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Old 10-27-2014, 02:33 AM   #1 (permalink)
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Prius Improvements? (considering aerodynamic modifications to 2005 Prius)

Hello,

I have a 2005 Prius that I'm considering making some changes to, and wanted to get some input from folks on the site as to whether these changes are worth the effort.

I'm not a hypermiler, I drive the car for work to my customer sites that are generally about 100-150 miles from where I live. I probably average one trip per week over the year, but in the summer I could go every day during some weeks. Anyhow the speed limit on the freeway is 70 and traffic generally flows at 75-80. I typically drive between the speed and the flow of traffic 72-74. Due to the speed, aero is my biggest concern, and I'm wondering if I can do some things to improve on the Prius to make it even more efficient.

Thoughts (at random as I type this):
1. smooth under body
2. mirror delete from the outside and install mirrors on the inside of the glass.
3. wheel covers (pizza pan type)

those are the easy ones

4. rear wheel well skirts
5. rear boattail
6. front bumper cleanup and grill block

I've used the template that you have on the garage tools and the angle of the rear hatch is not fitting the template very well. Not sure how to use the template I guess. My thought was to just use the existing angle of the roof and extend it out until it met with the rising angle from under the vehicle.

Anyhow, I'd just like some feedback as to whether these changes will make much difference and are worth spending the time and money to install on this vehicle.

I was inspired by this thread.... http://ecomodder.com/forum/showthrea...d-gen-978.html

I love the look of the tail and skirts, but I think the nose should be more blunt, and would probably be easier to build as blunt. Perhaps a splitter at the same ground height as the underbody and help keep the air from going under the car like the pointed nose will do.

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Old 10-27-2014, 06:55 AM   #2 (permalink)
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Be very careful with a grill block on a Prius, remember you have lots of heat-sensitive electronics under the engine bay. It is a little different animal than a typical internal combustion engine. I would add some sort of gauge (scangauge, ultragauge, etc.) to the list, it will be more accurate than the instant readout on the dash, and will also let you monitor other important things like exact temperature. Also, your aerodynamic streamlining template overlays should line up with the peak of the template right at the peak of your roof camber, they don't really mean anything in any other position.

----Edit:
I did a quick and dirty AST-2 overlay for you on a found picture of a 2005 Prius, you can see the back half of the Prius is basically ideal. Note this isn't a true distance side photo, you can see I am trying to adjust for the two different heights of the tires at the bottom, but it gets the point across. Basically from your roof peak down to the back edge of the hatch is all but perfect.
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Old 10-27-2014, 09:02 AM   #3 (permalink)
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The Prius isn't that sensitive to heat. I run a full grill block in winter and fall/spring, and one upper grill slat open on it in summer with zero issues for years now. That being said, I'd highly recommend a grill block.
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Old 10-27-2014, 10:00 AM   #4 (permalink)
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The 4 things that make a Prius good at hypermiling are:
- Staying in the "atkinson" bit of the valve timing, you'll probably need to go slower though, but I don't know. Perhaps get a ScanGauge to find out? I'm pretty sure thats why they put an 1.8 in the later models, to get the Atkinson thing going even at higher speeds.
- Pulse&Glide, the hybrid system makes this extremely easy to do, no need to operate a clutch, turn off the engine manually, etc.. If you drive in places with hills, its much easier to do this though.
-plug-in pack, replaces fuel with electricity, and keeps the engine in atkinson mode even at higher speeds, so better bang for your fuel. the battery cost is hard to justify though, unless you live in a place with crazy gas prices, put hypermiling scores above cost/mile out of principal, or get a really good deal on your batteries. (Perhaps assemble a pack of used cells or something.)
-stock aero is already pretty good. (Theres always improvements, but still, its not bad to start with.
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Old 10-27-2014, 10:42 AM   #5 (permalink)
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Yes, all of the mods you posted will help. Only you can determine if they're worth your time.

As for costs, the challenge with improving the efficiency of an already efficient vehicle is you have to keep them as inexpensive as possible because the payback period is very long.

EG: simple coroplast boat tail:

Quote:
My personal results will no doubt be different from yours, but driving 50/50 highway/surface road I saw an 18% increase in mileage.


From: http://ecomodder.com/forum/showthrea...mpg-24999.html

I'm a little skeptical that % improvement would stand up to A-B-A testing at constant speed highway driving.
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Old 10-27-2014, 11:55 AM   #6 (permalink)
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Thanks for the responses so far.

Scangauge: I already own one to watch the temperature. I had a full grill block at one point just covering the entire lower opening and it was perfect except in the height of summer when I had to remove it due to temperatures rising. I'm thinking of a way to have a choke cable or something allow for opening it so I can choose when its closed and when its open. I never re-installed it after I removed it, but now i'm interested in doing something a bit more permanent.

Quote:
I did a quick and dirty AST-2 overlay for you on a found picture of a 2005 Prius, you can see the back half of the Prius is basically ideal.
I'm not sure why your overlay looks so much better than the one that I did. I figured the Prius angle must be pretty good, and when I did the quick and dirty it did not line up at all....strange.

pulse and glide is not really an option. I value my time on these trips since they are so far away, so driving at 72-74 is really as slow as I want to go. Heck, if they increased the legal speed limit, I'd like to go 80-85 in order to reduce my time on the road for these trips. Even at these speeds I'm getting 43mpg average according to the dash as well as the scangauge. The roads are perfectly flat here in the central valley, so no opportunity for the pulse and glide on the hills either.

Wow, 18%! I'd be ecstatic if I could get anywhere near that improvement with one change. I did not think all the changes would amount to that much. That is quite encouraging. I figured all the changes combined might achieve 15%, now it makes me think 20% is achievable. That would put me over 51mpg at 74mph. (starting at 43)
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Old 10-27-2014, 12:13 PM   #7 (permalink)
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Well, I guess its all on aero mods then.
- boat tail
- cheap pizza pan wheel covers (expensive moon caps don't really pay back)
- rear wheel skirts (another few %)
- "wheel spats" or small boattails behind the wheel. (Doesn't the prius have those already though?)
- antenna delete or replace with sharkfin
- right side mirror delete (if legal?)
- replace side mirrors with more aerodynamic ones / smaller ones, if possible. add wideview internal mirror for safety.
- get the grill block back in, and with a moveable opening. Bowden cable for control seems like an excellent idea.
(Perhaps car manifacturers will add thermostat controlled grill louvres in the future. open when its hot, closed when its not. )
- Perhaps using some rubber strip material to fill in the gap between the body and the front wheels, but in such a way that it wouldn't rub in corners and over bumps. (easier and nicer-looking than hinging front wheel skirts.
- Belly pan (if not already fitted)
- Air dam (depending on how much stuff is under the car)

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Old 10-27-2014, 05:03 PM   #8 (permalink)
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An easy way to determine your car's aeromod potential is to note what your mpg is when drafting another vehicle. This is the mpg that you could get if you thoroughly aeromodded your car to reduce its Cd.

In addition to improved mpg and the equivalent of a horsepower boost at higher speeds, two side effects of reducing your Cd is that drafting has less effect on improving your mileage and the non-fan assisted air flow through the ventilation system is diminished. Both of these require the presence of a high pressure zone at the front of the car and a low pressure zone at the rear of the car to manifest. As you lower your Cd, you reduce the pressure differential between the front and rear of the car.
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Old 10-27-2014, 05:37 PM   #9 (permalink)
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Quote:
Originally Posted by basjoos View Post
An easy way to determine your car's aeromod potential is to note what your mpg is when drafting another vehicle. This is the mpg that you could get if you thoroughly aeromodded your car to reduce its Cd.

In addition to improved mpg and the equivalent of a horsepower boost at higher speeds, two side effects of reducing your Cd is that drafting has less effect on improving your mileage and the non-fan assisted air flow through the ventilation system is diminished. Both of these require the presence of a high pressure zone at the front of the car and a low pressure zone at the rear of the car to manifest. As you lower your Cd, you reduce the pressure differential between the front and rear of the car.
I've drafted behind some trucks, but not particularly close. The problem is that I've never done it at my normal driving speeds, so I can't really tell how much if from the speed reduction. I have noticed that I get a better draft off of a flatbed truck than off of one with a big box. Particularly if the flat bed has some random items placed on it to really disturb the air.

I once drove behind two or three trucks (mini convoy) just to see what would happen, and at 60-65mph behind them I got somewhere around 70-75mpg. It was not well documented, but was an interesting trip. Felt like it doubled my time to get to the customer, but was a good experiment.

For this drafting test, do you mean finding a friend with a car and following very closely, or just following somewhat close to any vehicle, or are you talking about following trucks?
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Old 10-27-2014, 10:31 PM   #10 (permalink)
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Best would be closely following a friend in a truck.

I was on I-5 once and a 18-wheeler pulled slowly past, and pulled over right in front of me. It had a half-height full width load which seems ideal as there is less buffeting. So I stayed in his blind spot for 80 miles.

It wasn't documented, but as basjoos says, it probably about doubled my mileage. But I picked up a lot of rock chips in the paint and I was a nervous wreck, so I don't do it anymore.

Be aware that the boattail that MetroMPG posted has a bad mismatch at the trailing edge of the hatch. The whole top or at least 50% will be in turbulent flow.

Instead of rear wheel spats, maybe something like this:



Instead of a tab running across the tread, divergent fences that force the wheelwell turbulence outward where it can be sheared off by the ambient flow, instead of under the car. Adjusted for adequate departure angle of course.

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