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      04-21-2013, 02:14 AM   #1
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3 inch DP vs. 4 inch DP for 335i?

BMS makes a 4 inch DP and Challenge, just to name one has a 3 inch DP. Which one is better for the F30 335i?

This is just about 3 inches vs. 4 inches. I know catless and high-flow cats play apart in the decision but lets save that for a later time.
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      04-21-2013, 10:09 PM   #2
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I got the Evolution Racewerk 4inch one, i would assume the 4inch would have better flow and being it less restrictive.
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      04-21-2013, 11:29 PM   #3
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Obviously 4" would be less restrictive but there is a point in which you would not see any more gain. 3" vs 4" pressure drop and velocities reach a point where they are not restrictive for the given vapor flowrate. As a general rule of thumb pressure drop is equal to the square of the velocity. Which means, say you dropped the velocity of the vapor by 2x, the pressure drop you actually dropped is 4x!

So, the area of a 2" pipe is 3.1 sqin while the area of a 3" pipe is 7.1 sqin. The area of a 4" pipe is 12.6" sqin. Bernoulli's equation could be used to calculate the pressure drop. Exhaust tweaking is all about reducing pressure drop/backpressure is what I figure.
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      04-22-2013, 01:12 AM   #4
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Good info guys. I'm also assuming the 3 inch dp will allow the N55 to keep some of the low-end torque where as the 4 inch dp will be better up top due to spool will be a bit slower?
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      04-22-2013, 02:02 AM   #5
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Quote:
Originally Posted by nsquared View Post
Good info guys. I'm also assuming the 3 inch dp will allow the N55 to keep some of the low-end torque where as the 4 inch dp will be better up top due to spool will be a bit slower?
Doesn't work that way for turbo cars. Backpressure is not needed for low or mid range power. Bigger downpipe faster spool and more power until efficiency maxes out.
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      04-22-2013, 08:44 AM   #6
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Quote:
Originally Posted by nsquared View Post
BMS makes a 4 inch DP and Challenge, just to name one has a 3 inch DP. Which one is better for the F30 335i?

This is just about 3 inches vs. 4 inches. I know catless and high-flow cats play apart in the decision but lets save that for a later time.


From a quality standpoint, the BMS one looks infinitely better....I just don't feel right running around without ANY cat at all, and since this DP deletes it all together, I'm reluctant to advise either.

If BMS threw on a 200 cell cat into it, I'd pay the $$ and do it for sure...most of the gains they advertise are from the CAT delete.
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      04-22-2013, 10:21 AM   #7
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Quote:
Originally Posted by shivaswrath View Post


From a quality standpoint, the BMS one looks infinitely better....I just don't feel right running around without ANY cat at all, and since this DP deletes it all together, I'm reluctant to advise either.

If BMS threw on a 200 cell cat into it, I'd pay the $$ and do it for sure...most of the gains they advertise are from the CAT delete.
Running a HFC is definitely preferred but it doesn't seem like a lot of companies other than challenge is having them.
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      04-22-2013, 11:31 AM   #8
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Just talked to a buddy who owns a car shop that builds a lot of cars and he says sticking to a 3 inch catless dp is better than going to a 4 inch dp in terms of making power on a stock turbo.
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      04-22-2013, 11:34 AM   #9
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Quote:
Originally Posted by nsquared View Post
Just talked to a buddy who owns a car shop that builds a lot of cars and he says sticking to a 3 inch catless dp is better than going to a 4 inch dp in terms of making power on a stock turbo.
that's such a generalized comment (from your buddy) I can't even begin to comment on it...
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      04-22-2013, 11:39 AM   #10
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Quote:
Originally Posted by shivaswrath View Post
that's such a generalized comment (from your buddy) I can't even begin to comment on it...
Well I didn't know how to write everything he said but it has to do with velocity trade offs with diameter and what not... Iono. Maybe I will have him write something on here when possible.
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      04-22-2013, 12:50 PM   #11
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I can assure you guys that the Challenge quality is top notch.
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      04-22-2013, 02:02 PM   #12
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Originally Posted by mikew2069 View Post
I can assure you guys that the Challenge quality is top notch.
Thats good to know. I wonder if Challenge offers thermal coating on their DP?
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      04-25-2013, 06:15 PM   #13
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3" with aftermarket exhaust, 4" with stock
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      04-25-2013, 06:58 PM   #14
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Quote:
Originally Posted by s1k0ne View Post
3" with aftermarket exhaust, 4" with stock
Why is that?
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      04-25-2013, 07:22 PM   #15
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So do you get a CEL with a new downpipe? Anyone have MPPK with a 3" HFC dp? That 4" looks way too big.
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      04-25-2013, 09:54 PM   #16
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We have the 4" Evolution Racewerks Downpipe and its been getting great feedback from customers so far!!

Mike
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      04-25-2013, 10:47 PM   #17
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Quote:
Originally Posted by Mike@N54Tuning.com View Post
We have the 4" Evolution Racewerks Downpipe and its been getting great feedback from customers so far!!

Mike
Can you explain why 3inch is preferred over 4 or vice versa?
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      04-25-2013, 11:17 PM   #18
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Quote:
Originally Posted by nsquared View Post
Can you explain why 3inch is preferred over 4 or vice versa?
Bigger the downpipe less restriction which equals faster spool and potentially more power.
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      04-26-2013, 06:48 AM   #19
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I personally think the difference from 3"-4"-5" or whatever is negligible. But it does look like a quality product. For my needs, I still prefer the HFC option. On my white car, I get a little bit of exhaust soot on my back bumper and trunk after a few days and that's WITH the hfc. It's got to be a lot worse with no cat.
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      04-26-2013, 09:01 AM   #20
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Quote:
Originally Posted by mikew2069 View Post
I personally think the difference from 3"-4"-5" or whatever is negligible. But it does look like a quality product. For my needs, I still prefer the HFC option. On my white car, I get a little bit of exhaust soot on my back bumper and trunk after a few days and that's WITH the hfc. It's got to be a lot worse with no cat.
I've had my ER for three months and I've never seen any exhaust carbon built up anywhere on my car. I do have a custom made exhaust . If anything I'm running leaner than before. My exhaust tip is always white. I'm also always running race gas.
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      04-26-2013, 12:30 PM   #21
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Quote:
Originally Posted by buildbright View Post
Bigger the downpipe less restriction which equals faster spool and potentially more power.
+1

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      04-26-2013, 12:34 PM   #22
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Quote:
Originally Posted by GET A GUN View Post
I've had my ER for three months and I've never seen any exhaust carbon built up anywhere on my car. I do have a custom made exhaust . If anything I'm running leaner than before. My exhaust tip is always white. I'm also always running race gas.
Your tips don't even turn black? That's pretty bad A
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