SPArKy_Dave 8,865 Posted May 16, 2021 You begin the spark early - BTDC (before top dead center). How early is based on fuel type (petrol/LPG flame propagation time) engine rpm, combustion chamber and piston design. So we light the fuse early, the flame spreads for a while, and by the time peak cylinder pressure occurs, your crank is around 12 ATDC (after top dead center). If peak pressure occurred any earlier than that, it would be like a sledgehammer driving the rod into the crank straight down. By peaking after TDC, the rod acts like a lever instead of a railroad spike. 3 Seamus, Outback Jack and gerg reacted to this Share this post Link to post Share on other sites
