Exhaust downpipe flange hole *Update: Video*

[Leedeth]

Well I found out that in the middle of my midnight drive, my exhaust pipe cracked, or has a hole in it.

Midnight drive thread: http://www.standardshift.com/forum/view ... p?p=143702

All this happened in the same night. I felt that the car was getting less responsive in the high revs. On a Straightaway I was redlining the gears from a stop. I got first alright but from 5.5k to 6k rpm I heard a loud hiss, but I thought that's what the engine usually sounds like, but it's more like a loud whine normally. It struggled to get from 5.5k to 6k. Then in 2nd the car really bogged down at 5k rpm, and basically started engine braking from 5.5k and going down, even though I was flooring the gas. I shift to third at that point thinking that maybe it was bad fuel, then same thing at 5k, and this time I hear a loud hiss. I thought it must be a tire so I pull over. And now I'm kind of freaked out, because it was 3:30am after all. It was all fine but I still hear that hiss. It was from the rear area of the engine bay, around the firewall. The hiss gets louder with more gas, but not RPM. The only thing I thought could be wrong at that point was the fuel line, but it didn't explain the hiss, and there was no fuel leak (or any fluid leak for that matter).

So I drove it home. Probably a bad choice, but I got home fine. I kept my RPMs around 4-5k so I could give it minimal gas...I thought a louder hiss would mean more damage. As you can imagine, I was scared shitless and thought I wouldn't make it home. Waited until my dad woke up in the morning and he figured it out pretty fast. He covered the tailpipe and the hiss got louder. Bingo.

A quick drive around the block also revealed that it had also worsened. I have almost no power, and it struggled to rev higher. And this was in second gear. It gets to 60km/h fine but its power is crippled; I don't know if it can go much faster than that. There must be something else wrong as well.

Only one good thing came out of this - my car sounds like it has NO2 all the time now. NAWS.


I didn't know that backpressure was THAT important...lol.

[Sypher]

hmm something doesnt seem to quite add up... usually when there is an exhaust leak, it doesnt hiss... it just makes the exhaust sound louder. Also, just because there is a hole or a crack in the exhaust system, it shouldnt affect power that drastically...

[hockeystyx16]

exhaust leak, run a can of seafoam thru it, when its blowing white smoke all over, get under the car and look for white smoke out of wrong places. have somebody give it gas to make the smoke more dense, easier to see.

its probably a leak at a flange, becuase old rusted flanges are the weakest link.

backpressure is bullsh1t. its all old and stupid and pointless. the less backpressure, the better. if backpressure was good, all race cars would have high backpressure exhausts, not run open headers.

[eaglecatcher]

exhaust leak, run a can of seafoam thru it, when its blowing white smoke all over, get under the car and look for white smoke out of wrong places. have somebody give it gas to make the smoke more dense, easier to see.

its probably a leak at a flange, becuase old rusted flanges are the weakest link.

backpressure is bullsh1t. its all old and stupid and pointless. the less backpressure, the better. if backpressure was good, all race cars would have high backpressure exhausts, not run open headers.

with backpressure, you lose hp, but keep torque, with less backpressure, you lose torque and gain HP.

at least thats how i've always understood it.

[hockeystyx16]

shove a finger in your ass and take a dump
all the backpressure questions will be answered.

engine is an air pump. the easier it can breathe in and breathe out, the more power it will make. its as simple as that, people way overcomplicate it when it comes to exhaust

and anything will be better than stock intake/exhaust. all it does is shut the car up with no regard for performance

[El Beefo]

with backpressure, you lose hp, but keep torque, with less backpressure, you lose torque and gain HP.

at least thats how i've always understood it.

The only way you can gain torque but loose hp is if you decrease your redline or change the power curve. HP is a function of torque ((TQ x RPM)/5252 = HP if I remember right), so if you increase torque, you can decrease the HP by making less torque in high RPM's, or simply not going as high in the RPM's.



Now, uh, back to the original topic...

[Leedeth]

Well, now I'm thinking it's something worse than just an exhaust leak. I tested it again. With a redline of approx. 6100, it maxed at 5500 in 1st, then 4750 in 2nd. I tried again in 2nd and it somehow dropped to 4500. At this point the car would buck as the power turned on and off. Getting to this point probably takes about double the normal time. In addition, the revs drop quite a bit faster than normal. It also idles pretty rough (not at fast idle though). I can also smell exhaust fumes in the cabin sometimes.

What could produce such a loud hiss and have all these effects? I think it might be a blown head gasket but I'm just guessing.

[hockeystyx16]

loud hiss could be a vacuum leak. mazdas have a very touchy vacuum system, mine will barely run and will die if i open up the vacuum system. some cars will lope or barely stay alive with a vacuum leak, some cars idle will run away. my managers contour idle ran way up when i opened up a vac line.

check your oil. if its brown or black your good. if its milkshake color, you have a BHG the job of the head gasket, besides sealing the block and the head, is also to keep coolant and oil separated. when you blow a head gasket, coolant and oil will mix, and when they mix, they make a milkshake/coffee with a lot of milk in it, color.

[Leedeth]

Oil is fine, coolant is fine.

Just checked the exhaust, there is a gap between the pipe leading from the manifold to the cat. The pipe there is two piece, and between those two pieces is the gap. It's about 1cm wide at this point on the bottom.

Took it to the shop and they said it would take $206 plus taxes to replace that pipe. Fortunately, there is a connection between the manifold and pipe so the manifold doesn't have to be replaced as well.

I don't think it's a vacuum leak, noise isn't coming from the intake.


What the hell is going on? I'm pretty sure it has something to do with backpressure. Keep in mind that this is a crap engine producing 88hp and 96ft.lbs. I think because the exhaust has a leak, it messes up the flow. It needs restriction in the intake and exhaust to be balanced. So now that the exhaust is less restrictive, the intake is acting as a massive restrictor and the engine is working against this.

Or maybe because the exhaust has a leak, the backpressure is disrupted, and now the engine has to work harder to push air through the leak instead of the pipe, because the exhaust gases want to take the easiest way out.

It will be fixed on Thursday, so if performance returns we'll know that's the culprit.

[hockeystyx16]

the downpipe to the cat is a flange joint. its made that way. trust me, if your downpipe was open, you would hear it. open headers are deafening.

now i thought about it, now its more like a clogged cat. unbolt the downpipe and drive around, see if the power comes back. clogged cats choke the motor, it cant exhale (so theres the backpressure theory)

restriction balance is nonsense. the engine is an air pump. the more air it can get in, the better. the easier it can exhale the better. theres no such thing as a balanced restriction. usually, intake work is accompanied with headers and catback, but by no means is this necessary or hurting the power.

vacuum leak doesnt have to be on the intake, it could be at any point in the vacuum system. luckily, mazdas vac system isnt that complicated, and you can get a nice new vac hose kit from hose techniques, brighten up the engine bay and solve any potential vac leaks

but from what you described, and a random lapse of whatever instinct i just had, a clogged cat is my call. the crap in there could break loose (if you drive hard, or hit a pothole or some shit and caused a hard jerk) it could break loose and block the exhaust flow.
unbolt the downpipe before the cat and take it for a drive. if it feels normal again, the cat is clogged. depending on your state, you would either have to get a new cat (expensive piece of useless crap), or you could gut your cat with a piece of rebar, or just weld in a piece of pipe instead of the cat.

have fun with the flange bolts. marinade them in pbblaster overnight and have a breaker bar ready.

[Leedeth]

I don't have a breaker bar, lol. I'm lacking in tools, my dad just has the stuff needed to do routine maintenance.

But the thing is, the whole exhaust piping is rusted. I didn't know what to call it, but the flange joint is the problem. It's rusted right through and made a hole in the bottom. I'll see if I can get some pictures and maybe a video of the performance.

It is very loud. The hiss is louder than the engine when it reflects off cars right beside. And yes, it is coming from that leak. Perhaps I do have a clogged cat, or at least one that's partially blocked or especially restrictive, since so much air is being forced out.

I'll get the vacuum system checked when I take it in. I have no idea what to check or do.

[hockeystyx16]

start the car and pop the hood. with it running, get out and listen for any weird hissing. it might help getting a 3 foot piece of fuel line, sticking one end in your ear and following the vac lines with the other end. kinda like that thing that doctors listen to your heart with.

and if your downpipe has a hole in it, i feel bad for you. get some earmuffs before you go deaf

[Leedeth]

I just posted about my new OEM cat-back exhaust in my MX-3 thread: http://www.standardshift.com/forum/viewtopic.php?t=8235


So all my power returned, and it feels like more returned than it had before the damage. I'm convinced that backpressure has an importance that you're missing. The shop said the cat is fine.

http://en.wikipedia.org/wiki/Aftermarket_exhaust_parts

Wikipedia says this: For maximum performance, the exhaust system should be made as free-flowing as possible. Often this means the exhaust should taper, being the same size diameter as the exhaust ports on the engine and increase in diameter as the exhaust travels outward. For maximum performance the exhaust pipes should be as straight as possible and have no devices such as mufflers or catalytic converters to reduce flow. But all of this is not truly practical for every vehicle. To be driven on public streets, cars must meet all noise and emission regulations, which often requires a catalytic converter and one or more mufflers. Also, a vehicle's engine often cannot make full use of a free flowing exhaust without replacing the camshafts and/or upgrading the car's ECU. Lastly, though a free-flowing exhaust increases peak horsepower, it will reduce the low-end torque of an engine, which is not practical for driving at low speeds or in traffic as well as gas mileage will likely be less because of less torque and the need to rev the engine higher.

How the hell can an exhaust leak give such a huge negative performance hit?

[hockeystyx16]

all they did was weld up the hole in the downpipe. damn, thats lame and cheap as hell. if the cat was fine, maybe your muffler went FUBAR on the inside or something.

there are all kinds of views on backpressure, some people believe in backpressure, some dont. i dont believe in backpressure. the whole point of exhaust is to let the engine exhale, not to block it from exhaling. if backpressure was so important, then a blocked exhaust would help your performance, not hurt it. think about that.

also, that think on wiki about losing low end torque and all that crap is BS. what really happens is inside your head. the stock exhaust isnt a big deal of restriction at low rpms, but up high, where your motor is puffing out a lot of exhaust, stock crap gets in the way.
free flowing aftermarket stuff makes it easier for high rpms a lot of exhaust to leave. so now, you wind it up, and you get more power, because of less backpressure. more power up high compared to relatively the same power down low, you brain will draw messed up conclusions.
this goes for most boltons. at high rpms, the stock stuff's restrictiveness plays a big part. and most boltons are there to let the engine breathe easier at higher rpms, thus making more power.

stroking, overboring, porting, etc, the internal engine work, is what really increases the torque. intakes, exhausts, all they do is let the stock stuff breathe easier.

sux that you paid that much for OEM stuff. you could get a aftermarket catback for like 200 dollars on ebay.

BTW those downpipe heatshields that mazda put on our cars, i hate those fvckers with a passion. on my PGT they are rusted and the insulation between them and the actual exhaust pipe is falling out, and every time i bump into it when im under the car, i get a bunch of that crap and rust in my face. not sure if thats what you meant by the downpipe, because if you had a hole in the downpipe, that would be as loud as running open headers

[Leedeth]

I found this a while ago and dug it out of my bookmarks:

http://www.nsxprime.com/FAQ/Miscellaneo ... theory.htm

"We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car."

It all makes sense now! I think what happened here is that since there was a leak in the exhaust, it allowed air to cool in the rest of the pipe, causing it to become dense and need the engine to push harder. So now the engine is trying to push air out a small hole which won't widen much farther, and through a cool pipe filled with dense and heavy air. That, and maybe the muffler was slightly blocked.

Missed something after all, but it wasn't backpressure...forget I ever said anything.

But ya, when I floored the gas, it only gave slightly more power. It was crippled and could just barely keep up with the acceleration of normal traffic.

I'm not really the one making the decisions about this car though. If it were my choice I would've gotten the pipe welded up (by the shop of course, I don't have a welder), buy the necessary exhaust components off ebay, and run it till it breaks, then install the ebay stuff.

The hole was in the flange just after that heatshield. Yes it was sure damn loud. Just take a look at that picture of the flange again. Huge chunks of it is missing, and the whole thing is rusted.

Doesn't downpipe usually mean the pipe from the manifold/headers to the cat?

What tools would one need to install a cat-back?