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#1
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flow bench measurements
I was just wondering... what makes an exhaust pipe rated for say 500 cfm? We measure carburetors, cylinder heads, and exhaust pipes by CFM. I feel like if there is enough pressure gradient, the flow could go up to sonic speeds provided a strong enough pressure gradient. Is there an industry standard pressure gradient a flow bench uses to get its numbers?
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#2
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You are correct. Different test pressures equal different CFM measurements on the same orifice being tested. There is no specific standard. Most cylinder heads are tested at 28" Hg, but carbs and such are rated different from manufacture to manufacture.
Test pressures need to be the same to compare apples to apples. |
#3
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That is what I was thinking... 28" is approximately 1 ATM. I figure that anything beyond 28" would be parasitic drag to the engine, but would get pumped through.
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#4
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Quote:
I think that 28" Hg is 28 inches of mercury vacuum (mercury in a tall tube). Pistons pull air through the carb, intake and heads, by creating a vacuum when they are moving down in the cylinder with the intake valve open and the exhaust valve closed. 30" Hg is a perfect vacuum, which is not achievable to my knowledge. By the way, I know of a man who was exposed to an almost perfect vacuum, and he was pulled through grating that had a pattern of openings only 2 inches square. Wasn't pretty! |
#5
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How about exhaust pipe flow rates?
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#6
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Performance mufflers have a CFM rating. The muffler will be the limiting flow mechanism in the exhaust pipe. In general, you'll need about a 2.5 inch dual exhaust setup to achieve around 500 hp I would guess. If you want to make up near 600+ hp, you better have a dual 3 inch exhaust system (and header pipes at least 2 inch in dia). For a 2.5 inch or 3 inch system, the muffler will be limiting. I think the walker super turbos on my 3 inch system are rated at around 400 CFM each.
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#7
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Wow, X-pipe gives 24 hp increase. I gotta get this setup!
http://www.allpar.com/history/mopar/x-pipes.html |
#8
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I've always considered the exhaust has been neglected to some extent in performance design...
With the heat of combustion, there's really more CF to be disposed of per M than is pumped into the engine. Of course, the cooling process as it goes down the pipe will reduce that volume to some degree. |
#9
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Quote:
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#10
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Actually exhaust systems are really a neglected part of our vehicles. Even the standard header design could really be better, however the perfect exhaust is virtually impossible to fit in a standard car. Heck just compare the power numbers from standard headers to 180 degree headers, it is amazing the difference. However 180 degree headers are a wicked pain in the butt unless you have a custom chassis, with no passenger compartment...
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#11
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180's
A friend way back in the sixties had a Charger with a 440. THat car was very quick for a stocker. Odd how 180 headers make a V8 engine sound like a six cylinder.
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#12
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A 2.5 inch or 3 inch dual exhaust should not be limiting as far as exhaust flow, but the associated muffler would be limiting. But as with the X-pipe design mentioned above, exhaust design can have beneficial results. Muffler design is also a factor, with such issues as: does the muffler design create resonant flow conditions, which helps with power. Bottom line is: Just have a free flowing exhaust system to maximize your power. Keep in mind, you can have free flowing everything, but if your air filter is restrictive, that can hurt your power output as well.
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#13
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Also the farther back the muffler is, the better off it will be because the farther back it is, the cooler the air and it doesn't take near as much..
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#14
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Quote:
Specifically, if a muffler is rated for 600 CFM, is that derived from a mass flow rate based on a one atmosphere pressure gradient, or just for the application's suggested engine displacement and rpm power band? |
#15
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Probably an industry standard differential pressure for determining CFM in a muffler, but I don't know what the delta-P is that they use. I doubt exhaust size affects throttle response. Exhaust piping diameter should only become limiting at high rpms anyway. I think the thumbrule of 2.5 inch dual system for a 500 hp engine, and 3 inch dual system for 600hp, while also using a performance muffler, is the way to go. Warning, you get above 2.5 inch, and your ears are gonna suffer a little. I use ear plugs. Whether 2.5 inch, or 3 inch, and whether a performance muffler or more like stock, we're probably only talking about a 25 hp diff at 6000 rpm anyway, and most of us would be in jail if we had our cars on the street at 6000 rpm for very long!
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#16
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Le Mans cars back in the '60's had their engines breathing back to front, inlet at the bottom on each side and exhaust at the top coming out of the valley that way 180* crossover was easier to accomplish.
Manufacturers place the mufflers as close as they can to the front to keep them hot so they don't corrode or rust out. Car owners would get upset if they were buying a new muffler 3 & 4 times a year. That is also why Cat's are right at the front so the heat helps the pollution process. Some manufacturers have experimented with stainless steel mufflers that way they can put them further back without the corrosion problems. We could all just run zoomies like the dragsters but the noise on the interstate would get a little loud. Two stroke exhausts run expansion chambers and taper off, some outlets are smaller than the exhaust port exit. |
#17
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I believe you loose the scavenging effects at the low end region.
On an aside, I got picked up for subs. If I remember correctly, your an old academy nuke as well??? |
#18
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Quote:
Like you say, it was essentially so the crossover could be achieved for the exhausts, but not in every case. I don't think Coventry Climax went that way, but they also went to a flat plane crank (as most or all did...) and so it wasn't necessary, so some of those centre exhausts don't cross over either. V12s similarly didn't need it. I can't think of many Le Mans cars. The Healey XR37 had both BRM and Repco-Brabham engines at different times. |
#19
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Quote:
7 fun-filled patrols!! Where and what were you on? |
#20
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Yeah thanks for that Ray, my mind wasn't with it last night but I knew there were more and I knew you would fill in the blanks.
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#21
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Not only that, I put in the ° symbol for you...
Don't know why you count on me like this though. |
#22
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I graduate a year from now, will do one year at NPS, and hopefully end up on one of the new SSGNs after the pipeline. You can now interview spring 2/C year for subs.
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#23
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Quote:
Junior year was tough! Didn't enjoy double E, classes were tougher, and so many girls to distract! |
#24
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Like these?
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#25
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Gee, I always wanted to visit Australia!!!
Lets see, those, or an aluminum Hemi? Decisions, decisions! Must be Ray Bell groupies!! |
#26
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Not sure where these were from, but I'd bet it wasn't my homeland...
I simply googled 'college girls' and picked on a likely photo to attract attention. I daresay you'd be ogling too much to bother responding if I'd posted a pic of our bronzed Aussie girls. Hemi V8 or Hemi 6? |
#27
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Quote:
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#28
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Yeah, they go nuts over the accents...
Just like we go nuts over the accents of Kiwi women and American women go nuts over our accents. The good bit is that there's more there than here! |
#29
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Must be why the GEICO lizard is so popular!
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