How much better are the Aussie heads for the 351C vs. the 4v vs the 2v? Im looking into purchasing a set of Aussie's but wonder if there are better out there. Like the aluminum heads etc.... any info would be greatly apprecieated. Im not made of money thats for sure and don't want to make a bad purchase because of lack of info.
If you are not building a dragstrip or opentrack car, then the Cleveland 4-V heads will not perform as well on the street as the Australian 302 heads. The 4-V Cleveland heads need a lot of engine RPMs to really make use of and support the high flow requirements of the large 4-V head's intake and exhaust ports and its very large valves.
The "Clevor" swap was popular prior to the early '90s --mainly because, other than 289 HiPo heads [nearly inpossible to find, and expensive if you did], or '69-'76 351W heads, there were really no performance heads available for small-block Fords before that time.
With the advent of the 5.0L Mustangs in '87 though, the performance market opened up to many manufacturers producing very high performance heads --both in cast iron and alumium, for the small-block Ford crowd. At that point, the Clevor swap or the use of [old] factory performance heads sort of faded away.
The stock 351 Cleveland 2-V heads have 76.2cc combustion chambers with 2.04" I and 1.66" E valves. 351C 4-V [and '70 Boss 302] heads have 62.8cc's with 2.19" I, 1.71 E [2.23" I, 1.71" E for '69 Boss 302 heads].
The Australian 302 Cleveland 2-V heads have 62cc quenched [closed] combustion chambers, no smog bumps in the exhaust ports and the intake ports are 10% larger than the American 351 Cleveland 2-V heads.
There is no "2" or "4" cast into the corners of the Aussie heads [near the valve covers] like you'll find on the 351C 2- or 4-V heads.
There is, however, one identifier on the Aussie heads that you won't find on the 351C heads; There is a casting alpha-numeric under the #2 cylinder's exhaust port on the head that should read, "AR-D1AE". The "AR" is exclusive to the Aussie head and is not found on the U.S.-produced Cleveland 2-V heads.
My current vehicles:
1968 Mustang notchback
1990 Mustang GT
2003 Mach 1
1969 F-100 short bed Ranger pickup
Back in the 80's the Aussie clev heads got a lot of press in car mags as if they were best thing since sliced bread. They offer only 1 real advantage over the U.S. 2V heads, a small quench style combustion chamber. As popular as Clevor conversions were at the time, the biggest complaint about using 2V heads was the detonation prone chambers made it hard to build in any decent amount of compression and run it on pump gas. As far as port flow numbers, they are very similar to U.S. 2V heads, which isn't all that bad for a street driven 351. They are about the same as the low end of aftermarket aluminum windsor heads.
For what the castings cost, plus the work to rebuild them for a performance engine, the benefit of the quench chambers over U.S. 2V heads is questionable and certainly makes the use of 4V quench heads a more realistic option. Contrary to wide spread opinion, the 4V heads can work just fine on the street and you don't have to rev it to 7000 rpm either. The key is proper cam selection and adequate compression for the cam. Just look at the '71 Boss, the fastest 1/4 mile of any of the Boss's and the main difference between that and other 351C's was the solid lifter cam plus 11:1 compression. The '70 4V was no slouch either, and both of these engines used 4V quench heads that had the same port and valve sizes. Later 4V engines that had lower compression for emmissions regs are IMO the main reason the 4V heads have the poor rep for street performance that they do. If you really want to improve performance over the U.S. 2V heads and the 4V seem like too much port size for your plans, go with one of the aftermarket C heads like the CHI 3V.
"It's not having what you want, it's wanting what you've got!" - Sheryl Crow
Also, keep in mind that the intake ports of Clevelands are about half an inch longer than those on Windsor heads (about 5.5" vs. 5"). That means that a 225 cc 4V Cleveland has a similar port cross area (which is what's important, not the volume) as a 205 cc Windsor head, which is a little big, but not huge for a street driven 351 CID engine.
2.19" valves are not all that big for a 351 CID engine either. They're about the same as 1.88" valves in a 302 (per cubic inch of displacement).
If you scale down both the valves and the ports, then a 351C 4V heads on a 351 becomes the equivalent of a 175 cc Windsor head with 1.88" valves on a 302.
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