N352 Heads & N351 Heads - Cast-Iron Confusion

Who needs soap operas when we’ve got racing? Mostly, the wild rumors, back-room dealings, and unsubstantiated gossip are confined to the team level at the tracks—who’s running a ported head, who’s doctoring their tires, who has the tech inspector in their pocket. Recently, through a mixture of miscommunication and product evolution, a manufacturer—Ford—became a prime player in the NASCAR Late Model rumor mill.

Although it’s the top attraction at many Saturday-night tracks, NASCAR Late Model Stock racing is not intended to be a professional-level class. The rules are designed not only to provide fierce door-to-door competition, but also to keep costs affordable for the regular guy. A big part of keeping the costs of racing down is consistency in the rule book. Even the slightest change can lead to competitors shelling out for a complete engine rebuild, chasing the absolute performance limits that the latest change will allow. The way Jerry Cook, NASCAR’s main decision maker for the Weekly Racing Series, sees it, as long as the competition is tight and every make of car has an equal chance of winning, there is no need to change a thing.

Running Change, or Redesign?

That’s why more than a few eyebrows were raised before the start of the ’02 season when Ford unveiled an improved cast-iron head and called it a replacement for the venerable N351—the only head legal for Ford engines in NASCAR’s Late Model Stock division. The head was actually considered an update, or running change, to the N351, so the part number would not change. Apparently, the original tooling for producing the heads had completely worn out, and Ford engineers had taken advantage of the opportunity to update the head while bringing the new tooling online.

Unfortunately (or maybe, fortunately if you are a Chevy racer), Cook and NASCAR quickly rejected the head on the grounds that it would upset the delicate competitive balance in the series. “With the Late Models, we’ve got the Dodges, Chevrolets, and Fords, and there’s one head for each model,” explains Cook of his decision not to approve the new head. “We saw no reason to change anything in (Ford’s) cylinder head from a competition standpoint. The cars are competitively pretty equal from what I’ve seen since we’ve gotten everybody down to just one head (each). When Ford came through with these modifications, it was simply something that we weren’t going to approve.”

This is the point where the rumors started flying. Some said the cost of retooling for a set of head castings was simply too expensive and Ford would draw a line in the sand: Either NASCAR would approve the new head or the Blue Oval would pull its support for racers in NASCAR’s Weekly Racing Series. Others theorized Ford would begin producing the old-style heads again, but it would be several months to a year before they would reappear on the market. In the meantime, a suitable (read: legal) head would become hard to find.

Thankfully, none of the rumors proved true. Ford gave the updated heads their own part number (N352), and pledged to continue producing the N351. As far as Ford and NASCAR are concerned, everything is—and always has been—just hunky-dory. But the solution of producing both versions of the head means that racers in a series not sanctioned by NASCAR but still required to use a cast-iron head may have a brand-new option open to them. To find out exactly what the difference is between the N351 and the N352 we traveled to Raceparts Distribution Incorporated in Cornelius, North Carolina, to have a look for ourselves.

Initial Inspection At first glance the only difference between the two heads is a significant chunk of the N352 has been carved away on top of the head to lighten the load. “The N351 head was originally designed and produced with a lot of extra material so engine builders could raise the intake ports,” explains Preston Miller, president of RDI. “Of course, there aren’t a lot of series out there using cast-iron heads that allow porting. So the Ford engineers simply went in and took that extra material away and left the ports where they were. There’s about 7/8-inch of material removed at the top of the heads above the intake ports, and that saves about four pounds on each head.”

Otherwise, the heads are almost identical externally. Without the part number below the exhaust ports, the 351 and 352 heads would be nearly impossible to differentiate with the valve covers bolted on. (If you are thinking ahead, the part numbers on the heads are raised, not stamped, and would be extremely difficult to alter.) The rest of the changes are internal, and most address dependability issues, not power.

Internal Issues

Internally, the N352 head features more support posts inside the water jacket to eliminate any chance of cracking around the combustion chambers when the head has been decked. The intake manifold bolt holes have also been reinforced and raised on an angle so that more of the stud’s threads will make contact with the head. The combustion chamber hasn’t been changed, but there does seem to be a little bit of material added between the valve guide and the back side of the intake runner bowl. Miller believes this is so the heads will more easily pass a ball test. Finally, there’s a little extra machine work done around the pushrod holes to open up those clearances. Nothing major here, and certainly not enough to warrant a rejection by NASCAR—it’s the modifications to the intake and exhaust ports that raised Cook’s eyebrows.

“Although the changes are so slight it’s difficult to see, both the intake and exhaust ports have been changed on the N352 head,” Miller explains. “Overall, both the ports are slightly smaller to better match the two-barrel carburetors used in Late Model Stock and increase the port velocities. If you look at the exhaust side, you can see quite a bit of difference in the shape of the ports, but there’s also a difference in area, too. The biggest exhaust header pipes they let you run in Late Model Stock are 1-5/8 inches (od), which is smaller than those exhaust ports. These smaller ports pose (of the N352) less of a restriction for the exhaust gasses as they exit the port and try to enter the header pipe. If you are in a class that is allowed to run 1-¾- or 1-7/8-inch exhaust pipes, the N351 head has a good exhaust port, but for 1-5/8 pipes the N352 head matches up better.”

The good news is Ford is adapting the durability improvements of the N352 head into future—and NASCAR approved—updates of the N351. Although the updated N351 head wasn’t available at the time of this writing, it should be hitting the market by the time you read this.

For his part, Miller says he sees the wisdom in NASCAR’s decision to disallow only the changes that would have given the Fords a competitive advantage. “It’s just a financial hardship. If a change is allowed that will make just enough difference that it will provide an advantage, people racing Fords will feel that they have to buy the heads. Then to make the most of them, a cam change and other adjustments are required, which will have people getting entirely new engines built. Next thing you know, the Chevy guys will be screaming for a change so they can keep up. It just opens a big can of worms that would have zero change in the quality of the racing itself. All you are asking the people to do is spend a lot of money, but the end result is that everybody goes up one notch equipment-wise and the quality of competition doesn’t change one bit.

“The bottom line is if you get something that works, don’t change it,” Miller notes. “If somebody has the money and ambition to race the absolute cutting-edge technology, they need to move up to another series.”