Winter fuel economy/power

[NoDOHC]

Have any of you noticed that Stock ECU RX7s make no power when it is cold and they also get terrible mileage? I try to log my mileage on every tank and have the following data for your curiosity.

86 GXL 5-speed (0.69 OD out of 88 GXL) 12 degrees BTDC idle timing:
Temp (F) City Mileage (MPG) Hwy Mileage (MPG)
90+ 25-28 29-31
60-80 24-26 28-29
50-60 23 (Average) 25 (Average) *Largest sample size
40-50 18-21 24-26
30-40 16-18 21-23
10-30 15-18 19-22

Using a G-tech for power comparison (numbers may not be accurate horsepower, but are comparable) I found the following:

Temp (F) WHP (G-tech)
80 121
60 130
50 134
40 120
30 108
10 92

I installed a new transmission and a Haltech E6X and after tuning (during which I had some horrible mileage tanks) I acquired the following mileage chart.

86 GXL 5-speed (0.72 OD out of 86 GXL) Haltech E6X, Racing beat headers into custom exhaust, 16:1 AFR cruising, 50 degrees BTDC timing at cruising loads and speeds:
Temp (F) City Mileage (MPG) Hwy Mileage (MPG)
90+ 29-31 36 *one tank only at this temp
60-80 28-29 31-35
50-60 28 (Average) 31-34
40-50 24-27 30-31
30-40 25-27 28-31
20-30 23-26 25-27

Using a G-tech for power comparison (numbers may not be accurate horsepower, but are comparable) I found the following:

Temp (F) WHP (G-tech)
80 143
60 155
50 160
40 162
30 165

Now I have the misfortune of driving my '91 in the winter and have collected the following data.

91 Coupe, Automatic Transmission (0.72 OD), Idle timing 17 degrees BTDC:
Temp (F) City Mileage (MPG) Hwy Mileage (MPG)
90+ 20-22 25-27
60-80 19-20 25-26
50-60 19 (Average) 23 (Average)
40-50 18-20 21-24
30-40 16-17 19-20
10-30 14-15 *Not yet tested

Using a G-tech for power comparison (numbers may not be accurate horsepower, but are comparable) I found the following:

Temp (F) WHP (G-tech)
70 110
60 112
50 108
40 102
30 98
20 94

My biggest reason for the numbers is to show the trend.

Cooler temperatures hurt mileage and power for stock ECU, but hurt mileage less and helped power for Haltech running a constant 16:1 AFR (cruising) and 13.2:1 AFR under WOT (except for above 6,000 RPM, where the primaries maxed out).

Does this mean that the stock ECU has terrible cold weather maps?

Comments/Data are very welcome

[jimieatgirl]

I know I drive 30 miles one way to work, I ran her down to E, put just 20 in her. 1.99gl. about 10gals. got half a tank, drove got to work 3 times and home 2 times. I needed more gas to get home.

put 15 in to see what will happen, 1.95 gal about 7.5gal... I got both ways twice... at this rate I will have to fork over 80 every two weeks, thats if the price stays low..

WTF? I did make it home tonight on an 1/8th bu shifting 1-3-5.. and never over 4g rpm.

Jimi

[Cyborg13]

that's strange in my turboII it feels like i have more power.. just stepping on the gas i lose traction in 2nd..... that may just be because of the cold tires and ground though.. yea that's probably it.

[NoDOHC]

I would think that the turbo cars are different, they really should make more power when it is cold (more dense charge air). NA engines should too (up to a point) and that it what I find strange.

[jimieatgirl]

I don't know about my HP, but I do know about the gas..I have a 91 n/a 5 speed.

stock header racing exhaust cat at the Y. I think its the junk they are putting in the gas..

[88turboii]

seems you get better gas mileage with higher temps, could be the fact youre not wasting gas due to letting your engine warm up

out of curiosity, what does your timing map look like in vacuum?

[NoDOHC]

The car is N/A so it is all in vacuum.

I tuned it based on TPS rather than MAP (This may be hard to translate)

As the coolant seals leaked more and more I kept advancing the timing, Most of the mileage numbers above were with 50 degrees BTDC (instead of about 40 in this map) due to the water injection from the leaking seals slowing down combustion. Before the leaking coolant seals, here is the cruising timing map (based on TPS):

1500 RPM
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
0 _ 38 _ 35 _ 29 _ 26 _ 22 _ 22 _ 21 _ 21 _ 20 _ 20

2000 RPM
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
42 _ 39 _ 36 _ 32 _ 28 _ 24 _ 24 _ 24 _ 24 _ 24 _ 24

2500 RPM
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
44 _ 42 _ 39 _ 34 _ 30 _ 28 _ 26 _ 26 _ 26 _ 26 _ 26

3000 RPM
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
46 _ 44 _ 42 _ 38 _ 30 _ 28 _ 27 _ 27 _ 27 _ 27 _ 27

I was running 16:1 AFR below 90% throttle, this timing was conservative in the high Throttle region.

[vex]

I'll load up my data from the past year. Unfortunately my poor mileage is from doing short runs in town most of the time. While during the summer I get the best mileage because I commute.

[Fidelity101]

you will have worse gas millage in a the car if you don't warm it up properly before driving around, warm up that o2 sensor!

[Cyborg13]

^^ I don't think that's quite true. You would just be wasting more gas going nowhere before you even leave the driveway. The 02 sensor would heat up faster when the engines under load anyway. and at lease your getting somewhere close to your destination in the mean time.

[Fidelity101]

I don't know about you but when my car is cold it doesn't run nearly as nice as when its warmed up, I'll just wait.

[NoDOHC]

I agree about the running poorly when cold, but I think that you do use more gas while warming up (although it is probably easier on the engine).

I am curious if more of you noticed a similar trend from summer to winter and if you found a good explanation or reason for it (other than the air being more dense and causing more drag at highway speeds or the required warmup time).

I am especially interested in other NA owners power numbers with varying temperature.

[Phoenix7]

you will have worse gas millage in a the car if you don't warm it up properly before driving around, warm up that o2 sensor!
I'm with Fidelity.

Could it be that the car is overcooling? THe car reaches operating temps, fan comes on and the COLD air just drops temps too much and runs rich?

I notice the NA drive badly when cold....it seems to run perfect when fully warm but off when it's still cold.

[Fidelity101]

Overcooling hurts the most at the rotor bearings. Typically by the time you reach your destination (if its local, quick drive) you are turning it on and off before it will reach operating temperature.

that and the summer and winter gas is different, but keep in mind the lower the temperature of the intake charge the more power you make, intercoolers work on this method.

[yeti]

I get bad fuel mileage in winter, it takes my car easily 10-15minutes to warm up.. and by that time I can be at my destination. It depends on how cold it is for me, if I want to try to have the cabin warm or not.

But my heater sucks and you can't use it until the car is warm even then it doesn't blow hard. Once with it on I drove about 30 highway miles and the car still was cold.. so I turned it off the the gauge finally went to were it was supposed to be. (who knows?)

[Fidelity101]

I get bad fuel mileage in winter, it takes my car easily 10-15minutes to warm up.. and by that time I can be at my destination. It depends on how cold it is for me, if I want to try to have the cabin warm or not.

But my heater sucks and you can't use it until the car is warm even then it doesn't blow hard. Once with it on I drove about 30 highway miles and the car still was cold.. so I turned it off the the gauge finally went to were it was supposed to be. (who knows?)

everyone does, and its always weighing the "well do I wait 15 minutes let it warm up, then drive 2 minutes or just say fuck it and go" and your car doing 30 miles on the highway doing highway speeds was getting a lot of cold air to the radiator and such so when you stopped moving the hotter temps began to show since there is no longer a mass quantity of cold air moving past the engine, engine bay, radiator.

[NoDOHC]

Colder temperature making for increased charge air density is completely true. This is why I have trouble understanding why I observed the decrease in power when i was expecting an increase in power (also substantiated by the observed power numbers with the Haltech ECU).

I am still hoping that someone on here knows the stock AFR in cold conditions. I have a wideband O2 sensor, but is is on the '86 which is getting a turbo transplant and will need the wideband.

I can't use it on the '91 anyway as it is snowing in Wisconsin and I don't want to drive an RX7 in the snow if I can avoid it.

[Terrh]

it depends how you drive the car

if you accellerate agressively all the time, warming it up will save you fuel.

if you drive the car gently all the time (or at least, when its cold), warming it up won't.
The o2 sensor (assuming it's newish and in good order) is warm enough to be working within the first 30 seconds. It's not the issues.

Fuel mapping with cold coolant IS, and if it's cold out the injectors are DUMPING fuel. I would think with the coolant at -40F, if you were to go WOT in a TII you'd run out of injector pulsewidth with stock everything well before redline. Cold engines need a TON of fuel to operate smoothly.

[Fidelity101]

It always seemed backwards to add a bunch of fuel to a cold engine, when the fuel itself will actually cool it. But hey, whatever works I guess lol.

[vex]

It always seemed backwards to add a bunch of fuel to a cold engine, when the fuel itself will actually cool it. But hey, whatever works I guess lol.

more air density = more fuel needed

[Fidelity101]

more air density = more fuel needed

not very backassawards after all lol.

[vex]

Sorry it's been so long since I could post up some data. Here's a years worth of MPG data:

http://spreadsheets.google.com/pub?key=pcz81xvfoH8RY-8lAK1BtZQ

Preliminary data view looks like there is a slight increase in warmer weather, but nothing dramatic. Perhaps 5 miles at the most. Given that, this is through all types of driving, and is more indicative of whether I was doing city driving or freeway driving. During the summer months I was on the highway more than not, and during the winter months around town more. That alone would account for the decrease in MPG.

[Fidelity101]

Fill up while you can at the pump during the colder days, fuel is more dense when its colder so you get more bang for your buck since they have no way of calibrating or standardizing temperature with pricing and etc.

[MaczPayne]

Fuel at gas stations are typically stored deep underground. I don't think that area of earth is affected much by surface temperatures.

[Fidelity101]

Fuel at gas stations are typically stored deep underground. I don't think that area of earth is affected much by surface temperatures.

it depends on the state, a couple don't allow underground storage tanks.

And Michigan is cold always so its win win for me :D

[NoDOHC]

Hey, that google spreadsheets thing is pretty neat.

I guess that when I say running around town I mean my 11 mile one way commute to work on country roads.

I ought to figure out how to use google spreadsheets too and post my mileage numbers

I tried to summarize the data in my earlier post.

The absolute worst full-tank mileage I ever got was a weekend of drag racing (1/4 mile) and an autocross = 15.1 mpg. (This does include some normal driving). I really don't know how you got 10 mpg (although the bad tps may have been part of it). I did get 12 mpg on a partial tank (filled up, went to track, ran 12 passes, went home, filled up, checked mileage).

[Fidelity101]

Do you have a stock ported motor? I would get 14 on my street ported S4 non highway, mostly city and dicking around driving.

[NoDOHC]

Engine Statistics:

'86 (Stock ECU)
ECU: Original N326 built in October 1985
Fuel mix: Running closed loop at 14.7:1 AFR (when it was actually closed loop)
Timing: 12 degrees BTDC Leading 3 degrees ATDC trailing at idle (+17 degrees from stock)
Mileage: 178,000-192,000 miles
Compression: 104-110 psi on all faces
Block: Never touched, never had intake or exhaust manifold off, completely stock.
Coolant Seals: One may have been leaking slightly
Oil Seals: Blew clouds of oil smoke above 7,000rpm
OMP: Adjusted to the max
Premix: 4 ounces/tank
Auxiliary Ports: Functional in stock form (open at 3500 at WOT).
Air pump: Functional
Power Steering: Belt Removed
Air Conditioning: Belt Removed
Intake manifold: Stock and untouched
Exhaust: Stock 3 cats (functional) Dyna-Flow Mufflers, stock Y pipe.

'86 (Haltech)
ECU: Haltech E6X controlling ignition and Fuel.
Fuel mix: Running closed loop at 16:1 AFR (Wide band input)
Timing: 0 degrees BTDC Leading 12 degrees ATDC trailing at idle (advanced until ping elsewhere, 0 degree timing split above 2,000rpm)
Mileage: 192,000-199,000 miles
Compression: 104-110 psi on all faces
Block: Never torn down, completely stock.
Coolant Seals: One leaking slightly at first, both leaking toward the end
Oil Seals: Blew clouds of oil smoke above 7,000rpm
OMP: Adjusted to the max
Premix: 4 ounces/tank
Auxiliary Ports: Modified but functional (open at 4500 at WOT).
Air pump: Removed
Power Steering: Removed
Air Conditioning: Removed
Intake manifold: UIM deburred and polished, LIM deburred
Exhaust: Racing Beat Headers, Dyna-Flow Mufflers, stock Y pipe.

'91
ECU: Original
Fuel mix: Running closed loop at 14.7:1 AFR (when it is actually closed loop)
Timing: 17 degrees BTDC Leading 2 degrees BTDC trailing at idle (+22 degrees from stock)
Mileage: 146,000-154,000 miles
Compression: 80-100 psi on all faces (cold)
Block: in-car porting of irons to maintain runner cross-sectional area (Minimal port timing changes).
Coolant Seals: Good
Oil Seals: Good
OMP: Not adjustable
Premix: 6 ounces/tank
Auxiliary Ports: Fully Functional
VDI: Fully Functional
Air pump: Functional
Power Steering: Functional
Air Conditioning: Functional
Intake manifold: Port matched LIM and UIM
Exhaust: Racing Beat headers, ???(Stainless steel, unknown manufacturer) presilencer Y-pipe and mufflers.

I would guess that your street ported engine will not get stock port mileage. (Although I have heard some say that a street port makes little to no difference).