My friend is building his Chevy motor and the guy at Comp Cams asked if he wanted a 4-7 swap. He said they switch the firing on those two cylinders so that you don't have 5-7 and 8-4 firing right after each other. Then all you have to do is swap the spark plug wires from 7 and 4. I was wondering why they didn't do this from the factory? My first thought was that it would load up the crank with 8-7 firing right after each other, but 2-1 fire right after each other in the stock firing order. Is their any disadvantage to this firing order?
Wouldn't you also have to change the engine balance? That's a pretty nasty subect with the concept of rotating and rocking couples (moments) and in-plane and out-of-plane balance. Makes my head hurt. Or does this swap have some magical characteristic whereby balance is ok left as is?
No need to re-balance. Supposedly there is an advantage to the 4-7 swap in terms of crankshaft harmonics. I'm thinking that there is also an advantage in cooling: instead of having 5-7 firing sequentially, and producing a "double whammy" of heat in the left rear corner of the engine, you get 4-2 firing sequentially and the double whammy of heat happens right next door to the water pump. Maybe that makes a difference, maybe not. In a parallel cooling system, it's probably an advantage, and in a serial cooling system it may make things worse.
The 4-7 swap is fairly common in drag racing and probably other forms of racing as well. It has been done in Pro Stock, Super Stock, etc. for over 10 years. It was a secret for awhile, but now it is advertised all over. I read and contributed to a discussion on another forum with Darin Morgan of Reher-Morrision Racing Engines regarding this subject. He simply stated they have found "measureable" HP (around 2-3) on all of their race motors by doing this. He said they "really don't know why, it just works" Was he hiding something? Who knows. I offered that maybe it had to do with air/fuel distribution in the plenum being affected by g-forces during a pass. Darin said that that wasn't the reason because it also shows up on the dyno. He also told of other experiments they had like "flat cranks" (throws 180* apart). He said that motor sounded like a buzz saw, was extremely loud, and hit a wall at a certain RPM where it just stopped making power no matter what they did. The 4-7 swap requires a roller cam unless you can find a core made specifically for the 4-7 swap for your motor. A roller cam starts as a billet, so you can grind the lobes wherever you want. I'm sure when the new Buick blocks are out we will be able to take advantage of this little gem. Have fun.
sounds about like what ford did with the 302. they had a smoother firing order for the luxury cars, but used the 351 firing order to make more power on the HO motors.
I read somewhere that the reason they didn't do it from the factory was because it would produce a "rougher" idling/sounding engine, and since not everyone that buys a car likes to have a rough race car sound, they left it that way. And like jeff said, it gives you around 2-3 hp.
Sweet! So he can have a mean sounding idle without as much valve overlap? I'm really curious what it sounds like. I have noticed that Fords have a unique sound to them, they seem to resonate at idle where other cars do it at 1800-2200 rpm.
