firing order engineering.....

[crashbox455]

american "reg'ler" V8 engines used a firing order of 18436572 for years. now on the Gen III motors, GM has "invented" the 18726543 firing order, despite the fact that ford has been using it since '88.

GM claims 7% less crankshaft stress by "moving" 4 of the 8 cylinders 360*. same crank layout, same piston position, different cam phase.

so in my dream machine motor, i'll be using a custom crank and cam...... gets me wondering.......



is it feasible to try and beat the OEMs at their own game? i'm building a performance machine, so 100,000 miles out of this engine is not the prime directive, by any means. i noticed that the "new" GM/Ford firing order doesn't solve the problem of the old 5&7 cylinders inhaling sequentially, it just moves the problem nearer the throttle body. sounds good, but i'll be using a single plane, with a central throttle body.

i've rearranged the crankpins on paper, and found that a 14326587 firing order will solve the intake problem, but i have no way of knowing what to consider in the way of "crankshaft stress".

any experience, guys? dead horse, maybe?

 

[Metalguy]

IMO, the best thing to avoid, from a crank windup POV, is to avoid having 2 cyl. on the same throw near the front fire sequentially. The Ford sequence avoids this for the worst throw, but both have in for the second one back.

I don't think it means much unless you're making a LOT of torque.

 

[crashbox455]

ahhh.... thank you, i hadn't even considered the crank twisting.

 

[GregLocock]

The Bosch book gives the following firing orders, using 1-4 for one bank, and 5-8 for the other.

16354728
15486372
18364527



Cheers

Greg Locock

 

[enginesrus]

Doesn't Ford use a different cylinder numbering system than GM? Looks like that fire order would add lots of stress to the crank.

 

[Andy330hp]

GM numbers odd on one bank, and even on the other, whereas ford goes 1-4 and 5-8 I believe

 

[crashbox455]

yes, but when you translate the Ford numbers into Chevy-speak, they come out the same- older 302's & 289's have the same order as SBC's, and the 302HO's/351W/4.6 have the same as the LS1.

thanks for the ref's Greg, i'll play with those tomorrow when i'm not so sleepy...... zzzzzzzz.......

 

[enginesrus]

Thats my point.

 

[crashbox455]

guess i should clarify.... i use the chevy cylinder pattern, for no good reason other than that's the one i learned first. so i translate anything i find into that.

i.e. the old Buick nailhead engines (401 & 425) number cylinders the way chevy does, but opposite- odds on passenger side, evens on driver. their firing order is 12784563, but if you start on the driver's front cylinder, using chevy pattern, you get 18736542- different than either chevy or Ford pattern, but same crankpin layout.

basically, i haven't found a V8 yet that has a different crankpin layout, so i'm betting there's a reason for it.

i still need to "decipher" the ones Greg posted, though.

 

[crashbox455]

Greg- the second order you posted works out to be the same as the Nailhead, but the other two don't work on 90* V8s.

maybe they're for straight-8's?

 

[patprimmer]

Crashbox

The obvious numbering system to me is front to back, as the journals appear on the crank. It's just so easy, no one could forget.

That is how Chevrolet does it, and how almost all inline motors I know do it (some old Peugeots numbered from back to front).

The only problem is with compound rod journals like radial engines and V12 aircraft motors.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I should clarify - Bosch does not specify whether they are 90 or 180 cranks. As I am sure you are aware there are (at least) two separate issues here - which pattern gives the best intake mixture distribution cylinder to cylinder, and which pattern gives the lowest crank and main bearing stresses?

Getting the first right is reasonably straightforward, I think. Getting the second right can be done by hand, to some extent, but I've also seen rather complex (computer based) analyses involving working out the bending stiffness of the crank and block, and taking resonances in the crank into account.

There is a third issue here - the subjective sound quality, but I think you won't be worried too much about that.



Cheers

Greg Locock

 

[crashbox455]

i thought they might be flat cranks, but then the firing order would have to be in pairs, and i didn't think they'd notate that as an 8-digit string in a book.

anyway, after MetalGuy's post, i realized that i have all four front cylinders firing together, 1432, which is probably very bad for the crank.

i'll probably just use a stock-type crank, and a custom cam to use the LS1 firing order.