This is long but i think very inforamtive. Also, like any of my other posts, if you think i am wrong then please correct me as i am not a know-it all of everything.
Here are some basics about port and polishing and cylinder heads.
- Getting good airflow through your cylinder head is a must if you want to create big horsepower. How increasing airflow gets you big numbers? First remember that your engine is basically a big air pump. To get the most amount of horsepower out your head, you need the max amount of fresh airflow into the engine and the most amount of burnt exhaust out of the engine, with the least mechanical effort. Extra intake and exhaust port restriction will create more work for the motor, the extra created by restrictive ports is called pumping loss. Look at is this way; if you suck a smoothie through a coffee stir straw it will be very hard to get any of that smoothie through that straw. If you switch to a larger straw you get a lot more smoothie with less work—life is better with big straw right? The extra work with the smaller straw, is the same as pumping loss. The more work that the motor has to do to get the needed gases in and out of the motor, means less hp for your wheels.
- Another huge factor is volumetric efficiency. Volumetric efficiency is the percentage of an engine’s displacement that is filled on each intake stroke. Look at it this way: If you have a motor that displaces 1000cc. If the engine can take in 800cc or air on the intake stroke, you motor has 80% volumetric efficiency. Your standard Honda motor has about this amount of volumetric efficiency. How you get this number is based on a motors port size and configuration to optimize volume and geometry. This is good for a stock motor, but when you want modify your motor for more power, your flow demands for that motor change as well. Your basic bolt-on mods (header, intake, exhaust and cams, some people don’t consider this a bolt-on but in my eyes they are) promote higher rpm operation also demand more airflow. Although not a must to get head work when your basic botl-ons are on your motor, they do like it more and will perform better with head work. That goes with just a stock motor, but you get the idea. More flow usually requires bigger ports and more cross-sectional area. The main effect of porting a head is to reduce pumping losses and increasing volumetric efficiency by reducing the restriction.
- In the porting process the intake and exhaust ports are carefully reshaped (or should be if the porter really know what he/she is doing) by hand. This reshaping consists of enlarging, straightening, and streamlining to get rid of as much pumping-loss inducing restriction, turbulence to increase the flow velocity of the cylinder head as much as possible while make as much HP as possible. Most of the time ports are straightened by a die grinder and a carbide bit to a line of straight configuration. This straightening process gets rid of any bends that may cause turbulence in the head. This doe grinder that they use also is used to get rid of too cutting marks, sand casting pits, and usual bumps and lumps that are made by the mass production of our cylinder heads. Another process to gain more volumetric efficiency out of the head is called, extrude honing. This is where thick putty like goo full of abrasive is pushed through the cylinder head, enlarging the ports, just like the natural flow of your motor would like them to be. There are limits to porting though. You can make ports to big. Symptoms of a head that has been ported to much are a soggy bottom end, not making power. The other is a lumpy idle. The type of porting for your car will depend on the type of set up you want to run. Turbo cars like smaller port, high velocity ports without a lot of overall port volume. Nitrous and supercharged cars like the bigger ports with more overall port volume. The N/A street motors like the smaller, high velocity ports, like the turbo cars. Drag N/A motors will tend to like the bigger ports, this gets rid of the bottom end, but they want top end so it really does not matter.
- this is another major factor in getting the most out of your head. Truly, a 50% of head flow gains can be found in the valve job. Stock valve jobs are usually either one-angle valve job or two-angle valve jobs. One-angle valve jobs, is just done on the seats surface, two-angle valve jobs are a seat cut and a smooth throat cut. The high performance valve jobs have three-angled cuts, one on each side of the valve seat.
1st- there is a throat cut typically around 60-70 degrees. This will help the ease of the air’s transition to the 45-degree valve seat cut.
2nd- there is a 45-degree valve seat cut, which is literally where the valve actually seats.
3rd- this is called the top cut. This cut is immediately after the seat cut and is typically 30-20 degrees. This cut also helps to reduce valve shrouding of the airflow past the valve (or before if we are talking about the exhaust valve) as the valve starts to lift of the seat. There are five-angled valve jobs, but I think for the most part they are not needed, that is why I am not going to get into the five-angle valve job process.
- another thing that should be looked into if getting headwork done is the quench zones of the head’s combustin chamber. The quench zone is the flat are of the combustion chamber where the piston comes in close proximity at TDC (top dead center). These quench zones promote more complete burning and the chances of detonation.
- milling takes of a thin layer of metal from the bottom of your head. This creates a flat surface, which promotes a better seal to the block. This also increases compression. Most head porter will slightly mill the head, usually .050”-.060”. This increased the chances of piston to valve contact. Remember though depending on how much you mill a head will retard you timing anywhere from 4-6 degrees. To counter act this you should get a set of adjustable cam gears. Which most have, who are getting head work done. Since headwork is more saved to do when building a motor up. Piston to valve contact can be catastrophic to a motor. This can be done mathematically, but I highly suggest claying the motor. I did a write up on this you can find in the advanced tech and theory forum inside the N/A forum. Or go here to check it out Claying a motor...
It is important to maintain at least .045” on the intake valves and .055” on the exhaust valves. You want .030” clearance between any part of the piston and the cylinder head.