Engine modifications

Probably the most under discussed subject when dealing with supercharging is basic engine modifications and general preparation.

When building a normally aspirated engine, you basically stuff high compression pistons in it, a high lift long duration cam, big valves, ported heads, etc. Then you go out hunting at the track or on the street. When building a turbo motor things are slightly different, but not as much as you might think.

Although turbo and super charged engines don't like or require high compression, there are some changes that you can take advantage of when rebuilding.

Compression and boost

Factory turbo engines are normally between 7:1 and 8.5:1 compression ratio. The compression ratio is the amount that an air and fuel mixture is compressed or pressurized during combustion. In general the higher the compression ratio, the more power is produced during each combustion cycle. The more powerful the explosion is, the more torque is produced as the piston is driven downward.

Static compression is a rating used while testing the compression of an engine while the engine is not in operation. In other words, you disable the ignition so the car won't start, and then crank the starter and read the compression gauge. This is also the rating that a manufacturer gives for an engine or piston design. Static compression can be effected by several means, One of which is camshaft selection.

Boost is compression as well, but it is produced outside of the engine and forced into the intake system. Combining the two types of compression gives you an effective compression ratio. There is a formula for doing this, but it is much easier to use a tool such as the Virtual Dyno program.

Most engine builders and tuners will tell you that the low compression high boost model is the most efficient. I don't really agree with that assumption for several reasons, but it does work effectively. So lets get into some specifics in building or modifying a force fed engine.

Most street driven turbo cars today are controlled by a sophisticated computer control system. As such, most are running between 7 and 8.5 to 1 compression with a boost of around 10 to 12 PSI from the factory. When building a turbo motor it is very important to have a clear goal of what you expect. You also need to consider how and where you drive. You have to be honest while doing this. If you decide to build an 11 second screamer and expect it to deliver 28 MPG in town at factory weight, your project will certainly disappoint you in one or the other requirements. You have to be realistic, and you have to have information available to you.

The computers job is to monitor boost, temperature, RPM, timing advance, and many other items. It listens for sounds of detonation and retards the timing to keep the engine from self destructing.

The drivability of a car is determined largely by the throttle response and low end torque. The higher the static compression of an engine, the better low speed response is as a rule. If your engine is 7 to 1 compression stock, you should seriously consider going up to 8 to 1 pistons when you rebuild. You can go higher, much higher in fact, but that takes an intimate understanding of control systems and combustion dynamics. For our purposes here, we'll stick to the 8 to 8.5 to 1 ratios. Lower ratios will require higher boost levels to get the same performance. They will also have some low speed problems sometimes called lag, as well as reduced on hand power at low speeds.

Block, head, and internals

Take a cheap insurance policy out while your buying parts and get a set of head studs. You're going to torture the pistons, rods, bearings, and head gasket, so lets get them as bullet proof as possible first. Your parts list should always include the items below as an insurance policy. :

Cleavite 77 main & rod bearings

ARP Head stud kit

Forged pistons

Shim head gasket if available (FelPro otherwise)

Polished rod beams

ARP rod bolts

New oil pump (high volume if possible)

All new timing gears, belts, chains, etc.

New cam(s) (most regrinds have reduced life)

Rocker arms or cam followers as needed

High quality gasket set (Felpro, Victor, etc)

With those parts in hand you are ready to move on to the next step. If you are not rebuilding the engine, it is still possible to get some serious power from your basically stock turbo engine. You will need to know what type of piston your engine has to determine how much boost you can safely run. Forged pistons are by far the best, but many stock turbo engines come with cast pistons. Also find out if your engine has a cast or steel crankshaft, since it will take increased stress as well. Steel is certainly best, but cast cranks are useable with some common sense and limitations.

Any time you replace pistons, you need the new pistons in hand before you can go to the machine shop. Pistons need to be fitted to each cylinder (and marked) even if you are not boring the engine to oversize, each piston must be fitted and each cylinder honed for proper clearance. I don't replace rod bearings without replacing rod bolts as well. I also have the big end checked and sized. For forged pistons the small rod end must be bushed, or at least checked if the engine already had forged pistons.

Polishing the rod beams relieves stress caused by seams and imperfections around them. Polishing greatly increases the strength of the rod, and reduces the chance of bending or breaking one or more on a missed shift. I highly recommend balancing all rotating engine parts. That means crankshaft, rods, pistons, flywheel, and balancer. If your engine has balance shafts (or silent shafts), you should consider removing them if an elimination kit is available.

Heads, cams & intakes

Cams for turbo motors work the same as any other car, but they are controlling a different set of circumstances. In a normal engine, the intake opens producing vacuum to draw in a fresh charge of air and fuel. A turbo engine does the same thing at low speed, but changes to a pressurized system under boost. That means the normal engine performance is determined largely by the camshaft profile. It does that by opening the valves to a specified height, and for a specified amount of time to get fresh air in and spent gas out.

In a turbo motor, once boost pressure builds, the camshaft still performs the same functions, but it is no longer limited to its original profile. This is because it is not drawing in air anymore, rather it is acting as a timer to shut the air off. Since boost is always present under full throttle, the cam becomes a cylinder volume limiter. The higher the boost, the more mixture gets forced into the cylinder. So the lift doesn't have to be nearly as big as most race cams, and the duration acts as the timer to determine how much forced mixture gets in.

Most turbo cams are short lift and duration compared to performance V8 cams. An average cam might be .440 lift and 272 duration for a turbo performance application. In a mild street non supercharged V8 motor it might be .480 lift and 292 duration. So why can a 2.0 liter 4 banger kill a 5.0 liter Mustang? Simple, a well designed forced induction system, and a well designed suspension to go around it. A forced induction system makes an engine feel and act as if it is much larger displacement under boost.

Here's a simple physics experiment for you to try. Get a piece of heater hose about 8 inches long. Get a funnel and a jar or glass. Put the funnel in the hose and hold the hose above the jar. Now pour water through the funnel and hose until the jar is full and have someone time it. Ok, now put the hose on the end of your kitchen faucet with the jar under it (empty the jar first). Now turn on the faucet to full and time how long it takes to fill the jar. What's the deal here? It's the same hose, the same stuff (water), the same length and hose diameter. What's the difference? Pressure, boost, forced induction, or anything else you want to call it.

So the cam timing stays the same, but the timing of the fuel mixture changes as boost pressure rises and falls.

To survive, your engine needs oil. It needs to circulate that oil continuously while the engine is running. It also has to gather that oil back in the oil pan as quickly as possible so it does not run out of its supply. Most cast blocks leave a lot to be desired when it comes to oil drain back. With a die grinder and some patience, you can clear the way for oil to return to the pan though. Take off all of the sharp pieces of flashing that are left inside the block. Make sure that the oil has a smooth journey down the inside of the block. It not only helps make sure the oil supply stays available, but it will give the oil more time to cool in the oil pan.

Porting

It's pretty easy to go overboard when porting a turbo engine. They simply don't need much, if any. The only real purpose for porting a street engine is to match the size of the intake ports to the head, exhaust ports to the manifold, and manifold outlet to turbo. Each of these will gain you a slight horsepower increase, but it probably won't be noticeable even at high RPM. It will make a difference, just not a difference that you can normally feel. Enlarging ports and polishing them can easily kill low end performance. It does that because a larger hole under the same flow rate slows the velocity of the charge. In other words, the mixture doesn't move as fast under the same loads as stock. That makes lag very apparent and can really ruin your day. Leave porting to someone that knows what they are doing, and what purpose the car will be used for. Don't trust just anyone with a grinder. Make sure they have a good track record first.