Friday, May 16, 2014
CR-V: Challenges and Lessons
So I recently swapped out the engine on a 2000 era Honda CR-V 4wd due to a broken timing belt and the internal damage already present inside the engine. First off, let me say this: Swapping CR-V engines is not a good first-time or learner project. As an experienced mechanic, I found myself flabbergasted and flummoxed at the outrageously high number of bolts, brackets, and extraneous seeming parts everywhere. Second- the replacement engine was guaranteed to have less than 50k miles on it but was imported used from Japan- this created challenges all its own. We won't discuss the freight damage that was evident once we opened it up... if its not one thing, its another.
Part of the complication in this engine swap came from a decision by Honda to use the same engine block and cylinder head in both Japan and the US but use different intake & exhaust manifolds, oil pump, timing sensor system, distributor and both timing and accessory drive pulleys. It is these variations in design that I want to focus on in this article, as it speaks of how car designers and engineers are tuning engines for the radically different driving styles exhibited by American vs Japanese drivers and road conditions.
I am going to skim through the differences between the countries driving patterns and how I perceive them to relate to engine design. I encourage using the MachineWhisperer Forum for elaboration and explanation, as the conversation can continue more easily there.
So lets start with the manifolds, since they are the most significant in design difference. The intake manifold on the American version of this engine has shorter runners, but has a big resonator attached to the plenum after the throttle plate. The Japanese version has longer runners and a smaller plenum. The Japanese intake is designed to make more torque at lower RPM for the shorter driving distances there, while the American version is designed for the higher RPM band used on the freeways and highways that connect the greater distances in the US. Length of intake and exhaust runners is directly related to torque and power output curves.
The exhaust manifold for the US model is made of thin-wall steel and has larger diameter runners than its cast- iron Japenese unit. This, to my mind, means that the exhaust in the steel manifold is cooling more in the manifold because it is able to transfer more heat sooner due to lower thermal mass and higher conductivity, as well as the expansion cooling of larger runners. Cast- iron on the other hand, has a much higher mass, thus it would store some heat in it and keep the exhaust at a higher average temperature and help reduce exhaust cooling at stops. The added heat in the exhaust benefits the catalytic converter by making sure it is fed hotter exhaust as much of the time as possible. Because of the more constant and higher RPM conditions in the US mean that the catalyst stays hot and may actually overheat if not for the steel manifold wicking heat out to the exhaust constantly.
The only difference I could see in the oil pump is a boss for a crankshaft position sensor that detects notches on a timing wheel on the US edition crankshaft timing belt pulley. This timing wheel and sensor are missing from the Japanese model which leads me to conclude a wider RPM range is used in the US and thus better control of spark advance angle is needed. In the distributor for the Japanese engine are two extra wires- I did not investigate as to their purpose, but may soon, just for curiositys sake.
Since the Japanese engine is tuned for lower RPM ranges, it then makes sense to use a larger diameter drive pulley to spin the alternator faster to make plenty of power for electrical accessories in the vehicle. The alternator sheave is slightly smaller on the American version of the crankshaft pulley. It is highly likely that Honda also incorporated a slightly different computer and/ or firmware to also get the transmission to shift earlier in japan, as well as change ignition timing and fueling curves.
Depending on what goals you have for engine performance, using some of Hondas' ideas can drastically alter how the engine behaves and be used to create a tuned race car or ultra efficient commuter. There are design criteria inherent to engine configurations that are tuned for torque low in the RPM band, or high RPM power, and by recognizing them under the hood, it becomes more evident how the engineers intended for the engine to be run. Even if you are not working on a Honda CR-V- these design parameters are a great place to start for changing the performance factors of an engine, or just understanding better what the relationship is between these design criteria and engine output curves.