Eplebnista

Is there a chart or a "map" of typical intake manifold pressures at various RPM's for a normally aspirated S4?

Thanks

Rob Edwards

Here's the map from 800 to 1500 rpm (16 to 21 in-Hg)

AO

Mine goes positive for some reason...

Eplebnista

Exactly. Because the supercharger output is offsetting manifold vacuum, you need to know what the baseline normally aspirated manifold pressure is to know if your supercharger is working, when the boost is coming on and how much boost you are actually getting at WOT. If your supercharger is making 6 psi and your intake manifold vacuum is the equivalent of - 6 psi, your boost gauge will read zero. The boost gauge is really only useful if you know the baseline.

Every healthy engine is going to be a bit different and I assume that vacuum is also dependent on load, but a chart or map of "nominal" intake manifold vacuum to use as a baseline would save you from having to take a bunch of readings with your supercharger disabled and then comparing it to the readings with the supercharger running.

Lizard928

He was trying to be funny......

Every engine is different. Altitude, air density, humidity, cam timing all are factors which affect manifold vacuum.

Eplebnista

I know he was trying to be funny. I have no problem with that.

I was driving a friend's supercharged car and I knew what boost the setup was supposed to produce at max output. Since it wasn't my car, I did not really flog it. I did notice that the boost gauge rarely went positive and I assume that the output of the supercharger was offsetting the manifold vacuum at part throttle but only rarely went positive.

Just thinking about stuff and trying to learn.....

AO

I believe you are incorrect. Manifold pressure is manifold pressure. Either it is pressurized or it is not. I don't think you have to net what the engine consumes from what the SCer makes.

Remember the engine is a giant air-pump and the SCer is only there to help you pack as much air into the combustion chamber as you can so you can add more fuel which gives you a bigger bang and you end up going faster.

If you have a supercharger and you're not making positive manifold pressure, then you most likely have a boost leak or a belt issue or a bad boost gauge.

Remember that at "Zero" you effectively have 1 atmosphere of pressure.

Does this make sense?

Eplebnista

Yes, it makes sense.

I think we are both right, depending on whether the "rated" output of a supercharger set up is the Absolute Boost or the Relative Boost.

As described elsewhere on the interblab: If the general understanding is that a supercharging set-up's output is an expression of "Absolute Boost," then your view prevails. If the general understanding is that a supercharger system's output is measured as "Relative Boost," then I am closer to the mark.

In either case, the only time that pressures upstream and downstream of the throttle plate are going to be near equal to each other is when the throttle is wide open or nearly so. Isn't this why superchargers and turbos have vacuum controlled diverter or dump valves?

Given the operation of the diverter valve, I would imagine that the impact of supercharging is likely to be seen mostly when the throttle is more open than closed and largely in the last third of its travel.

FredR

What you see on a boost gauge is basically a measure of what you cannot get into the engine [i.e. nett differential pressure]. The more "boost" pressure you have the more nominal driving force you have to get air into the motor.

On a n/a motor the pressure on the piston crown is basically -14.7 psig or 0 psia [a full vacuum]. The pressure outside the car is 0 psig or 14.7 psia. Thus the driving force is 14.7 psi [differential pressure]. If the compression machine lifts the pressure in the manifold to 10 psig the driving force is 24.7 psi and air flow is nominally proportional to the square root of this driving force.

Bearing in mind manifold pressure is measured inboard of the throttle, in a n/a motor, some proportion of the losses are incurred as air flows through the inlet conduit to the point of measurement and the rest is lost from that point onto the piston crown. If we assume forp urposes of this discussion the split is roughly 50:50, and translate that into a boosted motor, the ratio of the boosted pressure [24.7 psia] to the n/a pressure [nominally 7 psia] is 3.5 and the square root of this is 1.8 thus you see an increase of airflow of 80% thus a n/a motor producing 280 rwhp will [at 10 psi of boost] develop approx 500 rwhp. Do those numbers look familiar?

A supercharger will also devour about 30 bhp or so moving that amount of air whereas a turbo will not.

In the case of a twin screw s/c like Andrews, providing the machine is well sized it will move air directly proportional to the speed of the engine. When this happens the engine will produce torque more or less flat lined [table top torque curve] from tickover to top end. There are also factors like exhaust resonance, reversion and machine efficiency that have some impact.

When a centrifugal s/c is used head generated varies as the square of the s/c speed so at low engine speeds the supercharger has little to no effect but by the time the engine reaches about 3k it starts to cook and onwards and upwards from there [better for top end development]. This also explains why torque peaks much higher up the rev band with a centrif s/c.

Hope this helps put some perspective on the dynamics of what happens and why.

Regards

Fred

Speedtoys

Mine's now broken since Sunday like that too..

Speedtoys

Unless you have a _manifold pressure_ gauge and at 0 you are reading absolute vacuum.

mark kibort

a turbo will consume HP very similar to what a blower will consume. it all consumes crank HP wheather the pistons (attached to the crank) pump the exhaust gasses out and push on the blades of the impeller of a turbo, or the crank spins a blower that compresses the air flow, its all net losses. Less for turbo, but still significant.

FredR

Mark,

The principal difference between the two boost variants lies in thermodynamics. The supercharger is nett parasitic loss from the crank the turbo is not. The turbo is recovering energy out of the exhaust gas whereas the s/c is not. From a thermodynamic point of view the energy going up the spout is about 70% of that burnt - on the tubo you recover 10% of that [in round terms]. In that context I agree with you. This is why John Kuhn gets 50+ bhp free and gratis compared to a supercharger same psi boost level.

Of course it is all comparative when you consider some 600 bhp equivalent worth of heat is going up the chimney on a stock 928.

Best wishes

Fred

mark kibort

in the real world, we see idential engines with near 500rwhp make about 20hp more with a turbo vs a blower at the high boost rates. (being very general here)

BC

Incorrect. Completely different relationship to BHP measured at the crank and what of that number is being "consumed".

100s of HP difference at 18psi on a 928 motor. 28psi required to make 675 on a blower, and 18psi on a turbo is making 720.