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GM Piston Slap

       
     Pistons rock as they cross TDC where the thrust load from the rod angularity shifts from one side to the other. This makes noise. Harmless..but it is the source of piston slap in most all situations.

    To prevent this, pistons are designed with long, tapered, flexible skirts so that they can be fitted very tight in the bores when the bore and piston is cold. The taper of the skirt and flexibility of the skirt then prevents scuffing when the piston is hot. Also, the piston pin in OEM production  pistons is always offset to one side....it is NOT in the middle of the piston. By offsetting the pin in the piston, artificial thrust load is created to control the piston "rocking" as it crosses over TDC.

  

   Unfortunately, all of the above control techniques, common in past model engines to the extreme, create excess piston mass, cause friction and cost power and fuel economy. With the desire to build in as much power and free-revving capability and to improve fuel economy as much as possible thru friction reduction these design features are pushed in the other direction on modern engines.

    Piston pin offset has been reduced over the years to a bare minimum today to reduce the thrust load generated and reduce friction. Pistons have been lightened up considerably by shortening the skirts. This creates less rotating/reciprocating mass which is good for power, free revving capability and fuel economy. Light weight pistons are great but the skirts, by necessity, are short making it hard to make them both strong and flexible and the shorter ckirts make them more prone to rocking. 

      Unfortunately, when the performance and fuel economy oriented pistons are run cold they are very prone to "slap" until they warm up to operating temperature. 

    The piston designers and development engineers are always treading the fine line between piston slap cold and friction and power/fuel economy loss when the engine is warm. 

    It is possible that you are hearing piston noise from an engine that is on the "high limit" for piston clearance so that it makes some noise cold. The good news is that the condition is harmless and that engine is probably a little more powerful (due to less friction) than a "quiet" counterpart. The bad news is that...it makes noise cold.

      As an example of what the piston pin offset can do, it was common back in the early 70's to turn the pistons around "backwards" in the large displacement Chrysler engines to gain power. Those engines had large piston pin offsets to create thrust load to control the piston slap. So much thrust load and friction was created that just turning the pistons around in the bores was often good for 10 HP. The engines were very quiet with the pistons in correctly and they slapped like crazy, especially when cold, with the pistons reversed so as to reverse the pin offset.

     Racing engines do not have the pin offset and thus the pistons slap like mad...but no one hears them over the open headers....

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Positive Crankcase Ventilation (PCV) system

        An internal combustion engine is built around a series of hollow cylinders, in each of which is a moveable piston designed to glide up and down inside it. A mixture of air and gasoline is pumped through a system of tubes called the intake manifold through each cylinder's intake valve (or valves), where a spark from a spark plug causes the mixture to explode in the open space at the top of the cylinder called the combustion chamber. The pressure from this explosion drives the piston in the cylinder downward, where it causes the crankshaft to rotate. The rotation of the crankshaft not only pushes the piston back up into the cylinder so it can do all this again, but it also turns the gears within the car's transmission that eventually make the car move. Meanwhile, the rising piston pushes the air and gas left over from the explosion back out of the cylinder through an exhaust valve.

       However -- and this is where crankcase ventilation comes in -- a certain amount of that mixture of air and gasoline is pulled down by the piston and slips through the piston rings into the crankcase, which is the protective cover that insulates the crankshaft. This escaping gas is called blow-by and it's unavoidable. It's also undesirable because the unburned gasoline in it can gunk up the system and produce problems in the crankcase. Until the early 1960s, these blow-by gases were removed simply by letting air circulate freely through the crankcase, wafting away the gases and venting them as emissions. Then, in the early 1960s, positive crankshaft ventilation (PCV) was invented. This is now considered the beginning of automobile emission control.

        Positive crankcase ventilation involves recycling these gases through a valve (called, appropriately, the PCV valve) to the intake manifold, where they're pumped back into the cylinders for another shot at combustion. It isn't always desirable to have these gases in the cylinders because they tend to be mostly air and can make the gas-air mixture in the cylinders a little too lean -- that is, too low on gasoline -- for effective combustion. So the blow-by gases should only be recycled when the car is traveling at slow speeds or idling. Fortunately, when the engine is idling the air pressure in the intake manifold is lower than the air pressure in the crankcase, and it's this lower pressure (which sometimes approaches pure vacuum) that sucks the blow-by gases through the PCV valve and back into the intake. When the engine speeds up, the air pressure in the intake manifold increases and the suction slows down, reducing the amount of blow-by gas recycled to the cylinders. This is good, because the blow-by gases aren't needed when the engine speeds up. In fact, when the car is up to speed, the pressure in the intake manifold can actually become higher than the pressure in the crankcase, potentially forcing the blow-by gases back into the crankcase. Since the whole point of positive crankcase ventilation is to keep these gases out of the crankcase, the PCV valve is designed to close off when this happens and block the backflow of gases.

Remapped engines - ECU remaps

         Back in the good old days, the task of timing the ignition spark was performed by the distributor. The greater the RPM, the more the timing would advance. It was clunky and mechanical but seemed to work quote well. This did a reasonable job, but for the most effective power you would need to vary the timing to a greater degree than a fixed ratio advance curve. The electronic ignition system was born giving much finer control over fuel delivery and spark timing. A complete map of variables was entered into the ignition program and preset timing would be read from a table.

          Now air temperature, engine speed, engine load and even control over turbo/wastegate control & fuel delivery rates means that precise management of the engine ignition timing is possible and you can achieve the maximum power output throughout the rev range. 

So what does ECU remapping do?

          Electronic ignition allows the manufacturer to fine tune economy at popular road speeds such as 30mph, 56mph and 70mph where most cars spend a large proportion of their time. It is now possible to advance the timing if the throttle is wide open to give greater power or back off the timing when cruising at constant speed. When a manufacturer creates a timing map they build into it a big margin of error to cope with: adverse temperature ranges, minor faults & bad conditions. Manufacturers do not want people breaking down, suffering premature parts failure or to get a reputation for uneconomical cars so they build in a wide margin of tolerance. Different countries use different grades of fuel and have varying degrees of extreme weather conditions, all these factors add to the fudge that has to be done to keep all the cars working well across the globe. Each car that leaves the production line is also unique, some achieve 10bhp less and others can be 10bhp up on standard specs, depending on how well the components are machined and put together. So rather than put each car through a unique assessment and get a bespoke timing map, they adopt a standard one map fits all philosophy.It is also a fact that manufacturers use the remap to produce different power versions of the same engine and get lower insurance cover ratings and better fuel consumption. You start to see the fantastic scope for improvement, when you add into the mix the fact that the average TorqueCars reader will be adding better performing components to the car, you have a really strong case for a remap. Other things that the manufacturer builds in to their map equation is the possibility of user neglect i.e. infrequent servicing with items struggling like dirty plugs, bad leads, clogged air filter, partially

blocked injectors etc... The list goes on.

        Anyone who has added performance parts to their engine should consider a remap. If your car is a turbo model & has electronically controlled fuel injection, there are massive power gains on offer and TorqueCars would strongly recommend a remap. What are the hidden costs or drawbacks? You will need to be prepared to keep the car serviced more frequently, and, sometimes decreasing the service interval by half. When you fill up you are also restricting yourself to high quality fuel and you must be prepared to replace components that fail due to the extra work they are doing. When an engine is tuned to produce more power, you are also creating more stress and strain, so things will start to fail such as air flow sensors. Also turbos can wear out, other major components like pistons and bearings will need care and attention and you will find that the clutch lasts a shorter time. If you do not have a turbo there's very little to be gained by a remap alone - perhaps only a few BHP so TorqueCars' recommendation for non turbo cars is to modify everything else first (cams, pistons, increase compression, engine balance, air intake, exhaust, head work, bigger valves etc...) after these things have been done you can then consider a remap which will help you get the full benefit from them.

         On the subject of turbos, (briefly though as we have a comprehensive article on turbos in the forced induction section), a remap will often introduce boost from lower down the rev range and because of this the turbo is running faster and hotter. In this situation you must let the turbo cool down a little before shutting off the engine, otherwise the oil will degrade and you will have an expensive turbo. repair on your hands. Fitting a turbo timer will also help with this problem and keeps the engine ticking over. Of course the amount of power you choose has a bearing on the reliability and cost of running the car. Many people go for an off the shelf remap which are on offer all over the country and typically cost around £200-£500. This is better than the manufacturers map in that it uses tighter parameters but it is still a one size fits all job. If you are after big power gains and have changed major components like the turbo, waste gate and have done extensive engine work then a custom remap is the best option.

Switchable remaps.

           Things have moved on and it is now possible to have a few maps stored which you can select from. It is typical to have a valet mode to stop the boy racers at the garage thrashing your pride and joy on the "test run" to bed in the new wiper blades they have fitted. Then there is often an economy option to give very frugal fuel consumption, particularly useful when cruising on long journeys. Then you have the 'sport' or 'power' modes which give lots of power and often require high octane fuel. TorqueCars strongly recommend that you get a switchable remap - it may cost a little more at the outset but you will avoid many of the pitfalls of running a high power remap all of the time by doing this and get the best of both worlds. Please do not confuse a remap with the little "tuning boxes" you can buy for £50 which generally obtain little more than a £3 resistor and often do little more than fool the car into thinking the air is cooler than it is. If it was really this easy to get more power from a car then the manufacturers would have already done that themselves. Some cars are not easy to remap, I remember that the Rover MEMS & Toyota ECU's are a case in point. The Manufacturer holds the key to the ECU and locks their Map into their firmware. Some ECU's are simply not reprogrammable. So what can you do? Is there an option? Well thankfully there is and they are called piggy back ECU's or aftermarket ECU's. Piggy back & Aftermarket ECUs: Piggy back ECUs connect between your existing ECU and the engine sensor inputs and outputs. All work slightly differently and apply some or all of the following combinations.

            Some will adjust the sensor readings such as air temp, engine speed, crank position and effectively lie to the standard ECU forcing a more aggressive timing. Some will actually perform their own calculations and take over control of some aspects of engine management like turbo waste gate control and ignition timing. Some will take the standard ECU output and modify the signals sent to change timing and learn to guess the next output a split second before it is needed using the base ECU map and just enhancing it a little. Tuning boxes are relatively new and these alter readings going to and from the ECU giving extra power or economy. Aftermarket ECUs are often direct replacements for the car's ECU and will take over all the functions associated with it. They are generally faster and able to cope with a wide variety of additional factors such as water/methanol injection and a turbo timer. Be wary though, as some piggy back ECUs and aftermarket ECUs do not include knock protection. If this is the case then set your timing conservatively and use high octane fuel. As with all ECU upgrades you are still dependant on having the car in top condition as you are removing the tolerances built in for dirty plugs, cheap fuel, or minor electrical faults.