Porting Gains

This article was first published in 2003.

It's a thread that often comes up in discussion groups: is it worth porting modern heads? (So what's porting then? It's the modification of the shape of the intake and exhaust ports to promote better flow, in turn boosting torque and so power.) In general, the consensus re porting is these days often a 'no' - with some heads now CNC-machined from the factory and all heads very carefully engineered and nearly as carefully cast and machined, what's one aftermarket guy equipped with an air-grinder going to be able to do that's positive? But on the other side of the argument, in all race engines where the formula allows such work, porting of the cylinder head is a must-have if the car is to be a front-runner.

With the help of the Gilbert's Cylinder Head Service - located on Australia's Gold Coast - we took a close look at the gains available from working on a 6-cylinder Falcon head.

The results are surprising - and may well warrant a re-think in the prevailing wisdom. And significantly, while heads from different engine designs are not the same - and so rules from one head can't be directly applied to another - broad modification approaches are universal. So this story has far wider significance than for just Falcon owners...

But Which Falcon Head?

Owner Joe Salter wanted the best commonly available Falcon head that would be a bolt-on fit to his EA Falcon. The intention was to later turbo or supercharge the engine, but even with the addition of a puffer, more internal flow capacity was wanted. The first thing that Gilberts did was to perform some flowbench testing of dead-standard EB and AU heads. (Anyone modifying heads without a flow bench is very likely pissing in the dark - their aim might be good, or maybe not....)

The later model AU head was expected to flow much better, but in fact the testing showed just the opposite. As can be seen here, on the intake side the EB head outflowed the AU right through the range of valve lifts. In fact, using the naturally aspirated rule of thumb that relates airflow to horsepower, the stock EB six-cylinder head is a 330hp design! In other words, at a valve lift of 500-thou, it flows 128 cfm at a pressure differential of 10 inches of water. In predicted horsepower terms, the AU is back at 302hp....

But what about on the exhaust side? The exhaust valves and ports of a head always flow more poorly than the intakes, but how did the AU and EB heads compare? Well, again the EB head was well in front, with a flow at 500-thou lift of 85 cfm versus 81 cfm of the AU. But even more significantly, all through the range of valve lifts the EB was in front!

So the earlier EB head was the one to go for, right? Well, maybe not. Gilberts' head cylinder head magician, Robby Abbott, saw more potential in the later AU head, despite it being behind at the starter's line.

"I think that the intake of the EB head is too big in the bowl area," he said. "And when you flow them they have a bit of turbulence around the short radius - although both Falcon heads do have that. [The EB head] needs a little bit of widening across the short side radius but other than that it is quite big - too big. But these heads are good - they give good figures straight up but as far as modifying goes, there's not a whole lot left in them."

And the exhaust ports?

"The EB exhaust ports are a bit low - they don't really have enough short side material. There've also got a big boss in the way of the exhaust guide, and the diameter of the valve is a bit small."

Despite its poorer flow figures, Robby suggests that straight out of the box the AU head has some major advantages for modifiers. He points out that the AU head uses a bigger exhaust valve - 41mm versus the EB's 39mm. At 47mm the inlet valves of the two heads are the same diameter, but the later head uses valve stems that at 7mm are a little smaller in diameter, free-ing up more cross-sectional area for flow. The AU head also doesn't have a guide boss inside the exhaust port around the valve guide.

So what do all these things look like? One of the hardest parts of following head modifications is picturing what is being described - so let's have a look at some photos.

This is an unmodified EB Falcon exhaust port, with the large valve guide boss described by Robby easily able to be seen.

In comparison, the absence of this chunk of metal in the AU head can be seen here. Note that in its unmodified state the EB head above still outflows the AU in the exhaust side (and the intake too!) but it's the performance potential that's being discussed here.

The very large intake valve bowl of the EB head can be seen here...

...compared with the smaller bowl in the AU head. The larger the bowl (and so port cross-sectional area at that point), the lower the local flow velocity. Keeping the velocity high is critical to low valve lift cylinder filling.

Intake Modifications

Let's cut straight to the chase: the Robby-modified AU head basically kicks arse. The flow at 500-thou lift on the intake side has gone from 117 cfm to 140 - that's a gain of 20 per cent. Or to put that another way, predicted peak horsepower rises from 302 to 362hp! Vitally, there is an excellent flow increase right through the valve lift range - something much harder to achieve than just improving the high valve lift figures. For example, at 300-thou lift the flow rose from 90 to 112 cfm - a massive 25 per cent gain.

So how were these intake improvements made?

"One of the main restrictions in the AU intake is that the [cross-sectional] area directly over the short-side radius is a bit too small and so it's accelerating the air and creating turbulence - the port necks-down at that point. I've increased the cross-sectional area there, mainly in width although there's also a bit taken out of the roof. I've also streamlined the guide to get a bit more area in there as well."

To achieve the latter, the guide was removed from the head, placed in a lathe and a 5-degree taper applied to the section that's exposed in the port.

Robby goes on, "I've also used a 1.5mm wide, 45 degree valve seat that runs into a 30-degree top-cut with a small radius on the end, right out to the chamber edge where it meets the cylinder. Below that I've put a 60-degree cut, 3mm high into a vertical drop."

So those are the changes made in the valve seat and valve bowl area, but what about along the long length of the port?

"The intake port at the entrance to the head is stock - just cleaned up. I didn't find any restriction there - plus most builders are only going to be using the stock manifold so you're going to be limited by that. If you start increasing that area, as soon as you put the manifold on all your gains are going to be lost anyway."

So what do these changes actually look like?

This is the view up the unmodified inlet port. Despite the coarse finish and the casting parting line running down the ports, flow improvements come much more from careful shaping at the turn into the combustion chamber rather than just ripping out lots of material and mirror-polishing the end result.

In fact, from this angle the modified port looks much the same in shape - just cleaned up. Note, however, the taper that's been applied the exposed section of valve guide.

Looking down into the valve bowl of the unmodified AU, the sharp step between the original machining process for the insertion of the valve seats and the as-cast finish of the bowl can be clearly seen.

In the modified valve bowl those transitions have been smoothed and the tapering of the valve guide can also be seen. However, the two most important changes are not particularly visible in these shots.

The clearest indication of the changes in the valve bowl width come when we use some calipers to measure the width between the valve guide and the adjacent wall...

...and then compare that width with the modified head. Note the gap between the caliper and the valve guide - and it's significantly greater than the space created just by the tapering of the valve guide.

But the biggest change is the work done on the short-side radius - the sharpest change in direction that the air has to undergo on its trip into the combustion chamber. Here the arms of the caliper are pressed against the end of the valve guide and the short-side wall.

With the caliper spacing just the same as in the pic above, the large gap that can be seen in the modified head is indicative of how far back the short-side radius has been moved, both allowing the air to 'stick' better on its way around the corner and also opening up much more cross-sectional room for the flow.

Combustion Chamber Modifications

The other big improver in intake flow came from a minor modification to the chamber shape. "Once I'd blended the chamber - making it more like an EB head - we picked up flow right through the range," said Robby.

The blending involved the removal of the hump (circled) in the roof of the chamber, normally located between the inlet and exhaust valves.

While this material removal will reduce the compression a little, in this case it wasn't a big drama. "At a guess I've probably taken out 1 to 1.5 cc, so it wouldn't be even a [compression ratio] point. At a guess it maybe 0.1 or 0.2 of a point. One reason I wasn't really concerned about it was because this head is going on an EB engine so the pistons are going to be different to an AU anyway. If I'd left the combustion chambers how they were it would have more compression than standard - and this is going to be a turbo engine."

And on a naturally aspirated engine?

"The benefits in the flow right through the valve lift range outweigh the small reduction in compression. But if someone wanted more compression I can easily pick it up by skimming the head with the mill anyway. It wouldn't take much to make it up - maybe 10 thou."

Exhaust Modifications

So what was done on the exhaust side of the head?

"We've got a 1.5mm wide, 45-degree seat that blends into a 12mm radius. I've tried to straighten out the bend a little bit in the port, but I know that the water jacket is close so I am reluctant to go too far, even though it did pick up the flow. I've left about 2mm of radius and then I've laid the short side radius back into the port which picked up the flow considerably. The next step would probably be putting a taper on the exhaust valve guide."

Again, pictures tell thousands of words, so what do these exhaust port modifications look like?

The standard AU exhaust valve bowl shows the slightly coarse transitions from the valve seat to the bowl area, in addition to four different cut angles on the seat.

The modified head shows the different valve seat angles and how the seat has been blended into the bowl area. However, as with the inlet ports, without comparison calipers the differences are hard to see.

Using the calipers on the modified head (yep, we're doing it the other way around this time) shows that with this spread they can be just squeezed between the edge of the guide and the port wall.

While here in the unmodified exhaust port they suddenly look bigger, overlapping the valve guide! This shows how much material has been removed from the port wall adjacent to the guide - remember, as they haven't been tapered, the guide dimensions are identical in both ports.

The view up the unmodified exhaust port shows some minor casting dags...

... which were cleaned up.

So how effective were these changes? The Superflow 110 was put into action again and showed again that Robby had been able to achieve a flow gain at all valve lifts. The improvement in flow was especially good at high valve lifts, with a gain of 25 per cent.

"There's virtually no turbulence in the ports any more. Particularly between 200 and 400 [thou lift] there was a fair bit of turbulence in the standard heads on both the intakes and the exhausts. Laying back the short side radius on the exhaust has pretty well eliminated all the turbulence in that port as well. Not only did the ports flow better but getting rid of the turbulence more than likely picked up the flow on its own."

"I've also put a 30-degree back cut on both valves. Also, having that 7mm stem would also pick up the flow - although only marginally."

Conclusion

The gains on the AU head were very good - not only were peak valve lift improvements made but the benefits were seen right through the valve lift range. In addition, the modified AU head flow figures easily overtook those available from the EB head (and while we haven't covered it here, extensive experimentation was made on the EB head to discover its potential) while still keeping port sizes small enough to encourage high cylinder-filling velocities. The modified head also had an exhaust flow 76 per cent of the intake, an improvement on the 73 per cent achieved with the stock head.

Of course, the "potential power" change from 302hp to 362hp is simply an estimate based on steady-state intake flow figures, but it's also a method that has proved accurate on many engines, assuming that the rest of the engine (eg cam, compression, intake manifold, engine management) is optimal.

None of this would cut much ice if the porting cost was extreme, but Gilbert's charge only around AUD$350 for the sort of port job covered here. The 'before' and 'after' flowbench testing is another $80, and obviously stripping and reassembling the head is additional again. However, if the head is going to come off the engine and be reconditioned, then in this case having a port job done makes a lot of sense.

Hmm, maybe a re-think on the benefits of porting modern engines is necessary?

Contact:

Gilbert's Cylinder Head Service
+61 7 5532 2249
0413 157 983