I am new to this site so bare with me. I had 69 conv with side pipes have the need for the rumble of the exhaust from this 85 c4 we just bought . The roar at 1300 rpm inside the car is unreal. I have been checking all forums for answers what is cause of roar or drone as it seems to be called. Any corrective solutions will be appreciated. Bybassett

When I bought my 90 it had a pair of flowmasters that were all rotted out inside.  The car was loud and not in a good way.

1st thing I will do. Try to fix the poor installation of the existing system it has leaking connections Don' t know if will help but I will put some extensions to move some oF the drone away from the car. Is there any real explained cause for the drone around 1300 rpm? I looked at g lass pack package and may go that route before it's over. We have only had the vette for about 4 weeks and on fast track get it up to speed summer is on the way. It is being reprinted this week Thanks for the post. Byron

I was most successful in removing the drone sound, when I removed the single catalytic converter, and the front and rear Y-Pipes.

well, i had the exhaust system leaks fixed with new pipe and welding of same. put extension tips on, but it did not help at all. the roar inside the car is terrible at 1200 to 1400 rpm when i take off easy the transmission shifts into third gear at 21 mph and 4th gear at 41mph which drops the rpm right into that terrible roaring rpm zone and really annoying. is there a way programming the shift points? if i take off faster, no problem, but not a good thing to do all the time. i would like to go for a nice ride at some point without putting ear plugs in. i am considering the slim line glass pack mufflers.

I have owned an 85 with bone-stock exhaust since June of 1985. I've never considered it loud , objectionable, or "droney". If the Cat has been removed, and a true dual exhaust installed, this may explain the objectionably harsh sound at low RPM.

I study pipe organs. I think I understand what you're 'hearing' at the 1200-ish RPM range. On a pipe organ, it's called a "resultant".

At 1,200 RPM each bank 'speaks' at only 5 pulses per second. This is inaudible Very Low Frequency in itself, but various simultaneous frequencies create harmonics that can be heard in the audible range.  VLF is very powerful, and greatly influences sound in higher frequencies. 2 different pitches will couple to each other and produce a different frequency called a "resultant". Two specific pitches 'couple' and 'result' in producing a different pitch or harmonic.

Also, because of the firing order of the engine, there are two consecutive-firing cyls (45º crank apart) then the next pulse is 90º crank, then 90 again for each bank.. So each bank is un-even as well. Another variable to the harmonics.

The original cat converter, and the "H-Pipe" in the photos of the installation above join the two banks together. Now each pipe and muffler 'speaks' at 10 pulses per second at 1200 RPM. A much smoother, pulsing that generates fewer harmonics in the VLF range, and at least doubles the frequency of any resultants into less powerful higher frequency.

This is the primary benefit to the H-pipe: It makes the system 'speak' at a smoother, higher frequency, rate.

I hope this helps you understand the 'science' behind what your experiencing. To me, sound is fascinating because it's mathematical. All kinds of different sounds are produced as they mix together and hit the human ear. some are pleasing, and some are harsh. Think of a symphony orchestra. Or a pipe organ.



 

ND4g

|UPDATED|4/29/2014 6:49:30 AM (AZT)|/UPDATED|

I will try for sure thanks

 What I did was replace the cat. with a series 50 flow master 3 inch in & 2- 2 1/2 out lets used a set of muff eliminator pipes and no more drone mine was so bad I thought the back glass was gonna shatter