Air Restriction

[Nish]

I'm wanting to learn about the threshold of air restriction. How much restriction is too much? I do realize this would probably be different from motor to motor and how the said motor is tuned. How would someone find out the percentage of air their car would be un-able to run or would the car just run on a very low amount of power?

Another question is how does air restricting effect torque. Does it decline at the same rate as power, faster/slower?

I am not attempting to restrict an engine, the thought just popped into my head and I couldn't find an answer through a quick search. The main question would "what percentage of air restriction is too much?"

 

[Slash]

Most engines don't need the air they are fed, and get too much at full throttle. Old carbed engines are a good example.

 

[ITCC_Andrew]

Nish, you're looking for a relationship between:

-Volumetric efficiency (how much the air fills out/spreads throughout the cylinder)

-stoichiometric fuel/air ratio (14.7:1)

-minimum fuel displacement (how little fuel the engine can run on)


Now, imagine this: the engine only puts enough fuel in to match with a roughly 14.7:1 air:fuel ratio. If you have atmospheric pressure (non-turbo'd car) pushing air into a cylinder, it's going to have approximately 70-80% volumetric efficiency.


Now, if you put a massive restriction (50%, for example) then the air flowing into the cylinder will flow in at high speed, but with low volume. When the engine is rev'ing slowly, then the air is "slow and thick" (slow speed, hih volume) because the piston isn't going down fast enough to create 29" Hg. Vacuum. However, as the engine speeds up, the piston goes down faster for the intake stroke, creating more vacuum, and thus drawing air in faster.


Thus, the massive air restriction would normally cause the air to move twice (for example) as fast, but you wouldn't get as much air into the cylinder, regardless of how fast the air moves.


However, turbocharged or supercharged engines have above atmospheric pressure pushing air in, meaning that there's more pressure pushing more air into the cylinder... Although the turbo- or super-charged would be tuned to run with a greater volume of air at a greater speed.



So, basically, the air restriction changes the way the engines run altogether, but it truly does go on a case-by-case basis. An Impreza WRX STi with a huge turbo will not be too heavily influenced by a 5% restriction - it will slightly affect output, but not terribly.


However, a Formula 1 car with a 5% restriction in air flow could possibly cause it to simply stall the second it gets into draft.



Now, applying any and all of this to GT6 is basically ridiculous, because GT6 didn't bother to model anything except a reduction in power output across the board, essentially. It's more like an electronic system which applies the brakes when power output exceeds what it's supposed to.

 

[Nish]

I was largely asking as just gaining a bit of new information. I believe DTM has a horsepower cap right now is there any point where it would just be better to put lower power gaining parts or to simply restrict air flow for the season if you started off with a vehicle largely over the cap? Also how does it effect torque?

GT has many tuning issues and I dislike de-tuning further than 98% but I was wondering on real world applications, that's why I posted here to try to understand the realistic side to it.

 

[ITCC_Andrew]

I'm not quite far enough into my college education to know much more about how an air restriction effects it, but your guess is as good as mine...


So, I'll give an honest guess: with air flow being sped up because of the restriction, you might still build horsepower to a point, but it would be highly inefficient to have huge pistons and cylinders and so on. The weight of the pistons and cylinders would necessitate the usage of a smaller engine block. Or, in other words, hello DTM 4 cylinders, we've missed your light-weight value.

Edit: why use a Bugatti Veryron's 16.4, when you could use a lighter 2.5 litre?