Designing a header for fuel economy (for a Metro G10 1.0L)

[Daox]

Does anyone have links to more info on calculating lengths? I see Old Tele Man's formula here:

http://ecomodder.com/forum/showthrea...tml#post279769

But, that says at 2200 rpm I should have a target length of 98".

I'd like a calculator that shows the 1st wave, 2nd wave, etc. lengths. I'd like to summarize what we have put together into one post so I can finish 'step 2 - selecting the length'. Then, I'll move on to the collector design.

[Shawn D.]

Originally Posted by Daox

It will definitely need a flex pipe behind it. It may need a hanger around it too. You don't want the header pipes fatiguing and cracking either. The engine mounts are stock. I have no idea how much the stock engine moves.

You can firm up the drivetrain by using a torque strap (it's unloaded at low/no-load) and/or using polyurethane mounts if the Metro's are shared with a performance-oriented vehicle (entire mounts or poly inserts), or fabricating stiffer mounts (e.g. if the mount has voids, filling it with urethane casting compound; let me know if you want more info on this).

It would probably be better to use a flex joint instead of a flex pipe. If you orient it near the rotation center of the drivetrain, it will have to flex/move less.

[Shawn D.]

Originally Posted by Daox

Does anyone have links to more info on calculating lengths? I see Old Tele Man's formula here:

http://ecomodder.com/forum/showthrea...tml#post279769

But, that says at 2200 rpm I should have a target length of 98".

I'd like a calculator that shows the 1st wave, 2nd wave, etc. lengths. I'd like to summarize what we have put together into one post so I can finish 'step 2 - selecting the length'. Then, I'll move on to the collector design.

Check these out:

4T Header Wizard - Exhaust Header Software

Header Design Resources - Stainless Headers Mfg., Inc.

[RALPH SPOILSPORT]

Any thoughts on Extrude Honing the stock manifold?

[Daox]

We definitely do not want to extrude hone the stock manifold. We don't want more cross sectional area for this header, we want less. That plus we need the long pipes to get the efficiency increase.

[Daox]

Alright, I've put this off quite long enough. Lets go over the primary length design aspect.

First off, headers by ed basically says to use 'at least 36" long primary tubes'. Not every scientific IMO. But, it really kills the idea of a 'shorty' header.

Next up I gathered some information saying that the length of a primary tube should be equal to the volume of the cylinder its evacuating. The Metro engine has a 993 cc engine, so that gives us 331 ccs (20.2 cubic inches) per cylinder. With an inner diameter of .62 inches, we need a length of 67 inches to achieve that volume. Well, that definitely agrees with header's by ed theory.

Now, if we use Old Tele man's calculation, we end up with a optimal length of 92" long.


So, what is a guy to do? Well, first off, most of those lengths are unrealistically long. We have to put the catalytic converter right after the header, and ideally we don't want to push it farther back into the exhaust. The farther back it goes, the cooler it runs and then it doesn't really do its job. So, that alone is probably going to restrict us to somewhere around 40" long. My specific case is currently drawn up at 37" because... well thats just where it is. When / if I actually get to building it, I'll probably adjust the length based on the materials I order. I believe come in 5ft lengths for the straight pipe.

[iveyjh]

Looks like a 1" OD with .065 walls at 35" for primaries with 2" collector will work just fine and still keep you in the zone. This is based on a 163 cu in V8, I think, with length 70.53" cut in half. Looks right to me.
http://www.bgsoflex.com/cgi-bin/prog...d=162&rpm=5700

[Daox]

Okay, so far we have our tube diameter and primary length figured out. The next step is to look at designing the collector.

The collector goes on the end of the primary pipes and brings them together. You'd like to do this as smoothly as possible to maintain exhaust speed.

Now, I think we can over complicate this part a lot, but IMO we don't really need to. Are you going to have a super fully optimized collector if you omit details? Absolutely! But, as we're finding out there really is no "perfect" design anyways. That being said, we'll at least go over the things to avoid.

So, lets do the calculations to figure out what size inlet and outlet we need. Well, inlet is pretty easy as it'll be the three 3/4" pipes coming in. But, what size outlet should we have? To do this, we're basically going back to the first step of calculating pipe diameter based on horsepower. The Metro has 55 horsepower, all 55 will be flowing through a single pipe now (vs one of the three primaries). So, we calculate pipe diameter based off of that.



Per our chart, 1.25" OD 16 ga will flow enough power for 52 horsepower. Since we're targeting a slightly lower rpm range, 1-1/8" will work. 1" will give us better power at cruising rpms, but it will most definitely kill some higher rpm power, so lets say 1-1/8" for now. Of course we could always design it for 1" and cut the collector back if we don't like it like that.





Next, we have to figure out a length for the collector. Headers By Ed says they should be at least 4" long and 5-6" is preferable. As we're familiar with aerodynamics here on EM, this is pretty easy to see that a more gradual transition will create less turbulence and maintain more exhaust velocity.



Per this collector manufacturer, the angle of convergence should be between 12-15 degrees.

http://www.spdexhaust.com/pdfs/02-11...Collectors.pdf

[freebeard]

However, by slowly increasing the area through which the gases pass, the reflected wave can be stretched out. The smoother the transition from high pressure within the pipe to atmospheric pressure, the longer the duration of the reflected wave, making the system less sensitive to engine speed tuning.

[freebeard]

AutoSpeed - Ballistic Bellmouths

Testing Shapes

Three intake designs were tested on the flow-bench. First up was a straight piece of 3-inch tube, cut off square at the end. The actual ID of the tube was 73mm, giving a cross-sectional area of 41.8 square cm. At a pressure differential of 3 inches of water, the 215mm-long tube flowed 255 cfm.

Next up was a 3-inch pipe running a straight flare. Again the tube was 215mm long, with the length of the 11-degree flare being 75mm. The mouth opening was 115mm in diameter, giving an intake area of about 104 square cm. The airflow with this design was noticeably improved, with a measured flow of 318 cfm recorded.

This represents a 25 per cent increase over the straight pipe!

Finally, a custom-made bellmouth was trialed. Again 215mm long, the bellmouthed intake used the same flare angle as the previous pipe. This time, though, a 22mm tube was rolled around the top, being brazed to the edge of the flare. Plastic body filler was used to fill any imperfections, giving a very smooth radius entry into the tube.

And the result? 322 cfm was the measured, up over the straight flare by 1.2 per cent.

A megaphone made more difference than the bellmouth. His $1.84 stainless steel egg-cup has an internal diameter of 2.3". So a 1 to 2.3 inch megaphone would fit the 1—1 1/8" collector.