RE: Intake FAQ
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RE: Intake FAQ - 1/22/2008 10:58:51 PM
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my05civic
Posts: 1763
Joined: 8/18/2006
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honestly, an sri doesn't make considerably more noise than an cai. honestly though, with either a cai or sri, i don't think ANYONE can get a headache. the only time when an intake will roar is when you're merging onto the freeway (or passing a car) and pushing your car's rpms up there in the 4-5k range. if you're ALWAYS mashing the pedal to get up into the high rpms, you're wasting your time: a civic's engine is too damn weak; you're just burning gas.
if you're just cruising, or going at a constant speed, there is no reason that anyone should get a headache. the hum of the engine is just a little bit louder than stock.
from your jetta story, i'm not sure an intake (or exhaust...) is the kind of modification you should go for. if you wanna work on your car and not risk the headache, go for suspension upgrades first.
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RE: ========Intake FAQ======== - 3/3/2008 12:51:57 PM
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08civicsiguy
Posts: 1
Joined: 2/19/2008
Status: offline
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I know I’m a newbie to this forum, but I’m not a newbie to Civic's (I am on my third) or modifying every car, truck, motorcycle, or ATV I’ve ever owned to get more power out of them. I am an engineer, and I design aircraft engine combustion systems and thought I could better clarify this never ending debate over Cold Air Kits and fuel mileage. I posted this here where I thought it made the most sense and I hope you find this useful. If you find some minor errors try to take it easy on me, I threw this together over lunch today.
Some Basics:
Air flow can be measured in volumetric or mass flow. Air density highly depends upon pressure and temperature.
A Cold Air (C/A) kit increases power by reducing the temperature of the air entering the engine, thereby increasing the density, or amount of air for a given volume. Basically you are able to squeeze more air through the intake at full throttle because the air is denser. C/A kits also use higher flowing inlet filters & lower drag intake pipes to reduce friction/drag losses at full throttle.
Evolution:
1. Carburetors are manually adjusted by mechanics without ECU’s, typically without dynamometers or exhaust monitoring. This resulted in many automobiles running off of optimal fuel/air splits (F/A Ratio) with high emissions and owner operability issues requiring tuning at various elevations/ambient conditions.
2. Fuel Injection systems coupled with ECU’s use inlet VOLUME (with O2 sensors) to adjust fuel injection rates to better maintain the F/A Ratio near optimal/Stoichiometric ratio (14.7:1).
3. Modern ECU’s use inlet MASS FLOW sensors (with O2 sensors) to adjust fuel injection rates to almost absolutely maintain Stoichiometric Ratio.
Why you get more power from a C/A kit:
Every intake has a choke-point or limiting component that controls the maximum amount of air your engine can intake. If we arbitrarily assume it is the intake manifold for this example you can now move more air through your intake because you can fit more air through the “hole” due to increased density. You have not increased the volume of air because that is limited by the size of the “hole,” you have increased the density of the air at that limited volume.
Why you don’t get better gas mileage:
As previously mentioned… modern ECU’s use inlet MASS FLOW sensors to adjust fuel injection rates to maintain the optimal F/A ratio. After the C/A installation your ECU senses the increased mass/density and adjusts the fuel injection rate to continue maintaining the proper F/A ratio. Older ECU’s/automobiles merely measured volume and therefore did not sense the increased density/mass of that volume provided by the C/A. Your engine would run leaner… gaining perhaps as much as 5% fuel economy, but more likely 2 or 3%. (3% of 30 MPG = ~1 Additional MPG). This is slightly detrimental to your engine because it runs at a higher temperature due to the lean condition.
Why you don’t get worse gas mileage:
At less than full throttle the engine is only going to intake the required amount of air and not all that it can possibly get… which is why you get the benefit of a C/A at full throttle, but more importantly why you don’t get worse fuel mileage during daily driving according to the incorrect “more air from the C/A requires more fuel and equals worse MPG” theory I see posted frequently (correct in theory if the engine did not regulate intake).
Summary:
Ultimately the answer is that you will realize a slight increase in power depending on the style/brand ranging from 2 to 10 WHP, (probably between 3 & 5) at full throttle because you can squeeze more air through the bottle-neck/choke-point to mix with more fuel (scheduled by the ECU thanks to your Mass Flow Sensor) in the combustion chamber. You will not affect fuel economy (after your ECU adjusts) unless your car does not use a mass flow sensor, or if your ECU does not auto-adjust and you fail to reset the ECU by disconnecting your battery (resulting in the lean condition mentioned above).
Hope this is useful, sorry for the long-winded first posting!
FYI: Same theory applies to intake/inlet spacers on newer vehicles because the O2 sensor senses the gains from increased mixture velocity and/or turbulence via emissions and sells you out to the ECU to make adjustments.
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