Here’s the short block for the Salt Racer Motor Mk11. It’ s been 18months since the unfortunate demise of Mk1 on the dyno 3 days before leaving for the 2016 event.
Bit of a late start with 6 months till the 2018 event, but not due to pure slackness – I’ve been delving into the world of combustion theory and engine simulation programs, not to mention building flowbenches and cam analysers. Using these tools, the heads and cam from Mk1 were analysed and this data was used in an engine simulation program to create a base line model which agrees with the dyno results of the Mk1 motor.
This base line virtual model was then ‘tuned’ to optimise power in the target range of 8000-8500RPM. The simulation shows that the best case outcome with the 46/38mm intake/output valves and current port and chamber configuration is 110HP at the crank. But valve lift would need to be more than the 12.3mm that the Schleicher 340 produces – at least 13.5-14mm. And more importantly, the intake tract flow would need to be increased by around 20%. This is not as easy as it sounds: Flow optimisation is a black art practiced by people with years of experience! I have some learnin to do..
The target valve lift can be achieved by either replacing the 340 cam with a higher lift cam or increasing the rocker ratio or a combination of both. I’m having a shot at designing some high ratio Skrunkrockers… stay tuned.
Finally getting to the meaty end of the salt racer build.
While the crank is off getting balanced with the new pistons and rods, I had a look at the rocker geometry.
With the high lift cam (about 12mm at the valve) and recessed valve seats the geometry did not look good. With valve fully closed, the rocker nose was close to the edge of the valve stem end and moved just past the valve stem centerline at full lift. Not the best. Ideally, the nose travel across the stem end should be equidistant about the valve stem certerline.
The only way I could think to adjust this was to adjust the thickness of the spacers under the rocker posts. The stock item is 3mm. Replacing it with a 1.5mm spacer got things right as shown in the pic but only for that rocker! All the others were a bit different but acceptable with the 1.5mm spacer.
A stock pushrod was transmogrified into an adjustable one so that I could determine the correct length for the pushrods. I was going to have a shot at making my own tapered aluminium ones but just don’t have the time so it’s off to the pushrod man to have some made in 4130.
G Feeling like I’m on the home straight…frame is painted in special ‘skrunkwerks’ golden grey (stolen from Mercedes and tinkered with!).
Lots of werk fillet brazing and filling the various gussets to achieve a sculpted look. The engine case is painted in a 2 pack mat black while top engine cover and rocker covers painted in a 2 pack gloss.