Arm Motorsports Downpipes

[Bettertomorrow]

Originally Posted by McIntoshMotorSports

Shouldn't be too much of a difference with those downpipes since they are 400 cell just like OEM cats, they may be designed to flow a bit more but for $3K not sure if that's really justified imo.. most catted down pipes are 200 cell instead such as ARM's offerings; these types of cheaper catted downpipes will definitely cause a CEL and are not EPA certified but for my needs of strictly sound they work much better than stock by providing more volume but without the rasp and tons of smell of going full catless.

There was another thread that talked about how different material used in GESI cat flows more. Still on the fence about AA catted dp. Stock DP with MHD Stage 2 seems to hold up pretty well so far. When I see oddity in data log, I will consider it more.

One interesting observation is this.

Stock primary cat has 400 cell and stock secondary cat has 200 cell, total of 600 cell. When OTS tuners recommend DP for stage 2, there is no recommendation for removing secondary cat, meaning they may be targeting 200 cell (secondary) in the system.

Adding 200 cell DP makes total 400 cell.

Removing secondary cat via straight pipe makes total 400 cell, essentially the same as 200 cell DP with stock mid pipe, but no CEL light, emission compatible!

Of course, above simplification does not factor in materials used.

[TopJimmy]

I spoke to PTF directly and they said hi-flow or catless is "recommended" for Stage tunes but not required. Lower restriction does improve top end HP by 5-10.

For your "cells" measurement…the numbers don't add…the highest number wins (loses). Having a 400CPI (cells per sq/inch) plus a 200 further back still nets 400CPI of restriction. The secondary 200CPIs don't impede flow much since the 400 already slowed things down.

Really the higher cell count does a better job at chopping up the pops so the exhaust isn't raspy. Our stock 400CPI cats are barely restrictive, so it's not a bad move to stick with OEM.

[Bettertomorrow]

Originally Posted by TopJimmy

I spoke to PTF directly and they said hi-flow or catless is "recommended" for Stage tunes but not required. Lower restriction does improve top end HP by 5-10.

For your "cells" measurement…the numbers don't add…the highest number wins (loses). Having a 400CPI (cells per sq/inch) plus a 200 further back still nets 400CPI of restriction. The secondary 200CPIs don't impede flow much since the 400 already slowed things down.

Really the higher cell count does a better job at chopping up the pops so the exhaust isn't raspy. Our stock 400CPI cats are barely restrictive, so it's not a bad move to stick with OEM.

Thanks for info!
I am not a thermodynamic engineer but, if this is true, AA Single Midpipe and its dynos (increased HP & TQ) is false? If stock 400 cell accounts for most restriction and having 200 additional cel doesn't matter much, there should be no to negligible gain from AA mid pipe (vs what AA claims)? Something doesn't add up? If we look at exhaust as a whole system, from engine to tail pipe, any restriction (or lack of) should add up as the total flow?

[TopJimmy]

Originally Posted by Bettertomorrow

Thanks for info!
I am not a thermodynamic engineer but, if this is true, AA Single Midpipe and its dynos (increased HP & TQ) is false? If stock 400 cell accounts for most restriction and having 200 additional cel doesn't matter much, there should be no to negligible gain from AA mid pipe (vs what AA claims)? Something doesn't add up? If we look at exhaust as a whole system, from engine to tail pipe, any restriction (or lack of) should add up as the total flow?

When I added my AA Single Mid there was a definite increase in seat feel. I don't think AA's claim is bogus, it's just hard to prove without a lot of dyno work.

[Bettertomorrow]

My wishful thinking says, it will be great if one of vendors did flow test with a few configurations.

e.g.)
Stock All (1000 flow)
DP 200 + Stock Mid (1200 flow) DP 200 flows 20% better
Stock DP + Single Mid (1100 flow) Single Mid flows 10% better
Stock DP + Equal Length Mid (1050 flow) Equal Length Mid flows 5% better
Catless DP + Stock Mid (1300 flow) CDP flows 30% better

I think using above combinations, one could draw other educated estimates. Rough estimates since the quality of product is not factored in but it could still be a dull measuring stick.

[ALCOBENDAS]

I installed the new version with flexible... before I had the version without..

the fact is that the original bmw exhaust clamp does not fit into the long tube...
there is no way to join it to the intermediate part of the exhaust...

I had to put it with joint paste.. has this happened to anyone else??

the tube is thicker at the end than the short one and than the oem

mike@x-ph.com

[mike@x-ph.com]

Originally Posted by ALCOBENDAS

I installed the new version with flexible... before I had the version WITHOUT...

the fact is that the original bmw exhaust clamp does not fit into the long tube...
there is no way to join it to the intermediate part of the exhaust...

I had to put it with joint paste.. has this happened to anyone else??

the tube is thicker at the end than the short one and than the oem

mike@x-ph.com

Thank you for the mention, but we do not sell the catless ARM downpipe.

We currently only offer the catted version, I don't think I ever had any fitment issues with it.

I do know that the first ARM version had some complaints but it's been fixed after they released the new downpipes with the flex section.

[Jim45678]

Noob questions here.

After installing hatless resonated DP I smell some fumes inside the cabin. Not always but from time to time. I did only 60miles (100km) in the car so far so maybe its brake-in period ?

shall I check something or re-install? or its something normal for these DPs ?

my friends M4 with catless DP + MPE exhaust doesn't have any fumes inside.

[fabulousprofound]

Have you double checked your install for leaks?

For example, it could be leaking between the downpipes and exhaust mid pipe, and the fumes are coming through the gearshift/center console, but this is a guess.

[Wanted_M]

Did you remove the center mid-pipe cats too? If you did, for sure you'll get the smell from time to time at a stop light or at idle in traffic. Please make sure the shop used new gaskets for the DP to mid-pipe connection. MPE piping starts behind the mid-pipe cats not from DP..

[mike@x-ph.com]

Originally Posted by Jim45678

Noob questions here.

After installing hatless resonated DP I smell some fumes inside the cabin. Not always but from time to time. I did only 60miles (100km) in the car so far so maybe its brake-in period ?

shall I check something or re-install? or its something normal for these DPs ?

my friends M4 with catless DP + MPE exhaust doesn't have any fumes inside.

fumes inside the cabin is a sign of a leak, double check the installation

[Jim45678]

Thanks guys for the advice. I will do some more driving and see if it repeats or not. I did not make installation so i will go back to the shop.

[mike@x-ph.com]

Originally Posted by Jim45678

Thanks guys for the advice. I will do some more driving and see if it repeats or not. I did not make installation so i will go back to the shop.

[Jim45678]

Originally Posted by Wanted_M

Did you remove the center mid-pipe cats too? If you did, for sure you'll get the smell from time to time at a stop light or at idle in traffic. Please make sure the shop used new gaskets for the DP to mid-pipe connection. MPE piping starts behind the mid-pipe cats not from DP..

PS: which one of these gaskets should I get?



source: https://www.hubauer-shop.de/en/carpa...81089-katalys/

[Wanted_M]

For turbo #3 (qty 2) and for mid-pipe connection #11 (qty 2).

[ALCOBENDAS]

In my new ARMs, part number 11 in one of the tubes was not valid, the tube is very large... I had to put joint paste... wrong... anyone else with this problem?

[Marioec]

Is the rattle issue with the ARM Resonated downpipes solved ?

[Jim45678]

I have no rattle sound with ARM Resonated catless DP.
1000 km made on them so far and all is good.

[Marioec]

Originally Posted by Jim45678

I have no rattle sound with ARM Resonated catless DP.
1000 km made on them so far and all is good.

How are you liking them so far happy with the sound any rasp ?

what exhaust combo are you running

[Jim45678]

Originally Posted by Marioec

How are you liking them so far happy with the sound any rasp ?

what exhaust combo are you running

I'm very pleased so far, but I'm not able to pass 3000rpm for next 800 miles until brake in period for rod bearings.

cold start is louder but not too obnoxious
rpm build has deeper tone but checked only from 2000-3500rpm

I'm running fully stock exhaust system, just DPs replaced. Hopefully by end of month I'll be able to go WOOOOOOT so I can record some footage.

no rasp as of today

[MasterYous]

It's 2024, and ARM has continued to refine these DPs. I ordered a resonated, catless set early this month when they had them on sale. They no longer ship the o-ring, and customer support told me they're no longer needed since they have "solved" the rattle. The flex section is wrapped in a bit of chain for protection. You can see them in the attached photos after I transferred the O2 sensors.

I am running MHD with Stage 2+ since these are completely catless. Using the FlexFuel (around E40) option as well. The power is very nice (4th gear is a monster and even spun), and the burbles and braps sound amazing now. Also, the overall sound is much much deeper than with the stock DPs, esp at low RPMs. At high RPMs, there is just about no rasp. This is with the ZCP OEM exhaust.

I have not heard any rattle yet. The chain doesn't cause any rattle at all either. It's been only about 100 miles so I'll keep listening for it.

Overall, I'm very pleased with the quality and fitment of this product.

I just installed them over the weekend, and here are my installation notes:
- Fitment was near-perfect. You'll have to change the routing of the O2 sensor wires slightly both pre and post cat, but it's not hard.
- The separate flange for the longer pipe fits very snugly onto the long pipe. This is the same approach as the OEM flange, which is separate from the pipe so that you can rotate things to achieve the correct fit. Without this, I don't think it would have been possible to align the DPs to the mids, so I understand the approach they took. It does seem like they've addressed the main source of the rattling.
- They told me the bolts and nuts they ship are rust-resistant and heat-resistant, so I used them. I had ordered the OEM copper nuts but did not use them because they do not fit as ARM is using 13mm bolts, a bit larger than the OEM bolts and nuts. I did not use the lock washers. I don't even see the point, mechanically, given how much torque you are going to use to install the bolts. I got them snug, with the flange flush against the mids, so the gap you see in the photo disappeared.
- I needed a good amount of PB Blaster/penetrant to get the OEM nuts and smaller hex ISA screws to come off. Soak that stuff for a few hours before you try and remove anything. No amount of PB, however, would dislodge the bolts from the OEM flanges (see below).
- It was a pretty easy installation, except for the exhaust. I had to lower the entire damn thing and pull it backwards, because the OEM bolts were rusted onto the OEM flanges and no amount of penetrant or downward force would allow the exhaust to drop. The stock exhaust is HEAVY and you will need a helper, or if working alone, something to raise the middle and rear so you can reattach the mids to the new DPs. An exhaust jack or floor jack is almost a must.
- I needed a flex-head socket adapter and 13mm wrench to do the top bolt and nut (you can see in the pic). The wrench needs to be long and slender to get on the bolt head on the DP side of the flange.
- CLEAN the mids with a wire brush wheel on a drill or Dremel, if you are re-using the stock exhaust. The old gaskets leave ash and foil and you need to get that stuff out.
- Also clean up the thin compression OEM flange as much as you can. Use the PB or penetrant and then use the wire wheel to get the gunk off.

[mike@x-ph.com]