Say hello to the Mighty Mira!

[SVOboy]

The MAF should operate outside of any other crap. I think it really exists only to replace the IAT.

It basically just measure the amount of air entering the engine by weight. So I could see how the ECU would sense idle/wot as seperate things and for the rest use MAF and RPM to calculate fuel trim. But mehbe that's all BS.

[Steve Peters]

Hi all, glad to have found this group!
Being short on cash this Xmas, I chose to drive my 1992 Mira L200 from Alice Springs to Adelaide, (Usually around 1600 km).
Temperatures were (low for here) but around 37-39 degrees. So I pulled out the air-con radiator before leaving (otherwise the car would have overheated). I split the trip over 2 mornings, leaving at 4 am daily. I left with 2 full jerrycans to avoid the exhorbitant fuel costs mid-way.
I travelled while outside temperatures were between 23-34 degrees, spraying myself with water from a bottle to keep cool, keeping windows closed but interior fan on. I was pretty comfortable, other than a paralysed foot on the accelerator to keep a steady 110 Kph.
Well, I can say that the average consumption was 5.1 l/100. The only time it was lower was between 3:30 and 5:30 am, when it was dark and I drove at 70-80 to avoid kangaroos: then I did a refill and had managed 4.6l/100.

Now the trip is over, I have already put larger tyres on my Mira:
replacing the 145/70R12s with 155/80R12s gives an 8.5% increase in radius.

I bought second-hand tryres, testing them in case the tyres scrubbed or the vehicle mis-handled. Fine all round; even better feel to the car. (I will possibly have to add larger shocks to cope with the extra 1 kg/wheel unsprung weight).
The change to gearing is dramatic; I no longer shift to 5th gear in city traffic at 60Kph as was intended for a shopping-car Mira(?). I can now do as low as 70 kph in 5th, as long as there's no hint at an uphill drive.

I'm now preparing my trip back. I've fitted a Uniden GPS301 (picked up for $100) just so I get a correct speedo reading. I've got a cruise control ready to squeeze under the bonnet for the next 1600 kms of straight road. (There is a real risk of deep vein thrombosis when you cannot move your feet around for 10 hours at at stretch.
I am watching these posts intently for advice on sensible modifications.

kind regards,

Steve Peters

[Mighty Mira]

Well, I've found the time to post the photos of all the modifications. The next post will have a discussion. Here goes:

[Mighty Mira]

Ok, so in total the current modifications are as follows:
grille blocking (with approx 200mm * 40mm hole for radiator)
- everyone wonders about that. The fan goes on sometimes when traffic is stopped on really hot days. It never overheats. The thing to realize is that the resistance of a typical radiator is very high. Imagine a cluster of tiny straws, for example. Note that they will not flow as well as a single pipe with the same cross sectional area as the sum of all those little straws. The same applies here. Although if I really had a problem, I'd make sure to completely duct the little airhole so that all air had to pass through the radiator.

Air holes on the front of car are primarily for the benefit of the lay public, who appreciate the stylistic aspects of lots of air holes.

front air deflector
The first picture shows this. Basically just a rectangle of aluminium (0.6mm from memory) bent around, so that from the front it sits just in front of the wheel. Took less than an hour (but more than that to test, because I originally made them too long and they scraped).

partial boattail
-it's about as long as you can go and still pass Australian regulations for rear license plate visibility, which must have 45 degrees to the vertical unobstructed.
-I used some sort of aluminium from the local hardware store for the frame, some bolts to bolt it together, and pvc film (0.3mm from memory) taped around with industrial strength clear tape. It flaps a bit in the breeze. You can't secure via drilling for this thickness, you must use tape. But it works. The first time I used thicker stuff, and thought it looked marginally uglier (also aerodynamically uglier), because I used 3 sheets, one for top and two for sides. The heavier 0.9mm stuff I used the first time hung down because it was heavy, meaning I had too sharp an angle between the top of the car and the boattail.

In total, took the better part of a day.

full 0.6mm aluminium undertray
-this took about 4 hours work, and someone helping for about 2 hours of it. Needed jack stands, and rested all four wheels on the corner of a wooden pallet.
Rattles a bit like an airplane at idle (sometimes), next time I take it off I'm putting flame proof insulation to damp the vibrations down. It stretches from the front to the back, just cut out where the suspension is. The reason I used aluminium and not corflute is because my exhaust system zigzags all under the car and it was a false economy to cut it out and patch it with aluminium over the heat sensitive end - far, far, far too much work. As it is, it's two sheets screwed together. I've screwed it into the chassis in a couple places as well as under both sides. I've duct taped the two sides together (for smoothing reasons, not structural of course). If it leaks oil, I might remove some of the duct tape.

rear fender skirts
-These are a single piece of aluminium, screwed using small galvanized sheet metal screws. Note they attach to the mudguard at the rear, flare out quickly and take a slow angle back (much like a typical airfoil). No bracing other than the natural stiffness of the aluminium.

rear deflectors
-self explanatory. Held on by two screws.

removed left side external mirror
-In Australia, if you have an internal mirror you only need a right external mirror, automobile manufacturers stick the other one on for symmetry. So I removed it and fashioned something in it's place with some rubber, some aluminium and some bolts.

faux racing rims
These are the cheapest 12 inch wheel covers I could find. The same pvc film is screwed all around, then heat-gunned a bit as an attempt to shrink it. In hindsight, should have used the tape I used for the boattail. May do if I can be bothered.

In future, I plan to cover up the gaps in the front under the bonnet with expanding foam, and do the front wheel skirts. Front wheel skirts I plan to make with aluminium and fish pond rubber (very similar to inner tube material, but flat). Basically the aluminium strip will extend virtually to the front of the bumper, flare out a little, and also around the back. Probably about 100mm long in total, if that. Hardly enroaching into the wheel well.

I figure if I make the flexible material as big as possible (i.e. roughly the shape and size of the wheel well) then it will bend easily and not rub excessively or ride up.

[Mighty Mira]

Ok, so how does it go? I've noticed vastly improved ability to coast. Take a relatively shallow hill, and I can go in neutral down it and maintain a speed of about 80kph. Steeper hills get closer to 90kph.

And that's pretty good, for a fairly light boxy car. The wake in the rain is also vastly reduced.

The pvc boattail flaps a bit at speed, but I can't hear it. My only concern is as to whether the aerodynamics are messed up at all. I might restretch it a bit more and retape it, but I can't really be bothered.

When I'm travelling at 100kph, my subjective feel is that the engine is only working to do the excessive 4000rpm (or whatever it is) required of it, the load of actually working against the wind is fairly minimal. I can of course put larger tyres on it as the desert mira has done. I think that would be a good option.

The other option is pulse and glide. It's funny, I don't think I was all there when I bought the car. The idea was that with a tiny EFI motor of 660cc, roughly half the size of the civic, that it should be more efficient at partial loads such as when I modified it. I didn't realize it didn't bump start too well though - in the manual it warns not to, something to do with the catalytic converter. It seems to take some effort to bump start, and I don't like using up the vacuum in braking without thinking, so that I get the heart stopping no brakes feeling with another car looming up ahead of me.

The other idea was that it was very light (600 something kg) and also had a small cross sectional area, so that was a good starting point.

Anyway, pulse and glide. Now I realize that having a small engine WILL be an advantage, and I might not even need to get bigger wheels. A car in idle should expend a roughly proportional amount of fuel with respect to engine size, so a small engine should get very good results from a neutral, engine-on glide. It might also be an idea to adjust the idle to the minimal value such that it won't conk out. So, if I'm pulsing and gliding, the engine will be in a relatively efficient range when working, and when not working, the bigger tyres won't be helping anyway.

Anyway, couple a car that coasts very well, an engine that doesn't use much fuel in idle at all, and since I haven't been pulsing and gliding, I think I've got some better fuel figures to post. Especially considering that an engine forced to overcome friction at 4000rpm should be much more efficient at 700rpm or so.

All in all, it has been an interesting experiment. With a bit of paint, the car would look a little less science projectish. I'll probably get that done after I make the front skirts. And it is a good learning experience on a very cheap car, before I get something like a prius and give it the full aero treatment.

I'd like to thank basjoos for providing a bunch of inspiration, and a commute that made this all necessary. Thanks also those who have been logging bike miles such as svoboy, I've probably clocked up about 10km trips to local stores so far that would have been done in the car previously.

One thing I would warn, when working with pvc film, start in the middle, pull to the other side, move a bit down, pulling as you go. In this way you don't tend to get ripples. The best way to get ripples is to secure the corners first and then do the middle - it will either be too loose or have ripples that way.

[Mighty Mira]

One other thing I had planned was to do subtractive testing rather than additive testing. i.e. Do everything I know should work, then do A-B-A testing by removing the modification, leaving all other modifications on.

Doing this I'd expect to see more of a difference, reducing the error. That's because you are comparing Cd to (Cd + x). The smaller the Cd value is, the larger [(Cd + x)/Cd - 1] should be.

[basjoos]

If aussie regulations regarding licence plate visibility is your only concern, then why don't you build a boattail onto your hatch that would cover the entire back of the car and would flip up with the hatch when you open it. Just mount your license and rear lights into the back of the boattail. Design it right and you could also considerably increase your interior storage space.

[Mighty Mira]

Originally Posted by basjoos

If aussie regulations regarding licence plate visibility is your only concern, then why don't you build a boattail onto your hatch that would cover the entire back of the car and would flip up with the hatch when you open it. Just mount your license and rear lights into the back of the boattail. Design it right and you could also considerably increase your interior storage space.

That's what I would do if license plate visibility were my only concerns. Remember, I live in the city and my commute is at least half city (it's quite impressive that I'm achieving better than Australia's version of EPA rating for highway as well as city, for city I'm bettering it by about 40%.).

Other concerns are
-ability to park it easily in the city - one advantage of a kei car I've come to really appreciate.
-time/money calculation - the investment could easily be a week's worth of work.
-worries about lift generated by such a boattail. It's a light car, and the torque generated might be significant.
-crosswinds removing a lot of the benefit. As it is now, there is minimal area on the sides for a crosswind to affect
-I want to have things, once painted, eventually look, if not factory, at least competent aftermarket manufacturer-like.

My car is less an experiment in what can be achieved in fuel economy with no expense spared, than a synthesis of the practical and low hanging fruit in terms of payback periods and consumer acceptance. It is also a way to learn how to do this sort of things, hands on, in a car that is virtually a throwaway item. Considering the car cost about $4000k AUD, that's a year's depreciation with most other cars.

If I eventually decide on an experiment that is more "balls to the wall", I will probably be picking a car that is either already mostly optimized for drag such as the insight or the prius, having the bonus of safe, easy and zero fuel used in EOC. That way I can just do grille blocking, mirror removal, undertray, and front skirts. The only problem is that these cars already have the very tip of the rear facing horizontal, so it's not easy to extend via a boattail. In fact, to make it look professional would be a multiple of the price of any fuel saved.

Or: I will be picking a ute/convertible/something etc, that has the combination of overall low height, that height very close to the front and the back exceptionally low, in order to minimize total length. Think ford capri.

So thinking on this a bit more, I'd like to go further than I have in order to approach ultra low fuel consumption. But...
1. It must be similar in practicality to the parent vehicle.
2. It should conceivably be an investment with a fairly low payback period, 3 years at most.
3. Aesthetically, it should look somewhat professional.

One thing that must be investigated is if similar results to a boattail can be had by increasing the rear angle (shortening the boattail) and tripping the boundary layer into turbulence. If so, that might enlarge the range of vehicles that could be modified.

Practically, that means two things.

1. The vehicle must be modified to have as small a drag figure as possible.

2. The engine must have miniscule idle consumption at worst, or so-called mild hybrid capability for everything except stop start driving. Of course, regenerative braking is great with city driving, but a lot of that can be minimized with enough attention on stop lights.

The choice, as I see it, comes down to:
1. Accept the rear end of an insight or prius as given, seeing as modifying the rear as I would like is in the too hard basket, and just modify the things I can.
2. Go nuts with a capri, using a mild P&G with the relatively small engine it has.
3. Enlarge the circle of cars I'd be willing to look at a bit, and nut out all the hairy details of an aftermarket mild-hybrid conversion. Frankly, that looks difficult and expensive to get it to even the stage where it switches on and off and doesn't harm the engine when the throttle is below a certain point.

I suppose if I can start a trend or make it fashionable to have all the things that are currently considered stylistically ugly or unecessary (such as fender skirts, grille blocking, LHS mirror missing, etc), then maybe the auto makers will make something more extreme in terms of what the rear end suction is doing than the prius or insight.

Or maybe someone like JanGeo could help me out with option 3. Frankly, I need to measure my own car's idle fuel burn and evaluate the maths of P&G before I even seriously consider things.