My assumption before test-driving a Cobra yesterday, was that a supercharger would always be showing a positive pressure on the boost gage. On the drive yesterday the gage stayed pegged at "zero" - almost like it was under a negative boost - until about 1800rpm when the supecharger whine really kicked in and the boost finally took off. Is this normal - no boost in the lower rpm?
Also could not believe that Ford would have an economy shift light on a car like this! Or is it Government regulations to the extreme!
My boost gauge doesn't move in most normal daily driving. It only bumps up when I get my foot into it. I think that the bypass valve lets the engine run like an n/a in most driving situations, if i'm not mistaken.
And yes that shift light is a royal pain. Just like the traction control -
Yeah, the boost is only applied as a function of load. If you're going a steady speed, on level ground, then there won't be any boost. Tell the car to accelerate and the boost will engage. More acceleration means more boost applied.
As far as the shift lamp goes: I called SVT. The guy said that it was due to a govt regulation. Ford would have had to charge us another tax if the lamp wasn't there. It was driving me crazy (crazier?). I've heard that a "chip" program could be used to disable it, but my engine is still stock. So, I removed the instrument cluster (which was a bitch) and removed the lamp (which was easy). Now that I know where the lamp is, I think the job could be fairly simple.
If you'd hook up an actual boost/vac gauge, you'd see that you are, in fact, in a state of vacuum during most of your daily driving. I had a Thunderbird Super Coupe with a stock boost/vac gauge and a majority of the time, you're under vacuum just as a normal car would be. Opening the throttle any more caused the bypass to open and let the compressed air from the blower into the engine.
My '89 turbo Probe is much the same way, only you can actually *feel* in the gas pedal where the turbo will kick in. The pedal is light, to a certain point, which also can change with load. Driving downhill, the kneepoint is much farther down as you'd have to be trying harder to get boost. Uphill at
70 MPH in 5th, the kneepoint is almost at the top. Watching the boost gauge, resting the pedal on the kneepoint yields a 0 PSI pressure, but breaking that quickly throws the turbo into the mix and the gauge jumps up.
This allows the car to consume less gas, be more driveable around town, and not break as often. The car will be much happier without constantly being force fed. Just like anything, the harder you run it, the more likely it is to break.
The Eaton has a boost bypass valve that the PCM uses to manage the pressure. Under low load/normal driving (positive to zero vaccuum) the PCM lets the blower free wheel. When the load goes up the bypass valve is closed and boost is applied.
That light saved about $600 in gas guzzler tax. Ignore it, put a piece of black tape over it, or put in a chip. The Diablo Predator might be able to disable it too.
I don't think it has anythig to do with the blow-off valve. That is to prevent damage from things like a backfire.
The reason is because the Eaton and other top mounted blowers are draw through systems. They draw air from the throttle body. If it isn't open to get enough air it won't produce boost. This is one of the daily driver advantages of these blowers because they don't put as much parasitic load on the motor when you don't need the power. When you open it up to let enough air thorugh they will build up pressure.
Turbos act like this as well but for a different reason. They are driven off of the flow of the exhasu and not a belt like a blower. When the motor is not under a load it does not build up as must exhaust pressure and the turbo does not spin fast enough to make boost.
Not sure exactly what happens with blow though systems like the Paxtons and such. I would think that they make boost as long as the motor is spinning fast enough. Again the throttle body will block air flow so I guess it depends on where you chak for boost pressure. Before or after the TB.
The valve on the Eaton is actually a bypass valve not a blow off valve. The PCM uses it to manage the boost pressure under various load conditions. Under low load the valve is open allowing the blower to free-wheel and minimize the parasitic drag on the motor. This increases economy under normal driving conditions. The PCM also uses the valve at the rev limit, dumping boost to get the intake down pressure as the system pulls fuel.
The bypass valve acutally increases the effectiveness of this strategy.
On my 2k GT tt was a belt driven pump. I don't remember if it was the steering pump or a separate pump. I remember seeing it for the first time and thinking, wow, that's one less thing to worry about if I ever add a turbo or blower. I traded the GT in on a new Cobra. Next time I think about it I'll take a look.
Some folks disconnect the boost bypass solenoid. Some also do the "boost bypass mod" which eliminates control of the BPV under WOT. I've tried both mods and neither provides any gains in power or any other benefit under most conditions that I can tell.
These two mods got started with the Lightning crowd. What was happening was that the PCM was pulling boost after a burnout in the box during drag races. It took a few seconds for the PCM to give back the boost so the truck was at a disadvantage. These mods took control of the BPV away from the PCM so that the loss didn't occur while heating up the tires.
With the Cobra this problem never seemed to crop up so I suspect the PCM strategy was adjusted. Also, since the PCM uses the boost bypass valve to dump boost at the rev limiter, if it were plugged or disabled it's likely the motor would go !pop! when the PCM pulled fuel under full boost. A chip can push the rev limiter back to help protect this from happening but since there's no gains I, personally, don't see a reason to do this tweak.
Dan
2003 Cobra convertible With some stuff and things
[snip]
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.