Tech Articles

Head Porting - March 2001 with Updates

A Primer on Volvo B18/b20 Heads and Head Porting for the Vintage Volvo Enthusiast

This article originally appeard in Vclassics online Magazine in March 2001. A shorter version appeared in Rolling, the magazine of the Volvo Club of America, in February 2000.

Some modifications have been added to bring it up to date. Be sure to read the addendum at the end.

Last year I read an article in a Volvo club magazine summarizing the various methods of improving engine performance. Although generally accurate, in trying to summarize head porting in a few sentences some  misconceptions as to what should be done to improve the performance of a head may have been perpetuated.  The article summarized porting as " the process in which the intake and exhaust ports are opened up as much as possible. This allows more intake and exhaust to flow ..., which , in turn gives you more power. Increasing valve size ... also has the same effect. The larger the passage and doors, the faster you can get things in and out."

While these statements accurately reflect the general conception of head porting, they can be misleading, especially if used as a "how to" porting guide applied to a particular head such as the Volvo B18 or B20.  While it is correct to say that most porting jobs involve opening up some part of the port, many ports will show significant improvement if portions of the port are filled in. Likewise the flow will be reduced if the  ports are enlarged in certain places.  Both of these effects  can easily be demonstrated on B20 intake and exhaust ports.

In the past head porting was based on an educated guess as to the modifications that would increase flow. Everyone assumed that enlarging the ports would increase flow.  It was also assumed that engine power would increase if the ability to flow was increased. Anyone can take a grinder and enlarge the ports in a head, but flow and dyno testing show that this approach does not usually increase performance. The shape of the port is in fact much more critical than the size. In an effort to enlarge the port, a common result is the disruption of smooth or laminar flow.  ( Its crucial to know where port enlarge is helpfull and where it is harmfull - mainly a trial and error process.  Unless adding material to the port, material has to be removed to improve the shape and make all areas of the port work together. )

Modern head porting is designed to improve the performance of an engine, not just increase the flow of air/fuel. In many cases enlarging the ports will increase flow through the ports. In other cases it does not. Surprisingly,  increasing the ability of a port to flow  may in some cases actually reduce the power output of an engine, particularly in a useful RPM range. Here is where we see the disappointments that many people have with head porting, and the conception that head porting is only appropriate for race engines.

On race heads porting does usually involve increasing the volume of the port by "opening it up", but on a head destined for use on a street engine significant enlargement of the port may well hurt overall performance. There is a tradeoff here between port volume and charge velocity. Enlarging the volume of a port will normally reduce the velocity. Due at least partly to this effect, a head is ported differently depending on the intended use. Putting a head ported for race use (or a large port casting such as an R Sport head) on a relatively stock street engine will always yield disappointing results. In essence, the size and shape of the ports must be matched to the engine and its volumetric needs.  Effective improvements on a street head normally involve careful reshaping of the ports in specific areas without significantly increasing port volume.

Even on race engines bigger is not always better. To complicate the matter further, some types of dynometer testing do not accurately reflect the ability of an engine to accelerate. They measure  steady state horsepower as applied against a load.  As an example, the head I used to win the Volvo Grand Prix at Road America in 1998 had smaller ports and produced less horsepower than the head I used on the same block to win the Volvo Gand Prix two years before. But with a broader power band the smaller port head was faster out of the corners and allowed me to turn similar lap times. Since I was running with a standard ratio M40 transmission after my close ratio broke at Watkins Glenn the weekend before, the broader  power band of the small port head may have  made the difference.
For street use with stock displacement engines there is little evidence that significant enlargement of a B20's ports or putting in significantly larger valves will increase useable power, without other changes. However it  can be demonstrated that judicious reshaping and enlargement of certain areas of the ports, especially the diameter and shape of the transition area to the valve seats,  will improve both tested air flow and power in a street engine without significantly increasing the port volume or the RPM range where the power is made. With this type of porting there can be a significant increase in power without other modifications

Today any competent head porting effort involves careful testing of each modification on a flow bench as well as dyno testing to confirm that the port modifications not only increase flow but useable horsepower in the desired RPM range. Unfortunately this type of testing is beyond the ability of most individuals.  In addition, it is beyond what most of us can afford to spend to have someone not familiar with the nuances of a particular head  learn which modifications are most effective on a Volvo head. (What works on a Chevy does not necessarily work on a Volvo. You don't want the experimentation done on your head!)

.Another issue is the limitations of the casting. Given infinitely thick port walls, it would be relatively simple to design and execute modified port shapes that would result in remarkable improvements in port flow. Unfortunately, port walls are comparatively thin - no more than a 1/8th of an inch thick in most cases. Grinding the port walls thins the wall, and there is a limit to how thin the wall can be before it is prone to stress and thermal cracking. Often the changes we would like to make cannot be made because the wall is too thin in the area where material needs to be removed.  Thus, there are compromises that must be made between performance and reliability.

To make matters worse, the ports are not uniform from casting to casting. There are significant core shifts in the casting process. It is not uncommon to find a casting that is "off center" by .050 to .060 left/right or front/back. Thus the difference between the port wall location in two supposedly identical heads can very as much as .100 to .120, essentially the thickness of the port wall in some cases. Since the ports in any particular head are not symmetrical, there being two right intakes, and two left intakes, for instance, it is necessary to take the core shift left or right into account in porting different ports in the same head. You can visualize the core shifts by noting the different thicknesses of casting around machined surfaces on the block, the simplest being the head bolt holes under the valve cover. These machined holes are always centered, it is the casting that is off.

I just could not do this article without getting into the issue of which Volvo heads to use.
Unfortunately, I cannot give a comprehensive answer, and even if I did there would be disagreements. First of all I have not tested every head made for the B18/B20. When we get one in the shop we frequently do not know what engine it was originally on, so I don't have all the specs for all of the heads. Further, there were engines that were not normally imported into the US. Thus, I would not be shocked if there were some surprises out there. So let me just give you some general guidelines, recognizing that I am going to try and cover the issues most often asked.  

If you have a B18 engine, use a B18 head. The combustion chamber and valve spacing are just too big on a B20 head for the smaller bore B18. If you  have a 2 liter or larger engine, even if it is a bored out B18, use a B20 head. The B18 head combustion chamber is just too small, the compression ratio will be too high.

If you are rebuilding a 2 liter or larger motor, use a B20F or B20 Carb head. For SU's use the carb head as it has the benefit of the appropriate carb location rings, although an E or F head can be used.  (It is unclear to me whether there is more than one B20 Carb head casting.)  For Webers or FI use the B20 F.  The F head can be identified by a combustion chamber depth and total head thickness that is .090 more than the E head.

The exhaust port is the one that needs work on the B20. For a street head, don't worry about the intake port until there is a significant improvement in the exhaust port. A standard is that the exhaust port should flow 75 - 80 % of the volume of the intake port. In stock form the B20 exhaust ports flow only 60% to 65% in comparison to the intake. The exhaust valves on the B20 and B18 are the same size, 35mm, while the intakes on the B20 were increased from 41mm to 44mm. (This may be one reason why B18 engines seem to rev much higher than B20s.)  

The B20 E engine produces more power in stock form than the B20F largely because it has a higher compression ratio than the F (10.5 - 1 as opposed to 8.7 to 1) and has better exahaust port flow. The B18, B20 carb (there are at least two versions), B20E and B20F exhaust ports are similar, and have the same size exhaust valves.  However the B20F and late B20 carb head have thicker exhaust port floors and a more restrictive "throat " area below the valve seats ( looking at the head with the combustion chambers pointed up ). Both the B18 and B20E exhaust ports actually flow better than the B20F in stock form, not due to a better shape of the port, but because they have a larger throat (the area below the valve seat). Fortunately, what hurts in stock form actually helps when the heads are modified.  Because of the extra material, the B20F and late B20 carb head ext. ports can be easily modified to flow better than the B20E.

The 10.5 - 1 CR of the E head is just too high for normal street use unless the engine is worn out, a radical cam is used, the combustion chamber is enlarged, or a special knock sensing ignition is used that will retard the timing when there is detonation. Don't rely on a thick head gasket to lower the compression ratio. The engine was designed for a particular distance between the piston and head. Adding .090 in gasket thickness alters this relationship and can have negative effects on combustion efficiency. An F head can be milled to get a higher than stock ratio. Remember that increasing the bore increases the compression ratio and this should be calculated any time you are modifying the engine and head. Around 9.5 to 1 seems to be a good compromise.

Following is a list of 14 points that should be considered before having any head ported.

    1) bigger ports don't always flow better
    2) shape is more important than size
    3) port polishing is a waste of time (and money) and usually hurts performance
    4) just putting in bigger valves usually hurts the flow due to increased valve shrouding
    5) most intuitive modifications either produce no improvement or hurt the flow.
    6) modifications that work on other heads do not always work on the Volvo B18 and B20
    7) extremely small changes and combinations of changes are crucial
     8) many changes yield no positive results unless combined correctly with other modifications
     9) head porting must  be done in combination with valve and valve seat preparation
    10) head porting should also involve changes to combustion chamber shape
     11) the normal practice of flow testing bare heads (without carbs and manifolds) gives inaccurate and misleading results.
    12) choose a head that is in good shape and has minimal core shift
    13) for a street engine have hardened exhaust seats installed
    14) ask your "porter" for proof of the positive results of his modifications

And one more point. In most engines, head flow is the crucial factor in limiting the power that can be produced, regardless of what cam is in the engine and what intake and exhaust are bolted on to it. And this is certainly the case in a B20. For example, I've had a B20 engine, with the same head, valves, cam, carbs, compressin ratio, exhaust header, etc., produce a difference of over 50 HP depending on the porting done to the head..
In summary, basic improvements in street performance from head porting can be significant while being comparatively simple and inexpensive compared to other types of modifications, but trying to get the last few percentage points of improvement for the track is expensive, complicated, and extremely time consuming.  Overall it is a complicated process with no easy answers, but it is the key to significant improvements in engine performance.

Important Update   -  June 2009

As the above article was originally written almost 10 years ago, there have been changes.  The best way to summarize it is to say that head porting is an evolving science ( or art ).  Throughout the industry there have been significant improvements in that period of time and porting at VPD is no exception.  Although some of our heads have the same names that they did in 2000, they are not the same heads.  Each head has more work put into it, usually more material taken out of it in the "right places", and the results show it.  They also demonstrate the degree of improvement that can be obtained through experience.

An example is a race head originally done in 2001 which came back as part of an engine rebuild this spring. Before doing any work I put it on the flow bench to get a new baseline and compensate for any changes in flow procedure, etc., since it was originally tested.  The result was somewhat of a surprise since it would not have met the flow criteria of one of our type II or type III Street Performace Heads today -even in the exhaust were it had 38 mm vs the 36 mm valve size of the street heads. So it was updated and went out a much better head. On the dyno we got an increase of 20 HP. I also checked the old head from my 1800 racecar, a head that has been in service off and on since 1992, but which essentially remained in a 1998 design stage and which was the test bed for some of the ideas that went into the street head line. Essentially the results were the same - the street heads had evolved to the point where some of the flow numbers were higher. Which maybe should not be a surprise as dyno results from the stroker street engines show significantly more power than the older 2 liter vintage race engines.So it was upgraded, some new elements added, and we got an increase of almost 30 HP.

The current power output of our street engines is another consideration. As we progressed from 150 to 220 HP normally aspirated street engines, the heads had to evolve to keep up, and they did.  For those of you that had a head done several years ago, this may not sound like good news, but it really is. All of our heads have been done with the idea that they could be updated at a later date. So if you have one our heads that was done several years ago you might want to consider sending it back in for an upgrade.

John Parker

Vintage Performance Developments