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FedEx delivered this morning the new low-profile, polyurethane transmission mount.
Before noon it was in the car and I was out for a test drive.
My test drive was short because of rain. Aaand - it seems, seems... that fixed the problem. The car is much more comfortable to drive. No rumbling vibration in the transmission tunnel or footwell.
I need to tweak the exhaust pipe a bit; it’s slightly banging against the seat bolt... but other than that, I think the problem is solved.
(Note: trans mount bracket is tight and the tranny lowered away from the body.)
I no longer think it’s the balance of the engine.
I did the glass of water test and... it passed. In fact it passed with high marks.
At idle the water in the glass was clearly disturbed. As I brought it up in rpm, the water went completely flat. And I mean flat.
The only thing I can think of now is exhaust resonance. In order to fit the T5 I removed the center exhaust hangers located at the transmission mount.
The exhaust pipes are hanging only from the headers and the mufflers. Nothing is supporting them in the middle. I’m thinking under load that must be resonating. It’s not touching the holes in the trans cross member.
I do emphasize that the it always seemed to me that the vibration was in synch with the exhaust popping. Sort of a rumble.
The look pretty flimsy to me. Maybe it worked for a very mild 350.... but not so much for a high performance V8 ?
I will need to measure the exhaust pipes again but to me they look closer to 2 than 2.5 inches.
Muffs are some kinda Flowmasters.
Last edited by DorianC3; Dec 30, 2020 at 12:44 PM.
The look pretty flimsy to me. Maybe it worked for a very mild 350.... but not so much for a high performance V8 ?
I will need to measure the exhaust pipes again but to me they look closer to 2 than 2.5 inches.
This is what mine look like. They are nice and solid. Since you have aftermarket mufflers you may need to fabricate something, but you could start with these since the little tab inserts into the frame and then you bolt it down....it's all nice and solid. For that matter you could probably modify yours with thicker metal but not sure it's worth the hassle with the price of these being 28 bucks. https://www.corvettecentral.com/68-7...ounting-323289
Anyway, I have a 2.5" system from the front to the back and it doesn't rattle nor rumble. I went to essentially a stock parts 2.5" system with stock mufflers a couple of years ago after having headers and fabricated exhaust for 30+ years.
I added the pictures from the corvette central webpage in case the links change in the future. I rotated the support brackets so it's easier to visualize how they bolt onto the frame. I was surprised/happy how well it all fit to my car after 30+ years of a fabricated exhaust.
Last edited by carriljc; Dec 30, 2020 at 01:02 PM.
Those 40 series flowmasters are definitely going to drone.....likely what you are experiencing. If you could put an H-pipe just after the headers that may tone it down for you.
Been doing a lot of reading about C3 corvettes, glass bodies and, Flowmaster 40 muffs... and I think I’m experiencing droning.
Droning apparently is a resonance that is felt in the cabin, not just heard. From what I read, I t’s not uncommon in C3 corvettes with loud muffs and usually happens at a set RPM band.
Changing mufflers is one solution. Car Chemistry inserts is another. Finally a cross pipe can also help.
New mufflers and or exhaust pipes can get pricey quick, especially shipping the whole kit and kaboodle over here, Belgium.
Inserts behind the collectors, I think are the easiest and cheapest solution... but there will be a price to pay in power for the muscular 400 SBC.
Finally a crossover might help - splitting and diminishing the pulses. It’s not too expensive... excluding labor, which may be hard to find here. And it may make dropping the exhaust a pain later on.
I think the best idea is to start with the inserts and see if that makes a difference.
Last edited by DorianC3; Dec 30, 2020 at 05:23 PM.
You need to make a secure exhaust mounting for those loosy-goosy mounting brackets that you showed. I would suggest that you start with that first.
You can place the cross-over at the rear -- like underneath the spring area. That's where mine was before I changed to stock exhaust..and that's where I plan to cut and weld one in again. Should be easy there.
Last edited by carriljc; Dec 30, 2020 at 10:11 PM.
You need to make a secure exhaust mounting for those loosy-goosy mounting brackets that you showed. I would suggest that you start with that first.
You can place the cross-over at the rear -- like underneath the spring area. That's where mine was before I changed to stock exhaust..and that's where I plan to cut and weld one in again. Should be easy there.
Agreed. That bracket has to go. I used to have a crossover on my other car and liked it. I also like the idea of bone stock. On other hand I gotta figure out best budget here. But something has to be done about the hangers
