Dave W.

Does anyone know what's inside the black plastic covers on the balance shaft? Are they solid or hollow?
I'm thinking about using a lathe to remove a certain amount of weight from the balance shaft to counter the weight I'm removing from the reciprocating assembly. Since the covers make the whole thing round it's going to cut into the plastic cover too.

blown 944

just remove them and use a dampner rather than a regular pulley. my .02

thingo

I've only seen the Broadfoot method, no experience with them.

Voith

Old shafts have no plastic, that would probably make it a better candidate for removing material. Plastic can fall off and cause hole in the block.

Paulyy

Is it even worth doing that to the balance shafts?

Voith

If you put lighter pistons and rods in an engine, balance shafts will put out more vibrating energy than reciprocating mass. Thus creating secondary vibration that were designed to cancel.

In theory.

67King

Do you know how to calculate revised imbalance, and subsequently the revised balance shaft requirement? I don't, and I wouldn't mess with it unless I knew how to calculate it. I think you are asking for more trouble than it is worth. Keep in mind, it isn't grams you are using, it is gram-mm's (or ounce-inches), as inertia is what is critical, not mass.

FWIW, if you do a search for parts, you'll see that the 968, S2, Turbo, NA, and S all use the same balance shaft - only the 2.7L uses a different one. I'd expect that the differences between all of the ones using the same shafts are probably greater than your deviation from stock.

Chris White

Keep in mind that the S2 had much lighter pistons - so the same balance shaft weight may have worked out due to the longer throw / higher piston speeds. The 968 had lighter pistons and rods...so something is odd with the S2 and 968 using the same shafts.

I can offer a semi scientific method for 'adjusting' the balance shafts - figure out the reciprocating weight (piston, rings, pins and little end of the rods) of the stock set up and the set up you are using. Calculate the percentage difference and then cut off that percentage of the length of the counter weighted part of the balance shaft. If you get hear the mounting point of the plastic you will have a problem with it staying in place. The plastic is just an aerodynamic aid - remember that he balance shafts are spinning at twice the speed of the crank....up to 13,000 rpm

67King

I bet you are a lot better at riddles than I. I was sitting here thinking you'd take it off diametrically. <headsmack>.

Dave W.

Thanks for all the input. Yes I looked at a formula that calculates the imbalance and it's complex since the weights distance from center matters.
I already weighed the new pistons and rods. The new reciprocating assembly is 124 grams lighter per cylinder. That's a big difference, I don't feel comfortable keeping the stock shafts intact.
For comparison the weight of piston/rods from a stock 951 is 3 lb- 10 ounces
My new piston/rod combo is 2lb-11 ounces
Just for comparison a Mitsubishi DSM 1990-1992 6 bolt piston/rod is 2lb- 13 ounces
Stock Mitsubishi 2003-2006 EVO piston/rod is 2lb- 9 ounces.
The fact that Porsche used the same shafts in a few different engines tells me that the actual balance shaft weight might not matter to a high degree. I'm not looking for a perfect solution, but is there something better? Maybe the motor mounts in these cars are good enough to make up the difference?

Dave W.

Oh yeah, I looked at it as a percentage as well. Since I'm building a 2.8 stroker,
stroke increase 8.86%
weight decrease 15.53%
total difference in 'balance weight' 6.87%
OK now what? How do I measure the imbalance in the shafts?

EDIT; I just noticed Chris said to cut off from the length of the weight. Good idea! I was thinking of using a lathe and reducing the radius which is obviously complicated since it removes weight and radius.

Chris White

Yep, my way is the easy way and you don't have to do as much math!

refresh951

I messed with this some a while back and ended up aligning both shafts one tooth off in opposing directions and it made a difference. I had a thread on it IIRC.

Dave W.

I like that idea! But it still raises the question, if the plastic covers are hollow then I don't want to cut through the outer shell. If oil gets inside it'll throw off the balance again.

Interesting idea, I'll have to check out that thread.

Laust Pedersen

Since the forces involved are directly proportional to both a mass (weight) change and a stroke change, your straight forward percentage subtraction should actually work … except subtracting the two numbers results in 6.67%.
The contribution from each of the balance shafts is additive, so the lengths of the counterweights on each shaft should be reduced by only 3.33%.

Here is another thread on the subject:
https://rennlist.com/forums/944-turb...ft-belt-3.html

Laust