EGR Valves - Ported vs. Manifold Vacuum

Contributed By: Michael Baxter

There are two sources of vacuum which is used to power many systems on our FSJs. Manifold vacuum, which is the present in the intake manifold, is the main source. It powers systems like the power brakes, heater controls, diverter/air management valve, etc.

The manifold vacuum signal is high at idle and even higher when decelerating at relatively high engine speed with the throttle valves closed. At sealevel, the idle manifold vacuum is usually in excess of 20" HG.

20" HG is 20 inches or water column on a manometer. When the engine is off, the pressure differential between the inside of the intake manifold and the atmosphere on the outside of the engine is nothing or 0" HG. In other words, the pressure is the same on both sides of the intake manifold's walls. If we could pull all the air out of the intake manifold and form a perfect vacuum (as in outer space), the pressure differential would be 29.92" HG at sealevel standard atmosphere. 20" HG is a strong vacuum signal produced because the engine is really an air pump and it is working hard against the restriction created by the nearly closed throttle valves when idling.

The vacuum signal at idle would be even stronger if it weren't for the overlap period ground into the cam shaft. There is a certain amount of time (I believe 21.5 degrees of crankshaft rotation for each cylinder in the 360's case) that both the intake and exhaust valves are off their seats. Some of the air (and fuel) leaks out of the intake into the exhaust during this period. Especially at idle since port velocity is low.

I don't know exactly how much vacuum signal it takes to fully open an EGR valve. I would guess it would be fully open by 10" HG or less. If we hooked the EGR valve up to a 20" HG manifold vac. signal, it would be fully open.

Air (which is composed of gases) follows all the laws of fluid dynamics and flows via the path of least resistance. With the EGR valve open, a large vacuum leak is introduced and the path of least resistance is from the exhaust system, through the EGR circuit and into the intake. Since the exhaust basically contains very little oxygen or fuel, the engine will die immediately with little struggle.

An engine with a functioning EGR valve and no plugged-up passages in the EGR system, will not idle with an EGR valve connected to manifold vacuum.

Now it is possible you confused manifold vacuum with port vacuum. Port vacuum signal is generated via Burneulli's Principle in the carb's venturi. The larger the volume of air flowing through the carb., the stronger the vac. signal. At idle there is very little air flowing through the carb and the vac. signal is very small. Basically no vacuum. At wide-open throttle (WOT) and high RPM, the port vacuum signal is strongest. T

he EGR valve's diaphragm is hooked ultimately to port vacuum. At idle there is no signal and the valve is closed. At WOT, the valve would be fully open except there is a calibrated vac. switch inline which is referenced to manifold vacuum and will dump the port vac. signal, closing the EGR valve, when the engine is under a significant load.

The other system normally (when the engine is at op. temp.) connected to port vac. is the vacuum advance mech. in the distributor. Hooking the vac. advance to manifold vacuum will decrease WOT performance but, won't significantly hurt fuel economy. When cruising, manifold vacuum is relatively high and the timing is advanced. But at WOT when the manifold vac. signal is very small, there is no timing advance beyond the mechanical advance's max.

Another thing: hook those EGR valves up the way their supposed to be and make sure the whole system is operational. They reduce NOx which is an orderless, colorless chemical concoction which destroys lung tissue. Some modern cars don't have EGR valves anymore but, they still have EGR via careful cam profiles. The engineers time the overlap period so the fresh air/fuel mix is diluted with inert exhaust gasses.

James already stated advancing the timing would increase NOx production. So will lean air/fuel ratio, vacuum leaks and higher compression ratios. A correctly functioning EGR valve reduces NOx production greatly and doesn't affect performance. Which allows us to benefit from better performance without poisoning all living beings including ourselves.

-- Michael Baxter
From Reno, NV USA on 06-Mar-1999