Here’s a sneak preview of the high ratio rockers I’ve been working on.
With the stock rocker shaft position and my tapered pushrods, 1.45 is the highest obtainable ratio before the pushrods clash with their tubes. So the shaft will need to be moved which means custom shaft and posts. Shooting for a ratio of around 1.6 which will give 14.2mm max valve lift with the Schleicher 340 cam.
At a ratio of 1.6, the valves clash so I might have to sink the exhaust valve a bit or go for 37mm exhaust valves.
Port flow would need to be optimised to suit otherwise there’ll be bugger-all gains.
Material will 4340 high tensile steel. Seriously tuf stuff so can get the weight down.
Why not a lumpier cam instead?
Yes, a cam change would be easier. But other high lift issues start to rear their heads.
Lift velocity is limited by the 22mm tappet diameter so to make things work, need to go for smaller base circle diameter, which means smaller shaft diameter and more deflection. Also, you can’t fit the higher lift cams with decent base circle and shaft diameter through the hole in the case.
A friend loaned me a 2 piece high lift roller cam by one of the well known airhead aftermarket suppliers from back in the day. The shaft diameter is way small so can fit in the hole. But flexed so much, it morphed into 2 pieces
Soooo, to overcome the 22mm limitation and keep up area under the lift curve, people have used mushroom lifters and enlarged the case hole to get in a fat cam then fit a sleeve for the stock flange bearing.
In some cases, going for rocker ratio instead of cam lift has advantages for valve train dynamics which I don’t yet fully understand. One thing is that because velocity and acceleration are multiplied by the rocker ratio, ramp action is quicker and you get some more duration. However, this may or may not benefit your cause depending on the setup. It’s complex!
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.
Here are the final studio shots of the salt racer after the 18 month build. If you haven’t been following the build blog, here’s a quick run-down:
The frame was heavily modified and strengthened – most of which is hidden under the tank. The front end is from a GSXR 600, and rear suspension is handled by a BMW R1100RT swing arm supported by a Hyperpro shock. Hanging on the back of all this and tied back to the frame with a custom torque arm, is a bevel drive from a R1100S which provides the taller gearing necessary for the salt. The wheels are from a K100.
The carbon work was all done in-house including ‘ramstein’ – the top secret ram air system that you see jutting out the sides like bulls horns. Rearsets need to be way back and yes, no front brakes – plenty of room to stop without them. Besides, they can be dangerous on the salt.
The bike was fitted with the motor from luftkopf#1 since time ran out to finish the the hot engine that I had intended for the salt. The tank is an old fiberglass jobbie that I found. Harks back to 80’s superbikes. Lots of cutting and shutting to get the shape right for the bike.
The bike was finished just in time to make the 2015 DLRA speedweek at Lake Gairdner. What an amazing experience! There’s a story about it here.
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.