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Old 05-16-2010, 09:11 PM   #8 (permalink)
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Intake Restriction and Economy

I know exhaust was the main question of the post, but I see this a lot about the intake, and it's rarely really addressed, so I'm moving from lurking to posting around here...

The effects of intake modifications on pumping loss is very commonly misunderstood. The common assumption is that reductions in intake restriction will lead to reduced pumping losses. This is entirely FALSE at all engine loads other than WOT. On the typical NA engine, a certain amount of intake restriction is CRITICAL to running the engine, otherwise you'd be running at full throttle, all the time. In order to decrease power output from WOT at a given RPM, you need to decrease the pressure outside the intake valve, thereby allowing less air and fuel to enter the combustion chamber, which will then produce less power than would be done at WOT. The only remaining way to alter power output without changing pressure at the intake valve is by altering the fuel/air ratio. And since the cylinder/piston and intake valve essentially create one big vacuum pump, the way to reduce the pressure outside the intake valve is to create restriction in the intake path. That is THE function of the wonderful device we call a throttle: creating restriction.
So, say your car runs at 1800rpm and 10% throttle at 55mph. If you then go and modify the intake to be less restrictive, you may now need only 8% throttle to maintain this same condition. And the condition that you are maintaining is a certain pressure outside the intake valve. Therefore, in order to maintain the same output you have not changed pumping losses at all, you've simply altered how much restriction is being created by each individual part of the intake. So you are correct in thinking that decreasing intake restrictions would reduce pumping loss at a given throttle position, but what you need to consider is that a reduction in pumping loss on the intake side means more air is now entering the cylinder, and you're producing more power, using more fuel. So to maintain the same RPM/speed/load etc. as before your intake mods, you need to reduce the throttle opening to get back to the same total restriction you had before the mods. If you apply these principles to the performance end of things, the reason intake mods can improve top-end performance becomes clear. If you can now move the same amount of air at 95% throttle that you used to move at 100%, you now have headroom to increase power. (note: intake modification can improve performance below WOT, however any improvement in performance below the throttle position that moves the same amount of air as the pre-modded WOT will be due to harmonics, not reduced flow-restriction)

I would question the assumption that reduced intake restriction hurts fuel economy. Any reduction in economy with such mods is not directly related to the reduced restriction, since as we see above the TOTAL intake restriction must remain the same at a given load. Other factors come into play though; A) intake mods will often be CAI-related, which may reduce economy. B) Throttle position at load is affected, more importantly, it's reduced. Therefore your accelerator is more sensitive, and you may be giving more effective throttle than you used to be without even realizing it.

As mentioned, pipe size is critical in determining MPG loss/gain with exhaust modification. Proper scavenging lowers the pressure beyond the exhaust valve and can increase efficiency. Improper sizing can have the opposite effect, but it is all dependent on the desired efficiency range in terms of RPM's.
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