Header Design Theory - Fuelly Forums
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Old 10-12-2007, 11:21 AM   #1
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Header Design Theory

I have been working on cars for years and have always enjoyed improving the performance of cars by adding a set of headers. Now Im having a great time coming up with ideas for high mpg header designs. Currently there are no purpose designed mpg headers out there, but by selecting the right header it can be made to give up better torque and mpg.

Is a header more efficent than a cast iron maifold? Yes. I know many of you have achieved great mpg with the stock cast iron manifold, but there is even more mpg and torque that can be squeezed out of an engine by using a header.

A cast iron manifold is a corporate cost effective economic decision made by every auto manufacturer. If you make 1 million cars and spend $20 on a cast iron manifold, that is 20 million dollars spent. Now if a header cost $50, that's 30 million an auto manufacturer can save and still get reasonable performance and pass emissions by using a cast iron manifold.

There are a number of header designs out there, I will try to give an overview and suggest which of the headers to use and avoid designs that can acutally loose power.

The basic header is three parts:

The primary tubes, one for each cylinder that are all equal length.

The collector, that joins all the tubes together.

The tuned pipe, this is a pipe 4 to 14 inches long after the collector, some designs omit this pipe and just have the collector going right into the tail pipe.

Bigger is not better, but longer is. The primaries produce more torque at lower rpm by being longer and smaller keeping velocities high. Using a list of tables I made up from (bgsoflex.com/bestheader.html) I took a 1600 engine and looked for the best primary length at 3000 rpm, it is 39 inches. Now if you use the Harley exhaust pipe calculator at (nightrider.com/biketech/calc_exhaustlength.htm), it wants to you calculate your best header length by taking half of you overall rpm, that would be 1500 rpm and that primary would be 79 inches. Where that same 1600 engine tuned for a performance header would have primaries at 18 inches for 6500 rpm.

I drew up a list of c.c. to c.u. so you can play around with primary length at the Bowling and Grippos site at bgoflex.com, this calcualtor only uses cubic inches. To convert you take .061 X c.c. to get cubic inches.


Now shifting at 3000 is the key for great city mpg, but a number of cars out there like Honda, Acura, and the Miata, all turn 3400 to 3800 rpm at a highway cruise of 65 to 70 mph. The Toyota MR2 being worst at 4000 rpm at 70 mph. You should be able to tune to a higher rpm for cruise without having to make major modifications.

Now the off the shelf headers out there are tuned for 6500 to 9000 rpm. There are 3 types of headers you can buy: Shorties, Tri-y or 4-2-1's, and the classic 4 into 1's.

I'll start off with the classic 4 into 1's, they have been around since the early 60's. If you use the tables for a VW 1600 in the VW Hot Rod book, their calculations came from a book I wish I could get. "The Scientific Design Of Exhaust And Intake Systems" by Phillip H. Smith.

He recommends for a perfect broad range header to have 53.6 inch long primaries in 1 1/4 to 1 1/2 joining in a collector of 366 cubic inches and having a tuned pipe length of 6 inches and a diameter 50% larger than the primarie diameter, or 2.5 inches. This is old math from 1969, but the tables still apply to today. No one I ever have known ran a collector box of 366 c.u. That kind of collector would start at a 3 inch diameter and taper like a funnel down to 2.5 inches and have a length of, just off the top of my head 90 inches before you go to the 6 inch piece of tuned pipe. Today's collectors are about the size of a coffee cup and taped down to a 2.5 inch collector.

How well does a 4 into 1 work? It produced 7.5 more horsepower at 3000 rpm. My own personal experience with 4 VW's went from 26 mpg to 34 mpg, by just adding a 4 into 1 header, an 009 distributor which makes the timing advance more, and rejet the carb from a 25 to a 27 main jet.

Also one thing that helps a header collector work is rotational firing. Some call it a sequenced header, simply the pipes join in a square of four into the collector and in sequnce with the firing order spiral around in a circle, this makes a smooth torque band. There are headers out there that are not sequenced and still work, but provide a wavy torque band and dont work as well.

Headers work, here's another story about 4 into 1 headers. The big block Chevy has one of the best cast iron exhaust manifolds, its close to a modern shorty header in design, but by swapping out the cast iron manifold for a set of headers which had 44 inch primaries, into a 4 inch collector, tapered down to 2.5 inches by 7 inches long this engine produced 65 more pounds of torque.

My 57 Chevy had a 396 with headers, it was getting 14 mpg through a set of 2.5 inch pipes, the dual exhaust system that was on there was ratty, so I replaced it with dual 2 inch exhaust and rejetted the carb. This got me up to 18 mpg and still pulled to 6500 rpm.

Now the Tri-y or 4-2-1 systems have been around since the Mustang Racing program of the 60's, this is a system noted for its improved low end torque. This system works by taking the 4 primaries, pairing them off into 2 primaries and then joining them into a 2 into 1 collector. The collectors are small and lend themselves to more torque and better mid range than a 4 into 1. The 4-2-1'a are my choice for best header, if you can get them. Some manufacturers only make one type of header for one engine, some compromises may have to be made in your design choice.

The last design is a relativly new design, the Shorty header is just that. With the market range of cars broad, shorties came in the late 80's and were made to fit in tight spaces and give a little boost in performance. As we know from the explanation of longer tubes give more torque, shorties are being sold with very short primaries in the neighborhood of 13 to 18 inches. This would give a calulated tuning of 6000 to 9000 rpm.

Would I recommend a shorty, yes, some cars are limited by their choice in headers, but it is better to have a ballanced system, than some of the highly restrictive cast iron manifolds out there. Some cast iron manifold are very restrictive and actually make the engine push harder to get the exhaust out. By helping the engine have a freer path, this should net some better torque and mpg.

Now here's some comparison numbers, of stock vs. 4-2-1 vs. 4 into 1. You can view the stats at (racingbeat.com/2001header/headertest.htm) The first numbers are stock, then the 4-2-1, and the 4 into 1. These numbers are torque only.


This test was performed on a 98 Miata 1800, notice how the 4-2-1 made better torque than the 4 into 1 until 3500 rpm. Also notice that the 4 into 1 made better torque than the stock system, which is a factory 4-2-1, but has cast iron collectors.

So the secrete Im getting at here is the efficient design of a header and the flow control of the exhaust will make better mpg and a huge torque band. Get your exhaust pipe too big and all your mpg is going to flow out the tail pipe.

For my current test as soon as I get another car be it a Saturn or Cavalier, I plan to install a header and run it into the stock system. From there I'll take tests and see if it is worth to put on a larger muffler, or upgrade to a larger over all exhaust system diameter. Also I plan to do the other mods, swichable IAT, HAI, and LRR's.

Now I would love to take a set of headers and cut them up and really stretch out the pipes and see what numbers I would come up with. Until next time, I hope you have enjoyed the read and got some inspiration to play around with headers.

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Old 10-12-2007, 06:18 PM   #2
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interesting theory. why didnt you get into pipe diameter and having stepped primaries and have smoother merges in the collectors and all of those other important little tricks?

also what about the sequential vs non-sequential pairing of primaries in the tri-y setup? you are not going to open up the discussion there?? there is so much more than just length and the amount of collectors involved, particularly when most vehicles now are fuel injected.

i agree that any oem exhaust manifold out there has room to improve, but often times most aftermarket manifolds are designed to flow more and/or flow better, never flow the same volume but flow better. do you follow what i mean?

ideally for fuel economy, you dont want to cater to the flowing larger volumes. the right design for fuel economy would be designed for the more conservative right foot.

don't waste your time or time will waste you
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Old 10-12-2007, 10:51 PM   #3
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yea cast stuff isnt smooth at all...

pipe size is important, too small and its restrcting, too big and it causes weird air curents, thus slowing the air down, just right and its gettin smooth airflow with max velocity
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Old 10-13-2007, 03:15 AM   #4
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I think I may have had the same book on exhaust design years ago. I remember trying to design a FE header for a '67 TR4A (this was 1978 I think) and coming up with 4 small pipes running all the way to the back bumper and then going into a 4-1, just as you recommended. (2l, about 90 inch runner length I think. Aiming for a torque peak closer to 2k rpm) I think the longer runner length would make a clear FE gain but building it sure wouldn't be cheap.

My brother (slight nutbag) thought it would look better if we went out through the fender well and all the way down the passenger side with it to the collector. It might have looked better but I think I would have had trouble talking my girlfriend into climbing through my door when it was raining and the top was up.

Great post BTW.

"If you want to save gas I suggest you permanently remove the drivers seat and steering wheel. That seems to help." -Oscar Halverson

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Old 10-13-2007, 07:14 AM   #5
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Theory discussion

Yes, I didn't want to make the Header Theory too long, but on the collector design, if you are familiar with the Merged collector, it is this beautifully hand crafted aerodynamic merge of four pipes in a collector. The problem I have encountered with a Baja Bug, is when a friend of mine switched from a coffee cup style collector that had four tubes firing next to each other, the one firing pipe would send a signal back to the other three pipes and stop thier flow. This created good fe, but when my friend switched to a Merged collector, it has such an antireversionay effect, it flowed too much and his fe went down to 28 mpg. I also experienced the effect when I went from a muffler on my header to a stinger non restricted pipe, that I went right down to 28 mpg from 34.

Now from the Tri-y's I have seen in magazines, they seem to be sequenced according to the firing order, meaning on one rotation of the crank, two cylinders fire 180 degrees apart and go into the collector left-right in balance.

Primary diameter is every important on fe, from my bug experiments, going from a header with 1 1/2 primaries to a header with 1 5/8 primaries killed low end torque. Since allot of those headers out there want to be full race, ie. giant primaries, I found the cheaper headers have smaller primaries, cost can protect you from buying a header that is too big. But while messing around with the 1 5/8 header, I found that you can choke it down by placing a restictive exhaust after the collector. With the bug I went from a stinger, to a glass pack, then a small diameter turbo muffler, the later giving the best fe. That is the theory I want to use, I want to add a header to a stock system.

Now fuel injection, especially with a reprogrammed f/a ratio, I havent heard of anyone melting down from going too lean yet. I imagine if a system had too much flow you could run too lean.

I don't know if anyone out there has added a header and noted the results.
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Old 10-13-2007, 07:27 AM   #6
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Originally Posted by Scott View Post
I found that you can choke it down by placing a restictive exhaust after the collector. With the bug I went from a stinger, to a glass pack, then a small diameter turbo muffler, the later giving the best fe. That is the theory I want to use, I want to add a header to a stock system.
very interesting!! i think i might be willing to do this just because my oem exhaust is fine, but my header has a massive dent on one of the down pipes.

on my old car (94 civic with a 1.6 vtec motor) the only difference in getting an aftermarket header was the weight savings. power was bout the same and so was the fuel economy. getting a larger diameter exhaust hurt fuel economy though. it was considerably larger.

as far as the tri-y headers go, the ones made for power are the ones that pair sequentially, i.e with the next 180degree combustions. this just makes more power with the pedal down. the non-sequentially paired header pairs with the combustion that is 360degrees away. the non sequential header will be better for smoother, lighter throttle cruising. the idle with the non sequential is also noticeably smoother. most oem honda manifolds that have the tri-y are non-sequential, unfortunately they also usually do not have equal length or long primaries....

don't waste your time or time will waste you
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