Inliners International Forums General Discussion Engines headers on 261

I am building a set of headers for my 261 with 4 equal length 44" tubes into a 10" collector. I am thinking about the Siamesed ports. The firing on 4 and 5 is 240 degrees apart and on 2 and 3 120 degrees apart. I want the headers to scavenge properly and was thinking that if I use 1.625 pipes 1 and 6 and 1.75 pipes on the other two larger ports, the scavenging balance will be off. I'm thinking I should use 1.625 pipe on all ports with only a flare to accommodate the two larger ports. I think the firing and subsequent exhaust stroke on each siamese is far enough apart not to interfere with each other. Has anyone built tuned headers for a 235/261 and what did you experience?

Thanks
Warren

I wish I had the answer. There are other people here that are way more knowledgeable than I am. This is just an opinion, but if you are going to run a full exhaust system it won’t make much difference which way you go. Open headers, different story. Sorry I can’t be more help. Jay

If you build them that long they will be loud as hell, I built shorter ones as close as i could and used a dual pattern Crower cam with the intake and exhaust open at the same time and it ran great.

Hi Warren . . .

What are your plans for this 261 - racing or street driving?

regards,
stock49

Sorry, I don.t know how to post pictures on this site. I wanted to add a picture to members rides The engine is in an Bantam coupe running J/altered. The engine has a McGurk roller cam, triple 97s, a 120 mill on the head. I was running Fentons with a short extension last year, but want to get more scavenging to help that poor exhaust. No exhaust system. Sorry about the poor start to this thread.

Warren

No worries - so long as the thread gets going . . . Sounds like a great project!

I am a street guy (stuck in the early 50s') so I can't offer much in the way of experience here.

But I find the work of David Vizard instructive Exhaust Science Demystified. His work suggests that the primary tubes on the siamese ports most definitely need to be larger because they will carry twice the volume.

The article includes a chart showing header primary tube sizes based on CFM flow rates. The displacement of a 261 puts it on the lower left side of the chart in the neighborhood of ~1.25" primaries.

His experience suggests that getting the collector sized right (diameter & length) is just as important as the primary pipes - and that bigger is not better. His experience suggests that the collector not be left open - and that a track-tuned length of secondary piping is needed to finish the job.

Send me a PM. We should be able to get a photo in the Rides section.

regards,
stock49

W/r/t THE "Primary Ex. Pipe Sizing." chart?
My browser cuts off the legend identifying the scale of the displacement range, begins at 100 goes to 280.
Cubic what?

Click on the chart. It allows you to browse all the pics in the article full size. The scale is in CFM.

Thanks, but neither of my browsers will do this. You used CFM of 1 cylinder × RPM × VE ÷ 3,456?
261 ÷ 6 × 5,000 × 100% ÷ 3,456 = 63 CFM, which puts it closer to smaller than 1" ID. I don't think the VE will ever reach 100, and certainly not above 5,000.

That diameter provides enough area for a 43-1/2" cylinder at good exhaust velocity. The problem is that the Gen-2 individual (#1 & 6) port window in the casting is much larger than that, and it's a rectangle with rounded corners, about 1-1/4" tall × 1-5/16" wide. The area is about 1.52 In^2. assuming 3/8" corner radius, but the diagonal is 1.50" long (and longer if the corner radius is smaller).
The choices include:
1. use a primary tube large enough to completely enclose the port's diagonal measurement (with 18 gauge wall: 1.60" OD - the closest size is 1-5/8"), but too large in area, or (better, more work)
2. re-form (flatten 4 sides) of a short section of 1-5/8" OD tube to match the port outline's shape, then transition its outlet down and join to a smaller primary.

IMHO masking the port anywhere near the port face is an obstruction of greater effect than raising the gas velocity for better extraction. There is a well respected maximum taper (for megaphones) of 14° which should be used to minimize flow loss (anything sharper is restrictive), which would make the primary a baffle cone tapering from 1-5/8" down to 1".

Using another small engine as a model: the Triumph 650 (40" twin) has 1.50" OD (1.40" ID @ 18 ga.) primaries. This is not merely sufficient, the bike is slower with any larger tube up through 800cc (24" cylinders) and 60 hp. A 1.75" "TT pipe" kills power below 4,000 RPM.
That's 180 hp as a 6 cylinder...
The engine will probably be more tolerant of long overlap at low speed if the high exhaust exit speed helps kill reversion.

If I were offering advice on constructing a tube header, I would suggest using the largest possible radius tube leaving the flange (I found 3" radius), even if it must turn 100 or 110 degrees (back on itself to avoid steering, etc.) to get the best possible flow.

Your browsers are probably configured to block all pop-ups. The pics are displayed in a photo-gallery-focus pop up.

I can’t agree more. The Vizard calculations speak to the primary tube diameters for the majority of the running length. Your points on transitioning to that diameter are spot on and are echoed by Vizard himself when discussing some tuners' obsessions with ‘equal length’ length primary tubes: “A positive power-increasing attribute of differing primary lengths is that it allows larger-radius, higher-flowing bends and more convenient pipe routing to the collector in often confined engine bays”.

What I like most about Vizard’s insights is that he asks us to look at the volume of exhaust gas at hand. Swept cylinder volume in cubic inches can be converted to cubic feet by dividing by 1728. Multiplying that by the target RPM range yields the volume of gas to be expelled: 261/6 = 43.5 / 1728 = 0.025173611111 * 3000 RPM = 75.5 CFM
@4000 = 100.69 CFM
@5000 = 125.86 CFM.
Your point on the impact of volumetric efficiency echoes Vizards key take away on exhaust system design – when in doubt go smaller.

As for the question posed in the OP – the Vizard calculation suggest primary pipes sized from the lower left side of the range (for the single valve ports) and in the middle of the range for the siamesed ports.

I just read the linked article. A lot of good info there for sure. Twice the exhaust flow in the center ports would need a bigger primary pipe for sure. Based on that article and what has been posted here go maybe 1 inch on the single cylinder ports after being carefully necked down from a port match at the head. Same thing for the shared ports only down to 1 5/8 as best I can see on the chart. Not to be too far off topic, what about the people who either run a single manifold or a split manifold set up like Fenton’s, Langtons, or even the factory split set up. I would assume that the primary length could at most be considered the length of the farthest port from the manifold outlet which would be the “collector”. Would there be any way to use pipe size and length comming off a manifold to help with exhaust scavenging? Or are all the gains using split manifolds over stock just a reduction in back pressure? Jay

Indeed your points are well taken and I can't agree more with your advice/ideas . . .

Moreover, the Vizard article is bent-8 focused around 4-into-1 affairs . . .

But we're talking straight sixes here. In this regard I find the work of Phil Smith enlightening:


In the book he discusses a 3-into-2-into-1 design:


Perhaps the smaller single port primaries should transition into a common 'bank' primary somewhere before the final collector - a 4-into-2-into-1 design . . .

I want to thank everyone who chimed in. This has been very enlightening. For several reasons I am going to compromise. I don’t have the resources for extensive testing and no one here including my exhaust guy have the where with all to truly fine tune this system. I will go to the smaller pipe by transitioning down to that. On the end ports, I will start with an 8” x 1.625 pipe fit to the flange, this will transition into a 12 x 1.5” pipe and finally a 24 x 1.25” pipe. The siamesed ports will start with an 8 x 1.75” into a 12 x 1.625” and then a 24 x 1.5”. All this will go into a 3” x 10” collector and necked down to a 2” x 8” reducer. The Siamese pipe is approximately 140% the size of the end pipes or 70% / cylinder. Despite what has been said, both cylinders don’t empty at the same time and travelling at 1125’/sec, the tube should be virtually evacuated before the next charge arrives. This exhaust system is expected to work in the 3500 to 5000 RPM range. There is no need for clearance problems therefore the primaries will be a long gradual bend. If anyone sees major flaws with this compromise or has solutions that can work within the basic plan, please say so.
Thanks again
Warren

"cubic inches can be converted to cubic feet by dividing by 1728". They can, but it's not accurate for a 4-stroke engine.
It's 3,456 because each cycle requires 720 degrees. Divide your calculated CFM by 2.

AFAIK the best design for an L6 street car has been collecting the 1-2-3 and 4-5-6 separately, into dual exhaust, no crossover, for about 100 years.

You are correct on the arithmetic. Low RPM small cubic inch sixes just don't move much air.

The application is a J/altered drag car. Warren sent me a picture of his work in progress:


He's tried the traditional design already on the track (as he mentioned above) Fenton's with a short section of pipe.

What a cool picture! I would love to see the difference between the Fenton’s and the custom header in terms of how much more HP the header provides (and at what RPM) in an open exhaust situation. Everything else (with the exception of timing and jetting);being the same. I have a factory “Y” pipe for the later 250 Chevy used with the intergrated cylinder head. The 2 smaller pipes (2” In diameter) join the split manifold into a single 2.5 inch pipe. Holding the Y pipe up to the manifold shows that the 2 smaller pipes snake around to make it an equal distance from the manifold to the where they come together into the larger pipe. Having owned a pickup truck with that pipe on it those pipes were not routed for clearance reasons. After seeing the picture of the 3-2-1 header posted, could the factory be on to something?

Some of the power difference is based on how large is the overlap triangle area (time in degrees that both valves are open @ TDC × their average lift). The returning exhaust pulse has to have a "window" to enter the chamber and induce intake vacuum.
A very mild cam won't show much, but as the .050" duration goes past perhaps 230° the advantage picks up steam.

Those extensions should help the manifold.
Extra length develops more intake velocity and inertia, and also makes the 90° turn in the casting farther away from the port face.

Panic, very interesting about the “triangle”. Makes a lot of sense to me that to a point at least, the overlap amount would would be a big factor in how much vacuum that the exhaust wave could pull on the intake port. Too much overlap could in theory at least, pull part of the intake charge right out the exhaust pipe! Please forgive me for asking a question, when I try to figure my triangle do I multiply my overlapp in degrees a .050 lift x valve lift at that point? Jay

In a more conventional head where the intake and exhaust valves sit side-by-side the scavenging effect may well pull on the intake port during overlap. But we are talking about the last of the stovebolt designs here:

scavenging must first evacuate the un-swept combustion chamber volume before it can start to work on the intake valve which sits shrouded in a recess in the head and completely covered by the piston at TDC. IMHO Just evacuating the un-swept combustion chamber is benefit enough from scavenging. Moreover, I think the shrouding of the intake deters reversion until very low RPM allowing for more degrees of overlap (a bigger triangle).

The combustion chamber shape and type would have to have a major effect on what size triangle to run. What would happen if the wave hit the chamber before the intake valve was open, in a low overlap cam? Would it empty the chamber, leaving it in a vacuum for when the intake valve opens? Would that be better than not having a wave at all? Jay

Now we're hijacking Warren's thread and turning it into a stovebolt Cam discussion. Time for a new thread . . .

Very true, and I apologize. I hope Warren will send a picture of the finished header. Jay

Hijack, hell no. I have been on a lot of forums and this by far is the most intelligent conversation I have ever witnessed. The engine is out of the car and apart right now so it may be two months before the package is finished. Keith (stock 49) posted the above picture for me and will have a picture of the race car posted in members rides soon. If I haven't figured out how to post pictures by then, I will take advantage of his good will to get a picture of the finished product here. By the way, I have been drawing out charts of cam timing and valve action for the past couple of day because I figure it is integral to header design.

Warren

Sometimes when people have trouble posting pictures here I refer them the the Stovebolt site because they have a forum just for computer stuff and the site is very similar to this one. "The Short Bus" There is a thread about posting images and it used to be the same as here but I just checked and they have changed. Now it looks like they can just drag & drop like on the HAMB. You don't have to pay an online host.

I slept on my earlier reply and thought that one last post on the use of these Vizard CFM numbers was is in order:

His chart is based on flow-bench data and dyno testing.

The challenge is to estimate the CFM output of a particular engine so that the data contained in the chart can be put to use.

My personal take on that estimation is what I have offered here.

As I stated above – panic is spot-on about the arithmetic in converting cubic-inches to cubic-feet given a 4 cycle engine. So I went back and looked at my notes from when I was designing the exhaust for my early 50's Road Job. The use of 1728 as a constant is a sizing convention that also accounts for temperature – resulting in a handy formula using just displacement and RPM range. These models/conventions have been created over time to avoid what is actually a messy and cumbersome calculation when one tries to become more precise.

If an engine were simply an air pump (2-cycles) the amount of air entering the cylinder and the amount exiting would be equal - making the divisor for converting cubic inches to cubic feet 1728. But a gasoline engine is doing more than just pumping air – there is a compression and power stroke in between the intake and exhaust strokes – so only every other revolution is in play at a given RPM – making the correct divisor 3456. In addition, what is entering the engine (air + fuel) is not what is exiting the engine after compression & combustion (carbon dioxide + water). Moreover, the gaseous mixture is entering the engine at temperature A and exiting at temperature B - where the difference between B & A involves many variables.

When one tries to get rigorous in calculating a theoretical exhaust flow rate for a given displacement – one enters the world of combustion chemistry/efficiency and thermodynamics where temperatures are actually a gradient both on the way in and on the way out . . .

Here’s a good discussion on an engineering site ENG-TIPS that shows what happens when one attempts a more formal approach. Notice how quickly the debate splinters off in all directions depending the engineering focus: stoichiometry, volumetric efficiency, combustion efficiency, throttle position, effect of ignition timing on exhaust gas temperature (and therefore whether or not the engine is turning fuel into mechanical motion or just throwing heat – unproductively because of poor tuning). And then the thermodynamics guys chime in that exhaust gas is not an “Ideal Gas” and all of sudden what looks like some really cool applied-thermo formulas turns into many countervailing variables and caveats.

Which brings us full circle to the use of sizing conventions that are easy to compute and get us close enough to choose an exhaust pipe diameter and get on with building a system.

If you spend some time reading on this topic you will find several conventions in use. The guys from the bigger-is-better-camp immediately zero in on the effect of temperature deltas and peak EGT – for example the extreme 80-degree-intake/1800-degrees-exhaust(used in the post linked above) which yields an expansion factor greater than 4x - suggesting that 50 CFM of displaced flow becomes 200 CFM of exhaust flow due to heating!

But this maximum temperature exists for just a short time in a water cooled exhaust port. Dissipation begins as soon as the exhaust valve opens and continues as the exhaust slug moves further into the header and ultimately the exhaust system. Visualize a balloon blown up and tied-shut on a hot Sunny day – now toss it into the deep freezer . . . with a temperature differential of just 90 degrees the volume of air trapped by the balloon quickly collapses causing the balloon to shrivel. The same thing happens to a slog of exhaust as it moves from the exhaust port to the tail-pipe – and the temperature differential is much greater so the collapse is even more pronounced.

As a result the sizing convention needs to consider average flow rate – not the max or the min – which is a function of the displaced volume and the temperature differential. For a long tube header primary or head-pipe the mid-point or 2x can be used. Which is where the sizing convention comes from (Cubic-Inches/3456)*(RPM)*(TeF)=(Cubic-Inches/3465)*(RPM)*2=(Cubic-Inches/1728)*RPM after combining the constants in the formula.

Some suggest that if one is building say a shorty header or a log-manifold then a thermal expansion factor of 2.25x or 2.5x should be utilized because of the close the proximity to the exhaust valves. Still others argue that the tube should be sized closer to the peak expansion say 3x – but using super high exhaust gas temperatures is a whole other kettle of fish – starting with why is the EGT so high in the first place? Some will have you believe that the 2x factor is good enough for any application.

Others will remind that 100% volumetric efficiency is unlikely so the Cubic-Inch-Displacement value should be reduced to .9 or .85. And then the chemists will chime in and remind us that the combustion process for gasoline actually increases the expelled content by 7-10% depending on combustion efficiency. Considered in concert the VE and Combustion effects essentially cancel each other out.

For me the thermal expansion factor remains open for debate – but 2x seems a great starting point.

For the curious there are many scholarly papers out there discussing this EGT topic in extreme detail. The emissions police of the past have funded a lot of research in this area whilst trying to engineer catalytic converter placement into then contemporary exhaust system designs. This piece out of Aristotle University is an interesting albeit difficult read at times. For me the most important take away from the paper is that long-primary-tube exhaust systems retain the least amount heat – and failed to lite the catalyst in testing. The comparison charts are interesting – using the exact same 1.8L inline 4 they compare EGT and Tube Temps for a 4 into 1; a 4 into 2 into 1; and a 4 directly into the Cat design.

Stock49 - I have read a number of articles on header design but your summary above is the best I have ever read. Great Job.

Warren - I have built 2 different 4 into 1 headers and a dual header set for my 302 GMC. It has the same exhaust layout as your 261. I asked for Inliner help and summarized my experiences on the Inliners Bulletin Board topic:"GMC/Old Chevy Single Headre Design Questions", 11/13/10 - 11/17/10. Another Inliner discussion titled:" How to make an I6 exhaust header" appeared from 7/19/10 - 10/14/10. You might want to use the Bulletin Board's search feature to look these up. Two other good discussions appeared in the Landracing Forum by some famous inline 6 Land Speed Record holders. They are: "321 GMC inline 6 header", appeared from 9/2/07 - 10/11/07 at the following address which still works: http://www.landracing.com/forum/index.php?action=printpage;topic=2878.0 A second one titled: "INLINE SIX EXHAUST HEADER" appeared on 7/6/08 - 7/9/08 at same address but topic number is changed to =4160.0

Hope this old info helps you. Good Luck.

Frank Hainey , radar

All this reading, I'm exhausted (pun intended). Frank, your search started almost 10 years ago, what did you finally come up with and does it work? Pipe size, pipe diameter, primary runner length etc. I have done no porting on the head, but I am clearanceing around the exhaust valve at .300-.500 lift.

Thanks for the leads.
Warren

Warren - Nice pun.

The first header was a 4 into 1 with 1 5/8" and 2" pipes (to match the ports) 36 in long and a 3" collector 12 in long. I bought the tubing from Doug Robinson, (a Land Speed record holder) and he thought 2" pipes were too large. It ran well but 1 7/8" pipes might have been better as Doug suggested.

At the time, I thought the opposite knowing that twice a much gas is flowing out the middle ports. I made a second 1 into 4 header to match a newly ported head and to equalize flow rates. It had 1 3/4" and 2 1/4" pipes. The engine and ported head developed little power below 2000 rpm. I learned the hard way that big pipes do not work and equalizing average flow rates is not the answer. I now see that exhaust pulses are short, separated and flow at high speed. Two cylinders can be serviced by the same size or a slightly larger pipe compared to a single cylinder.

When the smaller header was put on the ported head the car's performance improved quite a bit even though the pipes were smaller than the enlarged ports. In fact the car turned the best time and speed ever: 15.87 sec and 88.68 mph with 3400 lb 40 Chevy Coupe.

Now I am running a dual exhaust system with each header having 1 1/2" and 1 5/8" pipes flowing into 1 3/4 pipes. Total length is 17 inches. 12 inch extensions are added for racing. The best I have done with this combination is 16.018 sec and 84.1 mph. This is slower due to :the driver is getting older, car has been on the road for 23 years, 11 years of racing at LA's Antique National Drags wears things out, and I may have reduced the smaller pipes by 1/8 or so too much. I went to dual headers because the 4 into 1 type has a lot of exposed pipe and they get very hot. I also like the sound of the duals. I am running Jaguar XJ6 1 3/4 exhaust pipe and Jaguar mufflers driving on the road.

I am now planning to build a Tri Y header. I am thinking about two sets of 1 5/8 and 1 3/4 primary pipes 10 inches long combining them into two 1 7/8 secondary pipes about 20 inches long. The secondaries would combine into a 2 1/2" collector pipe about 6 inches long and flowing into some sort of extension or megaphone. What do you think?

I have been trying (with little success) to interpolate what Vizard wrote for the Mini with its siamese 2-3 port and individual 1 & 4 ports in a "long center branch" header, but the separation time in degrees looks like too big to make sense of it.
I've wasted many an hour looking for 4-port L6 header designs from long ago, Ford Zodiac for example.

I've seen pictures of a forming tool made from a solid steel bar, shaped on a grinder to match the non-circular port shape (like stovebolt). Not enough text to tell for sure, but looks like:
1. cut a piece of your primary size to 6" length
2. insert (at least) the next smaller size pipe or sand inside to stiffen it
3. hold in padded vise or mandrel
4. grease the pipe ID and tool OD and apply BFH to convert the circle to a rounded square or rectangle.
This works best if the tube is slightly larger than the port ID, since the circumference can only change so much (stretching) before it cracks.

A friend of mine uses this tool to make headers for the Buick 215. They also have a rectangular port and the tool works well. The Buick has only one size port, it is a lot of effort to make two tools. He makes a 2" long piece to which you can weld on your own primaries. If someone did something like this for the 235/261, they could be used for any configuration of header. I just came back from the machine shop and we are making a set of dies so that you can use flanges such as those made by Hells Gate Hot Rods.

Warren

The #1 & 6 port shape appears to be 1-1/4" tall by 1-5/16" wide, so it might be easier to start with 2 pieces of flat stock: 3/4" and 1/2" thick (cheap & easy), face them together (spot welds?), make them 1-5/16" across and knock down the corners. Now do the same again but 1-3/4" wide for the #2-3 & 4-5. Duh.
Remember to hold the tool with a fixture or clamp to keep all fingers attached. Drill & tap the back end for a slide hammer to remove if captured.

Warren's drag car made our front page over the weekend: https://inlinersinternational.org/

great pic complete with blurred spectators . . .