A few pages back I had taken the AAN apart for cleanup and balance before reassembly. This presented it's own set of problems, since anyone that has had the opportunity to take an I5 in to balance knows the look of the machinist when 5 cylinder is mentioned. There really isn't a lot of information out there on balancing the I5!! I thought that I would touch on a few threads that I've read and what I have come up with.
First discussion is always whether or not the I5 is internally or externally balanced from the factory. ALL AUDI I5 ENGINES ARE EXTERNALLY BALANCED!!!!!!! I believe they are zero balanced cranks WITHOUT added weight for dynamic reciprocating balance. By this I mean we could spin the AAN crank just bare and it was within .5 gr on the damper end and perfect on the flywheel end. The AAN flywheel is very easy to see the welded weights that are added for that end and the early I5 damper/pulleys with the chunk of metal that the tool for tightening the bolt uses is also drilled to lighten that blob and come up with a 50gr balance shot.
The AAN serpentine damper is not as easy to see the weight, but it to has balance weight added.
The second discussion is always whether or not the I5 needs to be balanced with bob weights. Bobs are used on the counter weight portion of the crank to offset the reciprocating weight which is made up of the small end of the rod, wrist pin, clips, piston, and rings. Usually a few grs. of oil weight is also added. Because inline 6 cylinders are naturally balanced and I4s also have two pistons going up at the same time as two are going down and offset one another, neither one of these configurations need bobs while balancing. The same is true with opposing engine configurations like porche, suburu, VW etc, but V8s and V6s need to balanced using made up bobs. I5s NEED TO BE BALANCED WITH BOBS. The throws are 72deg apart from one another so there is never going to be exact cancellation of moments. The ends of the I5 crank are going to want to flex up and down in relation to the center of the crank. Ideally a counter balance shaft directly above or below the crank turning the same speed as the crank should be used, but obviously packaging something like that would be difficult. More than likely the shaft would have to be off to one side and would be better, but still a compromise. Audi opted out of using a balance shaft.
The next discussion with your machinist is going to be what percentage of reciprocating mass needs to be used in making up the counterweight bobs. The crank can be in static balance meaning that if freewheeling on it's bearings the shaft will not want to turn either direction when stopped because of a heavy spot wanting to fall down, but dynamically can be out of balance. Because of the pistons constantly moving up and down and the reciprocating weight always changing along with the inertia stops on the top and bottom of the stroke, the counter weights of the crank shaft have to be weighted to offset this weight dynamically. Because it is constantly changing, the engineers come up with a percentage of the reciprocating mass to add to the rotational mass to make up the bob weights. Most V8s use 50% of reciprocating for calculating bobs whereas 60deg V6s might use 38% reciprocating in their calculations. The point being is there are all kinds of books, articles, motorhead memories and other means of finding out what is supposed to be used, but Audi I5s aren't that easy to find that information. Sometimes if the information isn't there the machinist will reverse engineer by spinning the stock crank with bobs made up with the most common factor of 50% and see if it dynamically balances or not. If not bobs of 60% or 40% are used until it gets closer and closer to what the factory used. AUDI and most I5s that I can tell use 50% reciprocating as their factor in making up the bobs.
Here is a view of my AAN crank on the balance macine with the bobs made up.
A picture of rods being weighed to determine the big end weight versus the smallend weight.
The rotating weight is made up of the big end of the rod, rod bearings and an allowance for oil. This weight is taken as 100% and added to the percentage of reciprocating weight which in this case is 50% of the weight of the piston, rings, clips, wristpin, and small end of the rod. This makes up the weight of the bob.
If, like in the case of an Audi I5, the engine is externally balanced and one wants to have it internally balanced so that aftermarket flywheels, clutches, and dampers can be used the weight that has been added to the outside ends will have to added to the counter weights on the crank to keep in balance and be all internal. This can not only be difficult, but expensive since weight has to be added by drilling, filling with mallory or other heavy metals, and then capped with a weld or threaded for a plug to keep centrifical forces from throwing the weight back out. When building a built motor with aftermarket rods and pistons there is usually enough weight difference (aftermarket being much lighter) that the crank can be internally balanced without adding weight. This is especially true when building a stroker where the crank came from a diesel application and has counter weights that had to offset large heavy duty rods and pistons. If this is the case changing to internal balance is good, but that doesn't mean that external balance is bad. It just limits you to what can be used front and rear and will have to have the same balance as the damper or flywheel that is being replaced.
I used after market Scat rods with OEM AAN pistons and wasn't able to internally balance without adding weight so I opted for an external balance. I wanted to change from the serpentine setup on the AAN to the V-belt of the 7A and use the 7A damper pulley. For the rear I used an aluminum Fidenza flywheel. We ended up drilling into the edge of the flywheel to add weight to come up with balance.
Here are some links to some balance information that I used.
http://www.google.com/url?sa=t&source=w ... wM0RgxDWCQ
http://www.google.com/url?sa=t&source=w ... u2-M4ssn0Q
http://www.s2forum.com/forum/showthread ... 880&page=3
http://motorgeek.com/viewtopic.php?f=10 ... ead#unread
viewtopic.php?f=10&t=40946viewtopic.php?f=10&t=36778http://www.s2forum.com/forum/showthread.php?t=21805http://www.codecogs.com/reference/engin ... forces.php
http://books.google.com/books?id=mX1-OJ ... q=&f=false
http://www.autozine.org/technical_schoo ... mooth2.htm
