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Old 05-17-2001, 08:49 PM   #1
hayscoob
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Question Intake back fires? What do you think?

I have Iridium spark plugs and a Home made custom cold air intake using the factory filter box behind the throttle body. From time to time I get a small backfire sound coming from under the hood. Car runs just fine no pinging no missing etc. But this little intake backfire is driving me crazy. Anyone else having this problem? What do you think it is. could the car be running a little rich? Thanks...
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Old 05-17-2001, 10:06 PM   #2
8Complex

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I get some pretty massive backfires when I'm running crappy gas (87) with my intake.
http://8complex.mwsoc.org/intake.html
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Old 05-17-2001, 10:29 PM   #3
hayscoob
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Holy Crap 8complex! You got over 10k in posts. You da man....

But hey, I run 92 octane gas. The small backfires don't seem to hurt anything they are just anoying as heck. I thought maybe a hotter plug may do the trick. I dunno...
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Old 05-18-2001, 12:39 AM   #4
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I think the problem may be that your intake is a bit to long. If its to long, you may experience some air choke. Without gettin ginto the principles of resonance, and port tuning, its basicaly a pocket of trapped dead air withen the intake when you close your throttle. A Hick-Up if you will.

Intakes are very touchy things. You can liken them much to the speaker port on a large ported, or bandpass speaker box. Depending on the voluve of the box (in your engines case the volume of air the cylinder, and filter box), the frequency output of the speaker (or for your engine, the RPM at which maximum HP/TRQ is acheived), and the use of the speaker weither tight, or boomy (or for your engine, torquey or revvy) your port length and diameter will change to properly match your desired output.

Speakers work around the principle of Frequency. Each speaker depending on how its built, and how the voice coil is structured has a differnnt frequency in which it has a sweet spot. Likewise, your engine, depending on its size, displacement, and how the cam is setup, has a differnt tuning frequency. And in order to get maximum efficiency out of either, it must be tuned acordingly.

Frequency is a very interesting thing. AGin without going to deap into it (oops, too late) lower frequencys have longer wavelength, and higher frequencys have shorter wavelengths. So likewise, the lower the frequency, the longer the port, the higher the frequency the shorter the port. Now of course there are exceptions to that becuase of box design, and wave compresion. But if you had a driver atatched to a tube, and you wanted to tune that tube to 35Hrz, the tube would be the exact length of a 35Hrz wave, which, without doing hte math, so im going to guestimate, would be around 20ft. long. And for 800Hrz, it would be about 3 feet long. So you get the idea.

Now, how does this all relate to engines you ask, well, quite simple really.

Your engine move air as well, not in the conventional way of a speaker, but closer then you might think.

As each individual intake valve opens, it sucks in a small volume of air, and then then is a small pause, and the next valve on the next cylinder opens, and it sucks in a volume of air. In that process, your engine has just ran a cycle. A frequency cycle. At low RPM, this process happens slowly, creating a greater delay in the intkae of the air. So the resonance frequency of the air is low. Thus, for a low RPM torque engine, a longer intake, with a lower tuning frequency in required for better efficiency.
However, on the, literal, oposite end of the spectrum, a high revving engine has a very high resonance frequency. As the valves are being opened and closed much faster. And the air is being taken in, in very fast short spirts. Giving it a higher resonance. Thus for a high reving, high output engine, a shorter intake is required for better efficiency.

So now that you see the similaritys, heres part of what may be happening. In a speaker, it reaches a point where it will stall in its enclosure. The amount of air it is moving, becomes equal to the amount of air contained in the port fo the box. And the speaker justs sits there making no noise, as the air becomes trapped withen the port.
If your car reaches a certain RPM point, and hits that exact resonance of yout intake, that causes the air to stall withen the intake, it will cause the same thing, becuase the engine isnt going to stop on a dime like a speaker will (dynamic and all) it will force the air out, and a *pop* will insue. So it might be worth it to do some math on your intake, and find the best possible length for it. ANd do some experimenting, and see if you cant modify it in such a way to keep it from happening as often as it does.

Unfortunatly, like speakers, there is no such thing as a perfectly tuned intake. Unless you can make a telescoping intake that adjusts its length with engine RPM (ive now just lost millions from givng that idea away), you wont have a perfectly tuned engine. There are just too many variables. So you do the best with what you got.
Of course there are exceptions to this. Add an air box/plenum into the mix and it gets all screwed up.

Anyway, sorry for the long disortation, i siad i was gonna be short, sorry.

Hopfully you can get the problem resolved. I dont think it will really harm anything, but im sure its annoying.
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Old 05-18-2001, 07:27 AM   #5
ba_feitl
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I knew Buck-O was good for something ... hehehe


Brad
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Old 05-18-2001, 07:50 AM   #6
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That was interesting and instructive. I'll admit though, that I got a little lost with the talk about speakers. But I think I get what's being said.

Brian
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Old 05-18-2001, 07:37 PM   #7
hayscoob
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Yikes!

I think I will learn to live with it. Of course, only till the STi WRX comes to town.
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Old 05-18-2001, 09:36 PM   #8
White Wolf
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Umm You said that you changed the plugs. One of the things that I have come across when changing the plugs is that if you dont get the plug wires on tight, they work loose and the engine will missfire and backfire. I would start to look at the things you changed. Did you put the intake on before or after plugs? If you did the intake first did it backfire with the old plugs? If you put the plugs in first did it backfire then or after the intake was installed. I would look at your last mod and then work backwords. Maybe your intake needs a little help.
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Old 05-18-2001, 10:24 PM   #9
Mr.Preza
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I used to get that intake backfire too...I have a Filter on the throttle body with the hood scoop opened. It used to do it alot actually when i used to "fluff" the gas. Now i got my exhaust and that backfires. I cant win.
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Old 05-19-2001, 12:00 AM   #10
hayscoob
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White wolf,

By the way, cool handle...

I put both mods on at the same time, so I can't trouble shoot by going backwards. Basically my intake is kind of a meld of Cobb's intake and Larry's intake. I use the factory torque box and a 3 inch tube running to a heat shielded filter with a fitting on the end to gather cold air from the hole in the side of the fender. I can't imagine what it could be. I will however double check the plug wires to ensure they are plugged in all of the way. Thanks for the tip.

Go here to view the intake before I put the cold air fitting on. As you will see, not to complicated. It should work just fine. What you think?
http://home.pacbell.net/jeepfvr/mod.html

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