Crankshaft Harmonics 101
Guys, this is an interesting post. Unfortunately quite a bit of mis-information is posted herein. I have done some research on this topic so let me try to clarify some of the points brought up to help un-confuse:
1) The crankshaft assembly we have in our GT’s is just fine for its intended design condition. 550 hp was the engine power design point at 6500 rpm redline and as Dave correctly points out many owners have moded-up to 800-900 hp and have had no reported crankshaft issues. Will you have torsional issues if you push the engine (by whatever mod you care to apply) to 1300 hp (235% of design power) and turn the engine 7500 rpm (+15% overspeed) my guess would be yes! But the engine was not DESIGNED to run at that power reliably. So if one owner chooses to run to those outlying conditions and has a problem, why is the “sky falling” for the 99.999% of us other owners? Clearly it is not.
2) The crankshaft harmonic damper selected by the Ford design engineers for the GT engine is a “viscous” damped balancer. This is a sophisticated (and expensive) damper which uses a precisely tuned weight which moves within the damper annulus to attenuate crankshaft harmonics. The annulus is filled with a (relatively temperature insensitive) viscous silicon fluid. During engine development the team spent many hours of dyno testing to determine the various “harmonics” (dynamic resonances at power of the rotating crankshaft assembly with the rods, bearings, pistons, rings and flywheel) of our GT engine crankshaft. Typically the 4th order harmonic of the engine firing order, is the most dominant mode and the OEM damper effectively dissipates this mode. I would think it reasonable that the telemetry and measuring equipment necessary to map these harmonic modes up to the 12th harmonic would be beyond most if not all aftermarket balancer companies resourses. Thus I would conclude the Ford engine team knows more about our engine dynamics than most others and if a 20 pound damper was specified, then that is what the engine needs.
3) Engine torsional and bending harmonics are difficult to understand even for most engineers. Comments such as reduced rotating mass is always a gain and thus leads to less stress on the crank are not always true. Crankshaft harmonics are not “intuitive” and often counter to what you might think is the correct answer. Thus a 10 pound damper is not ALWAYS better than a 20 pound damper just because it is 10 pounds lighter. It depends on what mass is necessary to attenuate the quantified responses. Will the engine acceleration characteristics be faster with a 10# damper vs. a 20# damper? Possibly. But in my opinion given the torque output of our blown engine and the mass of the rotating components the engine must accelerate to move the car (i.e. clutch, transaxle gears, differential, rear axles, wheels and tires) the 10# delta in damper weight could likely not be felt by the driver.
4) The viscous damper also has the attribute of providing damping over a broad range of engine rpm. The other type damper is termed “elastomeric” as it uses a compliant rubber layer between the damper hub and harmonic tuning mass. This type of damper is less expensive, narrower in its attenuation properties and not as effective as the vicious style at high rpm. Ford has opted to use a “dual-mode elastomeric” harmonic balancer for the 2008 GT500 MOD5.4 engine. Note this IS a different damper than the viscous damper used on all GT engines and is tuned to slightly different discrete frequencies. The GT500 crankshaft is slightly different than that used in the GT and thus the harmonic balancer is tuned as best the dual-mode elastomeric damper can do, for that specific engine. I would see little advantage of swapping the GT500 damper for the viscous GT damper to save the reported weight difference only to have a less effective damping system.
5) One must also consider the environment in which the component is expected to operate. Our mid-engine GT sees little if any airflow around the engine. Certainly the airflow cooling characteristics of a MOD5.4 engine in a front mounted GT500 are dissimilar to that of our GT engine. All harmonic dampers absorb and dissipate torsional vibrations. They attenuate the detrimental harmonics by converting the unwanted oscillations into heat energy. The elastomeric damper is more temperature sensitive and needs cooling air to dissipate this generated heat. Thus an application in a front engine car with cooling air bathing the engine is fine for this type of system, but not so good for a GT application which has little airflow to dissipate the converted heat energy. The viscious damper as OEM supplied is the better application here.
6) At the engine rpm ranges ALL of us owners will ever use there is NO risk of the OEM harmonic damper bursting. Certification testing of the GT balancer was made to 10,000 rpm and held for 2 minutes. The GT viscous damper is just fine for all engine speeds up to at least 7,000 rpm. The damper selected for the GT500 MOD5.4 which IS different than the GT damper was proof tested to 12,000 rpm. Thus there is no need to worry about a damper burst. Claims to the contrary are just not accurate.
7) There have been statements to the effect that the harmonic balancer is somehow connected to the rpm limitation (red line) placed on the engine. Again factually inaccurate. The redline speed of the engine is determined by engineering based on the calculated drivetrain stresses (piston pin, connecting rod, crankshaft throw fillets, main bearing caps, etc.), bearing oil film performance, valvetrain stresses/dynamics and desired engine longevity. Will the engine turn faster than 6500 rpm and not fail? Absolutely! Many of the tuners bump up the electronic redline as part of the tune. Will the engine still have the durability aspects the 300 hour Certification run at the OEM recognized redline? Likely not, but that’s what we owners trade off for the increased rpm capability. And we are not likely to use all the engine longevity Ford built into the engine anyway with our limited driving use.
Sorry if this was long winded, but there appeared to be many different aspects to cover. To summarize, the viscous damper Ford supplied on the GT engine is just fine at attenuating the engine crankshaft harmonics throughout the rpm range most of us owners will ever use. Hope this helps others understand some of the complexities of this damper.
