I have a buddy who does not want to do headers but is wanting some performance gains but mostly just the sound. I recommended looking into the down pipes but I have zero knowledge in this area.

For those who have the catted down pipe version from afe, eos, etc. can you answer two questions for me:

1. Performance gains? (even if just through the "butt gauge")

2. Sound improvement? pretty sure he wants the system to be louder.

He cannot go catless due to testing here, so catted would be the only option.

 

Hi-flow cats will sound louder than stock but will have a harder time passing a real sniffer test. Requires a tune to ensure you can get it to match the aftermarket efficiency levels. This was tricky for me to get just right on my old camaro, but it was doable with lots of actual street adjustments. Performance wise, you aren't doing too much to the general flow rate after the exhaust manifolds. There's still going to be a partial obstruction and the tubes themselves are still restricted by the x-pipe afterwards and mufflers so overall the only gain will be whatever increase in flow rate is the cat over stock which will be a small amount. Considering the stock cats can flow for 650hp, they are not your typical restrictive cats to begin with.

If the test is just an ODB2 sensor check and readiness meter, then cats don't matter. Disabling the codes associated with the secondary O2 will be fine and will "pass" the ODB2 sensor check provided the readiness checks don't see the associated codes triggered.

 

Thanks Slade. So what I'm gathering is performance wise negligible, sound definitely gets louder?, and then it would be up to him to play around with the system to ensure it passes testing. That seem about right?

 

Yeah, that is about right.

It all depends on how you get tested over in your area. Sniffer or pure odb2 sensor.

If odb2 sensor, passing is easy and just go with catless or even headers.

 

I was trying to look up catless but I cannot seem to find a version of the catless anymore? Is it quite a bit louder? I think that is his primary concern.

I want him to do it so I can see how this setup works lol

If it was me, I've been thinking either catless downpipes or doing headers; why im posting question for him since I'm genuinely curious myself as headers are ++ money compared to the downpipes

 

Removing cats will bring up the noise out the back quite a bit. Headers having thinner walls compared to manifolds will bring up the in cabin bass note significantly along with noise out the back. Without cats and having straight pipe it was too loud for me at highway speeds on my ls3. No way to listen to music or hold a conversation with a passenger.

 

That was with the downpipe or headers your referring to?

Im just wondering if telling him the catted downpipes will do very little for sound

 

Catted down pipes won't do a ton for volume by themselves but they will add horsepower, especially when tuned. If you search around you'll see lots of folks have done that and gained 25-35 rwhp on a Z06. Obviously less if your buddy doesn't have a Z. If he just wants it louder and doesn't want to mess with potentially failing a sniff test he can just put an axle-back on. That's the easiest way to get a big volume boost but not really any power gained there.

 

The mid-pipes themselves don't add a ton of power on their own. The tune is what opens up most of the gains. If one were to tune the car pre mid pipe, then add them and retune, the gains would be very little. This is because the pipe diameter isn't increasing all that much and you are still limited by the flow coming out the exhaust manifold and whatever you have restricting you AFTER the mid pipe. If you want more horsepower, going to headers will add significantly more power AND noise as you are also shrinking down the wall size versus the manifolds and improving flow which brings more noise out the back. Max effort would be to do headers, x pipe, and mufflers designed for flow. Then there's no restriction in the exhaust flow.

Noise wise, the noise goes up because the walls of the pipes are thinner than stock ones and the lower restriction of the high flow cats will also add to the noise, expect a noise gain around the same ballpark as when removing the secondaries. Yes it's louder, but not by a whole lot.

 

He does, he has a '15 Z, already has different mid-pipe and catback (aew) but he is wanting it to be louder so we were discussing the down pipe option. He doesn't want to do headers as we have no one up here to tune for us, and the few that do work with imports predominantly so neither of us would take our cars to those guys.
From what you guys are saying it sounds like if he goes downpipe route for sound he would want the catless version as the catted version would be louder but negligible.

Mid-length headers don't seem worth the effort, and LT headers we got no one to tune them. Just brutal up here.

 

What kind of emissions testing? You could go the hp tuners route and just go after the codes necessary for the secondary O2 sensors and definitely pass emissions if a non-sniffer test. Would not be able to really unlock any of that horsepower without a real experienced tuner and dyno. Cats or not, messing with the primary cats will cause a code to be triggered unless you put in O2 simulators. I went through a few different cat options on my camaro until I just messed with the tune myself and got it within spec to actually pass a sniffer test.

 

Emission testing only occurs if you are ticketed by traffic officer requiring inspection, otherwise the only thing they look for are cats and that they function. My friend is purely after sound, that is why I was thinking downpipes for him but it sounds like that would be negligible from what you guys are telling me.

For me, I'd prefer to do headers and have no problem with 02 simulators but then I would get flagged by the police at some point and no way I'd pass an inspection which leads me requiring catted headers and then I would need a tune which we do not have here whom I'd trust to do it correctly. HP tuners I know about, but like you said without a proper tune I'm leaving hp on the table and if I'm going to spend all that money in parts that just seems idiotic to do so.

So I'm going to tell my friend if he goes catless downpipe be his best bet for sound, possibly catted but would be quieter than catless version.

As for me, I have no idea lol Ive wanted headers for long time but not sure that is an option and from what your telling me down pipes almost seem pointless.

 

If he wants just noise and a little hp gain (more like free up), go with a muffler system. It'll free up some hp and still get the car sound up and even make it sound "better".

 

He already has a catback and midpipe. Guess he's wanting it even louder, which is where the downpipe thread came from. I'll let him know it probably won't make too big of a difference if I'm understanding your correctly.

@SladeX Thanks

 

I would go with the catless downpipes if he already has an aftermarket mid pipe and axle back. Catless downpipes are significantly louder than stock and make a very raw sound.

 

The dyno sheets I've seen on here and in talking to the shop that did the headers on my car when I was going between the two don't agree with that or maybe it's semantics because I don't consider 10-15 rwhp much of a difference on a car with 650+ rwhp. Without a tune you will most likely get a CEL with catted downpipes at some point and definitely won't pass inspection with a CEL lit so I wouldn't plan on swapping out the cats and not getting a tune. You definitely will get a CEL with catted LTH because the cats move further downstream and take longer to warm up plus you have to turn off the rear O2 sensors. He's going to need a tune either way he goes here imho so might as well talk about tuned numbers.

The difference between a tuned downpipe car and a tuned header car is normally about 10 - 15rwhp from what I saw when evaluating this same question but there are usually other mods like a CAI on the car as well that provides more gain when combined with a tune when you open up the exhaust. Norcal's downpipes without cats actually are closer to 10 rwhp difference when compared to a 2" LT w/o cats based on his posts. The downpipes will have the same high flow cats as headers depending on the manufacturer. Those primary cats are a restriction but the manifolds flow really well so headers on a non-pullied car aren't going to buy you much over just the downpipes with stock manifolds. Port the manifolds with downpipes and you're really close to LTs. The cost to install downpipes is less than half of a header install too (8 hours of install time on headers typically) so they will be much less expensive. Another option may be the ARH mid-length headers with cats. Those will hook up to the OEM X-pipe and have a nice sound increase but not as much as full LTs.

If you want the most power possible and the loudest setup, yes, headers would be the ticket but they are not cheap and your buddy will have to dump his x-pipe he already paid for and likely won't pass a sniff test, it's a gamble. Paired with an axle-back it will be really, really loud at WOT in sport/track or over 4k rpms in tour. It sounds awesome but it's loud. IMHO, cats don't lower the volume much at all (I've had both catted LTs and non-catted on my car, not sure I'd do headers at all if I had to do it over again honestly but I wasn't after sound with my combo). If you want pretty close to the power of LTHs, sound that doesn't draw unwanted attention but is a tad louder than stock, and a much easier install then do the downpipes. That's how I'd evaluate it.

 

Appreciate the feedback.

He's not one to replace parts he's already added, so LT's would be out as he already has the AFE mid, so that would leave him with the mid-length headers or downpipes. Tuning is basically not an option for us up here, nobody reliable even remotely close, so that would leave him with doing catted down pipe or the catted mid-length headers. From my research I'd personally go with the mid-lengths, but since all he cares about is sound/noise it appears getting the catted downpipes might be worth him giving a shot. From what I've read all he needs is the downpipes and then straight big daddy 02 sims, and should be good to go.

If we had a legitimate tuner around here we'd probably both have LT's. had them on previous cars, damn I miss that sound.

 

There are quite a few solid remote tuning options. Especially for something as simple as headers.

 

True, it is more the point if doing LTs I'd want the most out of them and other mods, so I'd want an actual tune done. Unfortunately that is not an option around here.

 

That is what I’m referring to as a remote tune, not a canned tune. There are great tuners out there ie. people like Chris Crawford that will street tune via data logs through hptuners. I wouldn’t even be able to guess how many LT4s Crawford has under his belt, mine included.

Edit to add you would need a wide and and hptuners.