| Immortal D - Turbocharged D-Series Motor Honda Tuning shares an office with the staffs of Turbo, European Car, Car Audio & Electronics, Import Tuner, and, until recently, Sport Compact Car. When you have this many car geeks in one room together, day in and day out, you can expect friendly competition to arise every once in a while.
A few months ago, I found myself picking at the remnants of a bowl of pho' while arguing the limitations of a D-series motor with Import Tuner's Technical Editor, Scott Tsuneishi. While Scott drives a WRX now, his roots are with Hondas, and he still has a few DA Integra project cars in his stable.
Like a lot of Honda guys, especially ex-Honda guys, Mr. Tsuneishi sees the D16 as a cute little brother to God's (Honda's) gift to the world, the B-series. When the subject of a 400+ wheel hp, turbocharged D16 came up, Scott chuckled with condescending disbelief when I told him that not only could I build it, but that I would. This sounded like a challenge to me, so I set out to build the biggest, most powerful D16Z6 I knew how. That'll show 'em.
In this month's installment, we start the build by modifying the cylinder head. Ricky Ortiz from Industrial Flow in Carson, Calif., ported our head and got it ready for a full Skunk2 top end treatment. Skunk2's .5mm oversized valves, valve springs, titanium retainers, stage-one camshaft, cam gear, and intake manifold have been employed to aid in our effort. With a top end like this, combined with a stout bottom end and a sufficient turbo setup, we will easily meet our 400 wheel hp goal.
When building a completely custom motor, the top end is the only place to start. Before we can design our custom pistons, we need to know the final volume of the head's combustion chambers. We also need to know the specs on the valvetrain combination we'll be using. These two parameters define what the piston crown will look like by helping us find our ideal compression ratio and valve relief configuration respectively. Once the head is done, the pistons can be made and the snowball keeps rolling downhill from there.
The five major parts of a complete top end job are: the surface, combustion chambers, cold side, hot side, and the valve job. Each one of these is important in it's own right and should all be taken into consideration when building a cylinder head.
SurfaceThe deck surface is the area of the cylinder head that mates with the bottom end. Since single cam Honda heads do not have very much meat on the deck surface, Ricky informs us that he will usually only take off what is necessary from the surface to leave a flat, precise mating area for the deck. While it is common practice in the aftermarket world to mill heads to gain compression, the inherent timing, heat transfer, and durability problems in doing so lead us to believe that this process should be left to the domestic guys from which the practice originated. A custom set of pistons and modified combustion chambers negate the need for milling anyway. Stick to resurfacing, you won't be sorry.
Combustion ChambersThe combustion chamber is arguably the most important part of an internal combustion engine. This is where the magic happens. In a four-stroke cycle, there is only one stroke that really matters, the power stroke. The intake, compression, and exhaust stroke are just there to make the power stroke more powerful. Once the piston has compressed the air and fuel mixture into the combustion chamber, the spark plug will ignite the fuel and push the piston back down the cylinder while turning the crank. The crank turns the flywheel, the flywheel turns the clutch and transmission which turn the axles, hubs, and finally the wheels. Now you can see why having a misshapen combustion chamber can be so detrimental to a car's overall performance.
Ortiz started our Z6's chamber by opening it up to 78mm to match the bore. Ricky made sure to explain to us that by opening up the chamber, he is trying to deshroud the valves. When there are less obstructions around a valve seat, it is easier for the intake air to enter the chamber. Air takes the path of least resistance, so easier is better. Ricky also tells us that opening up the chambers to match the bore isn't necessary, but the extra couple of millimeters provide space to take material from around the valves which helps flow significantly in low lift (pre-VTEC) situations.
Cold SideThe cold side consists of the whole intake port from the manifold mating surface to the valve seat. Most head porters will tell you that this and the valve job are the two places that you'll make the most power. Since this motor will be pushing a whole lot of boost, the port job is a bit more extensive than what you would normally see on a street car. Be careful, though .5mm is a world of difference in a port, and while it might not be visible to the naked eye, it could make or break the motor's performance.
For obvious reasons, the floor of the port is called the short turn, while the roof is called the long turn. Air flows in layers. The air on the long turn will always move at a different velocity than that of the short turn. In our application, Ricky chose to widen the floor to give the short turn area more volume and in turn slow down the air passing directly over it. Getting the air on the short turn to be closer in velocity to that of the long turn cut's down on turbulence in the center of the port.
The forward (closest to the intake manifold) side of the long turn has also been raised in order to make a better angle for the air to enter the bowl and mix with the air from the short turn. This process was also carried out to complement the exit angle of our Skunk2 intake manifold. Ortiz explains to us that the injector pocket need not be touched; he tells us that flow is not affected by leaving this pocket in its stock form.
The bowl section is the 90-degree bend after the divider that directs the port down to the valves. Most head porters will argue that this section of the port can greatly affect your cold side modifications. Ricky opened up the bowl section to direct air to the backside of the short turn and complement the size of the Skunk2 valves. He told us that it is important to remember that the port should usually taper down in overall circumference from the manifold to the valve.
The last modification to the cold side is a long lasting argument among head builders, whether to cut down the valve guides or not. The reason why there is so much controversy over the subject is that less valve guide can lead to wobble, excessive stem wear, and eventual catastrophic failure. Ricky enlightens us with his experience and explains that a D-series has more valve guide to work with than a B-series and while longevity is compromised severely when cutting them down on a B, it is less of an issue with a D.
?Hot SideThe hot side is made up of the exhaust port from the exhaust valve to the header surface. Ortiz tells us that it is important to understand that exhaust gasses travel up, both because of heat and the direction it is being pushed in by the piston on the exhaust stroke. The primary objective here is to make an easy exit for spent gasses. The roof was raised a little to cushion the majority of the gasses traveling on the top side of the port. The valve guides were also cut down to make a smoother streamline on top of the port.
Valve JobA valve job is the process of cutting three or more angles into the valve seat and valve. Most commonly, there are three angles in a valve job. Some porters will use five but not commonly enough to make note of it. The first angle, the throat (or bottom) cut, is usually somewhere around 30 degrees. This first angle redirects the air from the port exit to the combustion chamber. The second angle, the seat angle, is usually around 45 degrees. The seat angle is the most critical of the three. The thickness of this 45-degree angle is a big compromise between flow and longevity. A short seat angle will flow better, but there is less mating surface to take the constant abuse of slapping against the seats at 9000rpm. A smaller seat angle will also negatively affect heat transfer through the valves. The last angle, the top angle, is usually around 60 degrees and is important to direct the air into the combustion chamber smoothly.
Words of WisdomA word to the wise from our cylinder head guru Ricky Ortiz:"Never contradict Honda's theory. All you need to do is clean up what they already did. Honda's engineers are paid a whole lot of money to be better than we are at building their motors. Sometimes you will find a loophole here and there, mainly for emissions or fuel consumption reasons, not because their port is inherently wrong."
