LT1 and Backpressure?
04-12-2004, 05:19 PM
#16
Registered User
Join Date: Mar 2003
Location: Pelzer, SC
Posts: 1,411
I started off with stock manifolds, cats, and a Flowmaster 80's series. I now have SLP shorties, no cats and Bullets with turndowns. The car is faster, and I am completely satisfied. Backpressure doesn't mean jack squat to me.
04-12-2004, 07:00 PM
#17
Registered User
Join Date: Aug 2000
Location: North Vancouver, BC
Posts: 4,305
As a proud owner of an Electric cut-out 
I can tell you something, capped I go through a full exhaust with STOCK MUFFLER, uncapped it goes from FLP Long Tube headers straight out into the calm world LOL.
Prognosis? I notice no bottom end power loss with cut-out open as apposed to it being closed, but I notice the car is 10X slower when I gun it with cut-out closed since its going through a stock muffler, but its quite as a bee with cut-out closed
Now, with big cams you don't want ANY backpressure IMHO because by creating backpressure you promote REVERSION inside the intake because of the huge durration & tight overlap, so you want to get as LITTLE reversion possible, you can help it by many means one being scavenging, no restriction boys. Think about it.
BTW cut-out closed I run 109-10mph at the track with all my mods & uncapped I run 116-117 mph.
Matt.
I can tell you something, capped I go through a full exhaust with STOCK MUFFLER, uncapped it goes from FLP Long Tube headers straight out into the calm world LOL. Prognosis? I notice no bottom end power loss with cut-out open as apposed to it being closed, but I notice the car is 10X slower when I gun it with cut-out closed since its going through a stock muffler, but its quite as a bee with cut-out closed
Now, with big cams you don't want ANY backpressure IMHO because by creating backpressure you promote REVERSION inside the intake because of the huge durration & tight overlap, so you want to get as LITTLE reversion possible, you can help it by many means one being scavenging, no restriction boys. Think about it.
BTW cut-out closed I run 109-10mph at the track with all my mods & uncapped I run 116-117 mph.
Matt.
04-12-2004, 08:21 PM
#20
Registered User
Join Date: Mar 2004
Location: New Milford : North Jersey
Posts: 315
what's up - everyone on this thread knows quite a bit - well more than me at least - i went from having a flowmaster put on my 93 z28.....then eventually getting a high flow cat...then eventually gutting my high flo cat...then eventually gettin a cut-out installed.....definatly less backpressure but i guess there is always some back pressure if you still have stock manifold n all that....point im trying to make...i just had a supercharger installed and i was driving on a highway by me and floored it coming out the toll.....my car was backfiring like crazy from the amount of exaust coming out the cutout and i got a call later from my friend telling me how i shot a huge flame out the driver side exhaust pipe......apparently because i had no (not enough) backpressure with the cut-out......so i plugged the cut-out and it definatly runs much smoother now and doesnt backfire when i get on it......again, i only shot a flame b/c my supercharger is putting out so much more exhaust - i never shot a flame out before...... just thought i'd share my story since it is related to what everyone is talking about...later everyone 
Giovanni
Giovanni
04-12-2004, 09:14 PM
#22
Registered User
Join Date: Mar 2004
Location: New Milford : North Jersey
Posts: 315
who said i didnt know anything about cars ? i kno enough to have my car running perfect at 8lb boost, so i guess i kno enough - i did some research though before the charger install to make sure i knew what i was getting into but all is fine right now
06-02-2006, 10:02 AM
#23
Registered User
Join Date: Jun 2006
Posts: 4
Re: LT1 and Backpressure?
you also have to rememberthat if this is a daily driver not having any back pressure will slowly distroy your rings which is a pain in the *** to fix and if you ask me Borla makes a great cat back exhaust then I would get a high flow cat to replace the stock one.
06-02-2006, 10:12 AM
#24
Registered User
Join Date: Jul 2001
Location: Kingsport, TN
Posts: 1,889
Re: LT1 and Backpressure?
A little quote by a man a lot smarter than all of us put together:
"If you have ever heard that an engine needs a little backpressure, you might want to ask yourself why an engine would want an exhuast system that literally pushes exhaust back into the combustion chamber rather than sucking it out. The simple answer is, it doesn't."
"If you have ever heard that an engine needs a little backpressure, you might want to ask yourself why an engine would want an exhuast system that literally pushes exhaust back into the combustion chamber rather than sucking it out. The simple answer is, it doesn't."
06-02-2006, 10:24 AM
#25
Registered User
Join Date: Sep 2005
Location: Bahrain
Posts: 2,271
Re: LT1 and Backpressure?
here is something i read
Backpressure myth (interesting)
Destroying a myth.
Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.
Backpressure myth (interesting)
Destroying a myth.
Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.
06-02-2006, 11:39 PM
#26
Registered User
Join Date: Jan 2005
Posts: 69
Re: LT1 and Backpressure?
BACKPRESSURE (by itself) is BAD for power production. So are the other forms of airflow impediments in your system. The rules don't suddenly change at the exhaust valve.
Keeping gas velocity high at the port (through the use of smaller header primaries) improves VE/torque through inertial scavenging. (Moving column of gas in primary keeps moving, and creates a low-pressure area that helps scavenge the cylinder and draw in air/fuel mixture during overlap.)
Smaller primaries result in a little more backpressure (This is where the myth comes from), but as long as the VE increase generates more torque than the backpressure saps, you are good-to-go.
Note that as RPM rises, losses to backpressure increase...so you end up with a lower-end RPM window where you see an improvement. As mentioned by others, this is generally desirable for a street-driven car.
Keeping gas velocity high at the port (through the use of smaller header primaries) improves VE/torque through inertial scavenging. (Moving column of gas in primary keeps moving, and creates a low-pressure area that helps scavenge the cylinder and draw in air/fuel mixture during overlap.)
Smaller primaries result in a little more backpressure (This is where the myth comes from), but as long as the VE increase generates more torque than the backpressure saps, you are good-to-go.
Note that as RPM rises, losses to backpressure increase...so you end up with a lower-end RPM window where you see an improvement. As mentioned by others, this is generally desirable for a street-driven car.
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