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Stage 3 | headwork and machining

| 4 - 5
You rebuild the bottom end to handle more power and abuse from your major modifications elsewhere, but the head is where the power is made. There is so much to consider while building your head, it can be staggering. Some people spend the better part of year considering various options to create the perfect head that will give them the power when and where they want it in the powerband. Hopefully, upon reading this section, you will have a good idea of what you want to consider and will be able to carry on an educated discussion with your machinist as to what you need and why you need it.

Be sure to have the head milled for the new head gasket!

A Good Rule of Thumb.
Head modifications predominately deal with airflow into and out of the combustion chamber - Bigger is not always better! Larger passages are intended to flow massive amounts of air through them. If you can not increase volume (such as with a turbo), you will lose velocity and not get as much air into the chamber at lower engine speeds. Conversely, if your passages are too small, you will have the velocity (which translates into torque and power down low in the RPMs), but will choke things out at the top end if they do not flow enough volume. There has to be a happy medium. If your portwork and mods are too aggressive, your car will likely be a slug down low, but come alive above 4000rpm. This can make things a bit annoying in a daily driven car. Imagine the car chugging and bogging in the drive through.

Cost | $300 and up
Probably the first thing that comes to mind, right? You will find vendors offering a "street" port and a "race" port and variations thereof. You don't want someone to just hog a bunch of material out of the ports without knowing what they are doing. Once it's gone, it's a bitch to put back and you might ruin the head in the process. Street ports are more conservative with the intention of maintaining civility in daily driving conditions. Race ports are more aggressive, with the goal of making the most power under race conditions when the engine is being revved to the limits. Experienced machinists will have a bag of tricks they can do to tweak the port in a variety of ways, so be sure to ask them about their style or features when porting smaller, high-revving engines.

Cost | $150 and up
Maybe you've heard people mention angles when talking about valve jobs. This comes from special machine work being done to the backsides of the valve and where the valves "seat" to the head when they are shut. Matching angles are cut into both the valve and seat to both improve sealing when closed and flow when open. A three angle is a great mod. A five angle is a bit better, but will cost more, and a full radius valve job, where the valve and seat are perfectly matched together as smoothly as possible, is the ultimate in valve jobs and is, understandably, the most expensive. Will you notice one more than the other? Not likely, but it's a good idea to have a valve job done. Pick the one that best suits your budget and goals.


Document statistics: Last modified on 2006-08-08 10:06:00 by DR1665

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