Characteristics and Limitations

The six is a reasonably light and compact engine compared to an iron V8, and can provide reasonable performance in a lightish car like an early Holden or Torana. Having said that, the red motor wasn't a particularly advanced design even when it was first released and it's ultimately quite limited in output, mainly by the cylinder head design. As we will see later, the cylinder head is the key to making power with the six. The other limiting factor with regard to power production is block durability. While they can handle as much as double the original horsepower without problems, further increases frequently result in a cracked or broken block, particularly when high rpms are used. Blown engines can easily make more power than the engine can reliably withstand - with enough boost power levels of over 600hp are possible if not sustainable. Turbocharging works exceptionally well on these engines; making 400 odd hp is easy, not only that but the power is made at rpm levels below those that cause block problems.

Chevy Six Engine
Chev six from the mid 50s. Chev influence on Holdens design is obvious.

Naturally aspirated, don't expect to get much more than around 220 - 230hp for a streetable engine. This mightn't sound like a lot compared to todays injected V8s, but it's not unusual for a light street car with a hot six to get into the 13s without resorting to nitrous. With exceptionally light cars like the early Toranas 12s are fairly easily achievable but the streetability of such an engine will be marginal. Its possible - if not easy - to get 300+ hp from these engines using radically ported 9 port heads and matching cam profiles, but at these levels the powerband is painfully narrow and the engine will be starting to become fragile. If you really do need much more than say 240hp from a Holden 6, it may prove more practical and cheaper in the long run to use a Jzed/Duggan style head. Not only will you get the peak horsepower, you'll get a wider powerband.

Why - Or Why Not - A Holden Six

I'll be blunt. If performance is the primary consideration and you don't have to use a particular engine to comply with rules then there are much better choices than the old Holden. Modern engines from Nissan or Toyota for example will out perform the old Aussie six by a massive margin - you won't even be in the race. Another older engine with a fair bit more potential - mainly due to it's bigger displacement - is the old Hemi six. All of these engines will outperform the Holden and almost certainly cost less on a horsepower per dollar basis. If however, you don't need to make a million horsepower, or you'll be competing against similar engines, or perhaps you want to build up a cool old-school street car then the Holden six might do the job nicely. Just don't kid yourself that you're going to show these young blokes with their turboed 2JZs a thing or two with the old six - you'll only embarass yourself.