Metric Mechanic - Porting the BMW Head

HiFlo ST/Sport Head Consists of the Following

• Major Modifications
  • Head is ported for an 18% flow increase.
  • Surface Turbulence™ is added.
  • Metric Mechanic Sport Cam is used (Refer to Dual Profile Asymmetric Design)
• Parts Replaced
  • New "Inspected" Rocker Arms
  • Rocker Shafts
  • Valve Springs (special)
  • Valve Guides (Aluminum Silicon Bronze)
  • Valve Guide Seals (Teflon)
  • All worn Valves
  • And, a Modified Oil Sprayer Bar on 6 Cylinder Heads
• Assembly
  • We carefully fit and assemble the head.
  • Then, hydrostatically check the head for compression leakage.
  • The valves are adjusted and the cam is set to fire TDC on #1 cylinder.

Modifications
• New valve seat angles
• Reshape the valve head
• Lift valve guide out of the roof of the port
• Ditch Cut
• Eyebrow Cut
• Anti-reversionary Cut

Objective
• Streamline and bounce air into the air flow path
• Remove an obstruction to the air flow path
• Direct air flow path around the intake valve
• Spread the air flow path more evenly in the port and keep the intake charge from traveling back up the manifold during valve closing.

The Main Air Path
Using a probe we can locate and measure low and high flow areas within the port. This information allows us to draw an accurate picture of the air flow path in a BMW intake port. When the intake valve lifts off the seat, most of the air flow will be in the bottom half of the intake port and will enter the combustion chamber through the high side of the valve seat area. (Illustration F)

In Search of Perfect Valve Seat Angles
On a three angle valve job there are entry, valve and exit angles. Of these three, the exit angle has the greatest effect at low valve lifts (when the valve is less than .200" open). The entry angle produces the greatest flow change at mid to high lifts (.200 to .400"). On the whole, the entry angle has a much greater effect on flow increase than the exit angle. Once the entry angle is set up correctly, the incoming air will "bank" off this angle into the air stream and drop right through the hole created by the valve and the seat. An entry angle error of as little as 5° will severely reduce air flow. See illustration G

Pulling the Valve Guide Out of the Port.
We use only silicon bronze valve guides that have been modified by removing about 10 mm of the guide from the roof of the port. The valve guide is shortened with a special cutting tool we use to install Teflon™ valve guide seals. Silicon bronze increases valve guide life and Teflon™ valve guide seals provide superior oil control over the life of the engine.

Increasing Air Flow over the Low Side Radius of the Port (The Ditch Cut).
The low side port radius is very tight on a BMW. Air flowing on the bottom of the port wants to run into the back side of the valve. After three months of intensive flow testing we discovered this cut. We nicknamed it the "Ditch Cut". This cut creates a slight low pressure area under the air flowing on the bottom of the port causing the air to start bending early so that it can make the turn at the low side radius. We spent many hours figuring out the right depth and placement for this cut. The Ditch Cut is second only to ideal seat angles in contributing to the greatest flow increase.

Increasing Air Flow on the High Side Radius (The Eyebrow Cut)
This is a very light cut done just above the valve seat on the high side of the port and looks like an eyebrow. Not all BMW heads like this cut. Before making this final modification to the intake side, we test each head to see how well it is flowing. That test will establish whether we make this cut. If flow is down slightly and the head looks like a good candidate for the eyebrow cut, we'll make it. If the head needs this extra cut, it will produce a worthwhile increase but it's a judgment call. Maybe that's a key to our performance, judgment based on critical observation instead of following simple mechanical procedures.

Anti-reversionary Cut
This modification serves two functions, first, as air passes from the manifold into the port, this cut creates a low pressure area and raises the air flow upward toward the roof of the port causing the air flow path to enlarge and spread more evenly through the port. Second, when the valve slams shut, air curls around the valve and heads up toward the roof of the port. Upon hitting this cut, some of the charge is captured and stays down near the port (instead of traveling back up the manifold) ready to become part of the incoming charge.

Modifications
• Reshape Combustion chamber walls
• New Valve Seat Angles
• Modify Valve Shape
• Lift valve guide out of the roof of the port
• Reshape short side radius

Objective
• Converge exhaust to the valve seat opening
• Complement the walls of the chamber to direct exhaust to valve seat opening
• Minor modification, stock shape is pretty good
• Remove an obstruction to the exhaust flow path
• Improve flow at port

This modification is not shown in the illustrations. The shape of the combustion chamber is good on all four and six cylinder heads except the early 121 heads. On this head the wall slightly shrouds the exhaust valve and impedes flow to the exhaust opening.

Ideal Valve Seat Angles
Of the exhaust valve angles, the entry angle is most important and the first angle that the exhaust sees. The entry angle aims the exhaust (flowing along the combustion chamber wall) into the valve seat hole. To be most effective, the entry angle must compliment the combustion chamber wall.

Ideal Exhaust Valve Shape
After running 45 tests on different valve shapes, we could not beat the stock exhaust valve. After we modified it by removing metal and reshaping it every way we could, we started adding Bondo to give it every form we could think of. (NO! We weren't actually considering modifying valves with Bondo, we were just experimenting with shapes!) We even substituted exhaust valves from other engines and tried high performance valves. Before all this testing, I would have never guessed that the stock exhaust valve would be the hot ticket.

Lifting the Valve Guide
We lift the exhaust valve guide 5 mm out of the exhaust ports. It delivers only a slight flow increase, but then, every little bit helps.

Reshaping the Short Side Radius
Re-curving the short side of the exhaust port was about the only significant thing we were able to do to improve the exhaust flow in the port. We tried to improve exhaust flow by porting the high side of the exhaust port but we discovered that anything we did only reduced flow. The factory has to be right sometimes.

Did we mention that we did over a thousand flow tests during initial development? Obviously, not everything we did worked the first time. The following is a short list of the things we tried that we thought would work but turned out to be air flow losers.

Multi-angle valve jobs
will not always produce an air flow gain. A very standard three angle cutter uses 30°/45°/60°. If you based these angles on the exhaust it would flow a little worse than stock. The 30°entry angle (the first angle the exhaust sees) hurt the exhaust flow more than the other two angles. It took many hours of flow testing to find the three optimal angles.

A Perfect Radius
with a valve seat angle (44°) cut into it. I thought that this would be the hot ticket to good air flow gain. As air banks off the radiused seat it diffuses in all directions and blocks off the air flow path.

Reshaping the Exhaust Valve will only cause air flow loss. Here are some modifications we tried that we thought were winners but ended up losers.

Removing the Ridge on the Head of the Exhaust Valve
with a 30° back cut will not only hurt exhaust flow but will defeat the anti-reversionary system used to keep exhaust from back flowing into the combustion chamber.

Radiusing the Margin of the Exhaust Valve
will reduce flow by 10% or more depending on how big you make the radius. I've read several porting articles that say you should radius the valve margin. Well, that may work on Fords or Chevys but, it diffuses exhaust into the entry hole and dams up exhaust flow on a BMW head.

Polishing
A port on a BMW engine will cost you about half a cfm air flow loss below .450" lift. Above this you will gain about a quarter a cfm. If you want your head to look pretty (on the inside), polish the ports. If you want it to perform, don't! Here's why. As the fuel and air mixture runs along the smooth walls of the polished port, it will be pulled out of suspension. The port then becomes damp with fuel. This fuel causes early carbon build up at the intake port and doesn't burn as completely in the engine resulting in higher fuel consumption and loss of power.

To get a better idea of what is happening, try this little experiment: Put a drop of water on a piece of glass and then blow on it. Now, put a drop of water on a sheet of 400 grit sandpaper and repeat the test. See, the water sticks to the glass more than the sandpaper. The walls of your ports act the same way. Here's the best reason of all not to polish your ports. It costs cubic dollars. We would have to add $200 - $300 to the cost of our four cylinder heads and maybe as much as $450 to the cost of a six cylinder head. That's a big price for a net decrease in performance. I don't think that it's worth spending money on this kind of cosmetic work. Maybe it seems that we're making too much out of this, but it's hard to convince people that they've wasted money when they think they were improving their engines. They were only improving their mechanic's wallet. After all, once the head is on the engine, no one can tell if the ports are polished.

This popular combination mates up our HiFlo head with our MM Sport Cam. Maybe you have head problems (cracked head, blown head gasket, etc.) or you just wanted more power. You may not think of a head as a bolt on item but it really is. When you pull the intake and exhaust system, it is almost as easy just to pull the head at the same time. Let's say you have a good sound engine and you're thinking of upgrading the intake and exhaust but there is a cork stuck right in the middle of everything, a stock head! To get the power you are after, you need to "uncork" the head. A correctly ported and cammed head is worth about a 20% increase in Horsepower. A correctly matched up intake and exhaust system will enhance that figure.

This combination will have little or no low end torque loss compared to a stock engine because of high port velocity and the design of our MM.

Note: We do not recommend installation of the HiFlo ST Sport Head on high mileage engines (over 100,000 miles) or engines that use more than a quart of oil every 750 miles

Introduction
At Metric Mechanic we use the Fall and Winter months to do research and develop new products. In the later part of winter '85, we began developing HiFlo ST "Baby" Six (ETA) Heads completing all flow testing by Autumn '86. At present we are at a 19% flow increase. Before getting into how we port the "Baby" Six Head, let's give a little background information on these heads.

Head Inter-changeability
We've seen 323i owners look high and low for a new head and usually pay about $1000 on finding one. Well, 323i and 325e ETA Heads are virtually identical. Both heads have seven cam bearing journals. The only difference is that the ETA Engine has a four bearing as opposed to as seven bearing cam used in the 323i Head. So, the ETA Head only used four of the seven cam journals in its head. To use an ETA Head on a 323i or to install a 323i cam in an ETA Head, all you have to do is drill out extra oiling holes for the other three cam journals. This rather easy modification saves hundreds of dollars because an ETA Head is $700 or under and readily available.

"Baby" Six Head Cracking
"Baby" Six heads like to crack around the bottom of the #5 cam journal (see picture). The crack will go right through the journal and oil that is feeding the cam bearing journal under pressure will be forced into the cooling system. Also, coolant under pressure in the cooling area of the Head will be forced up through the crack. This causes the oil to get into the radiator and coolant to get into the crankcase. So, if you see a light brown creamy milk shake in your oil pan and or radiator, your head is cracked and needs replacement. At the same time, you'll need new rod bearings because coolant is a lousy lubricant and so the rod bearings wash out. We have pulled some unbelievably terrible looking rod bearings out of "Baby" Sixes with absolutely no damage to the crankshaft rod bearing journals. When installing rod bearings, the rod bolts must also be replaced. They are stretch type angle torqued bolts. To do a 70° angle torque on new rod bolts requires about 45 ft. lbs. of torque. If you reuse the old rod bolts, it takes about 70 ft. lbs. of torque and bolt heads will feel like they're ready to wring off.

But, if you have the light brown milk shake in your radiator or oil pan, it's almost a sure bet that your head is junk.

So, before sending us your "Baby" six Head for porting, please check the #5 cam journal area of the head for cracks. (These cracks can be hard to see if the Head is not very clean.) The cracks in the picture have been marked with a black marking pen.