OK, me and a guy who "should" know, but I am doubting him has this argument going on. Facts: 440 CI engine 671 GMC supercharged @ 18% OD Hilborn 4 port injection on supercharger Question, when engine is at idle is the manifold under pressure or vacuum? I say there is vacuum at the manifold (under the supercharger) as well as at the injector hat (above supercharger) The guy who "should" know says no. He says there is a vacuum above the supercharger (at the injector hat), but a pressure below it (in the manifold). Remember, this is at idle with the injectors closed. Of course you get pressure at throttle when the injectors are open. This is the purpose of supercharging. What say you guys?????????????
there is vac at idle but the ? would be at what point over drive does it build pressure? boost builds under load so when does the point of pressure happen ?
I think your buddy's right!.. The only way to really tell is to hook up a vacuum/pressure gauge on the blower intake.... And a vacuum gauge at the base of the Hilborn injector... Some drilling and tapping is required...
Common sense tells me that the injectors have nothing to do with it and they still have to be open a little if the engine is running. My guess is that there is still a little pressure under the blower, even at idle.
Guys...think about it a minute! The only thing that keeps the engine speed down is manifold vacuum. If there was pressure there, the engine would rev up! But where would the air to create this pressure come from? The throttle plates are only open enough to let it idle. The whole intake system is at the same pressure (or vacuum) until the blower and carb output are greater than the engine's requirements, at which point the blower starts to build pressure in the manifold. At idle, or even at cruise, there is vacuum in the system.
Vacuum, absolutely. Until the butterflys open it will not make boost. I've got a boost/vacuum gauge under my blower and a vacuum gauge above it. I can show you sometime.
kerry i think you might be right if the blower is under driven but he's talking 18% overdrive, if the boost is less than the demand of the motor you will have a vacuum
What godspeedbear said. There can't be pressure until the butterflys open allowing air through to be compressed.
Vacuum. THINK about it. The throttle blades are open .010 in. Tremendous vacuum going on there. We run an alcohol BBC, 8-71, hat. We have a boost gauge on the manifold. It starts at zero. So it don't show vac. it does not show boost untilthe rpm climbs. And it idles @1500.
It's easier to think of this way- The engine can't build boost until it has something to push against (load). It's like asking how much force it takes to push open a door. If it's already open (no pressure required) we can't answer the question. But once you have 'work' to do, and a load can be placed upon it, then boost can begin to build. At idle, out of gear, you'll see vacuum under the blower. No load/no boost.
Vacuum at idle, thats why the carb guys have to reference the vacuum pots. I suspect that the more driven the pump (supercharger) the greater the vacuum at idle. If the blades are closed and it is driven 1:1 the then blower is turning at crank speed; if it is overdriven at 18% then it'll be spinning just that much greater than the engine and the suction should be proportional. Now, crack the throttle blades a little and all bets are off!
Definitely vacuum both above and below at idle. You can't build boost unless you have something to compress, and at idle there just isn't enough air coming through the butterflies to allow the supercharger to do that. The rotors are spinning in an atmospheric vacuum. Cool fact: Ever see a supercharged motor running with the belt off? A well-sealed supercharger still spins.
I knew the answer, but wanted some back up. As I told the guy, with .010 thousandths throttle leak,,,,,,,,,,,,,no way you can have pressure. Now when I had a pressure gauge on the car "back in the day", when I slapped the throttle open, I got prssure very quickly. Case closed...........and thanks.
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