Choosing The Displacement

If the engine is going into a registered street car, this decision may be already made for you. There are generally restrictions on displacement before engineers approval is required, and this usually means other modifications to brakes and steering for example will also be needed. Also keep in mind that later engines in registered cars usually need to have all the emission controls fitted and working.

Theoretically, maximum horsepower is basically determined by the flow capacity of the engine components, so there shouldn't be a big difference in output between say a 186 and a 202, though the smaller engine will produce its peak power at a higher speed. In practice though, friction losses will be more severe with a smaller, higher revving engine, and the bigger engine will have more area under its torque curve. The bottom line is this: use the biggest capacity you can - the peak horsepower mightn't be much higher than that of a smaller engine but the actual in-car performance will be significantly better. The exception to this is extreme high-rpm engines; a 202 with stock-length rods will be pushing the walls around quite a bit and the block itself will be right on the ragged edge of keeping it all together. For these applications you should consider whether the better structural integrity of a shorter-stroked, thicker walled engine would outweight the lost cubes. If you must run a 202 at very high revs I'd certainly be trying to get a much longer rod in it.

Whatever capacity you choose, for best breathing you should run the largest possible bore size (ie. 186/202) in order to minimise the chamber overhang problem that we'll touch on later. The other side to this is that these engines tend to be quite thin in the walls, so sometimes a small reduction in bore size shows a net benefit if it allows thicker walls to be used. There is another advantage to the 202, and that is the availability of a relatively cheap performance piston/ring/pin package in the ACL Race Series parts, though these are getting hard to come by.

Don't overlook the EFI 202 from the VK Commodore for something to transplant into a daily driver - these are as cheap as dirt and are very close in output to the XU1 - but more civilised.

Stroker motors are fairly popular, and can provide about 235cu in in the most common configuration. Usually they are based on the 221 Ford crank. The original Ford flywheel flange is cut off and the journal turned down. Then the rear journal and flange is cut off a Holden crank and a hole bored into the journal so it can be pressed onto the turned-down Ford rear journal and welded on. The other journals are then ground to suit the Holden rod and main bearings. Provided it's done properly it's quite durable. If you don't like the idea of a welded steel crank new cast units are also available in semi or fully finished form. Slightly modified stock length rods can be used, usually with Ford 250 pistons though others eg. Suburu can be used. A fair bit of work has to be done to make room for the longer crank throw; notches have to be ground into the sides of the crankcase, and the sump needs a bit of hammer work as well. Also the cylinder bottoms need to be relieved a little and the camshaft needs to have some flats ground into it for big-end clearance. Is it worth the extra expense and effort for about 15% more capacity? The extra cubes would certainly improve the performance at lower speeds, and the little Holden has always had a capacity handicap when compared with similar straight sixes from Ford and Chrysler. I tend to steer clear of oddball parts whenever possible, and a stroker crank would add a fair bit of expense. On top of this the rod ratio ends up being awful if you use stock rods and the camshaft relieving weakens it to the point that breakage is likely. But if the maximum possible torque from a low-revving naturally aspirated engine was the goal, I'd certainly consider it. To me though, it would seem better suited for duty as a mildish street engine than a competition engine.

As it turns out, the maximum practical capacity for a highly tuned engine is not much more than 202 cubes. The lack of big-end to camshaft clearance prevents increasing the stroke by much, and reducing the crankpin diameter would only exacerbate the crank flex. Similarly, it's not practical to increase the bore size, even with sleeving. The existing cylinder walls are already borderline too-thin, and the close bore-spacing means that significant bore increases would leave very little head-gasket support between the bores as well as very little room between the outer walls. In short, we're stuck with the small capacity and therefore must concentrate on making the engine live at the very high revs required to make good power.

Twelve Port or Nine Port?

We'll look at cylinder heads more closely later, but generally speaking it's difficult to get much more than 230 - 240hp from the twelve port heads, while the old nine port heads can be made to flow enough to make over 300hp. Despite this the twelve port head is often the best choice, especially for a street driven car, and at power levels within its limitations a 12 port with the right manifolding will outperform the 9 port because of its fatter torque curve. Keep in mind though that the only real benefit of the 12 port is it's ability to make good use of long, individual manifold runners. If you don't intend to use this type of manifold then a 9 port will work just as well.

Which Block?

Again, if it's for a registered car you may not have much choice. A track-only car can make use of a late blue or black block, complete with counterweighted crank and better rods. But if you want to avoid having to use emission controls you'll probably be stuck with an older (HJ or earlier) red block. These can be fitted with the counterweighted cranks with a bit of work, and it's also fairly easy to adapt the later 12 port heads to these blocks. There is nothing special about the old HP blocks, and while the XU1 blocks allegedly are beefier good luck in finding a block or the money to buy one.

Keep in mind that some of these blocks are over 35 years old, and most have been rebored a couple of times. They are also likely to have a fair bit of corrosion in the water jackets, so you might have to check out quite a few before you find a good one. Pull out the water pump and knock out some of the welch plugs so you can get a good look in there. Finding a good 202 or 186 will be the hardest, many of these will already be bored too far to be used for high performance applications.

Cylinder wall thickness is an important consideration with these engines. For very high power levels bore wall stiffness can be marginal, so you need to find a block that will clean up with the smallest possible overbore - certainly no more than 40 thou if you plan to make much power. A standard or very mild street engine could go 60 thou. over if necessary but generally keep the walls as thick as possible - thin walls are risky with high outputs and even if the engine doesn't fail outright the lack of bore stiffness will cost you power.

For a moderately powerful 186 or 202 (say 200hp or less) any of the 3.62" bore blocks will be fine. If however you will be building a very high output engine then the blue 173 is the block to use. These engines have "2.8" cast into the side, and are cored the same as the 202s. Not only that, but they also used the same main journal size as the 202, and that means you can use either the original 2.8 engines crank or the 202 crank. A 2.8 block that has already been bored 0.60" or so will have thicker walls than a "virgin" 202, and this additional bore stiffness will certainly help to increase output and reduce blowby. The biggest problem with these engines is their relative scarcity; they were only produced for a very limited time and many have unfortunately been scrapped. Yet another feature of these engines is the small-chambered 12 port head that was used; these are excellent raw material for a performance engine.