I read about some Drag Racers changing the firing order, on their Small Block Chevy's. I forgot which two cylinders were involved (One on each bank) but it said that all a person would have to do is change the Cam and switch the two wires on the Distributor cap. The Crankshaft was already in the correct position, so that wouldn't have to be changed. I haven't read any other articles about this changeover, maybe it didn't do that well on the street. Anyway, I was wondering if it was worth it and at what RPM range it worked best ..... Anyone try or know about this?
i "think" they swap 4 and 7 http://www.southernairboat.com/phpBB3/viewtopic.php?t=25732 http://www.hotrod.com/techfaq/113_0701_lunati_cams/edelbrock.html
Usually change #4 and #7 in the order. With tri-y type headers you can pick up about 15 HP. The cam grind remains the same just the position of the lobes for 4 and 7 are changed. Sounds a little strange at idle.
They call it a 4-7 swap and it supposed to smooth out the engine. I think it changes the fireing order so it sounds like 5.0 ford or 351W ford.
Because 5&7 are next to each other in the firing order,the is the chance of the intake not having enough time to fill the cylinders,also more heat in that corner because of the exhaust also being the same.This is not true of IR like 4 webers or IR fuel injection.18436572 Some report large gains 30 hp,some very little 6hp
Finally Cheby catchs up and realises that FORD RULES Essentially its the same firing order as the "Y" Block Ford engines. As I understand the theory behind it ,it allows for better exhaust scavenging thus more H.P. Oldmics
You beat me to it...I was told it wouldnt matter in our sprint car stuff since the injectors had their own air source per cylinder. Nascar boys also say the big lift/big overlap in their valve trains makes the problem more obvious. I wont even get into the scavenging exhaust part of their theory. Way too complicated.
It's mostly for durability and also helps maintain consistant tuning in extreme performance conditions. "GM adopted a special firing order in its LS engine series (Gen 3 and 4), which feature a 4/7 and 2/3 swap for the same reasons...to smooth out the harmonics in the pursuit of greater engine durability and to potentially generate more power. Power gain potential aside, the primary reason to address (and alter) cylinder firing order is to achieve a smoother-running engine (a smoother, more lineal acceleration ramp), with less harmonic effect (and crank deflection) on the crankshaft and its main bearings." "According to engine builders at Reher-Morrison, Pro Stock drag engines typically take advantage of a 4/7 swap. Swapping cylinders 4 and 7 in the firing order eliminates the fuel distribution and heat problems caused by cylinders 5 and 7 firing in succession. With the revised firing order, the two end cylinders dont have to fight for fuel from the manifold plenum. The result, in many cases, is a measurable power increase (typically 8 to 10 hp) and a smoother, cooler-running engine." - those HP numbers are on a 1300 HP engine, so by percentage it ain't much.
I don't doubt the firing order change has the advantages mentioned...but...When the 66 396 ,325 and 360 hp engines came out all the magazines were impressed with what they called turbine like smoothness compared to the other Muscle cars.I noticed this too back then.BBC firing order is 18436572 Some tuners also mention possible ignition cross firing between 5 and 7.
the car i work on , has a different cam setup , the firing order is 18724365 instead of the normal 18436572 order . on a nitrous motor , it helps keep the cylinders cooler.
The 4/7 swap does make the firing order the same as a Ford flathead or Y-block. Interesting to note, though, that when Ford went from the Y-block to the FE, they swapped 2 & 8, giving their engines the Chevy firing order.
sdluck is right on. I run one in my o/t NHRA Stocker drag car. Never ran the coventional way as this is a known positive so I have no back to back testing to report what it is worth. I know many that have dyno and track tested it and it never hurt horsepower. As has been stated, cam selection dictates how much it helps.
Paulie9fingers explained it just right. Big cube engine benefit a little bit. Cam grinders don't push it cause its more hassle for them. They need a diifferent cam core to grind and folks get confused about the firing order. Smokey
If you think about it, it changes the order to fire left, right, left, right, left, right, left, right, instead of L, R, R, L, R, L, L, R. It would smooth out the exhaust and I suppose somebody also made an intake manifold to take smooth out the intake flow.
Thanks for your info! I guess it would increase the horsepower IF: You modify the intake manifold too!
If I recall correctly didn't NASCAR experiment a few years ago with firing orders and header crossovers and it made the engines, and hence the cars sound like Indy cars? It was a great sound, but really seemed out of place given what we were use to hearing.
In response to a letter in Car Craft this month, retired GM engineer Don Webb notes that the reason for changing the firing order for the LS series motors was "minimizing crankshaft torsionals".
ive heard some race tracks depending on class dont allow these cam/motors with the altered firing order because its not factory firing order
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