The 4/7 Swap

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   I was watching Horsepower TV on Spike the other day (a typical winter Sunday morning activity, nursing a hangover with black coffee). They were working on a plain-Jane Chevy 350 SB which they would use to test various bolt-on parts to see what their dollar/horsepower gain was on their new dyno. 

One thing they  tried was a 4/7 swap cam, which  takes two paired cylinders (both in the same "stroke" position, but one doing an exhaust stroke and one doing a compression stroke") and switches the valve action on those cylinders so they do the opposite of whatever they were doing before on the same stroke. Then they flipped the plug wires on the two cylinders too. I can't recall what the HP gain was but it was enough to get my attention. Maybe 9-20hp? Did anyone see the episode?

I thought it would be a neat topic to bring up with all the gearheads on this forum. Just to chat about if nothing else. Would something like this benefit a SB Ford or is this only for Chevy engines because of some specific firing pattern they have?

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M3Fan Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: October-22-2005 Location: United States Status: Offline Points: 3185	Post Options Post Reply Quote M3Fan Report Post Thanks(0) Quote Reply Topic: The 4/7 Swap Posted: January-18-2007 at 1:30pm
	I was watching Horsepower TV on Spike the other day (a typical winter Sunday morning activity, nursing a hangover with black coffee). They were working on a plain-Jane Chevy 350 SB which they would use to test various bolt-on parts to see what their dollar/horsepower gain was on their new dyno. One thing they tried was a 4/7 swap cam, which takes two paired cylinders (both in the same "stroke" position, but one doing an exhaust stroke and one doing a compression stroke") and switches the valve action on those cylinders so they do the opposite of whatever they were doing before on the same stroke. Then they flipped the plug wires on the two cylinders too. I can't recall what the HP gain was but it was enough to get my attention. Maybe 9-20hp? Did anyone see the episode? I thought it would be a neat topic to bring up with all the gearheads on this forum. Just to chat about if nothing else. Would something like this benefit a SB Ford or is this only for Chevy engines because of some specific firing pattern they have?

eric lavine Members Profile Send Private Message Find Members Posts Visit Members Homepage Add to Buddy List Grand Poobah Joined: August-13-2006 Location: United States Status: Offline Points: 13413	Post Options Post Reply Quote eric lavine Report Post Thanks(0) Quote Reply Posted: January-18-2007 at 4:54pm
	Ive read that or seen the same show and I think you had a different cam and then of coarse a different firing order,it was pretty cool
	"the things you own will start to own you"

Munday Members Profile Send Private Message Find Members Posts Add to Buddy List Gold Member Joined: August-17-2004 Location: United States Status: Offline Points: 538	Post Options Post Reply Quote Munday Report Post Thanks(0) Quote Reply Posted: January-18-2007 at 6:07pm
	Fan this is fairly common on fords,3o2 and 351 came with different firing order,some think the order effects the cooling of the motor also.I've also seen cams that have you fire both up cyl. at the same time,this is more a circle dirt track deal. Munday

backfoot100 Members Profile Send Private Message Find Members Posts Add to Buddy List Platinum Member Joined: January-03-2007 Location: United States Status: Offline Points: 1897	Post Options Post Reply Quote backfoot100 Report Post Thanks(0) Quote Reply Posted: January-18-2007 at 10:15pm
	I haven't seen or heard anything about this before but it could be very interesting. The firing order on the 350 is 1-8-4-3-6-5-7-2. There is speculation that the 5-7 firing can be detrimental to performance because the two cylinders are right next to each other on the same bank of cylinders and firing just milliseconds apart from each other. The only problem that I see with this is that the same thing would then happen on the 4-2 cylinders on the opposite bank. Maybe then it has something to do with the physics of the direction of rotation of the engine that is increasing the HP when the firing order is changed in this particular instance.

The 4/7 Swap

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