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Old 04-07-2020, 12:22 PM   #1
Tfrasca
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Default 4" DP on a 16v?

I am building a 16v B230 with a BW s252 (I have a 1.00 twinscroll housing, but may want to use a .85 instead). My thought is that I want it to flow enough to make decent power at around 8k rpm. Not super concerned with peak power, but I'm assuming the 350 whp range.

I realize that a 3" down pipe would be fine for this, but I'm wondering if I stand to gain anything by using a 4" one. Better spool characteristics, and more flow at high rpm? I will be dumping the two 38mm waste gates back into the down pipe.

It would obviously be a tighter fit with the 4" tube, but I already have a 4" v-band for the housing, which is what started me down this road.

Thoughts?
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Old 04-07-2020, 12:24 PM   #2
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Pain in the ass fitment wise for zero increase in power.
I wouldn't.
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Old 04-07-2020, 02:32 PM   #3
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For one it would radiate more heat in the engine bay.
Power wise, I don't know and did not test it.
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Old 04-07-2020, 02:46 PM   #4
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how much power are you looking to make?

3" isnt really a restriction until you get up over 500-ish hp.
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Old 04-07-2020, 02:53 PM   #5
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if you can fit a 4" downpipe easily then sure, why not. Granted a 140 series engine bay is a little harder to work with I would imagine fitting a 4" downpipe would be a pain in the ass.
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Old 04-07-2020, 03:15 PM   #6
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I read several tests on DP size, it seems that below 500whp there is no real gain in going larger than 3".
I am building a similar engine as you and I decided to go with 3".

http://www.superstreetonline.com/how...aust-test-tech
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Old 04-07-2020, 04:27 PM   #7
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This is a test that some people did that have me thinking about the bigger down pipe:

https://m.facebook.com/story.php?sto...30429603800232

The gist is, bigger down pipe will always help spool and will sometimes help torque, and power to a lesser extent. The numbers and dyno graphs are pretty interesting.
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Old 04-07-2020, 05:24 PM   #8
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Hi Tfrasca.

If we do a quick comparison of those numbers:


I would expect that the hp gained per inch^2 of surface area would decrease with increasing diameters.
In other words, the relative gains from increasing DP surface area becomes smaller with increasing DP size. Do you people agree?

However, if we compare those numbers we see an opposite trend is the case, which leads me to believe they are not telling the whole story.
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Old 04-07-2020, 05:27 PM   #9
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Quote:
Originally Posted by Sjeng View Post
Hi Tfrasca.

If we do a quick comparison of those numbers:


I would expect that the hp gained per inch^2 of surface area would decrease with increasing diameters.
In other words, the relative gains from increasing DP surface area becomes smaller with increasing DP size. Do you people agree?

However, if we compare those numbers we see an opposite trend is the case, which leads me to believe they are not telling the whole story.
Why do you expect that? Over a certain size, absolutely. But if we believe these numbers, it just means that they didn't hit the size at which gains per inch^2 level out, then decrease. Or am I misreading your comment?
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Old 04-07-2020, 06:11 PM   #10
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Also, i missed the design of those pipes. Twin turbo which merges into one pipe with short harsh bends, which are way less pronounced on the 6.5 inch version. This could be a bigger improvement than the dp size itself.
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Old 04-07-2020, 06:22 PM   #11
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No reason to use over 3" with that build. 3.5" exhaust was enough for 800hp with "B6284T" and E85. No screamer.
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Old 04-07-2020, 07:15 PM   #12
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Quote:
Originally Posted by Sjeng View Post
Also, i missed the design of those pipes. Twin turbo which merges into one pipe with short harsh bends, which are way less pronounced on the 6.5 inch version. This could be a bigger improvement than the dp size itself.
Yeah, I wish they had done this test on a single turbo car. That twin setup looks horrible. But I still don't know if the results can be completely disregarded.
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Old 04-07-2020, 07:18 PM   #13
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Quote:
Originally Posted by Lankku View Post
No reason to use over 3" with that build. 3.5" exhaust was enough for 800hp with "B6284T" and E85. No screamer.
I don't doubt that at all, and I'm not under the impression that a relatively mild 16v redblock needs bigger than 3". I'm mainly thinking out loud about quicker spool with the 4" DP. 3.5" was enough for 800hp with your build. But you don't think 4" could have been better?
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Old 04-07-2020, 08:00 PM   #14
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I did swap to a 4" dp later on because goal was to reach higher numbers. Didn't go that route, but with 700-750hp the results have had the same relation to boost as with 3.5". No difference whatsoever. Biggest change after 700hp was changing the rest of the exhaust from 3" to 3.5". The first version had 523hp @ 1.5bar on regular gas, 3.5" dp, 3" front muffler and over the axle, 2.5" rear muffler and end pipe.

Many have had pretty decent numbers with 3" dp. It does not choke a ~400hp redblock. Power and boost go hand in hand if the engine is otherwise working.

Friend dynoed 575hp(not whp) from a 2.3l 16V with a 3" exhaust. Turbo was a Borg Warner, couldn't find the model. That result was with 2bar boost, he did drive it with 2.3bar too.

Last edited by Lankku; 04-08-2020 at 07:54 AM..
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Old 04-08-2020, 12:19 AM   #15
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ran a 3 inch into 2.5 inch system with a 650bhp 1jz. next owner took it to 900hp same exhaust. no need for 4 inch unless its a mack truck.
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Old 04-08-2020, 02:10 AM   #16
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Yea 4 inch is a bit over kill but if you're set on doing a 4 inch downpipe you can always do oval pipe.
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Old 04-08-2020, 08:03 AM   #17
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Quote:
Originally Posted by robyoung View Post
ran a 3 inch into 2.5 inch system with a 650bhp 1jz. next owner took it to 900hp same exhaust. no need for 4 inch unless its a mack truck.
If you have a screamer then the situation is a bit different because there is less gas going through.
Changing my exhaust size to 3.5" upped the power level from 727 to 789hp and torque from 870 to 935Nm(690lb-ft). 2 degree less timing on max torque and around 0.1bar less boost. No other changes.
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Old 04-15-2020, 11:38 PM   #18
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Well I ignored all your advice. It wasn’t that hard to fit, so I figured why not?
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Old 04-16-2020, 02:18 AM   #19
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Well I ignored all your advice. It wasn’t that hard to fit, so I figured why not?
Damn. Blunt.
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Old 04-16-2020, 04:37 AM   #20
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It's so big that it won't even matter that you necked back down to what appears to be 3" lol
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Old 04-16-2020, 09:03 AM   #21
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Quote:
Originally Posted by toms_elleck View Post
Damn. Blunt.
No time for niceties. Too busy overkilling stuff. But for real, I believe everything that people said, and I know a lot of them are way better at this than I am. But at the end of the day, I just wanted to try something. If I ever get bored, I want to make a 3" down pipe out of cheaper steel and run them back to back for comparison. But there's almost no chance I'll get bored.

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It's so big that it won't even matter that you necked back down to what appears to be 3" lol
Yeah, it was always going to go back to 3". No way I'd run a 4" exhaust. I looked at continuing the 4" until the v-band at the exhaust, about midway down the transmission, but ground clearance is already a pain on this car.
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Old 04-16-2020, 03:09 PM   #22
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Quote:
Originally Posted by Tfrasca View Post
No time for niceties. Too busy overkilling stuff. But for real, I believe everything that people said, and I know a lot of them are way better at this than I am. But at the end of the day, I just wanted to try something. If I ever get bored, I want to make a 3" down pipe out of cheaper steel and run them back to back for comparison. But there's almost no chance I'll get bored.



Yeah, it was always going to go back to 3". No way I'd run a 4" exhaust. I looked at continuing the 4" until the v-band at the exhaust, about midway down the transmission, but ground clearance is already a pain on this car.
I like the 2 stroke-y-ness of the design. Looks awesome. Honestly, I think youll be happy with it.

On the 122 I plan to neck down from 3" to 2.5 at the axle. But I have this 3" valve that might find its way in before the neck down.

Not sure if you care, or have thought about it, but throwing an external wastegate set to like 1/2 you max boost pressure down by the neck down might be cool. little boost activated cutout action.
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