i am running 2 inch TRUE duals with walker highflow cats, i wouldnt think that there would be too little backpressure, but i have lost 5 mpg and i want them back without thowing away a 300 dollar exhaust

 

There's no such thing as too little backpressure. If you lost 5mpg that sounds like an increase in backpressure. Are the cats installed the right direction? Are you sure you didn't lose it because your foot's in it all the time?

 

did you reset computer after such a major change?

are the o2 sensors new?

is the o2 sensor wiring faulty/loose/crimped/burnt?

have you messed with your timing?

you do not give much info in your posts, like any other mods? when was last tuneup? etc.

 

The simple answer is yes, you can have too little backpressure. Without a lengthy dissertation I'll try to explain why. The factory exhaust tract, starting at the valve, is a series or very rough, misaligned surfaces with sharp turns and edges. As high speed/velocity gas passes these imperfections turbulence is created. The higher the speed the more the turbulence. Backpressure keeps these gas speeds low and stabilizes the flow at the boundary layer. Once the backpressure is removed or greatly reduced the high speed gasses can actually "see" more restriction through the port due to turbulence or boundary layer separation. This is why many stock engines loose low speed torque when the exhaust system is "opened up". The 300 straight six head was not designed with high gas speed/velocity in mind. It becomes obvious the first time you run your finger down the exhaust port. The intakes are just as bad. You can easily test this theory by installing restrictors in the ends of the exhaust pipes. Just my been there, done that, 2 cents. Best wishes.

 

where can i get restrictors at? by the way, thats my only mod, no tune up lately, no new oxy sensors

 

i would suggest leaving the restrictors off and fixing the other things listed.

reset computer.

 

Twinscrew, you are mistakingly mixing backpressure into an equation with many more variables. There is no doubt that the stock port is badly configured, this is the reason for the problems you are describing. Backpressure is a bad thing and no ammount of it creates anything desirable. Every bit of backpressure the engine must fight results in a bit of power being wasted. Is it possible to make an exhaust pipe too big? Of course it is, there is no doubt about it. However, excessive reduction in backpressure is not the reason a large pipe doesn't work as well in many applications.

 

Reason a vehicle with an opened up exhaust looses lower end power is the diameter of the pipe, not the loss of back pressure. as SS said, its not good.

Increasing to a pipe to large will slow down the exhaust gas velocity and lower the efficeny of the scavaging, nothing to do with backpressure. backpressure is often misunderstood and mistaken for other things.

if u have a stock exhaust of 2" and move up to 3" pipe u are severly decreasing the speed at which the exhaust gas can effectivly move and lowering the scavagability thus low end power loss. its all about keeping the exhaust system "tuned" for maximium flow and speed.

 

Streak and Optikal I totally agree with what you're saying. It all comes down to keeping velocities as high as possible. However pipe size is only one factor in an exhaust tract that is as poor as the 300 six's. Slight backpressure is actually benificial when there are excessive port to manifold misalignment, casting flash, and the horrific short side radius that Ford leaves in the 300 head and considers to be within manufacturing tolerance. If Turdbird were to port his head, and by that I only mean to blend the bowls, shape the guides, ease the short radius, and then port match the manifold to the head; then and only then would it simply come down to velocity and tuning with regard to pipe size. Well almost. Have you guys seen the insides of some of these exhaust manifolds? Some have casting flash that makes me cringe and enough scale to destroy the catalytic converter if it ever broke loose. I agree with Quicklook that the other variables should be ruled out. However, I firmly stand by my original statement. When it come to the "stock" 300 straight six there is such a thing as too little backpressure, unless of course, you're one and only throttle position is WFO. Turdbird do you happen to be running your truck at Bonneville?

 

i have a spare 300 head and EFI manifolds up my apartment ill have to check them out.

interesting concept, so ur saying the little imperfections in the casting and ridges in the head create unbalanced turbulance?

 

Yes, precisely. The flow of gas is highest when the boundary layer is the thinnest. This is the layer that adheres to the surface and tries to remain static. On a highly polished surface the thickness of the boundary layer is very thin over which a gas passes very easily. The rougher the surface the more drag imposed on the gas due to the boudary layer separating from the surface and allowing turbulence to form below it. The higher the speed of the gas the more it is lifted from the surface and the more turbulence and drag created below it. So quite simply, a 2" pipe with a smooth interior surface will flow more than a 2.5" pipe with a very rough interior surface once the gas speeds become high enough. In Turdbirds case I am suggesting that if the gas speed is allowed to become high enough through the very rough stock head and manifold, that the positive pressure gas will actually "see" a smaller/higher drag opening through which it can pass. So will the negative return wave which helps to evacuate the cylinder of exhaust. In the stock 300 some backpressure will actually allow more flow through the head.

 

so if i have less flow through the head (which i think your saying) why am i getting bad mileage?

 

Reduced volumetric efficiency. The engine is an air pump. The easier the air moves in and out of the engine the greater the VE. You say that the only modification you made was to install a very low restriction exhaust. I'm saying that you may have decreased your volumetric efficiency by creating more turbulence in the exhaust port. You may have to make other mods to get the VE back up and will then realize increased performance and mileage. This subject should be a sticky in the exhaust forum because it is commonly misunderstood.

 

While everything said about fluid science is scientifically correct, this is a fairly poor practical application for it. I think that if you were to calculate the Reynolds number for the exhaust gas (a task I wouldn't be excited about doing) you will find that the very nature of the gas creates more problems than the type of exhaust that is employed several inches (if not feet in some cases) away from the portion of the exhaust that is where the problem occurs. You are talking about backpressure (or more appropriately, lack of it) like it is the reason there is a problem and it isn't. The port design is the problem. The 300 EFI head is supposed to be a high velocity design, but manufacturing limitations have also limited how well it is allowed to work. There is also no evidence that a reduction in backpressure is a problem with this engine. However, if that is the case it would certainly explain why my engine is woefully lacking in power and torque with a dual 2.25" pipes feeding a single 3". I doubt there are many 300's out there with better exhaust systems than mine, and there are even fewer that are putting out the numbers mine is in a similar configuration.

 

OK - sorry to hijack this thread a little, but my stock 85 300 carbed engine... I am thinking about going to an EFI exhaust manifold. That would give me the opportunity to go true duals, but I would rather do a "y" pipe and then what size exhaust would be best from there out. Also, delete the cat? and what muffler?

My old manifold is cracked and I'm mostly looking at fuel and overall ecomony - I have a cheap source for the manifold and pipes. Muffler will be $ from my pocket.