F8X downpipe catalytic converters - actual back pressure data?

[FTP20]

It goes without saying that removing the factory downpipes and primary catalytic converters is one of the most popular F8X (S55) engine upgrades but has anyone actually measured how much back pressure the original DP/cat combo creates vs aftermarket DPs with HJS 100cpsi or 200cpsi cats vs catless DPs?

Any example of this is the Porsche 991.1 Turbo S with a large diameter aftermarket performance exhaust and 100 cell sport cats. Maximum backpressure: 0.14 Bar va stock exhaust: 0.35 Bar (2psi vs 5spi)

Aftermarket DPs, catted or catless, offer minimal gains on the S55 with the factory tune in place likely due to the ECU's strict torque targets. This means that the big advertised power gains from DPs are often reported in conjunction with a good aftermarket ECU tune which really clouds the overall picture of what, if anything, the DPs contribute to the total power output.

Now that the F8X S55 combo has been on the road for over six years I am hoping that someone has real data to back up why so many F8X owners rush to swap DPs when looking for power gains. I am searching for ways to make my S55 more efficient and aftermarket DPs would be a simple and inexpensive upgrade but I need to be convinced of their value.

All of my comments pertain to cars with stock turbos

[1000hp]

How do the torque targets work?

Conventional turbo wisdom says the less restriction after the turbo the better. Backpressure is typically a non factor in turbo applications, since there is a turbo in the mix. (And actually not a positive in any exhaust system, as I understand it).
So whether or not the computer allows the car to make the torque without the tune, the aftermarket DPs may still be making substantial torque.

I think what you're after is flow bench data, which is probably available from the DP manufacturers who have actually engineered their own parts, rather than just copying someone else's design. Whether or not they'll give it to you is another story.

If you reeeally want to know, and can't get the data from the manufacturers, you could buy a bunch of different downpipes and rent a flow bench. I will contribute five dollars to your research budget 😎

Edit: I plan on doing catted aftermarket downpipes to improve spool. Hopefully the computer doesn't get in the way of that but, if it's limiting torque, I'd imagine boost control has a lot to do with that effort.

[TopJimmy]

I have a 6MT base package. In low RPM situations there was quite a power delay, and the stick shift lets you feel it a more than with a DCT.

When I upgraded from stock cats to ER Hi-Flows (200cpi) there was a much better feeling of power availability down in the low RPMs. I was still using stock exhaust to compare before/after. I get some fuel smell when idling, but the sport cats remove do the egg stink that open pipes have.

[1000hp]

Originally Posted by TopJimmy

I have a 6MT base package. In low RPM situations there was quite a power delay, and the stick shift lets you feel it a more than with a DCT.

When I upgraded from stock cats to ER Hi-Flows (200cpi) there was a much better feeling of power availability down in the low RPMs. I was still using stock exhaust to compare before/after. I get some fuel smell when idling, but the sport cats remove do the egg stink that open pipes have.

That's exactly what I'm looking for with downpipes. I have a 6MT also, and it definitely feels squishy at low RPM for a car that makes torque so early.

Do you have a tune or just the downpipes?

[TopJimmy]

Originally Posted by 1000hp

Do you have a tune or just the downpipes?

Both, FBOs and tuned. E85 really increases torque as well.

[1000hp]

Originally Posted by TopJimmy

Both, FBOs and tuned. E85 really increases torque as well.

Nice. I went from an S/C E36, to an M156 AMG, and now to the F80. And while this car is quick, it feels pretty spongy while the turbos are spooling. If I could get it to feel just a little more like an N/A powerplant, that'd be great.

[Twistm4]

So since this car is based on torque targeting, its not as much about peak hp gains as it is putting the car in a position to be the most efficient as possible.

Decatting the car fully will always relieve backpressure on a modern turbo DI car like ours. I tested varying intake mods and decat gains by aggregating logs, dragy data and virtual dyno data.

So the v dyno, with calibrations that were DEAD on what the car dynoed on a SAE smoothing 5 dynojet, all of my plots after decat were higher. The peak difference was not constant but they never were lower, even with the same tune. But what is most interesting is wastegate duty and vanos data did change slightly. Wgdc dropped a couple percent on average at the same rpm in same gear, and cam timing didn't need to be as overlapped to produce the torque target requested.

So.. yes there are decent gains. Decat the car is probably 3x the gain that an aftermarket intake would give in terms of inefficiency imo.

[1000hp]

Originally Posted by Twistm4

So since this car is based on torque targeting, its not as much about peak hp gains as it is putting the car in a position to be the most efficient as possible.

Decatting the car fully will always relieve backpressure on a modern turbo DI car like ours. I tested varying intake mods and decat gains by aggregating logs, dragy data and virtual dyno data.

So the v dyno, with calibrations that were DEAD on what the car dynoed on a SAE smoothing 5 dynojet, all of my plots after decat were higher. The peak difference was not constant but they never were lower, even with the same tune. But what is most interesting is wastegate duty and vanos data did change slightly. Wgdc dropped a couple percent on average at the same rpm in same gear, and cam timing didn't need to be as overlapped to produce the torque target requested.

So.. yes there are decent gains. Decat the car is probably 3x the gain that an aftermarket intake would give in terms of inefficiency imo.

Great info.
I'd love to see those plots, if you don't mind sharing.

Does the computer allow the torque to be produced earlier, as Top Jimmy's impressions suggest?

[TopJimmy]

To the point of Twistm4 this is why my first power mod was the ER Hi-Flow cats, followed by an AA Signature exhaust.

Since the M2-3-4 all have very good intake flow through the factory CAI style airboxes (do the charcoal delete) I thought it best to free flow the exhaust first. The full 3.5in cat-back Signature kit does have some added exhaust scavenge effect over using a single midpipe that still splits into a 2-can rear exhaust. If I had used full de-cat DPs with this exhaust it would be way too loud (but awesome at the strip!).

[1000hp]

Originally Posted by TopJimmy

To the point of Twistm4 this is why my first power mod was the ER Hi-Flow cats, followed by an AA Signature exhaust.

Since the M2-3-4 all have very good intake flow through the factory CAI style airboxes (do the charcoal delete) I thought it best to free flow the exhaust first. The full 3.5in cat-back Signature kit does have some added exhaust scavenge effect over using a single midpipe that still splits into a 2-can rear exhaust. If I had used full de-cat DPs with this exhaust it would be way too loud (but awesome at the strip!).

So to clarify, by asking the same stupid question again, you installed high-flow-catted downpipes on an otherwise stock car, and it felt stronger at low RPM?
Or did you already have it tuned in some way when you put the downpipes on?

I'm on a mission to de-numb this car, one part at a time.

[chetrickerman]

This is the map the factory tune uses to compensate for post turbo back pressure in its airflow model. It is likely pretty accurate as the BMW engineers modeled everything associated with the engine airflow. Keep in mind, this is only accurate with OEM downpipes

[TopJimmy]

Originally Posted by 1000hp

So to clarify, by asking the same stupid question again, you installed high-flow-catted downpipes on an otherwise stock car, and it felt stronger at low RPM?
Or did you already have it tuned in some way when you put the downpipes on?

I'm on a mission to de-numb this car, one part at a time.

Yes, sport cats with all other parts/tune stock feels a little better than just all stock.

[1000hp]

Originally Posted by TopJimmy

Yes, sport cats with all other parts/tune stock feels a little better than just all stock.

Cool. Thanks.
I will eventually do the tune, but for now I want rid of the mush. Thing feels like a 4cyl Camry coming off idle 😪

[1000hp]

Originally Posted by chetrickerman

This is the map the factory tune uses to compensate for post turbo back pressure in its airflow model. It is likely pretty accurate as the BMW engineers modeled everything associated with the engine airflow. Keep in mind, this is only accurate with OEM downpipes

How does it compensate?

[Twistm4]

Originally Posted by 1000hp

How does it compensate?

Lowers boost.

[1000hp]

Originally Posted by Twistm4

Lowers boost.

Thanks