vacuam

does a turbo or supercharged engine make vacuam?somebody asked me this today i realize once you get to a certain amount of boost the amount of vacuam would be very low,but on an underdriven blower or low boost turbo is there any?ive seen blowers on cars with auto trans and i know they didnt all have manual valve bodies or vacuam pumps,how does this work?

If you are asking about vacuum, yes, all non-supercharged engines make it. In relation to atmospheric pressure, of course. This is due to the piston descending with the exhaust valve open. A turbocharged engine when off boost functions essentially the same as normally aspirated (IE vacuum at idle) When on boost, the fuel/air mixture is forced into the combustion chamber under pressure, effectively raising the compression ratio. The intake vacuum makes more sense if you have hooked up a manometer (carb sticks) and thought about it for a few minutes.

A engine is a vacum pump, if you are forcing air/fuel into a cyl, it is still going to have to have some neg pressure in order to get the mixture into the cyl, this has got me thinking.
I will try to get in touch with the guy that builds our race car engines, he does all kind of neat things with the dyno, and sure he will have some thought

i told him when you stick your hand over the carbs of a blown or superchaged engine it tries to draw your hand in,wouldnt that be considered vacuam at least to that point before the blower?

Anywhere you're sucking air into the engine from open air (pulling through an air filter still counts) you'll get vacuum. It's just after the compressor that manifold pressure might exceed atmospheric pressure, which means that you get positive manifold pressure instead of manifold vacuum.

As mentioned, at zero boost you'll still get manifold vacuum, but as boost increases you'd almost have to get some positive pressure...

Shoot, actually now that I think about it you've still got to have some vacuum so the carb venturis still work.

Is the air pressurized before the carbs, or after?

Is it possible to run very high boost with carburetors? I'd imagine that with boost high enough you'd need pressurized fuel in order to atomize...

Now I'm really unsure. Jeff? Lorcan?

the air is pressureized after the carbs/fuel system, the limit on boost is going to be how much compression the engine can take without breaking the pistons, rods, cranks. this is why you have a waste gate, gives you the oppertunity to control the boost the cyl will get
you will still have negitive pressure until it goes past the turbo or blower.

With a blown through system, you get similar pressure differentials on either side of the throttle bodies to those in a normally aspirated system. With a forced induction system the pressure variation depends a lot on the size of the plenum chamber and won't be as great. I guess you could bleed some of the plenum air to the 'atmospheric' side of your vacuum operated transmission doodads or use a vacuum booster. Never gave any thought to putting a blower on a car with automatic an transmission.

-Duck

If the carb is "up stream" before the blower/turbo it sees a vacuum signal(and will shift) just as it would in any engine.
If the carb is "down stream" after the blower,it has to be in a sealed box (pressurized)then you open a whole other can of worms with fuel pressure regulators and all that stuff.
With an auto and "down stream" its manual valve body or electronic shift.

the local"blow-hard"was telling me a supercharged motor makes no vacuam period!i told him that untill the motor starts making enough boost to cancel it out,there is vacuam present.talked to 3 differant machine shops got the same answer i said,talked to dear ol dad(hes got this thing for blown Flat-bottem drag boats,no time for bikes anymore!ugh!)told me it has to do with "drive ratio" on blower,gave me a number for Blower Drive Service tech guy gave me a lot of neat info,most of which went right over my head!(ha-ha)but gave me pretty much the same answers underdrive/overdrive=boost!the local airhead still wont listen,but im satisfied!thanks for the help!happy wrenching!