Exhaust Headers - Myths and Secrets...

geoklass

Active Member
Sep 3, 2018
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Monrovia, California
Good decisions come from experience. Experience comes from making bad decisions. This is a famous quote from Mark Twain.

Selecting headers for your Mustang is all about making decisions. You will have to decide which style header is best for your combination; long tube, shorty style, mid length or front exit style. You will have to decide on the primary tube diameter. Most of these decisions will depend on what you are going to do with your Mustang. Is it a street car? Is it a race car? Is it a street/strip car? So, before you make a decision about your headers, you are going to need to decide what you want to do, or are doing, with your Mustang.

If it's a street car, you want headers that have good torque all through your normal RPM range. For me, my normal range is between 3000 and 6000 RPM, with the majority being in the middle somewhere. I would say that a 1 3/4" header would be about max for 90% of most hot street cars (up to about 450 HP). And I would use a Mid Length header and not a Long Tube for that application. The Mid Length costs less and is easier to install, and the horsepower differences is about 8 to 10 HP difference at most. Myth #1, equal length primaries are essential. Not only are equal length primaries not essential, even if they were, it's almost impossible to have the primaries all having the same length in the confines of a typical Mustang engine compartment. There is just no room in there. Another thing is that street cars do not run open headers on the street, they have a full exhaust system, and we have found on the engine dyno that when the exhaust system, the mufflers and the tail pipes are installed, the difference in HP between a Long Tube Header and our Mid Length header is almost un-measurable. Both our 1 3/4" Long Tube header and our 1 3/4" Mid Length header use a 3" collector (actually, the same collector is used on both style headers). When running headers with 3" collectors and a 3" exhaust system, what do you have? You have a collector that is over 5-feet in length (from the headers to the mufflers).

A little more regarding equal length primaries. Primary length can be calculated to maximize power for a specific RPM. Not an RPM range, but a specific RPM. For instance, if you wanted to maximize power for 6000 RPM, it would not be perfectly correct at 3000 RPM. Or at 4000 RPM, or at 5000 RPM. It would be perfect only at 6000 RPM. But a street driven Mustang has to have power from 3000 to 6000, so a compromise is in order. Somewhere in the middle is usually selected when calculating primary length, maybe a little shorter if you want to slant the power curve closer to the top, or a little longer if it's a heavy car and you want a little more low and mid range torque. Life is full of compromises.

Selecting headers for a race car, and in particular, a drag car. Drag racing requires a standing start. But you can also use better tires, as in Drag Radials or all out Drag Slicks. And most drag strips these days use concrete for the launch pad on the starting line. An all out drag car typically leaves the starting line between 4500 and 5000 RPM. The drag racer is not interested in low or mid range torque, he is never going to be below 5000 RPM. A beefed up automatic transmission probably has a 5000 RPM stall. He is probably running the engine past 6500 or 7000 RPM, too. Let's say he shifts at 6800 RPM, at which time the engine RPM may drop down to about 6000 RPM on each shift. He is running in a very tight RPM range. Tuning the primary length to about 6500 RPM would be an advantage for that racer. And a lot of these drag racers are not running an exhaust system, it's open headers all the way with them. And because they are usually making more HP than a street car, they can get away with larger primary tubing, such as 1 7/8" or 2" primaries (we offer headers with up to 2 1/4" primaries for Big Block Ford engines). The racer is going to lose some torque under 5000 RPM with the larger headers, but again, they are never running in that RPM range except at the launch. We have a customer that had been running a set of our 2" Long Tube headers, open, with no exhaust system. It was a supercharged 351W Small Block, and he was making about 900 HP at the flywheel. Just for grins, we built a set of 2" primary Mid Length headers for him. All of our 2" headers use a 3 1/2" collector, Mid Length or Long Tube. His speed and ET did not see any appreciable change. There was probably a small (very small) HP loss with the Mid Length headers, but the 15 pound weight reduction using the Mid Lengths over the Long Tubes made up for any loss in HP.

And finally, the Street/Strip Mustang. The majority of our customers have (or say they have) a Street/ Strip car. What is the definition of a Street/Strip car. It's a car that is used as a daily driver and also does a little drag racing on the weekends. Every day I get phone calls from guys that want to know what size primaries they need for their "Street/Strip" car. A guy calls, he wants a 2" header, but he also wants my advice. In theory, a Street/Strip Mustang is not a street car and is not a race car, it's a little of both. I start out asking the guy how many miles he drives on the street every year. He tells me that he drives 20 miles to work Monday through Friday, and 20 miles back home. That's 40 miles a day. There are 365 days in a year, and since he does not drive to work on the weekends, he drives 40 miles a day for 260 days a year. I'm doing some calculations while we are talking. This is just a tad over 10,000 miles a year. I then ask him how often he takes his Mustang to the track. His response is about once a month when the track is open (which in his case is 7 months a year). I asked him how many runs he usually makes when he is drag racing, and he thinks he averages about six 1/4 passes for each time he is at the track. Think about this for a minute. Six passes at 1/4 mile per event is 1 1/2 mile per event, times 7 events, equals 10 miles of drag runs per year. 10,000 miles on the street and 10 miles at the track is NOT a Street/Strip car. It's a street car with OCCASIONAL runs at the drag strip. This guy needs 1 3/4" headers. The light bulb goes off in his head. (A) he agrees and orders a set of headers that works for the vast majority of his driving miles and (B) saves him money over the 2" headers. I hope that all this makes sense to you. Feel free to comment or ask questions, or give RCI a call during the week at (909) 552-3690.

George Klass
RCI Custom Mustang Headers
Any Mustang (1965-1970) and any FOX chassis Mustang
Headers for any pushrod V-8 Ford engine.
www.rciheaders.com
 
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