Hey all!

SO I recently just installed an Agency Power midpipe and borla exhaust, and my fuel consumption has increased pretty drastically. And I realize you're all going to say it's because if a different driving style, but i'm driving the same way I used to before the exhaust. When I got the new engine, i was getting like 220-230 miles per tank, now I'm down to like 180-200 mpt. Could anyone please explain to me why this is reverse logic to what I was thinking would happen. With greater exhaust flow, wouldn't the engine be able to consume fuel more efficiently and effectively because it isn't being restrained by anything besides the intake now? Seems like, now it consumes more gas per combustion with the exhaust...

My brain is itching for a response.

Thanks in advance!

 

i noticed a drop in the amount of miles i get from a tank from the same exhaust combo, but it might just be the way i drive.

 

Haha, that's what I figured for a while, but now I just feel like it's due to something with how the engine is going through the combustion process.

 

I have been tracking the mileage of my car for years and even with just about every bolt on my mileage is about the same, even through three motors.

 

midpipe won't do anything for your FE. Take it from me, the guy who's owned about 10 different exhausts/midpipes.

 

Then why does it seem that my car goes through a quarter of a tank faster than it used to...I could go like 70 miles on the first quarter tank...Now it dumps down quickly to 25 miles on a quarter tank.

 

quarter of a tank is meaningless. You don't even have any real metrics!

 

its all in your head, or driving style. a catless mid and/or catback simply has no physical way of creating those symptoms

 

yeah, I seem to get a lower MPG just granny driving in the city to keep the heat away.
It's driving style. Just let it get cold out and it should go back up.

 

I see your in WA where the temps have dropped in the morning and night. So, consider the reasons listed below in this article

9 reasons why your winter fuel economy bites!


Those of us living in northern climes have already started to notice the seasonal decline in fuel economy, even with careful attention to sagging tire pressure (probably the best known effect of the mercury's slide).

Yet, despite diligent all-around maintenance and continued careful driving, cold weather fuel consumption can be dramatically worse than in warm temperatures.

How much worse?

Have a gander at these calculations for a Honda Civic hybrid at 60 MPH in varying ambient temperatures:

MPH-----AMBIENT-TEMP-----MPG (US)

60------------95----------52.98
60------------85----------52.62
60------------75----------51.16
60------------65----------49.12
60------------55----------47.22
60------------45----------44.67
60------------35----------43.05
60------------25----------41.54
60------------15----------39.41
60------------05----------38.09

Look at the extremes: the coldest MPG is 28% lower than the warmest. (Source.)

My own experience supports this: 12.5% worse mileage during the colder half of the year (Oct 15 to Apr 15) than for the warmest half (Apr 15 - Oct 15), on average 2002-2004 in my 1989 Accord. Comparing just the warmest months (Jun-Aug) to the coldest (Dec-Feb), the difference is even more apparent - 21.2% worse (2002).

Why so bad? Off the top of my head, I could think of a couple of reasons to explain it, but together they didn't seem significant enough to account for the magnitude of the change. With this mystery to solve, I hit Google. And here's what I learned...

9 reasons your winter fuel economy bites

1. More idling

This should be a no-brainer, yet parked idling cars are a common sight in cold weather. Resist the temptation to idle your car to warm it up. An idling engine gets 0 mpg. Consider also that idling the engine does nothing to warm up the tires and drivetrain.

Even in the coldest weather, you can begin driving after 30 seconds from a cold start - keep speeds low/moderate and use gentle acceleration until the temperature gauge starts to climb (source).

2. Low tire pressure

Of course you're smart enough to keep up your tire pressure as the temperature drops, right? A 10-degree (F) change in ambient temperature equates to a 1 psi change in tire pressure (source). Fuel economy declines 0.4 percent for every 1 psi drop (source).

3. Increased rolling resistance

Even if you're completely attentive to proper tire pressure, cold ambient temperatures will still cause your tires to return worse mileage. That's because a tire's shape isn't completely round - the sidewall bulges out at the bottom, and where the tread meets the road the small contact patch is actually flat. As the tire rotates, it constantly deforms to this shape, and this deformation requires more energy when the rubber is cold and hard. Rolling resistance at 0 degrees F is 20% greater than at 80 degrees (source 1, source 2).

4. Crappy road conditions

It's increased rolling resistance of another kind: driving through slush and snow. And then there's its wasteful polar (no pun intended) opposite: no friction at all! (A.K.A. wheelspin on ice.)

5. Lower average engine temperature

In the winter, an engine takes longer to reach operating temperature and cools off faster when shut off. Since the engine management system orders up a richer mixture when cold (proportionately more fuel in the air/fuel combination), more fuel is being burned overall.

A block heater can offset this problem (improving fuel economy by 10% in sub-zero conditions - source), as can garage parking, and combining trips (to minimize the number of cold/hot cycles).

Also related...

6. Higher average lubricant viscosity

Engine oil thickens as it cools. So does transmission and differential fluids and even bearing grease. Significantly more energy is needed to overcome the added drag these cold lubricants cause.

Using synthetic fluids can address this problem, since their viscosity changes less at extreme temperatures than traditional mineral fluids.

7. Weaker gasoline

Gasoline doesn't vaporize readily at very cold temperatures. So oil companies formulate fuel differently for cold-weather markets in the winter. Unfortunately, the changes that provide better cold vaporization characteristics also result in less available energy for combustion. You won't get as far on a liter of winter gas as you will on a liter of summer gas. (Source.)

8. Higher electrical loads

In colder temps, you use electrical accessories more often:

- lights (in higher latitudes it's darker in the winter)
- rear window defroster (because it's easier than using the ice scraper, right?)
- heater blower motor (I don't have a/c, so this isn't balanced out during warm conditions); heated seats/mirrors
- windshield washer pump (because it's easier than using the ice scraper, right? And for frequently cleaning off dirty road spray.)

9. More aerodynamic drag

No, I'm not referring to the layer of snow you're too lazy to brush off the top of the car (though that would hurt mpg too).

A vehicles aerodynamic drag is proportional to air density, and the density increases as temperature drops. For every 10 degree F drop in temperature, aerodynamic drag increases by 2%

 

what you said is correct, mostly, any batterypowered vehicle sucks in the cold due to the batteries, so that is not the best kind of car for such a test.
and I don't think my sarcasm was spelled out correctly with a smiley face.
you're car looks nice btw.

 

^ Take into account that this is basic info for all vehicles. For example, we all know that the warm up procedure is important with rotaries.

I didn't look long enough for a Gasoline powered chart. Nevertheless, it's an example that brings the point across. All 9 reason are valid for gasoline powered vehicles thou.

Thanks for the compliment

 

True, I understand all of this going into this discussion (although some of that was also news to me) but I feel like there has to be something more to that issue. For instance, I recently replaced my coils (not for BHR yet because I'm a poor college student working to make ends meet for now (6 more months until grad!)) and I'm starting to hear the same sound that was produced by my dead coils, a really high pitched squeal when the car turns on that lasts until the car lowers a little in idle. I'm thinking to replace my coils again (since I got the BDW coils which have a lifetime warranty on them) and seeing if that will help in the short coming of it. But even looking at your Fuelly average mpg, you're getting 18.5, mine was never above 10 on my old engine and now on my new one, it's barely reaching 15...So, I feel like I'm still working towards finding an issue in there. But I'm still interested to hear what you think would be the issue. There isn't a huge following of RX8s up in WA anymore (most of them have fallen off the planet) but would love some more insight!

 

It's not uncommon for OEM coils to be faulty right out of the box.
Your squealing issue is as simple as belt tension, worn belt, or a worn out pulley.

15mpg is very low if your not driving like a mad man. When was the last time you checked/cleaned your MAF? You might also want to take a look at your plugs and see how they're burning

 

Well, the plugs were supposedly replaced by Mazda when I got my new engine.

I'll look at the belts (but I feel like they would've warned me if they were worn when I got my new engine in. So I'll double check the coils and the MAF tomorrow or something. Hopefully I can figure it out. Because I don't drive that crazy, I'll really floor it a couple of times but other than that just cruising at like 65-70 to get to work and back, and I only work 5 miles from my house.

 

Unless you know the tech working on the car, never trust the dealer.
Always double check their work and make sure if you see something questionable you bring it their attention.

Your 5mi commute will definitely effect your mileage.

 

I'll check the plugs next time I have the car in the air, and I'll get new coils tomorrow and let you know how it changes.

 

really i thought that was normal for our cars?

i get from 13.3 to 15.1 mpgs on a tank and my commute is around 15 miles, plus i have almost the exact set-up and driving style

 

I usually saw anywhere from 180-230 miles before the low fuel light came on. Not an exact number, but that's a ballpark range. Going turbo didn't change the numbers much.

 

Yea seems about right and when it's cold in Miami (rarely) I see 12.8 mpgs lol

 

When did you change your midpipe and exhaust?
How many tanks have you gone through since the swap to determine the loss in MPG?
Do you know if gas stations in your area use summer and winter gas mixes?
How many miles are on your car?
When were your coils, plugs and wires actually done?
Did you try cleaning your MAF?
Have you tried using different gas stations?

 

13-15mpg is on the low side
Many owners are seeing much higher averages
Click on my fuelly link and you will see a good example

How you drive and your commute will be a large factor with mpg.
I went from driving at 80mph getting 16.5-17mpgs to doing 70-75mph and getting around 19-22mpg highway. These averages include about 30% city driving

 

In a colder environment where the air density increases, would it be logical to conclude that fuel efficiency is poorer but peak performance increases due to higher oxygen values as the ECU tries to maintain stoichiometric ratios by injecting more fuel during the combustion phase?

 

Yea I do about 30% city driving as well, 10 miles of my commute is hgwy but I'm in bumper to bumper traffic for like 30 mins. I have never done a full tank on just hgwy. And I keep my car up to date on it's tune up.

I have driven at low speeds and still nothing, lol like i said I thought it was normal

 

My comments to your questions are bellow in Red. Hopefully this helps answer a few questions.


How many tanks have you gone through since the swap to determine the loss in MPG? This is my 4th Tank since I started noticing my problem, and it seems to be getting worse with each tank
Do you know if gas stations in your area use summer and winter gas mixes? No, I had no idea about different mixes, until earlier in this thread.
How many miles are on your car? I have about 42,000 Miles on the body and 2,000 on the new engine.
When were your coils, plugs and wires actually done? I did the coils about 2 months ago and they said they did the plugs and they still have the OEM wires.
Did you try cleaning your MAF? No, not yet, I was going to when I got another batch of coils for free from Schucks.
Have you tried using different gas stations? Yes, same results