The Starting Point
The standard Ferrari 308 2-valve models (1980-82) with Bosch CIS fuel injection represented the nadir of the 308 series performance.. In 1983, Ferrari came out with the QV model (4-valve). The power of this engine brought the performance back up to the levels of 1975 (GT4); so much for progress! The only real performance increase came in 1985 when Ferrari released the 328 model displacing 3.2L. Finally, more power and torque. Our project goal was to take a 308 QV engine the next step beyond the 328. It is well know that the chassis of the 308 is a good platform for more power. The concept with this project was to make more power by increasing the displacement, but retaining the CIS fuel injection/Marelli ignition.
The Basic Engine Architecture
The seed for the idea of increasing the displacement of the 308 engine beyond 3.2L was planted at the time when we were re-building 360 GT Michelotto racing engines for one of our racing clients. We noticed that the bore centers and the main bearing saddle spacing and size for the 360 engine and the 308 engine were the same. While the 360 85mm steel, thin-wall, Nikasil coated liners wouldn't drop into the 308, we visualized a re-dimensioned 360-style sleeve working fine, as Ferrari had already done the bulk of the engineering. Cooling wouldn't be a problem nor would durability. It was obvious that while the crank from the 360/355/348 would physically fit, some additional modification would be necessary to make it work. The flywheel/crank bolt pattern was different, the crank nose too long and a few other issues would have to be addressed to work in the 308 block. In building this particular engine, the displacement of 3.5L was determined by using the Razzo Rosso 85mm liners and a 355 crank. (Since then we have designed, developed and manufactured a new crankshaft that needs no modifications. It's 10 pounds lighter and works with the original flywheel.) We already had tested the 85mm Razzo Rosso liners in an F40 project and they survived almost 30 psi of boost with no problems. Using them in this 308 engine would work. These liners, made to F-1 specifications, are works of art. Razzo Rosso can also provide an aluminum flywheel (see parts price list) with the revised bolt pattern and ignition pick-ups that would be compatible with the stock clutch bolt pattern.
New forged Razzo Rosso 10.5:1 compression pistons with custom valve reliefs and ARP/Razzo Rosso rod bolts/nuts finished off the special lower end parts. Some may ask why we didn't also use the Ferrari 355/360 titanium connecting rods; again it was a cost vs. performance equation that didn't add-up. Besides, the stock connecting rod was appropriate for this application since the old bolts/nuts were going to be upgraded. Ok, now the basics were set; a 85mm bore and a 77mm stroke – the same as a 355 – would yield 3,493cc. This increase in displacement would be a great start to the next stage of the project; adapting it to CIS injection and achieving our client's goal of 300 HP.
CIS Injection/Marelli Ignition Adaptation
Rather than trying to re-engineer these two systems to fit some "hot rod" engine characteristics that they weren't designed to handle, the goal was to modify the engine to simultaneously make more power, but stay with in the design parameters of the existing engine controls. This meant keeping the engine RPM range as standard. Peak torque and horsepower had to stay at approximately the same RPMs as original. This is because the CIS injection fuel curve is basically fixed. The Marelli ignition is a sealed unit and our only means of adjustment was repositioning the flywheel pick-ups for more or less total advance. The good news is that increasing displacement is just the ticket for making more power with a CIS injected engine. Trying to make power by using the traditional methods of more radical cam timing to crank up the RPM range weren't going to work here. CIS injection develops idle stability issues when the cam overlap is increased too much. We're not talking small issues either as some engines we have seen can have wildly oscillating idle speeds due to the "platen" fluttering out of control. The larger displacement engine would still need to breathe better so cylinder head porting, larger valves and optimized camshaft profiles would be essential modifications; the question was…how far do we go? In our experience, only engine simulation could give us the precise answers we needed.
Because Carobu has a long history of engine modeling with Ferrari engines, some data for our model already existed. While this undoubtedly helped speed things along, first hand measurement of critical areas is important. Equally critical is the knowledge and experience gained from doing thousands of dyno simulation runs on Ferrari engines. Engine simulation accuracy is dependent on precise measurement (input), program sophistication (calculation) and operator skill/experience to get good output. Our accuracy has been proven many times over by the dyno test results. The typical development loop consists of baseline dyno testing, measurement, simulation/model modification, parts ordering/fabricating, engine building and, finally, dyno testing. This loop is performed repetitively by top engine developers in F-1, NASCAR, etc. to constantly improve engine performance. We, however, were only going to get one shot at this loop so a lot of careful planning and Ferrari engine experience was needed to get it right. The initial modeling showed that (as expected) the intake flow needed to be improved. As a side note, the exhaust headers would be replaced with a new stainless steel set. After some time spent running through various scenarios on the model, it was determined that we needed to get the heads ported to see how much help we could get from the intake side.
