Anyone tested the Drag caused by swapping exhaust?
 

Notice how the Z's exhaust is designed, it's engineered so that the air passing underneath the car curves through the bends of the exhaust through the rear diffuser in a smooth way. This causes less drag at high speed.

By changing the exhaust to a two piece set up, i.e: Fast Intentions, Berk, etc. you are are creating essentially a parachute on the rear of the car. I would think drag is significantly increased by this design.

When you remove the CAN design, the rear bumper gets exposed and creates a huge gap which traps air underneath the car just as its exiting through the rear.

The better designs would be exhausts similar to the stock setup and/or the nismo setup. Stillen, Greddy have similar designs which leave less of a gap in the rear bumper.

Your thoughts?

You are absolutely correct... much like the square back of a VW golf creates a vacuum. However unless you are looking for absolute top speed of the vehicle, this may be just an academic observation.

:tiphat:

Interesting point, never really thought of that. Significant to affect necessary performance, not really.

Well just imagine how much drag you are creating at higher speeds? The higher the MPH the higher the drag your car is creating. I would think that any significant HP gain from these two piece setups systems would be offset by the increase in drag they create?

I know the two piece setup sound nice and looks cool from the back but not much engineering thought is put into these systems IMHO.

But the car goes much faster while standing still with the HKS style exhaust setup ;)

:icon18:

I've noticed that too. It's such a huge gap, and the back is mostly solid. There are several aftermarket exhausts that still use a setup similar to OEM though, like Invidia, Akrapovic, Ark DTS Type 1 (did that ever go into production?), and probably a couple more I can't recall.

One could probably just put a flat piece over the entire area to cover it up if the drag really is significant.

I haven't noticed any issues at speeds around 240km/h....

I see the difference being extremely minimal..

I'm going to sell my exhaust now and revert to stock. I need to make sure I'm as aerodynamic as possible.

:roflpuke2:

LOL funny but hypocritical and sarcastic at the same time! lol think about it aerodynamics and drag coefficient should be equally considered and are equally as important as HP gains, back pressure, etc when considering an exhaust setup! Otherwise, why get one at all! :p

Sounds like an opportunity for someone to develop a fireproof foam insert to fill the gap. Only way to find out is to line up a stock Z and a Z with upgraded exhaust and wind sail for a 1 mile race. :eekdance:

Blocking it with something would just make things worse IMO... At least the canisters are round and they are pretty slick.. A straight wall would make it even harder for the air to go around

The large majority of people who buy an aftermarket exhaust system are more concerned with sound and looks.

if you are trying to shave seconds of your best lap... why would you want to hinder your chance at going faster?

Same argument people make on this forum when a member gets new shoes for their car... "HOW MUCH DO THE RIMS WEIGH"??? Now why would people care about the weight of their wheels?

I personally dont think that drag will make a difference even on the straightest of tracks... Plus the power you gain would make up for it anyways. As for weight of wheels, that seems like a retorical question...

You may be underestimating the importance on minimizing drag as much as possible specially if you track your car. LOL yes, about the weight of the wheels; definitely a retorical question. :ughdance::tiphat:

I can't imagine the increase to the drag coefficient from swapping exhausts to be that bad. It's not like the stock exhaust is tuned to provide laminar flow, and that massive black piece of junk in the back of the exhaust is pretty bad design-wise as well (if Nissan actually wanted a "performance exhaust" they'd smooth the front of the curve to guide air better). Every gap and bump will contribute to the drag coefficient, or in essence increase the turbulence in the area.

Yes, the low-slung cans of the FI may be even worse in terms of causing turbulent air flow. But you're better off worrying about the exterior of the car, which will have more say in the drag coefficient, than what's going on with the exhaust.

If you wanted to have a drag car that absolutely had the best aerodynamics possible, you'd put an entire underbody diffuser that guides the air in a straight flow line from front to back.

Side note regarding the wheels: it's a well known adage that reducing unsprung weight is equivalent to three times its amount in sprung weight. So it's pretty good bang for your buck IMO if you're going the weight loss route.

:iagree: You typed exactly what I didnt want to type out.. lol

IDK.... my car still did the nissan specified 155mph (in mexico) with my aftermarket exhaust on... so i imagine at the speeds our factory cars are limited to that this small of a difference is neglegable. hell a nismo spoiler probably causes more high speed drag than this

plus any drag gains are probably negated by the massive amount of weight u will be dropping and the small hp bump when going with an aftermarket exhaust

i thought these cars were designed with zero lift.... meaning minimal air going under the car at speed. if that's the case the open bumper parashute effect the op talks about should be minimal.

and i too agree with phelan about adding a functional rear diffuser would further improve things. how about adding a small flap just aft of the most rear flange to the muffler area?

I think unless the drag is compared to a non modified z we can't really determine how bad or how little a difference drag makes. All I know that I lean towards it making more of an impact then not.

I remember watching a myth busters experiment about leaving your truck bed hatch open or closed while driving to increase MPG. It was found that you will lose about 3-5 mpgs if you leave the hatch down instead of up. All because the turbulence thats created in the bed of the truck while the hatch is down. So you can see how much harder the truck was working just to keep the same speed. :p

Again this argument only pertains to certain exhaust designs as previously mentioned.

robones- that's interesting and if there is significant air going under the Z (which i believe does not) would affect top speeds and mpg from the rear bumper parashute effect. :)

it would be nice to have a flap or a diffuser though....

Saving this spot

Kenchan: There is definitely a good amount of air going underneath the car. This is indicative by looking at the way the nismo rear bumper is designed to incorporate the rear diffuser and in a similar but less effective form the stock z bumper.

Oh dear, the mythbusters correlation.

Okay, there is a question of scaling here. The truck bed is quite a big surface area that changes / alters the air performance characteristics in the area. The exposed square footage of the exhaust is significantly less. Therefore while there is SOME effect, it is minimal at best. Do the proportion, take the truck hatch and the exposed area of the exhaust, and divide them to find the ratio. It'd be like...5% maybe, at best. edit: You also would need to consider the volumetric air flow in the area. The majority of the air is going up and over the car; that's how most are designed (but air still gets underneath, as I note later). Therefore there would be a slight bias, or increased impact, corresponding with aerodynamic improvement on the upper exterior of a vehicle.

To fully consider the question, you'd have to consider the better air flow through the engine and the reduced weight an aftermarket exhaust gives you, and trade it against the increased drag coefficient. To me, it's a waste of time, the drag coefficient is such a minor issue that the benefits of swapping vastly outweight the deficiencies. Or do a CFD model, run it through a theoretical wind tunnel, and then see what you get for drag coefficients of the vehicle with stock and aftermarket exhausts.

ken - what do you mean by significant amount of air? The Z isn't designed to push air around the vehicle, it lets it go through. The front "scoop", or intake area, isn't designed to limit air from going down. If there was no chance of air going through the bottom of the car, we wouldn't need race-type wings to increase downforce and keep the car planted at high speeds.

i dont have a numeric value but i dont think there's much air going under the car when compared to above it. so i dont think there much of this parachute effect from the rear bumper.

i think you're exaggerating a bit too much. you need to sit down and have some strawberry Pocky. :icon17:

^ well that's what I wanted to get out of you ;) read my edit. I agree with you, the parachute is magnified over the vehicle because the percent air flow above the vehicle, not below. :tup:

Can you hand me a pocky now please?

hummm... so PWJDM can make us a rear diffuser. :ugh2: :wtf2:

:icon14:

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[QUOTE=phelan;1651002]Oh dear, the mythbusters correlation.

Okay, there is a question of scaling here. QUOTE]

:p LOL I know but I made the reference only to make a point. Agreed, definitely much less air is directed through the bottom of the car then on the top.

:roflpuke2:
:bowrofl:

:inoutroflpuke: AWESOME ZED!!!! HAHAHAHA

Just some observations:

1. Kenchan is right, there is not much air going under the car
2. we have front spoilers
3. the differential is almost as low as the rear bumper therefore it will block off some of the air approaching the bumper "parachute like" area
4. If someone really wanted to, plug in an airflow sensor right behind the bumper between two cans and measure how much airflow there really is.

I'm not sure if this is relavant:
http://www.the370z.com/nissan-370z-g...ml#post1581307

Two words... "Flux Capacitor" Solved!

http://images.wikia.com/bttf/images/...flux_paper.jpg

IMO, the impact is negligible at best. There simply isn't enough airflow in that area to worry about. If i could get hold of a cheap air flow meter and find out i would.

I don't think it's really an issue of drag as much as it is lift, or reduction of lift. Remember the 370z is designed to have zero rear lift. This is probably done with the spoiler and flat (err... flat enough ) bottom. The fact is that none of us likely have the resources Nissan has in order to determine what effect there is from the various styles of aftermarket exhausts. All I know is from most of the articles and papers I've seen regarding race car aero, airflow underneath the car is at least as important as airflow above the car, if not more important. Just look at some of the supercars coming out now that generate hundreds of pounds of downforce without using a wing. Of course they usually have a completely flat bottom along with a bunch of other specific design considerations, but you get the idea.

That said, I think anybody really concerned with the issue would probably also think to buy or build an exhaust that's tucked up as high as possible in order to leave room to build a custom diffuser. Since we don't necessarily have the wind tunnels or computer modeling at our disposal, we really would have to stick to the academic. My gut says that messing with the exhaust in any way risks creating some rear-end lift. If that's a concern then you should plan on adding a rear diffuser.

But, I really think the only folks that ought to be concerned with it are those that heavily track their cars and want to do so with no rear wing or one as small as possible.