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#1
More than any other question I get everyday, "How big should the primaries be on my long tube headers?" is still number one. More than "Are the primaries equal length?", or "Are the headers stainless steel or mild steel?", or even "How much are the Long Tube Headers?" It's a legitimate question, probably based on the theory that if the primaries are too small it will hurt power. I used the word "theory" because in many cases, smaller diameter primaries may boost power. There are many questions that need to be addressed before you decide on the diameter of the primary tubes on a set of headers, such as maximum expected horsepower, is it a race car, a street car, a street/strip car, engine displacement, transmission (stick or automatic), vehicle weight, cylinder head, and on and on. If the engine is a Small Block Ford (289, 302, 351W, etc.) there is typically one common issue, and it's the cylinder head and in particular, the exhaust valve. The stock exhaust valve on the Mustang 5.0L (302) engine is 1.45" in diameter. In the vast majority of aftermarket aluminum cylinder heads, the exhaust valve diameters have been increased to 1.60". This is a pretty big improvement (about 11%), but 1.60" diameter is still smaller than 1 5/8" (which is 1.625"). And we must remember that the exhaust does not flow through a 1.60" diameter hole (the outside diameter of the head of the valve) but through the valve seat, which is probably about 1 1/2" in diameter, which is further reduced by having the stem of the valve in the way. So, the real question should be, "Why would anyone want a primary tube larger in diameter than the initial restriction found around the exhaust valve area?"

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A larger diameter tube moves more volume (exhaust gasses in this case) versus a smaller diameter tube that moves less volume, but moves the gasses at a higher velocity. Big tube equals more volume at a slower speed, small tube means less volume but at a higher velocity. On paper, it would look like the larger volume would make more power but the speed of the gasses leaving the exhaust port is important because of one issue, and that is "scavenging". As the exhaust gasses travel down the primary tubes, if they are moving fast enough, they can create a partial vacuum, which can then help pull the next charge with it. A large diameter tube, with it's slower moving exhaust gasses, may not have that advantage. Here is my rule of thumb. If you are running a light weight small block Ford powered drag race car, that leaves the starting line at or above 5,000 RPM, and you get your max power at or above 7,500 RPM, you will make more HP with a larger diameter primaries (1 7/8", 2", 2 1/8"), everything else being equal. On the other hand, if you are racing a small block Ford powered "street/strip" Mustang, that weighs 3,200 (or more) pounds, and have your maximum power at 6,000 to 6,500 RPM, you may get better results with 1 3/4" diameter primaries. With a "street/strip" Mustang at the drag strip, you need plenty of mid range power, from 3,800 RPM on up to your shift points. With the all out race car, that guy could care less about "mid range power", he leaves the line at or above 5,000 RPM and is never below about 6,500 or 7,000 RPM going down the track. At this point, we have run head on into the most important rule about primary tube diameter, and it is the word "compromise". It has everything to do with what you are going to do with your Mustang. Is it mostly a race car or mostly a street car? The majority of our customers like to refer to their Mustangs as "street/strip" cars. I ask them, "How many miles do you put on your car in a normal year?". I would say that the average answer is between 8,000 and 10,000 miles a year (except in Los Angeles, where it's usually 15,000 miles). My next question is, "How many times do your race at the drag strip?" (street racing is meaningless for this discussion). The typical answer is that the average number of times they actually race at the track per year is about 5 to 6 times per year, and they average about four to six 1/4 mile passes. If they make six passes at the track, six times times per year, that is equal to 9 miles at the track, and 9,000 on the street. Folks, that is not a "street/strip" car, that is a street car that occasionally goes to the drag strip. Even if they put 90 miles on the drag strip, that is till only 1% of the total miles Your headers need to fit what you actually DO with your Mustang. RCI builds headers with your choice of primary sizes of 1 3/4", 1 7/8", 2", and 2 1/8" for Small Block Ford cylinder heads (and up to 2 1/4" for the Big Blocks). By the way, we have drag race customers that are making up to 900 HP with 1 3/4" primary headers...

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RCI has done and continues to do dyno testing comparing primary tube sizes, but one fact continues to stand out. Large primary tubing may help HP in the upper RPM ranges, but it comes at the expense of mid range HP. On the other side, smaller primary tubing may help HP in the mid ranges but lose some at the higher RPM ranges. Of course, smaller and larger are relative numbers, how small is small and how large is large? Technically with small block Ford wedge heads, small is 1 5/8" and large is 2", a spread of 0.375" (a little more than 1/4" and less than 1/2"). It's even smaller if the header is a 1 3/4" (which is almost the defacto standard these days), in which case we are talking about a spread of 1/4". Like many things in life, there are compromises. The question is always "What am I willing to give up to have something else?" Again, the answer to that question is dependent on the answer to this question, "What am I going to do with this car?" If it's going to be a dual purpose car, which of the two purposes are going to be dominant over the other? If most of your driving is going to be on the street, and keeping the engine under 6,200 RPM, with only an occasional blast at the drag strip, that is a different header than one on the same car that is a race car, with an engine that peaks HP above 7,000 RPM and that is almost never driven on the street. I have to assume that this is obvious to everyone. And yet, too many enthusiasts seem to think "big" instead of "the right" size, and that also includes the race car crowd too. I have a customer that has a '90 Mustang, 351W, pretty tricked out engine, and it runs pretty strong (it's a street/strip car but mostly strip). He told me that the car weighs about 3,000 lbs. with him in it. He had been running a 5-speed and recently swapped it for a Powerglide. He had been running a 2" header with the 3 1/2" collectors (standard on the 2" headers). The speed and ET had dropped off a little after the trans swap, said it just didn't seem to pull as hard as it did before. We talked for awhile and I reminded him that he went from a 5-speed to a 2-speed transmission. The Powerglide is a great transmission, and is very consistent (my customer is a bracket racer), but it requires a little more mid range power. With the 5-speed he was using, he was shifting at 7,300 RPM which dropped down to about 6,700 RPM on each shift. This meant that he was operating in a power band between 6,700 and 7,300 RPM. The 'Glide has only two speeds, "low" and "drive". When he shifted at 7,300 RPM the RPM dropped down considerably more. The wider your power band, the more the header size needs to be dealt with. A typical street car needs to have a power band somewhere between 3,000 and 6,250 RPM, give or take. My customer allowed me to talk him into going with a 1 7/8" header (with the standard 3" collector) to pick up a little in the mid range and he said he can feel it pulling hard again. He lost about 1 MPH over the 5-speed but the ET came back to what he was running before. Another example of "everything is dependent on everything else". Next season we are going to try a 1 7/8" stepped to a 2" header (with a 3 1/2" collector), just to see if we can push the MPH up a little bit. We have been building step headers for years, like the 1 3/4" stepped up to a 1 7/8" and the 1 7/8" stepped up to a 2". I'm not really that pumped up about them myself, sometimes they help and sometimes you can't see much if any difference. On a street car, I have never seen an advantage, on a street/strip or a race car, sometimes I do and sometimes I don't. It's all just like the reality of life, "what is, is, and what ain't, ain't"...

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