odd exhaust question

[CamaroAJ]

not something i'm going to try but was wondering about if it would work. mounting a NACA duct 180* from normal to pull air out, mounting the exhaust to that in a side panel. now in theory i would help scavenge the exhaust out of the system at speed. now in practice would it work and if it did would it help with anything?

[wmhjr]

Not sure it would really help scavenge. Interesting idea though. But I think the reason x-pipes help to scavenge is because the exhaust pulses from one exhaust (positive pressure) help evacuate gasses from the other exhaust circuit. The surface area exposed (the 180) would probably be too small and too far downstream to help scavenge IMHO.

[Nessumsar]

From what I have read, there is speculation as to whether a reversed NACA duct actually does much; I would think it would be more beneficial to exit the exhuast underneath a cowl.

An idea I have wanted to do is bring the exhaust up through the trunk and have it exit out of the trunk lid underneath a cowl, this would help scavenge and provide more downforce at the same time.

[monteboy84]

I know a lot of road race cars have the exhaust exit below the spoiler in the middle of the back panel, I'm positive this is for aerodynamic reasons, but it'd be difficult to package on a street car.

-matt

ETA: Not only difficult to package, but the people sitting behind you at a traffic light probably wouldn't appreciate it

[wmhjr]

I know a lot of road race cars have the exhaust exit below the spoiler in the middle of the back panel, I'm positive this is for aerodynamic reasons, but it'd be difficult to package on a street car.

-matt

ETA: Not only difficult to package, but the people sitting behind you at a traffic light probably wouldn't appreciate it

Kind of like the surprises encountered during the initial deployment of the turbine powered M1 tank in Europe back in the 80s. Armor moving through small towns was so common that host nation civilians grew accustomed to pulling cars or even bicycles up right behind M60 series tanks. The turbine exhaust from the M1s generated a newfound awareness for the hazards of getting too close.

[Bryce]

When I studied aerodynamics in school, I was taught a reversed NACA duct would work as described above. Its due to the boundary layer flow. Actually disruptence of the flow.

I will have to look through my book and go back over the equations. But I doubt you would have enough flow at normal driving conditions to make this an effect exhaust scavenger.

[CamaroAJ]

normal driving no. track days or racing events is where it would work the best or possible the guys that run open road events like silver state and the texas mile.

[Bryce]

normal driving no. track days or racing events is where it would work the best or possible the guys that run open road events like silver state and the texas mile.

My rough guess would be closer to 200mph you might have enough flow.

[formula]

I still don't see the airflow getting high enough to measurably improve scavenging. Exhaust gas velocity is already, what, around 250 fps? So I suppose you might begin to see a slight effect above, say, 200 mph?

[monteboy84]

Right exactly, the way Naca ducts are designed, they aren't working as designed in most cases. We run them on our race cars for driver cooling ducts and such, but that's because they allow an easy air inlet without disrupting airflow, not because we're going fast enough for them to work in a way where air is drawn and forced into the ducting, we have fans for moving the air.

Similarly in your exhaust, it'd make for a nice clean exit, but you'll never go fast enough for there to be any scavenging effect. I'm not real sharp on aerodynamics, but the way I see it, you'd not only need to be going 200mph for the NACA to induce a vacuum without considering pressure from the exhaust, but you'd need to go fast enough where the vacuum effect exceeds at least half of the exhaust pressure for there to be a noticeable effect on scavenging. And then there'd be the debate on whether this would actually enhance performance, since many engines perform better with some level of backpressure.

-matt

ETA: I'm not trying to talk you out of it, I think it'd look pretty cool regardless of whether it's actually working.

[wmhjr]

Matt, you bring up a great point. Iirc, the scavenging advantage from an X-pipe is from the pulse from the off-exhaust assisting I can see the math resulting in using these ducts actually decreasing performance due to the lack of coordinated pulse timing, right? I mean, potentially.

[mrn2obelvedere]

From what I remember of fluid mechanics, the effect would be nil. I think even at 200+ mph it wouldn't be very much. Also consider that at those speeds, it takes a lot more power to go faster. In fact, the aero disturbance that such a system might cause could easily negate any gains.

This is all speculation of course, but I can't see how it would have any effect.

I also don't think this would have any effect on the exhaust pulses

[NJSPEEDER]

I would think the pressure of the exhaust being pushed out would prevent any draw from teh air being pulled by but it certainly would look cool.

-Tim