05 May Building a Reliable and Powerful Street Engine III
The power is in the Heads!
It is often said that the secret to a well performing engine is in the preparation of the cylinder heads. It is also commonly known that the combustion engine is basically an air pump. Aside from creating the airflow’s path into and out of the engine, the cylinder head also seals off the area of combustion, and determines the shape and speed of the combustion action. A cylinder head that presents little restriction to the incoming air-fuel mixture and outgoing spent exhaust gases will benefit engine performance. Such a cylinder head does not lend itself to creating restrictive pumping losses on a performance engine.
Typically, to achieve less airflow restriction, cylinder heads are set up with enlarged intake and exhaust ports, coupled with larger diameter intake and exhaust valves (compared with the stock units). It seems the key to a high output engine is to create the largest ports and stuff the largest valves that can be arranged within the space of a cylinder head. This assumption is wrong.
When planning and assembling a high output street (or race) Volkswagen engine, it is best to evaluate what your primary goal is for the car and engine. The first and most important consideration concerns cooling. Many of the aftermarket heads really don’t provide the same cooling capabilities as a stock-type casting. While these aftermarket casting heads do offer a number of advantages over the stock head casting in strength, space for larger valves (and valve springs, retainers, rockers, etc.), they can help to create an overheating condition that is hard to overcome. I have seen differences of 40 degrees F oil temperature between aftermarket heads and stock casting heads on similar engine combinations. The well-known reason for the cooling problems is that the many aftermarket heads don’t have the same number or configuration of cooling fins as the stock casting.
Don’t take this consideration lightly. I found myself very frustrated after sinking many dollars into a big set of Super Flow heads for the street, as the car could not be driven for very long before the engine began to overheat. Of course, improvements to – or replacement of – the VW Type 1 cooling system could help to solve any cooling dilemmas.
For a street driven hot rod that we are considering (using the common VW doghouse fan, shroud, stock offset cooler and an auxiliary oil cooler), a stock-type design casting head will work best for performance and cooling abilities. Not that an aftermarket head won’t work, because it will, just at the expense of less cooling capabilities. New Volkswagen casting heads are usually found under two part numbers today (dual port versions), 040 101 355 from Brazil and 043 101 375 from Mexico. The 043 castings can be identified by the large boss above the gasket rail (temperature sender boss for FI engine), and the lack of intake guide bosses in the intake runners. In addition to these VW heads, there is also the 044 casting, which offers a number of benefits over the stock Volkswagen versions. For heads that will not have the spark plug holes welded and relocated in 12mm x ¾” size, the 044’s have the popular 14mm x ¾” size stock, while offering stock VW fin configuration. The latest 044 castings even offer an intake flange with a larger area, purchased with CNC ports or ready to accept larger ports on your own without major aluminum welding.
Modifying Heads for Performance
Once the decision has been made regarding what casting will be used, it is time to choose what type of modifications best suit the performance you are looking to achieve with your street VW. The basics will be reviewed here, as the true details and secrets behind the nature of airflow and combustion are best discussed with a cylinder head specialist.
Each specialist has his own theory and practice that has earned his title as a “specialist”, and this experience can hardly be duplicated on paper. The important aspects to remember when consulting a head specialist are the following:
Horsepower is the product of torque and rpm. The more torque an engine can produce, in turn, the more horsepower it will produce. Within practical limits, the more rpm an engine can safely turn, the more horsepower it will produce. There is the catch, however; an engine with overly large ports will lose port velocity, typically resulting in less than favorable torque output. Conversely, an engine setup with small, restrictive ports will not receive the volume of incoming fresh mixture in order for it to turn higher rpm’s efficiently.
In addition, an engine equipped with very large valves (in proportion to the bore size) can actually produce less power than an engine equipped with valves better sized to the bore size and application (and keep in mind, the larger a valve is, the heavier it is, further limiting rpm limits due to the strain on the valve springs as a result of the increased mass).
An engine equipped with too small of diameter valves will generally need a liberal amount of camshaft duration to allow the engine to turn elevated rpm’s, however such an engine will have a very narrow and peaky powerband. It makes more sense to build an engine with a set of larger valves, and stay within the cam figures discussed in my earlier article. It is probably best to build a set of street heads that follow the tried and tested path to high horsepower on the street. I would suggest that, for a street machine at least, shy away from overly exotic modifications. Many very fast street VW’s are outfitted simply with stock-casting type heads that have been fitted with larger intake and exhaust valves, reshaped ports and combustion chambers, and dual valve springs (correctly set up for spring pressure and coil clearance).
For an example 2276 being built on paper, again, I would choose a head with the stock type cooling fins and have a cylinder head expert perform the following:
* Install 43.7 and 37.5mm stainless steel, stock length valves
* Weld up existing stock spark plug holes and relocate 12mm x ¾” holes
* Reshape the intake port, keeping in mind we are after port velocity
* Reshape the exhaust port, but only to an extent; an overly large exhaust port will allow spent gases to re-enter the combustion chamber during the valve overlap period, decreasing scavenging efficiency
* Reshape the combustion chamber in a fashion that will not only improve flow, but also provide the needed “squish” that aids turbulence in the chamber, improving combustion. The head specialist will want to discuss “deck height” and static compression ratio with you as well. Keep the following in mind: the more deck height (distance from top of piston to top edge of cylinder at TDC) you build into the engine, the less squish-area you will have, impairing efficient combustion (engines running a large deck height are usually very soft on response and power!)
* Assemble the valve springs and retainers according to the camshaft figures you provide. Each valve spring must be installed at the same installed height, and also checked for total travel to coil bind. For the street engine discussed here, a good quality set of stock diameter dual springs or possibly triple springs (similar to dual spring, except with a flat damper spring wound between the outer and inner coils in an effort to combat spring harmonics) coupled with titanium retainers will work well. The springs should be lightly deburred where they contact the shim in the head and also where they contact the titanium retainer. The head expert should also make sure that the valve keepers don’t touch each other once assembled (if they do, the keeper grooves on the valves will wear out very quickly!)
* Match port the intake manifolds that are to be used.
* Flycut the cylinder-seating surface for the desired chamber volume to work with your compression ratio target.
One more thing to keep in mind: quality head modifications never come cheap. Cylinder head work can make or break your engine. There are no “easy ways out” when it comes to cylinder heads, so be prepared to discuss your needs at length with the head specialist, inform him or her of what you are looking for and of what your budget is. If your planned budget doesn’t fit with the modifications necessary for your desired performance level, keep saving money.
