Stupid question of the day...

Why does the boost guage indicate a *negative* boost? The guage shows -10 to 0 to +10. When I'm at idle it sits at -10. When I'm trying to beat the neighbors grandfather in his grandfather-ish Lincoln, it might jump from -10 to -2.5, or even creep up as high as +1. When I stomp on it from a dead stop it jumps up to +6 and (sometimes) higher. Normal driving puts th needle somewhere between -10 and 0. Does a -6 negative boost indicate that my intake air is being expanded by 6 psi, rather than compressed as it should be through the supercharger? Do I not see benefit from the SC If not +0?, why not just start with a 'zero' on the left and indicate psi from there?

First of all what year and class of bonneville???

Then all the smarties can put their heads together.

Negitive boost......Vacuum ????

ken

you don't see boost until after +0, under 0 and the intake air is entering as if there was no supercharger I guess. The supercharger doesn't actually give a boost in horsepower until a certain RPM is reached because it'* not spinning fast enough.

The gauge is reading manifold vacuum below 0. This is what a non-supercharged car would read all the time.

Yeah to go a little more in depth, and because I like to beat dead horses...but only on Mondays, they don't expect it on Mondays....Anyway, yeah the negative numbers are how much negative pressure *read suction* the pistons are causing on the manifold. At idle its neither a good idea nor necessary for the supercharger to be pounding air into the manifold, you only need this when you're accelerating. At the inlet of the supercharger you have another valve you can't see that looks like your throttle body valve only smaller. At idle or cruising this valve opens and lets air around the supercharger so it never gets compressed, which is why it stays below 0. This is roughly how much power your car would have if you ripped the supercharger off, sad huh? Right now this gauge is handy in diagnosing engine problems. If it only read 0+ you wouldn't have the benefit of possibly figuring out what is wrong with your car. I'm only recently reading into this but here are a few examples:

Intermittent Fluctuaiton at idle - Ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft

Gradual Drop and Idle - Clogged exhaust, excessive backpressure (extreme cases engine will die at idle)

Mainly all you can use for diagnostics is at idle, above that and the diagnosis is skewed by the supercharger ramming air into the manifold and smoothing out the needle. Just my thoughts though.

Thanks for the clarifications.

Mandible