When rebuilding an engine there are several things that you need to concern yourself with when choosing your components. Be aware of; Brand names and what your local shop may be influenced by, including by what they can get versus what will actually work. I have had some cars come through the shop that were built by other shops and individuals--- and not all are created equal. When you have a turbocharged car compression ratio is extremely critical. When someone says they can get you 8:1 compression (pronounced eight-to-one) pistons, RUN! There is no such thing as an 8:1 or 9:1 piston. This number is figured out by a mathematical formula that takes the following items into consideration:
- Volume of the cylinder head in cc's.
- Volume of the head gasket (yes, it has volume--- small but important)
- Volume of the piston whether it is dished or domed
- Volume between the top of the piston and the top of the block at TDC.
With this information you can accurately compute compression. I have to admit that often if an engine has not been machined in the past, the compression ratio will often be similar from one to the other of the same make and model. Where the problem comes in is when it is bored, the head has to be decked as it warped, and you use different pistons. So what do you do? The best way to figure out what you need is what you are intending to do with the car. If it is going to be naturally aspirated, nitrous, turbocharged, supercharged, or turbo and nitrous, compression is a critical choice. With many advances in Motorsports arenas there are different theories as to what you should run. What we have found to work the best for a turbocharged car is between 7.5:1 and 8.5:1. With this, we have found the least amount of problems with detonation and the best combination of power, emissions, tuning ease, and reliability.
So what happens if I run 6:1 or 10:1 with a turbocharger? What we have seen is with higher compression ratios, the less boost you can run and the more prone to detonation it becomes. With very low compression a motor becomes difficult to make enough exhaust energy and simply will not make as much power. Also the other thing to remember is design of the pistons. We only deal with certain manufacturers of pistons for several reasons. Our best pistons are manufactured by Venolia and are our proprietary design that we have spent over five years developing. The reason we use them is that they also are the only manufacturer of pistons for top fuel dragsters and funny cars. The end result is also the fastest AWD DSMs run our pistons, including our race car which uses our part number EXM0103 and ring package EXM0104. For the street we use EXM0778, which is a non-coated version of our race pistons. We also deal with Wiseco and Arias depending on applications and again it is all based on what the engine is going to be used for.
So which ones do you want? How about factory replacements? If the car was and is going to be a completely rebuilt as stock vehicle, the OEM replacements are the best way to go as their cost is definitely lower, but their power capabilities are much less. Usually stock pistons are cast versus a forging that is used for aftermarket forged pistons. The same principals also apply to connecting rods. Again, you can rebuild a stock connecting rod, polish it, clean it, put new bolts in it, shotpeen it, cryo-freeze it---- the end result it has only so much power handling capability. The other problem is that the machining processes are not any less expensive because they are not aftermarket rods. So if you are looking to make more power and are looking at rods there are several different materials that can be used:
Usually, steel material rods are used as they are cost effective and have great strength. There are several types of steel rods such as powdered metal, cast, forged, and billet. Powdered metal rods are the latest creation by factory manufacturers as they are cheap to make and work quite well for certain applications. Many others are cast and some are even billet. When you are looking at an aftermarket rod there are different styles such as H-beam or I-beam and personally it does not matter which style is used as long as they have a good reputation. We have used Eagle, Crower, Manley, Carillo, Pauter, Cunningham, as well as several others and the biggest thing I look for besides the basic features is weight. If they are abnormally heavy, I avoid them. We are always looking for weight savings and any rotating assembly part that can be light without compromise in strength is a plus. Titanium rods have certain properties that are good and evil. First they are light and extremely strong. Secondly they are extremely stiff. Their application is often in naturally aspirated engines. The problem with them is that if you get detonation they transmit all of the shock to the bearings and crankshaft--- which can break the crank or the rods, so for nitrous or turbo/ supercharged I would steer clear. When it comes to aluminum rods, their application has always been drag racing. The reason is that they are strong, can withstand pressures, are light, and are flexible. Often in drag racing motors every ounce is expected out of the engine and since in a 4-cylinder motor a set of four aluminum rods that we carry weigh as much as three Crower rods---- it is a 25% weight savings. The only down side is that they have to be replaced more often as they do fatigue.
So which rods do you need? In many applications we use Eagle rods, which in a 4-cylinder we have seen them live in 500+HP applications. For something stronger, and more expensive, we use Crower and Manley as they have more power handling capabilities that would be necessary for certain applications. Again, it depends on your application and what you intend to do with the motor.
So to wrap this up, simply tossing JE's and some Crower's in your motor is not the answer, nor is Mahle and BME. The right answer is what kind of setup do you have and what components need to be matched to it. For our typical street combination or full race applications, give us a call and we can set you in the right direction that you will only have to do once---- and it will be the right way.
