By switching to the smaller battery you can still have enough juice to power & start your RC51 in street trim & you will shed 3.5lbs of weight carried fairly high in the chassis

New lighter battery (I paid $65 USD)

Battery Type: Yuasa YTZ7S
Voltage: 12
Capacity: 6
Dimensions: 4 7/16" x 2 3/4" x 4 1/8"
Weight: 4.6 lbs.
Metric Dimensions: 113mm x 79mm x 105mm
Metric Weight: kg
Polarity:
Acid Volume: N/A
Amps: 0.6
C.C.A.: 130
Stock battery (about $90 USD)

Battery Type: YTZ12S
Voltage: 12
Capacity: 11
Dimensions: 6" x 3 7/16" x 4 3/8"
Weight: 8.1 lbs.
Metric Dimensions: 150mm x 87mm x 110mm
Metric Weight: kg
Polarity:
Acid Volume: N/A
Amps: 1.1
C.C.A.: 210