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  #1  
Old 04-22-2010, 10:25 PM
bronco9588 bronco9588 is offline
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Default 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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Old 04-22-2010, 11:16 PM
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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.
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Old 04-22-2010, 11:52 PM
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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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Old 04-23-2010, 12:12 AM
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Quote:
Originally Posted by bronco9588 View Post
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.

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!
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Old 04-23-2010, 12:18 AM
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How about exhaust pipe flow rates?
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Old 04-23-2010, 01:00 AM
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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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Old 04-23-2010, 01:13 AM
cudabob496 cudabob496 is offline
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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
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Old 04-23-2010, 06:16 AM
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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.
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Old 04-23-2010, 06:44 AM
bronco9588 bronco9588 is offline
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Quote:
Originally Posted by cudabob496 View Post
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.
So the CFM for a muffler is where it should get the most power, but the system is not physically limited to such numbers?
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Old 04-23-2010, 09:51 AM
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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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Old 04-23-2010, 10:45 AM
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Default 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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Old 04-23-2010, 03:00 PM
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Quote:
Originally Posted by bronco9588 View Post
So the CFM for a muffler is where it should get the most power, but the system is not physically limited to such numbers?
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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Old 04-23-2010, 04:17 PM
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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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Old 04-24-2010, 07:21 PM
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Quote:
Originally Posted by cudabob496 View Post
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.
This certainly makes sense, but what about too much flow? I was under the impression that larger pipes gives you more power,but at the expense of throttle response.

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?
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Old 04-25-2010, 10:05 PM
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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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Old 04-26-2010, 09:46 AM
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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.
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Old 04-26-2010, 07:24 PM
bronco9588 bronco9588 is offline
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Quote:
Originally Posted by cudabob496 View Post
I doubt exhaust size affects throttle response.
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???
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Old 04-26-2010, 07:33 PM
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Quote:
Originally posted by Dart 360
Le Mans cars back in the '60s 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.....
Not only (a very few...) Le Mans cars, but also a number of Formula 1 cars. I can cite BRM (about '64 on), Repco-Brabham ('67) and Ferrari ('66 - '68 IIRC), but not Cosworth or most of the Ferrari engines, BRM went back to the conventional setup with their V12 and the Eagle V12 was always conventional.

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.
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Old 04-26-2010, 11:51 PM
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Quote:
Originally Posted by bronco9588 View Post
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???
Ya, Henry L Stimson Blue crew SSBN-655 out of Charleston, 1976 to 1980.
7 fun-filled patrols!!
Where and what were you on?
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Old 04-27-2010, 01:04 AM
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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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Old 04-27-2010, 05:11 AM
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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.
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Old 04-27-2010, 06:55 AM
bronco9588 bronco9588 is offline
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Quote:
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Ya, Henry L Stimson Blue crew SSBN-655 out of Charleston, 1976 to 1980.
7 fun-filled patrols!!
Where and what were you on?
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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Old 04-27-2010, 02:34 PM
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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.
Cool, wish you the best!
Junior year was tough! Didn't enjoy double E, classes were tougher, and so many girls to distract!
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Old 04-27-2010, 06:18 PM
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Like these?

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Old 04-28-2010, 12:20 AM
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Like these?

Gee, I always wanted to visit Australia!!!

Lets see, those, or an aluminum Hemi? Decisions, decisions!

Must be Ray Bell groupies!!
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Old 04-28-2010, 09:58 AM
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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?
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Old 04-28-2010, 05:44 PM
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Quote:
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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?
When I was in the Navy, I heard some great things about the Australian women! Of course, it took guys from the States to bring out their full potential!!
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Old 04-28-2010, 10:10 PM
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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!
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Old 04-29-2010, 12:15 AM
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Quote:
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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!
Must be why the GEICO lizard is so popular!
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