Whenever I close my cutout, just when I leave a nice neighborhood late in the night, my idle quality suffers greatly. Now obviously my cam selection is nothing modest and overlap is high. I do have relatively restrictive headers, mac mids, a horribly pieced together y-pipe which probably works just fine for bolt ons, and with the cutout closed, a highly restrictive muffler. I deduce this due to the fact that the car sounds like it has stock cats, no internal engine modifications, and just a catback when the exhaust flows through the Borla muffler.

I do plan on substituting some used Kooks stepped headers with a nicely configured merge y-pipe and a catback this is still quiet, but possibly flows better in the future, but for now, college student cashflow has me limited.

So, do we think it is possible I am experiencing a natural EGR when the cutout is closed? FYI this is a Borla adjustable catback with the cutout just over the axle.

 

no one?

 

Who did the tune?

 

PatG did the tune and at that time, I did not have an electronic cutout, but the flange was always open.

 

What else have you changed since then? I doubt its in the tune since Pat G did it.

 

Nothing at all has changed since then. The tune seems to be spot on for everything else. I do not think this is a tuning issue.

 

I dont think i would call it natural EGR......

if you had enough back pressure to cause it to run rough at idle I would imagine it would run pretty badly at mid and full throttle also as this is where pressure can cause problems. What is your cut out wired up to for power? Plus if you crack the cut out open just a hair does it start to idle fine?

 

I still think it is. The entire point of large cams is high rpm efficiency, that causes low rpm efficiency to fall. For instance, my cam, has ~18.5 degrees of overlap at 0.050, at low rpm, the exhaust hangs open way too long for the velocity of the exhaust pulses. At low rpm, everyone knows intake velocity is much less than that of high rpm intake velocity. That is a very valuable/critical force to counter the exhaust backpressure. With the slow intake velocity at low rpm, it cannot counteract the back pressure from the exhaust, IE when exhaust cut out it closed, and a natural exhaust gas recirculation is kept in the cylinder versus fresh fuel and fresh oxygen.

At mid to higher rpm, the force from the high velocity of the intake is much greater than the resulting force from backpressure of the exhaust, and keeps the natural EGR to a minimum. These engine dynamics, pulse tuning, play a crucial role in determining the efficiency of the engine.

Now, my muffler has never really been fully functioning. The previous owner, as well as the original owner, hardly ever had the block-off plate in the flange. I do not know what this has done to the muffler, but I can tell you as quiet as it is, it is very restrictive. Now, there are free flowing mufflers out there, any straight through muffler, but it would be impossible to mass produce and engineer a sound canceling straight through muffler that would work so universally.

Cut out is powered through the interior's electronics. I am unsure as to exactly where, as I have never been curious enough to chase it. I will test out the idle capability with it just barely cracked tomorrow.

 

I understand overlap. I just find it hard to believe that at idle the backpressue is effected enough by opening or closing the cutout to affect the driveablity of the car. Unless the exhaust is almost completely plugged behind the cutout and if that were the case the car would run like crap at all times with the cutout closed. If it takes the idle a few mins to get worse with the cutout closed then I would think that the muffler might be clogged and backpressure builds up. Again though if this were the case the car would be very sluggish driving around also.

 

I have no idea as to Borla's muffler internal design, but it must relatively restrictive. It is essentially as soon as I close the cutout, it is like an instant idle quality failure. The driveability is not affected at all, other than idle quality. I do not think the muffler is clogged, but mostly, a poorly designed exhaust system for this engine, exaggerated by the cutout closing, and somewhat relieved by the cutout opening.

 

i dont understand the question
u know egr stands for exhaust gas recirculation, wich is were exhaust gas is put back into the intake. even if u'r engine isnt scavenging the exhaust gases effeciently and leaving some in the cyclinder (which there is no way) u still dont call it egr because that is not the definition of egr system.

but to try and answer u'r question, if everything is smooth with cut-out open, and then not when it's closed. i would say u have to much backpressure with the system closed

 

...and how does that backpressure affect my idle?

 

I am not saying that definitely 100% that my cutout is causing a natural EGR, that is why I posted this thread, but what I was describing up above, the topic of overlap, and exhaust being pulled back into the cylinder/intake, IS natural EGR. Research the term natural EGR, it is not an EGR system, but a naturally occurring EGR, due to engine dynamics.

I am posting my thought process to verify or try and find the cause of bad idle. I have not yet had the time to test the cutout cracked, nor whether open loop may have something to do with idle quality with the cutout closed.

 

I am leaning towards a combination of open loop, when the engine is cold, and cutout is closed, the idle quality suffers. I drove around a while tonight with the cutout closed with the engine warm and the ac on and off. Idle quality was normal with ac on, and was VERY tolerable, just about as normal with cutout open, idle hunted a few times, not near as bad as open loop.

Tomorrow morning I will test cutout crack test during open loop startup.