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Old 02-21-2013, 01:50 AM   #23 (permalink)
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Location: Oregon, US
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ZoomFreakingZoom - '88 Mazda RX7 Convertible
90 day: 19.02 mpg (US)

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Originally Posted by Davinator61 View Post
My reasoning behind a WAI is that warm air would help the engine warm up faster and stay above 195 degrees Fahrenheit, in order to stay in closed loop operation more often, and to bring about a more complete atomization of the fuel droplets from the individual injectors as each one of them sprays a pre-determined amount of fuel (based on RPMs, load on engine, current engine temperature, etc., etc.) into each intake port of each cylinder in turn.

I've found that my 1991 Jeep Cherokee Laredo 4.0L engine runs best at between 210 and 225 degrees Fahrenheit, ESPECIALLY when it comes to fuel efficiency. As the cooling system is pressurized to around 13 PSI above atmospheric pressure, I'm not too concerned about overheating, as long as ALL links in the engine cooling system are free-flowing when they need to be, and properly maintained on a regular basis (back-flushing, thermostat operation checks in boiling water with a thermometer, replacing hoses when they begin to swell up at their clamped ends, etc., etc.).

Cold air slows down the proper atomization of fuel, even from the injectors on a multi-port system, requiring more fuel to compensate for the slower evaporative rate at colder air temperatures. Why do you think the cold-start fuel consumption, before closed-loop operation temperatures are reached, is much higher than when the engine is fully warmed up? This is due to open loop enrichment in order to compensate for the fuel droplets' tendency to clump together during colder engine operations during warm-up, especially right after cold start-up. Also, cold air would cause an engine to take longer to warm up to closed-loop operation temperatures, adding to the higher-fuel-consumption-after-cold-start-up issue.

No, cold air intakes are NOT the best for fuel economy, even though they are SUPPOSEDLY great for boosting high-end Horsepower and Torque. However, fuel efficiency appears to be the MOST optimal in about the 1/3-2/3 RPM range, or between 1650-3300 RPMs in the case of my 4.0L Jeep Cherokee Laredo, at an air intake stream temperature of around 90-120 degrees Fahrenheit. However, cooling the fuel to prevent vapor lock, which has occured on my vehicle already at least once, on a really hot day in which the fuel got really heated up to the temperatures inside the engine compartment, might be a good idea, if it doesn't interfere with improving fuel mileage to any appreciable level. Does this reasoning make any sense to you at all?

Yes the reasoning makes sense. I can easily see how it would warm up the engine quicker, but, at least for my driving habits, that wouldn't be very beneficial.

Comparing cold start efficiency isn't a good comparison because the engine is cold. A warm engine, even on a day with cold temperatures will not necessarily have a problem evaporating fuel due to the heat from the warm engine right underneath that throttle body.

Also, if you want to increase operating temperature, why not just change your thermostat?

This is my understanding: there is some point at which the intake air is "warm enough" to not have efficiency losses, and I never would have imagined that you would need to modify your intake to achieve that. I suppose it would make sense, if you do alot of cold starts, to do the modification to improve warm up time, but it would still cost you power when the engine is warm.

Even on relatively cool days (40 degrees), if I open my hood, it is plenty warm and I can't imagine the intake air being much colder.

You might consider editing this page

Because whoever wrote it clearly doesn't get it
1988 rx7 convertible. Streetported, racing beat header and presilencer replacing all 3 catalytic converters. Atkins 6 port sleeves. K&N cone filter intake. 22mpg average, 27mpg max. Spreadsheet (updated regularly):
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