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Bringing A GenIII LS1 Stroker Back to Life
By Norm Brandes
Performance engine builing is as old as the first Model T rebuild, but thanks to today's new engine platforms and a plethora of aftermarket equipment, many engine builders can be intimidated by the scope of the possibilities. But, as we highlight in this article, there are basic engine practices and philosophies that work whether it is a 1957 small block Chevy or today's newest modular V8 engines. In some cases, you can't even tell the difference.
The LS1 is a great motor for hot rodders, who are snapping them up as fast as they can. The LSx family of engines in general have a lot going for them: they're reliable, they make a ton of power and they get great gas mileage.
The problem is, they look horrible in a hot rod. The engine shown at the beginning of this article is a pretty tricked-out LS1 in a 1962 Nova that has been extensively reworked. And it is a great example of how far away from the stock, ugly LSx engines we can get. It looks right at home in the Nova, especially with the air cleaner in place. How many people would even recognize this is a stroker LS1 that originated out of a 2000 F-body?
Our solution is to make them look old school like you see here. But sometimes, the surgery required to make it beautiful is more than just cosmetic.
The subject for this study is a late model GM Gen III engine that was previously rebuilt by another shop. Our patient had been previously stroked to 383 cubic inches from a stock 346, and had reworked cylinder heads, an aftermarket camshaft and aftermarket lifters.
The initial customer-related complaints included low compression test results, significant oil blow by, engine coolant temperatures running hotter than normal, considerable loss in performance compared to new and higher-than-normal engine noise.
When receiving an engine whose health is this grave, we traditionally start out with an autopsy. This will give us a baseline of conditions this engine has already experienced, a review of the condition of its components, and an idea of the corrective surgery that we'll have to perform. By performing this analysis, we can better determine what can be salvaged, what is under spec'ed or inadequate, what modifications will be required, and how to integrate newer components and changes. This will give us a good roadmap as to where we were, where we are currently and where we are trying to go. Based on the customer's goal, we can accurately assess not only the engine program, but also the supporting parts including headers, exhaust system, fuel system modifications, fuel pump, computer program, injectors, gear ratio, tire size and transmission type.
Here's what we discovered during triage on this particular engine.
The visual autopsy showed:
Tops of pistons and combustion chamber exhibited an abnormally high amount of carbon coating;
Analytical Breakdown and Decisions
Piston skirts were galled and matching cylinder walls were galled and scratched. Rocking of the piston showed signs of high piston-to-wall clearance, which is hard on a short skirt piston, as in a stroked motor;
Rod bearings showed signs of detonation;
Engine oil showed signs of fuel dilution; and
Nose of crank and harmonic balancer/pulleys showed signs of being spun.
This GM Gen III aluminum LS1 block had cast-in-place liners for a stock bore of 3.898" Taking the bore out to a 3.910" would compromise the strength of the cylinder walls and, in all probability cause them to deform under high cylinder pressure, prevalent in this boosted application. Also, the column strength of the cylinder and walls to deck may yield under applications higher than 550 flywheel horsepower.
The structural rigidity of the main bore tunnel has similar issues in the OEM aluminum block making the GM Gen III 6.0L iron block (GM p/n 12572808) an acceptable, cost effective alternative. This block comes stock with a larger 4.000" bore, which will effectively un-shroud the valves and allow the cylinder heads to breathe better, thus making more power. Determination: New block required
The existing crankshaft was an aftermarket Eagle crank with a stroke of 4.000" (as opposed to the stock 3.625") that had been abused. Rods and mains showed signs of wear and would need to be turned. Scoring damage to the nose would have required welding and turning to size which would cause the crankshaft to be unserviceable economically. Determination: New crankshaft required
The connecting rods could have been rebuilt by resizing and re-honing them for oversized bearings. However, for our particular application, we determined that we would increase the piston compression height. This change required us to use a shorter rod than those already in use. Determination: New connecting rods required
The pistons were not serviceable; first, because of wear, and because we were increasing the bore size due to the new 4.000" bore block. After measuring the pistons, they were determined to be undersized for the current bore, and new pistons were selected for the new bore. Furthermore, design changes for a new piston allows us to go to a taller compression height with a dish, thus accommodating the additional volumetric efficiency increase due to the supercharger and allows us to keep our static and dynamic compression at a safe level that will be pump gas friendly. Additionally, the taller compression height promotes a more stable piston, with less wrist axis wobble as well as a more spread-out side load. Since this car will be street driven this will greatly add to the drivability and cold starts a race engine will not see. Determination: New pistons required.
Camshaft and Rocker Arms:
The existing camshaft was a custom grind from Comp Cams, specified by the original builder. While, physically, the camshaft's condition could allow it to be placed back into service, the cam lobe design on this camshaft has lift values that exceed the point at which our cylinder head flow data showed the ports stalling. Furthermore, the large amount of duration and tight lobe separation caused significant dynamic compression issues (very low) as well as poor vacuum signal generation.
This led to poor drivability issues, poor cylinder burn characteristics, and tuneability issues. Since this engine will see mostly street duties on pump gas, these characteristics are important.
The chart below highlights some of the key variables to consider when selecting your camshaft.
Note the relationship between dynamic compression ratios and the static compression ratio. In our case, not only did we have to be conscious of keeping the static ratio lower due to the engine being equipped with a supercharger, we had to keep in mind the effects of the camshaft on the dynamic compression ratio.
It became very evident that by reusing the existing camshaft, our dynamic compression ratio would have been far too low, not only costing overall performance, but also creating issues with drivability and engine component longevity. Furthermore, the existing camshaft had lift values of almost .700" when combined with the equipped 1.85:1 rocker arms. As shown in the cylinder head flow data table (Chart 1),
this was above the lift values where the ports stalled.
The new camshaft was selected to optimize the air flow capabilities of the ports. Furthermore, the installed rocker arm ratio of 1.85:1 was causing valve train stability issues. These were replaced with aftermarket 1.7:1 ratio rocker arms which will give additional spring longevity. Determination: New camshaft required.
Cylinder heads were flowed on a Superflow 600 flowbench and showed decent airflow numbers throughout the lift range, showing stalling at just over .600" of lift. Guides and valves were well within service limits, and the deck finish and thickness was correct for the application. The existing valve sizes of 2.080"/1.600" and existing chamber volume of 66.5 cc were determined to be sufficient for our supercharged application in terms of compression ratios. Determination: Clean and reuse cylinder heads
There are a variety of machining operations that are required to properly prepare components for assembly and it's important to distinguish between the standard performance engine procedures and the additional procedures used on this stroker build.
Standard Machining procedures that should be performed on any engine include:
Check preliminary tolerances
In this case we are using a GM Gen III LS1 6.0L block. Although it is a brand new block it is crucial to verify that all tolerances are within specification, so all dimensions are thoroughly checked. This holds true for all of the components that will be used in this build. The main bores, cylinders and decks were all within specs.
Bore and Hone
After the block was thoroughly cleaned and inspected, it was set up in a mill to be bored .030" over. From there the block was transferred to the honing machine where it was brought to its final tolerances and surface finish. Because proper surface finish and concentricity of the cylinder is crucial, a deck plate was used during this procedure. Piston type and engine application will determine piston-to-wall clearance. For this application we used a .0055" piston-to-wall clearance because of the supercharged application and forged aluminum pistons.
The block was then cleaned and the rotating assembly was mocked up. This allows us to check several things including swinging clearance, oil clearances, and piston protrusion height before final assembly, all of which are vital to performance and the life of the engine. These procedures will be explained in further depth later in this article.
The rotating assembly was disassembled and weighed for balancing. The bob weights are precisely calculated, even taking oil weight into account. The crankshaft is then balanced to within 1/2 of a gram, which is more precise than OEM tolerances and standard overhaul balancing. Balancing the crankshaft is accomplished by spinning the crank on a balancing machine with attached bob weights. The bob weights simulate the rotating mass of the piston, rods, and hardware. The machine uses a photo eye and sensors to locate the exact location and size of the imbalance. The machinist then removes or adds weight as necessary until the imbalance is less then 1/2 gram. Once the crank is balanced it is then de-burred and cleaned for assembly.
Ring gap sizing and filing
Ring gap is greatly dependant on piston type and application. All pistons come with proper guidelines to calculate ring end gap. Similar to piston wall clearance, ring gap is application-based. With this application, the top and second rings were filed to .022" and .015" respectively. After the rings were filed they were checked to the bores for proper gap. Once filing is finish, all rings were deburred and cleaned.
Additional Application-Specific Operations:
For this (and other) stroker applications, the basics aren't always enough. The rotating clearances need to be checked and in some cases, clearanced where necessary. Typical clearance on a street engine with steel rods is .050" to .075". In this case, additional clearancing was not necessary.
Note that clearance is checked by installing one rod at a time and rotating until the entire rotating assembly is complete. Some engines have oil pickup clearance issues that may need to be changed to an external pickup if severe enough. Absolutely no modifications were necessary for this engine.
Checking Effective Piston Volume
During mock up one piston was installed with its rings in order to check the effective piston volume. The piston was brought to a known distance down in the cylinder. A piece of Plexiglas with a hole drilled in it and a thin bead of grease was used to seal the top of the cylinder. From there a burette was used to measure the volume of solvent in the cylinder. The volume of the cylinder can be calculated knowing the bore and height by using the following calculation.
where H is distance down and R is the radius of the bore (bore diameter/2). The value of pi is commonly referred to as 3.141593.
This calculated value is then subtracted from the actual measured value to determine the effective piston volume. Although most piston manufacturers specify the effective volume, this method allows us to accurately confirm their specs. This process is also necessary if the pistons are modified from stock (i.e. milled or machined for larger valve reliefs). Additionally cylinder head combustion chamber volumes were checked. This is absolutely necessary to calculate accurate compression ratios.
Piston Protrusion Height
Piston protrusion is the distance the piston is above (+) or below (-) below the deck from the compression height of the piston and achieving the proper protrusion is critical. After initial mock up we found that, in order to achieve proper quench and compression ratio, both the deck surface and piston step needed to be modified.
Piston protrusion height was first checked by bringing the piston to TDC using a bridge dial indicator at the center of the piston. Next we rocked the piston with an indicator on piston top closest to the cylinder wall to determine the high and low sweeps of the piston. The measurements were then averaged by adding the two together and dividing by two, which gave us our established piston protrusion height. Although all the parts are new, piston protrusion height can differ slightly and can normally be corrected by switching rod and piston combinations. Although the difference is minimal, because it is from the slight deviance in rod length, piston compression height and crank throw, it is a factor we always check.
Decking of block
The block was then set up to be decked to achieve desired compression ratio of 9.25: 1. However, decking the block created too tight of quench between the piston and cylinder head, so to correct this, the piston steps needed to be machined by fly-cutting. Quench distance is particularly crucial in forced induction applications such as this, since it can directly affect an engine's propensity for detonation.
The pistons were set up in a Bridgeport with a specialty jig provided by the piston manufacturer. This allows fly-cutting the piston step in order to achieve .045" quench. Many consider .040" quench to be ideal, however, because of the blower application and additional rock in the piston we added .005" of extra clearance for cold starts. This procedure enabled us to achieve the exact compression ratio necessary and keep the ideal quench, giving us optimum power.
Since this is a supercharged application and our autopsy revealed a spun crank pulley, we knew that we would have to modify the crankshaft-pulley interface. We surmise that the damage to the nose of the crankshaft was due to the increased load from the supercharger and the lack of having a key-way between the crankshaft and the pulley. The factory method for securing the pulley is a press fit, and we feel that the load overcame the factory's press fit and spun the crank pulley. To eliminate any future pulley slip, we machined two keyways in the front snout of the crankshaft.
This was done by mounting the block upside down onto a Bridgeport mill, and locating the crankshaft square to the main bore centerline. The crankshaft was installed with shim stock under the bearings to clamp the crankshaft into the block. A degree wheel was installed to the rear of the crankshaft and zeroed in. The crankshaft was centered with the mill, and the first keyway was machined. The crankshaft was then rotated 180 degrees using the degree wheel and the second keyway was machined.
Rather than using a factory replacement pump or spending the time to modify a factory oil pump, we utilized a Melling high volume performance oil pump. This unit features a redesigned pressure spring increasing oil pressure by 10psi and an increased rotor volume along with an increased pump inlet and output openings. All of these improvements account for an 18 percent increase in oil flow. A ground, cast iron cover replaces the factory stamped steel piece which eliminates blow by and produces higher hot idle pressure. This pump will better serve the increased demands on this engine.
The above procedures may not be necessary on all stroker builds. However, on this build they will guard against the previously stated downfalls which lead to the demise of the original engine components. This engine is destined for a GM F-body car, and will offer the performance desired and the streetability required by its now completely satisfied owner.
About the Authors: Norm Brandes owns and operates Westech Automotive, Inc., a machine shop and vehicle repair service business located in Silver Lake, WI. He is as wizard in gaining performance while keeping vehicles within emissions standards. Keith McCord is president of McCord Consulting Group in St. Louis, MO, and is a car nut.
For more information and photos, view this feature in an expanded format online at www.enginebuildermag.com