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.
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:
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?
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
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.
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.