How necessary is the Downpipe?

[HarrisRacing]

I need a quick answer if anyone can tell me how much power can be had on the stock downpipe (ie-what gains are there?). I am tunable 2021, and installed ECUTEK and Flex fuel, but since I was waiting on my downpipe to show up, I went ahead and put an OTS stage 2 tune in the car to do some testing.

Dragy numbers:
100% stock 12.2 @ 115 (best of 116.0)
Added OTS stage 2 tune 11.85 @ 119.16 (HOT outside - 80 deg F and humid)
Now only a few days later installed DP and it's 45 deg F outside and the car is only running 120.2 mph now.

It's making me wonder if the stock downpipe is really as restrictive as everyone says.

Has anyone pushed the stock cat to see if it's really limited on like an E30 tune? Seems like Flex fuel only would make a huge difference and my car seems to be performing pretty well on the OTS tune since I bet it would be 120 mph on a cold day like today.

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[zrk]

You're going to significantly shorten the life of your turbo trying to force it to push that much boost while the cat is restricting it.

If you try to run a tune designed for port injection without port injection you'll run lean as fuck and burn up a cylinder. Not everything is purely about times/power.

This is exactly what some less experience folks claim things like, "JB4 is dangerous, you'll blow your motor up" or "Engine <x> isn't strong, it blew after <y mod>." If ya don't know what you're doing, don't do it.

 

[black_sedan]

Has anyone pushed the stock cat to see if it's really limited on like an E30 tune? Seems like Flex fuel only would make a huge difference and my car seems to be performing pretty well on the OTS tune since I bet it would be 120 mph on a cold day like today.

Pretty impressive numbers through a stock downpipe. Personally I wouldn’t do it especially with the cheap cost of a catless downpipe.

 

[razorlab]

125mph from just a tune on E50 is very impressive.

 

[NitroYellowMKV]

You are gonna damage your turbo and probably blow out your stock cat. there is a reason why they do not recommend stage 2 for stock dp's. Plus it sounds better catless. it is not expensive to go catless, or even a high flow cat, and on top of that, you can sell your oem downpipe (to me shameless plug) for more than a cost of a catless one or at least the cost of a nice catted one.

 

[HarrisRacing]

You're going to significantly shorten the life of your turbo trying to force it to push that much boost while the cat is restricting it.

If you try to run a tune designed for port injection without port injection you'll run lean as fuck and burn up a cylinder. Not everything is purely about times/power.

This is exactly what some less experience folks claim things like, "JB4 is dangerous, you'll blow your motor up" or "Engine <x> isn't strong, it blew after <y mod>." If ya don't know what you're doing, don't do it.

Can you please prove your claim of significantly shorten the life of my turbo? I don't think this claim is very accurate.

I was SPECIFICALLY asking what the stock power limits of the stock catalyst is because after inspecting it in person, I don't really see that it would be that big of a restriction on a stock turbo car (I mean look at the TINY turbine!). As long as the increased heat from horsepower isn't melting it and causing a plug (which yes could hurt your motor by burning a valve or other), then it should be fine. I assume the engineers at BMW overdesigned a catalyst to handle some serious abuse, especially with all the pops and bangs these things do), so it's likely oversized to boot. I mean the car completely stock is running 380+ whp through that cat with no issues. Even with the OTS tune pushing probably 420 whp (to run ~ 120 mph), it's flowing quite nicely and looked brand new when I took it off to install the downpipe.

I'll agree that asking another 100 HP to go through it may be a bit much and may cause integrity issues - I get that. Recall I actually installed the downpipe already, however I didn't see much in the way of gains and the logs show the same boost, timing, knock counts, etc. If it was the cause of significant back pressure on the system in my experience it should have picked up a lot more.

The post below with the E50 tune on stock DP was what I was looking for. I was a complete believer that these cars could run 10's with nothing other than a tire and an E30-50 tune.

I am just not one to blindly accept "you need this part...because". Even the tuner working on my car (a VERY reputable one in fact) said "I've never actually tuned one on flex on a stock downpipe...but I'm sure it will make more power with an aftermarket one". If it wasn't $800 more for him to make another tune, I'd honestly dyno test it.

 

[zrk]

Can you please prove your claim of significantly shorten the life of my turbo?

Do what ever you want. It's your car.

 

[Mason]

This is my turbo glowing because of the stock cat. Can’t dump the pressure or the heat fast enough. If everyone on here is giving you the same advise and you still don’t want to take it, then I’m not sure why you asked in the first place

 

[HarrisRacing]

Do what ever you want. It's your car.

I'm not trying to be rude, I just haven't experienced modern turbo failures from many other reasons than debris in the oil system or massive overspeed situations. Like I said if you plugged a cat and had a TON of back pressure the exhaust valves would likely be the first to overheat or a piston failure from terrible EGR effect happening.

I think as a science experiment, I could VERY easily tap a port into the the stock downpipe up by the turbine and install a pressure port to determine exhaust back pressure of the stock downpipe. I could easily TIG a port on the new downpipe for comparison as well. As easy as it is to swap the downpipe it may not a bad idea to make 1-2 pulls on camera at my power levels to see what the differences are on backpressure just outside of the turbine housing. I could also test to see if the sport flapper makes a big difference on backpressure.

Either way, it may not be worth the test for me or you, but if some people are limited to running the stock cat due to emissions, I would think this would be worth it to someone.

 

[Mason]

but if some people are limited to running the stock cat due to emissions, I would think this would be worth it to someone.

I believe most people in this situation run the AA high flow. No CEL and 50 state compliant

 

[HarrisRacing]

This is my turbo glowing because of the stock cat. Can’t dump the pressure or the heat fast enough. If everyone on here is giving you the same advise and you still don’t want to take it, then I’m not sure why you asked in the first place

Thanks for the feedback. This makes me have more questions:
1) Do you have an aftermarket downpipe on the car now? If so, does it glow when you do pulls also?
2) If you have removed this cat since then, was it burned up inside?

 

[HarrisRacing]

I believe most people in this situation run the AA high flow. No CEL and 50 state compliant

Great point.

 

[Mason]

Thanks for the feedback. This makes me have more questions:
1) Do you have an aftermarket downpipe on the car now? If so, does it glow when you do pulls also?
2) If you have removed this cat since then, was it burned up inside?

This was the only time back to back racing I’ve done with the car. It felt sluggish while it was glowing. Ran the car to let it cool but I bet it would have burned oil if I shut it off without realizing. It’s « ok » for turbos to glow under high stress but it will also shorten the life to get beat up all the time. I have catless sitting in my garage ready to go in this spring

 

[zrk]

I think as a science experiment,

A simplier one is this: blow through a straw, then jam a brillo pad in there and blow through it again.

Is it possible to push the same amount of air through it. Absolutey. Is it harder to blow through it with the brillo pad in there? Absolutely.

Anything exhaust flow will cause the turbo to work harder.

I'm not sure why you'd expect the horsepower or 1/4 miles numbers to change if catted vs. not catted if you don't change your boost targets. If the turbo is spooling to 22psi (or whatever) then it's spooling to 22psi. Same as my port injection example above. Sure you can crank the boost and hit 600hp with no supporting mods (there's a reason they're called supporting mods), but you're going to run lean and pop the motor.

Forcing the turbo to work harder means it's going to generate more heat. See @Mason's glowing turbo. Not only is heat bad for performance, but it's bad for the longevity of the component.

But again, if you don't want to hear it, that's fine. I'll be done in this thread now - it seems your mind is made up. It's unfortuate I already sold my stocker turbo though.

 

[BadOne]

I assume the engineers at BMW overdesigned a catalyst to handle some serious abuse, especially with all the pops and bangs these things do), so it's likely oversized to boot.

BMW is a German car manufacturer, in general, they don't over-design anything.
Everything is exactly to specifications and requirements.

Take a look under the car at the OEM rear power brace or at the OEM front power brace under the hood. These are just a couple of the examples of exact engineering that BMW does.

Nothing about this car says over-engineering, other than the B58 motor itself.

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