I have a friend with a '99 Civic EX. He has purposely introduced an exhaust leak by introducing a gap between two of the exhaust pipe pieces to make it louder (that's a whole different story).

I argue that low-end power will suffer because of problems with the back pressure but he argues that any losses should be negligible because it's after the catalytic converter and sensors. Who is right?

Image for reference; it is where exhaust assembly(#17) attaches with the muffler(#3). Near where the number 19 is written.

 

No one will know the answer unless you dyno it. Even then, the effect is probably so small it will be inconclusive.

But he loses this debate by default because he made the exhaust leak to make it sound louder aka terrible. At least buy parts and spend money to rice it out.

IMO there won't be any loss that is measurable. Look at all the race pipes and aftermarket mufflers. Not a lot of back pressure in those exhaust systems. Heck he might even make some gains because that cat is probably stuffed up by now and removing the muffler might help it a bit lol.

 

I had an exhaust leak last year in front of my catalytic converter. Turbo spool came a lot sooner and overall the car felt a LOT faster.

Noise was annoying though, so I changed a gasket.

 

No, it won't cause a damn thing other than noise. It's also painfully moronic.

 

Anything after the O2 sensors won't cause a problem with performance UNLESS you are presenting a blockage. A leak will do nothing to performance (there is no such thing as backpressure).

Now, where you might run into problems is during emissions testing. The test is typically run by measuring the gasses comming out of the tailpipe. If your test is like this, then a leak could introduce fresh air into the test sample and throw off the reading. I had one and it shut down the test when the guy transitioned from the 3k rpm test to the idle test. The sudden drop in pressure in the pipe caused some amount of fresh air to get drawn into the mid pipe.

Fortunately the guy running the test was smart enough and re-tried the test by gently slowing to idle instead suddenly slamming the throttle shut.

Now if your OBD2, them most likely none of this applies to you as they don't actually test the gasses. At least not here in Nevada. They simply plug into the ECU and read the saved data.

BTW: IF you have a leak before the O2 sensor, fresh air drawn into the system will cause the O2 reading think it's lean. This will cause the ECU to add more fuel making your actual combustion overly rich. The fresh air will still be present in the exhaust stream so it will continue to run rich.

 

Your loose connection at #17 could cause potential valve damage. I've had a lot of cars come in with broken exhaust manifolds, leaky manifold gaskets, leaky donut gaskets, etc. . Cars will usually run great but on an emissions dyno they test high for Nitrogen Oxide gases (NOx). If it's a Honda, Jeep, or V8 Chrysler, the culprit is leaky/warped exhaust valves 75% of the time.

 

I agree you wont see any gains unless your getting rid of a blockage, hence allowing more flow. Maybe if the inside of his muffler is falling apart it may be obstructing proper flow. In that case unbolting it there would help.

Otherwise hes just making himself breath carbon dioxide/Monoxide and if he decided to sit in the car and listen to the radio or make out with his girlfriend there both gonna die... Its about as smart as installing a normal battery in the passenger seat...

 

This^

...for a gap at 17 and 3.

 

Thanks for the replies, some interesting information.

But wondering, why would this cause a high NOx reading if clean air is being introduced into the exhaust pipe?

 

It won't cause any measurable effect. It'll just cause noise. Create a thread and you'll almost always find somebody who will say what you want to hear but it won't necessarily be true.

 

I think you are misunderstanding the post, or maybe I am. I believe he's saying an exhaust leak right after the cat at 17 (not down by the muffler like you were indicating) can cause damage to the valves. I'm not sure how that is but that is the statement.

Then goes onto say when they emission test they get high nox reading, from the bad valves, not from the exhaust leak.

Basically a cause and effect statement. The exhaust leak damages the valves. The damaged valves cause high nox readings.

 

/thread

 

I'm in for the noise, it's annoying as ****. Sounds like the car's about to die IMO.

 

yes it will. Will it be enough to notice? probably not. but the leak will disrupt flow.

 

If you have a garden hose thats open at the flow end...and you then put a pin hole in it....does it disrupt the flow?

If you're blowing compressed air out of a pipe and decide to put an extra hole in the side of the pipe, would that disrupt air flow?

An exhaust leak after the O2 sensors doesn't affect much. The system isn't closed. It's all "leaking" out of the tail pipe anyway.

On a car like a S2000, who's engine depends heavily exhaust scavenging to fill the cylinders, a leak may affect performance (probably not). On a stock D series civic...it won't do much of anything.

A big open gap like the one your friend created on his civic may actually help his MPG, midrange, and top end a tiny, tiny, tiny bit.

 

This is correct.


As for the how, it is my theory that the exhaust valves cool too quickly upon shut down which results in warpage. Again, this is just a theory and not scientifically proved, but the majority of vehicles I encounter with high NOx emissions due to leaky exhaust valves will usually have a pre-cat exhaust leak.

 

Pre-cat, sure. But we're talking about a leak at the axle-back here. That isn't going to do anything, good or bad.

 

In my experience, could just be me, but a leak towards the area where we are discussing causes a decent amount of condensation buildup/leakage which I find can cause rust at and around the flange. Then **** gets into the muffler and sounds absolutely horrendous. I think we can all just agree that it's just kind of stupid haha

 

Where's his proof?

 

No proof, but it seems from most of the replies here, that it doesn't cause a hurt in performance.

 

There is significant low end power loss if you have a leak in that location, ask me how I know.

 

How do you know?

 

Had numerous exhausts fall of off in that general area.

 

Doubting Thomas checking in.

 

not going to do a damn thing except make it louder, and any gains or losses would be around 3-4 whp, which is too small of a factor to even test consistently...

if you are looking for free power, aka no $$$, remove everything that is unnessesary, spare tire, carpet, sound deadening, etc. you could lose around 200lbs+ that has to be like gaining at least 10hp.