Sept - Aug Crack Spread Calendar +$100 after fees

Takeaways
CSC price fluctuates with a magnitude of ~10% on 5min time scale.
It looks OK on paper but in reality bid/ask differential is very large.
The cost of trading is ~$30 which is high.
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Update : Closed with profit of  ~ $100 after fees.

Exec Time	Spread	Side	Qty	Pos Effect	Symbol	Exp	Type	Price	Net Price	Order Type	CSC
											-1.274
5/16/2016 12:20	FUT CALENDAR	BUY	1	TO CLOSE	/RBQ6	16-Aug	FUTURE	1.6166	0.022	MKT
		SELL	-1	TO CLOSE	/RBU6	16-Sep	FUTURE	1.5946	DEBIT
5/16/2016 12:20	FUT CALENDAR	SELL	-1	TO CLOSE	/CLQ6	16-Aug	FUTURE	48.73	-0.35	MKT
		BUY	1	TO CLOSE	/CLU6	16-Sep	FUTURE	49.08	DEBIT
											-1.3982
5/13/2016 14:12	FUT CALENDAR	SELL	-1	TO OPEN	/RBQ6	16-Aug	FUTURE	1.5876	0.0221	MKT
		BUY	1	TO OPEN	/RBU6	16-Sep	FUTURE	1.5655	CREDIT
5/13/2016 14:12	FUT CALENDAR	BUY	1	TO OPEN	/CLQ6	16-Aug	FUTURE	47.39	-0.47	MKT
		SELL	-1	TO OPEN	/CLU6	16-Sep	FUTURE	47.86	CREDIT

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I propose here to trade Crack Spread Calendar which is the difference between  /RB and /CL calendars. CSC is  a capital efficient trade. The trade below is an experiment to better understand the mechanics of such trading.
Sept-Aug CSC = 42*(/RBU6-/RBQ6)-(/CLU6-/CLQ6)
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Exec Time

Spread

Side

Qty

Pos Effect

Symbol

Exp

Strike

Type

Price

Net Price

Order Type

CSC

-1.3982

5/13/2016 14:12

FUT CALENDAR

SELL

-1

TO OPEN

/RBQ6

16-Aug

FUTURE

1.5876

0.0221

MKT

BUY

1

TO OPEN

/RBU6

16-Sep

FUTURE

1.5655

CREDIT

5/13/2016 14:12

FUT CALENDAR

BUY

1

TO OPEN

/CLQ6

16-Aug

FUTURE

47.39

-0.47

MKT

SELL

-1

TO OPEN

/CLU6

16-Sep

FUTURE

47.86

CREDIT