The piston is the busiest component of an engine. It travels the most distance at the highest speeds & rates of acceleration. It is subjected to the highest temperatures that an engine sees, in the most hostile environment imaginable. It has to transfer the highest pressures of combustion to the crankshaft to deliver the all-mighty "horsepower" to the ground. It has to seal the combustion gasses in the combustion chamber (for which it is the bottom portion of) and keep the oil from the crankcase out of the combustion chamber. It also controls the "squish" band to coerce the gasses to go where we ant them to, and stay out of where we don't.
Stock pistons are generally a cast part that is capable of enduring the environments originally intended by the OEM when designing such a part. In a lot of cases, the part is "over engineered" to be as reliable as possible for a road-going vehicle that has a warranty. This is to the advantage of the engine builder up, but only up to a point. Recently, for example, two of the riders I support had their bone-stock 2005 GSX-R1000s crack the pistons right across the crowns. This was due to Suzuki pushing for maximum performance from a stock part. Under normal street applications, these pistons would probably last the life of the rest of the bike. However, under racing conditions, where the piston is subjected to the maximum designed environment (or over)100% of the time...and the occasional hard downshift...the material fatigued and caused an engine failure.
For this reason, most race engine builders prefer to run a forged piston. Forging is a form of metal reshaping using a die in the rough shape you want and, under extreme pressure (hundreds of tons) forcing the material to take on a new shape. This forms a much tighter grain structure and pattern that ends with a much stronger part for the same given volume of material. In many cases, a forged racing piston can be made lighter at the same time as stronger because of its higher density. Also, forging alloys are usually of much higher quality materials, adding better durability there as well.
Tooling costs to make a forging die are very expensive. You need to stamp out thousands of parts to make your money back on a single tooling die. Therefore, making a die to the net shape is practically unheard of, and what you're left with is piston companies that have bulk forgings for a large variety of pistons that need to be custom machined to suit. But, when you're a small R&D shop such as G-Force, only looking to make 16 to 24 pistons at a time, the selection is very limited. You can either run what has already been produced, you can pay for a tooling die, or you can elect to have the parts 100% CNC machined from billet. This is definitely more expensive than a traditional forged piston because of the design and machine run times. But, it is far cheaper than paying for a tooling die.
We looked at what was out on the market. The only pistons that were already made for the VFR & RVF were JE Pistons made for TTS. These pistons were made from a very old-style full-skirt forging, which adds friction to the motor at high engine speeds. The pistons we had were over 14 grams heavier than stock, which was completely unacceptable for our engine design parameters. Also, the rings used on the 58mm example we had were a very heavy, thick ring. The heavier the ring, the easier it is to unseat from the ring land at high revs, losing ring seal. The 58mm 444cc kitted engine we had (built by someone else) had the cylinder walls bored thin, needed to be re-sleeved on one cylinder and showed signs of excessive blow-by (poor ring seal). The motor eventually spun a rod bearing, which we attribute to the heavy piston distorting the big-end of the rod and breaking the oil film. We did not want to use these pistons in our engines.
We were out of ideas until we had a discussion with Kelley Roberts at REC (Racing Engine Components). Kelley had been our Cosworth distributor for over 10 years. He was also frustrated with the piston market in the USA, and disappointed that Cosworth stopped making/selling motorcycle pistons. In fact, I believe Kelley was responsible for the design of the last several years of motorcycle pistons that Cosworth did produce.
Kelley said he was extremely busy with a Hayabusa development project, but would make the time to make us some pistons. His design was definitely revolutionary at the time for a motorcycle piston, looking like a Formula-1 slipper pistons all the way. The +2mm piston was 3 grams lighter than a stock VFR piston. The wrist pin was also smaller and lighter for a net savings of nearly 7 grams over the stock VFR piston, pin and ring pack. This was an excellent move in the direction I wanted to go with reciprocating weight reduction.
It took a seriously long time to get the pistons. Kelley worked on the design, but didn't have the time or equipment to manufacture them at the time. So he contracted them out of a piston manufacturing shop in the UK. He refused to divulge his source to me, as if we would go around him in the future. Unfortunately, it was his "source" that eventually caused me to lose faith in REC as a supplier of pistons. We ordered 2 batches of pistons, each batch taking 8 to 11 months to get (an we still were shorted 2 pistons). Once we did get each batch, we were having troubles with them. It wasn't for months and months of testing and rebuilding the engines over and over again that we finally found the problem.
At first, what we noticed was an inconsistent power delivery past 11K. Some runs would pull cleanly, the others would drop off and flatten out. We decided to see what the crankcase pressure was doing, so we hooked a hose to the crankcase breather at the top of the rear valve cover. We put a measured amount of water in a "U: shaped section of hose, creating a manometer. When the motor made good power, this water level moved about 1~2 inches. When the power dropped off, the water shot out the hose, verifying uncontrollable ring sealing.
Our first thought was the tops of the pistons were expanding too much and closing off the gas pressure to the rings. We thought this because of the high amount of scuffing we found on the pistons in that region after only a short amount of running. We tried a method of "gas porting" by having little grooves cut in the piston to give the gas pressure somewhere to go if this clearance was getting closed off. The motor with that set-up ran really well, so we though that was it and stopped looking at that time.
When we were assembling another motor with new pistons, we discovered the rings were sticking in the ring grooves. We then studied the ring grooves very carefully. We found that they were manufactured with a taper, getting tighter at the rear. When the rings were installed by hand, they would float seemingly free in the groove. However, when we compressed the rings for installation in the cylinders, the rings would stick in the fully compressed position, barely providing any ring seal at all. Some pistons were worse than others, and the motors would have very erratic power delivery all the time.
We ended up buying a special cutter and having tooling made to hold the pistons exactly concentric and axial on the lathe to re-cut the ring grooves ourselves (our local machinist). This involved taking all the assembled motors apart and having all of the current running pistons, as well as all our spare pistons, re-machined. Once we did this, the rings sealed consistently and the motors started to produce great power numbers. It was shortly after doing this we achieved the 88+hp number on the Gen 3 motor.
When it came time for the Gen 5, we knew we were running out of pistons from REC and didn't want the headaches from before. My trust in Kelley Roberts was also tainted as I lost my faith in his ability to deliver a quality product in a timely fashion. This was/is a real heartbreaking thing for me as I very much respect his abilities as an engine builder and parts designer. I believe he honestly just got overwhelmed with his Hayabusa project (which is really amazing) and didn't have the time for us that we needed.
Jack Sparks was transfered from Carrillo Industries to JE Pistons (also PMI Group owned). Because of our relationship with Jack from the rods, we decided to give JE another shot. Mike Lohmeyer and Mike Norman flew down to Irvine, CA to meet with Jack at JE Pistons. Jack introduced us to the motorcycle piston engineer and we discussed the options for our project. It didn't look too promising as the only forging they had was the TTS-type full-skirt piston. We unfortunately left there without much hope in a piston from JE.
After coming back from JE, I was talking with an AMA Crew Chief friend of mine. He said he swears by CP Pistons, and gave me another person's name as a reference. Then, in conversation with a completely unrelated engine builder, CP's name came up again...and again...and again. So, I contacted CP Pistons with my design criteria. They responded in fairly short time and said it wouldn't be a problem to make what I wanted.
Again, I drove to Southern California and met with the representative I had been working with to discuss what we were looking for in person. He assured me he could deliver what I needed and he was certain he could make the piston lighter than what we already had. I left him samples of a stock piston, the REC 57mm Piston and the HRC F-III piston (donated by fellow 400Graybike moderator Steve Northrop). I specified all of the components I wanted from each piston, I received a CAD model within a few days that I was able to test fit into my assembly model of the whole motor to make sure we didn't have any clearance problems, etc. I gave the green light and the new pistons were in production.
Like everything we get custom made, it took much longer than originally quoted. When we finally received the parts, we did have to ship the parts back a couple of times due to attention to little details that weren't a big deal, other than time. Once we got the final pistons they were amazing little jewels. The attention to weight reduction by CNC machining every area that didn't need to be there paid off. The final pistons, depending on the compression height, were up to 12 grams lighter than the REC, which makes them nearly 18 grams lighter than stock. This is 32 grams lighter than the TTS/JE 444cc piston kit that we originally looked at. In the world of reciprocating weight moving at 14,500 rpm, this equates to massive reductions in forces to the crank and rods.
The end result is a light weight, low friction piston that is available locally in the USA in a reasonable time frame (4 to 6 weeks). The quality of the piston is some of the highest I have seen in 20+ years of building engines. The pistons are available to us in small batches with direct control over many design parameters to get exactly what we want for any given engine configuration. From an engine building standpoint, they are worth tremendous power increases from just installing this one component (see graph at right). They are well worth the <$2,000/set.
