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Old 11-20-2015, 12:30 AM   #11 (permalink)
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My mechanical engineer friend had a cracked exhaust manifold on his 4-cyl 2.7L taco, which in Virginia means you to fail inspection.
He has fuel economy logs going back to 2005 for this truck.
New dealer part was something like $800, a used one off the internet was $500. A pacesetter 4 into 1 header was $400.
He put the pacesetter header on in 2010 and it is still going good.
He says the header appears to have not hurt fuel economy at all, but is half the weight of the cast iron manifold and has increased low end torque and high end get up all around.

For the metro header, if were going to build one I would use 1'' diameter 0.62'' wall 304 stainless. Not sure how long the tubes would be.

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Old 11-20-2015, 08:18 AM   #12 (permalink)
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1" primaries may be too big. Looking at the amount of exhaust coming out in comparison to a 350 SBC...

43.75 CID/primary. Usual* header primaries are 1.625" for street headers, 1.5" are rarer but would push the torque peak down lower.

Metro 1.0 has 0.33 CID/primary, a 24.6% reduction in exhaust gas volume.

1.5" x .754 = 1.131" primaries.

I stand corrected! Now I don't know if you can directly compare like this. The size I came up with seems to show that 1" is the way to go if this math checks out okay!
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Old 11-20-2015, 08:26 AM   #13 (permalink)
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Quote:
Originally Posted by BabyDiesel View Post
1" primaries may be too big. Looking at the amount of exhaust coming out in comparison to a 350 SBC...

43.75 CID/primary. Usual* header primaries are 1.625" for street headers, 1.5" are rarer but would push the torque peak down lower.

Metro 1.0 has 0.33 CID/primary, a 24.6% reduction in exhaust gas volume.

1.5" x .754 = 1.131" primaries.

I stand corrected! Now I don't know if you can directly compare like this. The size I came up with seems to show that 1" is the way to go if this math checks out okay!
CID? When i run numbers for the volume in the pipe (OD not ID, unfortunately) i get 2.07/inch of length vs .785/inch.
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Old 11-20-2015, 08:58 AM   #14 (permalink)
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Thanks for all the tips guys, there is some good info there.

I do realize the Metro's exhaust ports are rectangular. I'll be getting to that. The initial image you saw was just me throwing together a rough draft so to say. There will be transitional pieces going from rectangular shapes to round.

This will be a long tube header design, this helps low rpm power/efficiency. I'm not exactly sure on the length yet. I'm still working that out.
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Old 11-20-2015, 10:40 AM   #15 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
CID? When i run numbers for the volume in the pipe (OD not ID, unfortunately) i get 2.07/inch of length vs .785/inch.
My apologies if I was not clear. CID is Cubic Inch Displacement. The 350 SBC will have 43.75 cubic inches of displacment per cylinder, therefore per primary. The Metro 1.0 has around 33 cubic inches of displacement per cylinder, roughly 25% less.
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Old 11-20-2015, 11:21 AM   #16 (permalink)
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Quote:
Originally Posted by BabyDiesel View Post
My apologies if I was not clear. CID is Cubic Inch Displacement. The 350 SBC will have 43.75 cubic inches of displacment per cylinder, therefore per primary. The Metro 1.0 has around 33 cubic inches of displacement per cylinder, roughly 25% less.
Ok got it, thanks for clarifying. So we would also want primaries that have roughly 25% less cross section? Around 1.375 - 1.4" inside diameter?

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Old 11-20-2015, 12:11 PM   #17 (permalink)
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Lets do step #1, selecting the primary tube size. This is no simple task, so I'm going to provide a bunch of info, and then take my best guess based on what I'm trying to design into the header. This design target is low to mid range power, 2000-3000 rpm is generally the range I'm targeting.

My info from Ed says that for the horsepower each cylinder produces (math is 55 engine horsepower / 3 cylinders = 18.3 horsepower per cylinder or tube) I should use 1.25" OD tubing. However, I'm on the very low end of the range of power for 1.25" tubing size, so its safe to say I could downsize that. Unfortunately, 1.25" tubing is the lowest on his chart, so that leaves some guessing up to me. This is also for a higher rpm producing header. I care little about that, I want more torque in the mid and low range. So, I'm going to downsize my tube.


Another source of sizing exhaust tube size is found here:

How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos

This chart is for exhaust piping in general, but we can use it for our header design. Again, his chart only goes down to 1.5" tube. However, I extended his chart down to 0.5" tube using his calculations in an excel document. The key when looking at this chart is each square inch of tube area gives ~115 CFM of flow.



According to this chart, I should be looking at 0.75" 16ga tubing.


So, wait a minute! These two estimates are very different. Why is that? I believe the two designers are targetting two different flow velocities. This is the speed of the exhaust gas inside the exhaust tube. Too fast of a flow velocity and you get excessive back pressure which causes the engine to have to push harder to get the exhaust gas out. Too low a flow velocity and you don't get good scavenging effect, your exhaust gas cools and then becomes more dense and more mass to push out the exhaust tube, and a few other things. Our goal is to balance these two.


To find the flow velocity of your engine and tube size, you'll need to do two calculations.

1) Find your flow rate for each cylinder. This is roughly equal to 2.2 CFM per engine horsepower (per exhaustvideos). Now divide by the number of cylinders.

2) Find your velocity. Velocity calculator here. Directions: solve for velocity, put your CFM in from the above calculation and your tube inner diameter in the other field.


Most articles say to target ~250 feet per second (FPS) for peak torque. Using Ed's sizing, our flow rate is around 200 FPS. Using the exhaustvideos calculation, its around 275 FPS. But, these are values at max rpm and wide open throttle (WOT). The thing is, for a fuel economy header, we're again tuning for a lower rpm, roughly half of peak rpm. This translates into roughly half the flow, and thus half the flow velocity. So, I could probably go with 5/8" OD 16ga tubing and call it good.


(Yes, my name is Timothy )

This seems ridiculously small, but if I measure the exhaust tube on my 1981 Honda CM400 which has a 42 horsepower 2 cylinder engine, the outlet's inner diameter is 9/16" (there are two exhaust tubes), so I don't think I'm too far off.

Sadly, I haven't been able to find any information that says "don't exceed this velocity", or "after X FPS you start getting Y psi of back pressure". So, I can't really design to that factor.

Now, when searching for actual parts for the header, I was only actually able to find 3/4" OD 16ga mandrel bent 304 stainless tube. This gives me 320 FPS at max power output (5700 rpm). So I should have ~250 FPS around 4450 rpm. That is pretty high for me, but since I haven't been able to find smaller tube it'll have to be good enough, and its certainly smaller than the stock setup. So, that is pretty much that... 3/4" 16 gauge stainless it is.

Also, please keep in mind that these are very rough calculations. This is by no means a super specific guide. Real header builders calculate tons more stuff like valve diameter, cam lift and duration, bore and stroke, and lots more stuff. This is by comparison very rough.
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Old 11-20-2015, 12:14 PM   #18 (permalink)
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Quote:
Originally Posted by Daox View Post
Thanks for all the tips guys, there is some good info there.

I do realize the Metro's exhaust ports are rectangular. I'll be getting to that. The initial image you saw was just me throwing together a rough draft so to say. There will be transitional pieces going from rectangular shapes to round.

This will be a long tube header design, this helps low rpm power/efficiency. I'm not exactly sure on the length yet. I'm still working that out.
We are all looking forward to what you come up with

Quote:
Originally Posted by ksa8907 View Post
Ok got it, thanks for clarifying. So we would also want primaries that have roughly 25% less cross section? Around 1.375 - 1.4" inside diameter?

Pi(R^2)
Since we are discussing volume, yes!

The math worked like this for me - A 1.5" primary SBC Header will have a primary cross section of 1.767". 25% of this would be 1.325". Since there is a greater focus on low-end, I believe Tim could go slightly smaller. A 1.25" primary gives ((0.625" x 0.625")3.14), which equates to a 1.227" cross section, a 30.6% decrease.

So 1.25" seems to be the front runner unless there is some specific exhaust information that we are missing...

Edit: Tim posted right before me. Throw all this malarky out the window!
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Old 11-20-2015, 12:29 PM   #19 (permalink)
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Haha, good shot at giving it a try. I had to dig pretty hard for a lot of this info, and the rest is really my best guess. There isn't a massive amount of info out there on header design to begin with and ALL of it is for 'performance' and/or racing applications. You have to take that data and try to wrangle it into some form that we can use for fuel efficiency. I hope I'm in the ballpark.
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Old 11-20-2015, 01:00 PM   #20 (permalink)
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Quote:
Originally Posted by Gasoline Fumes View Post
The G10 head has square exhaust ports, roughly equivalent to a 1"-1.25" diameter round pipe. I'm guessing you'll want primaries smaller than that. Suprf1y on TeamSwift or GeoMetroForum knows a lot about these engines too, I'd ask him for recommendations as well.
Do you know a good way to get in contact with him?

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