It depends on your drives' firmware and also the media used. If the media tells the drive it doesn't support 2.4x then it won't burn at 2.4x. The drives' firmware (internal software) also has to support this speed. If 2.4x isn't in the list of allowable speeds, again, it won't burn at that speed. Information telling what the drive and media are capable of is displayed in the right side window when in WRITE mode. Like this:
PIONEER DVD-RW DVR-109 1.58 (ATA)
Current Profile: DVD-R
Disc Information:
Status: Empty
Erasable: No
Free Sectors: 2,297,888
Free Space: 4,706,074,624 bytes
Free Time: 510:40:38 (MM:SS:FF)
Supported Write Speeds: 4x, 6x, 8x, 12x
Pre-recorded Information:
Manufacturer ID: RITEKG05
Recording Management Area Information:
PIONEER DVD-RW DVR-109
Physical Format Information (Last Recorded):
Book Type: DVD-R
Part Version: 5
Disc Size: 120mm
Maximum Read Rate: Not Specified
Number of Layers: 1
Track Path: Parallel Track Path (PTP)
Linear Density: 0.267 um/bit
Track Density: 0.74 um/track
First Physical Sector of Data Area: 196,608
Last Physical Sector of Data Area: 0
Last Physical Sector in Layer 0: 0