Hello everyone,

First and foremost, I'm new to this board and don't actually own a hybrid, but I am in the market for one. I recently found a thread on this particular topic and I thought I would write down some of my experience on the subject for everyone’s reference. I've always wondered whether or not there is any room for improvement in hybrid cars. The thread seems to confirm my suspicions that there is in fact something further to be gained from these cars. My experiences have all been in the high-performance realm of automobiles (mostly V8 muscle cars and the such), but I’ve always been intrigued by efficiency.

An internal combustion engine is basically an air pump in basic nature. So, the more easily you can facilitate the process of pumping air in and out of the cylinder the more efficient the engine will be. The technical term for this is Brake Specific Fuel Consumption (BSFC). Basically, the lower BSFC, the more efficient the engine is. I think there should be a small note about BSFC and horsepower/torque. While it is true that as horsepower/torque production go up, there can also be a corresponding reduction in BSFC, but this is not always the case. You can always make an engine larger to make more horsepower/torque, but it’s not necessarily reducing the BSFC, actually it’s probably raising it, but it is possible to make more horsepower/torque with less BSFC and the key is to make the engine more efficient. Now, there are all sorts of tricks you can use to gain a higher BSFC, but the easiest ones to address, at least from a non-technician or non-engineering standpoint, are the intake and the exhaust sides of the engine. The very things touched on in this thread. In both instances, the idea is to reduce the amount of “work” the cylinder and piston have to do in order to pull the air into and expel the air out of the engine. While smaller engines such as those in hybrids are less effected by “pumping losses” as larger engines, the results from Duc and other members show that there is room for improvement.

Speaking directly to the intake:
While the manufacturer (i.e. Honda, Toyota, etc.) has spent a great deal of time and money to optimize the infinite variables present on an engine at any particular moment in time (engine load, engine speed (rpm), operating range of the engine) there are inevitably trade-offs for the sake of noise, harness, vibration, and more often packaging. Most naturally aspirated motors actually run best with individual throttle bodies on each cylinder with long pipes running directly to the port. While this makes great horsepower and torque, it’s ungodly loud and packing these tubes is difficult at best. So, while the manufacturer’s engineers have worked long and hard to optimize the package as much as possible, there are some areas of improvement. Area in front of the throttle body, as Duc has revealed, is one of the biggest areas for improvement.

Generally, the idea is to reduce turbulence in the air stream of the intake. Smoother, or laminar, airflow helps create more torque and reduce BSFC. Usually this is achieved on most aftermarket intake systems by increasing the overall size of the intake system before the throttle body, as well as smoothing the direction or path the airflow must take as it enters the throttle body and intake. Careful consideration must be taken here, though, because bigger isn’t necessarily better in all circumstances. (This also applies to the exhaust as well, but more on that later.) Thus, most after market systems will generally work to achieve nice, gentle bends from the location of the stock airbox to the throttle body, trying as much as possible to straighten this path out. Commonly they use aluminum or steal piping, both of which can be detrimental to the ultimate power output of the motor for one factor alone. Heat. As was mentioned in the thread, a cooler air charge is a denser air charge and thus makes more power. Although, I’m not sure that a denser air change is necessarily more beneficial on a hybrid car. (Like I said my experience is mostly in larger cubic inch V8 cars.) I’ve actually seen quite effective intake systems built out of PVC pipe that you would find at a home improvement store. The PVC absorbs less radiant heat from the engine compartment and has a very smooth surface inside and out, which again helps promote a more laminar airflow path. Getting back to size; like I said bigger isn’t always better. While I’ve seen very large increases in horsepower in torque on V8 case by increasing the overall size of the intake system by 15mm or more, I don’t think this would necessary apply to the relatively small size of the hybrid internal combustion engines that are out there. It has been my experience that the existing throttle body on the engine is generally the best size for overall engine performance with a preference for torque, which is what most people need.

With that being said my ideal intake solution would be one that is made from a piping/tubing that does not absorb radiant heat easily. I would run it from the existing location of the stock airbox (which I’m assuming pulls air from outside of the engine compartment) to the existing throttle body with as few bends as possible, again trying to create smooth gentle bends. The best solution would be to match the inner diameter of the piping with that of the throttle body, so as to create a smooth transition between the two. If this not readily achievable, then the next best solution would be to find the next smaller size available in the tubing you’re using. You always want to go from small to large with regards to the flow of the air.

Now a quick word on air filters:
While I’ve always been a fan of K&N and similar type filters on all of my cars, there is one important aspect that must be reviewed by these types of “oiled” filters. If the filter has been over-oiled after cleaning, there is the possibility that some of that oil can be sucked into the engine while passing over the Mass Air Meter and Intake Temperature Meter, covering them in a thin layer of oil, or soot. This can contaminate the meters giving false readings to the car’s ECU and thus leading to reduce performance. Thus, the owner must be diligent in making sure to not over-oil the filter after cleaning it. Also, it is possible to clean the meters, but I WOULD NOT RECOMMEND DOING THIS TO ANYONE THAT DOES NOT HAVE THE EXPERTISE TO DO SO. There are several air filters out there that do not need oiling and also produce similar results with regards to airflow as a K&N oiled filter. You just have to look a little harder.

Now on the exhaust side of the equation:
As Duc and others have seen, reducing pressure in the exhaust system, backpressure, can provide noticeable gains in horsepower/torque while also helping efficiency. Again, the name of the game is to increase laminar flow of the exhaust as it exits the catalytic converter. Again, I’m not familiar with the inner diameter size of the flange at the catalytic converter on all of the cars, but I’m guessing its around 2” to 2 ¼” inner diameter. The trick would be to match the inner diameter pipe size of the new exhaust to the size of the exiting flange after the converter as closely as possible to again reduce turbulence in the pipe. Reducing turbulence increases torque. From the converter back you again want to reduce the number of bends if at all possible or at least reduce the angle of them (say from 75 to 60 degrees, etc.) so as to reduce turbulence. Keep in mind, though, that the more you “straighten” the exhaust system, the louder the exhaust will become. Bends lower the exhaust volume. In all cases you should try to have all of the piping mandrel bent to maintain laminar flow and try to have the pipe bent all as one piece from the converter all the way back to the muffler. The more section of pipes you have, the more breaks you have in the pipe to allow for disruptions in the laminar flow of the exhaust. With regards to size, as mentioned earlier, bigger isn’t always better. The goal is to keep velocity up while also increasing flow. You can image your exhaust pipe size like a straw. When you drink you have a straw that is maybe ¼” wide. You can easily suck up you favorite beverage. Imagine trying to repeat that with a straw that is 1” wide. Sure you may be able to get a higher volume of liquid into your mouth, but you would have to work a lot harder to do so and the velocity of the liquid would be slower. The same thing goes for the exhaust. It looks like people have found success with sizes ranging from 2” to 2 ½” so I imagine that’s close to the size of the catalytic converter outlet.

Now speaking of mufflers:
Again, I’m not an expert with mufflers and will never claim to be because the technology of aftermarket mufflers is always changing. I myself have always enjoyed the sound and performance from “chambered” mufflers such as Flowmaster and Borla. While others prefer the sound of “packed” mufflers such as the Hedman one mentioned in the thread. While it’s hard to say which muffler is better than other, you need to find out how the muffler flows with regards to cubic feet per second (CFM) or some other measurement and see how it stacks up against the other mufflers. Of course size and overall look of the muffler also play a big role in the decision. As I final note on the exhaust subject for the really hardcore modifiers, you could also coat the exhaust (and the intake if it’s made out of metal) with a heat reflecting or heat retaining coating such as Jet Hot. Coating the exhaust piping would have the benefits of retaining heat inside the pipe where it helps and smoothing the wall of the piping to again retain laminar flow.

I hope this gives you some ideas as to the benefits of modifying your hybrid for increased efficiency and power. Now get out there and try some and keep track of the before and after MPG!

Thanks for reading.
Brad