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Old 09-29-2011, 10:06 AM
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Default Question for those who know about forced induction

I couldn't find a FI section so figured I'd post this here....

I know a little about FI but not enough to be dangerous, and thought of a question last night....

In terms of measuring boost at the intake, how is this measured? Wouldn't this constantly fluctuate depending on engine load? Or is the rated boost the max setting before any additional pressure is bypassed/blown off?

I'm just wondering if say your car runs 10psi, is it 10 psi at idle, and 10psi at WOT or is there a difference depending on engine load? I guess there'd be no boost at idle? ....my head hurts now...
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Old 09-29-2011, 06:10 PM
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No....with a supercharger at idle, you get minimal, lowest rated, boost at the intake. As the engine RPM increases, so does the supercharger boost until max pressure is realized and the blow off valve kicks in.

Obviously, a turbo will provide the same thing but at idle, they barely put out anything at all and really don't produce big power until the RPM's are up there a bit, unless you are running dual turbos, one smaller than the other as the initial boost supplier.
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Old 09-29-2011, 06:17 PM
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thanks maddog, makes sense
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Old 09-29-2011, 06:20 PM
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You're welcome, bud.
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Old 09-29-2011, 09:19 PM
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Quote:
Originally Posted by snrusnak View Post
I couldn't find a FI section so figured I'd post this here....

I know a little about FI but not enough to be dangerous, and thought of a question last night....

In terms of measuring boost at the intake, how is this measured? Wouldn't this constantly fluctuate depending on engine load? Or is the rated boost the max setting before any additional pressure is bypassed/blown off?

I'm just wondering if say your car runs 10psi, is it 10 psi at idle, and 10psi at WOT or is there a difference depending on engine load? I guess there'd be no boost at idle? ....my head hurts now...
The intake boost pressure is measured using a MAP (manifold air pressure) sensor just like a N/A manifold, they're just rated to a higher pressure. The MAP with a supercharger does change drastically based on engine speed, not load, where a turbo is more about load than RPM. A supercharger needs RPMs to make power, a turbo needs exhaust velocity. The turbo really needs to be spun up for it to make power, and the exhaust velocity at idle is negligable, that's why there's turbo lag; but once it's spinning, it'll keep going as long as there's flow; so even at low RPM it can still be making max velocity where a supercharger would start decreasing MAP.

The max MAP is goverend on both setups by the BOV as it'll bleed down the excess pressure in the intake system. The turbo also has a wastegate which limits the max exhaust pressure that the turbo sees. This prevents surging if you are on/off the throttle and put more pressure/volume than the turbo can handle.

Using the 10psi scenario, a supercharger would be pushing maybe 1psi at idle and a turbo won't be pushing anything. A supercharger will bring the pressure on much sooner in the RPM range, but once it reaches its max RPM, that's it, no more climb because you're limited to a max RPM based on the pulley sizing (so it's making 10psi at 3000rpm). Comparatively, the turbo comes on a little later in the RPM band (normally) but it'll keep increasing spool speed based on the exhaust velocity until it reaches max volumetric flow rate based on the turbo/intake geometry (so it may start making power at 2500rpm, and reach max flow at 3500rpm).
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Old 09-30-2011, 12:16 AM
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Ive never seen any sort of blower make boost at idle. See

If you build boost at idle you would have to have the TB blade open of the force would wreck the TB or the blower. When your at idle there has to be some sort of valve open to save the engine setup. Its why in a boosted car when you let off the gas it looks like its building vacuum instead of boost because the blower is spinning but the extra air isnt making it to the engine
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Old 09-30-2011, 07:59 AM
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Thanks for the responses
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