gpslater

Has anyone been able to solve the aerodynamic problem created when the stock exhaust is removed and replace with a true duel exhaust. The problem is the empty space that allows underbody airflow to be trapped by the rear bumper. I recall reading a thread a while ago where some prototype airfoils were attempted but unsuccessful. I know the different true dual configurations make a solution difficult, but maybe some kind of foil-wrapped high-temp foam with a more aero-efficient face could be designed to fill the bumper area. Obviously, a generic device wouldn't be perfect, given the differing exhaust configurations, but I think it would help some. No engineering degre here and I'm not handy enough to even make some trial and error atttempts at this myself. Anyone?

Tony@Performance

iTrader: (16)

Jason actually is a firm believer in this. However unless you are going upwards of 120 or thee abouts, you won't get any side effects really. I mean you really have to be going pretty fast to have that become an issue. It's true, but for the normal Z owner, it's not something that will hold them back. Also, witha true dual, you are filling up much of that space in the back with two mufflers, etc.. like on the injen, so it's not something that really needs to be addressed. If anything the mufflers act like diffusers to an extent because they guide the air through the middle and then the sides of the rear of the car.
It's an issue not worth bothering over, it won't affect 99% of us

Tony@Performance

iTrader: (16)

oh and using a foam wouldn't be a good idea. foam is porous, (many tiny holes in it), and thus air will flow through it still, and that can actually harm the performance of a car, because you'll be catching air without letting it out. It would sure create downforce. Basically, that would be like creating a sponge for air instead of watter. a flat panel of carbon or thin aluminum is all that's needed, and reletively inexpensive. but again, it's not worth wasting time over

JoneZZZ

iTrader: (1)

There was a discussion on this a little while ago....https://my350z.com/forum/intake-exhaust/110717-aerodynamics-under-a-z-without-stock-exhaust.html

JimRHit and Chebosto were talking about fabricating a solution....check the discussion.....might want to PM them.

gpslater

Granted the drag effect might not be much for the average/casual driver, but I'm thinking that a lot of debris would get kicked up into the bumper, especially in winter. BTW, I was proposing a foil wrapped foam solution.

gpslater

Thanks JoneZZZ. I'll run this by them too.

Z1 Performance

iTrader: (564)

how about someone prove this is actually an issue first before everyone starts engineering solutions or hypothesizing......

Chebosto

iTrader: (43)

My main concern was when i swapped out my stock exhaust for the RSR, was that "whoa! there's now this huge void where my muffler used to be, and what's this? i think it was a diffuser to begin with" concern.

i've noticed from observing my own car during highway driving that yes, my back bumper does vibrate slightly. I didnt remove any mounting points for the rear bumper, so i suspect that it's due to the fact i have now nothing where there was something, and air is being trapped.

if you look at alot of the new body kit designs coming out for the Z, you'll notice several ducts,slots, and holes slightly below the support beam on the rear bumper. and i suspect this to aleviate the air being trapped. Now a generic aftermarket diffuser probably won't work for everyone. Hell, about 80% of the dual canister muffler designs out there will block optimum placement of any channel/diffuser you can use since you want to smooth the air immediatly following the rear axle, and the only clear exhaust choice is either with a custom HKS single exhaust, or the Greddy Evo2.

now, what some people have come up with is yes, ffill that void, under neath the heat shield with a block of something to force the air flow to go under the bumper flap- you may do with with your idea of a heat-treated foam. i was going to try to jsut cut out a piece of cardboard cut to fit, then lay fiberglass over it to make it rigid. either way should suffice, but i havent thought clearly enough on how to mount this. you'll need some sort of Xbrace underneath there so this doesnt come off..

the easiest way i think would to be cutting holes into the stock rear bumper, and that's probably what i'll do inconjunction with sometype of flat cover piece.

if you're worried about lift, then i suggest a wing of some type... cuz you know, what better place to picnic, that ontop of your car trunk?

uplz4588

i have the HKS single... so your saying that this doesnt effect me?

kosmic

for anybody looking to purchase an exhaust in the future, APS's apparently retains the stock underbody air diffuser. Doesn't help much for those who already purchased something else, though.

gpslater

Chebosto, thanks for your thoughts on this. Yeah, some horizontal slits in the bumper might just be the most exquisitely simple solution. Think I'll pass on the wing suggestion though; a picnic table or blanket will do just fine, thank you. lol

Tony@Performance

iTrader: (16)

Any single exit exhaust will run into this issue. however i really don't think of it as too much of an issue myself, it's really hyped up beyond what it is.

Also, even witht he stock exhaust, and diffuser, air will still get trapped in the rear bumper, there is no avoiding it. And with any sort of aftermaket exhaust, you will still get air trapped under the bumper. the only way to get rid of it is to build a diffuser panel under the car which in my opinion is too much of a wasted effort at correcting something that will not affect us too much.

I am looking to do a underbody diffuser in CF just for looks, for show etc... and i highly doubt that it will benefit me at all in making the car any faster or smoother etc....

speedform

It is an issue. I work as a designer in a major OEM car company (shall remain nameless) and that is a topic we discuss at great length towards the end of the vehicle design process. It's called a 'dirty floor'. Nissan went to great lengths on the Z to clean up the aero down there adding baffles and deflectors all in the effort to lower drag, which intern, improves gas mileage and top speed.
Exhaust systems like the Greddy (which I just installed yesterday) and the Injen leave a massive void where the stock muffler used to be. That void acts as an air brake at speed, causing a great deal of drag.
I track my car a lot and I'm betting (unproven of course) that a stock Z would be faster down a long front straight then my modified Z, just because of the increased drag caused by that void. Just my 2$.

Tony@Performance

iTrader: (16)

Our racing team never did anything on their cars, and they didn't even run mufflers in the back of the car. I don't think any of the race teams even touched that part of the car. so obviously it's not THAT big of an issue. of course air gets trapped there, but honestly it isn't much of an issue until you get into the much higher speeds that 99% of us won't be even getting close to.
Again, another topic that is really exagerated

uplz4588

even if we do decide to take the car up to 130+ MPH would it make the car shaky and unstable? or will it just act as a brake and make the car harder to accelerate w/?

gpslater

Look guys, all I did was pose a reasonable and IMO logical question. Geez, I'm glad you naysayers weren't in charge of approving the Wright bros. little project! If you don't think it's a worthy consideration, fine. I'm only looking for a constructive discussion. It is an issue. And I'll bet if someone comes up with a product that mitigates the "empty space" problem, a lot of folks are gonna want to buy one -- whether the problem is a significant performance issue or not.

Peace.

FLZ_Boy

Hmmmmm doesn't the Nismo Exhaust fill up the same spacing as the stock exhaust? (It looks like they are about the same dimensions)

speedform

Tony,
I don't believe the GandAm cup Z's are allowed to run an underbody diffuser even if they wanted to. Neither can anyone else unless it's sold as a factory option.

speedform

Yes it does but does not come with the aero extension that's attached to the stock muffler. Still, I think any east/west configured exhaust (Stillen, HKS, Nismo) would be better for underbody aero then a north/south configuration.
You know, this would be a great opportunity for a venturi- style aero piece to be bolted in down there.
Hmmm....I may have a talk with our shop guys.

Tony@Performance

iTrader: (16)

Very true about the race car rules, however, if it was such an issue with aerodynamics, then don't you think it would be prudent of the teams to stick with some sort of exhaust that fills that gap etc etc?
hp will not cure aerodynamics, and it that was such an issue the teams would probably have addressed it since that can (if it IS a huge issue) play havok with a race car.

It was a valid and construtctive point to bring up, but it's not practical for anyone to market anything that will "cure" this supposed problem.
The reason being: there are too many exhaust setups out there on the market, that mount differently, so how can you produce one cure all part for the car, if it won't fit everything? Heck, it could only really fit only one exhaust setup, so something like this would have to be a custom project all the way.