[gmcvt]

Hey, I dont recall seeing this but do you have Dual Mode or no?

[oldman]

I feel that on a M6 and the factory suitcase muffler, any removal of the front cats cause sound resonance issues. This seems to be made worst by LT with no cats. For some reason this is a unique combination, know not to work: M6, front cat removal and factory suitcase muffler.

[gmcvt]

So what are your thoughts on this then: Kooks 1 7/8 LT's, NO cats and the Borla 60608 to the stock NPP. I intend on leaving the NPP and hoping that I can smooth the LT's out quite a bit with the 60608. I have traditionally always ran hi flow cats with Long Tubes but I was considering not doing that this go around for two reasons: 1) From a volume/tone aspect, the exhaust combo of the 60608 and the NPP (having the ability to run it in "quiet" mode) I thought might handle the job. 2) I basically run corn exclusively, so I dont think I have as much reason to be concerned about managing excessive odor.

[oldman]

I think that the auto's AFM valves keep the strange resonance down, I also think that with no cats there maybe rattle issues with said valves. I have not tried LT no cats at all and the 60608. I will since I have lots of time...

[gmcvt]

Originally Posted by oldman

I think that the auto's AFM valves keep the strange resonance down, I also think that with no cats there maybe rattle issues with said valves. I have not tried LT no cats at all and the 60608. I will since I have lots of time...

If that’s true on the resonance, then it could be a problem because I have the AFM valves off as I do the DOD (or whatever it’s referred to). Hmm...

[oldman]

So to recap, and this is on a M6, but also note many ZL1 guys (auto and manual) say the same thing. LT with no front cats and then stock after that sounds bad. For me in specific M6 LT, no cats, the engine would NOT produce HP at say 1200 RPM, this would mean I can't use 6th gear to 67 MPH (from memory). As soon as I put cats in, problem was 90% solved and I think 57 MPH I could use 6th. Anyway even near stock with front cat deletes the M6 would have some strange low RPM resonance, so at 57 MPH to 65 MPH it was better to leave it in 5th gear. So many road here are 60 to 65 MPH I would only use 5th gear as the top gear.

I've tried the Borla with no front cats, and the Borla with and with out front cats. I think the Bolra works surprisingly well, we all know that resonators really don't work well, especially straight thru and short (Borla is both). I think the 2 and 3/4 helped a bit here. The car is near stock with the Borla and no front cats in terms of sound and low speed drivability, The car is stock with Borla and all 4 cats and stock suite case, but on my setup WOT the car is still CRAZY loud, I have start-up and rev sound clips posted.

I'm sure LT no CAT and the Borla will be OK in terms of drivability, I'll still run cats, I just don't want to tune my car with cats (afraid I may damage them).

[gmcvt]

So what are the catalytic converter’s doing to aid power at 1200 RPM. I assume this has something to do with airspeed but I just wouldn’t have thought the catalytic converter‘s would aid airspeed.

[oldman]

I really don't think so, so strange harmonic is going on. Many more post from ZL1 guys if you want to read them. So say it is a tuning issue, don't think so. 4 tuners including myself have work on the car, the car runs FINE with cats and stock suitcase, the car runs blah and sounds bad with no cats and stock suitcase. The car is awesome with cats 7200 RPM and LT / cats. All engine not some Mustang too big to loud exhaust on a stock engine. It is a high winding high horsepower sound that only a small block can make when it comes on cam. Reminds me of my old DZ302 Camaro.

[gmcvt]

It looks like there are some ups/downs and in/outs to the factory piece you cut out. I would assume by looking at the stock system that the black cross brace would have to be removed or at least lowered in some way. Looks like Gary H removed his, not sure if out of necessity of preference.

[oldman]

Brace(s) bolt back in

[gmcvt]

Sweet, ok, thanks. I think I'm gonna play it safe and do Hi-flow cats and the 60608. Then I will report back. Thank you again for this post, very helpful.

[MackSteelPrivateEye]

Originally Posted by oldman

To answer the question once and for all, can the Borla 60608 be put on a bone stock exhaust system?
Yes it has been asked and the posted pictures have the Borla front connectors too (between header or stock front cats) to the 60608. We are talking does it work on a BONE stock exhaust.

Why go with the 60608? It is one of the few actual straight through 2 and 3/4 designs. My only fear was if it was going to cure my low rpm 1100 to 1500 resonance issue for instance 40 MPH in 5th,45 MPH in 6h. Without primary cats my engine freaks out at these RPMs and gears.

So today CA offroad pipes, factory headers and the 60608 installed inback of the 2nd cats... results are surprisingly quiet and really no it truly behaves like stock. A surprise because to be honest in general short large diameter resonators don't do much, balance this out that the per piece Borla stuff are 2 and 3/4 (there systems are all 3"). So yup actually works.

So this probably concludes my quest after like 20 tries for a quite system that flows well and that has a high RPM engine sound vs just loud.
stocker headers, CA front cat deletes, move rear 02 to behind factory 2nd cats, factory 2nd cats, Borla 60608, and then stock exhaust back.

Oh, BTW, my ancient Lisle exhaust expander broke 10 years ago or so, I tried two lifetime expanders and they broke first use. I got a lisle (pronounced lyle) and it can actually take a impact gut and it expanded the pipe like butter on a hot cake.


Here it is:

How it sound?

[oldman]

at low engine speeds say 1200 RPM in higher gears, the car can't be driven and sounds terrible. Don't remove your primary cats unless you do something (maybe Bolra 60608) and keep the stock factory suitcase. I have an M6 SS, I've heard the same complaint many times for Zl1 (auto or manual). So it ain't a sound issue, it is a run like crap and sound issue. There seems to be NO issues other than been heck of loud if one uses any or no muffler, this is with the factory suitcase.

Look GM must know there is an issue hence the auto with active AFM, uses valves to close down the exhaust and create some backpressure in the exhaust. This happens only at very low throttle settings and RPM.

[FrostySS]

I was gonna go with LT's and no cats but have read similar complaints. I really don't feel like swapping out the factory suitcase. :sigh: Oh well.