Off throttle (engine breaking) I'm at 25in/hg, idle is around 17in/hg. If I'm getting off the line or part throttle on a hill, I'll read between 10in/hg to 8psi, just depends on throttle modulation. Near redline at WOT, it peaks 11.5-12psi.

Your car is normal.

 

I think you may have a leak - you should be running anywhere from 14 - 17 pounds boost at WOT with your pulley.

 

I 'have' the pulley, just too lazy to install it. Still stock pulley.

 

I'm not sure that "mods in the boot" count on a signatue!!
In all honesty, get it done, it will transform the car!!
Back to the op....
When power is not needed, like cruising at lower rpms...3000 ish, the bypass valve is open...reducing load on the motor cause the sc has no force to fight against when compressing the air....so the car can get MUCH better fuel mpg....when on boost, the intake temps are also hotter...resulting in the need for a richer mixture...so even lower mpg.
Since there is a varrity of places to plug in a boost guage, and varrying qualitys of them, if the car is performing good, i would not worry!! In a stick...you would just drive it ONE gear lower to keep the rpms up if you wanted to stay on boost....the auto SHOULD drop a gear when you floor it to make a pass...so functionality similar...just the momentary pause for tge downshift.....a manual does have some advanges!!

 

A few folks expermented in zip-tieing the bypass valve closed....
No real gain in performance, and you get a 5-25% (guess) in mpg depending on the rpms you would normally cruise at.
Rememer, a sc is not a turbo....it is ADDED power WHEN YOU NEED/WANT IT!!

 

The bpv works like a computer controlles wastegate on a turbo car...it keeps the boost down until you need it (although a bpv doesnt dump the boost it just reroutes air around the supercharger and in an over-simplification you're driving a naturally aspirated car when your right foot isnt digging too far into the go fast pedal).