The DG heads really didn't work well on modified engines, either.
As to John's thoughts on fins cracking without the metal bridge connectors between them, quite true, simple harmonic vibration causes a lot of failures in different areas. Another place was at the rear upper motor mount on the cases, ripped a part of the bolt boss right out of the upper case. That is why I never use an upper rear motor mount set.
It was also mentioned about spark plug positioning, and I said before that some of the Yamaha TZ heads used an offset chamber, with the spark plug aimed to the rear of the chamber. This brings me back to the development of the later h1R engines, before they went to the H1R-LC liquid cooled top ends. There were a few sets of testing heads made for those engines, and distributed all over the world to the teams. They were still air cooled designs, but, they had two spark plug holes in each head and a trench in the chamber. The trench and plug holes was symettrical, so, end heads could be swapped, end for end, and center simply rotated 180 degrees, so the testing would be symmetrical for the engines.
The testing revealed that when the plugs were in either spark plug hole, performance was nominally different, the engines ran the same with the plugs in either position. The glaring difference is, when the plugs were positioned in the rear holes, the engines started very easily, two serious steps, bump clutch, engine running. When the same plugs, plug wires were moved to the front positions, you literally had to tow the bike about 20 mph to get it to start. This is with NO other changes to anything, just actually moving the spark plug wire from the rear plug, to the front plug.
On some of the factory YZR Yamaha V4 500's, they had aftermarket heads, with pressure accumulators on them, like a shock oil reservoir. These reservoirs had a diaphragm in them, and a pressure valve in their ends, with a line going to the head right next to the plug holes. What the heads had were chambers that had head skulls in them that moved up and down a small distance in the head covers. When the engines were at higher rpm's, the chamber inserts moved upwards in the covers, lowering the compression, then, the pressure in the reservoir moved the inserts back down, for better acceleration.
They worked well for what they were, only one serious problem, mechanics had to use air impacts to set the spark plugs into the chambers, but, the plugs still came loose, and out of the plug holes after a short time, as there was nothing to stop the inserts from turning in the covers. Nice idea, no setting device for the chamber in the cover. I can't remember the name of these heads, someone from Europe might.
We did a lot of very strange testing 45+ years ago.
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