RE: Backpressure: The Myth and Why It's Wrong.

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RE: Backpressure: The Myth and Why It's Wrong. - 5/24/2006 12:00:01 AM    sacicons Posts: 7699 Joined: 6/26/2004 Status: online look at the exhaust system on an F1 car. theres little to no BP there, and they can run at 14-18k rpms for hours on end. try and get someone to explain how backpressure could soften the blow of the valves. they would just use softer springs if that were the case. and what about the intake valves? _____________________________ Yes, there's a "Replacement for Displacement". Its called Volumetric Efficiency. quote: ORIGINAL: Forty04 I choose the rear ending (in reply to ArmedSalad) Post #: 41

RE: Backpressure: The Myth and Why It's Wrong. - 5/30/2006 10:29:21 AM    Impreza WRX Posts: 335 Joined: 1/6/2006 Status: offline Well, I recall a "seen it myself" example with a slant 6 Dodge Ram that was held together by improbability. The exhaust was gone, the only part being the header, and the engine spit fire wherever it went. Exhaust valves were not closing all the way anymore so the engine was barely making enough compression to run. This is the example I got. If there is a difference, or the problem is different, let me know! _____________________________ LoooooOOOOOOOOOZZZ-ERRRRRRRRRRRR!!!! (in reply to sacicons) Post #: 42

RE: Backpressure: The Myth and Why It's Wrong. - 5/30/2006 10:57:34 AM    riceburner700 Posts: 6518 Joined: 1/21/2005 From: Loveland CO Status: offline for a split second the intake and exhaust valves are open during crankshaft rotation _____________________________ non-vtec ALL THE WAY cylinder head guru! (in reply to Impreza WRX) Post #: 43

RE: Backpressure: The Myth and Why It's Wrong. - 5/30/2006 9:25:06 PM    sacicons Posts: 7699 Joined: 6/26/2004 Status: online or it had another problem. as far as i know (or care) there hasnt been a slant 6 made in a long time, and so I assume this was an old motor, and you sound as if it werent in the best of shape, so chances are it was mistreated in many other ways, and it could have been carbon buildup, bent valves from cam timing being off, or a bad valve job. or any number of other mechanical maladies. let alone that an engine with only a header will bend valves and crack valve seats easily because when you shut it off after a decent run, the head and valves are really hot, and cold air comes back up the header and shocks the metal enough to crack or warp it. you ever seen a frying pan that was put into cold water right after coming off of the stove? it messes them up bad, and itll do the same to the valves and heads. _____________________________ Yes, there's a "Replacement for Displacement". Its called Volumetric Efficiency. quote: ORIGINAL: Forty04 I choose the rear ending (in reply to riceburner700) Post #: 44

RE: Backpressure: The Myth and Why It's Wrong. - 6/1/2006 10:38:13 AM    Impreza WRX Posts: 335 Joined: 1/6/2006 Status: offline Good explanation. That answers that. (in reply to sacicons) Post #: 45

RE: Backpressure: The Myth and Why It's Wrong. - 6/13/2006 4:27:43 PM    Kahuna Posts: 68 Joined: 3/12/2006 Status: offline I plan on making a custom exhaust for my Wagon, and I was wondering what size I should use. All the aftermarket mufflers I have found have only a 2.25" inlet. I believe my stock pipe is 1.75". Would I be ok if I went up to a 2.25" pipe, or should I stick with stock size? Thanks _____________________________ -1990 Honda Civic Wagon Project Car- (in reply to Impreza WRX) Post #: 46

RE: Backpressure: The Myth and Why It's Wrong. - 6/16/2006 8:04:33 PM    96civic285whp Posts: 565 Joined: 4/20/2006 From: Middletown Status: offline Well in my honda i needed more backpressure, I turboed the car and when i did i took out the cat. I had a tial 38mm wastegate with a .6bar spring(8.7psi). So in all i had a 2.5" down pipe and staight through exhaust. When the boost came up, it shot to 9 psi, then creeped to 10,11,12,13. Nobody could figure out what was going on. Long story short, even with the wastegate wide open, the exhaust was travling through the turbo and out my exhaust causing the boost creep. So i put the baffle back in my exhaust tip and that gave me just enough backpressure to make the exhaust go out the wastegate instead through the turbo, no more boost creep. (in reply to elfreaky70363) Post #: 47

RE: Backpressure: The Myth and Why It's Wrong. - 6/16/2006 8:27:48 PM    pWnEdU Posts: 1194 Joined: 6/4/2006 From: Care-a-lot Status: offline Boost creep is a whole 'nother subject, it's a problem with big exhausts on turbo'd cars... I've experienced it a lot before but it's easily fixed. (in reply to 96civic285whp) Post #: 48

RE: Backpressure: The Myth and Why It's Wrong. - 6/17/2006 11:12:23 PM    joe383   Posts: 50 Joined: 6/9/2006 Status: offline Back in June of 2000 or so when I was in high school I remember reading a article called exhaust science in hot rod magazine and they stated that to efficiently scavenge your cylinders during valve overlap that the gas has to be traveling at .45 to .50 mock so about halve the speed of sound. I am not sure how to do the math for this equation but if some one could figure it out it would make picking out header pipe diameter easer. I would assume that you would want this to be at peek HP but I don’t remember that much just the speed stuck with me. _____________________________ 92 DX 1.5 4DR Cold air OBX intake manifold High flow cat DC header Mugen fuel rail Blaster ignition coil 17” rims with 205-40-17 tires Soon to have ported Z6 head swap, Edelbrock throttle body, crower cam, and SI transmission. (in reply to pWnEdU) Post #: 49

RE: Backpressure: The Myth and Why It's Wrong. - 7/2/2006 11:49:55 PM    WhiteCivic15   Posts: 15 Joined: 6/28/2006 From: Albany, NY Status: offline ive got a 94 hatch w/ the 16v 1.5, its got the header from the 1.6 v-tec on it and a catback system from there. i dont have hardly anypower until i hit 3000rpm (it basically hesitates), so i was wondering if putting the stock header back on would fix this problem. (in reply to sacicons) Post #: 50

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