Switching 4 & 7 firing order - Duke, Clem?

[David B.]

Duke, Clem, others,

What do you think of the Comp Cams idea of switching #4 and #7 firing order with their certain oval track and drag race cams? They claim more HP, cooler temps, smooth torque curve (Summit Catalog, p.146). I would have thought that if this was a good idea, it wouldn't have eluded GM engineers. There must be some drawbacks or we wouldn't have the 18436572.

[Clem Z.]

Re: Switching 4 & 7 firing order - Duke, Clem?

the new GM LS series engines changed the firing order. 1-8-7-2-6-5-4-3. flipping the 4-7 also allows the 5-7 cylinder from firing next to each other and causing a fuel distribution problem you have with racing intakes with open plenums.

[Rod K.]

Re: Switching 4 & 7 firing order - Duke, Clem?

If you flip the 4-7 cylinders, you eliminate the 5-7 adjacent firing, but create a 4-2 adjacent firing, and the LS order results in a 3-1 firing. All the same problem with single plane mainfolds. The LS engines are all individual runners and injected, are they not, so it would seem the fuel/air flow issue would be moot. I'm still confused.

[Wayne W.]

Re: Switching 4 & 7 firing order - Nothing New

My 31 and 33 Cadillacs have that firing order.

[Clem Z.]

Re: Switching 4 & 7 firing order - Duke, Clem?

GM now sell kits to convert LS engines to a intake manifolds with a carb. NASCAR requires this engines in some of it racing series. several magazines have done tests and found no or little HP gain with the 4-7 switch BUT these were not all out race engines which may make a difference.

[Duke W.]

Re: Switching 4 & 7 firing order - Nothing New

Don't those vintage Caddys have flat crankshafts - like the old Ford flathead V-8? I recall reading a SAE paper when I was in grad school on working out the primary rocking couple balancing on a cruciform crank V-8s, and I don't think it was done until the late thirties at the earliest, and all previous V-8s had flat crankshafts with the associated unbalanced second order horizontal shaking force.

F1, CART, and IRL still use flat cranks to get even exhaust intervals on each banks. Ferrari road V-8s also have flat cranks.

Duke

[Duke W.]

Re: Switching 4 & 7 firing order - Duke, Clem?

It's another marketing ploy to get you to pony up for something "new" to replace something "old" - never mind that "old" works just fine.

I think it was the SAE paper on the LS1 where I read that the primary reason for the swap was better idle smoothness, the details of which are buried in the esoteric subject of crankshaft torsional stiffness and vibration. Who knows if this would smooth out the idle on a vintage V-8, but with a SHP cam you would never be able to detect a difference, and I similarly doubt for a base cam engine, either.

LS engines have a symmetrical manifold, so air distribution is probably indifferent to firing order.

It might make a difference on a vintage V-8, but without a sophisticated test setup to measure cylinder to cylinder air flow, you would never know if there was a difference.

Like most aftermarket stuff, it's just hype.

Duke

[David B.]

Re: Switching 4 & 7 firing order - Duke, Clem?

Interesting comments from everybody - especially the Caddys. When the subject of cams comes up, it ain't finished until Clem and Duke both take a whack at it as far as I am concerned.

Thanks all. I will leave the altered firing order concept alone.

[Wayne W.]

Re: Switching 4 & 7 firing order - Nothing New

Duke, Cadillac used the flat crank from 1915 until 1924 when the V63 was introduced. It was a dual plane crank. By 1927 under Owen Nackers direction the 314 was introduced that had all of the vibration problems solved.