blue flames?
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Senior User
Joined: Jun 2005
Posts: 173
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From: maryland
blue flames?
friend of mine has a 95 f250 with a 351 and 5 speed. has true duals with glasspacks. if he gets on it real hard in 2nd gear and lets off at night, it will shoot blue flames about 1 foot past the tip. but i have 3 or 4 other friends that do it too. buts its only the stick shifts that shoot flames. why do stick shift 351s shoot flames when u get on it hard and let off?
Postmaster
Joined: Oct 2006
Posts: 4,816
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From: Davis & Newcastle, CA
My best guess is that the high RPMs are pushing excess fuel into the combustion chamber, while the exhaust pressure builds in the pipes, and when they let back off of the gas the high exhaust pressure (rich with fuel) equalizes by blowing the built up pressure out of the pipes. Where is the ignition coming from? My best guess is that the exhaust temperature is so high that the fuel rich exhaust fumes are able to combust. This is all speculation.
-Chris
-Chris
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Senior User
Joined: Jun 2005
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From: maryland
the ignition is stock. everything is stock...but it seems all my friends fords that do it is only manuals. and 2.5 inch true duals with glasspakcs and 4 inch tips...i dont see how the egt would get too high...because it didnt do it with stock exhuast...he even put the stock exhuast back on to go thru inspection and now it doesnt do it. He was wondering so i figured id ask
Senior User
Joined: Dec 2006
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Timing is overlapping just enough to allow the exhaust valve to be open while the fire is still occuring. Unburned fuel in the exhaust is firing up. Old timers used to run a plug wire from the dist. to the exhaust tip where a spark plug was fitted. The unburned fuel from the cyl not being used would ignite out the pipe.
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Joined: Dec 2006
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It could be that your buddy is revving high enough to float valves. Dont rev so high or install stiffer valve springs. Or perhaps your buddies springs are not up to par. When the valves float the ignited fuel is exhausted. If the springs are weak the valves will float at a lower rpm
Fleet Mechanic
Joined: Mar 2005
Posts: 1,603
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From: St Louis
Something else to ask is if the Cats are in good condition or even in place. They may be overloading during the manual transmission reving up where the auto trans engines do not over load the catylitic converter. The newer trucks did have air injection as well and the valve float may be just enough to cause the flame front to continue down the exhaust all the way out the tail pipe. Some of the newer ford diesels had a recall because the filter to clean the soot out of the exhaust would plug up and cause a flame out the tail pipe.
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Senior User
Joined: Jun 2005
Posts: 173
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From: maryland
like i said in the original post, it has true duals with glasspacks. no cats. he can go to 4k and let off and it will do it. but all the manual 351s iv seen do it. his truck doesnt have air injection.
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Posting Guru
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From: Hawkeye Country
My 86 will pop blue flames, 351 with long tube headers with glasspacks and no cats, its an auto. It'll do it when I put it down into first gear manually and stand on till about 4500 rpms then let off. When its dark out I can watch the blue pops as it cackles and rumbles back down.
Also works if i shift it manually from first to second, standing on it till 4500 I shift out of first and wait a second before hitting second. Both my pipes stick out in front of my rear passenger side tire, if I'm running someone I'll let it pop flames at their car. If extremely loud exhaust straight in their window don't freak them out, then the fire will.
Also works if i shift it manually from first to second, standing on it till 4500 I shift out of first and wait a second before hitting second. Both my pipes stick out in front of my rear passenger side tire, if I'm running someone I'll let it pop flames at their car. If extremely loud exhaust straight in their window don't freak them out, then the fire will.
