How does the downpipe size (diameter) affect the spool-up and top-end power from a turbo fitted to a 13B? I'm debating what size downpipe to use with my HKS T04Z (1.0 A/R).

Having read some of the posts on this forum, I see that some people think 4+inch is the way to go. But is a 4inch going to make any difference on a HKS T04Z (1.0 A/R) compared to say 3.5inch or even 3-inch, as this turbo isn't really that big?

Having thought about this and read various posts, the restriction to exhaust gases, on entering the turbo's ports, is influenced by, in order of importance:-
(1) the design and A/R of the exhaust housing on a turbo, plus turbine wheel design/size
(2) the downpipe diameter
(3) the flow on the exhaust (diameter, muffler size/effiency, restrictions of bends etc)

But are there any negative effects of using a downpipe that is too big? Will it hurt turbo spool-up or power at 7-8000rpm?
For example, even if the downpipe was say 6inch dia, would the T04Z spool-up just the *same* as if it was 3.5inch?
Even at 7-8000rpm, the back-pressure in the downpipe would be negligable compared to the pressure/resistance-to-flow in the turbo's A/R 1.0 exhaust housing? Am I right here?

I would appreciate it if anybody could provide some good info, or has experience, on this?

Thanks for your help,

Adrian

 

You want to put as little back pressure as possible on the turbine. Back pressure will create a resistance to exhaust flow generally at higher pressures. I have 3" and don't have issues at 20 psi on a t04r. If you want to go much higher, anything larger is good insurance.

Josh

 

You must take into account what your plans are for use.. 3 inch is fine for street driving ( 1 bar or so ). If you plan on race or drag go larger for increased psi and respnse. G

 

as long as u can keep the same diameter with smooth turns all the way to the back then,

this is the size i suggest:

..

as BIG as humanly possible

 

Bigger the better. Yes there is diminishing returns. In other words, there is a point were your not gaining power or quicker spool, just making more noise. But in general, post turbo, the bigger the better. You'll spool faster and make more power, anyone who says otherwise is an idiot. And the T04Z isn't "small" lol. Anyone making 325 RWHP or more on a rotary WILL see better spool up times running bigger then 3" tubing.

~Mike................

 

Thanks Mike.
So who's running a 4-inch downpipe then? Are these people running GT42's and bigger, or are there some with 500R's, GT40's and T04Z's?

I'm thinking of getting 3.75 downpipe made up, as this is the approx the same bore as the Trust Power Evolution2 exhaust (with midpipe) I plan on getting. Has anyone fitted this one their car?

Any other recommendations for the exhaust?

 

Ok these guys are idiots that just believe what others have said forever and have never done any research into it, The truth is if you go with a hugh pipe your creating problems because there will be so much room the gas wont want to leave but if you make it the right size the gas will leave at the quickest speed it possibly can. And this thing people refer to as "back pressure" is non existent. the most I personally would go wold be 4in and thats only with like a t78. Youll be fine with a 3in exhaust.

 

Here this is the best article I could find with minimal work.

http://www.nsxprime.com/FAQ/Miscella...austtheory.htm

 

martin at speedline imports on MRC has a few for sale i think.....but make sure you get the updated version with the exhaust bung.

I have this system and personally i dont think your gonna benefit loads with that turbo. I had this put on my T70 which is bigger than a t04z and compared dynos and the difference was around 10-12 FWHP however i have a MAHOOSIVE turbo now and i needed a 4 inch system!!!

 

I'm running a full 4 inch exhaust on my 1.06 500R. Great spool (15 psi by 3500-3700 rpm) but man oh man is it loud

 

hmmmm.... I donno.. if that link reinforced what you said above.

 

Wow, quote ME and you'll find just the idiot whom I'm quoting now. Mr. interweb genius. We are talking turbo, being an engineer, and working at NASA not to mention being GARRETT trained retail/ turbo builder you sir are an idiot. Post turbo (meaning AFTER THE TURBINE) you want the LEAST exhaust restriction possible. I did mention a point of diminishing returns, were... your going to increase more noise then power, but you WILL NOT LOOSE POWER. A turbine works on pressure ratios, pre-turbine and post turbine. If you lower the post turbine pressure, then your ratio is greater. <--- That's a period (hense quicker spool-up, more power).

Speaking naturally aspirated, YES, totally different story and YES, back pressure is a misnomer. Key to N/A power is VELOCITY with respects to the point were things get choked and back pressure rises negating the velocity your tryig to keep up.

YOU need to research because you are WAY WRONG in respects to TURBO exhaust systems. There's guys on this board spooling 500~800 RPM faster and making 20~40 more HP going from 3" to 3.5" in the low (respectively speaking) power ranges of 350~400 at the wheels. Your quoted Link and Arther is un-educated to an extent as well. Though the majority of his page is valid, he does lack basic fundamental turbo knowledge and even makes a comment "Another object that might be sitting in your exhaust flow is a turbine from a turbocharger. If that is the case, we envy you. " thus given way to his lack of knowledge and lack of experience with turbo systems and how they work.

I'm not an internet quoter or idiot who repeats crap I've read.

Solid physics and engineering, can't argue with it, its NOT a matter of opinion or open for debate. If you think other wise you are ignorant and un-educated IDIOT.

~Mike...........

 

Tafkamb2, you are very ignorant. Its very simple, you want to get the biggest change in pre and post turbo space conditions to encourage the exhaust flow through the turbine housing. Optimum on a turbo car, being no exhaust.

 

So basicly you guys are saying that because theres a wheel in the exhaust of a turbo car the velocity theory is non existent. Ok you go with that.

 

as far as i know ONLY Normally aspirated engines require back pressure for low rpm performace.

TURBO vehicle might as well have no exhaust because the turbo ITSELF is the back pressure. (exhaust on turbo cars should just be for sound restrain) THis is why open downpipes create the best results.

 

If you look back at all of tafkamb2s posts, he is pretty much just a useless troll. I would just ignore everything he has to say, period.

 

Well said Mike. Obviously there is a serious lack of actual experience here...

 

but but.... i read on the honda forums.....

 

The entire discussion is POST turbine. Velocity important post turbine?? No, you want as little pressure as possible. <----that’s a period, again. You want a certain taper out of the turbine, then from there it can be completely open to the atmosphere or you can add an exhaust system, the bigger the better for quicker turbo spool up AND over all power from idle to redline. Pre-turbine, certainly you want to keep velocity UP.

You don't seem to GET that the velocity you keep reading about is important for N/A application or in a turbo car PRE-TURBINE. You want to keep the velocity up from the blow down phase to promote evacuation of the cylinder/chamber. Pulse tuning and velocity are keys to tuning here, but after the turbine, it’s all over with. The turbine has chopped every bit of sound and pressure waves so it’s useless after it leaves the turbine. The Hot exhaust lost much of its temperature because it was used to spin the turbine and it was absorbed by the turbine housings etc. It’s cooling rapidly and it needs to leave the entire system with as little restriction possible.

Yes the sarcasm was detected in your response, so either your pride is too large for you to swallow and admit being wrong, or you aren't capable of understanding what everyone here is trying to explain.

~Mike...............

 

For those who want a real world example.

I ran many combinations of exhaust on my car with the same turbo and engine set up. It is a little stock hybrid turbo- basically a clipped P-trim T04B 60-1 HiFi in modded stock S5 exhaust side/manifold w/ 60mm external wastegate.

With 2.5" Racing Beat downpipe/presilencer to 3.5" catback it would not quite reach 14psi full boost- so, no full boost threshold.

With 3" Racing Beat turbo back it would hit 14psi full boost ~3,800rpm.

With 3.5" JIC turbo back exhaust full 14psi boost at ~3,500rpm and lots more boost/power in the 1,000 to 2,000rpm range. But had boost creep past ~6,000rpm.

With 3.5" JIC downpipe to Racing Beat 3" rear section full 14psi boost ~3,500rpm, a very little less boost/power than full 3.5" system from 1,000 to 2,000rpm and no boost creep (so obviously not flowing as well on the top end).

All the systems felt like they had close to the same PEAK power, but the larger the exhaust the more low end power/boost the car had.

 

Man you are a **** quoting NA theory , this is about turbos you ********

 

Two words for you, pressure differential. Take some time, think it through, and absorb it

 

this man and zero r are the truth. its also kinda basic in a sense also less back pressure for a turbo car equals faster spool. wouldnt go too big either because it may be unecessary and price also goes up on something you dont need. im a go with a y pipe and dhm exhaust cut out right before my mid pipe resonator.

 

Thanks for all the replies, and info I've now got. It's interesting that going from a 3.5" downpipe to a 3" exhaust limits the top end power with a T04B 60-1. I guess with my HKS T04Z the effect would even greater, so I might as well keep the exhaust dia the same size as the downpipe. I just hope having a 3.75" downpipe/exhaust isn't going to be too loud for me (or the neighbours/police!). There's only one way to find out!

As for tafkamb2, I think he'll know better than to quote N/A theory on this forum again...

 

i recently heard a 3.5" straight through on a half-bridge .. IMO it wasn't loud enough!

my brother's fd is a 3" straight through and it's not even as loud at 6k rpms as my civic at 4k with stock mani and old muffler