diff --git a/beacon_chain/beacon_node.nim b/beacon_chain/beacon_node.nim
index 265cf1fa6..547b5189c 100644
--- a/beacon_chain/beacon_node.nim
+++ b/beacon_chain/beacon_node.nim
@@ -28,7 +28,7 @@ const
   topicBeaconBlocks = "ethereum/2.1/beacon_chain/blocks"
   topicAttestations = "ethereum/2.1/beacon_chain/attestations"
 
-  stateStoragePeriod = EPOCH_LENGTH * 10 # Save states once per this number of slots. TODO: Find a good number.
+  stateStoragePeriod = EPOCH_LENGTH.uint64 * 10 # Save states once per this number of slots. TODO: Find a good number.
 
 
 func shortHash(x: auto): string =
@@ -434,7 +434,11 @@ when isMainModule:
 
       info "Starting beacon node",
         slotsSinceFinalization = node.beaconState.slotDistanceFromNow(),
-        stateSlot = humaneSlotNum(node.beaconState.slot)
+        stateSlot = humaneSlotNum(node.beaconState.slot),
+        SHARD_COUNT,
+        EPOCH_LENGTH,
+        SLOT_DURATION,
+        SPEC_VERSION
 
       node.addLocalValidators()
       node.processBlocks()
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index ee188041f..7266e6148 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -40,11 +40,16 @@ import
 # TODO Many of these constants should go into a config object that can be used
 #      to run.. well.. a chain with different constants!
 const
+  SPEC_VERSION* = "0.2.0" ## \
+  ## Spec version we're aiming to be compatible with, right now
+  ## TODO: improve this scheme once we can negotiate versions in protocol
+
   # Misc
   # https://github.com/ethereum/eth2.0-specs/blob/v0.2.0/specs/core/0_beacon-chain.md#misc
-  SHARD_COUNT* = 1024 ##\
+  SHARD_COUNT* {.intdefine.} = 1024 ##\
   ## Number of shards supported by the network - validators will jump around
   ## between these shards and provide attestations to their state.
+  ## Compile with -d:SHARD_COUNT=4 for fewer shard (= better with low validator counts)
 
   TARGET_COMMITTEE_SIZE* = 2^7 ##\
   ## Number of validators in the committee attesting to one shard
@@ -89,10 +94,11 @@ const
   ## the validator pool
 
   # Time parameter, here so that GENESIS_EPOCH can access it
-  EPOCH_LENGTH* = 64 ##\
+  EPOCH_LENGTH* {.intdefine.} = 64 ##\
   ## (~6.4 minutes)
   ## slots that make up an epoch, at the end of which more heavy
   ## processing is done
+  ## Compile with -d:EPOCH_LENGTH=4 for shorter epochs
 
   # Initial values
   # https://github.com/ethereum/eth2.0-specs/blob/v0.2.0/specs/core/0_beacon-chain.md#initial-values
diff --git a/beacon_chain/state_transition.nim b/beacon_chain/state_transition.nim
index 2e4214bc5..8d39a82bb 100644
--- a/beacon_chain/state_transition.nim
+++ b/beacon_chain/state_transition.nim
@@ -517,7 +517,8 @@ func processEpoch(state: var BeaconState) =
   let # Previous epoch boundary
     # TODO check this with spec...
     negative_uint_hack =
-      if state.slot < 2 * EPOCH_LENGTH: 0'u64 else: state.slot - 2 * EPOCH_LENGTH
+      if state.slot < 2'u64 * EPOCH_LENGTH: 0'u64
+      else: state.slot - 2'u64 * EPOCH_LENGTH
     previous_epoch_boundary_attestations =
       boundary_attestations(
         state, get_block_root(state, negative_uint_hack),
diff --git a/tests/simulation/start.sh b/tests/simulation/start.sh
index 25518650f..e030965ff 100755
--- a/tests/simulation/start.sh
+++ b/tests/simulation/start.sh
@@ -25,11 +25,15 @@ mkdir -p $BUILD_OUTPUTS_DIR
 
 BEACON_NODE_BIN=$BUILD_OUTPUTS_DIR/beacon_node
 VALIDATOR_KEYGEN_BIN=$BUILD_OUTPUTS_DIR/validator_keygen
-SLOT_DURATION="-d:SLOT_DURATION=3" # Default is 6
+
+# Run with "SHARD_COUNT=4 ./start.sh" to change these
+DEFS="-d:SHARD_COUNT=${SHARD_COUNT:-4} "
+DEFS+="-d:EPOCH_LENGTH=${EPOCH_LENGTH:-8} "
+DEFS+="-d:SLOT_DURATION=${SLOT_DURATION:-3} "
 
 if [[ -z "$SKIP_BUILDS" ]]; then
-  nim c -o:"$VALIDATOR_KEYGEN_BIN" "$SLOT_DURATION" -d:release beacon_chain/validator_keygen
-  nim c -o:"$BEACON_NODE_BIN" "$SLOT_DURATION" beacon_chain/beacon_node
+  nim c -o:"$VALIDATOR_KEYGEN_BIN" $DEFS -d:release beacon_chain/validator_keygen
+  nim c -o:"$BEACON_NODE_BIN" $DEFS beacon_chain/beacon_node
 fi
 
 if [ ! -f $STARTUP_FILE ]; then