just as a bit of info I had a buddy that built a killer 468 cubic inch 13.5:1 compression ratio, BIG BLOCK CHEVY with a very radical solid roller cam,
(crower 01521 cam/brodix heads)
(first solid roller cam he ever installed) and he spent hours trying to get it to idle below 1500rpm, it sounded like crap,and didn,t sound right, and he was sure the cam had lost a couple lobes, he called me over and we ran down a check list, I had printed out, when we got to the firing order question, he starting cursing and said, I can,t believe I spent 6 hours chasing this problem, when its my own damn stupidity!
(he installed a custom ground 4/7 swap cam and was still using the 1-8-4-3-6-5-7-2 firing order he had used for 30 years, without thinking and had verified it several times, not the 1-8-7-3-6-5-4-2 required by the new cam)
I had a different friend install a 467 cubic inch Pontiac V8 in a 1967 fire -bird and he hooked the headers to the stock 40 plus year old exhaust, the car ran decent up to about 4000rpm where it seemed to hit a wall on power, a simple check showed he had over 7 psi of back pressure at 4000rpm so the engine just could not fight that, swapping to a custom 3" dual exhaust with an (X) pipe really woke-up the power potential, and before testing he was convinced the cam was too mild, which it was not. (crower 60213 cam/ edelbrock 2957 intake/850 holley/edelbrock 60955 heads)