INFO FOR THE TECHNO-GEEK AND PEOPLE WHO JUST WANT TO KNOW

Recently I got into a conversation with some engine tech-geeks and I said the “crank” determines firing order of our SIX. They looked at me as if I had cursed the Internal Combustion Gods! I argued my point with the following information. Our SIX has a longer crank than the V8 and has no “opposite bank” of cylinders to help offset vibration. Being longer it is more susceptible to imbalance inputs from the reciprocating mass, so firing order is set to negate those problems as much as possible. The V8 fires every 45 degrees while our SIX goes 60 degrees between bangs. In short, keeping an In-Line smooth as possible at all rpm is more difficult and some in-lines use a counter-balance shaft to help. Four cylinders almost have to us a balance shaft due to firing only every 90 degrees.

The cam obviously determines when both valves are closed for the cylinder to fire, but if one does not consider overall balance of the firing order, parts can separate! The factory engineers obviously know this and take it into account during design work. However, Hot Rodders tend to “modify” for specific purposes such as Bonneville and drag racing. Cam profiles (overlap, lift, duration, etc.) get changed by people who know how, to get more air-fuel IN and more exhaust OUT at the right time! On a street driven car “even” torque and smooth running at all speeds is wanted…NOT all-out power!

Think about the Ford factory 153624 firing order as 3 sets of 2 cylinders. 1 & 2 are front, 3 & 4 are middle and 5 & 6 are rear. The firing order balances by firing 1 (front) then 5 (rear)…then 3 (middle) then 6 (rear)… then 2 (front) and finally 4 (middle). After that, it jumps from 4 back to 1. This keeps the power pulses jumping around the crank spreading the load evenly from front to rear, to middle to rear, to front to middle, then back to front. I have heard of “high speed” freaks changing firing order to eliminate a high speed harmonic, but I don’t understand all I know about that!? All pistons reach Top Dead Center (TDC) at some point, so a cam could probably be made to make any firing order one wants to try, but I am not one of those guys, and I am pretty sure Ford knows what they are doing. I do balance the rod-piston assemblies though and keep everything hospital clean during assembly.

Believe it or not, at one time TTD (Test To Destruction) was the preferred method if engineers wanted to know…for sure! That was before computer programs. I do not have a bullet-proof room where I can run an engine to literal death! I do not have the ability to make a cam. I do not have a computer program to show me what profile the cam should have. I have no desire to do any of those things either. I do rely on Ford’s engineers to show me what works best…then I modify from there with reliability in mind.

A small block chevy fires 18436572. Notice 5 & 7 fire on the same bank, one after the other. If these wires are crossed the engine runs fine…to a point…then misfires or breaks. A friend who raced chevies learned this the hard way. Another friend raced a Ford SIX cylinder 240” that never broke and it was highly modified. Cam lift, duration, overlap, rocker ratio, cylinder bore, stroke, rod length and more were changed, but firing order was factory stock. Our little SIX will never out run a corvette or cobra but it can run longer and be funner! Fun, Funner, Funnest…still sounds right!

Charlie J. “old Geezer” Rising Star, Texas

 

Posted in Old School Old Geezer