exhaust manifold...or headers for FE?
 

I always figure car engineers do things for a reason...

but does anyone know which would be better for FE/MPG: headers or exhaust manifold, with regards to the Honda CRX HF? This car comes with an exhaust manifold (88-91), but the CRX DX and CRX Si models came with headers. Any ideas out there as to whether or not the HF's fuel mileage would be improved upon if it were fitted with headers? OR, would the DX (also 1.5) be improved upon (for FE) if its headers were swapped with an exhaust manifold? I've often wondered this but never asked until now. Thanks for any thoughts.

I had an '71 Pontiac LeMans with a 6 cylinder engine that I fitted with headers. The mileage went from 12-13 to 15 mpg highway. The engine was otherwise bone-stock.

depends on the engine and how well/poorly the manifold is designed. Most cars use a pretty emissions friendly manifold for both intake and exhaust.

generally speaking, aftermarket headers will improve both economy and power by allowing the engine to breath easier BUT you have to keep the pipe diameter down (both individual runner and cat-back) if you want to maintain low end torque (as in a FE minded vehicle)

this is the manifold for my cressida with a RWD mount straight 6:
https://www.fuelly.com/attachments/fo...010d73cdee.jpg
notice the front 3 cylinders run basically one pipe with each successive cylinder adding to it wheras the rear 3 are roughly equal length. this engine usually sees around 10% gains with headers and full exhaust...15-25 HP on a 200hp stock engine depending on the details. enough to feel it even with an automatic. FWD I4s are easier to make decent manifolds in the first place so you probly won't see anything that dramatic. you'd see more from just using high quality free flowing cat/mufflers but I could be wrong. got any pics?

All aftermarket headers I've seen for Honda's are tuned for max high rpm power. Exactly the opposite of what we want for mpg.
Here is a VERY good read:

https://www.gassavers.org/showthread.php?t=5326

Well, I can't say for sure. I don't know about the Si, but I know the DX did not come stock with a header. I have one I installed on mine, so I know it was a stock cast iron unit, previously.

The headers are typically set up for power. However, I haven't heard of anyone grousing about getting any worse mileage. I think I'm getting as good or better than I got previously, but I installed the header, right after I got it, so I can't say for 100%.

Typically, automakers design exhaust manifolds around cost first, and emissions second. And the cost factor is particularly important on cheaper cars like the CRX HF. For this reason, performance and/or fuel economy are almost never factors in the design of exhaust manifolds. With this in mind, about the only thing you can really do is experiment. You could try using the stock manifold from a DX/SI to see if this helps. Or maybe something like a DC Sport header. Any of these should probably be better than the stock manifold. And I believe that the exhaust port size on the HF matches that of the DX/SI.

I got the same mpg when i switched the header from oem to aftermarket on my 94 civic dx, its worth it. i felt a bit more power at the top end, looks better under the hood, and a ton lighter than the oem header.

Equal length headers are what you want so the gas pulses do not hit each other on the way out.

Just out header shopping.
 

If your car has a cast iron manifold, chances are a header my help, but here's some tricks I'v learned. I was just looking for a Honda Fit header and I found the factory equipts the car with a header. I was looking for a torquier 421 design, all I could find was a 4 into 1. The factory header had small long primaries going into a collector. The replacement performance header had short large diameter primaries going into a larger collector, with a larger opening at the collector, presumably to go into a large tube full exhaust system. This is a demonstartion, where the factory parts are better than the after marrket parts for fuel economy.

I just ran across some factory dyno numbers for a late 90's Saturn, where the cast iron manifold squeezes all 4 cylinders together and into the exhaust pipe with virtualy no collector. The dyno drops torque 9 pounds at 3000 rpm, the place you want to shift for economy. Now the same Saturn fitted with a header gained 3 pounds off torgue at 3000 rpm. It was a straight torque band versus a torque band with a 9 pound dip in the middle.

Now my idea was depending on what kind of car I get, if it had an iron manifold, I'd get a header for it, but keep the stock exhaust pipe and muffler. This way ballancing the exhaust, while still keeping enough back pressure for better mpg.

Also, most iron manifolds have the O2 sensor close to the engine. A header can move the O2 sensor 2 feet away from the engine. So for the engine to keep the O2 sensor hot enough, it will lean out. I saw this happen to a friend of mines Civic, he added a header and got 5 mpg better, course he added a fart can style muffler, loosing all his back pressure and loosing a noticable amount of low end torque.

Another restrictive factory cast iron manifold I saw was 90's Nissan Sentra's, they have a bow-tie looking manifold, going right into the exhaust pipe. I could just imagine the benefits of giving each cylinder 2 feet of pipe, rather than mashing them all together.

Another mpg exhaust trick I saw was O2 sensor spacers, it moves the O2 sensor back and out of the exhaust path, making the O2 sensor read colder and again leaning out the air fuel ratio, giving you more mpg.

If you go to an auto parts store, you can find exhaust reducers, I have read here a couple of guys trying smaller exhaust pipe diameters at the muffler. I believe in headers, just control the back pressure and that's a key to economy and torque.

I believe another reason automakers don't install headers at the factory is because they are just trying to get the exhaust out of the engine to the back of the car with taking the least amount of room up in the engine compartment. What I mean is the have so many things going on in the engine compartment that for them to stick a room consuming header in a tight area would be disastrous for them.

Could actually be better off going a tad bigger further back for more expansion room, so the same volume can move even though the exhaust gases lost heat. But, going too big, too soon, forcibly slows everything down.

Back pressure has but one purpose in the world, it IDs those that really know what they're talking about. Velocity is key, not back pressure.

I have a few experiences with "Back pressure"

Muffler baffle came loose internally, restricting flow... drove like crap, lost mpg.

Cat converter clogged... drove like crap, lost mpg.

Some idiot fitted a muffler 1/2 inch under stock size... drove like crap, lost mpg.

Dented exhaust before muffler on rock... drove like crap, lost mpg.

Dented exhaust before muffler on rock, part Deux... actually this happened to the feeder pipes before they joined a collector on a car where they passed under the sump from the front, maybe 18 ins from the exhaust ports... made power band very "notchy" and cut off the top end... BUT... inside the "notch" mpg increased slightly. Had no tach on that car, but it was smooth until 2000 rpmish, then got resistance and stumble, then resonated and pulled like a train between about 2500 and 3500, then stumbled coming out of it, sounded asthmatic in the 4000s and was hitting a brick wall at 5000ish.

since no one else has said it, and its been annoying me..... if you put headers on your 4 banger, it'd better be a V-4 and not an I-4.

and yes, back pressure is something I like for a massage, not my car's exhaust. For that I like velocity.

Had a random thought a long while ago while building up my protege; what rpm band will it primarily operating in? Figure out the intake CFM and exhaust CFM and design manifolds and piping to match that for optimal velocity. Sure, it'll suffer when not in that band, but for what you designed it, it'd be ...many percentages (I have no idea how much better) more efficient than one designed for the entire rpm band. Nascar stock cars tend to stick above 6000rpms and have tuned their open exhaust to promote free flowing, high velocity.

Just a random thought.

^^V4? i never knew they existed in cars?
i think i have to go with everyone else on this that the 4-2-1 header is the best. it gives you low end torque where you really need it in our honda's. i'd go with a DC or just some cheap ebay header.

I've only seen V4s in older (1970s) econboxes in Europe. As far as I know only bikes are made with them now.

Cheap headers are probably NOT going to give you low end torque, they will be too big, they're made for 9000 rpm dyno queens.

yea thats what i was thinking too, and why not put a header on a inline 4?

what kind of header do you recommend for a 1.6 civic? would it have to be custom made? :confused:

If that's a D16, I'd look to see if I could order a D13 or D14 header that would bolt up, or I'd look for a "rally" header, 'coz rally performance guys know better than dyno freaks, or I'd get the cheap eBay header anyway and hammer it a touch flatter on the inside curves where possible. you want about 1 inch primaries, and if you're lucky you'll find 1.25 primaries which are more ideal for 2.0L motors, but will probably find ones with 1.5 inch primaries, because they can sell more to the all knowing "bigger is better" crowd. However, it's area that counts, and deforming 1.25 inch tubes to the same area of 1 inch tubes should work (And due to a quirk of fluid dynamics, may flow better than a header with the same curve radiuses made in 1 inch round tube, because the flattened short side radiuses may flow better)

oh putting a header on a I4 is a good idea. But headers? I've never seen one. header, headers. there's a difference. That's what I was getting at. Currently I have a simple 4-1 header on my protege, but its stock from mazda, (different year) and according to the sheets, only adds power up top above 4000rpms, so not terribly useful for FE.

I beleive a tri Y type header is best for FE as it frees up low to midrange torque. I have one of these on my wagon and it seems to have helped a bit...

Tri Y is also known as the 4-2-1. 4 combine to 2, and 2 combine to 1.

If you have a honda D series engine fro 88 on up, the 2 part 4-2-1 96-00 civic EX (d16y8) OEM header is widely available on craig's list for 15-20 bucks and it will fit. I put one on my hatch with a 99-00 civic si cat back and a new 2.5" cat. It took 35$ worth of help from a muffler shop to get it shortened a bit and I installed it myself. The car produced noticeably more torque and quite a bit more power in that last 1500-2000 rpm before redline. I might have picked up 1 mpg or so. I have the welded assembly type oem header, there are also cast iron ones that weigh quite a bit more. Many 99-00 civics have the cat right on the head built into the exhaust manifold. If you replace that type of set up you need to move the o2 sensors back down the exhaust and this requires either buying Honda extending harness(s) or soldering your own extensions.