Installing downpipe without getting tune have effect on motor?

[esrojo]

Hey! I have a 2025 FL5 with an Apexi front pipe, Eventuri inlet, and Eventuri air intake already installed. I just purchased a TSP downpipe, and I’m wondering if installing it now-without getting a tune until April—would have any negative effects on the engine. I understand I won’t be getting its full potential yet, but aside from that, is there anything I should be concerned about?

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[Phenom3030]

I asked Google’s Gemini the same thing. I wouldn’t do it. At least get a new base map accommodating the new down pipe. Jester04 (jestertuned.com) was able to get me a base map last Sunday evening after I installed my
full exhaust parts.

TLDR: at least the bold text.
The short answer is: You will likely trigger a Check Engine Light (CEL), and your current Phearable Stage 2 tune will not perform correctly.
Installing the 27Won downpipe without updating your tune (specifically your Phearable map) is generally discouraged for the FL5. Here is exactly what will happen and why you should wait:
1. The Guaranteed Annoyance: Check Engine Light (P0420)
The 27Won downpipe uses a high-flow 200-cell catalytic converter. While it is high quality, it flows significantly more air than the restrictive stock unit (which has roughly 600-800 cells).
* The Result: Your secondary O2 sensor will read this increased flow as a "catalyst failure" and throw code P0420 (Catalyst System Efficiency Below Threshold).
* The Trap: Even though you have a tuner (Hondata), the EPA has forced tuning companies to remove the "Disable O2 Sensor" checkbox from their software. You cannot simply "turn off" the light anymore. You generally need a tune that is properly calibrated for the new flow dynamics (or a defouler, though those are hit-and-miss on the sensitive FL5).
2. The Performance Risk: "Boost Creep" & Throttle Closures
You mentioned you are currently running the Phearable Stage 2 tune. That specific file is calibrated for the backpressure of the stock downpipe.
* Physics: When you remove that backpressure with the 27Won pipe, the turbo spools faster and flows more air at the same wastegate duty cycle.
* The Conflict: The ECU (expecting the stock restriction) might see boost rising faster than its targets. To save the engine, it will react by closing the throttle plate or pulling timing to limit torque.
* The Drive: Instead of feeling faster, the car might feel "jerky" or inconsistent at wide-open throttle as the ECU fights the new hardware.
3. Wasted Potential
The FL5 ECU is "Torque Based." It targets a specific torque number, not a specific boost pressure.
* Without a Tune: If you add a high-flow downpipe but don't tell the ECU to target higher torque, the ECU will simply lower the boost to hit the same stock (or Stage 2) torque target. You essentially get zero horsepower gain from your expensive downpipe—just more noise and a Check Engine Light.
My Recommendation
Do not install the downpipe yet.
Since you already have the hardware coming, you should:
* Email John Vega at Phearable: Tell him you are adding a 27Won Catted Downpipe to your current Stage 2 setup.
* Get the Update: He likely has a specific revision (or a "Stage 3" file) optimized for high-flow downpipes.
* Install Together: Flash the new map the moment you bolt on the downpipe. This ensures your O2 sensors are happy, your boost targets are safe, and you actually get the +20whp that downpipe is capable of.

 

[WeudS]

I wouldn't trust AI for this one. One of the primary selling points for the TSP downpipe is that it does not trigger a CEL without a tune.

 

[Phenom3030]

I wouldn't trust AI for this one. One of the primary selling points for the TSP downpipe is that it does not trigger a CEL without a tune.

Didn’t realize TSP’s offering didn’t need a tune. My query with AI was specifically for the 27Won downpipe.
In that case I might try installing it w/o a tune. But I’d feel more comfortable if I had one. Just seems like one of those major upgrades that should be accompanied with a tune.

 

[nattypiso]

Didn’t realize TSP’s offering didn’t need a tune. My query with AI was specifically for the 27Won downpipe.
In that case I might try installing it w/o a tune. But I’d feel more comfortable if I had one. Just seems like one of those major upgrades that should be accompanied with a tune.

The TSP high efficiency Downpipe does not need a tune, and will not give you a CEL, but it is recommend for optimal performance. Reason being for not needing tune or experiencing any CEL is because it has a huge 5” 400 cell GESI G-Sport cat.

Typically your smaller cell aftermarket downpipes, like 27won (200) or RV6 (300) will trigger a CEL and would actually require a calibration or tune, and you’ll most likely trigger the common P0420.

Despite having a big 5” 400 cell cat, I am pushing around 570 HP / 450 TQ with no issues.

 

[J_D]

In terms of performance you will see no changes. The FK8/FL5 works on air charge and torque tables. Even if it can flow better it won’t take advantage, because it reaches its target and stops making power.

If you have the GESI cat you won’t have a check engine light. Everything else will trigger one.

your exhaust note will likely change

 

[ModJPB]

In terms of performance you will see no changes. The FK8/FL5 works on air charge and torque tables. Even if it can flow better it won’t take advantage, because it reaches its target and stops making power.

If you have the GESI cat you won’t have a check engine light. Everything else will trigger one.

your exhaust note will likely change

Thanks! That explains why a company that dynoed the tsp downpipe didn't gain any hp without a tune.

Do you know if the L15B7 and L15CA engines do the same?

 

[nattypiso]

Thanks! That explains why a company that dynoed the tsp downpipe didn't gain any hp without a tune.

Do you know if the L15B7 and L15CA engines do the same?

Not familiar with those engines.. but I do know that TSP also sells a smaller version of the HEDP (4.5” 400 cell), but just designed for the 1.5T. Since it’s a GESI cat, I would also assume no tune required and no CEL.

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