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Old 11-08-2003, 11:36   #1 (permalink)
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backpressure and hurt valves

ok, i was talking to mone of my friends today about my exhaust. normally i run the stock exhaust manifolds thru the stock cat and then into a 3" flowmaster after-cat('94 F-350 w/7.5LV8). lately i have just been running the cat and no muffler. my friend was telling me that if i dont run a muffler i run the risk of hurting my valves due to a loss of backpressure. i personally think that just running a cat alone will produce enough backpressure as to not hurt the engine. i have never heard of this being a problem. is my friend right? or am i right? or are we both wrong. thanks for the help.
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Old 02-06-2005, 15:39   #2 (permalink)
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Re: backpressure and hurt valves

i think that your prob right, i have a 77 ford f250 highboy with a carburated 400M in it, i have been runing about 2' of exhaust past each manifold for about 2 years now and the motor runs just fine, but the electrical, thats another story. if you or any of your friends know anything about electrical stuff you can go to the forum under interior/electrical forum entitled "please help me with my wiring nightmare!"
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Old 02-19-2005, 15:58   #3 (permalink)
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Re: backpressure and hurt valves

ive heard you have to have backpressure on your exhaust or you burn up your valves. but i never knew why. can any one explain that to me? and if you do explain the term open headers, and how that is better?
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Old 03-12-2005, 14:31   #4 (permalink)
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Re: backpressure and hurt valves

i asked my auto teacher about the backpressure thing and he said you dont want any more than 1lb back pressure, and the less the better on a 4 cycle. but on a 2 cycle thats another story. the only thing to worry about is you need to keep cold air from getting to your valves after you shut your motor off. like a long exhaust pipe , a cat, glasspack, mufler, anything to keep the cold air off the hot valves or they can warp or crack
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Old 03-18-2005, 19:37   #5 (permalink)
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Re: backpressure and hurt valves

I'm not sure about the new big blocks, but I know that newer, emissions SBF's need a fair amount of back pressure to run right. I have an '86 F150 4x4 w/302 and I took the rear cat off, hollowed out the front one and just ran glass packs. The truck ran like complete crap. I took the glass packs off and put on a Flowmaster 40 series Deltaflow muffler and it ran a lot better. I did some research into that and found out that the cam timing of the emissions engines needed some back pressure to operate correctly. It doesn't make any sense to me, but I guess I'll believe them. But the valves won't get hurt if you run a full exhaust plus a cat. Look at race engines that run open headers. A lot of those engines run a season or more before being pulled apart and freshened up and they dont have any valve problems. I'd say if you ran just the stock manifolds, then I'd worry, but you have nothing to worry about. Just my opinion.
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