Not true anymore.

DA support has stated that unregulated A123 (6.6v) is fine. I've run this for over a year without issue
3W manuals state up to 7.5v (5 cell NiMH) is acceptable. They state 5 cell pack is preferred in fact.
The newer Evolution/MVVS engines accept 6-8.5v. Their manual states 5 cell nicd/nimh minimum and they recommend a 2S lipo/LiIon.
Saito recommends a 5 cell/ 6v minimum pack for the FG series gassers.
Zenoah does specify 4 cell only - meaning their ignition probably cant take over 6v.
I don't know about ZDZ or any other makes.


Bottom line - dont just assume, read the manual.