flash968

ah - my bad - i have never been good with names - i actually have to repeat somebody's name back to them as soon as they say it, or it's gone

in the 80's i "dated" a number of women named babe, sweetie, and cutie

SpeedBump

ROFL

whakiewes

There are two different forms of porting and polishing; gains differ accordingly. First is port matching which is making the manifolds or the head even with whatever the difference is. Typically both the intake and exhaust manifolds are about .5mm larger than the actual head ports. On the exhaust this doesn't matter as its small going into large, but on the intake this creates a restriction. The idea is to remove that restriction. At the same time there are casting flaws in the aluminum head and you want to work those out well. A good start for this is two part - first purchase a porting and polishing kit from someone such as Standard Abrasives. Second is you need atleast an intake/exhaust manifold gasket and the manifold. Tools needed would be an air compressor, grinder, and engine builders blueing liquid (for the love of me I can't remember the correct name). Keep in mind with this modifications you are only matching, not shaping. I wouldn't attempt this for the first time on a 968 head, and in all honesty I would still pay someone. I did this on my Audi Coupe Quattro's head and it came out well, but it took me nearly 10hrs to be satisfied.

The other form of porting and polishing is shaping. This is where you alter the actual ports to increase performance and alter performance. Many people attempt to catagorize this modification in power but fail to look for the overall gains. Making the ports larger doesn't always net gains, sometimes it nets loses. In some motorcycle applications they actually make the ports smaller and shape them for better directed high end airflow (power at 15,000 vs. 4000). This is not something that can be a DIY typically. The correct way to do this is using a flow bench to view and alter the work. Builders go about this kind of porting a couple of ways. First is by sonic testing the head to insure its durability as well as to view the thickness between water jackets. This is of upmost critical importance as in many engines there is only a matter of millimeters between the ports and water jackets. Then the head will be flow benched and a digital diagram will be shown of what can be improved via the software (kind of cheating for newbies...old timers still do it old fashioned with original flow benches with tennis ***** and such). Sometimes all the port can take is just cleaning up casting marks - such as BMW's S14 engine. The idea is to make the port as direct as possible while still allowing the proper mixture of fuel and air. Some heads like my Audi CQ's head have nearly a 90 degree angle from entrance to the combustion chamber. BMW's S14 has closer to a 20 degree angle so not much can be gained from altering the shape. Some builders choose to port the head as far as possible then using ceramics and clay shape the ports perfectly. I am not sure what makes what different, but these are the typical methods.

Polish ONLY the exhaust side. 140 grit is pretty good for the intake side. I start with a grinding stone, then go to 80 grit, the 160 grit. Polishing is done on the exhaust via a 160 grit wheel, then two grades of steel wool type material, then a fine cloth polishing pad. I also polish the combustion chamber to keep carbon from forming. Be careful doing this though as you don't want to remove hardly any metal to keep the dish volume the same or VERY close. I never touch the combustion chamber with anything less than 160 grit.

Another thing to consider on the head is a valve job. There are a set of angles that are present from the factory. On non-performance engines there is typically only one angle, 45 degree's. Valves are cut at 46 degree's to allow proper seating. A performance engine such as BMW's M-series, and possibly Porsche's come factory with a 3-angle valve job. 45 degrees that the valve sits flush with, 30 and 60 to ease the flow of air into and exhaust out of the combustion chamber. High performance engines such as race engines and high performance street can utilize a 5-angle valve job. This adds two more angles to make as close to a rounded entrance/exit as possible; 15, 30, 45, 60, and 75 degree's. There are two ways a builder can go about doing this, hand or machine. There can only be a very small amount of metal removed as too much will result in improper valve seating and a loss of compression. A 3-angle would be possible as a DIY, but a 5-angle needs to be performed by someone who has the experience to not mess up.

Typically on a performance engine you can see between a 20% and a 40% increase in CC flow by performing quality porting and a 5-angle valve job. On a non-performance engine (such as a Ford Focus) anywhere from 30%-60% gains can be seen. Keep in mind this is NOT a 20-40% increase in power...don't we all wish. This is a 20-40% increase in the volume of air that a port and hold and flow. Typical power increases are between 5 and 10% depending on what they are accompanied with. So on a 250hp engine, gains between 15-25hp could be had from doing about $2000 worth of head work.

Wes - two years ASE and performance engine building trained

300guy

Thanks Wes,

good info. I plan to port-match, remove casting marks, and finish to 140 grit or so.

I'm considering a 5 angle valve grind... still asking around for a good shop in my area (Ottawa, Ont.) Are there any durability issues with a 5 angle grind?

thanks,
Colin