When does a firing order swap on a SBC become an advantage??

[CSH]

I know on some big HP SBC they do a firing order swap. Is there a point where you decide when it will work? Why does it work? Is it a rpm related choice? Is it a better pulse through the manifold(sorry may not be good choice of words) What cylinders? Interested to know on 580 HP engine is there a gain to be made??

Is there a good book you guys know of to read for this info?

Thanks..

[Ken0069]

I know on some big HP SBC they do a firing order swap. Is there a point where you decide when it will work? Why does it work? Is it a rpm related choice? Is it a better pulse through the manifold(sorry may not be good choice of words) What cylinders? Interested to know on 580 HP engine is there a gain to be made??

Is there a good book you guys know of to read for this info?

Thanks..

There has been some discussion on this here in the past and on another board that I run. Seems as though there are only marginal gains as far as HP is concerned and on my new 18* 434 SBC I had decided NOT to order one. However, the guy that does my machine work says they are worth 3 to 5 HP on engines that turn up over 7k RPM plus they make the engine smoother as the harmonics are supossidly different with the 4/7 swap. There is also a 2/3 swap (I think) but I have NO info on that one.

Anywho, I was told by some of those in the know that if money wasn't an object to go with the 4/7 and if money was tight, not to bother but I found out that Comp only charges $40 for this option so I went ahead and got that swap for this new engine. With me it wasn't the HP gain as much as the advantage of less harmonics in the upper RPM range. I hear they idle better also.

I'm surprised no one has commented on this? Maybe you could do a "search" and find some of that other info that has been posted. Drop the "Search" console down and type in 4/7 swap in the keyword slot and see what you come up with.

[CSH]

Thanks Ken for your reply I would have thought a lot of answers and comments would have been posted on this topic. Thanks again.

[bigjoe1]

I have used and sold about 10 or 15 of these 4 and 7 swap cams for Chevys. They all run real good but, I have never done a back to back test.The 4 and 7 swap makes it the same as the Windsor Ford. Go figure. JOE SHERMAN RACING

[Lem Evans]

I'm thinking that the 4/7 deal is the flathead Ford order and the new LS-6 sbc is the one with the 351W/5.0 order .

[Ed-vancedEngines]

Ok,
WE have all about beat this subject to death in this forum in other threads and also in other forums.

What most people do not mention in their explanations of it being like making the Chevy into a Ford firing order is this;
Yes' The number sequence is the same, however it is far from the same firing order as the Ford. Ford numbers their V-8 cylinders as right side 1234 front to rear & left side of 5678 front to rear. Chevy numbers their cylinders as left side of 1357 front to rear and right side of 2468 front to rear. I know that we all know this, but this difference seems to escape the explanations of making the Chevy firing orders to be the same as the Fords.

IN a way I am like Big Joe in this, just have not done as many. I do like the swap cam concept. To me it makes perfect sense, but I have never been able to do any back to back testing or to test them vs other designs or the regular firing orders in the same exact combinations.

I am a firm believer that it may actually show a slight power increase in some combinations, and may not in others. Different intake mainfold configurations, different induction designs, different header designs, can all perform different depending on the rest of the supporting cast of all other components.

Just becuase a certain Cam design maybe be all the current rage in Pro Stock and helps them, does not necessarily mean it will be the all time super star in designs in other performance apppications.

The C Swap (4/7-2/3) (Same as LS 1) cams are harder to get because not very many cam core providors have them available. I know of only two and both of those also grind their own cams.

The 4/7 swap cams is avaliable now through just about everyone. Back in 1984 when an old friend of mine was doing the R&D and developed it into working engines, after much experimenting with all other possible swaps of swapping only two cylinders, NHRA would not permit it to be used in Pro Stock, at that time. Now it is very common.

IT may be now too common, because it gives the idea when people read about it and talk with cam companies, you get the impression that the old style and original Chevy Firing order cams are not as good. Last year in Texas Outlaw 10.5 w racing, the second quickest car in the state was still using a 4.840 bore spacing block (out dated by the 4.900 and 5.00 and 5.20) And in that 765 cu in engine was a standard Chevy firing order cam.

I do like the C swap cams 4/7-2/3 firing order in sheetmetal tunnel ram all out applications, because I theorize it to be putting less stress of torsional harmonics on the crankshaft. It cost lots more money.

I can see that changing the Chevy firing order to be a 4/7 swap can possibly aid in rear cylinder cooling becuase of the 5 & & cylinders firing in sequence, however it moves the cylinders firing in sequence to be the 2 & 4 which is in the beginning of the coolnat fow path, where the water is coming into the engine and has not yet collected heat from the cylinders. Likewise as far as coolling goes swapping to the 4/7 - 2/3 will make the 1 & 3 cylinders to fire in sequence which can be at the beginning of the water flow pattern if utilizing a reverse flow colling. OR at the end of the coolant flow pattern if as a regular flow pattern.

I have seen cam companies and also seen in some places where the merits of swapping to a 4/7 swap gives a better or more equal fuel distribution. I can not agree with this. All swapping the 4/7 does is to move the fuel distribution problem away from 5-7 and move the same problem to the 2-4.

I have looked thoroughly at trying to figure some way in the V-8 Chevy to avoid having two adjacent cylinders firing in sequence and I am convinced there is none. I hope someone were to prove me wrong. I am also convinced that two adjacent cylinders with a plenum style manifold will surely interfer with air and fuel flow in the last one in that sequence. While I do not own any high dollar testing equipment to verify my theory in this, I do own a thinking cap and do ocassionally use it.

Ed

[MadBill]

Re your insightful comments, Ed:
o There are available firing orders for the existing SBC/BBC crank geometry which do not result in sequential firing of adjacent cylinders, but AFAIK, they end up firing all 4 cylinders on each bank in 90° sequence. Darrin M. mentioned that they had tested with same in a thread here about a month ago. Intriguing, but a little scary re balance, vibration, etc., especially for anything running at WOT for more than 7 seconds or so...

o Re plenums and 'charge robbing', about 20 years ago I worked on an EFI natural gas 4 cylinder engine with Rochester Products Division of GM. We built a small diameter/volume (for, we hoped, good transient response) log plenum, long-runner manifold with a TBI at the front, and motored the engine on an electric dyno to look at 'cranking pressures' through the rev range as an indicator of V.E.s. The tuning was very evident (up to 15% gain) at the two expected RPMs, but so was a 2.0% per cylinder pressure drop-off from the #1 to the #4 cylinder! Obviously even with considerably larger plenums this could be one of the mechanisms affecting plenum size experimentation.
When I eventually get my 496" semi-nostalgia cross-ram aluminum BBC on the pump, I intend to plumb a digital or sensitive analog gauge (say a 30 "H2O Magnahelic) between the runners in question and see what shows up. A later planned swap to front-mounted TBs and EFI may further exacerbate the #7 starvation situation, unless I use a 7/4 swap cam to put the relevant cylinder immediately adjacent to the TB...

[Lem Evans]

We have a lot to agree on...
1 - subject has been "beat to death"
2 - manifolding determines benefit relevant to HP .
IMO....the most advantage of the "swap" sequence may be crankshaft life with high Hp and/or long stroke shafts . The "flats" only had about one main bearing so things needed to be good Switching from the Ford to Chevy format makes my head hurt.....but... it seems that what the "flat" does is fire the four corner cylinders and then fire the four center cylinders . Makes me wonder what dumb Ford engineer thought it should change...in that..I do not see the down side of the "old" design .

[rmcomprandy]

Just information -
the production Chevy firing order, (except LS engines), and production Ford firing order, (except 351W & H.O. 302), is EXACTLY the same, even though the numberins ARE different.

The firing order is determined by positioning within the V-8 engine and NOTHING to do with the numbering except as a reference.

The 4/7 swap Chevy IS the same as a 2/8 swap on a FORD and IS the old FLATHEAD firing order.

The 351W & H.O. 302 firing order is different and is the same as an LS Chevrolet engine; 3/5 & 7/4 swaps with a Ford sequence and 4/7 & 2/3 swaps with an LS Chevy sequence. They ARE exactly the same cylinder firing order. Manufacturer numbering has nothing to do with their comparative locations within the engine.

Argue forever whether they have any advantage or not but, at least know what you're arguing about.

Someday I'll even learn how to spell, lol.

[Jay Allen]

Chevrolet *A* Fire 4/7 swap is the same as the Ford Pro-Stock firing order.

Chevrolet *C* fire 4/7 & 2/3 is the same as a 351W/302 HO Ford. This is also the LS-1 firing order.

Ed it right. There is no power per say to be had. But in an all out endurance application, I can see the "C" fire being easier on the crankshaft. But if you are really looking for power, this would be far down on my list.

Good Luck!

[EngineTech1]

I guess you guys didn't see the test they did today on whichever show it was on Spike TV, Horsepower TV or something like that. They swapped cams of the same spec in a SBC to a 4/7 swap and picked up 30hp. What do you think of that???

[Lem Evans]

I guess you guys didn't see the test they did today on whichever show it was on Spike TV, Horsepower TV or something like that. They swapped cams of the same spec in a SBC to a 4/7 swap and picked up 30hp. What do you think of that???

I think it was good "TV" or magic manifolding

[Jay Allen]

Well if it works on spike tv, then we should all do it! LOL

How's it going Dano? Congratulations are in order! BTW, a pair of heads you did a while back came through these parts, very nice work! Nice job!

[Cobra]

The 4/7 swap may be more related to other benefits than horsepower increases! Decreased crankshaft deflection, a smoother torque curve, and gains in acceleration are significant, independent of any horsepower gain! As for this swap being reported to work better in high RPM engines, is this true or is there a lack of testing on lower RPM engines?

[MadBill]

Well, GM made the change on the Gen III SB engine, which by volume mostly goes in trucks, for reasons of improved main bearing load distribution and then followed suit on the Gen V (?) BB (100% truck apps.), so RPM wasn't a big factor for them...