In theory, a well designed header and exhaust will "Increase the ability of the engine to scavenge exhaust gasses from the cylinders, thereby allowing MORE air and fuel to enter the cylinders".

Lets think about this...

The trapped exhaust is gone - so it no longer is a factor in the combustion process. With the burned gasses gone, a greater volume of mixture is present to be consumed. It stands to reason more oxygen is also present, therefore a more powerful combustion process will take place.

The air/fuel charge being larger than before, obviously more fuel is being consumed, thus the mileage factor will change for a specific RPM range. Or will it?

At part throttle, it stands to reason that the same amount of work is being done by the engine than at a wider throttle position with standard equipment. The RPM's and power will be the same, so the fuel consumption may or may not be a factor since the throttle does not need to be open as far...

The burned gasses previously trapped in the cylinders are absent, what effect do they have on the mixture? It seems to me they would have a "damping" effect on combustion, but the mix of air and fuel might be a sliding scale.

Most exhaust has some amount of unburned hydrocarbons in it (fuel).

Without the remaining CO and CO2 from unscavenged exhaust, the air charge from a better scavenged engine may result in more complete combustion, in any case it will not be retarded by the presense of already burnt air/fuel, and therefore a more complete and thus hotter combustion will happen in there...

This means higher temperatures. THAT means a different heat range for your spark plugs. What this might mean in terms of a diesel I am not entirely sure yet - but diesels are speed and power regulated entirely by fuel management.

One thing I know for sure through experience, adding headers means you have to re-tune your carburettor or EFI system to maximise it. If you don't - you are likely to torch off an exhaust valve!

Headers will directly impact your fuel/air ration, and like a welding torch - adding oxy will raise the temperatures. When headers are first put on, the common sight to expect to see is for them to glow cherry red. The main step in header tuning is to richen the fuel mix.

You have to get that burn temp back down... If it is that high it is far from optimal.

THIS APPLIES TO MAIN JETS, not just the idle mixture screw(s)...

Having said all of the above, I bet (and hope) that Eric chimes in here.

~Wolf

 

In theory, a well designed header and exhaust will "Increase the ability of the engine to scavenge exhaust gasses from the cylinders, thereby allowing MORE air and fuel to enter the cylinders".

Lets think about this. What does it mean to the whole engine?

The trapped exhaust is gone - so it no longer is a factor in the combustion process. With the burned gasses gone, a greater volume of mixture is present to be consumed. It stands to reason more oxygen is also present, therefore a more powerful combustion process will take place.

The air/fuel charge being larger than before, obviously more fuel is being consumed, thus the mileage factor will change for a specific RPM range. Or will it?

At part throttle, it stands to reason that the same amount of work is being done by the engine than at a wider throttle position with standard equipment. The RPM's and power will be the same, so the fuel consumption may or may not be a factor since the throttle does not need to be open as far...

The burned gasses previously trapped in the cylinders are absent, what effect do they have on the mixture? It seems to me they would have a "damping" effect on combustion, but the mix of air and fuel might be a sliding scale.

Most exhaust has some amount of unburned hydrocarbons in it (fuel).

Without the remaining CO and CO2 from unscavenged exhaust, the air charge from a better scavenged engine may result in more complete combustion, in any case it will not be retarded by the presense of already burnt air/fuel, and therefore a more complete and thus hotter combustion will happen in there...

This means higher temperatures. THAT means a different heat range for your spark plugs. What this might mean in terms of a diesel I am not entirely sure yet - but diesels are speed and power regulated entirely by fuel management.

One thing I know for sure through experience, adding headers means you have to re-tune your carburettor or EFI system to maximise it. If you don't - you are likely to torch off an exhaust valve!

Headers will directly impact your fuel/air ration, and like a welding torch - adding oxy will raise the temperatures. When headers are first put on, the common sight to expect to see is for them to glow cherry red. The main step in header tuning is to richen the fuel mix.

You have to get that burn temp back down... If it is that high it is far from optimal.

THIS APPLIES TO MAIN JETS, not just the idle mixture screw(s)...

Having said all of the above, I bet (and hope) that Eric chimes in here.

~Wolf

PS: THIS is an "engine theory" topic, please do not move it to the dungeons of exhaust and so on...

PPS: Porche was notorious for minor tweaks because of things like this

 

They make the engine more efficient. Engines are just big air pumps. The more air you can either let in or let out will make it more efficient. Headers let more air out, thus the engine doesn't have to work as hard to push the air out.

 

Can't say that the charge is "larger" than before. The engine's displacement and volumetric efficiency dictate that. Yes, with headers the VE should be higher, but not enough to actually increase the amount of volume in the cylinder.

However, the work that the engine is doing to get the spent gasses out of the engine is reduced (pumping losses) and the velocity of the escaping gasses can be higher and smoother. On that note, sometimes headers will not work as efficiently on some engines as others. Examples of this are:

1. 221-351W where the exhaust port is so stupidly (new word) small that the factory exhaust manifolds are sufficient to support the flow. The most flow is only as great as the smallest orifice.

2. FE with 4x4 exhaust manifolds. Ever tried to stick a finger in that #4 exhaust manifold port? You can't do it. Headers work wonders on the FE for that reason. The head port is large enough to flow some, but the manifolds hold you back.


Agreed. I can't think of a better way to put it.

The words Exhaust Gas Recirculation come to mind. Exactly what you are describing here. EGR is used to keep combustion temperatures down to reduce the amount of "Oxides of Nitrogen" (a so-called greenhouse gas) in the exhaust. EGR also is used to curb detonation. You are dead on the money with the dilution theory here.

Yes, "colder" spark plugs are recommended with headers. Is it that important? Not unless you run into overheating plugs (I'll get to that later). As far as diesel goes, the only reason to use EGR is to reduce the combustion temps. As most folks know, the process of combustion in a diesel is significantly hotter than combustion in a gas engine (unless running lean) and oxides of nitrogen emissions are WAY higher. Back on topic though, the more air you can get out of a diesel, the more you can get in on the next intake stroke. More air+more fuel= more power. But then we get to the same concept of the air/fuel ratio...........

Without more fuel, the gains will be minimal when installing headers. With the scavenging effect and ability to intake more air, more fuel IS required. This is especially true on a gasoline engine because a leaner burn is a HOTTER burn. Hotter combustion temps cause detonation and higher emissions. Not to mention they can be harmful to your engine.

A diesel on the other hand doesn't require more fuel, but what's the point of essentially adding air if you don't add fuel? A diesel engine is not going to run higher combustion temps with less air/fuel ratio....it's just not going to run up to it's potential. There is no "lean" with a diesel, only the ability to run at a certain speed/load. The more air you can get in/out, the more fuel you can get in and make more power....but beware, too much fuel for a given amount of air you are able to get in causes high exhaust temperatures and eventual meltdown.

Cody

 

The problem with headers though is that on a closed loop engine they also increaes the hydro carbons in the exhaust and cause engine to lean out some anbecause the exuast is bleed off quicker, there is a loss of energy for piston to capure from expanding gasses too and the result is reduced lower RPM torque and throttle responce. Modern engine are well tuned and caution is advised.

 

but if you read what wolfie wrote, you would see that what you stated was useless drivel, because he already said that the carb needs to be adjusted to compensate for the lean run.

so you will get better performance, because the engine is now breathing better.

 

One thing about you is that you can always count on you trying to start something. Yes you can adjust/rejet carb and may gain some top end power but low end performance will suffer. I have used header many years ago on street rods when carb were still main stream and I know them well and likely better than you.

 

i do not try to start anything. i simply state what i know works. and when i see somthing that i know is wrong, i coment on it trying to clear up any confusion. i do not try to cloud things up, or confuse people with statements that may, or may not be true.

 

OK you two....now this forum....CHILL...do not make this personal....




Strike two

 

To comment on the modern engine and headers, low end will not be lost if the engine is tuned to work with the exhausts better breathing potential. A computer controlled engine is easier to tune for this. Reset the PCM, and it will compensate and retune itself and make more power. Something as simple as headers are well within its ability to tune. True, they are tuned well for the parts they are sold with, but there is always room for improvment.

Its really tiring to hear about loss of low end from better breathing exhaust systems all the time. Better breathing positively affects EVERY RPM if done right.

cleatus12r has all the info you need in that single post. Efficiency is everything.

 

A computer controlled engine WILL make up for the efficiency gain caused by headers and a free-flowing exhaust. Modern EFI systems are capable of supplying up to a 50% (25% short term/25% long term) increase in base fuel strategy (although the check engine light will come on at 15-17% long term depending on make). A free-flowing exhaust is NOT going to make that big of a difference in fuel trims.

On the note of this statement:Not exactly. The computer controlling of fuel trim adjustment happens within a fraction of a second. The oxygen sensors used in modern vehicles are very fast switching and can detect the presence of more/less oxygen in the exhaust stream very quickly. Once the computer "sees" the state of the oxygen sensor (<450mV-lean/>450mV-rich), the computer compensates. Not only are we compensating for a rich or lean condition and adjusting it accordingly, but the computer is already programmed to "make" the exhaust go rich and lean (200mV to 800mV on a narrow-band sensor) to actually make the catalytic convertor work and do it's job. So you see, the air/fuel ratio in a modern EFI system does NOT sit at 14.7:1 anyways. It fluctuates between 13.8 and 15.5 under light load conditions and it does it very fast (10-15 times a second on some cars).

I hear you, brother. Every time I read about "more backpressure is better" and "it breathes too good and I lost low end torque", I just want to laugh (or break something).

I've got quite a bit of experience with free-flowing exhaust and can tell you that everything I've been around that has had a free-flow system has run like a totally different animal.

You'll never hear of me speak otherwise.

Cody

 

On exhaust ports - in '77 I was at a shop as a trainee where the bosses project was a "C" production 240Z race car.

He had ported the intake side, but didn't consider it much of a gain to port the exhaust. ('PORT' as in "ENLARGE")

A few months later, he was setting up a new head for the car, intake AND exhaust exactly "Gasket Matched" to the manifolds.

Porting is a serious modification. Done badly, you can cause a crack to the water jacket or an oil passage. Some porting experts will even take an old head and saw it apart to see how much material they really have around the passages.

Valve guides can be ground flush so that the valve stem is the only thing in the gas flow (much narrower), but on exhaust valves its at risk of melting the stem, since the guides protect the valve stem from some of the heat.

There is also overkill! If you ever see a set of factory four barrel heads for a 351 Cleveland - that's overkill... The ports on those heads were already so huge there was zero swirl factor on the intake, and no way you will ever run a high enough RPM to fully utilise them (IMO).

I have big hands, and I could just about stick my whole hand in the ports on either side of one of them...

So really, a header is only effective up to the point where the flow restriction is in the heads themselves. A well set up engine will have large ports matched to the headers for optimal breathing throughout.

Gasket matching is essentially done by marking around the ports with india ink, dye, or permanent marker.

The performance gaskets for the headers are then lined up on the head(s) and a scripe is used to mark the outline of the port cutout of the gasket.

The scratched outline then shows in the dye, and the ports can be relieved to the exact outline of the passage through the gasket - which hopefully is an exact match for the headers.

Inside the ports is another story, and larger valves are usually fitted if someone is going to this extreme.