Backpressure
10-13-2005, 02:23 PM
#41
!LS1 11 Second Club
Along with trusty old cfm list:
2 “ Straight Pipe 283 CFM
2 ¼ “ Straight Pipe 365 CFM
2 ½ “ Straight Pipe 521 CFM
2 ¼” Typical Bent tailpipe 268 CFM
2 ½” Typical Bent Tailpipe 417 CFM
2 ¼” Inlet/Outlet_ Glass Pack Tips- No Louvers- Smooth 274 CFM
2 ¼” Inlet/Outlet- Glass Pack Tips-Louvered 133 CFM
Same as above set for reverse flow 141 CFM
2 ¼” Cherry Bomb 239 CFM
2 ½” Cherry Bomb 294 CFM
2 ½” Inlet/Outlet Dynomax Super Turbo 278 CFM
2 ½” Inlet/Outlet Ultraflow Bullet 512 CFM
2 ½” Inlet/Outlet Gibson Superflow 267 CFM
2 ½” Inlet/Outlet Flowmaster ( 2 Chamber) 249 CFM
2 ½” Inlet Outlet Flowmaster ( 3 Chamber) 229 CFM
2 ¼” Inlet/Outlet Thrush CVX 260 CFM
2 ½” Inlet/Outlet Maremount Cherry Bomb 298 CFM
2 ½” Inlet/Outlet Hooker Aero Chamber 324 CFM
2 ½” Inlet/Outlet Hooker Max Flow 521 CFM
2 ½” Inlet/Outlet Borla Turbo 373 CFM
2 1/2" Inlet/Outlet Magnaflow 284 CFM
Standard OEM 2 ¼” Inlet/Outlet 138-152 CFM
Standard OEM 2 ½” Inlet/Outlet 161-197 CFM
2 “ Straight Pipe 283 CFM
2 ¼ “ Straight Pipe 365 CFM
2 ½ “ Straight Pipe 521 CFM
2 ¼” Typical Bent tailpipe 268 CFM
2 ½” Typical Bent Tailpipe 417 CFM
2 ¼” Inlet/Outlet_ Glass Pack Tips- No Louvers- Smooth 274 CFM
2 ¼” Inlet/Outlet- Glass Pack Tips-Louvered 133 CFM
Same as above set for reverse flow 141 CFM
2 ¼” Cherry Bomb 239 CFM
2 ½” Cherry Bomb 294 CFM
2 ½” Inlet/Outlet Dynomax Super Turbo 278 CFM
2 ½” Inlet/Outlet Ultraflow Bullet 512 CFM
2 ½” Inlet/Outlet Gibson Superflow 267 CFM
2 ½” Inlet/Outlet Flowmaster ( 2 Chamber) 249 CFM
2 ½” Inlet Outlet Flowmaster ( 3 Chamber) 229 CFM
2 ¼” Inlet/Outlet Thrush CVX 260 CFM
2 ½” Inlet/Outlet Maremount Cherry Bomb 298 CFM
2 ½” Inlet/Outlet Hooker Aero Chamber 324 CFM
2 ½” Inlet/Outlet Hooker Max Flow 521 CFM
2 ½” Inlet/Outlet Borla Turbo 373 CFM
2 1/2" Inlet/Outlet Magnaflow 284 CFM
Standard OEM 2 ¼” Inlet/Outlet 138-152 CFM
Standard OEM 2 ½” Inlet/Outlet 161-197 CFM
10-13-2005, 02:54 PM
#42
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This thread made me lol.
If you don't know much about cars, just find the best trap times for a car with similar mods and see what he is running for exhaust.
If you want to get technical, think of your engine as an air pump (because that is what it is). You torque (which makes horsepower) is based on how much (volume*density) air you can fill in your cylinder. You want the exhaust gas out of your cylinder as quickly as possible. Hot air in the cylinder is less dense, so you want all of the exhaust gas out. At lower rpms back pressure can actually keep hot exhaust gas in the cylinder. This is in no way benificial (except for emmissions).
The amount of exhuast flow you need is directly correlated to the amount of air you have coming in. At some point there is no more air to push out, and if you get that far, exhaust velocity will slow down and stall (which is why SOME people might lose low end power with a cutout), which is also not good because you have that exhaust gas just lounging around sippin on coffee waiting for the next cycle.
Back pressure means nothing, the only gains from running a more restrictive exhaust is if your intake sux ***** and you need to keep the air moving.
So a good rule of thumb is with a pretty stock engine, 2.5" y-pipe setup w/ muffler is fine. Running longtubes, 3" orp and a cutout on an otherwise stock engine you will probably not gain anything down low.
Also a good rule of thumb, if you lose power opening a cutout your car sucks and you need to mod it some more.
If you don't know much about cars, just find the best trap times for a car with similar mods and see what he is running for exhaust.
If you want to get technical, think of your engine as an air pump (because that is what it is). You torque (which makes horsepower) is based on how much (volume*density) air you can fill in your cylinder. You want the exhaust gas out of your cylinder as quickly as possible. Hot air in the cylinder is less dense, so you want all of the exhaust gas out. At lower rpms back pressure can actually keep hot exhaust gas in the cylinder. This is in no way benificial (except for emmissions).
The amount of exhuast flow you need is directly correlated to the amount of air you have coming in. At some point there is no more air to push out, and if you get that far, exhaust velocity will slow down and stall (which is why SOME people might lose low end power with a cutout), which is also not good because you have that exhaust gas just lounging around sippin on coffee waiting for the next cycle.
Back pressure means nothing, the only gains from running a more restrictive exhaust is if your intake sux ***** and you need to keep the air moving.
So a good rule of thumb is with a pretty stock engine, 2.5" y-pipe setup w/ muffler is fine. Running longtubes, 3" orp and a cutout on an otherwise stock engine you will probably not gain anything down low.
Also a good rule of thumb, if you lose power opening a cutout your car sucks and you need to mod it some more.
10-13-2005, 10:29 PM
#44
TECH Resident
Originally Posted by 300bhp/ton
easy way. 310rwhp is going to be circa 350bhp at the engine.
so 2.2 x 350 = 770cfm flow rate.
Now lets assume you'll add a few more mods and get it upto ~400bhp, thats about full bolt on for an Fbody.
400 x 2.2 = 880cfm
You don't want to go too big, but allow room for future mods.
Or an easier way.
3" Y setup for 500bhp or 2.5" dual.
So you should know what size tubing you have and whether its a Y or dual.
Find out what muffler you have and speak to the manufacture and ask them what it's flow rate is in cfm, or more precisly scfm (use Google if you want to know more about the rating).
If it's a Y setup, then all the gases go thru 1 silencer so it must flow at the minimal of the total flow rate. Here 880cfm.
Lots of people make silencers which are good enough, Borla, dynomax my fav Powerstick. But remember not EVERY muffler from one company flows the same.
The Flowmasters people complain about only flow under 300cfm (if memory serves) which is why they rob power. A Hooker aero chambers (from the cat back) is good for upto 375bhp, so only a few bolt ons will max it out, with a Y setup.
Duals are different, as each bank only has to contend with 4 cylinders, so simply half the required flow capacity. In this case 440cfm would be sufficent.
This is why the same Flowmaster work fine on Mustangs, they have duals. So the muffler only deals with gases from 4 cylinders and Mustangs generall produce 230-290rwhp, so less than your typical Fbody.
If you have too much flow from a muffler it will just make the exhaust louder and yield no extra power.
A 3" Powerstick flows around 2000scfm (YES it is that good), so these can be used in a Y pipe no problem. In a dual setup it would be wasted potential and you'd be better off with the 1000scfm 2.5" muffler. As it wouldn't loss any power on a 500bhp motor, but would more affectivley muffle the sound.
so 2.2 x 350 = 770cfm flow rate.
Now lets assume you'll add a few more mods and get it upto ~400bhp, thats about full bolt on for an Fbody.
400 x 2.2 = 880cfm
You don't want to go too big, but allow room for future mods.
Or an easier way.
3" Y setup for 500bhp or 2.5" dual.
So you should know what size tubing you have and whether its a Y or dual.
Find out what muffler you have and speak to the manufacture and ask them what it's flow rate is in cfm, or more precisly scfm (use Google if you want to know more about the rating).
If it's a Y setup, then all the gases go thru 1 silencer so it must flow at the minimal of the total flow rate. Here 880cfm.
Lots of people make silencers which are good enough, Borla, dynomax my fav Powerstick. But remember not EVERY muffler from one company flows the same.
The Flowmasters people complain about only flow under 300cfm (if memory serves) which is why they rob power. A Hooker aero chambers (from the cat back) is good for upto 375bhp, so only a few bolt ons will max it out, with a Y setup.
Duals are different, as each bank only has to contend with 4 cylinders, so simply half the required flow capacity. In this case 440cfm would be sufficent.
This is why the same Flowmaster work fine on Mustangs, they have duals. So the muffler only deals with gases from 4 cylinders and Mustangs generall produce 230-290rwhp, so less than your typical Fbody.
If you have too much flow from a muffler it will just make the exhaust louder and yield no extra power.
A 3" Powerstick flows around 2000scfm (YES it is that good), so these can be used in a Y pipe no problem. In a dual setup it would be wasted potential and you'd be better off with the 1000scfm 2.5" muffler. As it wouldn't loss any power on a 500bhp motor, but would more affectivley muffle the sound.
10-14-2005, 01:00 AM
#45
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Engines don't need back pressure, they need scavenging. Scavenging results from exhaust exiting the combustion chamber at a high enough speed that it creates negative pressure in the chamber so that when the intake valve is open, the negative pressure pulls air/fuel in. On a mostly stock engine, open headers (or manifolds) can reduce velocity enough to reduce the scavenging effect, which can hender bottom end performance. At higher rpms, the exhaust velocity is high enough create the scavenging effect regardless of open headers. Modified engines with more compression, larger cams, NOS, etc move exhaust at a higher velocity by nature, and are less likely to lose scavenging at low RPM. The least amount of backpressure without losing the scavenging effect will make the most power.
GENERALLY, since this is sort of a black art, figure on roughly 2.2 cfm per horsepower required to make the most power (at peak, again, low RPM could be off due to the whole scavenging thing ).
2.5" Dynomax Ultra Flo Bullets flowed 1393 CFM on my flowbench.
3" flowed more than the flowbench, but the max we've seen before it pegs is 2200 CFM.
GENERALLY, since this is sort of a black art, figure on roughly 2.2 cfm per horsepower required to make the most power (at peak, again, low RPM could be off due to the whole scavenging thing ).
2.5" Dynomax Ultra Flo Bullets flowed 1393 CFM on my flowbench.
3" flowed more than the flowbench, but the max we've seen before it pegs is 2200 CFM.
10-14-2005, 04:09 AM
#46
TECH Addict
Originally Posted by seadoo
okay, so what about about 430rwhp (mild H/C)?? but I like low end trq. tire shreadding ya know. im still thinking 2.5" X-pipe
Going with 2.5" dual setup myself. Currently designing it first though.
10-14-2005, 05:23 AM
#47
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There is a definite science to it all. For the most power you want the appropriate diameter pipe for the amount of horsepower that you car has. I found this write up though:
We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.
As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.
Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game. For engines making 250 to 350 horsepower, the generally accepted pipe diameter is 3 to 3 ½ inches. Over that amount, you'd be best off going to 4 inches. If you have an engine making over 400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch exhaust. Ah, the drawbacks of horsepower. The best alternative here would probably be to just run open
exhaust!
We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.
As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.
Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game. For engines making 250 to 350 horsepower, the generally accepted pipe diameter is 3 to 3 ½ inches. Over that amount, you'd be best off going to 4 inches. If you have an engine making over 400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch exhaust. Ah, the drawbacks of horsepower. The best alternative here would probably be to just run open
exhaust!
10-14-2005, 02:18 PM
#48
TECH Resident
Originally Posted by 300bhp/ton
yep that'd do it just fine.
Going with 2.5" dual setup myself. Currently designing it first though.
Going with 2.5" dual setup myself. Currently designing it first though.
10-14-2005, 05:40 PM
#49
TECH Addict
Originally Posted by seadoo
thanks, can you give some muffler options please? im stuck on that too. I want mildly tame, but still pretty load, and nasty load at WOT
Personally, I'm going with these: Powerstick as the flow great, are small and compact and should offer the sound I'm after.
Stainless Works also do a similar muffler in stainless steel, but they are twice the price.
10-15-2005, 02:41 PM
#51
TECH Addict
Originally Posted by seadoo
does the short powerstick mufflers flow better than the longer ones?? same sound?
It will undoubtably affect flow but not by much, and the 2.5" muffler flows double whats required on a TD setup, and th 3" double on a Y. So it would be unlikely to affect performance.
10-18-2005, 10:39 PM
#54
Once, I ran open headers on my 3rd Gen formula and it would run hot all the time (it blew the radiator up). Someone told me that running open headers emptied the combustion chambers too fast and my stock injectors couldn't keep up therefore causing a lean condition. I don't know what to say about that. I also ran open pipes on my F**d truck and it failed due to heat related issues (so I'm told by local mechanics). Does the timing need to be adjusted when running open headers?
10-18-2005, 10:44 PM
#55
11 Second Club
Originally Posted by team39763
Someone told me that running open headers emptied the combustion chambers too fast and my stock injectors couldn't keep up therefore causing a lean condition.
I've heard for awhile. 