Downpipe intake surprisingly good - Alfa Romeo Forum
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(Post Link) post #1 of 4 Old 11-05-13 Thread Starter
MGG
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Downpipe intake surprisingly good

Due to high shipping costs to New Zealand making cheap parts expensive I have decided to have a go at making an alternative to the GTA intake pipe for the JTS.

Firstly I had to remove the resonator box to find out the diameter of pipe I would require. The air box requires a pipe with a 3.5" OD; hence a DN90 downpipe was suitable.

I drove to the local hardware store without the resonator fitted and man what noise. At about 2000rpm under load the car sounded like a boy race's Subaru; I found this amusing but also slightly embarrassing with a car with as much style as a 156.

Anyway I made the intake pipe shown in the images below. The intake is at the same height as the old one so I shouldn’t have any issues going through puddles. Personally I think it sounds good, the car sounds a little deeper under load but not boy racer loud.

Performance wise there may be a difference but only a slight improvement; there are certainly no negative effects. I feel that there may be room for improvement by trying to reduce turbulence behind the front bumper with an air dam of some sort. Oh and a lick of black paint wouldn't go a miss.

Hopefully once I install the full Supersprint exhaust system I have on order and get the car remapped there should be a noticeable difference.
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Hi MGG,

Interesting post and good work. A couple of questions. Why o you suspect hat here is turbulance behind the bumper and wht is an air dam? Thank you.
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(Post Link) post #3 of 4 Old 23-08-13 Thread Starter
MGG
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An air dam is basically a wall that stops air dead in it's tracks resulting in high air pressure on the outside of the wall. Consider putting your hand out the window and rotated it around, there is more force on your hand when your palm is facing the direction you are driving in. An engine can develop more power if the air pressure at the intake is increased as more air is available to create a bigger bang with the addition of extra fuel.

The engine also works best if it receives a steady flow of air as it does not have to constantly change the amount of fuel injected into the cylinders. If you consider a sail boat in a steady breeze verses a gusty breeze and how much more difficult it is to sail in gusty weather.
Now consider the bumper as being a giant wall with holes in it; if you were to stand behind a hole you would get a reasonably steady flow of air with a few fluctuations and if you stood beside a hole you would feel a buffeting much like if you put the windows down in your car. The two places where the air would be steady are right in front or behind the wall away from any holes. If you were to measure the air pressure you would find that the air pressure is higher in front of the wall.

That was rather abstract but I hope it kind of explains why I think there is turbulence behind the bumper.
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Nice job, though I'd put a coat of black paint on it even though I'd probably never see it again ,lol, a bit OCD.

I've done something similar, but with that bendy tubing from eBay, and took it up to behind the left hand vent thingy next to the grill for a bit of ram air effect.
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air , downpipe , filter or housing , good , induction , intake , jts or ts , surprisingly

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