Exhaust Hissing when in lowering rpms

[Dububster]

Hey guys, so I have been hearing this sound for a while now and always thought it was an exhaust leak, but after many hours of looking for said leak and not finding one, I ended up taking it to an exhaust shop and having them take a look at it. It turns out I was right, and there is no leak. So the sound I'm hearing is when I'm accelerating in the lower RPMs I can hear this hissing sound whenever I am driving next to a wall or something like that. I have a gruppe-s catless uel header and a perrin Catback. This sound goes away when I put my stock header back on to check, so it makes me think it could be just rasp, but when I have looked into that header catback combo, I've only seen people been saying it has zero rasp. Does anyone have any input on what might be going on?

[Project_1796]

I have the Tomei UEL header, a overpipe, and a motive front pipe with stock catback, I do hear the noise, it's the exhaust AVCS activating, you can try tuning the overlap or let it be

[Dububster]

Oh, interesting, I had no idea that was a thing. I really appreciate the insight.

[EAGLE5]

A head without insulation can make a hissing sound. Eventually my head failed, and it was a totally different effect.

[Dububster]

Originally Posted by EAGLE5

A head without insulation can make a hissing sound. Eventually my head failed, and it was a totally different effect.

If the head wasn't insulated, wouldn't there still be a noise when the stock headers were put back on?

[EAGLE5]

Originally Posted by Dububster

If the head wasn't insulated, wouldn't there still be a noise when the stock headers were put back on?

Dunno. I never tried that with the stock header. Possibly. Probably if you take out the cats. Also more likely with thinner metal aftermarket heads. On my gen 1 Borla, it was super noticeable. Sounded like snakes.

[Project_1796]

Originally Posted by Dububster

Oh, interesting, I had no idea that was a thing. I really appreciate the insight.

AVCS is almost like Honda's VTEC system but activated by different means, subaru has the ability to control both intake and exhaust camshaft timings on the FA20 for valve overlap, depending on the exhuast manifold design, the engine noise & exhaust note will sound different depending on how it's tuned. The stock exhaust manifold is more of a equal length design, so when you place it back on noise goes away.

[Dububster]

Originally Posted by Project_1796

AVCS is almost like Honda's VTEC system but activated by different means, subaru has the ability to control both intake and exhaust camshaft timings on the FA20 for valve overlap, depending on the exhuast manifold design, the engine noise & exhaust note will sound different depending on how it's tuned. The stock exhaust manifold is more of a equal length design, so when you place it back on noise goes away.

Gotcha, I'm very happy to hear it's normal. I have my car tuned with the oft. Would that affect it at all, or is it strictly the headers?

[Dububster]

Originally Posted by EAGLE5

Dunno. I never tried that with the stock header. Possibly. Probably if you take out the cats. Also more likely with thinner metal aftermarket heads. On my gen 1 Borla, it was super noticeable. Sounded like snakes.

What do you exactly mean by not insulated head? And would there be any other symptoms of that?

[Project_1796]

[QUOTE=Dububster;3432453]Gotcha, I'm very happy to hear it's normal. I have my car tuned with the oft. Would that affect it https://www.comeanddriveit.com/engin...avcs-explained

Here's more info on the avcs system. There's also a post here on the forums about different cam profiles for exhaust manifolds.
Take a peek here at steve99's tune tweaks under avcs section
https://www.ft86club.com/forums/showthread.php?t=74754

[GraysonGreene]

Originally Posted by EAGLE5

Also more likely with thinner metal aftermarket heads. On my gen 1 Borla, it was super noticeable. Sounded like snakes.

Same here. I bought my FR-S with gen 1 Borla UEL's and replaced them with Tomei UEL's when the flex pipes went and with both it has hissed when off throttle and coasting. Can't hear any other time because 3" Greddy Catback go brrrr

[EAGLE5]

Originally Posted by Dububster

What do you exactly mean by not insulated head? And would there be any other symptoms of that?

The stock header has heat resistant insulation, fuzzy stuff, that surrounds the pipes, followed by a heat shield. Aftermarket headers don't have this shield or insulation, though you can wrap them and even stuff insulation into the wrap, and I'm sure that would cut the noise.

Originally Posted by GraysonGreene

Same here. I bought my FR-S with gen 1 Borla UEL's and replaced them with Tomei UEL's when the flex pipes went and with both it has hissed when off throttle and coasting. Can't hear any other time because 3" Greddy Catback go brrrr

Mine hissed so much, @Ultramaroon thought I had an oil leak or something.

[Ultramaroon]

Originally Posted by EAGLE5

The stock header has heat resistant insulation, fuzzy stuff, that surrounds the pipes, followed by a heat shield. Aftermarket headers don't have this shield or insulation, though you can wrap them and even stuff insulation into the wrap, and I'm sure that would cut the noise.



Mine hissed so much, @Ultramaroon thought I had an oil leak or something.

Changed my whole understanding of how a lightweight tuned header should sound.

[Dububster]

Originally Posted by EAGLE5

The stock header has heat resistant insulation, fuzzy stuff, that surrounds the pipes, followed by a heat shield. Aftermarket headers don't have this shield or insulation, though you can wrap them and even stuff insulation into the wrap, and I'm sure that would cut the noise.



Mine hissed so much, @Ultramaroon thought I had an oil leak or something.

See mine only hisses when I am on the throttle in the lower rev range. I take a video and post it.