Anyone tested the Drag caused by swapping exhaust?

robones

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?

TerribleONE

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...

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robones

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.

phelan

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.

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凛 ('Rin') - 2009 Nissan 370Z
With silence comes peace. With peace comes freedom. With freedom comes silence.

TerribleONE

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

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edub370

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

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kenchan

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?

robones

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.

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

kenchan

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....

Red__Zed

Saving this spot

robones

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.

phelan

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.

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凛 ('Rin') - 2009 Nissan 370Z
With silence comes peace. With peace comes freedom. With freedom comes silence.

kenchan

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.

phelan

^ 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.

Can you hand me a pocky now please?

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凛 ('Rin') - 2009 Nissan 370Z
With silence comes peace. With peace comes freedom. With freedom comes silence.

kenchan

hummm... so PWJDM can make us a rear diffuser.