Hi guys.

Does any can explainme the relationship between headers and muffle diameter v.s. torque?

Currently I am running a 1989 F-150 5.0 FI 4x4 with the following system setup.

Long headers 1.5/8 going in to a 2.1/4 collector.
2.1/4 tubes going in to a staright custom made muffle with dual inlest and dual outlets.
After muffle tube goes up to 2.1/2 and 4" exahust tips.
No cat.

Sound is great and think performance is quite good also, but I do not know if any change in tube diameter will help me with low end torque.

Is this kind of setup to unrestricted for the engine? Any sugestion or advise?

Regards
Ray

 

The myth about 'some' back pressure helping low rpm torque is just that.

This is the best description on exhaust systems I've ever read:

[It seems like there are two separate issues that are often confused. One is backpressure in the exhaust system and the other has to do with a scavenging wave that is returned from a tuned exhaust system to help evacuate the exhaust gasses and pull in a fresh charge during the EVC/IVO overlap period.

NO backpressure is desirable in any engine system. Go plug your exhaust up (ol' potato trick in the exhaust pipe) and see how well your engine runs. Approx 1" hg backpressure = 1 HP lost. Backpressure only causes a mechanical loss during the exhaust stroke to expel the gasses. A properly designed engine will run best with no backpressure. If for any reason, you should find an improvement in engine performance with an increase in exhaust pressure, it is an indication that something else in the system is not correct. Put a tap into the exhaust system just before the exhaust enters the muffler. Pressures in excess of 1/2 psi should be considered restrictive. In other words, you have potential horsepower to be recovered and employed for more useful purposes than pushing out the exhaust gas.


The scavenging signal returned from an exhaust is entirely another matter. This is the result of a high pressure wave traveling at a much higher speed than the exhaust gas particles (upwards of 1450 fps). As the wave reaches the end of the collector, in a properly tuned system, a negative wave is reflected back to the exhaust valve. A collector pipe, tuned to the rpm of choice will "time" this negative pulse to reach the valve at the appropriate time and offer a negative pressure to the cylinder far in excess of the negative pressure developed by the piston during the intake stroke.

A properly "tuned" and designed exhaust system is a challenge.]

 

George,you state that back pressure is negative power.I want to go 3" from my collectors all the way back,but my buddy who is an exhaust man says to go 1 7/8 or 2" because to much flow through the exhaust will actually decrease power because the heads won't flow enough exhaust.He states that restricted exhaust is better for your engine in producing power,no relation to a very high performance engine needing flow.Your thoughts. John

 

He's wrong, but if he believes in that old wife's tale there's nothing you'll ever do to convince him of that. Any exhaust restriction creating back pressure is a reduction in HP, pure and simple. As that article described, there's big difference in scavaging and back pressure. Unless he does exhaust for diesel trucks, he may not have the bending capability for 3" exhausts.

Ask him to go to a local airport and talk to some small aircraft owners who have had to muffle open exhausts to comply with FAA noise abatement legislation around many cities. Those are generally small displacement engines that use(d) the largest diameter straight exhaust pipe that would fit in the aircraft design. Those people can give you first-hand experience about the effects of back pressure and loss of performance.

The issue of exhaust size is generally determined by application. If you're after a quiet exhaust, by the time you muffle a 3" exhaust down to where it sounds like a standard production car, you've negated the advantage over smaller systems. Dyno tests on 3" exhaust systems behind performance engines have shown minor hp reductions with performance mufflers that will pass the 95 decibil idle limit now imposed at many Eastern dragstrips.

I recently went from 2.5" exhaust off of full headers with 3" collectors to 3" pipe off the headers and 3"x14" mufflers with turndowns after the mufflers. I effectively increased the collector 36" and the increase in performance was massive. Nice legal rumble at idle and off-idle, almost as loud as open headers under acceleration. But I like that sound.

 

ok

Thanks a lot for your comments, then I know I have no major problems with back presure so far, however with new heads, cam and big valves I may

It can be a really interesting experimento to plug a presure gauge at the end of each header (just near the collector) and to check presure under difrerent circunstances (heavy load, climbing, de-acelerating, idling) to try to stablish kind of a chart were the system has the best performance.

Does any have made something like this? What the result can be?

What about the efect on toque? Now HP fact is quite clear, the lower the restriction the better in terms of HP and scarving wave has to be set up with proper time at valves (properly tuned headers), but what about torque? For sure it has to have an efect on torque if HP is moving out of what is standard.

Regards
Ray

 

hp is only a number derived from torque. Torque is the only true rating of an engine's performance, even though we all use hp as a benchmark. So, if it's good for hp, its good for torque.

 

Couldn't agree with ya more, george. However, theres a little more to it. You want the velocity of the exhaust to be high in the headers, aiding in scavanging at low rpms. As long as the backpressure doesn't increase, there is no loss of performance. The idea is that the mass of the outflowing air through the header will create a lower pressure by the exhaust valve, by the exhaust gasses wanting to continue flowing while the valve is closed, thus drawing more exhaust out when the valve does open. So an ideal system would be headers tuned for the RPM range you are going to run at, with no exhaust after that (open headers). Failing that, for sound concerns, the least restrictive exhaust after the headers is best. Put simply, the velocity only matters in the headers, after that, you want to eliminate backpressure. After all, the lower the backpressure, the faster the gasses will flow through the headers!

 

rusty70f100, we're in total agreement on the principles of exhaust scavaging and back pressure regarding all internal combustion engines.

I've explained this to many people, with little success to those from the 'Smaller exhaust creates better torque' congregation.

Only reason I mentioned that maybe the exhaust guy didn't have the equipment to bend 3" was a story from another forum. Poster said the muffler shop oracle carefully explained to him how 2.5" exhaust on a BBC was proven by dyno tests to be the most effective system he could run. One of the employees, thank somebody for honest people, told him on the side that their equipment couldn't handle 3". I think that may be the case with a lot of shops who nix 3". Corvette, LS1 F-bodies and other manufacturers of high-end vehicles use 3".

 

George,I in no way meant to seem doubtful.He does have the capacity to bend 3".I haven't had a chance to talk to him about his whole reasoning,but you are right that there is a general feeling out there that to big of pipe detracts from H.P..I personally right now run single 2 1/2 " in front of my pass tire,and it really opened the engine up.I have been posting to figure out a goodlooking and operating dual system.I didn't mean to detract from your opinion. John

 

John, doubts and questions are some of the things that establish facts. The internal combustion engine has been around for a long, long time and, due to its relatively inefficent operation driven by manufacturing cost considerations and eco legislation, always has plenty of room for improvement. That's what keeps the aftermarket thriving and develops these forums.

In my opinion, the Internet and this type of forum have done more to further understanding of our hobby in just a few years than car and trade magazines accomplished in 50 years. The magazines publish articles and answer questions that are by necessity confined to promoting the products, good or bad, of advertisers who support them. And with their power of the written word, often form erroneous opinions in the minds of many people. These forums have no such restrictions. Not yet.

 

Well guys

I have to agree with Georgedavila regrading this and many other forums. This is really an added value for all of us therefore is our rispozability to keep it as such.

Now is much clear fo me how the headers work and how they help my enginee to perform better, also I should assume that headers could lead to a decreased chamber temperature (I do not know really but I assume that) and would help in a way to avoid pre ignition, is my asumption right?

Now it also leads me to another dumb question.

Headers has to be matched with the intaque? I mean, having les srestriction should create a unbalanced equation because intaque system is not big enought.

So shall I assume that bottle neck is intacke system now?

Regards
Ray

 

Chamber temperature is dependent on cam, fuel mixture and timing. Headers will not provide cooler chamber operating temps.

Headers alone do not create any 'imbalance' in the engine. They merely facilitate the exhaust cycle. The cam is your next step as all the induction system does is service the cam and you spec those components, heads, intake and carb, to what the cam requires. If you're planning on building the engine for higher performance, headers are a great first step.

 

george,i finally was able to talk to my buddy.He says you are right
about most things but he figures you are hiding something.Mainly being that yes you can run even no restriction,no mufflers or tails,and run with better response,but the fact is that most shops have to conform to emissions,and the public should also do their part.He feels you are stating facts that don't take into consideration the environment(read what you will) and the laws restricting shops.You may not agree with these regulations as I don't,and even my friend doesn't,he usually explains exhaust related stuff according to his shop regs,but the epa has it's own rules.The main thing to consider with soundthrough any muffler is the amount of exhaust and the rate it goes through determins sound.It is alot like a in home air purifier,CFM.That's his wording,he agrees that my truck runs better in every way having bigger exhaust,and when I finally break down the engine and do it my way,the bigger exhaust will perform better.It still doesn't make anymore sense to me,but that's what he says.He's been doing it for 15 years now and is a great mechanic. JOHN

 

"George,you state that back pressure is negative power.I want to go 3" from my collectors all the way back,but my buddy who is an exhaust man says to go 1 7/8 or 2" because to much flow through the exhaust will actually decrease power because the heads won't flow enough exhaust.He states that restricted exhaust is better for your engine in producing power,no relation to a very high performance engine needing flow.Your thoughts. John"

Can't argue with his rationale regarding smog **** legislation or noise, but he started off telling you he recommended smaller pipe for better performance (above) and that restricted exhaust is better for producing power. I wouldn't consider that honest advice if he knew better.

 

George,I would consider it good advice considering he could loose his business license and certification if he did things his way,also your way.I wouldn't put that on one of my friends.according to EPA law he can't even advise people to remove their own cat.convertors.He agrees with you that no back pressure is great,but not practical to those who have to pass emission tests.