Today's aftermarket engine part manufacturers are making everything under the sun from stock replacement parts to wow, that's possible to build? In a nutshell it seems like we have the parts covered for just about every application of the small-block Chevy we want to build. So where to begin, or better question is, what is the weak link to your current engine? Usually we find out when it's to late, like in my case of a broken crank. This was all my fault because I was pushing the limits of what the last crank could do. Some of you may say it was a bad crank and I have to disagree – it was my driving habits.
In my case, repetitive 6,000-plus-rpm shifts with almost 456 hp and 481 lb-ft was pushing the capacities of the cast crank material. The good news is that the engine never self-destructed, but it did send main bearing material all through the engine. One of the positive things in all of this is that we can now provide you guys, the readers, with some details on how to beef up the bottom end and squeeze some more power out of that old small-block.
Although a plethora of available engine parts may be a logistical nightmare for novice engine builders, companies like Lunati, Comp Cams, Summit Racing, and Airflow Research (AFR) all have tech lines to help you. Mix and match the wrong parts and you'll wind up not making any more power than you started with, so start by doing some research. Questions I would ask include, do you want a stump-puller engine that will not rev up past 5,500 rpm or do you want an engine that revs to the moon, but has trouble getting the truck moving? Or is it a combo of the two? For me I definitely want the engine to rev, but I also want the engine to pull like a freight train from off idle to max rpm. The best way to achieve this is by choosing parts that work well together. Let's talk about the 383 bottom end and different rod lengths, etc.
Popular 383 kits have a 3.750 stroke crank, 5.70 rods, and 1.425 pistons, and if you add up the total of the combo you get a 9-inch deck height. This kit is popular because pistons are available in different CC's from dished, flat-top, and domed. The Lunati VooDoo Engine Kit we choose has a non-twist forged 4340 steel crank with a 3.750 stroke (part number 70137501), 6-inch H-beam rods (part number 70160001-8), and a shorter piston compression height of 1.130 inches (part number 201006K1) kit with rings and pin.
The Lunati Balanced Kit comes with main bearings and rod bearings that are designed to fit the standard small-block. So why did we choose to go with the longer 6.0 rods versus 5.7 rods? The 5.7 rods with the 3.750 stroke have what is called sidewall load. When the 5.7 rod and piston is at top dead center (TDC) or bottom dead center (BDC) the piston is forced to the side of the block causing increased friction. The 6-inch rod will have the wrist pin sit higher in the piston which optimizes the rod ratio for higher rpm's and puts less stress on the cylinder wall because of the decreased angle the rod is at during its cycle. The downside to the long-rod build is that the piston has smaller ring-lands due to the higher wrist pin location, so it is not as conducive to a boosted application.
This is measured from the center of the crank to the top of the block on both sides, left bank and right bank. A quality machine shop like Pfaff Engines of Huntington Beach, California, will be able to check your deck height and machine the block according to where your requirements for clearances and compression need to be. Most, but not all of the Chevy Gen II one-piece rear main 350 blocks, come with a 9.025 deck height. Machining or zero decking the block is done by removing .025 off the block, making the deck height 9.000 inches total on both sides. Now this can be problematic or a blessing depending on the parts utilized. For example, some stroker kits will take into consideration that machining the block down is expensive and some people are not going to do it, so they might spec the pistons so that they are less than .025 in the hole.
Piston Compression Height
Many people confuse piston height with deck height. Piston compression height is a measurement taken from the center of the piston's wrist pin to the top of the piston. A 5.7 rod will have a larger piston compression height than a 6.0 rod with the same stroke crank. To figure out what piston compression height you need per your rod size, you need to calculate the following. First, divide the stroke 3.750 by 2 = 1.875; add this to the rod length + 5.70 = 7.575 minus the 9.0 deck height = 1.425 piston compression height, this is for the shorter rod. The 6.0 rod for a 383 is calculated in the same way: 3.750 divided by 2 = 1.875; add this to the rod length + 6.0 = 7.875 and minus the 9.0 deck height = 1.125 piston. Lunati uses a 0.005 taller piston that will put the deck height of the piston at 9.005 or +0.005 out of the hole if the block is zero decked or machined to 9.000. Now, if for some reason you decide to not machine the block, then the Lunati piston will be 0.020 in the hole.
Dish, flat, and domed are the three common types to choose from, but for the most part we are looking at the either dished or flat. Dished pistons are just that – dished to reduce compression and are measured in minuses (-18.6cc). Flat-top pistons are flat on top and increase compression and have valve reliefs on top. They can measure plus or minus, but most common are minus (-4.9cc). Domed pistons protrude into the combustion area of the head and are measured positive or plus (+12cc).
Oh man, where do I start? Rather than getting into the complex makings and materials of head gaskets, let's just talk about thickness and bore. The Fel-Pro head gaskets have great descriptions of what they are and will oftentimes give you a compressed thickness and gasket bore size. The Fel-Pro 1044 head gasket has a 4.200 bore to clear most stock/bored SBC pistons and features a 0.051 compressed height. A thinner Fel-Pro head gasket 1094, similar to this one, has a 0.015 thickness with 4.100 bore and will increase compression because of the decreased space between the combustion chamber and piston (squish area).
Squish is referring to how much space the piston has before making contact with the top of the head and combustion chamber. It is named that because the head's combustion chamber is designed to compress the incoming combustion charge at the last moment and squish the charge toward the spark plug. This squish effect gives the charge turbulence and in effect gives a more complete combustion and burn. A target of 0.041 squish is the goal for our 383 build.
Like the squish area, the quench area is the distance in relation to the piston and the top of the flat part of the head. Quench is referred to as a way to reduce the compressed mixture temperature between the piston and the cylinder heads. Both are very similar and can often interchange depending on whom you are talking to. Ideal quench and squish on a small-block is .035 to .045 depending on the application. Believe it or not, increasing the piston to head squish/quench clearance too much can increase ping or detonation.
You want as much compression as you can get away with to increase horsepower. In California, 10.5:1 is about the limit on pump gas because our gas changes blends from summer to winter. In a nutshell the summer/winter mix changes the gasoline's power output, thus decreasing engine power and possibly introducing ping or detonation. So how do we figure out compression and stay within a reasonable range for a street driver and great performance? It all starts with the bottom end; that includes the piston, deck height in relation to how far the piston is "in" or "out" of the hole, stroke, bore, head gasket thickness, head gasket bore, and what size combustion chamber is on the heads.
A piston that is -18.6cc and is out of the hole 0.004 is measured in a minus number (-0.004) with the online compression calculator provided, the same negative or minus CC number with pistons that have valve reliefs or dished. Example, bore = 4.030 with our .030 over Lunati 383 pistons, stroke = 3.750. Head gasket bore depends on the gasket manufacturer; let's say we are using the 1094 Fel-Pro gasket. The head gasket measures 0.041 thick compressed and 4.100 bore and if we are theoretically using a 64cc heads, this brings us to compression ratio of 9.6:1, but that only leaves us with .037 piston to head clearance.
Since our target compression ratio was higher than 9.6:1 we started to look at other options. So Gordy of Pfaff Engines and I started to look at flat-top pistons with -4.9cc valve reliefs to raise the compression. Calculating the same bore and stroke with a Fel-Pro 1044 head gasket (.051 with 4.200 bore) was getting us closer to 11:1 compression with the 64cc heads I already had. If we replaced the heads with 69cc heads we could bring that number down to 10.4:1, a perfect street compression. This would also give us an ideal .041 squish because the new flat-top pistons were out of the hole .010. Go online to the many online compression calculators to figure this one out. Here is one of my favorites: www.csgnetwork.com/compcalc.html.