Its no secret that our C4s' exhaust tends to drone like a mother. I've heard many ideas on the subject. Some say its lack of sound deadening and the big open area under the rear glass. Others say removing the cats will increase drone. I've worked in a Corvette resto shop for 6 years and driven nearly every style of Vette under the sun since I was the shop's inspection mechanic. I noticed older Vettes don't drone and they have less in the way of insulatioin. In fact it wasn't until the introduction of cats in '75 that I began to notice a change in the Corvette's exhaust note. The sound became more flat and devoid of character. When louder mufflers were installed that dull exhaust throb became a drone.
I think the merging of the two exhaust banks is the source of the drone. Mustang 5.0 guys also struggle with drone because they also have merged exhaust (h-pipe). Now this goes against the held theory the merging the two exhaust streams supposedly quiets the exhaust. The only reason pulling cats increases drone is because they act as additonal sound suppression. Has anybody run true duals on a C4 and noticed the disappearance of drone? Any thoughts?

 

I had a 2-1 in the 89 and had drone, have some with 3"true duals on the 90. Cats on both cars...I beieve its partially due to the fact there is no pipe after the mufflers, gives the effect that cutting the exjaust off after the muffler like in the musclecar days ("turbos and turndowns"), but who knows...We sit just a few feet from the muffler as it is.

 

I am not convinced the mustang drone is caused by the h-pipe, at least not directly. I suspect it is simply a poorly designed piece, that if replaced with a good x-pipe would solve it.

Have you ever heard a helicopter fly? It makes that wack-wack-wack sound. Or a high performance prop-driven airplane? The roar off the props? Well, in both cases, the sound is caused by the tips of the respective props breaking the speed of sound.

Something similar is happening within the L98 exhaust system. It is the torture of the air flow. It bends in tightly, it gets squeezed together in bad ways, then ripped apart in even worse ways.

In short, it's restriction. The air is being compressed, essentially.

The LT1 OEM exhaust is prone to droning, but no where NEAR as bad as the L98 OEM. The LT1 OEM is dual, with an h-pipe, IIRC... and I think the resonator is a common chamber, too.

The key here is the tuning... and I honestly beleive that GM got in over their heads with the L98 system, and couldn't get out. That us, until the ZR1 and then LT1. They both use dual cats and dual exhaust. That collision never occurs.

If you live in an area that is less emissions restrictive, convert over to an LT1 style exhaust... dual bullet cats, where the precats are, then mount up an LT1 system, be it Corsa or what not... you will gain 20+hp, I will bet, and will be quieter for it.

then again, the Corsa system is well tuned, and will get rid of the drone, too. I haven't heard it in an L98 yet, but I bet it works good.

 

Its not the fact that there is a crossover (H or X), its where its placed, its pipe diameter, etc. Things that can be altered. Mustangs are engineered to be ridiculously loud from the factory, Ford knows that these guys put on a new exhaust almost immediately and they've set the car up to be loud as soon as its opened up.

GM puts things there for the stock car, once you start changing it all then you've screwed up their engineering that went into it (as little as GM does) and you have to figure it out yourself. X pipes, resonators, special mufflers and such can either increase drone or kill it altogether.

 

I thought the drone was caused by the fiberglass vibrating or something. That does make sense though. Would running headers straight to the mufflers with no cats help?

 

i highly doubt it

 

I just this week installed 3" x pipe no cats with 2 bullet mufflers, and 2 hooker aero chamber mufflers. The drone is real bad from 1200-1800 than gets quiet above that Very quiet at wot. I hate it at cruise i am going to have to change something. Before that it had factory cats no res or mufflers. sounded like a readneck pickup but no drone. I do think I picked up 20 hp will find out at the track this week. In a 94 car

 

add some bullet cats.

 

An H pipe never killed resonance for me, doubt an X would either.

 

I agree with vader.
I hated the drone in my 90 until today. It has a stock exhaust with the exception of the mufflers, they are aftermark.
All that said, if you don't like the drone I recommend you read this post:
http://forums.corvetteforum.com/show...ight=resonance

I did the same procedure on my mufflers and man what a difference. The drone at 1700 rpm is completely gone. I don't know what brand mufflers I have but here is what they look like with the plug in.