I recently installed my RB header, and I've noticed a drop in fuel economy, down from 17 mpg city to 15 mpg. This has been observed consistently for more than one tank now. Has anyone else observed a drop in economy with the header system? I was under the impression that it would actually increase my economy, or at least have no effect.

Now, the '79 carb runs fairly rich because of the way that the thermal reactor works, so perhaps I could go to a later carb that runs leaner and gain some economy that way? The jets in the carb I have now aren't any bigger than the later model's have, so it must be something else in the '79 carb that makes it run so rich. Could I swap the internals like the air bleeds, jets emulsion tubes, etc from a later model into my carb?

Comments and suggestions are appreciated.

 

My experience with headers is on both my 80LS (12A), and my 84SE (13B) is that addition of headers doesn't do much for increasing fuel economy, and here's why;

1) when you install the headers, you're usually doing it to gain performance. Once installed, you're so thrilled at the additional 'breatheability' that you have, that you often put your foot into it to enjoy the investment of you hard-earned money. This doesn't help fuel economy.

2) The headers, while making the extraction of exhaust gases more efficient, increases total airflow through the engine, since a freer flowing exhaust will have impacts on the total amount of air that can be ingested by the intake system. You free up the exhaust, you also free up the intake. More air requires more fuel for stoichiometric balance, therefore fuel economy drops.

These 2 reasons are the primary reason why I've noticed not a whole lot of change in my fuel mileage, particularly a drop with my 84SE. On the SE, I think there is a better control over fuel mix given the EFI, however I KNOW that I tend to drive it harder because it's more fun to drive that way. Plus, the '6'-ports don't open until 3800 (!).

I have been able to see some increases in fuel mileage if I am very, very light on the throttle, getting sometimes as much as 22 mpg for mostly highway driving. My city averages at around 16-17, and when I drive like a madman, it drops to about 15. Never much lower than that. Also, I tend to use cruise control a lot on the freeway on the SE, which helps to conserve fuel usage.

Hope this insight helps. Headers, while providing more performance, don't tend to change fuel mileage for the better, as you can see. I think your findings are in-line with the rest of us on this.

 

Yes I have observed the same thing. While having driven my SE for years with the RB headered system, it became time to smog my car.. so I reinstalled the stock exhaust system. Immediately my butt dyno sensed more low-end torque (with loss of response though) over the full RB header system (which has much better top-end power) All naturally due to a change in the power curve.
Having more low-end to mid range power, the car pulls slightly more without having the need to press down on the gas pedal as much, hence having slightly better fuel economy over the full RB system.
Currently I have been enjoying a setup which I have used before.. stock exhaust manifold to a custom made downpipe connected to the RB presilencer...similar to the RB setup for the 2nd gen. Now I enjoy the best of both worlds.. better low to midrange torque and only slightly diminishing the top-end power (I dont drive my car around keeping it at redline all the time anyways) ..makes me wanna go ahead and sell the header
For those who are on a budget I would simply recommend a cat replacement pipe, since the stock catalytic converters are the true power robbers of the exhaust system.
Im tired... hope this made some sense

 

this would be exactly what i'd would suspect at first thought. as long as nothing else has changed on the car, i'd suspect your driving habits with the new power or an exhaust leak somewhere.

BTW ... you might want to retune your stock carbie as far as jets and emulsion tubes and what-not ... maybe you should shoot a PM to Sterling or Rx7Carl, but i don't see why your stock 79 carbie shouldn't give you what any other would.

 

Weird...My gas mileage went up from 14 MPG to 17 MPG after I installed the exhaust on my otherwise bone stock 85 GSL, lol .

~T.J.

 

I agree with Racer, headers are over rated

Ray

 

My milage went from 15mpg, to between 18 and 20mpg after I put on the new exhaust.

 

I really didn't notice a difference when I installed mine. It was a full racing beat exhaust system. Headers, presilecer, muffler.

 

Gas milage didn't change at all for me.

I've got RB headers, presilecer.