Ken, Fred, others, I'm Really confused...

[Jay 02 TA ws6]

I'm sorry to post this again, but I really want to make sure that you guys see this question here. I am really confused by the information in this post... how exactly could open headers / straight pipes result in less torque?

http://www.f-body.com/index.html/sho...=&threadid=720

[Joe 1320]

Don't confuse torque with horsepower. A properly tuned exhaust system produces an anti-reversion effect where the exhaust pulse from one cylinder, helps pull the exhaust pulse from the next cylinder. Too much back pressure and the exhaust event looses momentum. As the exhaust leaves the head, it is still burning. It then continues down the pipes and as it cools it condenses slightly. There is a period of time that the exhaust and the intake valve are both open, creating a slight loss in momentum. Too much back pressure reinforces the loss and the previous exhaust event doesn't help "pull" the next event out. Too little back pressure and the momentum is lost again. When the exhaust is designed properly, low RPM cylinder filling is enhanced by the exiting exhaust gas "pulling" out the next exhaust event, without high rpm loss of power. Open headers produce great high RPM power, but don't benefit low end torque as well as a properly designed after the headers exhaust system due to the anti reversion effect. There have been headers designed with an anti-reversion cone enginerred into the primary tubes to help the situation, but it isn't as effective as the after header system. There is also benefit from header collector extensions for automatic equipped cars. The idea is to extend the collector to the transition point that the exhaust gas starts to cool and condense. It is at this point that you would reduce the size of the tubing to help create the suction effect that the exiting and condensing gas would produce to enhance the cylinder filling of next intake event. Again all of these points are meant to increase torque below the engine's torque peak while not disturbing upper RPM flow or power. This probably isn't described as technically as Fred would state it, but you get the idea.

[wolfdogs]

if you f@rted without backpressure...... your stomach would come out your (_!_)


hahahahahahaahahahahahaha



(d&r)....









kerm is bad
wolfman is nasty
kenC suks.






im gonna find the wright up for ya.

[Jeff_in_Atl]

So make sure you wear underwear?

[Joe 1320]

Kerm's in rare form tonight.

[Jason97T/A]

Hey Ken or Joe, would a flowmaster muffler cause backpressure?

[Injuneer]

EVERYTHING causes back pressure. "Back pressure" is resistance to flow. Simply put, it is caused by pressure loss in a flowing system. Simply flowing a gas through a pipe causes pressure loss, due to the friction of the material on the wall of the pipe, and the turbulence in the gas. Bend that pipe/gas around a corner and it causes even more loss. Put a sudden reduction or a sudden enlargement in the pipe, and it causes pressure loss. Put the baffle of a typical muffler in the way, and there is pressure loss.

The more pressure loss there is in the exhaust system, the more pressure is left in the cylinder at the end of the exhaust stroke. That means the incoming air has to push against the residual pressure, and not as much air gets pulled into the cylinder.

In general, there is no way back pressure can increase low end torque. Reducing back pressure is the best thing you can do.... but there is a caution. Reducing back pressure increases the amount of air that gets pulled into the cylinder. And as Joe points out, there are even greater benefits to keeping momentum up in the exhaust system IMMEDIATELY outside the exhaust port.

But, there is a caution. The lower back pressure increases the air fill in the cylinders, and that leans out the mixture. You have to tune for it. And here is where the "stories" about reduced back pressure arose. With a carb, to correct the mixture, you need to rejet. It is not "self correcting" like a modern EFI engine. With electronic fuel injection, and IN PARTICULAR with a mass-air system, the computer sees the extra inlet air flow and adds the extra fuel. There should be no loss of torque, or anything else.

A speed-density EFI system can be a little touchier. It can not sense the airflow increase. A speed-density system calcualtes the volume of air that is flowing based on RPM and cylinder volume. Then is needs to look up "volumetric efficiency" in a table of RPM vs. MAP. Increasing air flow increases volumetric efficiency... so the speed-density system needs the "VE" tables tweeked.... 93 owners beware.

I think that "loss of low end torque", is addition to being a holdover from the days of carbs, it also a "perception" problem. You open up the exhaust, and it makes the car louder. You step on the gas, and it makes a lot of noise, but doesn't go faster in proportion to the increase in the noise level. Gee... when my car made that much noise before, it used to have more "grunt"... I must have lost low end torque. Well... no. You just need to press the pedal down a little further. Floor it, and is HAS to make more torque than it did before, because of the better cylinder fill. But.... percepetion again.... you don't gain a whole lot of airflow at low RPM, just because you opened up the exhaust. Pressure loss is a function of the volume of gasses you are putting through the exhaust. Very little gain at low RPM, huge gains at high RPM. So, its perception again...it really hauls a** at max RPM, but it seem, "flat" at low RPM.... NO... it just gained a lot more on the top end, and maybe nothing on the bottom end.

David Vizzard has a good writeup on exhaust systems in his "How To Make HP" book, and he confirms... you will not lose anything with reduced back pressure, as long as you tune for it.

The next logical extension of this thinking is to remove the exhaust system completely... run without mainfolds or headers. Now you are in trouble. As mentioned above, you need VELOCITY in the first few inches of the primary tube to build inertia, and develop the concept that Joe is talking about, where during overlap, the fast flowing exhaust moving out of the cylinder is actually helping to suck air into the cylinder - scavenging.

But even on headers, bigger MAY be better, but not always. If you make the primary too large, you WILL LOSE low end torque. The velocity in the tube is too low, and there is no "scavenging" until you get up to very high RPM. The solution here is the "step" header design. Make the first few inches of the tube, coming ouit of the head port, smaller in diameter than the rest of the tube... keep velocity up. Downside of this is you gain on the bottom end, and lose on the top end because pressure loss in that few inches of smaller tube becomes a problem.

The issue of burned valves also stems back to the days of carbs. Lean the mixture out with improved air flow, and you can burn the valves. Again, not a problem with EFI, because the mixture doesn't lean out. You can burn a valve with EFI if you develop a large leak in a header primary tube. I've seen a case where the EGR tube broke off #8 primary tube when the engine torqued over excessively and the tube hit the body. That hole was enough to cause a burned valve, because now there is air that can get sucked into the hole, and expose the valve to higher oxygen content... and oxygen is what "burns" valve.

One minor point on "anti-reversion". I always thought that refered to the tendancy of an overly large primary tube to cause some exhaust to be pulled back into the cylinder at low RPM during cam overlap. The velocity in the tube is so low, that there is no momentum, and gasses start to flow backwards into the head, when the piston starts to drop on the intake stroke, instead of having the high exhaust gas vleocity pulling the air into the cylinder. The solution to this is an "anti-reversion" design, that adds the cone that Joe is taking about to keep the velocity high just at the exhaust port. I had a set of JBA 1-5/8" headers that has both the "step design" (1-1/2" at the head, opening up to 1-5/8" about 4" out.). And a weld bead built up in the opening in the tube that butted against the head, to form an "anti-reversion" barrier. Those headers produced great low end torque, but IMHO opinion seriously strangled the engine at higher RPM.... and that was with a simple bolt-on LT1.

IMHO, "reduced backpressure causes loss of low end torque" is a combination of urban legend.... and a bunch of old dudes like Ken and me who remember the days of carburetors, when that statement did make some sense.

[Jay 02 TA ws6]

Originally posted by Jason97T/A
Hey Ken or Joe, would a flowmaster muffler cause backpressure?

Any muffler restricts the flow of exhaust...

What they are saying right now is that some restriction is good, because it can actually help pull the next exhaust pulse out. Correct guys?

[Jay 02 TA ws6]

Thanks a lot for clearing that up, I really appreciate it!

[Injuneer]

Yes... any muffler has a pressure loss. Some just cause more than others.

No... the concept of "higher velocity" only applies to the header primary tube. Once you hit the collector, you have lost the benefits of scavenging.... no longer a factor. Get a really great header, and bolt a short muffler on it.

Note my "exhaust system".....

[Jeff_in_Atl]

Man how long did it take you to write that. I still say this kind of stuff needs to be save somewhere like the FAQ section. Good stuff Fred.

I do have a question though, I thought the speed-density system did measure the airflow. It did this by sending a voltage across a "hot wire" that went through the air flow tube. It then measures the resistance of the wire. As air flow across it, the air cools the wire which changes it resistance. The more air, the more cooling effect. The computer has a table that knows the resistance values and converts this value to air flow volume.

When Ford and GM first started using fuel injection Ford decided on the speed Density metering because it is less restrictive than the mass air. GM went with the mass air because it is a more accurate method of measuring the air flow. The speed density has a bit of time delay because it takes time for the air to cool the wire. In 89 (88 for Ca models) Ford went to the mass air because of the delay and the hesitation this delay caused. Gm went to speed density because it flows better which is why I assume you went with the speed density on your car. I believe the F-body switched back to mass air flow in 94?

BTW when a liquid or a gas flows along a surface it does slow from the friction. This is called parasitic drag. The friction does cause a slight turbulence too.

[Injuneer]

Originally posted by Jeff 95 Z28

I do have a question though, I thought the speed-density system did measure the airflow. It did this by sending a voltage across a "hot wire" that went through the air flow tube. It then measures the resistance of the wire. As air flow across it, the air cools the wire which changes it resistance. The more air, the more cooling effect. The computer has a table that knows the resistance values and converts this value to air flow volume.

No. What you are describing is the MAF, or mass-air sensor. It heats the wire, and the wire resistance is part of a Wheatstone bridge. By measuring the amount of energy (BTU's) required to keep the wire a fixed number of degrees above the inlet air temperature, it calculates the "mass" air flow (pounds per hour, or grams per second). It knows how many BTU's are being lost from the heated wire, and it knows the specific heat of air (BTU/pound-degF), so it can calculate the small mass (pounds) of air actually touching the wires, then "extrapolate" that small sample to the total amount of air that it knows can flow through the MAF. That "extrapolation" is in effect, the calibration table, and that is why you DO NOT want to mess with the flow distribution of air in the MAF sensor... it only samples a tiny part of the air, and needs to assume that small sample is moving at the same velocity as all the other air in the meter.

Speed denstiy works differently, to allow the PCM to calculate the MASS (pounds) of air entering the engine. First it knows the cylinder volume. The theoretical VOLUME (cubi feet) of air entering the engine = cylinder volume X RPM/2. But that is only theoretical. Cam overlap, pressure loss, exhaust scavenging, etc. all affect the ACTUAL volume of air that gets to the cylinder at any RPM and engine load. So the engineers test the engine endlessly, and determine the ACTUAL volume of air entering the engine, then divide it by the THEORETICAL volume of air (or displaced volume of the engine), and that number represents the VOLUMETRIC EFFICIENCY.

A stock engine might only see a VE of 80-85%, because of the heads, intake, etc. A well designed race engine can approach 100%... and purpose specific engines, like a NASCAR Grand National or similar can actually exceed 100% of theoretical volume. It does this by using techniques like exahaust scavenging and intake wave tuning to increase cylinder pressure above atmospheric.

So... the speed part of speed-density is a calculation...

Cylinder volume X RPM/2 X VE = air flow VOLUME.

But we need to know how many POUNDS (mass) of air is in the cylinder, so we can add the correct POUNDS of fuel. Since we have volume, we can convert that number to mass, IF we know the DENSITY. Density can be calculated by the perfect gas law... density is directly proportional to absolute pressure, and inversely proportional to absolute temperature. So, if we measure th inlet air temperature, and the manifold pressure, we can calculate the density part of speed-density.

flow Volume X density = flow mass.

The inlet air temp sensor is a resistance device. Specifically, a thermistor. That means the resistance of the sensor is inversely proportional to temperature.... high resistance indicates low temperature. As temerature increases, resistance drops.

When Ford and GM first started using fuel injection Ford decided on the speed Density metering because it is less restrictive than the mass air. GM went with the mass air because it is a more accurate method of measuring the air flow. The speed density has a bit of time delay because it takes time for the air to cool the wire. In 89 (88 for Ca models) Ford went to the mass air because of the delay and the hesitation this delay caused. Gm went to speed density because it flows better which is why I assume you went with the speed density on your car. I believe the F-body switched back to mass air flow in 94?

BTW when a liquid or a gas flows along a surface it does slow from the friction. This is called parasitic drag. The friction does cause a slight turbulence too.

GM didn't like mass-air because they were using the Bosch MAF sensors, built to GM specs, and they SUCKED!!! Very unreliable, susceptible to dirt buildup (they actually heated the wire red hot when the engine shut down to try and burn the dirt off the wire), and a major flow restriction due to the screens at both the inlet and outlet sides, and the huge cooling fins GM forced Bosch to put in, based on some operating tests alledgedly run in Death Valley at 130degF. Removing the screens and hacking the fins out of the Bosch MAF sensor paid BIG dividends.

But GM eventually developed their own MAF design, and that is what you see in the 94 and later F-Bodys. Rather than a wire, it uses a thin film resistor of some sort, which is less susceptible to dirt buildup. And the sensor is nowhere near as restrictive, with only one "screen" - and that screen isn't really a screen, its a honeycomb made by cutting and pulling a very thin piece of paper into the required shape.

[wolfdogs]

gr8 thread guys.

and gr8 i can quit diggin for the old right up...which..i think fred contributed to back then also.......but he didnt know quite as much as he does now...

[Injuneer]

You know my motto.... "Dazzle 'em with brillance, baffle them with bullsh**"......

Or any combination of the two......

[Joe 1320]

Fred, all I can say is...... dayum.

We also all need to remember that engines are also run at other throttle openings besides wide open. There is a slight anti reversion effect created by an after header system as well. As a muffler adds restriction as the gas flows toward it within the exhaust pipe, it also restricts the flow in the opposit direction should the gas try to reverse. The theory can be demostrated by a simple straw and a glass of water. Take a half full glass of water and insert the straw, gently blow a small amount of air into the straw, but not enough to blow the air out of the straw and into the glass. You'll see the water level within the straw fall below the level of water in the glass. So far you are seeing an effect of "backpressure" that the water in the glass is exerting on the straw. Suddenly release the air pressure from the straw and the water rises above the water level of the glass and settles back down. Notice that the water doesn't shoot out the top of the straw. It won't because the same force that "pushed" back against the air you were blowing into the straw, prevented the water from going up and out the straw when you released the air pressure. Usually this effect in an exhaust system is reduced exponentially as more positive pressure is produced from a WOT exhaust event. There is some benefit using this principle in cruise situations and mild throttle openings.