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Cylinder Head Porting

Basics of porting

by Julian Edgar

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A bit over 20 years ago I lived on an island that had lousy roads.

Since I was there for a year, I decided to leisurely rebuild the engine in my car – a 3 litre BMW. I pulled the SOHC six cylinder down and then, with plenty of time on my hands, decided to clean up the head. Mostly working by hand with a high speed grinder and sandpaper, I smoothed the ports and slimmed the valve bosses. I guess that I spent perhaps 50 hours working on that head – 50 hours that was largely a waste of time. Why? Well, with the standard, mild cam being retained, it’s very unlikely that I gained much extra performance.

In fact, to go further, in most performance car modifications, re-working the head is about the last thing you do – not the first. However, if you’re after every bit of engine efficiency, head modifications are still valid.

Flows

The amount of power that an engine can develop is dependent upon the mass of air that it can breathe per second. In fact, engine builders can often estimate quite closely the maximum power that is achievable from an engine simply by measuring on a flowbench the airflow that can pass through the cylinder head; some workshops using the following equation for naturally aspirated engines:

0.43 x maximum port flow (in cfm at 10 inches of water) x number of cylinders = peak hp

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This equation explains why some workshops state that a particular head “flowed X horsepower”. This is a little deceptive because the actual airflow efficiency (how much air is consumed for how much power is developed) varies from engine to engine. When they are tested on an engine dyno, good engines consume about 1 cfm for each horsepower developed. Also remember that peak power is not the most important aspect of a road car engine – average power across the working range is critical.

In the same way, it’s not the flow of peak valve lift that is important but again it’s average flow across the range of valve lifts. As one cylinder head porter said: “If we gain 10 cfm at low valve lifts and lose it at high lifts – great!” This is because the valve spends more time at lower lifts than full lift, and so low lift flow is more important. However, gains at low valve lift will not increase the “horsepower figure” that the head flows!

The restrictions to flow in the cylinder head comprise both the intake and exhaust ports, and the intake and exhaust valves. Multi-valve engines have large ‘curtain’ areas – the area found by multiplying the circumference of the valve by its lift. Thus in many cases, the ports are the major restricting factor, rather than flow past the valves. Note that this large valve curtain area means that reverse flow can easily occur when the piston moves from BDC towards the closing point of the intake valves, causing a reduction in torque at low engine speeds in engines with lots of valve overlap.

Flow Benches

As mentioned, a flowbench can be used to evaluate the volume of air that can flow through the port and past the intake valve. The flow measurement is carried out at a fixed pressure differential – normally very small (10, 20 or 28 inches of water). The flow is expressed in cubic feet per minute (cfm) and is measured at various valve lifts, invariably expressed in thousandths of an inch.

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The larger the “hole” (the cross-sectional area of the port and curtain area of the valves), the higher will be the airflow measured on the bench. From this it could be assumed that bigger holes – and so higher airflow readings – will always be better.

However, this is definitely not the case.

In addition to the mass of fuel/air that is breathed, the combustion pressure – and so developed torque – is dependent on the burn characteristics of combustion. Manufacturers build swirl and tumble into their head designs, resulting in more complete combustion.

Factors determining the swirl and tumble characteristics include the amount of ‘turn’ that the port has, the angle from the horizontal at which the port approaches the short side radius, and the shape of the valves and the chamber. Simply making the port larger without taking into account these airflow characteristics will not always yield increased engine power.

Another important factor is that a larger port area will result in a decreased flow speed. This may result in decreased cylinder filling at low engine speeds, reducing bottom-end torque. Effectively, the larger the diameter of the port, the higher peak torque will be moved in the rev range.

To indirectly indicate port area, port volume is often measured. With the valve in place and closed, the port volume can be measured using a burette and kerosene. As a rule of thumb, an increase in 3 cfm measured head flow should not involve the removal of more than 1cc of material (ie a 1ml increase in port volume).

Porting

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Major porting involves the careful removal of metal from the passages, enlarging them and often also changing their shape and even position. We do not recommend that you undertake any major porting of a modern engine. While there are some professional workshops that successfully carry out this type of work, these workshops are always equipped with a flowbench that is used extensively. Further, most of these professional porters have developed techniques which they will not share with mere mortals like you or me! Also, even professional workshops can spend many hours porting a head with only limited success.

An example of only moderate gains can be seen in the porting carried out on a four-valves-per-cylinder Honda CRX head. Prior to the porting, a spare Honda head was sectioned so that the porter could see casting thicknesses – carving into a water jacket isn’t unknown! The head was then treated to many hours of reshaping of the combustion chamber and intake and exhaust ports.

So what were the results on the flowbench? On the intake side, at all valve lifts over 0.275 inches there was a gain in flow, peaking with a 9 per cent improvement at 0.450 inches lift. On the exhaust side, flow was the same as stock or lower until a valve lift of 0.250 inches, with the biggest gain being 18 per cent at 0.450 inches lift. However, as already indicated, the valves are at their maximum lift position for only a very short time.

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In contrast is the porting of a Suzuki Swift GTi cylinder head. Already an efficient engine developing 58 kW/litre (high for a naturally aspirated engine without variable valve timing), the G13B four-valves-per-cylinder head responded well to extensive porting. The intake flow was substantially improved, with flow at 0.100 inches valve lift up by 28 per cent. This declined to a gain of 14 per cent at 0.250 inches lift, dropping to only a 1 per cent gain at 0.350 inches lift. On the exhaust side, flow was very substantially improved all the way from 0.100 – 0.350 inches lift, being up an average of 36 per cent!

Home Clean-Up

When compared with this type of professional (and expensive!) porting, how effective is some simple cleaning-up of the heads by a relatively unskilled amateur? With this approach, obvious casting marks, steps and jumps in the port walls are smoothed. The port shape is not changed and the port position remains standard.

Click for larger image

While not carried out on a 4-valves-per-cylinder engine, ‘before’ and ‘after’ figures are available on an AlfaSud 1.5 litre flat four. In standard form, the 1.5 litre engine develops 75kW (100hp) from its 1490cc – that’s 50kW (67hp) per litre.

Four different porting modifications were undertaken:

  • The sharp lip on the short radius from the intake port to the valve was smoothed

  • the rough surfaces of the ports were smoothed

  • the sharp edges within the combustion chamber were radius’d

  • the intake manifold was port-matched

Each cylinder required about five hours’ work - so 20 hours for both heads.

The flowbench results showed that the inlet ports improved in flow by an average of 9 per cent, with improvement recorded at all valve lifts from 0.100 – 0.500 inches. The peak improvement was 12 per cent at both 0.400 and 0.500 inches lift. On the exhaust side, the average gain was 3 per cent, with the peak improvement being 7 per cent at 0.500 inches lift. Again, at no valve lifts did exhaust flow fall.

Conclusion

The minor cleaning-up of the ports and combustion chambers is still a viable proposition on current engines, but very extensive (and expensive) porting can be carried out on sophisticated heads with no guarantee that the improvement will match the money that is spent. However, if the head(s) have been disassembled and more power is being sought, it costs only little to have them tested on a flowbench so that at least informed decisions about head modification (and cam lift selection) can be made.

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