A Failure Of One GT500’s 5.8-liter Trinity Engine

 

Background

When the 5.8-liter Trinity engine was introduced as the new powerplant for the 2013 Shelby GT500, it was met with open arms by enthusiasts and tuners alike after the new power figures were made public. After consumers started receiving their cars, aftermarket automotive developers started improving the power output from the already awesome 662 horsepower figure through the use of computer tuning, supercharger pulley upgrades, and intake and exhaust enhancements.  However, there is always a learning curve when it comes to new hardware. Before we dive into this engine failure analysis of a 2013 GT500’s 5.8 Trinity, let’s take a short look at the history of the events that helped these engines come to life.

The supercharged 5.8-liter Trinity engine as found in the 2013/14 Shelby GT500 Mustang.

The supercharged 5.8-liter Trinity engine as found in the 2013/14 Shelby GT500 Mustang.

The four-valve 4.6-liter Modular Ford engine was first introduced for the 1996 Mustang Cobra after the high output pushrod 5.0-liter was laid to rest in the previous Cobra iteration of the vehicle. The cylinder head design was updated in 1999, and finally the supercharger was added in 2003 with the 4.6-liter Terminator engine found in that year’s SVT Cobra.

The Mustang was redesigned in 2005; shortly thereafter, the 5.4-liter version of the supercharged four-valve engine found a home in the Mustang in 2007 with the introduction of the Shelby GT500 version of the Mustang.

In 2013, the 5.4-liter displacement was increased to 5.8-liters by adding stroke to the crankshaft.

What Could Have Happened?

This particular engine in as-delivered failed form to the engine builder.

This particular engine in as-delivered failed form to the engine builder – a pile of parts with no immediate reason for the failure.

The power packs offered from a wide variety of aftermarket tuners and performance companies can substantially increase the engine performance of the GT500 for an investment of roughly $3,000.

This can require swapping the lower crankshaft pulley with an overdrive pulley, installation of an off-road X- or H-pipe, cold air intake, single blade throttle body, colder spark plugs, then flashing the PCM with a new calibration, with improved performance measured in the neighborhood of 700 horsepower at the rear wheels once the modifications are performed.

However, power level increases on factory engine components can come with a cost. In the example shown here, the owner of this 2013 Shelby GT500 had the above mentioned parts installed to increase the power output, only to result in an engine failure. Further investigation of the engine’s internal components revealed that the factory hardware was not strong enough to handle the increased cylinder pressure. This information was unknown to the owner and the failure occurred during elevated acceleration in fourth gear.

After the engine was disassembled some differences were noted with the valvetrain. In previous versions of the naturally-aspirated four valve Modular engine, the lash adjusters were similar in size. The intake valve lash adjusters on the 5.8-liter Trinity are smaller than the exhaust lash adjusters. This is due to the dynamics of a forced-induction engine requiring more force to open the exhaust valve than the intake, due to the positive pressure in the intake manifold helping the intake valve open.

Note the difference between smaller (intake) and larger (exhaust) valve lash adjusters. The more robust pieces are required to help open the exhaust valves in the forced-induction environment.

Note the difference between smaller (intake) and larger (exhaust) valve lash adjusters. The more robust pieces are required to help open the exhaust valves in the forced-induction environment.

Also, when compared to the 4.6-liter four-valve naturally-aspirated cylinder heads, the quench area within the combustion chamber was relieved to improve the flow exiting and entering the cylinder. The intake and exhaust ports are also larger due to the displacement and application. In this engine breakdown, when the connecting rods failed some of the exhaust valves were also bent, leaving markings on the cylinder heads.

Comparison of the 5.8-liter (left) and previous 4.6-liter (right) combustion chambers.

In some of the pictures you can see that the connecting rods were bent which resulted in ultimate failure for one of them. Since there was no discoloration on the big end of the rods, oil starvation was not a failure mode. Moreover, no detonation was present since none of the spark plugs, pistons, or combustion chambers showed any signs of heat. After speaking with the owner it was clear that the engine was accelerating through the engine range, therefore failure could not have occurred from over-speeding the engine.

Some of the internal damage. Note the bent connecting rod and broken block skirt.

The engine block will also require significant reconstruction since the connecting rods broke out the skirts at the bottom of the block. Some of the shrapnel that went through the crankcase as the failure occurred also damaged the cylinders, and this will require the cylinders to be bored out and a new set of sleeves to be placed into the block.  There is the possibility that the cylinders could clean up as they are bored and honed, but that will be determined at a later date. Some engine builders would throw the block away and start from a clean slate, but this particular owner does not have that luxury in this case.

Note the combustion chamber contact in the cylinder with the failed connecting rod.

Note the piston-to-head contact in the cylinder with the failed connecting rod.

Summary

In conclusion, the 5.8-liter Trinity engine and internal components were designed to handle the power output specified by Ford, but can be at risk when power is added. While this does not happen in all cases (many Trinity owners are pushing substantial amounts of power through their factory engines) parts failure can occur if the strength of the rotating assembly is not improved to match the elevated horsepower levels. The future for this particular Trinity engine will include fixing the engine block, heavy-duty connecting rods, and new pistons. Stay tuned to Enginelabs.com for future articles and information regarding this engine as it undergoes the reconstruction process.

About the author

Corey Blue

The first engine Corey built was for his 70cc Honda dirt bike at the age of seven. As he got older, he had the opportunity to work on big- and small-block Fords and Chevys for truck pulling, and modified his 32-valve Ford Modular 4.6 Cobra to generate 900 horsepower. Corey is a graduate of Missouri University with a degree in Science and Technology for mechanical engineering and I interned for Roush Industries working on the 2011 and 2012 Shelby GT500 Mustang.
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