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Old 06-26-2010, 12:16 PM   #1
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Default Serious Question about Engine Performance

This is something I've been pondering over for a while and I want someone to tell me why my theory is wrong.

A naturally aspirated engine is operating at full efficiency at 0 vacuum. So the engine is pulling in as much air as possible to combust as much fuel as possible.

So lets take out fluid dynamics for a moment.

Would this mean that any intake/filter combination that allows an engine to reach 0 vacuum is the most efficient intake setup? I know all these N/A guys that spend way too much $ on intakes with giant pipes and filters but if you hook a vacuum gauge up they reach the same 0 vacuum situation with their stock intake.

So does it make any real difference to have a large, high flow intake?
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Old 06-26-2010, 12:37 PM   #2
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Pressure(load) is distinct from flow.

While pressure difference drives airflow, you can have obstructions which inhibit airflow and have a minimal effect on gauge pressure. This really only occurs at much higher rpms, and the tradeoff of many aftermarket intakes is that they lose the useful midrange torque of stock intakes that either cancel out or take advantage of helmholtz pulses, caused by the valves opening and closing.

In just about any intake tract the biggest restriction will always be the intake valve runners, and port/polish work is much more beneficial for this reason. This is the kind of work the factory can not justify (unless it's an Integra Type R), but most engineers will design an efficient intake, albeit one that is biased towards a powerband that the majority of drivers will find useful.


On a turbo car your attention should always be devoted to the exhaust, where returns are much greater due to higher pressure differentials. 30psi across the turbine spools faster and makes more top end than 25psi differential. It's easy for a poor, stock, or small diameter exhaust to create 5psi of backpressure...good for torque n/a, bad for flow in a turbo application.
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Old 06-26-2010, 12:40 PM   #3
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Ok, so fluidity is much more important than absolute pressure. That makes sense as air channeled correctly will flow more at a given pressure than turbulent air.

I didn't know that about factory intakes actually factoring in the pulses. Thanks!
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Old 06-26-2010, 01:11 PM   #4
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Quote:
Originally Posted by D.dutton9512 View Post
Ok, so fluidity is much more important than absolute pressure. That makes sense as air channeled correctly will flow more at a given pressure than turbulent air.


I'm not sure what fluidity is besides saying that yes, air is actually a fluid...but what you're really concerned with is MASS FLOW RATE. This takes into account both pressure and flow, and is what really makes power anyhow. You still have to consider volumetric efficiency of the motor, which is where flow restrictions come in, but the actual amount of air MASS that goes through your motor dictates power output, as long as you are providing enough fuel.

Once you've got a decent tune on a standalone ECU you can actually tell when anything you change on your setup (exhaust, intake, cam or cam timing, ported head) makes a difference since your AFRs will go leaner the first time you go to tune it afterward. This happens because more air is going through the motor, and you now have to ADD fuel, and this is where you see the power increase of the mod.

This generally brings up the point that needs to be made with bolt-ons. Lots of companies claim a part makes so many ponies, which it might be capable of, but it's all worthless without proper engine management. Yes, you may see some gain before it goes too lean, but being able to account and tune for the improvement is where you win...or find out you just wasted money.
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