CONVERSION BASICS
Turbocharge your Naturally Aspirated Engine

By Martin Donnon
(c) Copyright Express Publications. Reproduced under expressed permission. No copying permitted

Turbocharging a normally aspirated engine is not a Job for the inexperienced. Thankfully, though, it is getting easier. Turbocharging has always presented its own set of unique challenges to the budding enthusiast. There are all sorts of considerations that need to be taken into account, from the thermodynamic properties of the basic engine structure to the capacity of the fuel system. Back in the good old days, when pushrod sixes with triples ruled the streets, turbocharging was pretty much a hit and miss affair. Even the expert workshops in the pre-Sigma turbo era melted their fair share of otherwise perfectly good engines as soon as they added a turbo. It was a guessing game with very little information around as to 'what worked with what', or even how to go about tuning the engines to make them reliable. It was a brave soul that strapped a turbo onto their Series 1 626 back then.

Much has changed since then. Engines have become much better than they were 20 years ago. Turbochargers have improved dramatically, not only in their overall design, but also in their availability. Most importantly, the controls that keep the engine in one piece - the fuel and ignition systems - have improved from prehistoric carburettors and points-based ignition to mappable, electronically controlled engine management. You don't need to be a rocket scientist to successfully turbocharge an atmo car these days.  Here's our basic guide to building a useable street turbo conversion. ENGINE In the case of a modern and fairly high-tech engine - which is all we are going to deal with in this piece - you need to remember it's always going to handle a well set-up 100 per cent torque (and possibly power) gain. Don't expect too much more reliably, but for most that sort of limit will be enough.

 

Don't think that boost kills engines. It doesn't. Two things do - idiots that rev them too hard and idiots that allow them to detonate. If you're beginning to get the feeling that my opinion of the factory engine is quite high then you're right. Materials and basic component design have improved so much in the latest generation of engines that only the least optimistic would want to pull them down for a strengthen. Still, if you must then so be it. Just remember that, set up correctly and with a realistic power goal, you should never need to go to this level.     With some modern power plants there are perceived problems with static engine compression being too high from the factory for a boost application. You need to remember that, while being a contributing factor, there is more to an engine's detonation propensity than just the compression ratio. The solution to turbocharging a high-compression atmo engine is to run lower boost. Modern cylinder head design and piston crown shape is good enough for most small-capacity 10.0:1 compression engines to get away with up to 0.8bar of boost pressure.

TURBOCHARGER Matching a turbocharger to an engine is quite possible by using a compressor map and performing some basic calculations. This will get you close, but, when it comes to real-world turbocharger performance, there is nothing that will give you the same result as actual experience. The trick is to seek expert advice from someone who knows what works through experience. Don’t muck around with second-hand turbochargers or odd brands. Go straight to the Garrett range of ball-bearing turbochargers and take your pick. There is one for almost every application, so matching isn't really a difficulty. Always choose an exhaust housing size (should you actually have the choice) that is a little small. It's this size that determines how quickly the engine comes onto boost. Too large and you will have a high-boost threshold. Too small and you will have an engine that pulls sharply onto boost, with a strong torque curve. Keep in mind very clearly what sort of result you are looking for. Personally, I like the hit of a big turbocharger. I also don't recommend them for originally non-turbocharged street engines.

 

MANIFOLDS While there is much to be said for the use of tuned-length exhaust manifolds on maximum-output turbo installations, there is also much to be said for simply making something short that will fit into the space available. In some cases, its even possible to place an adapter onto the end of the existing cast factory manifold and mount the turbocharger from this. Just remember that the turbo needs to be mounted fairly high to ensure adequate engine oil drain back to the sump. Space is going to be your biggest restriction, so ensure whatever you have in mind is going to fit. You also need to keep the hot parts of the turbocharger away from temperature-sensitive components (fluid reservoirs, cables etc). Material selection for a remanufactured manifold should be either stainless steel, or thicker and heavier steampipe.

INTERCOOLING Spend as much time determining the best way of intercooling your freshly converted turbo engine as you did in selecting the turbocharger. There are no points to be gained from cutting corners here, with the efficiency of the charge-air cooling system determining the overall success of the installation. Don't even consider a turbocharger conversion these days without an intercooler. It’s simply a waste of time. Spend your turbo money out clubbing instead. Remember that the atmo car's cooling system has been carefully designed to perform under hot conditions without a massive intercooler partially blocking the airflow to the radiator. You must respect that when you size you front-mounted intercooler and do everything you can to keep the airflow from the back of the 'cooler travelling through to the radiator. Ducting the air generally doesn't go astray.

 

Too big an intercooler can cause overheating problems for the engine on hot days. Too small a 'cooler can cause detonation problems for the engine. It's a delicate balancing act. The trick then is to get the most efficient intercooler core for a given size possible. Don't worry too much about the pressure loss, as you aren't or at least shouldn't be trying to run massive boost on a factory engine. Rather, worry about the ability of the core to perform the all-important job of cooling the airflow.   FUEL SYSTEM With up to 100 per cent additional torque available, you will have to change the fuel system. Injectors will be too small and usually the fuel pump as well. Use the rough rule of thumb that you don't want to run more than around 85 per cent injector duty cycle and size your fuel system from there. Our advice is to go the smallest injector upgrade possible that will give you the power you require. Massive injectors do little more than make life difficult for the management system. With fuel pumps it's a matter of bigger being definitely better.

There are many ways to skin the fuel pump cat, with the most common being to install one of the well known and respected Bosch range of pumps. There are many options here, so speak to a competent workshop. Much like the fuel injectors, you don't need to go overboard when sizing the pump. Consider how complicated your new car is and then consider how well aftermarket management is going to control it. Regardless of what you may hear, there are very few aftermarket management systems that can do the jobs of idle control and aircon/power-steering load as well as the factory unit. It doesn't matter whose car, either - that stuff isn't simple to get right. Use the factory computer for what it's best at (all the niceties) and leave the aftermarket computer to simply control the fuel injectors and ignition system. It's a technique called piggybacking, but uses a full management computer rather than an 'interceptor'. This technique can give excellent results, if done by someone that understands what sort of end product they are looking for.

 

In the rare case (these days, anyway) that your management system can be 're-chipped', go this route, and have a competent tuner set it up to take the turbocharger and larger fuel system into account. There aren't too many that can do this sort of work, but if you are close to one, and they are confident of giving you the results, then go for it. That's basically all there is to it. Obviously, there are a lot more parts and complexities involved than those we have mentioned, but these are the tricky ones; the places where it is possible to go off course and give yourself an undesirable result. Keep our advice in mind when modifying your turbo engine and give yourself a really nice, street-drivable result.

 

(c) Copyright Express Publications. Reproduced under expressed permission. No copying permitted