Hi guys, about to buy some headers for my 80 F-150 4x4 351M 2 barrel, and some people say yah its a mith, others say no, you need backpressure or youll blow your motor. But if thats true, why do the Monster trucks run 20,000 Dollar motors with open headers?? So it must be a mith right, and what about the guys with straight pipes, they get more power to right?

Taylor

 

You definately will not blow your engine. It is a myth. Backpressure is restriction, nothing more.

 

They build motors for the monsters to run like that . The Factory ford don't .

 

so it would not work for my motor, is that what your sayin lostin, if it wont, will it blow my motor, damage to a certain extent where i can not drive it, and or have to rebuild the heads/piston

 

Backpressure is real. It is caused by a restriction, or restrictive exhaust system. Some are more restrictive than others.

What you have to watch out for in changing the exhaust system on a carburated engine is the fuel mixture. Remember that gas flow starts at the carb and continues thru the exhaust system. Exhaust flow also affects intake flow. If the carb is jetted at a certain level of tune for intake/exhaust flow, and you open the exhuast to a higher flow rate, it could cause a lean condition in the fuel mixture. This is what burns valves, pistons, and causes detonation, pre-ignition, and engine damage. It is not the fact that you have reduced the backpressure, as this is what you want to do to achieve better exhaust flow, but the fact that you changed the system tune in doing so. You may have to re-jet the carb to compensate.

You do not have these problems on computer controlled engines as the O2 sensors see the changes and correct the mixture automatically. jd

 

Backpressure is not a myth...but you wont blow your motor.

I cut the muffler off of my F150, and noticed a severe loss of torque immediately. However...I noticed a pretty good gain on the top-end.

 

Yeah Back Pressure is real, but it comes from something stopped up. [1982] Case in point 1976 Dodge charger wouldn't run but about 40-45mph on the interstate. got it home pulled the Cat. and ran a rod thru it and cleaned it out, then she was wide open again. Now that's some back presure when the Cat. gets stopped up. I have seen warped valve stems from running open exhaust on stock engines tho.

 

Maybe a little more detail would be helpful - since the headers will most likely improve exhaust flow, they typically will lean out a carburated engine somewhat. If the engine is set up w/ timing on the edge and the mixture gets leaned out, you can destroy the engine - so its not like that's impossible. However, if you pay attention to what's going on and make sure the carb is jetted properly for the setup you have, and that the timing is proper, you will be fine.

You can expect to lose torque and power at the lower rpm range and gain torque and power at the high rpm range. Just how much of each will depend on how much change in airflow there is to the engine.

The whole exhaust thing is art as well as science, since there are a lot of factors that go into tuning a compressible flow system - length, diameter, shape are all important, and depending on your desired outcome you need different length, diameter and/or shape.

 

so yall are telling me "dont do it myself, let a pro do it" right, if i were getting headers im getting cutouts, then a nice rumble with mufflers, do any of yall have experience with cutouts???


Taylor

 

Ok, backpressure to a large extent is a myth. Yes you will have backpressure that is measurable on most vehicle when they are idling or running on a dyno. But a vehicle in motion, that another story. Ever notice how many vehicles seem to actually get more power at certain speeds? I mean you figure, they are fighting wind resistance, maybe even inclines, but at certain speed/RPM combinations, you barely have to touch the throttle and it goes?

Here is what is happening:
If the exhaust exit is properly placed, and the system is well designed, the air rushing past the tailpipe will form a slight vacuum in the exhaust system. This can dramatically increase torque and exhaust scavenging and also fuel economy. So much for the backpressure myth.

Many people have claimed vehicles do not run well if the exhaust is too free flowing, allegedly because the engine requires backpressure. What they haven't figured on is another explanation. Each motor/exhaust setup has a sweet spot. This is the RPM range between the peak torque output and the peak horsepower output. Now as the engine approaches the peak torque output something is happening in that exhaust system. Each pulse of exhaust gases creates a wave of pressure in front of it, and a wave of vacuum behind it (these 2 opposing effect are the result of what we call inertia.) If these pulses are moving fast enough, they will force vacuum in the combustion chamber, pulling the spent gases out, and pulling the piston upwards. It also creates vacuum in the other port on the same bank. (It can have the same effect on the opposite bank as well if you have a single exhaust.) As the engine increases in speed, this effect gets stronger, which improves the engine output efficiency.

There is a point at which this effect becomes most prominent, and this is the engines' peak torque output. Revving the engine any higher will not produce an additional increase in power per stroke or torque. However, this scavenging effect is dependent upon exhaust velocity. Now if you increase the pipe size too much, you will decrease this exhaust velocity. Lower velocity means less scavenging effect. The larger exhaust will require more exhaust flow to get the same effect. The result will be that the engine will produce torque at a higher RPM. The torque at lower RPMs will decrease, especially as you come of of idle. Now if the exhaust is too large, you can actually have the opposite effect. Inertia works two ways, and objects that are stationary will tend to remain stationary. This includes the large mass of exhaust gases in an overly large exhaust. The sheer volume of air will tend to resist motion, and the engines' power has to overcome it. So in effect, by making the exhaust too large, you have actually created backpressure.

So your headers will only cause a problem if they are too large or are feeding into overly large pipe and mufflers. Monster trucks also produce a lot of power with the help of superchargers and they tend to rev a bit. By boring and stroking big blocks one can produce the same effect by brute force. Many of these trucks are far from ideal setups, and they could be improved upon. But when you are putting out 1200 HP (this is a hypothetical figure), it makes little difference whether the engine could be further fine tuned. Thats still more that double what is required. But a stock engine requires more careful planning since street use is still important, and that requires a careful balance between top end output and bottom end torque.

 

Thinking most are mistaking "backpressure" with "velocity". Air speed is key. There is no reason to have any restrictions in an exhaust system, but is essential to maintain the proper air speed, especiall in a vehicle with a carb. since air speed is critical for signals within the venturi.
Air speed vs. pressure. Two completely different thing, and often confused.

 

Very well spoken. One will find by close observation that the greatest restriction in most vehicles is not the exhaust or intake systems, but rather with the valve configurations. Believe it or not, most are too large to achieve good velocity induction at low RPM. And between 20 year old engines brand new engines, this is the greatest area of improvement that has occurred. Multiple smaller exhaust and intake valves results in better exhaust and intake induction and velocity, and broader powerbands. By having multiple valves, you prevent inefficiencies at both ends of the powerband. Many use hydraulic lifters and or variable valve timing to increase the intake and exhaust velocities at low RPM, which in turn creates venturi and inertial effects that keep things moving in the proper direction. As RPMs increase the valves open wider due to the effects of the hydraulic lifters, and timing at which they open may also change if the engine has variable valve timing. This allows for increased flow at higher RPM which in turn produces more power and less waste. The knowledge that smaller valves and multiple valve produced more power is not really new, except to most tuners. Automakers began making engines with larger displacements and smaller valves back in the late seventies. The problem is there are always tradeoffs. Smaller valves restrict the power in the upper powerband. But technology allows these problems to be overcome.

I have a truck that uses an early jet valve design that dramatically improved the torque output. It is a 1990 B2600i with a 2.6L Mazda G6 engine, which was a new block at the time. Basically, there are 2 intake valves, but only one opens fully at low RPM, as engine speed increases, the second intake valve opens more fully, allowing the engine to breath better to meet the needs of the engine. This rather small 4 cylinder cranks out at 125 hp and 150 ft.lbs torque, which when compared against the Mitsubishi block it replaced, was 23 extra HP and an extra 18 ft/lbs of torque. This torque also occurs lower in the powerband. To put in in perspective, a 1992 Ford F-350 with a 5.8LW produced 200 HP and about 310 ft/lbs torque. Mazda built a prototype V8 that was basically 2 of their 2.6L engines fused together. It had a displacement just over 5.2L and produced 280 hp and 320 ft/lbs of torque. This showed how powerful this technology could be. Had Mazda not been struggling financially, they might have gone somewhere with this. Todays engines are smaller displacements and produce a lot more power than previous generations, and technologies such as these are the keys the automakers use to get them there.

 

Not true. Very high pumping losses have been measured due to high levels of backpressure. The piston(s) have to do extra work to get rid of the exhaust gasses.

You have got to be kidding. This "tailpipe in a vacuum outside the vehicle theory" has been kicked around for years, and has no validation.

No, it is the velocity of the gas, not a "pressure wave." You can't have pressure and vacuum at the same time for obvious reasons.

No, you have decreased the velocity of the gasses. They are still free flowing, and once engine rpm has increased the velocity will increase and the torque band will have moved up with engine rpm. jd

 

If you have that much backpressure at idle you have problems more than that will allow you to do any dyno pull.

"Air rushing past a tailpipe will cause a slight vacuum"
Hell I must be good as I have dual 4 1/2" pipes pointing
straight back, I don't need brakes as the vacuum should slow
the truck down.

 

http://www.burnsstainless.com/TechAr...ry/theory.html


Pete