6G72 Underdrive Pulley - Long read, but worth it
BEFORE READING ANY OF THE INFORMATION ON THIS PAGE I MUST MAKE YOU AWARE THAT EVERYTHING WRITTEN BEFORE YOU IS FROM RESEARCH ON THE SUBJECT, COMPONENTS, AND DESIGNS. I AM IN NO WAY “AUTHORIZING” ANYBODY TO TAKE IT UPON THEMSELVES TO FOLLOW MY INFORMATION BELOW. THIS DISCUSION IS MEANT SOLEY FOR A CONSTRUCTIVE RESOLUTION TO THE DILEMA OF THE UNDERDRIVE PULLEY ISSUE.
Known information:
6G72 is an internally balanced engine
Use a “Harmonic Balancer” to reduce secondary vibrations
Same engine platform as a 3000GT (used as reference)
Underdrive pulley does not have any type of Harmonic Balancer
Stock crank pulley/harmonic balancer
Roughly 7” diameter and 6lbs
Underdrive pulley
Roughly 4 7/8” diameter and 1.4lbs
No harmonic balancer
The breakdown of the “harmonic balancer” on the 6G72 platform is fairly straight forward, but seems to be misunderstood with other variants of harmonic balancers used. Most of the balancers found on the market, and used on stock engine applications have an internally balanced (with a fluid or other substance) to balance out the lower end of the engine to prevent serious damage. Often these are components found on an engine design that is not internally balanced.
Take for example in the early 90s when Chrysler used the 6G72 platform in the Lebaron model. This was an early stage of this engine’s development which is probably noticeable from many things used during that time. One major thing that is most noticeable when compared to the platform in the 8th Generation Galant (and applicable to this discussion) is the harmonic balancer as pictured below. The first thing you will notice with this unit is that it is completely separate from the actual crank pulley. You will see this is also a “sealed” unit type of harmonic balancer. What is so different about the Lebaron unit from the one being used on the newer 6G72 platform in our 8th Generation Galant?
http://i228.photobucket.com/albums/e...ebaronVIN3.jpg
The newer platform has gone through some design changes, as evident with this particular area. The harmonic balancer is nothing like the one used during the very early 90s and actually combines the harmonic balancer and crank pulley together as one unit (pictured below). Now on first sight this unit does not really appear to be any different in design other than what I previously noted. However, upon a closer look of the overall design you can see some very pronounced differences. Since the engine platform is now internally balanced (which will be made even more evident as you read on), the harmonic balancer/crank pulley design still dampens the secondary vibrations…but how?
http://i228.photobucket.com/albums/e...t/GalantV6.jpg
When you look at a common harmonic balancer as I stated before it would usually be a sealed unit, so if you cut it in half you will see the fluid type substance used to balance, and dampen the vibrations. When you take the harmonic balancer/crank pulley from our 8th Generation Galant and cut it in half you will not find a sealed unit, but a mostly metal core with some rubber. Where is the rubber, and how is it assembled you ask? Simply look at the photo below (the black “slash” area on the pulley resembles the rubber). The unit pictured is the one used on the 3000GT 6G72 platform, and with little cosmetic difference, identical to the one being used on the 8th Generation Galant platform.
http://i228.photobucket.com/albums/e...t/dampener.gif
http://i228.photobucket.com/albums/e...3000GTVINH.jpg
The way the newer unit is being assembled begs to be thought on even more when you bring underdrive pulleys into the picture. The main concern many people seem to have, and even those who have run them with detrimental damage to the engine, is that the UPD is not balancing the lower end as it was designed to. Secondly the pulley is usually machined from a solid piece of material, less rotational mass and smaller diameter. The way these are made makes it nearly impossible to incorporate a harmonic balancer. But, here is the kicker to everything we have known. Go back to how the original unit is being assembled and pay close attention to where they have imbedded the rubber on the pulley. You will see the solid center section of the pulley (which bolts directly to the crank), is surrounded by the rubber insert, and then around that sits the actual drive belt pulley. If you are looking at how this all works, then you are starting to imagine the crank rotating, with a quick thrust the drive pulley will have a bit of “twist” as the rubber absorbs some of the twisting motion )this is also known due to the way these units will deteriorate). This rubber will also help to reduce any secondary vibrations from going through the lower end along the crankshaft because the rubber helps to dampen those.
Now that you start to understand the mechanics of how the newly designed harmonic balancer/crank pulley works we have to look closely at the balancing portion of this component. As with any engine, internal or external balancer the components still need to have a certain amount of “balance” as they will destroy any engine. When you take your engine in to be rebuilt, or if you are doing it yourself anybody who has done engine building for optimum performance will balance their engine. The key component being balanced is usually the crankshaft and machine shops require two additional items to make a quality adjustment. The two items needed are the flywheel/flexplate and the crankshaft pulley, for an externally balanced engine. Now, since the 6G72 platform we are looking at now is an internally balanced engine design your machine shop only needs one element, and guess what it is. (Reference thread, Page 3, Post #48) Your crank pulley could potentially throw the balance off as they are mass produced with different weights in the rubber (build up areas), the plastic used in the pulley and so on.
Now the real dilemma comes when we throw an underdrive pulley onto the crankshaft without any rubber dampening, or most importantly no balancing. In order to make such a pulley work you would need to fabricate some form of dampening on the pulley to reduce/remove the secondary vibration. One idea here is to have a piece of thick rubber cut to sit between the face of the crank, and the back of the pulley with a hole slightly smaller in diameter than the crank bolt. In theory this would help absorb those secondary vibrations, preventing them from “ringing” on the underdrive pulley and essentially amplifying the intensity of the vibrations. The only downside to this idea is that you would likely not get a sufficient torque reading, or even if you did there is still the possibility of misalignment for the pulleys, and the crank bolt working itself loose.
Don’t forget this isn’t all that will need to be done in order for this to work. The crankshaft and pulley will need to be taken to a machine shop to be balanced as one unit. In order to achieve the best results you cannot leave out the rubber dampener concept as the slightest imperfection in the rubber used could throw the assembly out of balance, resulting in serious damage.
