CONCORD, N.C. – Rumors continue to persist regarding engine builders taking liberties with stock engine parts to gain a competitive advantage for their customers. This has never been more true than in the areas of Briggs & Stratton Animal cylinder heads and the use of electric discharge machining (EDM) to reduce the blowdown time of Yamaha engines run in can classes. WKA is aware of our member’s concerns and has been taking steps to determine whether or not the rumors are well founded and how to address them if they are true.
Let’s begin with the Yamaha cylinder and blowdown time. Blowdown time in a two cycle engine is defined as the difference between the opening of the exhaust port and the opening of the transfer port. This difference can be expressed in fractions of an inch or degrees of crankshaft rotation. WKA uses the fraction of an inch method and measures the opening of each port using a dial indicator and standard LAD tool. The blowdown time in a typical Yamaha with a stock
bore is normally between 0.390” and 0.410”.
There is a popular belief that reducing the blowdown by raising the top of the transfer ports will increase performance in restricted Yamaha engines. There is also a perception that using EDM is undetectable. Rumors of this modification first surfaced in the summer of 2010. When WKA learned of this rumor, our Technical Inspectors began to measure blowdown time in the Yamaha classes with the last Manufacturers Cup event in 2010 at New Jersey Motorsports Park. They have continued this practice at Manufacturers Cup events to the present time. At no time since September, 2010 have they found a single cylinder outside of the expected range.
Even with this information, there have been members who were requesting that WKA establish a minimum blowdown time of 0.385”, as the rumors persisted that there were illegal cylinders being used at divisional and local events. Unfortunately, some of the early Yamaha Y3 and Y4 Yamaha cylinders that have been bored out may have a blowdown as low as 0.370”. This is due to the port angles in these engines, even those these are legal cylinders and have never been altered. (It is interesting that no engine with a blowdown time of less than 0.385” has appeared in post-race technical inspection at a Manufacturers Cup event.)
All of this said, rumors persist that illegal engines are being run at a Divisional and local level. As a result, WKA has developed a method to determine whether a Yamaha cylinder with a blowdown time of less than 0.385” has been altered in any or is legal. Any SFI/WKA Certified 2-Cycle Technical Inspector who finds a cylinder with a blowdown time of less than 0.385” and suspects that it has been modified to achieve this effect can submit the cylinder to WKA, and the Association will render an opinion as to whether the cylinder is unmodified or has been altered to achieve the blowdown.
Animal Cylinder Heads
Rumors also persist that some engine builders are gaining significant horsepower gains by altering the short side radius on the Animal intake port. The WKA Technical Manual strictly prohibits any modifications to the ports in this engine. Section 704.22 states that ports “must have [the] stock configuration. No porting or modifications of any kind allowed. There must be a single angle with a defined edge at the transition between end of intake port and the bowl behind the valve. No media blasting”
The first step in testing this theory was to dyno test a stock, unrestricted Animal engine with a known stock cylinder head. Then the cylinder head was removed and the intake port was fully ported as if it were going to be run on an Animal Modified engine. This test would determine the total horsepower potential of a fully ported intake passage. The test showed a .25 horsepower increase at 6,000 rpm and one horsepower at 7,500 rpm. The conclusion is that, if a fully ported head showed these results, it is not logical that just removing the defined edge would make a measureable difference.
To further test the belief that visually stock heads could be modified to produce significant horsepower gains, WKA technical inspectors impounded the cylinder heads of the seven class winners at the final Gold Cup event held during the 2011 competition year. These seven heads were thoroughly inspected, and no modifications were identified.
The next step was to test these heads on an independent flow bench. Also included in the flow bench test was a known, completely stock Animal cylinder head. The results were surprising. First, the known stock head ranked third out of eight heads in the test. Even more surprising was the fact that the cylinder head with the highest flow in the test came off of a restricted engine where head flow is less important than it is on an unrestricted engine.
Here is the bottom line. Although there was a small variation in their flow bench test results, all seven of the impounded Animal heads came off of winning engines, The conclusion is that there are other factors at work in winning races in Animal classes including chassis set-up and driver skill. Although there may be altered heads in the Animal classes, there is no evidence that these heads are winning races and reaching post-race technical inspection.
Of course WKA will continue to carefully inspect Yamaha and Animal engines in post-race tech. However, there is no indication that there is rampant cheating in these two areas.