Noob question again: Exhaust, make a power gain?

[LaserSVT]

I have used the Googler on the interwebz and found a few old threads on here and a few other sites but the results were inconclusive. One guy with my setup gained nothing on the dyno in peak HP but said nothing of the under the curve gain. Another claimed 15 HP. One with open exhaust saw 40 hp but also did a manual conversion so I would imagine most of that is the trans.

So with a tune and a free flow exhaust does the exhaust make a performance difference? Is there any peak HP gain and does it effect the curve at all? Like quicker spool up with more HP in the middle of the power band?

Sorry as I have no back to back first hand testing since mine already came tuned and exhausted.

[hahnmgh63]

Quicker spool up is a fact with any open exhaust on a Turbocharged car. So as you open up the stock exhaust your are going to get a little bit of the spool up depending on how much you open it up. A little bit on the peak but most of it is going to be the Turbos being able to spool up to the same boost at a lower RPM than before.

[LaserSVT]

Thats what I kinda figured. Quicker spool so mure power under the curve which translates to quicker acceleration. Most of my builds are with positive displacement blowers or centrifugals. Turbo modification experience is a tad more limited. Couple RX7s and Z32s really. Mt S4 2.7T I just pretty much winged it with an APR chip and an obnoxious exhaust. Sounded faster so I figured it was. LOL Learned a lot in 14 years but am still behind the curve with turbos.
Thanks for the input.

[G2]

Do we assume the care is "chipped"? If so, an exhaust makes sense. If not, get a Tune/Chip for it first. Three times the power and 5X the torque....

These cars have a very decent exhaust for the power level. I've driven others (RS6) with various exhausts. May make a slight improvement with throttle response. Certainly sounds great. Last thing a weak trans and tuned car needs is even more torque. If one needs torque that low, suggest dropping down a gear and using the solid mid range power that's available. It's better for the drivetrain. Keep in mind the trans is only rated for 415lb/ft....that "area under the curve", has teeth, and bites. Ohhhh... and it feels so good right?

But naturally, a bigger exhaust will help. I'd surmise a Tuned car would pick up 25+hp, mostly easily. And probably more so in Torque.

Let us know how it works out.

[LaserSVT]

Let you know how what works out? My car is chipped and exhausted and has been for several years.

[Turbowned]

I think from what I've read that the biggest improvements can be found in replacing the downpipes with high flow cats, or catless downpipes. You can also gut the factory cats as I don't think the shape of them is terribly inefficient. I have seen a couple spots on the factory exhaust where it looks a little pinched but overall it's very efficient-looking.

A full 2.5" setup from the turbos back would likely offer the biggest gains; can go through Milltek for the downpipes and catback or there are some other more expensive choices. I reckon anything bigger than 2.5" is unnecessary unless you are running much larger turbos, which we all know isn't possible without a 6MT swap or turbo relocation.

[Bigglezworth]

A full 2.5" setup from the turbos back would likely offer the biggest gains; can go through Milltek for the downpipes and catback or there are some other more expensive choices. I reckon anything bigger than 2.5" is unnecessary unless you are running much larger turbos.Very accurate statement and well supported by science. Many a hot rodder over the years has gotten caught with putting on pipes that are too large for their power plant and are heartbroken when they dyno things expecting a xx% power gain only to realize they have actually lost power. Smokey Yunick learned this many years ago when the motorsports industry was trying to extract more and more power out of FI and NA engines.

[G2]

Very accurate statement and well supported by science. Many a hot rodder over the years has gotten caught with putting on pipes that are too large for their power plant and are heartbroken when they dyno things expecting a xx% power gain only to realize they have actually lost power. Smokey Yunick learned this many years ago when the motorsports industry was trying to extract more and more power out of FI and NA engines.

Certainly agreed for NA motors. Recent and Personal experience on a new 5.0L Mustang, saw a marked loss of low RPM torque with a very large exhaust and headers. With shorter gearing it was fun making it howl. But even the reduced gearing didn't make up for the loss of torque. Lots of fun once "on the cam".

Turbo? Doesn't seem to care how big or open the exhaust is. The exhaust back pressure and other related harmonics so crucial to NA motors doesn't quite apply in the same way.

[G2]

Let you know how what works out? My car is chipped and exhausted and has been for several years.

Sounded like you were shopping for an exhaust. It's apparent it was simply back tracking, in a sense, reverse engineering the current setup to determine what add'l HP/TQ may exist.

With that said there are other factors such as weather conditions that will make far more difference.

I will likely install a Billy Boat exhaust when it's time. The OE / SE pipes are a good hold over. Be neat if someone had backpressure values to share.....

[Bigglezworth]

A decent 10 min read on this topic from another forum I visit a lot that covers it all for those in the know and those green on the matter.

http://www.nsxprime.com/nsx-faq/exhaust-theory/

[G2]

Reasonably well put in general terms (scanned it), but pretty thick sarcasm. I guess it adds humor...

This seems to point out where their knowledge hits the wall: "Exhaust gases leave the engine under extremely high pressure". If it did, the engine wouldn't run very well. Exhaust gas has to be at far less pressure. All engines will reach a 'pumping velocity' limit, where an equilibrium will prevent further RPM gains. On the street, we notice power decreasing and "grab another gear".

Maybe they meant 'velocity' instead of 'pressure'. Velocity certainly does apply, and what makes turbo's work. Fast gas leaving the cylinder head. Engine pressures are usually expressed as 'BMEP'. This is the actual pressure exerted onto the piston. It's been years, maybe decades now, but about 400 PSI (combustion pressure) was my calculation with 1 bar of boost in a low 'VE' rated engine (2 valve). Or about 1300 lbs of force exerted onto the piston for a 4 cyl.

In the racing world -- Porsches with short exhausts are a great example -- turbo's literally dump open header. Once can peek in to see the turbine. One of my cars has a similar setup, where the wastegate dumps "open header" style. It's probably more of a noise maker, but is fun at nite with flames coming out of it.

Far as turbo's go, this rabbit hole is deep with math: https://www.princeton.edu/cefrc/Files/2012%20Lecture%20Notes/Reitz/Princeton-CEFRC2.pdf

On a side note, if someone wanted the best possible flowing exhaust, best thing to do is keep the heat in. Far cheaper. When that dyno shows up we'll do a comparison. :revs:

[Bigglezworth]

On a side note, if someone wanted the best possible flowing exhaust, best thing to do is keep the heat in. Far cheaper. When that dyno shows up we'll do a comparison. :revs:Bingo. As the author eluded to, you can coat the entire exhaust system with an insulative material such as header wrap or a ceramic thermal barrier to keep the heat in, but unless you have lots of cash burning a hole in your pocket (some owners of these cars seem to certainly have a lot...), it's probably not worth the expense on a street driven car.

[LaserSVT]

^ Thank you. I am saving that PDF to my PC. :)

[LaserSVT]

Sounded like you were shopping for an exhaust. It's apparent it was simply back tracking, in a sense, reverse engineering the current setup to determine what add'l HP/TQ may exist.

.....

Yes sir, in a sense. Just curiosity as I do not have my car at the moment to strap to a dyno and see whats what. I have built several turbo cars over a decade ago but being Z32s (last 300Z) and FD3S (last RX-7) they have very restrictive exhausts and a simple high flow cat and catback made a world of difference both in the "meat" of the power band as well as 20-30 total gain.
I had found it odd that a few people here saw zero gain which is what me question the area under the curve. These cars have so much going on in their brains I didn't know if the system would ramp back the turbos due to the more flow to keep power at whatever the target was or if the system would allow them to reach more boost at lower RPMs due to the freer flowing nature of the exhaust. Those old Japanese systems were much easier to fiddle with. LOL

[Turbowned]

I miss the ol' "strap a 3 inch turbo-back and a boost controller on it and pick up 30hp" days with my RX-7 and MR2. I used to ignorantly run a big blow-off valve back then, so all the excess pressure would vent to atmosphere and the turbo would fall on its face after lifting off throttle. Live and learn.