Gone all anoraky and trying to find engine specification data for a 916, 748 and 996. Specifically gudgeon pin diameter, crank pin diameter, crank pin radial location, con rod length and barrel dimensions. Having looked in a number of reference books and at a few web sites I'm at a loss to understand how you achieve the 748 to 853 conversion and if as I have been told, what it takes to get an 890 conversion using 96mm diameter (955) pistons. As far as my tiny mind can work it out, the crank pin must be on a different radius to give the right stroke if as I believe all the con rods are the same length and the barrels must be different heights. This is a purely academic exercise for now but any information would be greatly appreciated. Thanks, Andy
Isn't the 853 conversion 916 "size" piston & barrels with the 748's shorter stroke crank (61.5mm as opposed to 66mm) I'm sure one of the tuners (Sigma maybe) wrote an article on this, but sorry no idea where you'll find the data your after.
Andy, at the risk of telling you something you already know the 853 uses the same rod length and stroke as the 748 at 61.5mm stroke but uses 94mm bore pistons and cylinders (same size as the 916) instead of 88mm, the 890 option uses 96mm bore pistons (same as the 955) and 61.5mm stroke. 916/955/996/998 engines have 66mm stroke. IIRC the rods are all 124mm, crank pin 42mm, gudgeon pin 20mm but you need to check. I seem to remember that the longer stroke cranks from 900ss (68mm) could also be used to increase the displacement. A more common upgrade on the 916/996 was to use 100mm over bored cylinders to take the 916/996 up to 1037cc but keeping the standard crank and stroke at 66mm
Thanks Denzil, that's confirmed most of what I had found out. All I need to understand now is how the positioning of the crank pin changes the stroke apparently without changing the length of the barrel. I do know the 748R gudgeon pin is 20.something and the RS barrel is very slightly shorter to increase the squish and that if you want to do the 890 you need to omit a gasket under the barrel. Well at least I'm a little less in the dark. Still not convinced the light at the end of the tunnel isn't some bastard coming the other way with a torch. Thanks guys.
Here's some information from the past about Sigma's 853R conversion 748R / 853R - Sigma Performance Their website still remains and you could even try contacting Neil Spalding via Twitter @Spalders Someone else could be Chris at CJS, he has done some 853 conversions and might be able to help.
Yes, Wilf was working on both mine and Koen's refreshed bohemoth yesterday. Currently looking at a number of options that fit work commitments and GCSE timings so Carole and her daughter can come too. After Cartegena I am very aware of my riding standard is at the lower end ability and I need to pick the right event. It's very likely I would be a mobile chicane at an open pit lane session which isn't much fun for anyone. Andy
Yes, thank you for that link, there is a lot of the background information I was looking for. I seem to have anticipated some of the potential issues with my RS and the power and torque being produced is exactly what it should be. Andy
Andy, its a sessioned trackday, early mid feb, they have separate acu group so no worries there If working on pace and technique, always best to use circuits yiu know imho. If going for a craic and just to have a augh and enjoy riding, go wherever you always wanted to