Is there such a thing as too little backpressure ? How does one know how much is needed, if it's needed ? How would it be measured ? I am looking at getting rid of the crossover pipe on my Ford 292 and it has been suggested that I be careful not to have too little backpressure. Thanks, Mark
Back pressure is not what it's about. Scavenging is what it's all about. Making something breathe too easy may diminish the scavenging ability of the system and may reduce exhaust efficiency. This, in turn, may reduce power output. Some people translate that to a need for back pressure. It ain't necessarily so. It's not nearly that simple...
Not always. If your system is too large, you will cancel out the scavenging effect, leaving spent gasses in the combustion chamber, to contaminate the next intake charge, reducing combustion efficiency, and thus power output.
Here's a good example. A while back, I had a customer come in, complaining that the 350 that I built him was down on power, and that I must have built it poorly. I carefully observed the paint marks on the adjustment screws and distributor base, and saw no changes. I gave it a quick test drive. I noticed two things. It was down on power, and it had a totally different exhaust note. A quick check revealed that the customer had replaced the 2-1/4" dual exhaust system that I had carefully bent up and welded, with a 3" dual system. He thought bigger was better. He was wrong. A dual 3" system is the proper size for an engine making 740 horsepower. One expensive lesson learned.
Like most things, the answer is "it depends" on science and engineering. A dragster running wide open throttle at high revs is happy with open pipes. A 4x4 rock crawler running just above idle is happiest with narrow pipes. For regular driveability, you have to match your pipes to the displacement. Just like carburetors, bigger isn't always better.
Thank you, and also you have to take into consideration a lot of other things such as cam, induction, and on and on. It's not a simple thing. But suffice to say a well thought out set up with power all through the rpm range will usually have a correctly sized exhaust and a modicum of back pressure to optimize the scavenging of the cylinders.
"Less backpressure" is not the same thing as "larger exhuast." The comments above about scavenging of the combustion chamber and exhaust ports is correct. The level of scavenging going on depends upon several factors, only one of which is the size of the exhaust runners and the rest of the exhaust system. This is primarily for a N/A application. Turbo and supercharged engines generally do not have the same concerns and larger is better in those cases.
Good, I am glad we have some thinkers here - I am very tired of having the "Backpressure" argument with people who know everything. Scavenging is what it's all about, empty the cylinders, help to fill the cylinders and make lots of torque. Keep the velocity of the gases up and you will avoid reversion. Simple. Primary and secondary tube size and length is more related to horsepower output per cylinder than displacement however. Example, a 550 hp 340 inch small block that makes its power between 5000 and 7500 rpm will use a bigger primary tube diameter (1 7/8" or 2") in a shorter length (32", maybe shorter) than a 400 hp 440 inch big block that makes power between 1500 and 5000 rpm (1 3/4" at 36" long). The 340 would like a 3" exhaust system while the 440 would be happiest with 2 1/4" or 2 1/2" pipes.
Ok here's the question again, in case you guys forgot. What exactly do you mean by cross over pipe : Would that be the Y pipe joining both sides into one for a single exhaust? Or would that be a H pipe that ties both sides together?
Not so simple, for it to function properly you don't want to much to where you pass raw fuel through the cylinder during overlap.
Yea the y-block crossover pipe is pretty hogus bogus... not sure what the Ford engineers were thinking on that one.
Though I can not claim any of the above answers are "wrong", I do believe that some are trying to take a simple situation and think it to death. A 292 Y block with the stock left side to right side to exhaust pipe type system will absolutely run a bit better and sound a bunch better with dual exhaust and glass packs. Yes. Do it. Don't look back. Eliminate the cross over and run duals with free flowing mufflers. You do not however need any part of the system to be larger than 2", and 1 3/4" would be enough. Exhaust theory and "perfect scavaging" science is pretty much wasted on a stock 292 cube Y block in a pick up truck.
More fuel\ for the fire ! http://www.bgsoflex.com/bestheader.html http://www.flowmastermufflers.com/index.php?page=54 .
The engine as it sits now is completely stock and very tired. I can see that the existing exhaust would be restrictive, I was wondering if the restriction was by design. I plan on building another engine with mild performance parts, adding maybe 50-75 hp. The plan is to use long tube headers with 30" glasspacs. The question is do I go with 1 1/2", 1 3/4" or 2" pipe out the back. Thanks, Mark
When "most" people speak of a cross-over, they are referring to the "H-pipe" or even "X-pipe" arrangement under the car, not something block-specific under the hood (like the Y-block pictured).
They were thinking on an otherwise slow Thursday afternoon: "Corporate wants us to get a Y-Block in the F100 before the weekend, what do we do?!"
thats in a pick up, the 292 in my Galaxie doesn't have that set up. The downpipes join somewhere around the tranmission and run a single pipe out to the rear
That's what I said to her, " do it like this baby, you will get lots modicum" http://m.dictionary.com/d/?q=modicum&o=0&l=dir
I do trust you, your advice has been most helpful. I know what to do, just trying to better understand why I'm doing it, so I can do it as well as a good carpenter can.
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