Is there a correlation between engine vacuum and a free flowing exhaust?

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Old 08-04-06, 08:18 PM
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Is there a correlation between engine vacuum and a free flowing exhaust?

A friend of mine has a 92 Nissan Maxima with the DOHC V6 engine, 5spd. He's having a problem passing emission due to a high Nox reading. When he applies vacuum to his EGR valve the diaphram moves and it holds vaccum without bleeding off. If he does it with the engine idling it will stumble and stall. That tells him the EGR valve itself is good.

I suggested he needs to diagnose that controls the EGR valve, and with the Nissan it's a EGR solenoid valve. The solenoid valve is control by the ECM and the vacuum source for the solenoid valve it fed directly off the throttle body right below the throttle plates. The solenoid valve is a normally open valve and the ECM provides a signal to close the valve which blocks the vacuum source that's going through the solenoid to the EGR valve.

He has check and double check to make sure he doesn't have a vacuum leak by visually checking all components and spraying starter fluid all over the vacuum hose to see if the engine rpm changes while at idle. In which it did not.

He has 18" of vaccum at idle and he's pretty sure the EGR solenoid valve curcuit is working correctly. He lives in the state of Cali and he's been fighting this emission problem for a couple months. He stated Cali DMV keeps giving him a 30 day extension just as long as he shows evidence that he's trying to correct the problem.

We're both car guys, and like most car guys if you're having a problem with something you're not firmiliar with we go searching. During his search he came across this particular website

http://autorepair.about.com/library/faqs/bl491b.htm

The reason he came across this link is because he has a modify exhasut system on his Maxima and he's thinking his exhaust could be causing his vacuum to be lower than normal while cruising. Which is causing his EGR valve not to open completely. The state of Cali is checking his emission at 15mph and 25mph on a chassis roller.

For arguement sacks lets forget about the many possibilities of what can cause a high Nox reading but discuss can a free flowing exhaust effect your vacuum reading? if so how?

We know a mild to hot performance camshaft can effect engine vacuum but I just don't see the correlation between engine vacuum and free flowing exhaust.
 
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Old 08-04-06, 08:49 PM
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Well,at least theoretically a restrictive exhaust could reduce the vacuum. If there is nowhere for the exhaust to go then the cylinders would not completely evac which would then cause less vacuum in the cylinder.

Inversely, I would suppose a free flowing exhaust would allow the vacuum to be maximized/ Especially since you get a scavenging effect with a freer flowing exhaust. Actually I believe an exhaust that is too free flowing will reduce the scavenging effect and actully cause a similar situaon (but not as much) as the restrictive exhaust concerning the vacuum levels.

This is one of the old things I learned long ago which is if your exhaust is too free flowing or too effective at scavenging, you will burn valves. It actually draws unburned fuel/air into the exhaust during any valve overlap at the end of the exhaust/ beginning of the intakes strokes. (before stellite valves)

I don't kmow if it would have that much effect on the vacuum but it seems it would affect it to some degree.
 
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Old 08-05-06, 04:12 AM
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[QUOTE=CandiMan]


For arguement sacks lets forget about the many possibilities of what can cause a high Nox reading but discuss can a free flowing exhaust effect your vacuum reading? if so how?

QUOTE]

I think I know what cause high NOX and EGR circuit problem of your friend Nissan but ...... since I'm really bad about arguement on what cause what and why,where and how thingy, I'll take a rain check on this .
 
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Old 08-05-06, 07:14 AM
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nap, that's interesting. I guess there can be some correlation between a modify exhaust and engine vacuum. I wouldn't believe it until i see it. He's either planning on re-installing his OEM exhaust, inwhich he has to hunt around the junk yards for. Or modify his modified exhaust to increase restriction. I've been dabbling in cars for 10-12 yrs and at one point even made a living turning a wrench. So this will be new to me.

New guy, even though I didn't want to debate what could casue a high Nox reading, if you have a possible insight on this particular Nissan I'm all ears. Post here or drop me a PM. I'm guessing your possible insight is not related to a free flowing exhaust, but I'm still all ears.
 
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Old 08-05-06, 07:27 AM
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Simply stated, a vacuum, as we utilize it, it actually a lower than atmospheric (or ambient) pressure compared to ambient pressure.
If there remains anything in the cylinders, they will not be able to lower the pressure as much as a completely empty cylinder. As elsewhere reffered to, an exhaust can have a scavenging effect which actually can impose a slight vacuum to draw the exhaust from the cylinder, thereby reducing the pressure even more.

2 stroke motorcycles are the easiest to see this theory at work. The expansion chambers that were designed for the racing crowds actually increased this scavenging effect and since 2 strokes do not rely on an exhaust stroke to evac the cylinder, the increased scavenging effect of the expansion chamber increased the amount of fresh fuel/air charge into the cylinders.
I has a down side though in both 2 and 4 stroke engines. It can actually carry unburned fuel into the exhaust system causing myriad problems. Melted cat convertors, OČ sensors getting fouled and a plethora of other problems may arise from too much flow.

Like I posted before, I don't know if the difference is measurable, ( I would think it is but the amount of change may require a much more accurate gauge than most folks utilize) but the effect is definately quantifiable.
 
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