Preface: I'm not trying to start a bench-test "which one is louder?" thread. I'm really looking for input from anyone with personal experience with both exhaust systems.

So, I currently have the Fujitsubo Legalis-R stainless exhaust with JIC y-pipe. Obviously I know how this setup sounds. I am thinking about trading with a member on the board who has the HKS Hi-power dual exhaust. I have the opportunity to hear it in person first, but it's a 2+ hour drive so I want to get as much info as possible before hand.

From what I can tell from Youtube/google/streetfire videos, the HKS system seems to be quiet at idle and lower RPM, similar to the Fujitsubo. However the HKS seems to be a higher pitch sound (i.e. frequency) in general than the Fujitsubo. I can't really tell which one will be louder in general (i.e. decibel level).

Anyone who has heard / owned both in person, how would you compare the sound between the two systems?

Performance differences: Anyone notice a difference? I'm mainly looking to change up my look and the sound a bit, but I do not want to sacrifice any noticeable HP or TQ.

 

if its a straight up trade and both are in great condition i say go for it. If i can drive 17hrs for 06 headlights both ways u can do it for 2hrs and your fujitsubo.

Im sure you wont notice any difference power wise. If you do lose power it might be in the lower rpm but will be made up in the mid-high range due to the hks being true dual vs single(fujitsubo). Looks wise hks has your exhaust beat by a landslide.

 

I don't know, both sounds will sweet for sure and perform. In the end, it's just how these looks. I just like how stockie looks, but it's your call.

 

You'll get better performance from the Legalis (single pipe) if your non-FI due to more backpressure.

True dual is only TECHNICALLY pertinent if your going or have FI. But honestly, exhaust gains in general for the Z are little to nothing anyway, so i wouldnt worry about performance.

As far as the sound, the Legalis will be quite a bit quieter than the HKS, and from what i have heard, the HKS has a higher pitch, which im not a fan of. Thats why i just ordered the Injen SES (very deep note) as opposed to the HKS.

IMHO, i would hold on to the Legalis for better performance (little bit) and better sound, but if your interested primarily in looks...get the HKS.

 

single over dual for an N/A Z due to more backpressure isnt exactly true. True dual exhaust from the dynos i see come out on top when it comes to most power in an N/A Z (Injen,Bassani,Borla,HKS,Motordyne). Nismo which is a single did very poor in the dyno test when compared to the hks and most true dual.

When it comes to sound that is up to the OP, but for looks, im sure most will agree HKS is the clear winner

 

^ Not to sure about that. The Nismo tests the worst among almost any single. Was any other single tested? Perhaps a straight through w/o oem setup?

Im seriously not convinced a dual can beat out a single on a non-fi setup.

 

From what I've seen the smaller diameter true duals make the best power N/A (Injen, Bassani, HKS, etc). The single exhausts (single exit) are also fairly decent (JIC, Tanabe). The worst IMO are the dual exit singles, such as the nismo.

edit: I based this mostly on what I've seen on the HR motor. I'm believe this also holds true to a considerable extent on the DE though.

 

Everything you said is false. Backpressure is bad bad bad bad bad bad in every application. The difference in performance will have nothing to do with single vs dual. It has to do with piping diameter bends and how the exhaust merges(HKS)

 

What are you talking about?

Ill tell you what, before i say anything more, why dont you explain your logic to me about backpressure?

Also, tell me why finding the prefect diameter is important and why you think bends play a role. ( i know they do, id love to hear you explain your statement.)

Oh, and also please explain to me the "myth" of loss of torque due to lack of backpressure. Id love to hear that as well.

Enlighten me, please.

 

Cubic Feet per Minute, or CFM. This is the common measurement for flow rate so I'll use that. Flow rate is calculated by:

Area x Velocity = Flow Rate

Now lets say flow rate is 100 CFM through our imaginary pipe.

1 x 100 = 100 or 100 x 1 = 100, flow rate is the same in both, except one example has very small area and very high velocity, and the other has very large area and very low velocity.

When fluids flow (gases are fluids as well), the layer of fluid closest to the surface stops moving, the layer immediately above that moves slowly, and anything above that flows nearly frictionless. These layers are very small, however, they do grow as the velocity increases.

The flow rate output at the engine head, stays relatively the same regardless of the pipe you attach to it. It's once you attach the pipe of fixed diameter, or cross sectional area, that the equation becomes important. A large pipe will have slow velocity, whereas a smaller pipe will have higher velocity.

Velocity is good, but, refer back to the reading. What happens when velocity increases, the non-moving and slow moving layers of flow increase in size. Remember the pipe is entirely enclosed in all directions. If the velocity continued to increase, the layer would continue to increase, until flow would choke. This is what "backpressure" actually is. It's the resistance the pipe presents to the flow of the gas, and in 99.9% of cases, this is ALWAYS a negative.

The perfect exhaust would have maximized velocity and zero backpressure, or negative back pressure. It gets even more complex though. The engine doesn't flow the same amount of exhaust all the time. At idle it flows much less than at redline. One may pick a pipe diameter that is ideal for low rpm, but may choke at high rpm. Another scenario would be where the diameter has good velocity at high rpm, but too low velocity at low rpm.

Picking exhaust piping diameter is critical to where the engine will spend the bulk of its time. A track car spending the majority of its time in the 5000+ RPM range, will want a larger diameter pipe to maximize gains in this range. A daily driver will want more power in the 2000-5000 range, so could settle for the lack of gains in the last band of RPMs in exchange for better gains in the middle.

The further away from the engine all this piping diameter stuff happens, the less of an effect it has. Higher velocity exhaust flow at or near the engine, i.e. headers, could aid in scavenging during the valve overlap. Picking the right pipe diameter, and length makes a HUGE difference here. Further down the exhaust path, it's far less pronounced. Exhaust gas scavenging is where the torque loss comes from, it really has nothing to do with backpressure. If the flow rate is too slow, it won't scavenge as well, meaning the exhaust pulse leaving the chamber won't be able to help pull fresh air in during overlap. Theres plenty of articles on this on google, so I won't get too in depth.

As far as piping bends go. Anytime a flowing fluid has to change directions, it presents a restriction, even if the cross sectional area of the pipe is kept constant.

TK

EDIT: Sorry for oversimplifying, don't want to step on any engineers toes.

 

^^^ good post

 

the HKS is louder under every condition vs the Fujitsubo

 

thank you posting something that makes sense, saved me from having to

 

havnt had any experience with the Fujitsubo exhaust but i just picked up my HKS and it does have a deep growl to it and isnt that loud compared to some others i've heard. But than i added the test pipes to the exhaust and at this point i'm a little nervous when going around town not to get on it to hard, its that loud.

 

So did you trade the exhaust OP?

 

^ bends and restriction is one part i had correct.

Great writeup, i appreciate you setting me straight.

Im def one to admit when im wrong, and i love to learn these things.

It seems as though i am mixed up in terms, perhaps velocity and flow rate with backpressure...im not sure. Most of what i learned on this came from articles, so its sad that apprently they were mistaken, making me look like an ***. Esp lack of pressure behind causing a loss in torque. This was one i was sure i was correct on.

So in basics, what your saying is the perfect diameter must be found to mate the smallest area possible therby higher velocity, but not to high as to cause backpressure? Interesting.

Its funny because im training here in Houston with a large LNG (oil and gas) company to be a sort of "process" engineer at an LNG terminal of theirs in Lousiana. Ive never heard the term "fluid" used to describe the LNG in its gas form. Perhaps the term "fluid" is used simply to decribe the movement of gas more than its molecular state?

 

Correct. Liquid would be the proper term for its molecular state. The term "backpressure" often gets misused, quite a bit actually. A lot of times people are talking about the same thing, with just a simple error in terminology. Backpressure and small pipe diameter do have a strong correlation, but the torque benefits offered by the smaller pipe are related to the stream velocity.

When it comes to comparing one exhaust system to another, just try and picture the flow through the piping and muffler. Generally speaking, applying a rule of thumb here, a muffler with a complex flow pattern, will tend to flow less than a straight through type. So the pattern for most single, transverse muffler systems to make less power than true duals, definitely makes sense.

Terry

 

Gotcha!

Ya, out of interest in this term, i just spoke with the mechanical engineer that is training me, and he says the term "fluid" is not used to describe a gas, itself, but rather its movement through piping, as in when our LNG is in gaseous form and its moving in a "fluid" movement through the piping.

Its (LNG) molecular state in gaseous form is called "vapor", which of course in essence is close to the same as "gas", or rather in between liquid and gas.

Thanks again, Terry!

 

If you havn't already seen this thread, check it out.

https://my350z.com/forum/intake-exha...k-for-n-a.html

 

/\ This has got to be the best, informative post i have ever read...thank you!!