Leon from A1 Hi Performance in Myaree, created an enormous amount of interest with Ford fans several years ago when he built the awesome 600hp Cat Killer smallblock for his TE50. This time it is Holden fans that will be on the receiving end of a remarkable combination.

As the waves of LSX technology continue to sweep over the entire performance market, there are those who still prefer the sound, reliability, torque and power of the General’s original gem, the 308. After the advent of fuel injection and the shift up to 355 cubes the Holden mill was destined to be a classic and literally thousands of performance 355s have been built across the country. On average, a well-built and modified 355 will produce between 450 and 470hp and generate around 400 lbs/ft of torque. In most cases it is that mob of torque hitting low in the rev range that puts a smile on the owner’s face and sets these engines apart from many of their late model cousins.

However, when Leon puts his thinking cap on and a wry smile creeps across his face then you know something is up. His favourite catch cry, “A little knowledge is a dangerous thing,” has rung through on many occasions so we were somewhat prepared this time. His aim with this project was to take a 355 Holden engine, develop a set of Yella Terra -9 cylinder heads for it, fill it with some good gear and then incorporate a few of the ideas he had been concocting over the last couple of months. Ultimately, Leon wanted to establish a new benchmark for both power and torque from the 355 by applying the same principles that held true in the Ford smallblock years before.

To hear this engine idle you could be excused for thinking that very little had been done to it at all. Just like the big cube Pro Stock engines of the NHRA it has a very low and smooth idle that belies its true potential. Crack the throttle though and the response is violent and explosive; this motor snaps like a two stroke motorbike and has an exhaust note all of its own. The short motor consists of a Crankshaft Rebuilders crankshaft, Scat conrods and SRP forged flat top pistons – solid but far from radical.

The trick to this engine is that there are lots of tricks! Each area has been analysed and then modelled using sophisticated software to ensure that the whole is greater than the sum of its individual parts – perfect engine synergy. The first case in point is the intake manifold. Forty hours were spent re-shaping and sizing the plenum an intake runners of the Torque Power manifold, so that their lengths, volumes and cross-sectional areas were perfect to suit the intended rpm range of the engine. Initially, the runners and plenum of this particular manifold were smaller than the engine required but that suited Leon as he was able to re-shape them accordingly. In standard form the manifold runners flowed 250cfm at 28-inches of water but after porting that figure was up to just over 290cfm – enough to make use of cylinder heads that flow around 300cfm of air (or approximately 600hp).

This was then matched to a set of carefully prepared Yella Terra -9 aluminium cylinder heads, which were ported to create the required intake charge velocity (through a specific rpm band) rather than simply to make them larger. Leon confessed that there was only mild port work carried out because the heads only needed to flow 300cfm for the job at hand to produce around 600hp and keeping the ports smaller yielded great advantages in the areas of torque generation and throttle response. This same care and attention was applied to the intake and exhaust valve sizing.

Considering that the solid roller camshaft has less duration and overlap than a Crane 288 flat tappet camshaft the ultimate power figure of 550hp is remarkable. Consider also that the engine maintains a steady 14-inches of vacuum at idle (a dead stock engine typically sees 18-inches and an engine running a large overlap camshaft may only see 5 or 6) and you will begin to appreciate just how much of an all-rounder this 355 actually is. With that much vacuum at idle there is no problem running air conditioning or power steering and there is certainly no need for a vacuum tank to operate the power brake booster. On the flip side, Leon has no doubt that using a camshaft with around 10-degrees more duration than the current shaft would net an easy 600hp on dyno. The trade-off would naturally be in the areas of idle quality and a narrower power band. The relatively small camshaft (278 and 284-degrees advertised duration) does allow the engine to build significant cylinder pressure, which, coupled with the static compression ratio of 11.5:1 is responsible for that magnificent torque curve and contributes to the way that the power curve hangs on well past 7000rpm without nosing over. As we have discussed in previous technical articles, it is the area under the curve that counts with camshaft lobes and solid roller profiles, with their steep ramp rates, offer tremendous area within a relatively short duration.

It is important to note that the engine was dyno tested with Ultimate 98 fuel and tolerated 28-degrees of ignition timing without any sign of detonation. For race duties the engine would be run on 101 octane race fuel as the slick tyres would create more load than radial tyres on the street would be capable of producing (see the torque curve for a graphic representation of this!).

On dyno, the motor proved almost impervious to change when it came to extractors. The first pair tried was from Pacemaker and had an interference design culminating in a 3-inch collector that was 18-inches long and backed by a pair of Magnaflow mufflers. Leon mentioned that it was virtually impossible to model an engine with interference style headers because each cylinder is accounted for individually and therefore its exhaust pipe was a given length back to a common collector. Computer modelling does not cater for the scavenging and cross pulsing that occurs when the exhaust gas from another cylinder collides with the cylinder in question. Switching to a set of custom-made four-into-one race collectors only yielded a modest 5hp gain. It seems that the engine’s displacement, relatively mild cam and under 7000 rpm combination did not require these exotic headers.

In terms of efficiency, the dyno soon revealed why this mild-mannered marauder produced around 100hp more than a typical 355. With Brake Specific Fuel Consumption (BSFC) dropping into the 0.39 region at peak torque, this is one very efficient engine. BSFC is a measure of how much fuel the engine requires to develop a given amount of power; the lower this number the less fuel required and the more efficient the engine. A motor running in the .39s (particularly a street motor) is operating very efficiently. Another litmus for efficiency is Volumetric Efficiency, or the amount of air consumed by the engine as a percentage of its displacement. For example, an engine that completely fills its cylinders at a particular point in the rev range would have a VE of 100% at that rpm point. The dyno data reveals that this 355 peaked at 113% VE, which implies a ‘supercharging’ effect due to the timing of various pressure pulses in the intake and exhaust cycles and a carefully matched port design and camshaft lobe profile.

It was interesting to note that the engine was pulling 2-inches of Mercury (Hg) in the manifold at full throttle. That is a vacuum equivalent to 27.2-inches of water and would often indicate that the carburettor is too small for the application. To check this theory, the 750cfm Demon was swapped for an 825cfm carburettor and although this vacuum dropped slightly, there was no difference in measured power.

The hunt for power continues however and the casebook has not been closed on this engine just yet. On the way is a brand new piston package that will accommodate a brand new low drag ring stack to reduce frictional losses in the engine and free-up more power at the flywheel. A new style of crankshaft and a set of super-light titanium conrods should see this engine scream like a Superbike. Leon certainly achieved his primary goal of creating a genuine 550hp from a daily driven 355 and the dyno graph shows just how far ahead this engine is compared to a typical hot 355, in both power and torque. It is the engine’s mild manners that set it even further apart because it is a true street engine that will deliver 10-second performance with ease.

If there is one lesson to be learned from this motor it is that a total understanding of the interrelationship of every component in the engine is essential before a successful combination such as this can be created. The current parts list may not be exotic (although the future list certainly is) but those parts work together in completely calculated harmony. The 355 bar has been raised but it seems that Leon is not content to leave it at that, the new version of this motor should be even more interesting and will apply modern technology and materials to the old 308 workhorse. We can’t wait to hear it fire into life.

Re-printed from Volume 16 Number 1 of Perth Street Car Magazine with permission.