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Discussion Starter #1
I have noticed a marked difference in the performance of my car on these 105 degree afternoons make the cross-Dallas jaunt from DFW to east Dallas to pick up my daughter from school. On LBJ/635 in the stop and go with the AC on max/recirculate engine is noticeably sluggish during acceleration.

Once I am on the side streets of Lake Highlands/Lakewood area of Dallas, with some hills, performance off of line at a red light really is very sluggish.

In the evenings, middle of the night when it is down around 80 and I have had to make those late night undertaker runs to the Funeral Home the performance is 100% better.

I am assuming that the effect of the heat on the oxygen levels, and humidity is taking a toll on the air intake, and the A/C not cycling is the nail in the coffin so to speak? (Fuel economy has dropped too, down to 34.5 from 36.8.

Anyone else who does a daily commute in the heat seeing this as well?
 

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I don't really notice any sluggishness, but the drop in fuel economy makes sense because it's hot as hell here! My A/C has been on full blast since I got the car. I pretty much sit in 75 traffic 90% of my driving time so the fuel economy hasn't been great. Hard to have good fuel economy when you aren't moving more than 2 mph.
 

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I am assuming that the effect of the heat on the oxygen levels, and humidity is taking a toll on the air intake, and the A/C not cycling is the nail in the coffin so to speak? (Fuel economy has dropped too, down to 34.5 from 36.8.
Yeah, that is basically right. As I understand it, it boils down to air density, as described by Boyle's Law. This basically affects all internal combustion engines, not just the Fiesta's duratec.

For a fixed volume and pressure, the warmer a gas (e.g.: air) gets, the less dense it becomes. Less dense air means less oxygen to burn, a leaner fuel-air mixture for full combustion, and therefore, less power. This is why car modders with an eye on performance will often add cold air intakes (CAIs) and/or intercoolers to get the coolest air possible for their engine intakes.

As you yourself noticed, in addition to less power from less dense air on hot days, drivers will generally also run their air conditioners, which will parasitically tap a few more hp/ft-lbs from their engine output while the chill compressor is in operation. In a car with a lower power-to-weight ratio, like the fiesta, or my 98 civic, this can be hard to ignore, especially when driving up hilly grades at speed. Back when it still worked, I would often turn off the AC in my civic when driving up from 5500ft to 8700ft elevation on my commute home from Denver to the foothills. For me, it was worse, because they say you should derate a naturally-aspirated (e.g.: non turbo/supercharged) engine for 3% of its horsepower every 1000ft above sea level you take it.

If anything, ambient humidity might help counteract that, as it forms a little steam when exposed to heat, but the effect is almost certainly negligible.
 

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I know about that climb out of Denver...

My family in Arkansas is in the bus business, and I have driven many ski trips...that climb out of Denver on I-70 in our old MCI-9 coaches with the non turbo 8 cylinder diesels climbing up to go to Berthoud pass, sometimes we would have to pull over and let the engine cool down, before continuing the climb...If we were going up and over through the Eisenhower tunnel, we would almost always stop at the pull out on the other side of the tunnel to let the engine cool down. It sure was nice to get new buses with 6cyl inline turbocharged engines with electronic controls, you could zip right up and over....

I don't notice the dramatic loss of power in my wife's 09 Focus that I do in the Fiesta....The 2.0 Duratec definately has more balls than the 1.6L
 

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...that climb out of Denver on I-70 in our old MCI-9 coaches with the non turbo 8 cylinder diesels climbing up to go to Berthoud pass, sometimes we would have to pull over and let the engine cool down, before continuing the climb...

I don't notice the dramatic loss of power in my wife's 09 Focus that I do in the Fiesta....The 2.0 Duratec definitely has more balls than the 1.6L
Yeah, absolutely. Almost every drive home in the summer, I see one or more vehicles pulled over with their hoods up on the steepest grade stretches. It's an automotive gauntlet, weeding the weakest with cruelty in the summertime. It's probably a contributing factor to my 98 civic's headgasket leak at 210K miles after an unexpected overheat from a mud-clogged radiator from the spring thaw and dirt road outside my house...

I really want to see Ford offer the Fiesta with either a 2.0l or the upcoming 1.6l ecotech I4. The lack of this option moved me to pick up a '10 mazda3 2.5l 167hp/167ft-lb 5-door 6-spd, but its 500lb of additional weight over the fiesta means I'll be paying a lot more at the pump, in the long run, on my mountain climbing commute... Just not enough hot hatchbacks to choose from in the US, so I want to see the Fiesta do well enough for ford to consider some beefy engine options, (and bringing back the focus hb) -- could be my next car!
 
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