Introduction to Heads.

- or, "The poor man's guide to fluid dynamics"

You have probably heard the claim before that the head(s) are the most important part of an engine build.

That's because it is true.

Unfortunately, it is also true that it is almost impossible to just go out and buy the right heads for your engine.

With the right headwork and a set of ratio rockers, an otherwise stock engine can outperform a significant percentage of "performance engines," possibly the majority!  I have seen the dyno figures of engine making incredible amounts of horsepower through their huge (ported) heads, but usually only ever see one number. While the engine may be making 200HP at 8000 rpm, what is it making at 4000?  Often it isn't even running well - this would be OK with close ratio gears and a low enough diff ratio, but with most VW transmissions this would run like a hairy dog around town! And drink like one, too!

I know I harp on about this a bit, but big ports and flow numbers do not equate to a powerful engine. Yes, the maximum or peak level of power an engine can make can be limited by, or enabled by the size of the ports, but if the ports are too large for the application, the engine may not be producing usable levels of power everywhere else in the rpm range!

People advertising heads often quote "capable of making xxHP" this is usually a calculation such as 1cfm @28" = 1HP which is kind of fair enough - but if the rest of the engine is not going to flow efficiently at that level there is no point.  CNC machining has become affordable enough that the old "hog out the ports as big as you can get 'em" has found a whole new marketing strategy - but the mass-produced ports may not always be optimal, nearly always include completely removing the exhaust boss, and are too smooth for optimal flow.  You heard me right - we actually learnt back in the 70's when flow bench data became available to the amateur scene (rally cars back then,) that a polished intake port flows less than a sand-cast one of the same dimensions!  The same should probably be true for exhaust ports, but the reduction in heat transfer and coking of a polished port more than makes up for it - plus there is usually more room for enlarging the ports and a lower port velocity is not such a problem (exhaust gases commonly exceed the speed of sound, hence the sonic tuning.)

A careful examination of the figures in HTHRVWs shows that a stock VW port outflows a lot of aftermarket heads!  Especially if a little sensible work is done on the combustion chambers after milling for larger cylinders and a good valve job performed!

The trouble with this is usually the cost of having someone with a flow bench perform the work, but the basic advice of HTHRVWs of unshrouding the valves, laying back the non-plug side and basic cleanup of the ports can be done by just about anyone and with a good valve job will produce good flow without decreasing port velocity (or increasing port volume) too much.

The original CB044 heads produced very good numbers from a port that was not overly large for most engines, but the extra metal around the combustion chamber and below the rocker and head studs left significantly less fin area for cooling.  I didn't hear any stories of overheating however,but they are not available unported at the time of writing.  A little unshrouding to the cylinder size on these or several alternatives such as CB041 or 043 heads should produce a good head.  Without a flow bench it would be hard to be sure though.

Ok, so the work on the chamber in HTHRVWs makes sense to you and you are aware of the value of keeping the deck height, therefore squish volume, as small as possible while the port volume will set the compression ratio (either by porting or skimming) - what about the ports, huh?

Basically, we want the port size to be as small as possible while maximising flow.  The main things here are to try to keep the cross sectional area constant, avoid any sharp bumps or turns, and avoid any "step" where the port suddenly becomes larger (or to a lesser extent, smaller.) We avoid the sudden steps by creating "venturi effect," essentially we smooth the change in and out to accelerate the air through the restriction. This enables to move air a little faster than the normal point where turbulence tends to occur, around mach 0.5, but only at that single point at the centre of our venturi. The required diameter can be calculated and you would be surprised just how small the optimum is for most engines when bore/stroke and rod ratios are combined with the appropriate cam timing - about that of a stock port or little more. An engine up to 2 litres turning up to 6k would most likely be happy with no increase in the maximum CSA of the port, just the appropriate cleanup as above. Strokers used on the street only require an average port diameter of 34mm to turn 6-7000+ rpm.

You can easily reach these engine speeds and flows using off the shelf CNC ported heads, but you will probably lose a lot of torque throughout the range. Remember, peak HP is what you are making at that RPM, torque is what you make the rest of the time (99% or more of the time.) Personally, I'd give up a little off the top end to get more for the 99% of the time when I really need it.

So, I'll probably get 041s with the correct bore and clean up the chambers. If they are no longer available I might get AJ Simms Pocket port.  The Simms' have the advantage of the valve job already being done, but unfortunately he also takes the exhaust guide boss out and trims the guides.

Whatever heads you end up with, they will probably need some polishing not the intakes or exterior, the chambers and exhaust ports. This reduces heat absorption, coking and the risk of detonation. Smooth off any sharp angles or roughness with sandpaper or stones, then polish until you can see your face in it.

- and that was just the intro! At a later date I will put some detailed information here.