The science behind going completely catless without tune
 

Any automotive engineers out there, can you please explain to myself and the community what will be going on inside the engine and exhaust when you strip the system of all catalytic converters without a tune? I knew the car would stink a little, but can you just explain why exactly and how the cars reacts (naturally aspirated) because of it.

Simply in car engine spends less power pushing exhaust gases out through more freely flowing with less airflow resistance exhaust duct (due cat delete), leaving more power on table to actually do it's main job of moving car. Without tune gains are less, but noticeable/measurable as well. With tune you also optimize & fine tune engine various fuel/intake/exhaust parameters like cam/ignition timing, fuel ratios, to better exploit now exhaust with less resistance, gaining more.
I'm no engineer, but it seems to me very obvious, not sure if there any extra science needs to be inquired for this question, except maybe for how to project parts to optimize best to get most gains of aftermarket exhaust bits, or get less resistance while still keeping cat function or how to better exploit with tune improvements on exhaust duct and so on.

That's why it smells more without a cat. The fumes are coming out of the car without any catalyst material there to convert the pollutants.
:)

It will smell differnet
You will likely fail most emmission tests
you may get large fine if caught with no cats at all
You will get a CEL code P0420
possibly slightly more power
It will be louder and possibly raspy/drone unless you add resonators.
It wont blow up
The ecu will detect any change in intake air flow via MAF sensor and slightly changed AFR at primary 02 sensor and adjust fuel trims to compensate/correct differences in commanded and measured afr. so it runs the afr thats in the maps
exhaust/intake CAM timing is not learned it just runs whats in the maps based on load and rpm
ignition timing is only reduced on knock detection else it just runs whats in the maps based on load/rpm/ intake air and coolant temps ect.

As the previous guy said you need tune to optimize the engine parameters for the better flow in exhaust (and disable the cel code for cat efficiency) to extract extra power.

as long as it won't reduce engine life that's good, because it will be driven without a tune for awhile. Oh, and I've already done it, and it sounds excellent.

TL;DR... not factoring in performance gains, will it be ok to run catless headers as a daily driver?? Obviously the tune is ideal, but for right now would it be fine just so I can get some rumble?


Hey guys, sorry to revive this dead thread, but I’ve been doing a good amount of research on this topic and I feel like I’m getting conflicting answers. I want to get UEL headers and a tune eventually, but is the statement above accurate? Will the engine be fine running without a tune? I’m not too concerned about power right now and will most likely get a tune within the next several months, but my budget is allowing me to get a UEL right now and all I really want is the subi rumble lol. I’ve read a couple of times that the engine will start running lean due to increased airflow... but then I’ve also read that the ECU will adapt and there isn’t much to worry about..

Ill try to help, but I haven't looked at ecu stuff for a while.

I think the ECU can adapt up to +-20% fueling to adjust afr. But it would take about a hundred miles to even out by my estimate. And that's only really effective at closed loop rpms and loads.

Once you hit open loop the adjustment remains, but only in the rpm bands you have been in under closed loop conditions, which is not in the fun zone. That means you may be running a bit lean up top where there are no adjustments made.

That being said, you CAN do it, but there is always an amount of risk. Also, I would imagine your engine would run like crap going from an equal length header to an unequal length header without a tune. I haven't done it myself or looked into it much, that's just my logic from knowing how pressure waves work.

As an alternative, there's gotta be a catback out there with the right resonator/muffler combo to give you the sound you want if that's all you are going for.

Think of the engine as a giant air pump. When you restrict the intake or exhaust, less air flows, which puts more strain on the engine to move the air. The catalytic converters are basically a mesh that restrict the airflow, as well as mufflers.

Spuds- ok got ya, thanks a lot for the good info! Definitely will probably hold off on that since this is my only car :/ but trust me if there was just an exhaust I could get to have that rumble I would. But I guess the UELs are the only way to get the sound I want. It’s irrational, but there’s just something about that sound that always gives me those good goosebumps :)

Spawn- hmm I see what you’re saying, and that makes sense in terms of why the headers would improve the power. So from what you said, would doing the catless headers actually help with engine life expectancy (even if negligible)? I’m assuming this would be the case as long as the ecu and engine are on the same page.

Thanks for the replies guys. All of my car knowledge comes from forums and I have very little practical experience, so hearing it from you all definitely helps :) sounds like I should just play it safe and get the tune with the headers.. gah I hate my impatience lol

I’m the opposite. Can’t stand the boxer 4 UEL sound. My dream would be a Porsche boxer 6 sound. I used to like the rumble but I guess that was ruined by the gazillion WRXs that are all over the place.

Oh god i would be a happy camper with that H6 soundtrack... future goals for sure my man, I can agree with that! Fortunately my area is full of a lot of modded Hondas... so hearing the wrx sound is refreshing for me lol

rare phenotype is attractive. its why beards go in and out of style

I've always loved the sound of a four-banger missing a cylinder. That's muh jam.

Catless with no no tune will give you a small jump in power, but not much. Without a tune the ECU only allows for something like 5% more power with hardware mods. Having no cats allows the tuner to use more timing advance (if you advance the timing too much with the cats in place you will damage the cats and lose a lot of power and damage the engine).

Catless with no tune won't hurt the engine but if you don't do something about the P0420 than you will have a CEL and lose cruise control. Catless tunes turn off the CEL for P0420 as well as increasing engine output over that 5% limit