Catless Downpipe + JB4 Combination Issues (No Full Boost + Partial Throttle Closure on WOT)

675430VME · Aug 20, 2026

I dun suggest a catless downpipe unless you can get a tune. The exhaust temperature with a catless downpipe is way too hot, without a proper tune you might burn the rear end like some of the Emira cup cars in China. Catless downpipe push the heat away to the end of the muffler and that is how we are able to throw flame with our exhaust.

abel1789 · Aug 21, 2026

Kaz said:
Yes the silver bar spins and mates with your valve motor stem.
It has 1. tension 2. end points
Every time you start your car that motor turns and needs those two requirements otherwise limp mode.

ah okay got it. what a complex system. on other vehicles, the exhaust valve flap is purely meant for sound. Even if it didnt work, it woudnt put the car into limp mode.

Guess lotus just have to do it differently, sorry its AMG. lol What a pain in the arse if you ask me.

abel1789 · Aug 21, 2026

675430VME said:
I dun suggest a catless downpipe unless you can get a tune. The exhaust temperature with a catless downpipe is way too hot, without a proper tune you might burn the rear end like some of the Emira cup cars in China. Catless downpipe push the heat away to the end of the muffler and that is how we are able to throw flame with our exhaust.

in your experience, would a catless downpipe put the car into limp mode despite throwing CEL codes?

675430VME · Aug 21, 2026

abel1789 said:
in your experience, would a catless downpipe put the car into limp mode despite throwing CEL codes?

It won’t. With our experience it is something else.

abel1789 · Aug 21, 2026

675430VME said:
It won’t. With our experience it is something else.

i was just wondering why wouldnt a catless downpipe cause the car to "overboost" seeing that there is no backpressure right after the turbo? Wouldnt the ECU be trying to retard the timing and lower the boost constantly to keep it in line with factory boost levels?

I'm wondering who wouldnt it put the car in limp mode despite showing CEL's relating to 02 sensor readings.

Am I right to say that a catless downpipe (which throws CELs) vs a catted or stock downpipe (which throws no CELs) would have no difference in performance (boost levels mainly) if a JB4 or stock tune was used, other than of course showing the annoying CEL? I'm excluding a full ECU tune for now, because that option is NOT easily available and accessible.

bmwzimmer · Sep 27, 2026

Hey all, I’m installing the JB4 but will just run the default Map 1 since I’m on 91 octane here. Before I do that, I’m installing a 300cell mach 5 downpipe and their race exhaust with valve relocation kit.
I’ve been struggling trying to get the damn OEM cat out from the top. I read someone removed additional brackets to make room while others had to remove the exhaust manifold (seems like a lot more work). Any tips guys, is the items I circled in blue near the top or red near the bottom the correct brackets to remove? TIA

F1 SML8R · Sep 27, 2026

I haven't personally done it, but several have reported it's the bracket on top circled in Blue. If you have already loosed everything and have been playing the wiggle game with the downpipe, you should be able to tell if that top bracket removal gives you a few extra centimeters of clearance. Just know, it'll still be a knuckle scraper tight removal regardless (again, according to others).

Milk242 · Sep 27, 2026

bmwzimmer said:
Hey all, I’m installing the JB4 but will just run the default Map 1 since I’m on 91 octane here. Before I do that, I’m installing a 300cell mach 5 downpipe and their race exhaust with valve relocation kit.
I’ve been struggling trying to get the damn OEM cat out from the top. I read someone removed additional brackets to make room while others had to remove the exhaust manifold (seems like a lot more work). Any tips guys, is the items I circled in blue near the top or red near the bottom the correct brackets to remove? TIA

I was able to remove it after I removed this bracket. I had to take off my rear right wheel and wheel liner to get to that brace. Also make sure you remove both the diffuser and the middle rear aluminum tray. After that, I just had to keep moving it in every direction until I found one that worked that let me get it out of the service hatch (that took awhile)