My quest for better fuel milage
 

Hey all-

Firstly, I've heard the "if you want MPG why not get a -----" many times. I'd rather drive the 7.

I'll start off by giving a short sum up on my car-

1988 Base, got for free in Nov 2009 with a blown engine. Obtained an S5 block for $100, rebuilt and swapped it in, July 2010. By December 2010, this car became my daily, and I've continued to fix it up in anyway possible.

Like everyone on this forum, the car didn't get too great of milage. My first measurement was 17 mpg, from fill up to fill up. I had to get better than THAT!

I started reading up on what people did for better milage... make sure I'm in closed loop, fully inflated tires, etc etc.

So, here's where I'll start:

I made a closed-loop indicator light. I figured I'd be keeping the car for ever, so I placed it in the gauge cluster, here:

http://a1.sphotos.ak.fbcdn.net/hphot..._1694719_n.jpg

[IMG]<object style="height: 390px; width: 640px"><param name="movie" value="http://www.youtube.com/v/eXQdM1YlKl4?version=3"><param name="allowFullScreen" value="true"><param name="allowScriptAccess" value="always"><embed src="http://www.youtube.com/v/eXQdM1YlKl4?version=3" type="application/x-shockwave-flash" allowfullscreen="true" allowScriptAccess="always" width="640" height="390"></object>[/IMG]

I found that a few things were keeping me from hitting closed loop. First, My O2 sensor wasn't reading according to the blinky light troubleshoot test. Installed a Bosch singlewire and got rid of the code.

Also, my radiator temp switch was missing, and I'd read that an absence of this switch (and permanent open circuit) would cause it to not go into closed loop. I live in FL, so it never get's super cold here, so I just soldered the wires together making the circuit closed.

Between these 2 things, I finally got her to get into closed loop. Here's what I've noticed (and what I'd read about factory ECU closed loop). It won't get into it unless it's >1700 rpm for about ~3 minutes. It also comes out around 3500, when the secondaries come on. My biggest disappointment however, is that if you clutch in and let the engine fall to an idle, you'll break off closed loop operation, and be stuck waiting another ~3 minutes before getting back into it.

My daily commute is from my town to another town 30 miles out, almost purely interstate. However, there's a bridge (Courtney Camble Causeway) that goes across the Tampa Bay at the 1/3 mark of the drive. I almost ALWAYS hit a light red in this stretch. Sometimes I'll coast to the light, push the clutch in & hit neutral, but hold the throttle while left foot braking. I basically bump the idle to ~1300, and am able to come to a rest, but accelerate away from the stop light still in closed loop.

I'm going to build an idle speed raiser here soon. I feel this will benefit me in that I can coast more on the highway, and get more "free" mpg's where there's no load on the engine. It will be controlled inside the car, but made in a way that it blends in (no gawdy cable dangling around the interior, or such.) I could always raise my idle speed by the idle speed screw, but this is really half assed, and I'd like to be able to return to "normal" idle for if I'm in city, or stop & go type driving. Stay tuned on this-

Some other mods I've done-

Sourced out all the "factory lightweight" parts- Aluminum hood w/ proper hinges, aluminum spare (had steel before), acrylic sunroof vs metal.

Most of an S4 aero package:
http://a4.sphotos.ak.fbcdn.net/hphot..._8207596_n.jpg
Aluminum belly pan
http://a1.sphotos.ak.fbcdn.net/hphot...3_361445_n.jpg
Sport Wing
http://a8.sphotos.ak.fbcdn.net/hphot...6_282308_n.jpg
Rear sport Aero
http://a5.sphotos.ak.fbcdn.net/hphot..._8181604_n.jpg
S4 Aero Mirrors
[IMG]http://a7.sphotos.ak.fbcdn.net/hphotos-ak-snc7/s720x720/313009_10150377684797049_676422048_9831107_1657204 20_n.jpg
[/IMG]
Car is lowered, and has a Volvo 940 lip. I'd like to windtunnel test with & without the lip to see if it is better or worse than the Sport front aero (which is near impossible to track down!!)
http://a3.sphotos.ak.fbcdn.net/hphot..._6450810_n.jpg
Made these panels which go over the frame rails/unibody to make the car more aero underneith. I bought them at Home Depot, it's a 4' x 8' sheet of PVC sheet. $19. Once I get an reoccuring exhaust leak fixed, I'll be covering the exhaust/drivetrain. This brings up a few issues, as I don't want the heat staying in there, I'll want easy access to the transmission if I drain it, and I don't want the car to be too loose and easy to go out of control at high speeds.

Another thing I did was remove a diode (D4 in the circuit) from the headlight switch. This lets me drop the lights down & shine through the FTPs for less frontal area on the interstate. My whole route is heavily lit up, and I drive later in the evening when traffic is very light.

With all this said, my mpg's per tank have lingered in the 20-21.5 area. My first fill up with the PVC panels on raised the # to 23.2. I think I can get into the 25's with a full underbody pan, and the closed loop idle bump-up.

That's all I've got for now, I know this thread's about as counter-RX-7 as you can get, but that's OK. I've had fun doing it, and I can drive my car daily without too big a hit on my wallet for fuel.

Thanks for reading!

Landon

remove passenger mirror. lowering the car helps a LOT. put a front lip on helps as well.

Awesome, thanks-

I've been contemplating removing the mirrors out right and putting in replacements inside, similar to this

http://ecomodder.com/imgs/aerocivic/Mirror.jpg

However I don't know what laws apply here, and I don't want to tear into the door panels. I'd probably mount them by the exterior mirror holes.

slight edit but Fixed!..lol..should get you to 25mpg though!

SAFC. then lean it out were you can.

sorry to sound like a noob, but what exactly does this closed loop mean? and how does it affect your mpgs if thats what youre getting at? or maybe im just missing something... haha

Closed loop is when the ECU takes the input from the O2 sensor, and adjusts the air/fuel mixture accordingly. It's a type of control system using active feedback. When your car doesn't go into it, you loose about 2-3 mpg on the highway.

The SAFC is a splendid idea. I've considering doing the RTEK for the NA w/ a wideband for safe tuning, but just haven't pulled the trigger yet.

i put an SAFC on my last S4 (contemplating it on the new S5). the problem is that the stock S4 isn't rich to start with, so you're looking at maybe -2% here and there. if you do the math, 2% is like .5mpg.

i had a WB on my S4, and basically below 3500, the richest it goes is 14.4, even @wot. i DID however have some good results playing with the timing it, liked about 4 degrees advance from stock.

i noticed the latencies on my car too, but it was more like 3 seconds, and not 30 seconds.

i was looking for power more than MPG, but more MPG is better!

If you had a turbo charged car, you can map all the vacuum and the void and run different maps. I know a guy with a tuned FD and he can get around 30mpg i think.

mine runs lambda at idle and at cruise and the beauty of that is that my lambda happens to also be 15.5 : 1

also only needs to be slightly rich of stoic under high load , unlike petrol

LPG turbo FC, impco 225 FB and closed loop commander
17 L per 100 km highway
24 L per 100 km city
15 psi daily

in australia 100-110 RON LPG = 1/2 cost of the low octane 92 RON pump petrol

makes a turbo FC as cheap to own as your average modern multicam 2L

PS

has the OP tried using the 3.9 auto diff yet?

Before I added fatter and larger rims I was getting closer to 30mpg but never quite hit it. I was confident that if I was to put on some vert rims 30 would be doable for hwy if you stayed below 60 mph on the hwy.

If I had a newer haltech Im sure the sequential injection would help a ton as well. But 25mpg with the haltech is no problem with my current config. I would expect you could get an NA to do it no problem.

Of course all bets are off as soon as you get into the throttle ;)

Landon why didnt i know that you had the headlight functioning in the down position? That would work wonderfully for me with the HID kit!!!!!!!!! I insist that you come over once i get the engine back in the car and help me do the same :)

Ive also been contemplating a full undertray for the 7 as well, but i was planning to use sheet aluminum from the racecar bodies and all as i figured it would be the thinest/lightest and most durable material to use but of course i havent gotten there yet.

I've gotten a factory 1988 GTU to 23 MPG. Driving was mixed, I'd say 80% interstate.

It's interesting to see what the S4 GTU/Sport aero package does, but I think the closed loop and fuel mapping will make the biggest difference.


My 1986 GXL gets 17 MPG, and that's with no O2 sensor, so take that as a baseline as no open loop.
It's lowered and has Vert wheels.

In my 88 n/a I went from 18 mpg to 22 by just installing an MSD. Smoother idle too and lower emissions too.

What's this? I searched and couldn't find anything but two posts, one was this one the other a haltech thread

You can do the same thing with a narrow band AFR gauge.

In closed loop you ems looks to the narrowband oxygen sensor to calculate fuel. It attempts to bounce it from slightly lean 15 afr to a 13 afr or right arround 14.7 :1 which is the air fuel ratio gasolene burns best at.

In open loop the ems looks to pre determined "programed" maps that say at this rpm/ at this mas air/ ect ect use this millisecond for open on the fuel injector.

not the best explanation or spellchecked but its what you get.

Download the haltech manual or some other ems system if you want a real explination.

Concerning the mirrors, If you don't mind the look, I would just go with a universal (or FD) APR GT3 mirrors. They're available at evasivemotorsport.com for a fair price. I am considering doing the same. And I like the idea of the underbody tray, but would aluminum sheets fair better or is PVC probably the best way to go?

Aluminum will weather better and shouldn't get brittle or crack like PVC might. I don't have a lot of experience with it though.

My closed loop indicator is REALLY simple. It's one of the LED's you use for the trouble code tester, plugged into the middle to pins of the trouble code output plug, near the battery (green connector, 6 pin)

The bulb is already wired for 12 V (has a resistor inside) and can be purchased at radio shack. I believe it's only $3. The part # is 276-011. Here it is: http://www.radioshack.com/product/in...ductId=2062548

http://fc3spro.com/TECH/HOWTO/ERRORC...KI/connect.jpg

This is from memory, but I believe the red wire goes into ABR, and the yellow wire into GL. I just lengthened the wires and brought them inside the cabin, through the unused spot on the speedometer cable (there's a small flat circle on the rubber plug where they fit through once drilling through.)

The reason I went with PVC over aluminum on the sides is because of cost. $19 for a 4'x8' sheet was very reasonable, I was also able to roll it up like a poster & get it into my interior without damaging anything (I've fully restored the interior, so I'm really picky about it). I don't see a problem with it so long as it doesn't drag on speed bumps/etc. I'll use aluminum sheet over the exhaust & drive train as it won't distort/melt from heat.

I actually did consider an 88 vert rear end (I've always heard they're a little taller) But opt to keep the stock 4.10? gears just so I can have *some* acceleration out of the NA car, haha.

Some good ideas in here though! I'm currently in the process of making mirror block-off plates, as well as internal mirrors. Found a proof-video of a metro that did 35. Here is is

[IMG]<object style="height: 390px; width: 640px"><param name="movie" value="http://www.youtube.com/v/Qa3Fqp1W2QA?version=3"><param name="allowFullScreen" value="true"><param name="allowScriptAccess" value="always"><embed src="http://www.youtube.com/v/Qa3Fqp1W2QA?version=3" type="application/x-shockwave-flash" allowfullscreen="true" allowScriptAccess="always" width="640" height="390"></object>[/IMG]

About a 2-3% increase. I'll have some pictures of this soon. I'm also doing the idle speed increase mechanism I'd talked about earlier. I'm using a generic choke cable from the parts store. I'll have pictures of this as well, and also an explanation of what I did.

Thanks for all the replies guys! I honestly thought I was going to get a bunch of flames, but this thread has just been awesome. So thanks!

Landon

+1 weight reduction. Get rid of those stock seats unless you have Infini seats. :nod:

How do you know when it's in closed or open loop? We could hook up the lights, but not know what they mean.

So the light has several modes- When rich (at idle w/ AC off, wide open throttle, or cruising above 3800 with secondaries running) the light will be solid, indicating rich.

During deceleration and idle with A/C on (because the BAC is opening for a leaner idle to compensate for the compressor load) the light will be off, indicating lean.

When cruising, but not in closed loop, the light flashes, but the time the light is on is about 3x longer than the time it's off. Almost an onnnnnn-off-onnnnn-off pattern.

When cruising in closed loop, the light flashes, but this time the time on = time off. on-off-on-off. It's this that you're looking for to make sure closed loop is working

How about a vacuum gauge? That will really tell you the best and worst "areas" to drive in.