ELI5: Why do I need to delete the cats to get rid of the dip?

[DiscoQuinn]

Explain it to me like I'm 5 years old:

-Why do I need to delete the cats to get rid of the torque dip? Can't I just replace the software regardless of the cats? I'm confused the correlation.

[humfrz]

Originally Posted by DiscoQuinn

Explain it to me like I'm 5 years old:

-Why do I need to delete the cats to get rid of the torque dip? Can't I just replace the software regardless of the cats? I'm confused the correlation.

Because I said so, period.

[PulsarBeeerz]

Something something exhaust pressure waves something cancelling out scavenging effect at certain frequencies something reduces timing advance opportunities. Its a hardware fix, not a software fix.

[churchx]

Imho cause and effect is slightly mixed up.
Most aftermarket headers, including catted ones, still need cat readiness CEL disabled in ECU tune, as while on catted ones there is reduced chance for it to pop up, it's still there.
Most get aftermarket headers for sake of more performance and they also prefer to pay less. So if you need to get aftermarket tune anyway, most customers prefer (thus "vote" with wallet) to have extra performance and cheaper price of catless header.
To get rid of torque dip one needs to have aftermarket header (not all of them help with that though, most UELs do help, few ELs also (eg. Ace's), but IIRC few didn't, both EL & UEL).
There is nowhere stated that you need to delete cats to get rid of torque dip. Just that most aftermarket headers made/sold/bought are catless.

[Irace86.2.0]

You don't need to get rid of the cats to delete torque dip.

A) Get a custom tune. The tuner can lower the stock torque so torque is flat.

B) Same as above, except you can add E85 or other bolt-ons.

C) Get NOS for the dip.

D) Get boost for the dip.

[DiscoQuinn]

I don't have access to E85 in my area. It seems so frustrating that everyone tells me to go E85, and most of the information is based around the US market solutions.

I wrote a stage 1, OFT tune to my ECU, but stage 2 requires a CAT delete when the cats do nothing for the engine tune and only emissions. So I'm confused why so much information suggests changing the headers to delete the cats, when it's not the cats causing the dip?

[Ohio Enthusiast]

As far as I understand the cats actually are restricting power. I don't have an explanation to offer, though. This is supported by the fact that de-catting the OEM header performs about the same as aftermarket headers once tuned (so it's not a restriction in the shape/diameter of the OEM header).

[2020BRZtS]

You can do this for free: Shift before 3500 RPM

[Brz-123]

As @churchx said above, it is not about the cat, it is about the long tube headers / manifold that gives the performance gains and eliminates torque dip. Making a catted long tube header is expensive and hence most people prefer the decatted one. You can buy a catted one from FTSpeed, if you like and can spend the money.

Same hardware aka OE header will not give you any further gains as its short tube.


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[RToyo86]

More airflow and more aggressive timing.

Gains have been shown on 2017+ header with punched out cat and custom tuning. The power curve will vary a bit between different headers and custom tuning.
You don't nessessarily remove the dip on a graph but add power everywhere which is noticable.

[therealstoly]

Originally Posted by DiscoQuinn

I don't have access to E85 in my area. It seems so frustrating that everyone tells me to go E85, and most of the information is based around the US market solutions.

I wrote a stage 1, OFT tune to my ECU, but stage 2 requires a CAT delete when the cats do nothing for the engine tune and only emissions. So I'm confused why so much information suggests changing the headers to delete the cats, when it's not the cats causing the dip?

I run the OFT Stage 2 w/ the FT86 UEL catted headers on 91 octane. I've never thrown a code and have passed SMOG without any manipulation, reverting back to stock tune, swapping hardware, etc. As mentioned, and in 5-year-old simplistic terms per your request, the torque dip is primarily a by-product of having equal length headers.

The best solution is a 1 2 process. Get UEL headers to change the hardware and flash Stage 2 to maximize your hardware change. Catted UEL headers will be fine. One trick told to me at the very beginning was to always disconnect the battery when doing anything with sensors. Something about the ECU having some degree of learning built-in. I did so for both my headers and CAI. By the time you're done installing and hook everything back up, you shouldn't throw any codes when you start it back up.

[NoHaveMSG]

Myth. UEL does not have more torque. The headers with the best torque curves are all EL.


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[OwlDance]

Wow there's so much incorrect information in this thread. So in as simple terms as possible.

Cats restrict airflow. Engines are tuned to as much power as possible with the given airflow. Trying to force more power with tuning will result in bad things happening to the engine. Engines blow up if tune is bad. OEMs rarely leave power on the table for NA engines, which is why tunes and bolt ons without tunes gain little to no power. They are as good as you can get for factory equipment. Big company doesn't spend hundreds of millions on RnD for a 500$ piece of metal to randomly do better or a random company to tune better without sacrificing something. That's why stage 1 tunes do little to nothing on our cars.

Properly designed equal length headers will always be superior to UEL.

The torque dip is a result of the short length headers and the cat creating bad airflow in the mid RPM range. The above two points are proven by the ACE350 and JDL 421 equal length headers proving better power across the board over the vast majority of other headers. ACE350 has a peak where the valley was before. Both are catless.

[Brz-123]

Originally Posted by therealstoly

I run the OFT Stage 2 w/ the FT86 UEL catted headers on 91 octane. I've never thrown a code and have passed SMOG without any manipulation, reverting back to stock tune, swapping hardware, etc. As mentioned, and in 5-year-old simplistic terms per your request, the torque dip is primarily a by-product of having equal length headers.



The best solution is a 1 2 process. Get UEL headers to change the hardware and flash Stage 2 to maximize your hardware change. Catted UEL headers will be fine. One trick told to me at the very beginning was to always disconnect the battery when doing anything with sensors. Something about the ECU having some degree of learning built-in. I did so for both my headers and CAI. By the time you're done installing and hook everything back up, you shouldn't throw any codes when you start it back up.

UEL only gains you the boxer rumble aka exhaust sound due to uneven pulses.It does not give any mechanical advantage.

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