Racing Green TVR News
Further thoughts on the halftime bearing upgrade for Speed 6
31/12/2008
The half time bearing on the TVR Speed 6 has been known as a potential problem largely due to loads imposed on it by the requirement to run the timing chains tight. This has been discussed in the previous articles and the problems can generally be summed up as being due to over tightening of both the upper and lower timing chains and the fact that the halftime bearing assembly is only splash fed with engine oil. Uprating the bearings is a sensible precaution but the choice of bearings for this application are very limited indeed due to the size of the relevant components. The upgraded dual race bearing TVR E6326 RG is a known approach but there are problems associated with angular contact dual race bearings not least the requirement for absolute accuracy in bore/shaft alignment.
The fundamental issue is that the halftime bearing sprocket in its current design is not ideal. It is both extremely heavy and its dimensions/hardness prevent the use of the ideal bearing for this situation, a needle roller bearing. Needle roller bearings offer massive load capabilities as well as reduced size requirements. The AJP V8 engine as used in V8 Cerberas and race Tuscans uses very high quality timing gears running on needle roller bearings and these give very little problems and show virtually no wear in normal use. Racing Green TVR has redesigned the halftime sprocket and shaft assembly with this approach in mind. A lightweight sprocket has been designed utilising 2 needle roller bearings running on a modified shaft. The halftime shaft diameter has been increased as well as incorporating a pressurised oil feed to both needle roller bearing races. The internal bore of the sprocket and the shaft are hardened to 60-62 Rockwell to allow the needle roller bearings to run directly on the sprocket and shaft. There are numerous benefits to this design, the main one being that the bore for the bearing in the halftime sprocket is machined in one operation ensuring absolute accuracy and concentricity.
This new halftime sprocket, shaft and bearing assembly will however only be available in the Simplex chain variant as running on Racing Green TVRs uprated Speed 6 engines. It is estimated that this new sprocket design together with the uprated Simplex chain kit will reduce the weight of the timing chain assembly by approximately 1400 grams. The value of the Simplex chain approach should not be understated as this massive reduction in rotating mass is of huge benefit and significantly reduces loads on the engine resulting in more available horsepower to the rear wheels. The original Duplex chain design is over engineered when compared to the new lightweight simplex chains used in todays high performance engines. The Racing Green TVR Simplex chain kit for the Speed 6 engine gives a significant performance increase as well as massively reducing loads on the TVR engine.
The all new halftime sprocket design will be available from the end of January 2009.
