crabman

Why did they change the firing order when they went to the LS1? What are the advantages of the old one versus the new one? I got to thinking about this when a fellow posted that one of the things that makes the LS1 and 2's exhaust sound a little different was the different firing order. Now I knew they had changed the order but never thought of why or what the ramifacations are. Educate me please.

VETDRMS

hubes

not positive, but i think it was mostly to make it run smoother

SWCDuke

There were at least a couple of different reasons, but the one I recall was GM said idle quality was better with the new firing order.

There's no fundamental change in sound since the basic firing pulses on each bank still have the same uneven spacing.

I don't think there is any advantage to changing an old SB to the new firing order, though cam grinders say there is a power improvement.

Of course if they said it makes no difference, they wouldn't sell new cams to all the suckers.

Duke

crabman

Hymmm, seems like most of the pluses and minuses would have been known for some time, right? Like decades? Didnt we make some tradeoffs going from the old to the new one? We wouldnt have used the old one without reason unless this was the case. Or is this something they just figured out recently with the aid of computer modeling etc? I have spent some time searching for this but have come up dry. Inquiring minds...

SWCDuke

Since two pistons are at TDC simulateously on a 90 degree V-8 with a cruciform crankshaft, one can select from several possible firing orders.

The best firing order will minimize crankshaft stress and vibration, but these phenomena were not understood at a low level of detail in the early fifties, so the original SB firing order was probably selected based on the best engineering intuition of the time.

Computer simulation tools and better testing techniques in the last 20 years have led to a much better understanding of the very complex loading, stress distribution, and dynamic behavior that occur in a crankshaft.

Since the LS-series was started from a clean sheet of paper, it was an opportunity to make the change at no cost, and their evaluations - both computer simulations and test data were likely the driving factors in the firing order change.

Duke

crabman

Ok, that makes sense then. Thanks.

GOSFAST

If "they" didn't keep changin' things "they" couldn't keep sellin' things.
Pretty soon we're gonna have "platinumized" heads on the market(haven't got a clue what that means yet). We've found no gains at all from the firing order swap, at least retro'd to early engines. This is the same input I got from at least one reputable cam grinder. Thanks.

SWCDuke

I recall reading that there have been some claims for "more power" retrofiting a vintage engine with a cam to support the "new" firing order.

On a carbureted engine, manifolding and carburetor calibration can certainly be an issue, and most of the currently available manifolds were developed with the traditional firing order, so changing it could make for a gain or a loss unless the manifold and carbuetor calibration was reoptimized for the new firing order.

On an EFI engine which should have very even distrubtion of both fuel and air, there is just no basis for one firing order having a power advantage over another. It comes down to crankshaft stress and torsional behavior.

Duke

wesmigletz

Duke, do you think there would be an additional cooling benefit to having the adjacent firing cylinders moved from the rear of the block to the front? My machinist recommended I go with a 4/7 swap cam due to the 4.00" bore and heavily ported heads on my 283. If I remember right, it was about $25 extra for the change from Isky. Wes

SWCDuke

My gut level intuition says it probably won't make any difference, but then I think #7 has the worse cooling on vintage SBs.

Back then the cooling passages were designed pretty much by engineering intuition. Nowadays they use CFD modeling to analyse cooling flow and heat transfer.

I don't recall GM mentioning cooling as a reason for changing the firing order on the new SBs- just remember reading "idle quality".

I also don't see any harm changing the order, and the price isn't that much, so it's up to you if you thing it's worth $25 to try something different.

Duke

wesmigletz

Thanks for the reply Duke. I'm leaning towards giving it a shot. Wes

comp

let us know what you pick

REDC4CORVETTE

They say it was the arching that existed on the 5.7 between cyl 5 and 7 .
The simulation showed at low compression and Idle 5 and 7 were the problem cylendars.
Who realy knows?

GOSFAST

So far, we've seen no difference whatsoever on dyno tests. G.M. should concentrate on getting their business out of a hole, and stick to the "tried & true". The only firing order change that ever produced results was when Ford carried it over from the early 302's to the later ones. This helped "unload" some excessive pressure from the front main bearing area. It didn't produce any add'l power either, but it did help the bearing life. Thanks, Gary in N.Y
PS In "street-cars" leave the firing order as it is, and don't mess with the oiling system. No galley restrictors, no plugged bypass's, and no HV oil pumps. The factory keeps changing things, to keep selling things. Period

KyRP2NITe Corvette

LS motors run individual coil packs so no arching between 5 and 7 can occur... that would only happen in distributor applications with extremely high spark volts...

steveC5

Just a guess as I don't know anything on engines. Maybe for the "reduced power" mode? Perhaps the old firing order wasn't possible to work on 4 of the 8 cylinders?

71coupe

Per Smokey Unick, they still had issues with the #7 after eliminating the spark crossover problem. He suspected that the #5 cyl was robbing the #7 of it's intake charge, causing a lean condition. This would only be the case on a single plane intake.

cardo0

There's quite a lot of design engineering, modeling, testing and evaluation to produce a new engine let alone from one of the world's largest if not largest car mfr. Like a full engineering staff and testing program. Dyno time? More than i'll ever get to use or match. And i won't speculate on how the timing order was chosen before or now. There were inline 6's, inline 8's, and now Chevy/GM is back to I5's & I6's - why? Yes there's V6's, V8's, V12's and now V10's but never saw a V4 - always a flat 4 or inline 4 - why?
Reality is that i hope to build just one more mill in my lifetime and the firing order as is will be good enough for me. I'm much more worried 'bout a machine shop i can trust for accurate block work than a custom camshaft grind to change the firing order to prove something. Hey u can even have the cam ground to turn the engine the opposite direction if that's what pushes ur piston - and its been done for special applications. But lets keep our feet on the ground guys. We all have better things to do than to prove the big guys wrong. Improve the firing order? Look's fine as is too me and i don't want to play there. Too many ways to say oooopps. Like do i want buy a good crank or design my own? Have a good used block machined or cast my own? Hey u can research for years but u will need to stop somewhere and build the thing. BTW i've read Smokey, Vizard, Lingenfelter, G. Jenkins, the GM Power Manual and they don't change firing order for more power. The reality is i need to keep looking at the firing order cast into the intake just to remember it.
Well if u c the need to change then good luck and post ur results.
cardo0

INMYBLOOD

engineering intuition engineering intuition engineering intuition engineering intuition engineering intuition engineering intuition engineering intuition engineering intuition engineering intuition
Duke that hit my funny bone