I can say this on the trucks (or at least mine) I'm running ASM headers on my 4.8 with 2.5" duals and cutouts. I picked up only 2 rwhp with the cutouts open but a gain none the less. Yes I believe I have some tuneing issues also. Finnaly a thread that will make grandpa think things do change and old school "backpressure" is not the answer to everything. If my exhaust is to open(that's what some people claim about us true dual guys) why would I gain when I make it even less restrictive by opening the cutouts?

 

"backpressure helps keep fuel economy up thats all i gotta say"

I hope that is all you have to say

 

My humble uneducated opinion is that backpressure = more torque at lower load/lower RPMs, but lower peak power. This is a drivability/perception thing.

Open tubes/no backpressure = better peak power but lower fuel economy and lower available torque at lower RPMs and part throttle driving.

Just my observations.
BadAss30th <img border="0" alt="[Firebird]" title="" src="graemlins/formula.gif" />

 

I'd like to chime in regarding the cam before heads issue. When I started having oil pump problems at ~8000 miles I decided to swap the cam at the same time since removing the oil pump got me over halfway to the cam. At the time I had intended on staying with stock heads. I also had the stock exhaust manifolds. I debated for a while as to whether a cam change was worthwhile with the stock exhaust manifolds ($$ wasn't there for headers at the time). I decided to go ahead and do it. Based on my exhaust limitations as well as the plan to stay with stock heads, I felt that the Hotcam would work well in my setup. After some tuning and rev limiter increase to 6600rpm I can most definitely tell you that there is a noticeable power increase. This is SOTP since I haven't dynoed, but its valid. Would I have gotten more with headers? No doubt, but in my situation I think it made sense.

 

I'm going with SStrokerAce...
"My general rule is Backpressure is not good at all. The design of an engine should be done at the lowest possible engine backpressure that you can obtain when it's intalled. That WILL get the the most HP and TQ everywhere in the RPM band."

This is also the impression I get.

 

Whats backpressure <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" /> <img border="0" title="" alt="[Razz]" src="gr_tounge.gif" />

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by BadAss30th:
<strong>My humble uneducated opinion is that backpressure = more torque at lower load/lower RPMs, but lower peak power. This is a drivability/perception thing.

Open tubes/no backpressure = better peak power but lower fuel economy and lower available torque at lower RPMs and part throttle driving.

Just my observations.
BadAss30th <img border="0" alt="[Firebird]" title="" src="graemlins/formula.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">This mostly has to do with tuning issues. If you have cutouts then this is all about where you put them and on top of it what the ECU is tuned for. The colector length is VERY important and if you change that with the cutouts you could very well lose TQ. In fact Keith Dorton had an article in Cicle Track about losing more low end TQ than high rpm HP with more backpressure, Figure that out?

My general rule is Backpressure is not good at all. The design of an engine should be done at the lowest possible engine backpressure that you can obtain when it's intalled. That WILL get the the most HP and TQ everywhere in the RPM band.

 

Granted, less backpressure equals higher rpm hp, but low-end torque is lost with some headers. Point in case, I have a 94 Dodge Ram 5.9L. I added the Mopar Performance Computer and my power band came in strong around 1500. I added the Mopar Performance Headers(Throley Tri-Y, and now my power band comes in strong around 3000.

 

"Efficiency" and "More Power" DO NOT mean the same thing. An efficient engine means maximizing output while minimizing input...in other words making the most HP and Torque while using the least amount of fuel. To accomplish that you must restrict the exhaust flow to the point of complete combustion of the fuel/air mixture. An engine is quite simply an air pump...it is designed to pump a certain amount of air...that is accomplished in part with the exhaust. When you reduce the restriction the engine will indeed be able to pump more air but there is a limit to how much any given engine can pump (normally aspirated). (Turbo-charged and super-charged engines are a whole other ball of wax). When you reduce the restrictive exhaust you will, in theory, make more power but at the expense of efficiency. More unburned fuel will be pulled out of the cylinders due to scavenging. Another way to put it...You can't have your cake and eat it too! Either maximum power OR maximum efficiency.
A prime example would be installing 1 7/8" headers on a stock motor compared to installing
1 3/4" headers on the same motor. There is a great possibility you would in fact lose some torque with the 1 7/8". That size header would be too large for the design of the LS1. If you stroke and poke it then yes...the 1 7/8" would make release more power than the 1 3/4".

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Mach Go:
<strong>"Efficiency" and "More Power" DO NOT mean the same thing. An efficient engine means maximizing output while minimizing input...in other words making the most HP and Torque while using the least amount of fuel. To accomplish that you must restrict the exhaust flow to the point of complete combustion of the fuel/air mixture. An engine is quite simply an air pump...it is designed to pump a certain amount of air...that is accomplished in part with the exhaust. When you reduce the restriction the engine will indeed be able to pump more air but there is a limit to how much any given engine can pump (normally aspirated). (Turbo-charged and super-charged engines are a whole other ball of wax). When you reduce the restrictive exhaust you will, in theory, make more power but at the expense of efficiency. More unburned fuel will be pulled out of the cylinders due to scavenging. Another way to put it...You can't have your cake and eat it too! Either maximum power OR maximum efficiency.
A prime example would be installing 1 7/8" headers on a stock motor compared to installing
1 3/4" headers on the same motor. There is a great possibility you would in fact lose some torque with the 1 7/8". That size header would be too large for the design of the LS1. If you stroke and poke it then yes...the 1 7/8" would make release more power than the 1 3/4".</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Backpressure is not scavenging. Scavenging is similar to intake tuning except it happens on the exh. In fact if you were worried about the exh scavenging taking unburned fuel/air out of the combustion chamber then you would do a few things. The first would be to reduce the valve overlap. If you have this problem then you are running to much cam for your application and desired Tq peak. Second you could run a numerous amount of things in the Header/exhaust system to prevent this. A easy example of this is a stepped header.

Backpressure just limits the abbility of the exh gasses to escape. Think of it like water in a stream being forced up into the current at high speed by a pump.
If you did a drawing of the Force then you would have two arrows facing each other giving you a net Force less than the Force excuded by the exhaust gasses.

Your though that the exhaust pumps air into the cylinder is interesting. Yes at the end of the exhaust valve opening it will create a low pressure area in the cylinder to help start the intake charge filling. The beauty with todays engines is that we have combustion chambers and intake pors that pormote swirl that allow us to not have a draw through condition that you are worried about. With proper cam timing you can have the exhaust system create a low pressure area and no draw through. If you have too much duration or to small a LSA then you will have too much overlap and then you can have draw through.

Yes, an engine is a fluid pump. Remember that we don't just have air which is a liquid, but we have wet flow that includes fuel. A 350 Cubic Inch engine can pump more air in a full cycle than 350cu in. It's called a volumetric efficency above 100. In fact to get the "limit" of around 95 ft lbs ber L of TQ you need to have a Volumetric Efficency at TQ peak of above 100% A good engine can act like a larger pump than it is.

As for the Header Diameter. Your whole reason for not needing the larger diameter headers on a stock engine has everything to do with exh port velocity. The stock engine is not producing as much HP as the moddified one. The by product of HP is a higher pressure, higher volume exhaust charge. With the correct diameter you can keep the velocity up to promote scavenging for a certain HP level. If it's to small then it will be restricted because there is too much going into to small of a hole. If it's too big it will not produce a good scavenging effect because it does not move fast enough to promote this. The correct size is on the top of a bell curve. With the vertical Axis being HP and the horizontal axis being diameter. Only one size will work and the rest will cause a loss in power.

I hope that explains that.

David Vizard was once quoted as saying "I don't have opinions, I have a dyno." His opinions are not that at all, but the results of thousands of dyno hours. I only quote him because the facts don't lie, and he has repeatedly proved people who belive backpressure is needed wrong, time and time again.

Bret

 

Yea!!! What Stroker said!!! <img border="0" alt="[judgement]" title="" src="graemlins/gr_judge.gif" /> Now we are beatin a dead horse. <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

 

Hmmm.... In four-stroke engines, backpressure is bad. You actually want NEGATIVE pressure in the head pipe during the end of the exhaust cycle. WHat? Negative Pressure?

As the ex valve opens, there is a VERY high pressure charge coming out of it. It acts like a sound wave. First High, then Low.

When the gases hit a larger crossection, the pressure drops, just like sound energy. When a high velocity sound wave goes past a crosspipe, the cross-pipe pressure goes down (Bernoulli).

Combining these two techniques, and using some math involving exhaust duration, speed of sound (at ex temp), and head pipe length, you can actually drop the pressure in the exhaust port below atmospheric. It effectively SUCKS out the remain charge (The Suck Zone <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> ).

However, if the timing of the exhaust wave events is screwed up, you LOSE power. You end up with high pressure in the port when you are trying catch a low wave. You are better with a full exhaust system than one that is miss-timed.

So while backpressure is NEVER good (you want a vacuum)it is worse to screw with the timing...

 

Don't mean to beat a dead horse

 

I'll put in my opinion too. Reducing back-pressure is always good. However, just running open headers don't always have the largest power gains. I know that running a tuned X-pipe setup can increase HP than strictly open headers (by increasing scavenging).

From a dollar per dollar take on it, I think a cam will get you more HP/dollar.

On the other hand, I think headers offer a higher reliability & driveability per HP (assuming you get the right header for your car.. taking into acount ground clearance, cat set-up, etc). A cam puts extra wear and tear on the valve train, that's why some people have to replace valve springs every so often.

I think if I start doing major mods (in about 2.5 years when the warranty is up...yes, its important to me) I'll probably do a cam first.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Dipstick:
<strong>Granted, less backpressure equals higher rpm hp, but low-end torque is lost with some headers. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Yes, it is, but it's important to note that low end torque is lost due to lower exhaust gas VELOCITY not due to lower exhaust gas PRESSURE.

The engine is an air pump. Plain and simple. It works by pumping air through itself which just happens to explode while inside the engine adding kinetic energy to the pump. (That's why you need the starter to get it going. no kinetic energy) Anything which restricts the engine's ability to pump air reduces it's efficiency (HP) regardless of if that restriction is in the intake system, the exhaust system, or is due to internal friction. These are all called "Pumping losses" They are the wasted energy which must be used to overcome the forces of friction and inertia within the engine itself before energy can be transfered to the rest of the drive train. To put it simply, if there is high pressure on the positive side of the pump, the pump wastes more energy pushing gas out. If there is low pressure on the negative side of the pump, it wastes energy pulling air in. That's plain and simple physics. There's absolutely no way to change those rules.

When somebody complains that headers reduced their HP, what they are saying is that they did not properly match their exhaust system to their cam and heads. If you put on a set of huge tube diameter headers with a collector which is tuned to start scavenging at 6500 RPM onto a car with a cam designed to produce power between 3500 and 6000 (like a stock cam) then the velocity of gasses will be too low due to the cross sectional area of the tube and there won't be enough inertia to pull the gas out of the cylinder before the valve closes. Also, instead of a negative impulse hitting the valve during valve overlap (scavenging) a positive one will be hitting it reducing the efficiency of the engine. Backpressure in the exhaust system can not physically help an internal combustion engine EVER. It just can't. A poorly chosen exhaust/head/cam setup absolutely can rob power from an engine, but anyone who thinks that backpressure is helping them hasn't done their homework (read: hasn't studied the physics of making power with an internal combustion engine)

This thread started off with a great quote from David Vizard. Anybody who doesn't understand, or believe for that matter, what he is saying should sit down and read the whole book. It's very informative.

 

Oh... By the way...

Performance modifications which improve performance, by definition, increace engine efficiency almost 100% of the time. The only deviation would be simply allowing the engine to turn more RPM like by installing stiffer point springs in older engines or bumping up the rev limiter on an LS1, or by changing the gear ratio.

When people notice a mileage drop after a performance modification the reason is simple. They are driving the car harder because it's cool and feels or sounds good to do so.

Any mod which creates more horsepower at a given RPM, that is allows the engnine to make more kinetic energy from the same ammount of chemical energy also allows the engine to create the same ammount of kinetic energy from less chemical energy. That's really really simple algebra there. 7th grade stuff.

 

Here's more proof why backpressure doesn't help.
Car mods b1 cam, lid, ported TB, LS1 edit tuning, big ford 9in 4.11's, alum. flywheel, and exhaust below.

Here's my exhuast setup. TTS 1.75 Longtubes, Custom y-pipe- twin 3in Flowtech Cutouts bolted onto header collectors, reduced to 2.5inch then to flowmaster 3in collector with Car Chemistry 3 disk 3in Insert, to SLP Loudmouth 21in Dynomax bullet. CC Insert installed in attempt to quiet exhaust ended up restrictive as hell and not exactly quiet. <img border="0" title="" alt="[Sad]" src="gr_sad.gif" />

Closed exhuast...339rwhp 339rwtq

Cutouts open...371rwhp 368rwtq

Power increased over closed exhaust at every point.

Hmmm Backpressure......BULLSHIT.

Exhaust Scavenging is what people misconceive as Backpressure.