DP diameter - bigger is better?

[TheRuss]

Hi folks, I'm currently looking for a set of DPs for my 2009 N54 135i (UK model) and was originally looking at a set sold in the UK, but the manufacturer is currently making a "V2" version with a cast elbow at the turbo outlet, but they're downsizing the piping to 70mm as they say it apparently will help torque on these cars without causing any drop in power vs 3" tubing.

I'm just wondering what the collective knowledge on this was - whether that sounds right, or if it's just a case of bigger tubing = better?

Thanks

Russ

[RimasRS]

I heard a lot people saying that 3 inch loses a bit of torque comparing with 2.5 inch but better for HP.

Can you share the source of downpipes you want to buy. I would be interested also.

[onefast1]

I really think it depends on what other mods you intend to have done. If you are simply running a tune with some DP's, intercooler, air intake. I don't think it really makes a difference.

If you are going to custom tune, run higher octane, change out the entire exhaust. Then I really do believe a true 3" or larger DP will yield additional gains.

[TheRuss]

I'm currently running Cobb Stage 1 Aggressive (on UK 98/99 Octane fuel) with BMW Perf Exhaust, but intend to add DPs and FMIC in future.

Not sure about intakes, as I'm not sure if there's a better option than the stock airbox? Not that I've seen...

[TheRuss]

Originally Posted by RimasRS

I heard a lot people saying that 3 inch loses a bit of torque comparing with 2.5 inch but better for HP.

Can you share the source of downpipes you want to buy. I would be interested also.

Sending PM now. Not sure how the cast elbow in them might help, either.

[brusk]

From what I've read most are 3" into 2.5" where the mid pipes are and that if there is a gain with 3" over 2.5" it's very minimal. Therefore find the downpipes that you like and fit your budget and put then on and be happy. The most important thing about downpipes is to go in knowing which ones fit properly and which have fitment issues. If your doing them yourself or paying somebody there's nothing worse than running into unexpected problems and losing more time or money than anticipated.

[TheRuss]

Originally Posted by brusk

From what I've read most are 3" into 2.5" where the mid pipes are and that if there is a gain with 3" over 2.5" it's very minimal. Therefore find the downpipes that you like and fit your budget and put then on and be happy. The most important thing about downpipes is to go in knowing which ones fit properly and which have fitment issues. If your doing them yourself or paying somebody there's nothing worse than running into unexpected problems and losing more time or money than anticipated.

Very true. Whilst it'd be easier for me to get the above mentioned V2 pipes over here, I've never heard a bad word about the AR Design DPs, so I'm very tempted with those, as fitment is supposed to be perfect.

Only thing with the AR ones is that I'd be so tempted to get them coated, even though I'm pretty sure it'd provide me with very little benefit on the road.

[mzman]

I agree with what brusk said. I have 3" pipes on mine just because I was able to get a good set with perfect fitment for a good deal.

I have noticed no decrease in low in torque or power. Just faster spool, more sweet noise and a good bit more power. Getting rid of the cat is the biggest difference going for you, not the 0.5" difference in pipe diameter.

[flinchy]

Regardless of any other factor, smaller dp and you WILL lose potential power ad efficiency.. Any backpressure on the wheel and it will be detrimental to performance

If they are saying smaller = better, they are used to tuning NA.. As that's how NA motors work.

[RimasRS]

One more thing going all the way 3 inch would make more difference but just that 50 cm pipe and then the rest of exhaust going back to 2.5 inch I don't believe if there will be any difference in downpipe diameter

[RimasRS]

Originally Posted by flinchy

Regardless of any other factor, smaller dp and you WILL lose potential power ad efficiency.. Any backpressure on the wheel and it will be detrimental to performance

If they are saying smaller = better, they are used to tuning NA.. As that's how NA motors work.

lower backpressure lower boost... If there is a difference...

[Elsabor67]

I've had both 2.5 and 3" DP's. Initially I had Raceland DP's (2.5) and the sound was raspy but performance was there and it was very spot on. The car felt very lively and breathed easier on the exhaust side.

After getting a helluva deal on a set of AR Design DP's (Both the Racelands and AR's were catless) I swapped out the Racelands and in went the AR's. Initially, a very deeper tone settled in and the turbos felt more open with the diameter change. Spool up was slightly quicker with the AR's and felt I could push the car more before or at redline.

Either way you go I really don't think you will have any issue. It also depends what you are looking to gain or what is your main HP goal. If you are looking at anything over 400BHP, I would go for the 3". Anything below that, you will be fine with the 2.5's

[TheRuss]

What's the deal with mounting brackets then? I notice that the OEM DPs have brackets to hold them, as do the horrendously expensive Akrapovic ones and another expensive DP (can't remember the name). However, the AR Design, BMS etc don't have these brackets. I wonder what effect this will have on things, as I suppose the pipes are kinda just hanging from the turbochargers.

That said, I imgine some people have been running hard with these pipes for thousands of miles?

[Elsabor67]

I have. Tons of 100+MPH runs on the autobahn, runs on the 'Ring and Hockenheim and drives everywhere. The mounting bracket really hasn't been a factor at all and I haven't had any issues whatsoever.

[TheRuss]

How many miles have you driven with the pipes total then? (Approx.)

I assume the ring etc were pretty hard drives?

[135Pats]

Originally Posted by TheRuss

What's the deal with mounting brackets then? I notice that the OEM DPs have brackets to hold them, as do the horrendously expensive Akrapovic ones and another expensive DP (can't remember the name). However, the AR Design, BMS etc don't have these brackets. I wonder what effect this will have on things, as I suppose the pipes are kinda just hanging from the turbochargers.

That said, I imgine some people have been running hard with these pipes for thousands of miles?

I wouldn't worry about the bracket, I'd venture a guess that something in the neighborhood of 30-40% of members here are catless, and that's just not a complaint or issue that I see brought up. Catless downpipes don't weigh much, so we're not talking about a ton of stress being exerted due to lack of a mounting bracket. Now an intercooler i'd want some sort of additional support, but downpipes don't worry me in that regard. People have been running catless downpipes on these cars for years now, if it were an issue you'd def. see a redesign from at least one company other than Akra, which is just obsessive in terms of quality and attn. to detail.

[Freon]

I'd probably not worry about the diameter so much. A catless downpipe is going to have a lot less restriction than the factory pipe. Dual 2.5" with no catalyst is not half bad, probably plenty for 400whp without being a serious bottleneck.

Generally with turbos the larger the exhaust the better after the turbo. The pipe sizing is more of an issue for naturally aspirated cars, or for turbos manifolds/uppipes (between the cylinder head and turbo only). On a turbo, you just want the absolutely lowest backpressure possible after the turbo. The "pulses" and acoustic tuning doesn't matter much after the turbo as the pulses are destroyed by the turbine wheel. Lots of race cars dump exhaust after the turbo straight out of the side of the car, like out the fender or front bumper. One or two feet of 4" pipe, done.

I won't say it is absolutely zero effect or 100% positively always Best, but most advice seems to point to least restriction post turbo as possible.

[flinchy]

Originally Posted by RimasRS

lower backpressure lower boost... If there is a difference...

backpressure = hotter and slower turbine = lower boost/power

[RimasRS]

Originally Posted by flinchy

backpressure = hotter and slower turbine = lower boost/power

Yeah and most people reporting that adding intake and/or freeing up exhaust gives better performance on higher RPM's but takes away low torque/power.
Also on my earlier turbo cars I had same experience even adding intake drops boost...
Though not willing to play YES NO, NO YES...

Each with his own...

[Freon]

The turbine operates largely based on pressure ratio (absolute) before and after itself. The lower the pressure after the turbine, the greater the power and energy transfer from exhaust gas to the turbine wheel. So the less exhaust backpressure you have after the turbo, the better, at least in a steady-state condition. In general it should be good for spoolup and topend power. I.e. if you have 44psiA in the exhaust manifold and 22psiA in the downpipe just after the turbo and before the catalyst, you have a pressure ratio of 2:1. If you replace the downpipes with catless models and reduce pressure after the turbine to 19psiA, your ratio has increased to about 2.3:1. That's more energy transfer to the turbine wheel.

Things get a bit complicated (i.e. arguable) when you talk about shifting gears and momentum of exhaust out through the exhaust system as you briefly shut the throttle plate... I would hate to generalize here.

Rimas, your experience with lower boost with an intake probably has more to due with not being correctly retuned than anything. Boost control is a closed loop system with a LOT of feed-forward in the solenoid duty and whatnot. Changing parts can upset the tune a lot and cause it to perform worse even though you've made a largely positive change in airflow or design efficiency. The system is too complex to create a causative link from your experience such as, "adding intake results in lower boost".