4>1 extractors into stock cat back noise levels

[goose202]

Hi all.
I'd planned to fit long tube 4>1 extractors and cats like the REX items onto my stock cat back on my L77. I've got the S2 bimodal mufflers with a home made controller that I'd assumed would keep noise under control fairly well even with extractors and big cats with the valves closed.
However a local tuner recommended against this if I'm keen to be able to keep the noise down. I don't mind a bit of a note with the valves closed but I don't want to hear it cruising at 60 or 110. Does anyone run a setup like this who can comment?

 

[07GTS]

using stock from the cats back will remain almost like stock sound maby just a little better sound under load, with light cruise loads will be like stock

 

[_R_J_K_]

I'd planned to fit long tube 4>1 extractors and cats like the REX items onto my stock cat back on my L77.
However a local tuner recommended against this

I think your tuner is against it because having a stock catback with extractors kinda defeats the purpose of getting extractors - particularly ones that look to be designed for high flow like those ones (they're pretty spaghetti-ish compared to most other LS extractors). By all means use the bi-modals, but you should replace everything else as well if you want it to actually do something. You just have to be smart about what parts/exhaust you pick, you can have a larger exhaust and still have it be quiet.

If you're dead set on keeping the stock catback, then don't bother with the extractors.

 

[RevNev]

I think your tuner is against it because having a stock catback with extractors kinda defeats the purpose of getting extractors

The engine will make more power off the stock manifolds, piped (zero cell) cats with a 2.5" bolt on. The stock exhaust necks down to 1 7/8" in a couple of places and is way too restrictive for a V8. On a stock exhaust, it's the high flow cats in a header/cat fitment making a bit more power not the headers as the worst feature of the stock manifolds is the cat arrangement.

 

[goose202]

Appreciate the comments, I'll deal with the rest of the exhaust later, but in the meantime I was more concerned about the real difference in noise through a given system with stock headers + cats vs extractors and big cats.

The tuner's comments were specifically about noise. His comments were that you can make good power on the shorty HSV extractor/cat style and would be quieter.

 

[RevNev]

I was more concerned about the real difference in noise through a given system with stock headers + cats vs extractors and big cats.

The bigger or freer flowing cats will generally make it slightly louder at certain RPM and engine loads, as cats act as resonators in the stock exhaust. I haven't heard a stock exhaust with headers/cats but VF HSV exhausts (2.5") with headers/cats are more notey and louder at higher RPM.

 

[Ginger Beer]

Headers will typically give you more engine bay noise due to the unshielded thin walled pipes

You can negate this with thermal insulation, either wrap (cheap) or the the sheilding (expensive)

Out the rear the header only change in sound will depend on a few things, pipe size size being one of them, just pick the correct primary size for your application as the change in sound is fairly minimal for only changing headers

The areas that really effect sound out the rear are the resonators and mufflers, with cats coming in after them, large bodied resonators and mufflers are what you need

You can still have a free flowing exhaust that isn't loud and is still compliant

People also focus alot on catalytic converters, but, with some planning and correct size choice (large body) you can still run compliant cats and make power

By compliant I mean Euro??? your year model is

Me, I run 2 400cell large bodied cats on my 2002 VX SS because I hate the smell of an uncatted car (100 and 200 cell cats don't actually do alot IRT converting jack squat and they stink like catless IMO)


Opinions may vary

 

[RevNev]

with some planning and correct size choice (large body) you can still run compliant cats and make power

Compliance is determined by a full ADR emission test and altering the cat arrangement deems it non compliant until proven otherwise. Many think that a cat in the exhaust stream maintains compliance but it's no more compliant than catless with the same penalty applied. Piped stock cats tend to pass a vehicle inspection in SA and the typical bullet style 100 and 200 cell cats result in a work order in most cases.

 

[jproz]

Does anyone run a setup like this who can comment?

For comparison, mine is a VF2 SSV Wagon with 1 and 3/4 headers (4>1 style, not long tube) with high flow cats into the standard exhaust. With the bi modal valves closed, the answer is yes, it is pretty quiet.

Cheers

 

[Ginger Beer]

Compliance is determined by a full ADR emission test and altering the cat arrangement deems it non compliant until proven otherwise. Many think that a cat in the exhaust stream maintains compliance but it's no more compliant than catless with the same penalty applied. Piped stock cats tend to pass a vehicle inspection in SA and the typical bullet style 100 and 200 cell cats result in a work order in most cases.

I've had 2 cars engineered and both required large bodied 400 cell cats to pass the sniffer

A guy I know has had his LS swapped R33 Skyline thru the testing station at Botany, it took clever tuning and 400 cell cats to "just pass", although he did have a larger cam than mine and was running E85, the RMS don't care what type of cats are installed, they only care about what the sniffer says

I've got friends in the Hwy, from chatting to them if they can smell the exhaust they will sticker the car, same as if it's loud, overly lopey, to low, if it is a slow day and they haven't got their quota from speeders, they will hit the next easiest target, which is illegal mods

I have also never had anyone look under the car to inspect the cat in NSW or QLD, maybe that is a South Australian thing?

Maybe I'm just lucky and fly under the radar, maybe they run my plates and feel sorry for an old guy reliving his youth