Article: Why a 2.5" MAF to TB pipe is better than a 3" for a CAI
Hi, I have here a small article that I wrote myself on why a 2.5" MAF to TB pipe is better than a 3" pipe. If there are any corrections that need to be done, point them out please. Opinions wanted, thanks!
Why a 2.5" MAF to TB pipe is better than a 3" for a CAI
Cold air intakes allow for greater fuel efficiency, and greater performance. The theory is that if cold air enters the combustion chamber, it will ignite cleaner and bigger because of the greater mass cold air has than warm/hot air. But what happens if you use a 3" pipe from the MAF to throttle body instead of the stock sizing of 2.5"?
Obviously Nissan has done their work on the intake side. On the GA16DE engine, the air intake is designed to produce a 'ram-air' effect. The housing box which stores the filter has a velocity stack that is 3". But go from the MAF to the throttle body, it is reduced to 2.5". Why do you say they did that? They wanted to increase the VELOCITY of the air coming in. Think about it this way. It sort of acts like a compressor. It compresses the air, making it rush in faster, and also somewhat cooling itself because it is not effected by external temperatures.
If the pipe from the MAF to throttle body is INCREASED to 3", this negates the effects of the cold air intake. Sure, you are sucking in 'outside' air (I say outside because it is not cold anymore due to the piping), but it will not be as cold if the air were 'compressed' with the 2.5" pipe. This is the reason why. The pipe from filter to MAF is 3", which houses the cold air. If the pipe from the MAF to the throttle body is also 3", it negates the effects of the cold air because the velocity of the air traveling thru the MAF to throttle body pipe SLOWS down and the throttle body is only 2.5". Therefore the throttle body is unable to 'suck' air in effectively thru the filter to MAF piping because the MAF to throttle body has increased in size and air volume. It can only suck in so much air, but pushing it in will yield greater results. It is just sucking on the air which is present inside the 3" MAF to throttle body pipe, and not sucking the air from the cold air extension (MAF to filter pipe).
Restoring the MAF to throttle body pipe to its original 2.5", and the cold air intake is in full effect. Instead of the throttle body sucking on warm air present in the 3" MAF to throttle body pipe (warm air because there is not enough velocity to effectively 'cool' the pipe, and also the heat soaking effect from the engine'), the throttle body uses the velocity and relies on the piping AFTER the MAF which is cold air.
Experimentation
To experiment and prove my point, I got a 3" MAF to throttle body pipe, and used silicon couplers that were 2.5-3". After the MAF, there was a pipe leading down to the fender with filter. Temperature was 60 degrees F. Normal driving early in the morning, approximately 14 miles.
After the drive, I checked upon the temperature of the intake pipes. Despite the cold weather, the pipes were WARM (this was done using my hands only). The pipe after the MAF to filter was warm (it shouldn't have been) and the MAF to throttle body pipe was even warmer. I can only conclude one thing. That the velocity of the air was not fast enough to effectively 'cool' the intake, thus making it act like a semi-warm air intake.
After this, I put on a 2.5" piping from the MAF to throttle body, and kept the same 3" piping after the MAF to filter. Drove same way, temperature though was 70 degrees. Slightly warmer. Despite the warmer temperatures, the intake pipes were significantly COOLER than before. The MAF to filter pipe was all COOL, and the MAF to throttle body pipe was also cool. The intake manifold was barely warm. This shows that the ram-air effect works better having a 2.5" stock size pipe for the MAF to throttle body instead of a 3" pipe.
In conclusion, having a 2.5" piping effectively compresses the air, making the intake rely on the air piping from the MAF to filter, instead of relying on the 'stored' air inside the 3" piping from the MAF to throttle body. And you don't have to take my word for it, take HotShots word, or Mike Kojima on Sentra.net
I wrote this because I noticed some people (Place Racing, and other manfacturers that I am unaware of, EBAY) were using or have a 3" pipe. To utilize the effects of a cold air intake, the MAF to throttle body pipe MUST be 2.5", no bigger.
To utilize the effects of a cold air intake, the MAF to throttle body pipe MUST be 2.5", no bigger.
You had good points, but the main restriction is the cylinder head so it doesn't really make much of a difference. Good job though It was written out very well and made sense.
Last edited by GA16DE200sx : Feb 9th, 2006 at 12:11 PM.
While 2.5" might be perfect for a stock engine application with bolt-ons, it does not address the potential that a modified engine has. So saying that 2.5" piping is the biggest you should run is somewhat of a blanket statement that does not hold true in all cases.
For example, IF you are running a ported head, higher compression pistons, a custom header and exhaust, an SR20 TB, and other modifications, then the 3" inlet pipe is the better choice.
The same holds true for exhaust systems, and while I've always been an advocate (if not the first) of the 'no bigger than 2 inch diameter' rule for the GA16 exhaust, I will be the first to admit that is not always the case, depending on the engine's specs and the applications. For N/A racing use, I actually use 2.25" tubing on the GA16 exhaust, even though I have adamently suggested otherwise in the past. But street and racing are two totally different things, regardless of how fast you think you drive on the street...
While 2.5" might be perfect for a stock engine application with bolt-ons, it does not address the potential that a modified engine has. So saying that 2.5" piping is the biggest you should run is somewhat of a blanket statement that does not hold true in all cases.
For example, IF you are running a ported head, higher compression pistons, a custom header and exhaust, an SR20 TB, and other modifications, then the 3" inlet pipe is the better choice.
Bob
That's true, thank you for the clarification. The article only applies to stock and bolt-on mods. Of course serious modifications such as ported heads, cams, pistons, etc. would require a bigger inlet because simply the 2.5" inlet would be to a point where it would be restricting airflow. Thanks for the heads up!
It was written out very well and made sense.
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