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Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Mon Jul 08, 2019 6:54 pm
by cmfalco
On the subject of heads and air flow, the first photograph shows the inlet tract of my 1928 Ariel (top) and 1963 BSA Gold Star (bottom), both 500cc. I shot both on a tripod from the same distance, carefully aligned along the axes of the inlet tracts, so they are the correct relative size in this photograph. Horsepower is directly proportional to air flow into the engine so you don't need a flow bench to see why the Ariel's smaller asthmatic inlet tract kept it to only about 20 h.p. while my Gold Star produced a measured 41.7.

I adjusted the two inlet tracts to be the same diameter in the second photograph, showing that even then the Ariel's inlet would be more restrictive. Both engines are of the same basic design by the same person, Valentine Page, so these photographs show the results of 35 years of cut-and-try development work done without the benefit of a flow bench.

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 12:15 am
by Richard Kal
Very interesting Charles!

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 4:45 am
by david.anderson
Unfortunately there is far more to horse power than flow numbers. Gas velocity is very important and is the main driver of cylinder filling. Gas velocity in large ports is slow. Large ports may (but not necessarily) produce more power at maximum revs as the gas velocity builds up but are often down so much power at lower revs that on the race track the smaller port bike will out accelerate the larger port bike and be in front at the end. The look of a port can also be deceiving as to what will perform best. (the Baisley ported A10 heads look like they would never flow but they outperform all others.)
The real advantage of the late goldie heads is the raised port floor where the gases turn the short side radius far better and the flow around the valve is far more even. The changes to the goldie head with the development by BSA is very interesting to study. The late goldie heads were the best developed 2 valve head ever until the XR750 heads which were initially a copy of the goldie, however Harley further developed the ports into a cobra shape for even better flow and performance.
It is also a well known fact that the goldie head left the factory with a step from the carburettor to the port. Most goldie heads had the step removed by ignorant tuners who lost a couple of horsepower with the small port enlargement. The reason why the step in the port produces more power has never been determined, although there are many theories (such as better atomisation of the fuel). From the Roland Pike storey:
“One of the strangest occurrences to do with carburettors was when one day Reg Wilkes sent an apprentice from the test shop into our, main shop for a 1 3/32 GP carburettor and he misread the size and gave Reg: a 1 3/16th carburettor and they immediately got quite a jump in power. They reported this to me and I went to the shop and the test was repeated and there was no doubt it was quite a gain in power. I suggested to Reg to open the port to match, immediately we lost what we had gained plus a bit more, so we made a thin sleeve and pressed it in and the power was back. We repeated this on other engines always with the same results. A number of private owners of DBD Gold Star noticed the carb being bigger than the port and opened up the port thinking to gain power, but not having a dyno were not aware of the results. Later on we fitted a venturi behind the carb and picked up even more power. The smallest diameter of the venturi could be 80% of area of the carburettor. We tested this on several engines and it always worked and seemed to improve carburation. The venturi had to have the classic included angles of 22 degree in and 7 degree out.” Theory has it that the step from carb to port will not flow as well and will rob power but with the goldie that is not the case. The step from carb to port was maintained as the ports were enlarged and bigger carbs were fitted.
It is also well known that the early Norton Commando heads with 28mm ports will outperform the 32mm port combat heads. The combat head may produce more power at 7000rpm and produces better numbers on a flow bench, but it does not start to work until the engine is spinning at 5000rpm. So there is a high demand for early unmolested 28mm Norton heads. Alternately the new Fullauto Norton small port heads outperform the standard head by a big margin. The gas velocity in the ports of those heads is much higher than in the original heads. In fact the Fullauto heads were used by noted classic racers such as Kenny Cummins with the as cast ports to win races.
Ariel also produced some factory short stroke singles that were basically the HS engine with an 85mm stroke and a bore of just over 86mm. Those bikes produced 44 bhp on the dyno at 7000rpm and as far as I can determine, the standard 1 3/16” HS head was used. I have built myself a short stroke single but opted for a longer rod stroke ratio than Ariel did which I hoped would provide more dwell time at the top and bottom of the stroke to enable better cylinder filling and a reduced ignition advance. The bike does perform with a reduced ignition advance however my bike is fitted with an enlarged port head. The bike will spin willingly to 8000rpm but has lost a lot down low. I have had to reduce the engine sprocket from 23 teeth to 21 teeth to make the bike road ridable. So I am currently looking for an unmolested 56 – 59 VH head. (the current new cast heads have an altered shape in the bowl which would require welding up and do not flow as well as the original. I have a new cast head.) From experience I will not enlarge a VH inlet port beyond 32mm, preferably less.

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 5:31 am
by cmfalco

Thank you for taking the time to write that post. Unfortunately, to even begin to discuss flow measurements in any kind of technical detail requires a book. However, those two photos can substitute for 2000 words of that book. People who understand flow measurements will see from those photos that the GS head flows a lot more than the Ariel head, and also will know that h.p. is proportional to air flow.(*) Those who don't understand flow measurements will still see from those photos that the GS flows a lot more air than the Ariel.

(*)Yes, the precise story is more complex, but that would require a book.


Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 10:10 am
by brenton.roy
David, any photo's or further explanation of the venturi?

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 10:13 am
by brenton.roy
Apologies Charles, not intending to hijack your thread.
Yes, the ports are massively different. The link between Ariel and that other mob makes it an interesting topic.

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 12:02 pm
by Damien Towning
Many years ago i came across an article somewhere about Val and inlet ports. I can't quite put my finger on it right now but I do remember it influenced my own work on a little single cylinder race bike I have. Something about them putting cones in the inlets at some point. Can't quite recall but the comment somewhere above about the trade off between top end and acceleration is spot on. Not knowing better at the time the first modification I did to the motor in the bike ( little 150cc job ) was to open up the inlet port to enormous size. Which resulted in a fast bike that took forever to arrive. Then came endless tinkering with jets and carbs of various sizes on that enormous port to try and get it off the line before everyone else was gone. I think most people could relate to this. Then I came across that article about Val and the ports. What the bike has now is two smaller carbs on a big port. Is setup with a progressive throttle linkage so they open at different rates. Years later I did see a picture of a small pea shooter race bike from one of the 50's European circuits with a very similar setup to. Was a fuss to tune and setup though. But it did run well once it was sorted. Makes a great sound and at full throttle some sort of interesting boost effect when both carbs are open. Never quite worked out what was up with that but taking it as a happy side effect!

**Maybe it was not Val but Turner.
**May have actually been the Roland Pike story ... !

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 2:58 pm
by Vincent.vanGinneke
Hello Charles, did you perhaps put in the same picture twice at the top of this thread ?
I dont see any difference between them....

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Tue Jul 09, 2019 5:22 pm
by cmfalco
Vincent.vanGinneke wrote:I dont see any difference between them....
If you hold a ruler against your computer screen you'll find that the Ariel inlet tract has a smaller dia. in the top photograph, as it does in real life, but that I enlarged it to have the same 1-1/2" ID as the Gold Star tract in the second photograph. The first photograph accurately represents the difference in areas of the two tracts but the second allows a comparison of what the areas would be if the tracts had the same IDs, i.e. even then the Ariel would flow a lot less.

The basic physics behind these photographs is simple. All the power from an engine is generated in the head, and it can only make as much power as is in the quantity of the air/fuel mixture that's in the combustion chamber when the spark ignites it. If there's more of the mixture, the power will be that much higher. The amount of mixture is simply given by the flow rate of the mixture through the inlet tract times the amount of time the valve is open (volume = rate x time). So, if the flow rate is increased, the amount of mixture that makes it into the combustion chamber is increased, and thus the power is increased. At its essence, it's really just that simple. Bigger port = more flow = more h.p.

Where it gets more complicated is if you want to know the amount of mixture, and hence the h.p., more accurately, as well as how it behaves with rpm. The flow isn't a constant. It starts from zero when the valve first begins to open, increases to some maximum that is limited by the lift of the cam, then decreases to zero when the valve just closes. Since air has mass, this means inertia has to be dealt with, which means effects will be different at high rpm (short times) than at low rpm. Another factor is cam overlap since that allows some of the entering mixture to pass straight through and exit the exhaust port without contributing to the power (pulses in the exhaust pressure complicate this further since they can keep some of the fresh mixture from escaping when on resonance). Another factor is the shape of the port. If it is made such that there are "dead" areas where the flow is small then the total area that contributes to the flow will be somewhat less than the geometric cross section.

What is important to keep in mind is that the factors in the previous paragraph just refine the basic truism that the h.p. of an engine is directly proportional to air flow. This means that if you increase the air flow you will increase the h.p. Using "American" units, a good estimate of the h.p. of a racing engine (e.g. with cams having lots of lift and overlap) is h.p. = 0.35 x flow (CFM), where the flow in Cubic Feet per Minute is measured at a test pressure of 15 inches of water and with the valves at maximum lift.

While the above simple equation provides a good estimate of the maximum h.p. it doesn't predict the details of h.p. vs. rpm. As an example, you can imagine that a short, small ID inlet could flow the same as a long, large ID inlet. In this case the equation predicts that both would produce the same maximum h.p. However, because of dynamic effects not contained in the equation the longer tract would have its maximum at a lower rpm. What this means in practical terms is if you were building a bike for the IoM you would want an inlet port that gave max. h.p. at max. rpm, but if you were building a scrambler you would want a different port that gave max. h.p. at a lower rpm. They might flow the same CFM but the length and diameters of those two ports would differ because those factors affect the dynamic conditions of filling the cylinder whereas a flow measurement gives a static result.

To summarize, it's easy to get baffled by all the information and misinformation printed about flow measurements. But, if you showed the first photograph to a large number of people involved with engines and asked which one produces more h.p. I seriously doubt a single person would pick the Ariel. You can simply look at those two inlet ports and know the bigger one flows more and hence the h.p. is greater.

Re: Heads: 1928 Ariel vs. 1963 Gold Star

Posted: Wed Jul 10, 2019 5:29 am
by david.anderson
The venturi mentioned above is part of a quote from the Roland Pike autobiography. The venturi is a 22 degree reduction in the port size down to 80% of the carburettor throat size, followed by a 7 degree increase in size until the port size is re-established. I do run the venturi on one of my bikes and it did clearly help and reduced spitback at low revs due to a large valve overlap. As for the top end, the seat of my pants could not tell, but roll on tests between 2 points did show a marginal improvement.
Just a bit more on ports. From memory Superflow gives us a formula that suggests that for every extra cubic foot of flow there is an extra half horsepower. But with regard to port size the idea of a large port being ideal is not the case. The real driver of power in an engine is the velocity of the flow, and that applies to the inlet and exhaust tracts. It is only with a high velocity flow that complete cylinder filling can be achieved, and with high enough velocities, figures well in excess of 100% filling are achieved by formula 1 cars. So the port must be shaped to flow without turbulence to maximise the flow.
Initially the main driver of the cylinder filling is the outward flow of the exhaust gases which sucks in the incoming charge during valve overlap and it is not until the piston is about half way down on the inlet stroke that the suction by the piston takes over. The piston then travels to the bottom of the stroke after which it starts to rise. When there is a high velocity flow through the inlet port, the momentum of the incoming charge continues to fill the cylinder as the piston travels up on the compression stroke and if you are running a 56 HS mk1 cam in the VH, the inlet valve does not fully close until about 80 degrees before tdc, (100 degrees abdc) ie the piston has travelled more than half way up the compression stroke and the cylinder is still filling? Well it can only still be filling if the velocity and momentum of the incoming charge is high enough. Otherwise there is a lot of spitback out the carburettor as the incoming charge is forced back out the carburettor due to the piston rising which greatly robs power.
That raises another point in that the most important event in the valve timing is the closing of the inlet valve. So to get the best out of the engine it is necessary to be able to alter the valve timing in steps of a couple of degrees at a time to get the highest possible cylinder filling.
Roland Pike in another part of his book mentions that Ariel complained to BSA about the poor power output of the huntmaster which on the dyno was down a few horses on the A10. BSA took a look at the huntmaster and fitted it with the smaller diameter exhaust pipes of the A10 and there was a corresponding immediate increase of a few horsepower. The smaller pipes operate at a higher velocity which aids extraction. Ariel however decided they preferred the looks of the over large exhaust pipes even though it robbed power.
While I am by no means an expert at porting I have spent a lot of time on a flow bench with various VH heads. I have used flow beads to check for dead areas and to reattach the flow and have filled in dead areas with plasticine, and have used cotton flags to look for turbulence and try to eliminate it, as well as to find out where the main flow areas in the port are. In that regard probably 90% of the flow is over the top of the valve, where much of it then rushes out the open exhaust valve during overlap (more power lost)
I have found that a good multi angle valve seat will do more to increase flow than many hours of shaping in the port. 90% of the gain to be made in the inlet port is in the bowl ie within 20mm of the valve seat. David Vizard has some excellent details in that regard. The reason why a 3 or multi angle valve seat and possibly a 30 degree back cut on the valve is important is that it improves flow at the most restricted part of the port. Think about it, the valve only opens to .430” with a HS cam and follower and only .318” with a standard VH cam, however for most of the valve opening duration it is open much less than that. So for the majority of the valve opening the most restricted part of the port is around the valve seat and not the size of the port that is feeding it. So that is the main area affecting flow and the main area for the flow to be improved.
To improve the remainder of the VH inlet port the floor needs to be raised considerably in the vicinity of the short side radius, and that also means the roof needs to be raised. That is not possible with the VH as there is inadequate depth of material around the guide and the spring seat is too close. It is also unfortunate that with the alloy heads the centre of the short side radius and the centre of the radius to the roof are not the same in the VH inlet port which makes it very difficult to improve flow and reduce turbulence in that area. That is where the late goldie is so good as it has a raised floor and a large short side radius, with the radius of the roof having the same centre as the floor.
We can all learn a lot from the late gold star ports but there is much more than a large port size to be considered in achieving more flow and power. Would enlarging the black Ariel ports increase power or is the slow valve timing such that there would be no benefit. But a 3 angle valve job would certainly help.
I have wrecked a couple of heads with hogged out ports so I would never recommend an amateur take on porting unless it is with the aid of a basic flow bench and a lot of time is spent first with flow balls and cotton flags in an attempt to understand how the port is flowing before any alteration is done.
This site where small high velocity ports are promoted is worth a read even if it is over the top. Do not consider filling the ports of an air cooled engine with epoxy.