He's already noticed a loss in low end with just an hks catback. I don't think going 3" piping will help it any. Heck, I used to start boosting at 2.5k rpms with the stock system and now it's at 3k with the helix system. I threw on a Helix tmic on a stock wrx (at the time the tmic was too good a deal to pass up) before the helix turboback and it started boosting at 3.5k rpms. Took that off and now I'm back to 3k.

As for the poll, it's like asking which would you rather prefer, Stainless steel or full titanium, utec or accessport, rota wheels or volks, etc etc. An opinion vote based on popularity.

For example, if you were to run a poll on the VF22 turbo vs a VF34 turbo, in which spools faster, the majority would say the VF34. Or which turbo would u rather prefer a VF22 or a VF34, the majority would say the latter.

However, since u said there is a vast misconception about the VF22's turbo lag, the 2.5" vs 3" poll is merely based on just unfounded opinions lol

 

Jesus, haha. You are terrible...

I throw in a white towel and give up, LOL!

The facts are in front of you. I cannot do much more.

Enjoy your undersized 2.5" downpipe and watch me try to use a 4" downpipe.

 

Yay I win! lol :banana:

 

3" is not necessary for the stock turbo. DWG or belmouth is not necessary for the stock turbo. Explain why there is no discernable difference in dyno readings between cars equipped with a 3" flat plate HKS DP and a flavor of the week 3" belmouth/DWG DP on a stock turbo car.

Marketing, that's all it is. All these high grade stainless steel parts will already outlast our cars lifespans. Why pay extra for it?

If you have the extra money and it will make you feel better, then buy the metal substrate hi/flo stainless steel belmouth DP for $4-600 and convince yourself that you will make more power than a $100 used HKS DP with your stock turbo.

 

Gotta let the engine breathe. Bigger pipe is better............More air leaving the turbo is more entering the turbo. It equals everything out. Why not go bigger??? If you go 2.5" you are just limiting yourself. But if it makes you happy then do it. Its your car. Imprezer was just giving his opinion. And you where giving him somebody elses opinion. Go with your gut man. smaller pipe = less flow = less hp. IMO

COMMON SENSE

 

This should be stickied. downpipe faq. lol

 

Proof, por favor. The dyno readings will for sure be different. I am looking for dyno sheets to prove you otherwise.

 

The stock turbo doesn't have a 3" exhaust outlet. Why do you need a 3" downpipe? Look at a picture of a Stromung, Maddad or Crucial racing DWG downpipe. It tapers from the turbo flange out to a 3" pipe. You could say it has a bottleneck, but it doesn't make a difference. The exhaust outlet of the TD04 is already a bottleneck in the exhaust system. It is what it is.

The 2.5" pipe already provides adequate flow with the stock turbo. Your bigger is better theory suggests that 3.25" or even 4" would be more ideal than a 3" DP. Why do you think most manufacturers make 3" pipes? Because it's a good, one size fits all, compromise. Go read some threads on people that tried running with no piping from the turbo back and see how that worked. Yes, the turbo provides all the backpressure needed, and you don't want any back pressure after the turbo, but come on, how much air is that little TD04 pushing. 2.5" can breathe just fine with a TD04 and not be a restriction.

Also, the wrx is a naturally aspirated car until the turbo kicks in. As a daily driver, it can use all the help it can get under 3k rpms.

I sold my old scoobysport DP because I thought a full 3" DP would be so much better. I regret selling it because the power delivery was much better IMHO on my car with the scoobysport. The car seemed more drivable around town. I felt a small difference on the topend with a full 3" belmouth TBE. Not worth it to me. Now, I'll admit that a custom tune with either setup could have changed my perception one way or the other. I was running an off the shelf Cobb stage 2 map with both.

The only reason to go 3" is because practically every DP is 3" and if you ever want to upgrade to a big turbo that will be pushing a lot of air, you'll have much more to choose from and be ready to go.

 

Imprezer, you know dyno readings are subjective. I'm sure if you hunt around long enough you can prove your point and I can prove mine. However, at the end of the day, the HKS DP equipped cars and the belmouth/DWG cars with similar supporting mods will all be making right around the same amount of power on the same dynos.

 

I agree that dyno readings are subjective. I disagree, however, with you main point that 2.5 and 3.0" downpipes would make same power. Especially something like "HKS" design where the wastegate passage is small.

Unless I get a car and dyno it with both downpipes, I cannot "prove" my point to you. Neither can you. So, let's just agree to disagree. People can read both yours and my arguments and make their decisions.

In regards to larger than 3.0" exhauts I will say that yes, going bigger for downpipe is better, but of course, there is a limit. Most downpipes in JAPAN are 80mm, which is around 3.2". There are 2 companies that I know that make 90mm downpipes and exhausts. When you go larger than 80mm, fitment becomes an issue. Also, combusion engine is a pump. So, it pumps out certain volume of exhaust gasses. When going bigger, there is a limit, at which exhaust system diameter becomes over sufficient for the exhaust gasses the car products. That is why theoretically, there is no point of going bigger than size X. In most cases with EJ20, it is 80mm, in my opinion. For EJ25 I recomment 80mm MINIMUM.

I will try to find an article where a perfect exhaust system for turbo car was described. It was something like a foot long pipe funneling out to double the size of the turbo outlet.

 

keep in mind me and him were talking only about a stock turbo wrx, lightly modded. I have the feeling you keep thinking we're talking about bigger turbos in which case I don't think anybody will argue with you that 3" piping would be better.

Let me ask u this, what size piping is a stock sti's downpipe?

Also, if you look at the VW 1.8t's with their puny stock turbos, look at how many downpipe manufacturers there are making 2.5" piping vs 3" piping.

 

It does not matter if turbo is stock. 3" downpipe will STILL make more power than 2.5" downpipe, especially one with OEM like flange that blocks off everything. I have seen it with my own eyes on the dyno and we agreed to disagree on that already, haha.

STi downpipe is 2.25" IIRC. But don't start saying that if OEM is 2.25" then 2.5" is ok. That is another agrument. Performance vs. emissions vs. noise vs. production costs vs. fitment.

About VW's. I don't know those cars, so I am not going to say anythign about this.

 

wat would be a good turbo if youve got a built engine(pistons, rods, studs, portnpolish, valves retainers, etc. 560 cc injectors, fuel pump, fuel rails, 3 inch turbo back exhaust and upipe?

 

haha, 4" TurboXS FTW!


Overkill....meh

 

It depends what you want to do and few other things like the displacement of your motor, which you left out.