You can't be sure in that way.

Use only fp8 without cls (configure arc.ini yourself :P ) to test actually

so i get error zero byte ???

the setting for arc.ini

[External compressor:fp8]
header = 0
packcmd = fp8 -1 $$arcpackedfile$$.tmp $$arcdatafile$$.tmp
unpackcmd = fp8 -d $$arcdatafile$$.tmp $$arcpackedfile$$.tmp
datafile = $$arcdatafile$$.tmp
packedfile = $$arcpackedfile$$.tmp

i tried different fp8 but the same...

I use this is fine

[External compressor:fp8]
packcmd = fp8 -8 $$arcpackedfile$$.tmp $$arcdatafile$$.tmp
unpackcmd = fp8 -d $$arcpackedfile$$.tmp.fp8
datafile = $$arcdatafile$$.tmp
packedfile = $$arcpackedfile$$.tmp.fp8

ain't working on mass effect...