TheCorvetteKid

Hi everyone,

I've been thinking about replacing the stock exhaust system on my '89 L98 with a custom made true dual exhaust setup. I'm picturing an all stainless setup coming off the stock manifolds and using something like a pair of universal Random Technolgies 2.5" or 3" in/out round catalytics piped out to a pair of stainless muffers (haven't decided on a brand yet) or muffler eliminators.

But before doing something like this, I have to ask a few questions.

Would 3" diameter piping allow for the use of a pair of Random Tech catalytics, or should I stick with 2.5" tubing. Does anyone make a single unit "true dual" catalytic converter (LT1 style) that will fit in the place of the stock cat, or am I stuck using two individual cats? Would a single catalytic per side allow me to pass emissions, or would I need a pair of precats like the stock system? Are there any special considerations with respect to the O2? If I used two cats, which one would I connect the A.I.R. tube to?

If anyone has any pictures of their true dual setup, please feel free to post them. I'm sure I'm not the only one who's wanted to do this, so I know that alot of you out there can help out.

Thanks very much in advance.

Pete K

I cannot answer the part of the question regarding the converters. I have none. I have been running the mid america, 2.5 inch true duals since 1991. Very pleased. This exhaust has been good enough for high 10 sec et's on the spray. You may be able to start with that system and add converters as needed.

-=Jeff=-

here are some pictures of the Dual 3" on my 89





Components:
All Mandrel Bend 3" tubing
Hooker 1-3/4" Pirmary tube headers into a 3" collector
Dynatech Bullet 3" cats
Stainless Works X-pipe
Dynomax Ultra flow 3" Welded Mufflers with LT1 tips

I have recently swapped the X-pipe for a custom Bassani Resonator with an internal X-pipe to try and cut down on interior resonance.

I also just saw this from Random tech:
C4 Converter/X-pipe 84-96
Stainless DualCat X-Pipe System (for use with headers)
813015

It has the X before the Cats and after the cats it goes down to 2.5"

This may also work quite well..

TheCorvetteKid

Hi guys,

Pete K - I like your idea of using the Mid America true dual and maybe cutting/splicing in a set of catalytics in order to meet emissions. It would certinaly be a very cost-effective way of going out what I want to do. But, their setup is only 2.25" piping (you mentioned 2.5").

Jeff - HOLY JEEBAS!!! That's one heck of a setup! Looks a little tight under there, but I'll bet the performance speaks for itself. I'm impressed - thanks for the pics.

But I have to ask both of you guys, and Jeff you touched on this subject, what about resonance? I know that some people complain about resonance with the stock system (and it's not every car that suffers it, only certain gearing/tranny combinations), but I don't know what it's like with a true dual setup. How do you guys find it?

Thanks again for the advice. Keep it coming!

-=Jeff=-

With 3" it is worse then 2.75 or 2.5

With my current set-up the resonance is not that bad, but I also have a worked motor which does not help in the resonance area..

I am swapping the rear gear this weekend and can get current exhaust pics.

Pete K

I guess you are right about 2 1/4 size. I assumed all these years it was 2 1/2. Oh well, it works fine anyway. Resonance is quite annoying in my car. I do not like it but I tolerate it. On my stock car I picked up 3 tenths and 2.5 mph with my system. This is why it stays on the car.

TheCorvetteKid

So if I hear you guys right, you're saying that resonance is definitely an issue with a true dual setup.

Any way to minimize/eliminate it? Does that cross-over 'X' pipe do a good job of it?

Pete K

In my opinion, you will never eliminate or severely minimize it, The shape of the car, the fact that it is plastic and the big open area behind the drivers seat all help to amplify noise.

Performance nut

This is interesting. A project I would of liked to have done had it not been for those pesky CARB kids. Probably could get it to look stock enough to where a non-Corvette expert wouldnt know the difference, but pass California emissions sniff test...

-=Jeff=-

Yeah it will always be there but depneding on set-up it could be at different dB levels.. Also each person's tolerance for acceptable resonance varies.. the Resonance I had was acceptable to some, but not me..

My set-up now is much better and has shifted to a 1400-1800 rpm instead of 1500-2000 rpm

conv90

Pete, are you referring to this?
http://www.madvet.com/shop?frame=1.36.928
thanks
-Beppe-

1991Z07

I don't have any pictures of mine...but I used the LT1 Corsa system. All I had to do was buy the hangers for the trans and the rear from GM (about $30 total) and then had the muffler shop bend new pipes from the resonator to the headers (in stainless).

This is a 2.75" system, and has no resonance, sounds GOOD when you mash the fast pedal, and I can pass emission here in Texas with it. I have 2 Random Bullit cats on the end of the headers.

If they make it look as close to a "stock" system as they can, you should pass a visual and the cats should let you pass the sniffer test. You'll need to get air tubes on both cats and have a new pipe bent to feed them. This will help them burn off faster and give your car a good exhaust for up to 500 HP.

-=Jeff=-

I thought corsa was 2.5"

MYCoupe

Yea, the Corsa is 2.5", but it flows straight-thru at the resonator and muffler.

1991Z07

You're right...I just thought it was bigger for some reason.

The OD of the Corsa pipe is just big enough to slip inside the 3" OD side of my Random cats...just figure they were a little bigger than that.

The sound is still awsome...

Here's a clip for 'ya Dell Diamond Burnout Contest - 3.2MB Streaming Video File

VR'92

The quickest way to convert a L98 to true duals is to buy or just pick up a stock LT-1 exhaust. It would be easy to bolt on and wouldn't cost much at all. Like someone else said the nessecary hangars from GM were $30 bucks. A Muffler shop could then do the rest for you for maybe $50 at the most. Probably would be a decent gain too. I see memebers hear on the forum giving them away all the time.

gcrouse

Here's my setup; not intended to be pretty, just highest flow with very few bends:
Hooker SuperComp 1-3/4", ceramic coated and wrapped
Dual 3" Carsound converters, heated O2 sensor
Dual 2-3/4" Pipes, 20ga steel, mandrel bends
Magnaflow Muffler 3" dual in / out

More pics on my CF page
http://temp.corvetteforum.net/c4/gcrouse/

89Corvette6spdFX3

If you add about 20lbs of Dynamat, it might be a little quieter.

65Z01

You could use a 2 1/2" pipe system and weld in adapters to go from 2 1/2" to 3" for the cats.

Bombervette

gcrouse : wow, thats a hot car you got. just the way its built up i KNOW its fast and can handle at the track.