#7 - Port Volume
It’s Effect on Torque and HP.
By
David Vizard
Dyno testing by David Vizard and Mervyn Bonnett
What you need to know to establish just the right port volume for your small block Chevy or Ford engine build.
(And in case you wondered we will deal with port velocity in a more universal manner to cover all engines in Porting School #8)
Question – how many times have you heard it said that an engine is nothing other than a simple air pump? If this was really the case then output should equate directly to the flow numbers alone. In a nut shell bigger numbers would mean bigger power. If it were that simple I for one would be out of a job. Unfortunately reality is somewhat different. The physics involved toward building a successful high output 4 cycle engine is far removed from that of a simple air pump. The principle reason pushing the so-called ‘simple air pump’ scenario into complexity is the dynamic ‘stop – start’ nature of the flow through the engine. The piston motion and pressure waves force rapidly changing rates of flow and air pressure at key points within the induction and exhaust tracts. Under such circumstances effects caused by both the momentum and the pressure waves generated completely alter the picture to the extent that for a given displacement and rpm, there is a certain size (cross sectional area) of port that is best for the job. Anything more than a few percent bigger or smaller is not.
Except for removing the march belt drive system here is our T & L built 383 small block dyno mule ready to go. Other than looking it there is nothing exotic here – it’s just a well specced unit that T&L (click on one of their ads for more details) can replicate for you - minus headers – at less then $4800 in turn key form.
What I intend to do here, courtesy of the good guys at Dart, is to determine the effect various port sizes have on a typically moderate budget street/strip small block Chevy engines power curve. Because of the close resemblance of the small block Fords port dimensions this also closely applies to the Ford offering. But before we start on the tech stuff let’s look at why, in the US, port sizes are quoted by volume in cc’s.
Back in the days when there were no aluminum heads to speak of building a set of race heads involved a lot of grinding work on production iron heads. I remember porting my first set of serious Chevy race heads intended for a Lola T70 back in about 1968. It seemed as if I had forever in those heads. They proved a winning factor on the track but the amount of time involved meant they were not the cheapest of items on that car to produce. This was the scenario for most pro head ported back then and, to justify what they were charging the customer, it sort of became the norm to quote the before and after port volume to highlight just how much work had gone into the porting process. Also since all the heads involved on small block Chevy’s had a 5 inch intake centerline length quoting the port volume also gave a loose measure of the port cross sectional area involved. Because of the ports changing shape as it progresses from the manifold face to the valve it was not entirely practical to quote a cross sectional area as there was always the question of the position of the quoted area. So, right or wrong, the port volume method of viewing port sizes became convention. So long as you do not loose sight of the fact that it is really the port cross sectional area that is the criteria here then you won’t go far wrong with the port volume method but bear in mind that these days ports of lengths other than 5 inches are now commonly quoted in cc volume.
The Test Engine.
The engine for our tests here was a T&L 383 built in spring 2006 and used extensively as a dyno mule. During it’s time it has been used to test cams, rockers, heads and a variety of induction systems. At this point in time it has about the equivalent of 500 road race miles on it. The bottom end was an all Scat/KB deal. The crank was a Scat 9000 series cast steel stroker (3.75 inch) with Scat stroker clearanced rods and KB forged pistons. In our case the KB’s, with the test heads involved, delivered a 9.5/1 CR.
The intent here was to run four pairs of the heads courtesy of Dart on this engine. I say courtesy here because this project was largely pushed through by Dart’s Jack McIness who felt that it would be in the best interest of potential customers to have it demonstrated that bigger in the port department is not always better. These heads, of the latest Pro 1 Platinum style, had intake port volumes of 180, 200, 215, and 230 cc.
Here is a shot of Darts Platinum heads chambers with and without valves. The design of these heads is the result of a lot of R&D on both wet and dry flow benches and the dyno. If this technology is to be converted into results on your motor it makes sense to choose the right port volume for the application.
At this point it may appear what we planning is an easy test to do – just take a strong performing engine and run 4 sets of heads across it. Unfortunately without a little more thought this is were things can go awry. To get meaningful results we need to look at things in a little more depth. What can be a major factor toward achieving meaningful results arises from the way flow increases with increasing valve lift and port volume. At low valve lift values, say around 0.050 to about 0.150 the flow has little to do with the port size because the limit is set by the still minimal through-flow area between the valve and the valve seat in the head. Only when the valve lift exceeds about 20% of the valves diameter does the port size/flow efficiency begin to influence flow as seen on the flow bench. This, for our test heads is amply demonstrated in Fig 1. From the curves you can see that the majority of the flow increases with increasing port volume occurs at the higher lift value. This is so to the extent that any test that fails to lift the intake valve to access the additional flow would be totally skewed in favor of the smaller port heads.
To achieve meaningful results here meant selecting a cam and valve train that combined a high valve lift and short duration for a wide power band. These two factors are not quite mutually compatible and this meant having a valve train with good dynamics. Here we selected a custom Comp single pattern 276 Xtreme roller hydraulic grind.
Another consideration here is that a cam with a relatively short duration would also be needed thus allowing the low speed attributes of the heads to be determined. Using a cam with too much duration would prevent the engine from running decent at low speed so any head volume that favored low speed would look worse than it possibly was. For this reason it was felt that the greater lift of a moderated duration roller cam was best suited to the test parameters involved. To meet these needs Comp ground a custom single pattern Xtreme profile (#3192) shaft on a 106 LCA. This hydraulic roller profile has 276 degrees of ‘off-the-seat’ duration and 224 degrees at 0.050 tappet lift. This, coupled with a peak lift of 0.605 when paired with a set of 1.6/1 rockers, got the job done.
All the heads to be tested had 72 cc combustion chambers (64 cc ones are also available) which, with the combination of deck height, piston valve notches and gasket thickness gave our test engine a 9.5/1 CR. Had we opted to test the 64 cc items the CR would have been bumped to 10.7/1.
As for the induction on our mule motor an influential carb and manifold decision had to be made. Whatever was used had to be able to deliver results at both ends of the rpm range. In other words the validity of any port volume tests is influenced by the intake manifold and carb selection. Unless high flow capability is seen here the differences in cylinder head performance, especially at the top end of the rpm range, will be masked. The intake selected, based on previous positive experience was Dart’s 180 degree 2 plane design.
But what was planned here was far from just bolting up the same intake to each set of heads. Remember each head has a different port size opening at the manifold face. What we want to test here is the effect port volume has on power not what effect port matching has. To deal with the size differential the 180 cc heads had a small chamfer applied to the intake ports of the heads as the manifold was slightly larger. For the other three sets of heads the manifold runners were opened up over about the first inch in to match the port size of each successively larger head.
Feeding the fuel to the system was an AED Holley with some 950 cfm flow capability. With this induction system the engine had access to sufficient air flow for a good top end while still catering for whatever low speed the smaller port heads might deliver.
So you can see where we stand on this let’s consider what our alternatives may have delivered. If we had used a single plane race intake such as a Victor Jnr. or the like, the bigger port heads could well have shown a greater top end advantage over the smaller ones. On the other hand a race style single plane could have compromised the smaller port heads ability to deliver a stronger low speed output. Conceptually at least, the induction system used proved to be a globally effective compromise.
Port Sizes.
So why is port cross sectional area important? If the area is bigger the flow surely goes up and that’s what we want is it not? Sure the engine wants as much airflow as possible but air has mass and weighs much more than you might think. During my lectures I have used the 100 foot cube question on literally thousands of professional head ports – many of high repute and, in I don’t know how many years, not one has come close to the right answer. Let’s see if you do any better here.
Imagine a cube with 100 feet down each side – that’s 100 feet long, 100 feet wide and 100 ft high Fig 1. Without stopping to calculate it, if you happen to know how, guess (assuming standard temperature and pressure) how heavy the air is within that 100 foot cube.
The above drawing shows the scale of what we are dealing with here. That’s my GMC Sierra extended cab truck parked on the 100 foot cube. The chances are you totally underestimated the weight of the air in that cube. I’ll tell you now – the answer is thirty eight (I am writing the number as words so you don’t immediately spot it). Now that is not thirty eight lbs nor thirty eight kilo’s but thirty eight tons! Yes it would take no less than 17 GMC Sierra’s to balance a set of scales against the weight of that 100 foot cube of air. Now if that surprised you don’t feel like the Lone Ranger here as, to date, only two people in 20 years have come even close to guessing anywhere near the right answer.
So why am I bringing up the point on the weight of air. It is solely to put into prospective that the medium we are dealing with is far from near massless. When that air is moving at 600 feet/second it has considerable momentum. Just so you can visualize the amount of energy here is an example. The total intake port length of a ProStock engine at 10,000 rpm has only slightly less energy than the muzzle energy of 0.177 caliber pellet from a reasonable high powered air rifle (about 10 ft-lbs).
Shown in the photo on the left are the 180 cc and 230 cc ports for a comparison of the smallest versus the biggest. The illustration on the right shows the difference in the average area of each of the ports being tested.
So air is heavy – if we add to this the fact that the energy is equal to half M(mass) times V(velocity) squared (1/2MV^2) we can see that when port velocity goes up the port energy goes up far faster and when port velocity drops the port energy drops far faster. Putting that into prospective if a port is made 20% too big the port energy drops by 44%. In basic terms that equates to a 44% drop in the ports ability to ram a cylinder by means of it’s velocity derived momentum. When it comes to combating reversion, especially at low speed port velocity is very effective. Kill the velocity below a certain level and you effectively kill torque at the lower rpm levels while not necessarily garnering any power advantages at the top end.
So what we can say at this point is that a significant proportion of an engines flow through depends on port velocity as well as the generation and utilization of pressure pulses. This means even a little excess in terms of port area can hurt power even though it may, on the flow bench at least, flow better.
Fig 2. As these curves show big ports do deliver bigger flow numbers but only at high lift. If the valve train does not access that lift then the extra port volume is totally detrimental to output.
As can be seen from the flow tests above (Fig2) the bigger port does flow more up at the higher valve lift numbers. Having established that the big port flows the biggest numbers we are now left with the question as to whether that directly translates into extra output or is velocity a sufficiently active player to have an overriding influence on the results?
Dyno Time.
At this point we have a lot of output data to consider and to make this easier to assimilate I have put the torque curves and power curves on separate charts. A point worth noting here if you are attempting to convey relatively small differences in torque and hp over an entire power band width of an engine is that it is easier to see output differences at the low end by looking at the torque curves but for the top end differences are easier to see if you look at the power curves. Now I have made that point let us look at the low speed effects (Fig 3) when the port volume was changed on our test engine.
Fig 3. The results here clearly show that smaller, higher velocity ports, favor low speed output. These results also show that going too big (blue curve of 230 cc port) on the ports, for the job in hand, produces worse results almost everywhere in the rpm range.
As we can see the curves clearly show that smaller, higher velocity ports, strongly favored low speed output and clearly produce the best results to 3400 rpm. Above 4500 rpm the 180 cc ports lost out to all of the bigger port configurations. These results also show that going too big (blue curve of 230 cc port) on the ports, for the job in hand, produces worse results almost everywhere in the rpm range. The only point at which the 230 cc port was better on our test engine was above 6200 rpm as shown on the power curve graph. At this rpm the power curve was about all over anyway so any advantage this far up the rpm range would not show any advantage on the drag strip. What we can say then is a 230 cc port – for this application- is simply too big. Since this port has the biggest flow numbers but the lowest velocity we can conclude that it is necessary to get the right compromise between port size for best flow and port velocity for best momentum filling.
Fig 4 Here it can be seen that the 215 cc port (green curve) equaled or bettered the 230 cc port (blue curve) everywhere so proving bigger is not always better. Combining what we see from the torque curves and the hp curves the 200 cc runner (red curve) appears to give the best average numbers over the rpm range tested.
Before finalizing on our conclusions here let’s give these results another look. From the torque curves Fig 3 we see the 180 cc ports (black curve) produced the best output up to 3400 rpm peaking at a stout 482 lbs-ft. We can also see the 200 cc port (red curve) was not far behind at the lower rpm and from 3400 rpm up it ran up with or close too the bigger ports. If we look at the torque curves and also consider the hp curves in Fig 4 we can see that, for our 383 incher’s cam and intake combination, and the intended rpm range, the 200 cc ports produced the best curve. The 215 cc (green curves) heads delivered the highest output at some 478 hp as apposed to 457 for the 180 cc heads, 472 for the 200 and 475 for the 230’s. The downside of the 215’s over the 200 was that to deliver this extra 6 hp they give away up to 10 lbs-ft of torque between 2300 to 3200 rpm.
As for the 230 cc port runner heads they failed to deliver any worthwhile superiority anywhere in the rpm range on our test engine. Indeed the smaller 215 cc port heads beat the 230’s everywhere! This, in case it was needed, is near conclusive proof that an engine is not a simple air pump! Nor, for that matter, is bigger better. Had we targeted an engine capable of more rpm or one with bigger displacement then the bigger port heads would have paid off. Experience with ports in the 230 -245 range show that every bit of the port size is needed if you are building a 440 cube small block Chevy. If we look at a comparison on a pro-rata basis a 235 cc port on a 440 inch small block Chevy is only equivalent to a 186 cc port on a 350. A worthwhile point of reference here is that since the port length of a small block Ford is also very similar to the small block Chevy the results seen here transpose pretty well to the Ford.
So how do you decide what port volume your small block should have for best results? As good a rule of thumb as any is to base the port volume, and we are only talking traditional 23 degree (or – in the case of Ford 20 degree) heads here, on the projected power output. However let me caution you that that if you rate your engines final output too optimistically(and an excess of optimism is a problem most racers have) you will end up with a port that is too big and the target output will not be reached. What this means is you could end up sabotaging your own efforts here. If you go with the recommendations in the chart Fig 5 you should have a good starting point for port volume selection.
With Darts selection of port volumes they pretty much have the range from a relatively mild 350 (180 cc ports) to a rampant 440 incher (230 cc ports) covered. One more point worth noting for those higher hp engines here is that these Dart heads are really easy to port. Doing so can get the port volume right where it needs to be along with more flow.
Summing Up.
Before winding up this feature there are two points I want to make clear. First that a port
a little too small will be a far better deal to drive than one that is a little too large. 20 cc extra in a small blocks intake port can easily cost 25 lbs-ft and sometimes as much as 40 lbs-ft at a point in the rpm band that is most often used for a true street driver.
Also a port that continues to increase in flow at more than say 0.050 above the maximum lift that will be used shows almost conclusively that the port is too big. That is a real mismatch of port size to cam/valve train spec.
For those of you considering building a small block Chevy a mention on Dart’s Pro 1 Platinum heads output capability seems in order. Our 383 test mule was later reconfigured with the as-cast 200 cc heads, a 280 degree street Hydraulic roller cam and, with a 10.5/1 CR delivered a best pull of 500.3 lbs-ft and 502.1 hp with complete street drivability.
David Vizard
Other articles in this series can be found at:
#1
Porting School #1 - Why engines need airflow
#2
Porting School #2 - Super Cheap Flow Bench
#3
Porting School #3 Budget Bench Calibration
#4
Porting School #4 - Budget Bench Electronics
#5
Porting School #5 Identifying Primary Restrictions
#6
Porting School #6 - Secrets to reduce valve shrouding
#7
Porting School #7 - Power & Port Volumes
#8
Porting School #8 Optimal Port area's
#9
Porting School #9 - 5 Rules to Goof-Proof Porting!
#10
Porting School #10 - Pushrod Pinch Point Power Issues
In addition to the Porting School articles there are directly related cylinder head development subjects at the following sites:
Wet Flow :-
Six Wet Flow Mistakes
Combustion Dynamics:-
#1:-
Turbulence and Combustion Dynamics
#2:-
In cylinder Turbulance and Combustion Dynamics
#3:-
Turbulance and Combustion Dynamics - Part 3
#4:- Coming soon
#5:-
Turbulance and Combustion Dynamics - Crevice Volumes - Stealth Power Thief