front and rear diffusers / skid plates

[sasquatch]

hey i was thinking of getting a sheet of aluminum and putting it under the car so the air going under the car would flow smother and faster... has anyone ever done this sort of thing before? i was under the car with a measuring tape and a straight edge and found out i could make it work fairly easy... just a few slight bends in a metal break and i would have them all made up... the only thing that im very worried about is the heat from the exhaust... do you guys think the heat from the exhaust will cook the floor in my car or even worse boil the gas in the tank cause im sort of taking all that air flowing over the exhaust from the first bend at the tank and sealing it off... not all of it but most of it... do you think if i heat wrap all the stuff in the engine bay and then leave the mid pipe exposed then heat wrap the stuff from the tank back then it will be ok? maybe to get some heat out from the pipe before it gets to the tank i can get some aluminum angle iron or C channel or somthing and strap it to the mid pipe to act as a heat sink... any other ideas or do you think ill be ok? btw ill be using a 2.5 inch down pipe with 3 inch the rest of the way...

[muscle_Car1]

You can thermal wrap the whole length if you wanted, it keeps the velocity of the exhaust up. The lower plates would also increase mpg due to cleaner airflow.

[littlecar]

Exhaust wrap and wet winters don't mix. The wrap gets wet, doesn't have enough time to dry out completely, combine that with exhaust heat. The result is really quick rust. Your exhaust will rot out in about a year. Ask me how I know.

Your best bet is leave a channel for the exhaust and plate up the rest.

Or just run exhaust up through the hood. :shock:

[Aaronbrook37]

For a belly pan to be effective, you don't really need it to run all the way from bumper lip to bumper lip. Once the air is on it's course underneath the car, it requires less guidance to keep it there, especially when it's travelling at upwards of 70-90MPH in track/freeway conditions.

Here's the key to flow/aerodynamics... you want to make boost. Yes, I said it... you want to make your car boosted in terms of air flow. In technical terms, you want a positive pressure difference so that not only does air flow through/under/over your car, but you want to actually have a small degree to which it is pulled through the air. This post will help you accomplish that.

The key to modifications for airflow is that you need a balance of pressure, and a balance of airflow. If you were to seal off the entire area underneath the engine, instead of just stopping shy of the exhaust downpipe and oilpan, you would create a region of medium-high pressure behind your intercooler and radiator. In other words, it would be more difficult for air to escape from behind your thermal exchange systems for both charge air and coolant. As a result, they work less efficiently... if it all.

The Front of the Car

If you think about the way that an OEM hood works, it provides little means of escape for air. If you were to clamp a hood to the bottom of the engine bay as well, there would be NO way short of leaks and a small aperture for the air to escape. Because the high pressure of the air in the engine would resist movement, it would practically eliminate the flow of air. In order to have the airflow you want (and need) through your FMIC and radiator, the engine bay's flow needs to have serious low pressure... aka a super easy way to escape. It's for this reason that bellypans in the front end need to provide an exit channel for hot air.


This turbocharged Maxima has a side/top mount underneath that hood scoop. Believe it or not, it actually doesn't get any airflow at all. The reason for this is that the region behind/below the intercooler is so highly pressurized when driving, that the incoming force of air isn't sufficient to force the movement of air through the intercooler. Basically, although your car may be traveling at 30-50mph, the intercooler effectively receives maybe a tenth of that.

It's for this reason that the question of bellypans is tricky. It's also the reason that, if done properly, hood "propping" and venting can be quite effective... in the right places. It's also the reason that you won't need to drill any big holes (ricers call these velocity holes) in your front bumper with a hole saw.


There is no point in maximizing air going through the front of the intercooler if it has no way of getting out the back. That's not to say that these can't be useful, they just have to be done consciously! It's like doing a Damkid swap with OEM exhaust... not a smart move. Your intercooler and radiator don't need backpressure, so it's smart to eliminate them before adding additional air through the front. By putting a bellypan under the entire motor, you effectively choke the escape of air, and will kill your motor by overheating it and the charge of air.


Here's a contextualized example from a source I don't remember. Sorry for the next user I'm about the offend. Most people here would say this is an inefficient design because of engine bay heatsoak. However, the problem with it isn't that... it's instead that there may not have been any attempts to decrease the pressure behind the intercooler. For example, if there's a whole bunch of stuff (ie the engine bay, the transmission, cables/hoses, etc) then the flow capabilities of that intercooler will be restricted. This is a similar issue for some TMIC Subaru's... sure... they have a huge scoop that pushes all of the air through the intercooler, but if the air can't dissipate from underneath the intercooler, it's effectively a great source of drag... and a very useless heat exchanger. This is of course saying nothing of heat soak.

This is the reason that old-school ram air intakes work so well. What's underneath the intake scoop isn't an intercooler or an engine bay, it's a friggin carby. That's about as much vacuum as you'll find in an engine bay, so it makes sense to have a scoop similar to the ones below. This is the same reason that bellhorns work for turbos.



In addition, the other great way to increase flow is to put in hood air escapes. But you need to do them in the right place, because vents at the front won't help air escape as much as they will permit air to enter, depending on how they are setup of course. At speed, you're going to want the rear of the hood (closer to driver) to be vented, so that air that spins around the outside of the intake manifold/brake booster region has a place to escape in the event that it isn't pushed down through the steering rack and rear bottom of the engine bay.

Thank you Ian for the donation... here's an example of how I plan to do it if I ever need flow assistance through the engine bay. Literally 60 second MSPaintjob. Look at the cowl area, and you'll see my sneaky OEM-style way of hood venting. All you have to do is shred a cowl, and pop it into your hood. Easier said than done, but this is the cleanest way of solving pressure problems in the engine bay, especially if you're adding belly-pans.
Again, this says nothing of the heat dissipation effects that such mods achieve.




When these vents are put in the right place, they will not only allow heat to escape, but they will actually PUSH it out with force. AKA positive pressure... consider it as up to 0.2PSI of boost, but not for your intake charge, for your airflow. Talk about hyperdrive.

Another issue are lip spoilers... they can actually cause more interference than assistance with downforce in the wrong applications. Check out the reading material at the bottom for more details and scientific testing on that.


Yes, these can be useful. However, what is there behind the lip? Restriction. If there's a biggggg lip spoiler without any means of channeling that air towards the back of the car, it creates a huge problem area of airflow directly behind that lip. Sure, it'll give you some downforce, but it will also slow you down if down wrong. This is how you get stability, downforce, AND flow simultaneously.

The Middle of the Car

If you were going to go all out with a bellypan, then the middle of the car is where the law of diminishing results would apply. Although it isn't by any means perfectly flat, the middle of the car (aka intermediate pipe region) is where you'll get the least result from your work. The most difficult regions of car aerodynamics are when the air hits the car, and when the air that's already hit the car hits the back of the car. In essence, the front fascia and the rear hatch/spoiler/bumper. It would be worthwhile to aluminumize the middle of the car, but there are two issues. One is rattling... the aluminum would have to be very securely mounted, because it could lead to serious noise through two means. The first is that the aluminum could create a noise chamber from the sound of the exhaust and road. The second is that it could itself create noises when the chassis and suspension flexes in the corners. I would avoid doing the "driveshaft tunnel" region as it's known to me. Doing the region underneath the front seats makes sense, even sealing it at the sides of the car, or bottom of the rocker panels and doors. However, you would be better off pursuing maximum effect from the front and back of the car first.

The Rear of the Car

The rear is very similar to the front, albeit with reversed flow characteristics. In other words, you want to make it as easy as possible for air to ESCAPE from the bottom of the car. One way people do this is through diffusers. The extreme way to help with air flow would be to eliminate the spare tire well, redesign the fuel tank in a sloping manner (or in the trunk) and to sort of "port and polish" the rear end of the car. Since this isn't practical:


What you're stuck doing is dealing with the OEM design of the rear end. Similar to the front of the car, one to two feet of bellypan would serve it's purpose well here. You don't want to mess with the movement of the torsion beam though, and obviously messing with the flow of the exhaust is also a terrible idea. You may notice that a lot of your race cars in Forza Motorsport (aka "Forza" rear bumper mod) will have exhausts that exit through the rear bumper. This is for the exact reason I describe: Making the rear exit of the car as slippery and friction-free as possible.



Not only do they decrease the restriction of airflow (imagine what it would be like to be air stuck in the rear bumper) but they also provide some stability the way that vortex generators claim to do on Lancer Evolutions.

[Aaronbrook37]

I actually smashed 10,000 characters LOL! Mike, consider this your birthday present from me to you. Here's the final part with links/sources.

Conclusion
With airflow and bellypans, you want to make the travel of air as easy as possible. In order to do this, you have to ensure that air can not only get in, but also get out. Maximizing flow to intercoolers and radiators cannot be reconciled with a full bellypan for the reasons stated above. In order to have the best flow of air throughout your cooling system(s) as well as your car, you should pursue various airflow aids described above, which are based on the Julian Edgar chronicles on Autospeed.

The two other good sources I recommend are the crazy mad scientists on Ecomodder, and any other hypermiling establishment. They get into the thick and thin of it by reducing wheel drag, hatch drag, body drag, glass drag, and basically everything else. This short little blurb of mine won't make your brick into a raindrop (aka perfect aerodynamics), but it will certainly help. It's very likely that you can increase your fuel economy by 10-15% if you were to perform every single means of improving airflow. An extra 5MPG is easy with a slippery well-pressure-balanced Festiva and some careful 45-55mph driving. And the amount of chloroplast/aluminum you would need to add couldn't possibly be more than five pounds.

Very Useful Links on Aerodynamics and Flow:

http://www.autospeed.com/A_2159/cms/article.html

http://www.autospeed.com/cms/A_2160/article.html

http://www.autospeed.com/cms/A_2162/article.html

http://autospeed.com/cms/A_107773/article.html

http://autospeed.com/cms/A_2455/article.html

http://autospeed.com/cms/A_2456/article.html

[sasquatch]

wow great wright-up!!!!! thanks aaron

in the rear of the car to clear the exhaust and axle it has to make a small step down about 2 or 3 inches... will that step down create a spot for the air to pust up on the car (like a wing or upside down spoiler) and create lift? i know that would be bad at high speed lol...

also.... i dont want to cut holes in my hood... i think it looks goofy on a festiva... maybe i could just cut a big hole in the plate that keeps the air from the engine bay getting in to the vents up near the windshild where the air inlet is for the cab... i never have it on anyways... lol

[bhazard]

I also recommend a visit to ecomodder. Those guys know it all.

[drddan]

A truely :confused2: eye popping write up.

Good candiate for a sticky or put into the articles!

[Aaronbrook37]

wow great wright-up!!!!! thanks aaron

in the rear of the car to clear the exhaust and axle it has to make a small step down about 2 or 3 inches... will that step down create a spot for the air to pust up on the car (like a wing or upside down spoiler) and create lift? i know that would be bad at high speed lol...

also.... i dont want to cut holes in my hood... i think it looks goofy on a festiva... maybe i could just cut a big hole in the plate that keeps the air from the engine bay getting in to the vents up near the windshild where the air inlet is for the cab... i never have it on anyways... lol

I wouldn't really worry about a downward taper causing lift... I don't think it really can. It wouldn't be that significant, especially since your car is already lowered. Make it smooth enough that stuff won't build up on the aluminum, and that it's an easy transition. Put the front-most part of the belly fan farther up on the car, and make the downward movement continuous. Use your port and polish skills compared with basic airplane logistics. Since there's no air traveling over the rear bellypan (it's the chassis there instead) it shouldn't be that much of an issue. The only real issue with shrouding the belly pan (short of jacking issues, which you've already explained to me over MSN how you plan to avoid these) is that the reduced room for air underneath the car creates a bit of a bottleneck, and will push airflow towards the fronts of the rear tires. The other problem of course would be risks of bottoming out if you were on really rough terrain. Even though the wheels will always be lower than this setup, it could get uncomfortably close if you were shrouding the entire rear suspension.

For this reason, you may wish to start behind the torsion beam instead... dunno, it's really your call.

As for cutting the upper firewall, that's not a bad idea! My only concern with that would be circular air flow within the sub-cowl, and potential heat issues getting into the glass and interior. Might cause a noise issue as well. That and if you ever wanted to cool off, you'd be enjoying the sweet smell (and taste) of eau d'engine. Here's a solution if you don't want to cut your hood for venting:



Personally, I find that tackier and lamer than the example I provided with Ian's car. I agree that hood vents are pretty aggressive on Festiva's, but they can be done tastefully, as has been demonstrated several times on this board.

[sasquatch]

well ill be thinking long and hard about it the next few days... you have given me ALOT to think about

[sc72]

air flow

Thanks for the write up,it does make sense,we did this back in the early 70's when we had the first gas shortage,people waited in line for hours and some were killed,well we used a 70 chevy pickup as our test bed and yup we covered the whole bottom of the truck from front bumper to the rear bumper and yes we did run into the over heating problems but that was solved on it's own one day when the front engine shield blew off going down the road and the temp came down like a rock,so knowing the two were connected we revised the pan to begin 6 inches up the foot box and run to the back bumper and as for the under hood heat and air pressure,how about some custom hood louvers,just a thought.
Oh ya for all the work we did and the P.B.R. we drank, we achieved about 1.5 mpg increase and we had fun doin it.

[Festiva_tunner]

also.... i dont want to cut holes in my hood... i think it looks goofy on a festiva... maybe i could just cut a big hole in the plate that keeps the air from the engine bay getting in to the vents up near the windshild where the air inlet is for the cab... i never have it on anyways... lol

That may be the best bet it would keep the stock look while providing the air escape needed. The cowl area already has a pressure zone due to the wind breaking over the windshield.

[festivaWES]

Aaron...those wheels ont hat civic are sick....

[Damkid]

^^If i had a set of wheels like that i would not hesitate to trade you for a plain set of steel wheels :puke:

[blkfordsedan]

Didn't someone take a FMS fiberglass hood and mod the front bulge? If I remember, they cut it & dropped it down to make a heat extractor like the "terminator" SVT Cobra Mustangs.