Effect of low end torque on MPG - Accord
 

Greetings,

I am looking into upgrading a few parts on my car that improve air flow and torque to the engine. I know normally that once those increase, so does lead-foot-itis. I am too set in my driving ways for it to be a deal, so the 'thrill' of more power does not appeal to me (on a daily basis...sometimes I am in a rush).

I drive a 2009 Honda Accord V6 Automatic.

One thing I know is that with a $30 Intake Manifold space, I have increased my torque 6 ft-lbs at peak, as well as throughout the entire curve (http://i205.photobucket.com/albums/b...47/pics001.jpg). The affect this has had on my mileage is to go up from 28 - 29.5. I did no ABA testing, but have a few thousand miles logged in each configuration. I am willing to accept several months of data points as my reality. The down side with my car is that it turns off 3 of the cylinders for fuel economy which has other problems. As a result of that (intermittent lean error code and / or cylinder misfire) I have removed the spaces and saw a drop of MPG from 29.5 - 28.1 (as measured in the app Torque). I feel very confident that this part improves my gas mileage.

There are a few other parts I can install which will also improve torque between 5 - 10 ftlbs at the lower end of the curve. Specifically a larger intake manifold. The impact is here: http://i20.photobucket.com/albums/b2...no_overlay.jpg.

I believe the biggest impact of this will be on my Variable Cylinder Management (VCM) mode that the engine has. My car will shut off three of the cylinders and I basically do a lot of driving in an I3. However, wind, slight grades, etc, cause the car to slow. If it is too much acceleration, then the other 3 cylinders will kick in and the instantaneous mileage will drop to 13 - 16 while catchiing back up. When it accelerates with VCM on my mileage is 22 - 26.

So, using the above behavior of my car, plus the already observed impact that the one part had, I believe that the larger manifold will have a similar 5% increase in fuel economy. That said, I also know more air = more gas. I am not sure, however, it is more air. I say this because the intake will be the same, so the size of the straw remains even if the lungs are larger....but this is where I am a little hazy.

I have tried asking in my forums about the impact, but people measure mileage based on miles per tank (oooh yay) or measure things with their butt dyno. The ones who respond tell me their impressions (#confirmation bias) but don't have the data to back it up.

Given all of the above, please let me know your thoughts. I will answer as many of your questions as I can to get mine answered.

I wouldn't normally expect a larger manifold to help low end, but would have been hopeful about the spacer. The dynos are a disappointment though, starting at 3500rpm for the larger manifold, and 2900 for the spacer, doesn't really give you insights into low end/ cruising behavior. If they were going to help low end I would think they would try to showcase that

What about the spacer dyno looked more promising than the manifold? The only thing I can really tell is that you can assume that the torque curve is being shifted towards lower RPMs and can see it drop off at the top end (where my engine might be 6x a year). That said, you are correct. The spacer has mad phenomenal changes in my mileage.

I have not found a dyno for my car, involving performance part upgrades, that start in the 2K range. I don't know if they just don't care or if it is a limitation of their testing method.

Not specific to your vehicle, but typically larger plenum, larger diameter, short runner manifolds are better for top end, and small diameter, long runner manifolds(or stock w/spacer) help low end.

Check the port matching with the spacer?

Thats awesome! I have been wanting a spacer for my car for a year and a half but never bought it ($150). Dyno results show up to 27 lb.ft. peak increase. I am making this a priority.

torque
 

HOT ROD Magazine did an article about exhaust headers,and they wrote that it was the header primary tubes which were responsible for the engine's torque,which mirrored the unit air charge,which reacted to scavenging during valve overlap.
If you already have variable valve timing,HONDA may have already optimized all the manifolds for the operational driving range.