Retro 1. Mk11 engine specifications

This is the first in a series of retrospective posts dealing with the development of a BMW R100 ‘airhead’ motor (Race motor Mk11) leading up to a record breaking run at the 2018 DLRA speed week at Lake Gairdner in outback South Australia.

We’ll start the series with the final engine specs then go back and look at how some of these were achieved.

Donor bike: 1987 BMW R80; Donor engine: 1993 R100 horizontally opposed twin ‘boxer’ engine.

1. Top end

Chamber shape: hemispherical
Displacement: 980cc (stock)
Bore: 94mm (Stock)
Stroke: 70.6mm (stock)
Compression ratio: 12:1

Valve gear
Valves: OHV, 2 per cylinder
Valve actuation: pushrod via 1.39 ratio stock rockers
Inlet valve diameter: 46mm stainless steel
Exhaust Valve diameter: 38mm stainless steel
Springs: Dual race springs with titanium retainers and stock collets
Pushrods: Custom Skrunkwerks dual tapered aluminium alloy
Lifters: lightweight lifters from Wank Germany

Spark plugs: 1 per cyclinder
Pistons: shortskirt, high compression from Moorespeed. Reduced compression height to suit longer conrods.
Conrods: 15mm longer than stock from Moorespeed.
Fuel mixture delivery: 1 x 40mm Lectron carburettor per cylinder.
Inlet tract and ports: Custom Skrunkwerks Design
Exhaust: 2 into 2 Custom Skrunkwerks design

2. Short Block

Case: Stock
Crank: Stock. Rebalanced for pistons and rods.
Cam: Schleicher 340
Cam sprocket: Adjustable from Nova Racing
Oil pump: Low volume from Moorespeed.
Ignition: Ignitech programmable. Silent hektik Inductive pickup on crank. Total loss system (alternator removed). 2 x PVL single post coils.
Clutch: Heavy duty Sachs diaphram spring with K75 pressure plate.
Flywheel: Stock
Sump: Extension from Motoren Israel with screw-on filter and provision for oil cooler.
Oil cooler: From 2005 Triumph Bonneville plumbed into Motoren Israel sump extension
Crankcase ventilation: Custom Skrunkwerks design slash tube to exhaust sytem.

3. Output

Power: 90rwhp @ 8200 rpm (approx 100hp at the crank)
Torque: 65 ftLbs @5900 rpm

As measured on a Dynojet dyno in full inertia mode with SAE J1349 correction applied.

So there you have it, the bare bones. Stay tuned for Retro 2. where we look at what was done to increase compression ratio.

5 replies
  1. adrian
    adrian says:

    Would love to hear your comments. Comments will appear on the page once they have been approved (warding against spam etc).

  2. Lorne
    Lorne says:

    No doubt against what you have achieved Adrian. Can you utilize data recorded to extrapolate a target final drive/ wheel diameter for that “optimum” max speed when at best engine rpm’s? This reminds me of a time ago chasing water jet impellor shape to maximize bollard pull on a hydraulically driven power train in an amphibious tracked craft. So gratifying when you achieve that perfect rpm.
    (aka Arktasian)

    • adrian
      adrian says:

      Hey Lorne, Thanks for your comments. Sounds like an interesting project that you were involved in.

      Yeah, I have some data points now so can calculate a rough drag coefficient which is always the hardest thing to quantify with these things as you would know. Fine tuning would still need to be done on the salt and still difficult with the shaft drive compared to my Triumph buddies who were changing one tooth up or down on rear sprocket from run to run.

      At least now I have meaningful numbers to help reduce the size of the ball park..


  3. Jim K in PA
    Jim K in PA says:

    Adrian- late to the comment party, but wanted to say thank you for adding some details here as well as your posts on Advrider. Lorne and I have discussed my plans for a salt racer, but with some more “tech” in the form of electronic FI and ignition, and then some “respiration inspiration”. I’m a long way from where you are, and look forward to reading more about your challenges and triumphs (lower case t).


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