BigBlockChev572

iTrader: (7)

Never really understood why an engine must have "load" for a boost gauge to generate a PSI on the gauge. With any FI you can freely rev an engine without load and 0 pressure will result on the gauge. However, put it in gear and hit the throttle and pressure reads. Isn't the physics all the same within the intake manifold where the gauge is reading with load and without as far as RPM goes and air flow characteristics? Higher Pressure going to a lower pressure (throttle opening) so what would cause a gauge to see differently?
Thanks!

Adrenaline_Z

Without a load, the power requirements to maintain idle speed is very minimal.
The engine will only use the power necessary to overcome the losses to spin
the motor.

Boost adders are designed to bypass building manifold pressure when the
engine does not sense load conditions.

If you can imagine the throttle blade almost fully closed and the pistons drawing
on the manifold, the pressure within will be low in relation to atmospheric.

When you hammer on the pedal and the throttle blades open, the manifold
pressure goes closer to atmospheric, and with the addition of the turbo, or
s/c action, the manifold pressure will increase further. This is the gauge action
you are seeing.

squealingtires

iTrader: (14)

I believe the bypass valve is responsible for maintaining the apropriate manifold pressure. The one way valve closes when the throttle opens and the engine builds boost. It would be impossible to build boost at idle because if you did you wouldn't be idleing anylonger. The same principle of the throttle body.

Adrenaline_Z

You might be thinking of the waste gate. The valve I am referring to will
prevent boost from building when the throttle blades are closed.

Without this device, the engine will free rev:

http://www.rpmoutlet.com/boost%20bypass.htm

squealingtires

iTrader: (14)

Thats it... wastegate. I couldn't think of the right word for some reason.

Country Boy

iTrader: (24)

I think his question is why does a turbo need load to build boost...

BigBlockChev572

iTrader: (7)

Or a Supercharger. Any type of FI. When a S/C engine is revved without load (clutch depressed, in neutral, etc) the boost gauge only goes to 0 even WOT. However when load is applied the guage will read "positive" boost with the throttle blade is open enough.

sytyjedi

iTrader: (6)

Because the engine is doing the work it needs (revving the motor) without needing boost. When you put a LOAD on the motor, actually making the engine produce power to move the cars weight, then you see boost. You're not doing any work by revving the motor. Work is distance over time. Once you start moving the car you create work.

Simple as that.

BigBlockChev572

iTrader: (7)

Yes, but why is that? You have the same pressure balance equal to the atmosphere at WOT load or no load in the intake manifold. Correct?

Mark98SS

iTrader: (1)

The wastegate controlles the boost so it doesn't apply any until there's a load

BigBlockChev572

iTrader: (7)

Or the Bypass valve on a S/C. What makes the byspass valve close when load is applied? Don't mean to sound like a dick asking all of these "why" questions, just trying to get deep into it.

Mark98SS

iTrader: (1)

It reads vacuum. The Procharger anti surge valve is similar, it's connected to the vacuum hose system of my car & opens to bleed pressure when off/part throttle

John_D.

iTrader: (2)

With the turbo, you need a certain amount of exhaust flowing to spool it up. Just reving the engine with no load won't create enough exhaust flow. If you held it wot for a 5+ seconds it probably would, but I don't think you want to do that with no load on it...

A blower can develop boost with no load because it's rpm dependent, instead of exhaust flow dependent.

DeltaT

iTrader: (5)

I can understand this question and some of the answers for a turbo. But the SC is mechanical - it produces boost (pumps air faster than the motor can consume it) at a certain rpm, period. The bypass valve closes as soon as the motor goes from a vacuum condition to a neutral or boost condition, because there is a spring in there that closes it anytime manifold vacuum is less than about 5 inches. I believe if I revved my motor with the blower, I would see boost even in neutral. You just have to give it a decent rev. I can test this when I get my starter back on.

Jim

89Formulaws6

iTrader: (52)

i never delt with a blower so i can't say much a turbo, diff story. its been covered above but the turbo won't spool in neutral cause theres no load on the engine to create the flow, maybe if you have a dinky turbo tho

but from waht you guys are saying i am thinking is the bypass valve is almost like a blowoff valve vents excess pressure when the throttle is closed

not sure what else you had for questions but let me know

Adrenaline_Z

Bypass valve = blow off valve. Same function different name.

The engine will create higher pressure in the manifold without a load, it will
simply happen in lower amounts. The pressure increase will show as an increase in idle speed.

Some people will think of boost as anything over zero (postive pressure) on
the vacuum gauge, when in fact it's a relative association.

IE: 12 in./hg. is a higher pressure than 10 in./hg. yet it is below atmospheric
at sea level (29.92 in./hg. at 0 degrees C = 1 atm)

An engine will only create enough power to maintain the current load, therefore
the idle RPM will rise to produce a higher load.

What do you think will happen to the power output of the engine if you reduce
the weight of the drum on a chassis dyno by half? Will the power go up;
go down, or remain equal?

EDIT: Drum weight is calibrated to acquisition software.

whiteghost

iTrader: (2)

you are able to generate boost on a turbocharged engine revving it in neutral if your turbo spools quickly enough. On my old mitsu evo i could see 7-8 lbs of boost if the throttle was opened quickly in neutral. For reference the engine saw full boost (27lbs) at 2800 rpm in a third gear pull, 19lbs @ redline @ 8000.

bc56f100

iTrader: (2)

Yea I can get about 3-4 psi out of mine in neutral by just reving it quickly! Of course my heads are ported out for high flow on the exhaust side!