CNG does not burn hotter. It burns cooler. Peak flame temps, 3254F for CH4 (methane), 3614F for C3H8 (propane), 3591 for C8H16 (gasoline), 3729F for C12H26 (diesel).

We have had a Crown Vic, two Honda Civics and now a Chevy Cr*palier, all dedicated factory CNG. One difference about the heads may be higher compression, which could also be done with different pistons. Without the HC, mpg and hp goes down about 20%. The reworked valve seats and valves probably have to do with the "dryness" of CNG, even without lead, gasoline, and especially gasoline-ethanol blends have some sulfur and lubricity. Because octane numbers are much higher and energy density so much lower with gaseous fuels, high compression is a must and dual-fuel setups a loser.

People keep equating LPG and CNG. They are quite different. "L" means liquid and "C" means compressed. The 3600psi at which CNG is dispensed creates a lot more safety issues. Tanks, valves, fittings, piping are all much more expensive. Because the tank is inside with passenger car installations, there is also a kevlar blast blanket between the tank and backseat. Is is worth it? It depends. I would never convert, but as we have, bought factory units. The premium for the Honda Civics was about $5k new.