header length n tuning

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PAINKILLER
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header length n tuning

Postby PAINKILLER » Tue Dec 01, 2015 4:52 pm

We regularly field questions regarding headers.

Common header questions are centered around tube diameter, tube length, collector diameter, collector length, and so on. Some folks will tell you none of this matters. Others will tell you it all matters. We tend to agree with the latter, but when it comes to the racing world, one thing is pretty clear:

Experience and knowledge are absolutely paramount.

To answer to your questions, we went straight to two well-respected expert organizations in the biz—Hooker Headers and Reher-Morrison Racing Engines—for the lowdown on headers.

Exhaust Scavenging 101

Inside a header (with the engine running), “waves” of exhaust passing through the tubing are more or less manipulated to extract the burned gases from the combustion chamber. Simultaneously, those waves of exhaust help the intake side by pulling in a fresh air-fuel charge. If properly designed, the header set will provide a beneficial pressure balance between the intake and the exhaust.

An engine benefits from two different types of scavenging: Inertial scavenging and wave scavenging. Header design can have an effect upon both.

Inertial scavenging of gases begins as soon as the exhaust valve opens. Here, exhaust gases move past the valve and exit through the exhaust port into the header primary tube (and eventually working their way into the atmosphere). Hooker says the best power is produced with an exhaust gas speed of approximately 300 feet per second (as an example, if an engine has a 36-inch primary tube, it would take 1/100 of a second for the exhaust “pulse” to pass through the tube). Hooker notes that once the exhaust valve is closed, these gases still continue to move down the exhaust tube at a rate of 300 feet per second. But as it travels down the pipe, the exhaust cools and actually slows in speed. Behind this “pulse” is an area of low pressure that is expanding as the exhaust flows away. Once the last of this exhaust gas reaches the end of the primary tube, the area of low pressure has increased in size to include the full length of the tube for that particular cylinder.”

Wave scavenging does not involve the physical movement of exhaust gases up and down the header tubes. Instead, it deals with the sound waves created inside the engine. Hooker says that when an exhaust valve first opens, a pressure wave begins to travel—moving in excess of 18,500 inches per second (more than the speed of sound), which at sea level works out to 13,397 in./sec., depending on several factors). This sonic wave is moving quickly—much quicker than the ever-expanding exhaust wave we discussed above. When the sonic wave arrives at the end of the primary tube, a negative shock wave is generated, and this wave travels backward toward the exhaust port (because it is reflected).

What the header builder has to do is time the arrival of this negative pressure wave to occur just before the exhaust valve closes and while the intake valve is opening. If the negative wave arrives too soon or too late, the power potential of the next combustion chamber cycle is diluted.

If the engine has no collector (for example, a one-cylinder powerplant or an engine equipped with zoomies) the exhaust enters the atmosphere once it has left the primary tube. But in most headers, the exhaust enters a collector. If the low-pressure area we talked about above spills into another primary tube for a cylinder where the exhaust valve is just starting to open, that low-pressure area will help to pull exhaust gases from that cylinder. Here, the engine will gain an advantage, because there is less residual exhaust gas remaining in the combustion chamber (which can foul the incoming charge of fresh fuel and air).

By manipulating the diameter and length of a specific tube, a header builder can influence the size of that low-pressure front.

Header Diameter and Length

Reher-Morrison Racing Engines tells us there is a direct relationship between the diameter of the primary header tube and the exhaust velocity:

“The key when selecting a tube diameter is to find a happy medium between the free-flowing characteristics of large tubes and the superior scavenging of small, high-velocity tubes. Header tube diameters normally range from 1-3/4-inches to 1-7/8-inches for smaller, low performance engines up to big 2-3/8-inches tubes for large displacement, high-horsepower applications.”

Remember when we talked about how inertial scavenging helps to pull the exhaust from the combustion chamber? This also helps to draw in the air-fuel charge, with the end result being higher volumetric efficiency.

Hooker notes that by varying the length of the primary tube, you physically change the time it takes for the vacuum pulse (low-pressure area) to reach the header collector. Essentially that’s what “tuning” header tubes is all about. This tuning can help to complement the components you’ve selected for the entire car (camshaft, torque converter, transmission gear ratios, rear axle ratio, tire diameter, and so on). Header tuning can also have an effect on how the car works at a specific track.

As a rule of thumb, the need for a long primary tube is reduced as the speed of the engine increases, because there’s less time between cylinder firing, and the low-pressure area we talked about has less time to travel to the collector.

Getting the tube diameter and length right is extremely important. With all other factors being equal, the proper header tube size will allow you to obtain a superior scavenging effect with an increase in exhaust speed. Some folks will even tell you that the exhaust valve diameter determines the header tube diameter (not exactly correct). To a point, a header manufacturer can control the speed of the exhaust gas by changing the diameter of the primary tube. The smaller the diameter of the tube, the faster the exhaust will flow (keeping in mind the exhaust flow slows as it cools). A clever header builder will recognize that by varying both the length and the diameter of a given primary tube in a header, those tubes can be tuned to provide the largest amount of inertial scavenging.

As you can see, it can get complicated.

So where on earth do you start? Reher-Morrison offers excellent advice:

“At lower engine rpm, long tubes help maintain good exhaust scavenging and increase torque output. As engine speed increases, exhaust gas velocities increase and a shorter tube length tends to work better. The best you can do is find a header tube length that offers the best compromise between low and high end power. This is also why header length can be an effective aid in tuning the rpm range of a racing engine. Most racing engines will work best with (primary) tubes between 28 and 30 inches long. It is also very important that all header tubes are as close as possible to the same length. The tubes from the rear cylinders are closer to the collectors and need a few extra twists and bends to be as long as those (tubes) at the front of the engine.”

When it comes to cheap, universal “fits-all” headers with seriously mismatched tube lengths, Reher-Morrison points out these unequal tube lengths actual create a different tune for each cylinder. Hooker Headers adds there can be as much as a 50 horsepower difference between 5,500 and 6,500 rpm when compared to a set of pipes with more or less equal lengths. That’s why primary tube length is significant.

What about zoomies (the kind of headers you see on a supercharged funny car or dragster)? The truth is they will actually run pretty well in a couple of different power bands (for example, from 3,500 to 4,500 and then again from 7,000 to 7,500 rpm). But Hooker Headers tells us they can give up something like 25-35 horsepower or more between those two ranges.
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GCRedfisher
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Re: header length n tuning

Postby GCRedfisher » Tue Dec 01, 2015 5:09 pm

Good read! Once again, the more you know,,,,,,,,,,,,,, :thumbleft:
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Re: header length n tuning

Postby jeepinocala1111 » Tue Dec 01, 2015 9:07 pm

Another thing to throw in is how the engine is setup. Example you build a low end engine not made for high rpm it will run better with some back pressure. One of the best examples of header/exhaust length is any two stroke engine they are tuned through the exhaust. I used to race a perelli 100 cc street cart engine that was tuned at 16000 produced some good power when wound up almost nothing below 10000 rpm it was a laydown cart basically you lay on your back with the engine on your side looking down the steering had a v cut out for vision on one handle there is a lever it changed the exhaust length. At lower rpm the exhaust had to be at a different length to get going once you are going good you simply let go of the lever the exhaust length changed it was like a whole new gear changing the power band. It's called a slippy pipe. Anyway that is an extreem example. I laugh every time I see a boat with open exhaust cause I used to watch at a friends dyno in Daytona as the hp dropped with cheap headers and no backpressure and they would spend a fortune on their engine and 50 bucks on their exhaust.

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Re: header length n tuning

Postby PAINKILLER » Thu Dec 03, 2015 12:28 am

Psalms 101:7 No one who practices deceit shall dwell in my house; no one who utters lies shall continue before my eyes.

bert
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Re: header length n tuning

Postby bert » Thu Mar 24, 2016 7:40 pm

long tube or short headers
engine set up with sbc with cam for torque

SWAMPHUNTER45
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Re: header length n tuning

Postby SWAMPHUNTER45 » Thu Mar 24, 2016 7:53 pm

With a SBC if your gonna spin it up on a gear drive use a standard length header. If it is a low rpm direct drive then long tubes may benefit you.

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Re: header length n tuning

Postby bert » Tue Mar 29, 2016 8:28 pm

thank you
the reason why i ask is sbc 400 cam with torque 237 gear box and spinning a 6 blade saber
i want to break free with ease , right now i have the headers that look like shortys
just trying to get most of my set up and run the hill with ease

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Re: header length n tuning

Postby SWAMPHUNTER45 » Thu Mar 31, 2016 2:24 pm

The amount of gain or loss from standard headers vs long tube would be minimal.

Wallace Racing actually has a computer program which inputs bore, stroke, cam data and target rpm. It will output data on both the headers primary and collector diameter and length if you were interested in having custom tuned headers built.

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Re: header length n tuning

Postby air-roberts » Fri Apr 14, 2017 9:15 am

my experience has been long headers cause long provides torq and thats what you want in airboating
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Re: header length n tuning

Postby Warden » Fri Apr 14, 2017 11:21 pm

It all matters , however after you pic your tube dia, and length , you will make most of your gains or loses with your collectors ( type/ size ). I used to stress over tubes being equal length ! Its not so important in reality ( i won more races with unequal ) but won more Dyno shoot outs with equal tube length ! I would think a try Y collector with 2 or 3 step tubes would be air boat friendly in general that being said , we have seen 28hp gains and losses on the chassis dyno with 340 inches of chev SB all at the colector and in 3" to 7" length changes ! Smaller dia. is almost always Faster accelerating ! Without knowing the head flow and cam #s & rod length , ( rod to stroke ratio ). Headers will be just a guess due to avalibility and price ! Some of the computer softwear is spot on with header design theses days ! I would think airboats would be the nicest thing ever to design and fab headers for ! It looks like U folks have room to spare ! W

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Re: header length n tuning

Postby SWAMPHUNTER45 » Sat Apr 15, 2017 8:13 am

I get to spend a lot of time in and out of the Branch shop with engine builds and one of the biggest problems that I observe is headers that are not properly matched to the ports. Often times the header is built totally improper in relation to the marriage of the pipe to the flange with Cadillac builds.

With a flange to pipe fit, your best flow will come from a flange that is exactly matched or just slightly larger than the heads port. Where the problem frequently lies is that the header builder will shape and insert the pipe inside the flange. Just this week I saw a boat that had a improper flange, the Cadillac engine uses a square port and this header was made using a round port flange. The header pipe was then inserted inside the round port flange and welded up. Now these headers looked fantastic externally but the loss of performance and potential for damage is huge with this set of headers.

The mismatch of port shape and internal restriction will not only reduce power but will cause heat to build up under load and pave the way to burn an exhaust valve. The reverberation will be horrible for performance and the header gaskets will blow out or burn thru very quickly.

If you have a choice the best port match will use a header pipe that is swedged and welded to the exterior of the collector. Very few header builders invest the time, labor and skill to build their headers to that standard.

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Re: header length n tuning

Postby NorthWestFloridaBoy » Fri Jun 30, 2017 6:28 pm

I went from using block huggers and true dual 2.5" exhaust on my direct drive 350 to block huggers going to 2.25" x-pipe under the oil pan then out to 2.25" duals and picked up 200 more rpm on my last boat. I'm a firm believer in a good exhaust and scavenging. Here's a link to the original post of it on here from NINE years ago...

www.southernairboat.com/phpBB3/viewtopi ... =48&t=5189


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