GRNcamaro

i have read a few articles about exhaust back pressure today and was wondering is it fact or myth that a lack of back pressure will damage your engine. some say with new fuel injected cars its a myth where as carb cars it was true. the most common idea behind it seems that a lack of back pressure results in a lien engine which causes burn valves. but from some reading fuel injected cars dont suffer from this because computer compensate. so whats the truth?

myslowcamaro

cant find the thread or the author, but one of our top gear heads in the caliber of rkrause, injuneer, etc, wrote a post regarding this. wish i could find it, it was an excellent read.

anyway, i beleive at wot, zero backpressure on our fuel injected cars is the goal, and something was mentioned about having problems if the backpressure built up over 5 psi, that would damage the engine possibly.

many have dual cutouts acting like open headers, hell some down in alabama run open headers. while you still have some bp from the headers, its nothing like a full exhaust. i think no headers is what you'd have to worry about unseating the valves or whatever.


edit-not the one i was looking for but, injuneer, 3rd post down
http://web.camaross.com/forums/showt...t=backpressure

GRNcamaro

thanks slowcamaro i really apprciate it.

sandman63

Yeah i've heard Injuneer say that multiple times.

Demus

It's a total myth.

With less back pressure, the engine will draw more air in due to a scavenging effect. This much is not disputed. Silly people will assume more air, means the mixture will get leaner. It will not.

In fuel injected applications, the MAF very accurately measures how much air is being used by the engine. This allows the computer to calculate how much fuel must be added to reach the desired AFR.

In a carb'd application, if the engine sucks in more air we're going to see more vacuum. With more vacuum, we will get more fuel.

It's as simple as that.

LiENUS

The reason people believe back pressure to be necessary is two things.
1) If you ran open manifolds you would end up with burnt valves. My understanding of the cause of this is that the fresh air mixes with the unburnt fuel and is close enough that it will reignite the fuel a second time in a more uncontrolled fashion and hit the exhaust valve while it is unseated, the exhaust valve must be seated to properly cool itself otherwise it can be damaged.
2) people have found in some cases that if they switch from a large exhaust pipe to a smaller exhaust pipe they end up with more low end power, and in some extreme cases they can also end up with an increase of peak power. This is caused by velocity, a smaller pipe builds velocity faster which helps suck burnt fuel out of the cylinder whilst a large pipe doesn't build velocity as well until l you are further up the rpm range, and in the case of a smaller engine with a very large pipe (think hondas with fart pipes only the pipe is that big all the way to the engine) you end up losing some peak power because you never reach a point at which you are actually building up velocity in the exhaust system.

Injuneer

With an exhaust leak close to the valve or no exhaust on the port, the exhaust valve "burns" because it is hot enough for the iron in the valve to start to combine with the high O2 concentration.

You build scavenging velocity with a small diameter primary tube, not a small diameter exhaust pipe. Scavenging can be aided by reducing the diameter of just the first 2 or 3 inches of the primary tube.... i.e. - "stepped" primaries. Once you get past the primary tube, you don't want pressure drop, that can be caused by high velocity.

There's a good explanation of why backpressure is a myth in the David Vizzard book, How To build Horsepower.

usafcrewchief01

so true

rskrause

I really like Vizards writing, so I second Fred on that.

Just one comment: carbs do not respond to more intake vacuum with more fuel. Think about it. If they did, the car would draw max fuel with the throttle closed. The fuel metered by a carb is primarily a function of flow thorough the venturi. More flow creates more vacuum in the venturi and more fuel is metered. But manifold vacuum is low under these conditions (open throttle). A larger carb does not change manifold vacuum at idle. But because it has bigger throttle bores, there is less velocity through the venturi and it becomes difficult to get it operating properly at idle and light throttle, i.e. when vacuum is high. The reason carbs have idle jets is that at idle, there is very little air flow and hence little vacuum in the venturi, but the manifold vacuum is high. The idle jet is just behind the throttle plate, so fuel is metered in response to manifold vacuum through the idle jets, but not the main jets.

Rich

GRNcamaro

im gona have to stop by the book store today and see if they had that book becuase it sounds like a great read and that i can learn alot from it.

GreenDemon

Backpressure (as a torque increaser) is a myth. Exhaust scavenging and velocity are not. And btw when I was at UTI I had an instructor who had run open headed (yes headed) engines without any negative effects. Obviously it was not a quest for more power but just an experiment to see what would happen. Not sure how long they ran them for, but it would seem that on a running engine there would be too much flow for fresh air to reach the valve, not to mention the oxygen rich intake charge on the other side of the valve. I would tend to think that most burned valve problems would come from the fact that the engine probably isn't running properly when you remove the entire exhaust. Just my thoughts, anyway.