Bill Ball

Actually, I think Mark is right about the vacuum behavior being to drop idle fuel pressure. Once on the gas and the plate opens, the vacuum source, which is just past the throttle plate goes atmospheric.

BrianG

Sorry Bill.... but that's not correct... every inch of the induction tract is at essentially the same pressure (vacuum/boost) value, from the throttle plate to the intake valve. That pressure is proportional to the throttle position. The only circumstance where "atmosphereic" pressure is the correct value is at WOT.

I would challenge anyone to predict the actual (different) vacuum values at various points along the intake tract, if I'm wrong......

scott863

Just when you think you know quite a bit, someone brings up STOICH, thought perhaps Stoich was a nasty tech guy, or some questionable area of central cal!

Bill Ball

OK, I'll hook my vacuum/boost gauge up to the same line as the FPR and do the experiment in the morning. Since the SC is not in the car in the moment, it should be valid.

BrianG

Excellent idea. Remember, we are talking about "seady state" data points, so note each "throttle position" after 5+ seconds at that position.

(if you think this conversation is nuts, just wait until someone launches off on "transitional pressures")

One must use some care in attaching to a vacuum port on the throttle body. I believe that there is at least one "timed" vacuum port on the throttle body. This is a port that is actually AHEAD of the throttle blade. This type of port is generally used for an ignition timing signal. If you tap this type of vacuum port you will find "near-0" vacuum at idle, and a sudden jump to high (10+") vacuum at tip-in. The vacuum with then behave normally and fall proportionally as the throttle continues to open.

Bill Ball

Yes, there are more than one port on the throttle body. I have it hooked up correctly. Doing a little more reading about throttle position and manifold vacuum, it seems maybe my memory and Mark are going to be wrong.

worf928

For S4s (except MK's), the FPR/FPDs are hooked to the port behind the plate. The tank-venting valve is hooked to the port in front of the plate. The air pump is hooked to a port on the bottom of the throttle body but I've never investigated its position relative to the throttle plate.

I'm glad BB's going to do the test. I was about to go out last night at midnight when I saw these posts and find a 928 to test (shouldn't be hard) to confirm what I wrote in an earlier post. It was 9 degrees outside and the garage's not much warmer. CA is better for winter testing. Dat's fo sure.

BrianG

Not "wrong" Bill.......... just "at variance with the facts".

mark kibort

You have have to issues here . german to english translation and 928 vs 911.
emperical testing has some value here. (and some basic logic)

if im wrong, I will be the first to admit it. in fact, i would love to know more about the system. I am no expert on the topic. im just reporting on how ive used the system and short cuts to my advantage in racing. what about the fuel cut off switch, why do i have it disconnected. obvioulsy the engineers had a good reason to design it right? well i dont use it for a very good reason

mk

mark kibort

you must have forgotten , PV/NRT.
also, the air mass moves to differential pressure. the pressure will never be the same all the way through the system it will be different levels of gradients based on location and speeds of the air based on diameters. its not as simple as you think. the throttle plate as it starts to open, has a very fast rate of flow, for which several dynamics are happening. the venturi effect. the port location is key the fact that on the 84s US, there is a port that drives the timing advance and a port or a few ports on the other side of the throttle plate that drives the fuel regs and other idle vacuum driven systems.
Mk

mark kibort

let me attach the VERY accurate Sunx sensor that I have been using to determine pressures around the exterior of the car for aerodynamic analysis.

ill put it on a few parts of the intake if i can find holes to tap on and show you that you couldnt be further from the truth

first off

Mk

mark kibort

The throttle plate on the throttle body is strattled by 2 ports. one on the atomopheric side and one on the manifold side. the outside port will not have any vacuum generated until some fllow is passing, only because it is in the ventruri area of the throttle body. it is used for timing advance on the other older cars. the faster the flow, the more the vacuum here.

The other port on the manifold side, gets near manifold vacuum, until the plate opens. if you see a throttle body, you can see that the port is located at the widest point of where the throttle plate opens. this means there will be a differential pressure of the atomospheric side and and the manifold side. it will be like a gradient bubble around that area. the pressure drops around it and at a part throttle condition, the opening is only an inch or so from the outside pressure. It cant be athmospheric and it cant be under as much vacuum as the manifold. its somewhere in between. the wider it gets. air speed will slow as it opens, though density and temp of the air will change. again, air moves to differential pressure. the outside pressure is pushing on the throttle body. once it opens, the flood gates are open. the only reason you dont have max power at a crack of the throttle, is that it has a pressure drop across it. even at max throttle, the entire intake is a series of pressure drops.
so, you will never have the same vacuum in all areas of the intake.

all someone needs to do, is get vacuum pressure at the throttle body port we are talking about. then, take vacuum off the line that the brake booster gets vacuum from. look at idle and then compare at part throttle . you will see my point then!

MK

mark kibort

my vacuum port driving the fuel regulators attach just behind the throttle plate, like everyone elses and the 84US.
i think the tank venting port is actually on the vacuum side of the manifold to have those vapors injected to the intake, using a little valve that is controlled by vacuum on the vacuum side of the throttle plate.

MK

mark kibort

So, to fully understand what is going on with the throttle body, here are some pics.

Second two pictures: the tiny ports that i have shown on the top of the throttle body side (idle screw side) lead to the vacuum retard) it is on the top (manifold ) side of the throttle plate, but once you just crack the throttle, it actually becomes on the bottom of the throttle plate. (so, certainly no vacuum there, except for some venturi effect) One picture is closed with me blowing air into the port, and the other one is with the throttle slightly opened (cracked at best) and you can see the port is now actually on the other side of the throttle plate!

1st picture: the larger port on the vacuum side (manifold side or top side) of the throttle plate sees vaccum at idle. once you are a little past idle conditions, its very close to the outside ambient pressure, less the losses of the AFM and throttle plate. after that the vacuum will get progressivly higher as you get toward the intake valves in the heads! so you can easily see that there will not be a large amount of vacuum after idel conditions, as at idle, the throttle plate is closed and all the air is passed through the idle screw opening.

Ill get some very accurate readings from my Sunx sensor of this off the vacuum port that drives the fuel regs at idle as well as a couple of other emission devices. all throttle bodies have this port near the throttle plate, even my S4!

MK

worf928

The 'hunting' behavior you describe is I thinkapplicable for idle and steady-throttle application only. I have hooked up an ARM 1 to the 02 sensor output and seen that the hunting only occurs at idle or steady-throttle. The hunting stops if the throttle is moving. I have not however observed the behavior carefully enough to be certain of this. Nor have I disabled my WOT switch to determine what it would look like at WOT with no WOT signal.

But, I am certain that the hunting behavior is periodic and increases in frequency with RPM. At high RPMs the hunt is very fast - 10+ cycles per second. Unless the sample frequency of the EGA employed by a dyno is quite a bit higher the dyno-generated a/f chart isn't going to reflect the hunting.

And in either case, the dyno chart I saw with a near-constant 14.7 a/f ratio 'returned to normal' when the WOT switch was replaced.

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