Fiat SOHC Performance Tuning. 1. Introduction. Despite having very humble origins as a power-plant for the 1969 front drive revolution Fiat 128, in my opinion the SOHC is quite a spectacular engine and Aurelio Lampredi's finest mass produced work. Notwithstanding it's 'inferior' design non cross-flow cylinder head the SOHC engine can be made to top the magical 100hp/l figure, which is certainly a benchmark for engines of this era. Aurelio was a very clever man and he intimately understood the relationship between the moving piston and the column of air that follows it, he did after all design some of Ferrari's finest Formula 1 engines of the 50's and 60's, and he used all the lessons learned from that time with Ferrari when he joined Fiat and designed the gem we know as the SOHC (Single OverHead Cam) During the early '70's the SOHC became the engine of choice for many of Italy's finest tuning houses and some managed to develop quite extraordinary power in the engines 8 valve form, topping 100hp/l with mechanical fuel injection. Many tuners also developed cross flow DOHC 16 valve cylinder heads to fit atop the standard block for various racing classes and Hp went even higher. However, this sort of engine build is well beyond the scope of this discussion as I'm not writing about a multi valve, dry sump, mechanical fuel injected, 12000rpm, $30,000 engine that everyone drools over but no one is ever going to build. I figure what people want to know about is what 'works' when you mix and match some of the standard components. Most home tuners want a few tips on how to do the fairly basic task of assembling an engine and while they're at it, improve a few things and get a bit more power out of it in the process. I'm certainly not reinventing the wheel with any of this, achieving a good result is possible by always choosing best practice, understanding some basic tuning theory and following Aurelio's lead. In standard form the SOHC eclipses it's 'big brother' the Fiat twin cam. When you consider that around the same time the 125 special was making 100hp from 1608c, (approx. 62.5hp/litre) with much larger inlet ports / valves and far higher valve area / displacement ratio, the 'inferior' SOHC with it's non cross flow design holds it's own to produce a healthy 75Hp in stock trim from 1290cc's. (58.125hp/litre) The SOHC was also massively over-engineered in it's bottom end, it uses the very same con-rod bolt hardware as it's 1608 counterpart as well as the same sized main and big-end journals. The sohc engine in a form that is true to it's original 1116cc's was still a current production engine in Eastern Europe as late as 2009, albeit with only Euro 1 emissions compliance, that's a production run of over 40 years. Not many mass produced engines can boast that, certainly not Fiat's twin cam. It only lasted through to the mid 1980's before it underwent major design changes (reverse flow) and then ceased to be produced altogether before that decade ended, a lifespan of just 25 years when it was completely redesigned as the 16 valve engine. So what does all of this have to do with building a performance SOHC engine, well not much and also lots. I'm hoping it gives some of the uninitiated readers a glimpse into my love of Lampredi's work, and also my reasons to build heavily modified engines based on a 40+ year old design, that some would term term ancient and obsolete. I think it's far from that. These engines are amazingly strong and capable of a reliable 8000rpm in the longer stroke options and up to 10,000rpm for the short stroke options. 'Ultimate' SOHC. What is the 'ultimate' SOHC engine, using a mix and match of stock components is a question I'm asked very often. The trouble is ultimate can mean different things to different people. Some may want or need to stay with one of the smaller capacity options for licensing, or racing rules, or for the sake of originality. Which capacity combination of the SOHC is the ultimate? Are we purists or are we racers? Maybe the majority of us are somewhere in-between, so where do we take this discussion? I think all X1/9 owners would agree, that the first thing they would want from their X1/9 is a bit more power! It certainly would be nice to match the cars excellent road holding capabilities with a similar level of performance. Being Fiat X1/9 owners would mean having a tight budget and needing to keep a ceiling on the overall cost. It also means retaining the original engine configuration so we can retain the original transmission and drive train. We don't want to re-engineer the car, why would we want to change anything about it. The chassis and the balance is just about perfect from the factory, we all just want a bit more power. As you have a budget it means you can only afford to spend money once to do, or have the job done, properly the first time. If your prepared to put in some effort, a well built "street" SOHC can make an X1/9 quite a quick car. There is no need for engine conversions, no need for 16 valves, no need for a turbocharger if you only need 90 to 100 hp/l at the crank. Any X1/9 owner heading down this path has actually come to their first crossroad. It's the first choice you need to make in our journey along the road to your personal ultimate SOHC, and that's choosing to stick with Lampredi's finest. I'm certainly glad you have as I think too many people consider power-train swaps without really knowing the true potential of this little gem. To me the SOHC engine is the heart and soul of the X1/9 and replacing it - especially for anything non Lampredi - is almost sacrilegious. What is the goal? This is what you need to answer and be realistic about it. An achievable target for your budget, your requirements and expectations, and your level of skill. Any performance improvements to the engine would be expected to achieve OE reliability, provide a measurable improvement and be cost effective. Transmissions don't like hard launches with the sort of power level we can get with carburettors or fuel injection, and turbocharging can double that. A forced induction option would need some thought and money spent in this direction to maintain the OE reliability factor. X1/9 owners are an eclectic mix and you can find a wide variety of different builds across the world by owners with vastly different goals. Collectively we would have covered just about every combination of mix and match including turbocharged variants of the SOHC. I'm going to try to break down and collate information into what I think are some of the achievable levels or stages of improvement. Starting from some bolt on cost effective options that have a measurable effect, up to what I would consider the most reliable way to assemble a 'mix and match motor' using components sourced almost exclusively from the Fiat parts bin. At the end some discussion about engines and output with aftermarket components. Turbo options exist and will be discussed but limited to stock factory components using OE manifolding. Simply getting outright power isn't the goal of this book, it's a cost vs benefit analysis of the components that can be used. To be informative it's going to need to offer some advice on component selection, have some current OE part numbers to help locate useful parts, include some of the aftermarket options and measure their relative cost effectiveness and performance potential compared to the standard components. It also includes some tips on choosing a combination of components that will perform to expectations. To help decide what those expectations are, you need a grounding in some of the basic and slightly more advanced aspects of performance tuning this particular engine type and this is where we are going to start. Disclaimer. For the sake of keeping all the information in one thread, it's going to mean quite a bit of duplication of things I've already posted here on Xweb, and on other forums ... and I'm also going to have to ask everyone to allow me the indulgence of having this thread to myself. If anyone feels this thread deserves another to discuss any possible content inclusion or problems, then please go ahead and start one. It's simply to allow me to edit this and keep it concise, please comment somewhere else. Please remember that this is a live on-line work in progress. This is a first draft, and will be available as a pdf. file when complete. I'm also going to quote other members of this forum directly in this thread, so if you've said or done something and I think it's a good turn of phrase or very clever, I'm going to use it and quote you as a reference. When I use pictures, most of the pictures will be mine that I have taken, but some will have been lifted from around the web. If you see a picture and it's yours and I haven't quoted you as a reference and you would like me to reference you or not use it, PM and it's not a problem.