The science behind going completely catless without tune

[steve99]

Originally Posted by guybo

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

Ive no idea what those dssport guys are on about, regarding the 5% limit on power increase, i think they were smoking something.

However i suspect it was an interpretation by the person that wrote the article from info relayed by tuner.

They probably meant you wont get more than about 5% increase by just chamging bolt on components without altering the tune. Not that the ecu somehow limited power increases to 5%

[Tokay444]

Originally Posted by steve99

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.

The ECU will not change the AFR. It sees the freed up restriction as more maf, and adds fuel to the extra air to maintain the same target AFR.

Originally Posted by Spuds

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.

You're describing what the ECU will do if it detects a vacuum leak in the system, by way of short term and long term fuel trims.
All the air entering the system in a catless car it metered by the MAF, so little to no additional trimming will be needed, so unless the fuel injectors only support 20% more fueling demands over stock, that's a bogus number in this respect. However, it wouldn't take a very large leak to swing your fuel trims over 20%...

[Spuds]

Originally Posted by Tokay444

The ECU will not change the AFR. It sees the freed up restriction as more maf, and adds fuel to the extra air to maintain the same target AFR.



You're describing what the ECU will do if it detects a vacuum leak in the system, by way of short term and long term fuel trims.
All the air entering the system in a catless car it metered by the MAF, so little to no additional trimming will be needed, so unless the fuel injectors only support 20% more fueling demands over stock, that's a bogus number in this respect. However, it wouldn't take a very large leak to swing your fuel trims over 20%...

Yeah, I was probably getting confused by how maf can read incorrectly with aftermarket intakes because of the shape. O2 sensors don't work the same way.

I thought I remembered seeing a limiter on how much fuel trim was allowed. That's where my 20% upper and lower limit came from.

[guybo]

Originally Posted by steve99

Ive no idea what those dssport guys are on about, regarding the 5% limit on power increase, i think they were smoking something.

However i suspect it was an interpretation by the person that wrote the article from info relayed by tuner.

They probably meant you wont get more than about 5% increase by just chamging bolt on components without altering the tune. Not that the ecu somehow limited power increases to 5%

That's what I said. Bolt on no tune- the ECU limits to 5%.

[Bkdshadow]

Thanks a lot for the replies guys. I’m thinking I might just put on the header since at least I’ll eventually get a tune at least. Plus I just DD this, not using it for any tracking or stuff like that. Very good info, glad to be a little wiser than yesterday

[Tokay444]

DD'ing with a MIL light on would aggravate me.

[Bkdshadow]

Originally Posted by Tokay444

DD'ing with a MIL light on would aggravate me.

I have this obdII port thing from my last car that I can plug in along with an app that monitors a good amount of things, and I believe I can take off the CEL. I’m a bit picky myself when it comes to that.

[Tokay444]

Originally Posted by Bkdshadow

I have this obdII port thing from my last car that I can plug in along with an app that monitors a good amount of things, and I believe I can take off the CEL. I’m a bit picky myself when it comes to that.

You can reset the code, but you won't be able to turn it off. It will return until you have that specific MIL light muted with an actual tune.

[Ultramaroon]

DDing without a cat would aggravate me.

[cjd]

Originally Posted by Ultramaroon

DDing without a cat would aggravate me.

Breathing is SO overrated.

[Trueweltall]

Originally Posted by Ultramaroon

DDing without a cat would aggravate me.

Honestly it's not bad at all, you never notice it until you park it in the garage, sit at a drive through, or you know stand behind it while it's running.

[steve99]

Originally Posted by Tokay444

The ECU will not change the AFR. It sees the freed up restriction as more maf, and adds fuel to the extra air to maintain the same target AFR.

..

Yep im saying same thing , the ecu will continue to target same afr. All im sating is initially if the mods cause any fueling change the afr may vary slightly the ecu will detect the extra air flow and quicly alter fueling back to target using fuel trims

[steve99]

Originally Posted by guybo

That's what I said. Bolt on no tune- the ECU limits to 5%.

Im just saying their is no 5% hard limit in ecu like those guys seem to be suggesting

[steve99]

Driving with the mil\cel on will mask any other engine issues so not a good idea