L98auto

Sorry, I know this topic has been beaten to death. I don't want to start a long thread about which exhaust combinations drone, how Corsa is the only way to go, how Flowmasters are terrible etc., etc. I'm more curious in the cause of C4 drone. I have extensive wheel time in C2 Corvettes and they don't drone despite having a glass enclosure behind the driver. I've driven C3s and most don't drone. What is it about the C4 that causes drone? All the earlier generations mentioned have mufflers in similar locations which rules that out. I'm aware of exactly what drone is, but specific causes in the C4 puzzle me (especially since it doesn't appear to affect earlier cars. Thoughts about the science behind it welcome, not looking for a brand shootout. Thanks for any insight into this.

jmgtp

billschroeder5842

Yes, this is correct. Drone is not a C4 specific thing but a combination of your exhaust parts. I've had all sorts of cars drone on me that were not Corvettes. My most recent drone experience was a 2015 Challenger Scat Pak after I put on a non factory exhaust.

Tom400CFI

Tall gearing + loud exhaust = low frequency boom inside the cabin, regardless of car.

L98auto

I'll certainly concede many other types of cars are subject to drone. It seems like virtually every aftermarket system or muffler drones on the C4 outside of Corsa. C2s I've driven aren't like this. I driven sidepiped C2s, stock C2s and C2s with turbo mufflers (not Flowmasters though) with no drone. Maybe its just down to the C4s structure/shape that makes it very prone to this.

L98auto

Sorry, I just missed that post about tall gearing. That's a good point and a definite change the C4s had over most C3s. I could see that low rpm lug as a culprit to that low drumming boom.

BowerPower

I haven't experienced done in my 85 c4. Mine has dual exhaust all the way back with no mufflers, cats, or crossover pipe. The pipe size is stock so about 2.25 and the engine is mostly stock. It kinda drones at 1300 rpm but barely and I dont drive at that low of rpm.

maybe I'm missing something here.

badduxx

I put those walker quiet flow mufflers on mine. The quiet is worth it to me for a daily driver. I only race on Wednesdays LOL

Hot Rod Roy

NOT TRUE! It's a matter of resonances. We want large, free flowing exhaust systems for maximum performance, but to smooth out the pulses in the exhaust flow, there needs to be some volume of chamber size to cancel these resonances. Look at the air intake system on many new cars today. They have mysterious chambers attached at precise locations in the intake ducting to cancel the resonances in the intake system. That's the art of muffler design!

The volume of the "chamber" behind the seats of our C4's is apparently a good resonating chamber!

I have a Corvette Central cat-back system on my '84, that's intended for the '85 (straight tips). On first installation, I had drone. This system is a single 3" dia, pipe from the cat that splits into four muffler outlets. The area of the four outlets exceeds to area of the 3" pipe! I tried reducing the area of only one of the four outlet tips a little bit, and viola!! No drone, and a nice tone!!

gerardvg

The standard mufflers do not cause a drone, i went back to standard mufflers to get rid of the drones from replacement mufflers.
Aftermarket mufflers are pretty much a straight though design, the standard mufflers have a dummy outlet on one side forcing the air through the baffle to other side outlet.

I found the crossover pipe didnt help, anything with slight throttle is where the drone was.
That part throttle is used 95% of the time, full throttle or no throttle is the only time it does not drone unless you have standard quiet mufflers.

fake

Yea these cars are design by enginers who try to eliminate drone. Leave them stock and you will not have any problems!

Churchkey

High engine load @ low rpm = drone the simple fix is to downshift to get the engine rpm past the 1500 - 1800 drone range.
If auto trans do not use OD until 50+ mph.

If you need to cruise in the drone rpm range fabricate a Helmholtz resonator to eliminate the drone or change the rear gear for more engine rpm @ cruise speed..

RetroGuy

I'm guessing the only safe exhaust modification one can make to their C4 is to replace the resonator with an X-pipe, and leave the stock mufflers alone to avoid drone (other than a Corsa system). I also wonder about those on this forum that claim sound deadening foam products placed inside their car, actually are telling the whole truth saying it worked wonders reducing noise/drone.

ddahlgren

If the resonant frequency of the exhaust matches the one for the car it will drone. Rev more no longer match rev slower same thing. Adjust the exhaust so they never match no drone.

barchetta1

This is why I programmed my transmission to to shift into overdrive until 70. I have straight through mufflers and really dont have much drone even in overdrive. I have the friggen 2:59 rear end so thats why 70mph. Cant wait to get a nice rear end.

RetroGuy

Every guy appreciates a "nice rear end."

rattlec4n

My stock '85 exhaust drones a bit around 800-1100 rpm when driving at city speeds if I forget to take it out of overdrive, but otherwise is drone-free. I'm not looking forward to having to replace it some day and possibly fight this battle.

cv67

Good way to extend the life of your trans, no advantage to running OD under 60,really.

Raised the power band up quite a bit....so now it drones around 2500, fairly quiet at 3k
As said, drive around it.

auburn2

A few things that could help:

1. Put boom mat or something similar down under the carpet.

2. Do not delete your cats.

3. Add resonator(s) to the exhaust. Resonators are straight through mufflers, sometimes packed with fiberglass around the sound chamber (they are a lot like old school glass packs). If used as the only muffler they will be extremely loud and "mellow", however if used in series with other standard mufflers these will reduce low frequency noise. Depending on your set up it may be difficult to find room for them.

3. Use stock exhaust manifolds instead of headers.

2LZ

I have stock manifolds and the "dreaded OBX cat back" system. LT, auto, 259's. It drones some at the lower cruise R's when I roll my foot into it heading up a grade in OD.
I did the sound deadening mat this weekend when I put in the rear carpet kit. Helped tremendously.