Finally got the truth about my bathroom rough-in situation

I went through somethig similar last year with my 1950s ranch. I had one guy telling me I needed to rip out the entire stack because the wet vent for the shower wasn't "up to modern standards," even though it had worked fine for sixty years. Like you, I looked at the actual physics. My main concern was a 3-inch cast iron main that had some scaling issues, which mattered way more than wether a vent was exactly 18 inches above the floor before turning.

I ended up using a few Fernco couplings to transition to PVC and just focused on getting the slope right—about a quarter-inch per foot. The expensive quote wanted to reroute everythign through the roof again with a new 3-inch vent, but I stuck with the existing 2-inch galvanized pipe since it wasn't clogged. I used an Oatey Sure-Vent AAV for the sink to help with the air flow because the old horizontal run was defintely sluggish. Two years later and I haven't had a single gurgle or slow drain. Sometimes those massive quotes are just a "cover your back" price from contractors who don't want to deal with old geometry.

I took the same "if it ain't broke" path on my upstairs guest suite and ended up regretting it when we swapped the old 3.5 gallon per flush toilet for a Toto Drake. Those modern high-velocity flushes move water so fast they actually created a vacuum that sucked the P-trap dry in the neighboring shower becasue my venting wasn't "textbook." I thought those 1.5-inch lines were plenty, but the sheer speed of the discharge changed the pressure balance in the wall in a way the old physics didn't account for.

I ended up having to cut back into the finished drywall six months later to install a Studor Mini-Vent behind an access panel. It was a massive pain compared to just doing it right during the initial tear-down. While I am all for saving money, sometimes the "overkill" from those expensive quotes is just an insurance policy against how modern, low-flow fixtures interact with old-school plumbing geometry. If you are keeping the old heavy-flow fixtures, you are probably fine, but once you upgrade the hardware, the old setup often fails to keep up.

You're spot on about the physics of the stack. Code is often written for the worst-case scenario, not necessarily every single renovation. Since you kept those old runs, did you check the internal diameter for mineral buildup? One thing most guys miss when they keep the original galvanized venting is how much the internal bore shrinks over fifty years. Even if the physics of the layout works, a 2-inch pipe can act like a 1-inch pipe if it's scaled up inside. It can cause just enought friction to slow down the air behind the slug of water, even if you’ve got a decent slope.

Spot on. Those "F-you" quotes are real. Most of the time, guys just don't wanna deal with the headache of matching old pipe to new stuff, so they bid high to make the gut job worth their time. I've seen plenty of 2-inch vents work better than a poorly sloped 3-inch modern setup anyway. Just keep an eye on that lavatory line. If you start hearing a 'glug-glug' sound after a long shower, it's usually that horizontal run finally catching up to you. Did you end up snake-ing the whole line while it was open just to clear out the old gunk?