Aero mods to '06 PT Cruiser
 

I needed a larger vehicle for my expanding family (son #2 arrived 5/26), so after many a test drive my wife and I decided on a PT Cruiser. My two least favorite characteristics when we rented one for a family vacation were the mid-range pickup and the less than stellar highway mileage. I held out for a 5-speed manual, which has helped on both fronts.

However, given that the PT has ~0.38 Cd and ~24 sq. ft. of frontal area, I suspected that there were improvements that could be gained on highway mileage without making dramatic styling changes or spending a fortune. During my commutes to work at my previous job, I drove 33 miles across town, mostly interstate, at 60-65 mph. With just me in the car and the rear seats removed for weight reasons, I could get at best 28 mpg. Aero pig!

My job and commute have changed (I now work a leisurely 5 miles from home. Yay! I even rode my bike one day last week: great for mileage!). However, we recently went on a 1500 mile round trip family vacation to Myrtle Beach, SC, and I thought that this would be a great time to try to bump up the highway mileage. Regrettably, my only means to date of attaining accurate (?) mileage numbers has been the empty-the-tank-and-refill-it method. Not bad for long trips, but worthless when one is trying to assess individual improvements. So, I have no idea what the contribution of the following elements was to the sum improvement. There was an improvement thought. With a car full of two adults, two kids, and a car squished full of vacation gear (I knew a roof carrier would kill me), I saw an average of 30 mpg during our highway stints. This was including stretches in WV where I had the pedal to the floor (still in 5th) to keep up on several of the climbs. I also traveled at 70-80 mph, which should induce far more drag than my typical commuting speed. All things considered, I was pleased with the average. I recently convinced my employer that we needed Nology?s Desktop Dyno to obtain reliable before / after mileage numbers for our high performance exhaust systems. Once I test several vehicles to gain some confidence in its accuracy, I plan to use it on the PT with just myself in the car, traveling at 60-65 mph, to see what kind of mileage I see.

I?ve attached pictures of the following elements, if anyone has interest.

I bought some 2.5? ID flexible tube from McMaster Carr and routed this from the brake cooling duct to the air cleaner intake. I had previously installed a drop-in K&N air filter (and saw about 1.5 mpg highway increase at the time). I was not impressed with how the air intake to the air cleaner necked down and sort of pointed at the fender liner. This way I know that the smallest ID in the intake system is the throttle, and that I am getting air reasonably unaffected by underhood heating. Given how crowded it is under the hood, snaking this thing around was a bit of a challenge, and I used a longer run than I wanted, but it worked. I had to remove a plastic deflector behind the hole that sort of diverted air towards the relative vicinity of the driver side rotor. Given that I do not make repeated high speed stops in this vehicle, I saw the weak brake cooling attempt as pretty worthless.

I removed the factory front airdam. It was small, but frontal area is frontal area! It also ended just after the inside edge of the front tires, which I felt was poor. Given the short front overhang (looks cool!), the oncoming air is not managed around the front wheels at all. Due to some SAE studies I have read, and after studying wheel air deflectors on a range of cars, I felt like I could do better. So, I cut out some deflectors from cardboard, refined them, and cut them out of black ABS. They extend the side fender line to direct air around the wheel, and extend under the car to keep the air out of the wheelwell / front suspension. I even tried to mimic the curve of the fender in the outside corner of the deflector.

Much to my wife?s embarrassment, I am not too proud (or wealthy) to dumpster-dive. I probably need a ?I brake for cool cast-offs? bumper sticker. In any case, I found an abandoned, slightly cracked lower engine cover by the side of a road a couple of years ago. I suspect that it was from an Audi A4. I snuck it into my trunk, and then garage, where it has rested since. I found that if I cut the ?ears? of it off, where it would have merged into the wheel wells, it would fit wonderfully under the PT. It even had pseudo-NACA ducts to direct air toward the power steering cooler that mounts to the bottom of the fire wall. I put this rascal on with four stainless bolts and it was one of the easiest mods I have ever done. Its sure nice in the rare instance that a mod does not get more involved as it goes!

Lastly, in an attempt to correct the beastly rear-end lift of this thing, and in hopes of drawing some air into the huge low-pressure region behind the blunt rump of this thing, I made a simple flat spoiler. ?07 PTs have something similar, though perhaps more subtle. I made a cardboard pattern first, that protrudes 20-25? above the body (taller in the center). I then transferred this to some 22 gauge stainless sheet that I got from work (I work for a really cool place). I made it so that it just drops in the slot where the top of the rear hatch mates to the body, and the gasket there holds it in place. After getting it situated, I found that it did not try to work itself out during the trip (though it needs to be repositioned when I open and close the hatch).

There it is. From here, I plan to add some underbody cladding, particularly after the rear axle, as the nooks and crannies around the spare tire and rear bumper scream ?parachute? to me. I also hope to fabricate some airfoil shapes to enclose the flat parts of the rear axle that are just parked out in the exiting air stream. Speaking of underbody cladding, how does Coroplast hold up to rain and nasty winters? Do you folks pop these panels off during the winter, or use them for templates and make aluminum panels or something? In any case, I find this stuff fascinating and hope that there is something in here that is useful for someone else.

Bman

You need to work on the tranny lube, engine lube and some fuel treatments also. I hope you are not taking air into the engine from the brake area - that dust will wreck havoc with the engine - the intake from the front fender area is generally used because air around there is calm and cleaner than else where and generally cooler or at least at a stable temperature. The air dam in front keeps the air from getting usder the car and hitting all the spots they forget to make smooth to air flow - really sucks when the axle sticks out in back like a plow! Also pump up your tires with more pressure. I have a Scion xB so don't talk about aero until you see what a box does but I get in the low 40's average with trips in the high 40's low 50's sometimes except like last night when I got all but 2 of 13 lights RED in 19 miles of driving so I only got 46mpg.

I'm very impressed with the mods you've accomplished.

Have you put any thought in making rear wheel skirts?

is the rear spoiler for efficiency or downforce? it looks like a nascar style spoiler....

Wings add drag - vortex generators may be the way to go - I'm trying to figure out a way to mold VGs into a rear foil so that it mounts on the rear roof area like the the spoiler that an xB can get as an option. I think a bunch of telltails on the back end of the PTC would give you a better idea of what is going on before you start changing air flow - the back end is pretty rounded compared to my box.

Thanks for the comments and complements, folks.

JanGeo: I am running Redline Synthetic in the tranny. I'll go to synthetic motor oil when I am out of the warranty period and I can run it for more than 3k. I'm sure that you have noted from the pictures that I am not drawing intake air from near the brake, but rather have made better use of a pseudo "brake cooling duct." It sounds like you are doing quite well with your box! What "fuel treatments" are you referring to? I'm not sure that VGs would have a drastic enough effect on the rear of the PT. As I understand it, a vortex does a good job of holding flow on a surface (and creating a core of low pressure). The rear of my car is really quite blunt (no sloping rear window, etc.).

Matt: Thanks much. I'm thinking I might try the flat wheel cover idea first. I've seen some interesting wind-tuft pictures of a Gen II Eclipse with and without flat wheel covers, and it can make quite a difference with the airflow down the side of the car. I'd like to do a wind-tuft study on the roof of the PT to see what the before / after is with my spoiler.

NotJared: As mentioned, the spoiler is intended to both negate the horrible rear lift at highway speeds, as well as "trip" the air into the low pressure area. In theory at least! :-) I did find it very stable through the twisty mountains of WV, at 70-75.

Yeah I would be careful about the redline oil - sometimes certain ones are for race use and not intended for long term use in engine or tranny I think. Some Scion owners are using it but I added some old Slick50 gear treatment to the tranny original oil and mileage jumped up several MPG and then a month later changed to Synlube and could feel the difference in a city block of driving. Gas treatment as most here will tell you is 3 oz acetone to 10 gallons and about 2.5cc of Torco GP-7 two stroke synthetic oil to 10 gallons of gas. It sometimes takes a little driving to see it take effect but what I have seen is that it raises the slow light pedal driving mileage a lot but not the blasting down the highway mileage much.

The Vortex generators are supposed to return the air to the dead zone behind the vehicle a little more efficently than anything else and improve cross wind stability and reduce drag. If you look at your rear window from the side it slopes if you look at my xB it is vertical.

It all looks like great work.

There is one caution I want to pass along. One of the reasons the manufacturers take the intake from the fender area (and other places) is to avoid ingesting water from rain, puddles and snow. A few years ago, Ford tried to raise the highway FE of the Exploder by giving it a ram air intake of sorts just below the front bumper. It had the side effect of sucking in water from road puddles (not off road, mind you) in high enough volume to create an incompressible hydraulic ram in the cylinders breaking the connecting rods, etc. The destroyed engine was not covered by the warranty nor insurance. More than a few owners had to sue Ford for making a 'V8 powered wetvac.' Your mileage may vary.

JanGeo: When your talking about the synlube making a huge difference, it was when you replaced your transmission oil, with the synlube, right? Do you have any quantification of the difference, as well?

I'm a big fan of using good quality lubricants everywhere i can, But Jan, i have been doing some looking around for info on synlube and haven't come across much postive info.

https://theoildrop.server101.com/cgi/...c;f=1;t=011708

The link is from www.bobistheoilguy.com a very large site dedicated to oil and lubricants. Thousands of users if i'm not mistaken and many oil chemists and oil analysis labs hang out and trade information.