As Count Vohn mentioned when he did his catback, and I have the stock cats removed. the throttle response is TREMENDOUS. you know before you stab the throttle there is a little delay and the engine spits out a bark? Now the response is INSTANT, it feels like a cable throttle on a snappy carburated vehicle. I can only imagine how it feels with ligtweight crank pullet and the tune.
That is amazing!!! Get that tune and pulley on there!
I dont know if its the stock mufflers but the car definitely doesn’t feel any faster than before lol. They did kill the throttle response though…before it was instant, after adding the stock mufflers back it has that delay again.
I have a stranger very mild bucking when cruising though. feels like the throttle is modulating a couple percent in a 1.5-2second period…Will check fuel trims see if I have a vacuum leak or something…I really dont think the rear O2s throwing faults would have any effect ?
Well the s8 exhaust system stock is actually decent, not perfect sizing but it works and has a crossover. On the s6 you see better gains because there are worse bends, no cross over, and I believe slightly smaller diameter.
I mean if you removed the stock cats you should still be seeing some gains unless something else is going on, perhaps they are higher in the rev range though. Until you get tuned it probably won’t be huge. You should feel a difference though as you did remove some restrictions. A well designed exhaust does make a difference, but this is defitnely better than something like the milltek where I believe people were losing or seeing no more power over stock.
I really think unless you plan on doing full blown headers at some point and/or a blower than 2.75 is more than great! I defirnely noticed a gain and better throttle response on my last s6 with my simple exhaust setup and stock cats.
I did a throttle adaptation this morning, seems to have taken care of the slight bucking.
Chasing down a small rattle from the right rear, may be just the crap in the trunk.
something crazy i saw is my exhaust actually grows. the tips will grow out 1" past the bumper when hot, then retract flush when cold again. Wouldnt bother me so much if the rear marmon dindt end up touching the heat shield aroudn the spare tire well!
Well, after the throttle adaptation ran fine to work and halfway back from work. But then car started bucking again…anyone have an insight as to why? I dont think the secondary o2 sensors have anything to do with fuel trims on these cars?
the 02s are pretty damn important. If one is bad your car will not run well. Did you not replace all of them when you were in there? Or did they test out ok?
When you say bucking what you mean? Can you describe behavior more?
So I’ve been on a wild goose chase for quite some time with a rear o2 (b2, s2 to be specific) acting up and throwing CELs.
Here are the faults that I see. The post-cat o2 reports that it’s too rich, meanwhile at the exact same time the pre-cat o2 reports that it’s too lean!
Further in that thread I actually went on several mid-range drives and logged data using VCDS and believe I caught the rear o2 acting “lazy” once I got back on the throttle after coasting for several hundred feet. Graph seen here. It is interesting that when this happens, both the long term fuel trim at idle and long term fuel trim partial values both go to 0% for bank 2. I suspect that in certain cases when the rear o2s aren’t functioning correctly it causes the ECU(s) to ignore those fuel trim values all together. I don’t notice any bucking like you describe, but I could very well be leaving some HP on the table. Thankfully I don’t drive it all that much since I’m fortunate to work from home otherwise I would be very concerned daily driving it.
I could be WAY off the mark here. This is literally just a hobby of mine. I’m again due for PA state inspection soon and told my guy @ German Automotive Specialists in Plymouth Meeting that I’m going to throw some cash for labor at it for them to really dig in a diagnose what is going on. I’ll be sure to update everyone once I have more info from them.
Sensors were all fine, i only replaced the top moun ones, b2s1 and b4s1.
The rear o2 will always throw cat efficiency codes since i dont have cats in the favtory location anymore. I know a tune will take care of this as they just code them out, and ecu thinks everything is dandy.
The bucking is very very minor, i will log some data over the weekend. But the tach shows small jumps of 25 rpm or so, just small tiny fluctuations that 9/10 people would just assume its road irregularities if they felt it at all.
It pccurs in a sinusoidal like fasion, every 2 seconds or so, “hunting” would probably be the better term.
Good news on the oil analysis and oil cooler. Usually though the oil cooler will leak oil into the coolant and not the other way around. Oil pressure is usually between 70ish psi at cold start/high load to 15-30 psi at idle after warmed up so the oil is under much higher pressure than the coolant.
The stock ECU will usually run the readiness monitor check for oxygen sensors and cats while driving at highway speeds. It needs to see like 2K-2.1K rpms consistently for about 30 seconds so I am wondering if it is trying to run the checks and that is the hunting that you are feeling. Needless to say I would get a tune ASAP.
By the way, I have heard differing opinions on the responsibilities of the rear O2s. According to the Audi Engine Diagnostics 2 course if there is a six wire/wide band front oxygen sensor then the rear O2s are just for catalytic monitoring. Other people have said that even though the front O2s really control the fuel trims that the rear O2s will give a feedback reading to the ECU that I guess is taken under advisement as a check/balance.
The heat expansion is a real thing. The exhaust can grow over an inch easy… Throttle adaption and a small bit of driving is needed. keep in mind the car will go though the warm up phase and then keep in mind the TCU will also keep adapting.
The stock mufflers are a pressure exhaust system. Where there needs to be a pressure build up situation before there is an exhaust pulse out. I will see if I have a cut out of the mufflers. The S4 has the same style muffler that the S8 and S6 have. That will have a huge impact on things
Also 02s control driveability as they kick the car into enrichment mode. If you feel the car bucking you need to make sure you are getting no 02 codes. Even a heater code will mean the 02 can be dead. Be careful. IF it keeps up log the 02s and make sure they are all active.
Yeah agreed Justin and Jimmy. Coming from a full exhaust with stock cats no res using stock mufflers even stock tune I definitely felt a difference. I think you and jimmy are nailing his issues with no tune and hunting caused by cat delete. I would also double triple check o2 status.
I am imagining he is running into some issues due to 3 vs 2.75 sans primary cats on the stock tune. While the exhaust system may not be perfectly ideal the jhm tune should be able to balance out some of the items and get the performance to match for a better overall experience.
At this point it might be worth him throwing on some magnaflows as well but I’d argue tune first to baseline and go from there.
hunting issue seem to be solved for now…left bank inlet pipe to throttle clamp was left loose during install…doh…snugged it up seemed to run smooth on the way to work this morning.
Well an oil leak is back. Valve cover, driver side, bottom rear that real tight one. Leaking alot enough to cause significant smoke.
Im hoping the fasteners just loosened up after some heat cycles and i can just snug it down tomorrow and retest.
I tightened them to OEM spec before.
Hoped its not warped valve covers
This is common. Usually with the VC bolts. You tighten them down and then after 4 drive cycles you check and re tighten as needed.
That gives me some piece of mind. Lost my little homemade tool for getting into there, picking up a few t30 shorty bits at harbor freight tomoro
I just installed wheel studs along with 20 mm rear and 15mm soacers, maxed out vcds lowering and she is sitting pretty mean. Went for a spin, got out and saw the smoke coming out…i almost lost it lol, just take a few breaths and think…dang she looks good at least is what i thought
Hmm tried to tighten it today…that fasener is already quite tight…yikes…so why is it leaking them? Snugged down the rest of the cover, will test drive later today or tomorrow And see…
Dissapointment.
Snugged em all down, went for a drive…lots of smoke…30 mins lter. Lots of smoke.
Took the bank 2 inlet air hose off, yup fresh oil around that rear lower bolt. The bolt infront of it is dry.
Ok 1) why? 2) how do i correct this?
Brand new oem gasket, i made triple sure that area especialy was surgicaly clean.
Maybe the cover is warped but gatdang i see there is plenty of compression around that bolt…
From my experience when you do valve cover gaskets you always need to add a generous amount of sealant. epically in the corners. They are prone to leak. Generally I add a healthy amount on the lower part of the cylinder head and in the lower corners. If you didn’t add any sealant it wouldn’t be a big surprise that there are leaks.