Im also replacing exhaust on my 86 and I need to know the largest diameter of pipe I can run through rear end assembly. He told me 1 3/4" is largest that would fit. I believe someone here ran 2 1/4".
Im not sure of the space because Im not that familiar with the independent rear suspension.

 

I used to know this, and my google fu is failing me at the moment. Isn't the stock exhaust 2"?? So we know 1.75" is a bogus number. I think folks have done 2.5" without modification.

I also seem to remember people doing 3" underneath the rear assembly. Don't recall any complaints regarding ground clearance, either.

Anybody recall those old threads?

 

When I did the '85 a very long time ago I used 2" thru the cradle, and the guy that did it had a real understanding of the cradle and its moving bits.

We had no issues, and I reckon 2 1/4" or maybe more, if the person doing it has that "special" imagination. Clearing the brake caliper bleeders, and getting that mounting pin in just the right place, really takes some 3D thinking.

The OE thru cradle pipes were 1 3/4" on my car, and it was still original.

 

is the stock system 2" or 1.75"?

 

Stock IS 1.75".

 

Jagzilla's correct. 1.75" And VERY restrictive at that size! My old Bonneville ran 1.75"!

 

The biggest clearance issue I have with this car is the front lip/dam/spoiler and the breakover angle, because the car is low and has a long wheelbase.

There is no issue routing the exhaust under the cage, it changes nothing about the clearance or breakover angle, and the only thing it would have an affect on is diff clearance, but lets get real, that is of little importance on this car.

 

I know the title of this thread is "Largest Exhaust Pipe Fit Over Rear Axle" but, others have mentioned running it under the cage, so I thought I might chime in on that.

The custom exhausts on both my XJSs run under the rear cage. The one on JagZilla is 2.25", and the one on SpeedDemon is 2.5". Neither have scraped in that area so far. However, the setup on SpeedDemon has only had 100 or so miles put on it since installation, and to be honest, the lack of ground clearance for those pipes is a bit scary (maybe 2.5-3"). This is mostly due to them not being run in the optimum location as they pass under the cage, rather than the diameter of the pipes themselves. Ideally, you want the pipes to be in that sweet spot just to the sides of the lower tie plate, just below the lowest reach of the fulcrum at full suspension extension.

 

I really like the discussion on this thread. An exhaust is one of my day dreaming hopefully soon to be a reality projects. For the time being I'm still working through the must get dones. The exhaust question I'd like to pose is. What would be the ideal pipe size for possible future mods? I would like to have in place an exhaust that could accommodate the additional flow of better exhaust and or Intake manifolds. Of course the addition of these items is questionable due to their rarity. I've come to think of finding these parts is like spotting Bigfoot. Even if you spotted them you would soon find out that capturing them would be cost prohibitive. So, back to the exhaust question. Would something like a 2.25 size pipe be feasible for both a modified scenario or a mostly stock but sounds better one?

 

As I stated earlier, I have both 2.25" and 2.5" pipes on my cars. The 2.25" has a higher pitch sound, and, is more in line with how I think a V12 should sound. The 2.5" is deeper and throatier. However, the systems are very different from each other, with Flomaster Delta 40s and Monza resonators being on the 2.25" setup, and Dyno Max Super Turbos, and 18" glasspacks being on the 2.5". Also the flowmasters seem to allow more exhaust sound into the passenger cabin, while the Dyno Max Super Turbos insulate it VERY well on the inside, yet, they are still aggressive sounding outside the car. Without identical mufflers/resonators, it's like comparing apples and oranges.

As for diameter vs HP, this link sheds some light on that, although it may be a bit conservative, as I've seen claims of 2.25" dual exhausts supporting 450 HP.
The complete performance exhaust system. An explanation on how to setup the exhaust system on your street rod.

 

I went today and had the cat's and resonators removed. What a difference!! I can feel the improved flow immediately. I think that the cats restricted the flow. It has a good sound as well. I call it the audio tech.
Im lucky here in BC as we don't have emission control or safety inspections.

 

I'm going to do that too while I wait to build/find the right exhaust. About what did the shop charge to make this mod?

 

The shop charged $60. I sold the cat's for $90 to local scrap dealer. More power and $30 for fuel. It came in handy as Ive been on the throttle a bit harder than normal just to hear that sound that I missed.

 

Awesome. It is now my plan.