heinrich

Wally is correct also. I had considered this initially, that the forces on the journals must be far greater from the pistons. I am no engineer though

T_MaX

As far as the history goes, I was not aware of it, nor do I see this as being very productive or professional.

Yes I agree one SC’d car showing excessive front bearing wear may not be anything to worry about. But in order to begin a study on any problem, a potential problem must first be identified and the failure analysts documented.

Simply put, all the forces that are on the crank from the factory are balanced in one way or the other. Doing something as simple as removing the air pump without making compensations could cause the system to become unbalanced.

Should this issue become a problem, there are belt tension solutions already proven that could be applied without adding belt width and allow for a lower belt tension with zero slip. Another option is to run a cog belt like what is used for our timing belt which wouldn’t require as much tension.

John Struthers

History aside, big slugs vs huffing, I agree with Curtis.
If Marc, has indeed found, observable wear on a engine front main having only 5K worth of wear since installed and the only significant change to the torsional stresses has been the installation of an SC then it is something to be watched/looked for.
How many people queried the validity of Mark Anderson's issue with main bearing oil starvation under high G racing conditions?
As I recall -could be wrong- they tried dry sump oiling, overfilling, ect... before drilling the crank.
This is now the approved method for preventing oil starvation, right?
Why don't we just take this as a heads-up/ caution from Marc and wait for more data to come in? No harm in that!
I don't see this as a "See! I told you SC'ing was a bad idea!" thread. And Marc, also, agrees that we need more examples.
Quite some time ago I asked about possible torsional imbalance problems, in a round about response, to a smog pump removal thread. I believe a Guru said there would be no/is no problem , and while I have faith in our Guru's... there was zero automotive engineering to back his wisdom.
Hence, I still have the weight of the pump/bracketry and parasitic drag from Pattycakes' still installed smog pump. Gaining a little HP percentagewise is not,
IMO, worth the risk of blindly following undocumented advice.
Again, IMO, do I seriously believe that the SC belt is causing deflection in a well supported, Porsche spec., forged crank? Offhand, no! But, I also remember my 7th grade science teacher demonstrating Archemeides' principal, and learning the old saw: "give me a large enough fulcrum and lever and I can move the world". I have usefully applied that principal many times in life to move enormous objects to the astonishment and dismay of others. While my engineering skills/knowledge might not fill volumes ... I do notice that there are some interesting loads -which I presume to be balanced- all of which are tugging at the 'NOSE' of the crank. Keeping Archemedies in mind, I can't help but wonder what really happens to the crank/supports when we add or remove torsional loads. When bad things happen to close tolerance spinning pieces of machinery - and I'm just talking in 100's of rpm's not 1000's- self destruction is the rule of thumb.
Being interested in a SC and saving for a kit, I would rather add a modified bracket with additional rollers/tensioners ( IF NEEDED ) at the outset.
In the much valued words of 'Number 5', " Input Stephanie, Input!"

Z

Yes, and it's never been an issue that i know of. The one supercharged car was running way, way more boost than 5psi, it ran that boost for quite a bit longer than 5,000 miles, and they were the original bearings with around 140K miles on them when they were taken out and inspected. They were in such good condition that the same bearings were put back in, and the car's been driven at the same high boost levels as before for probably about a year and another 12K miles since then.

I agree with what Wally has said, and also wonder about one more thing. If it was due to the belt tension, what about the supercharger bearing? After all, that belt tension was acting on the supercharger bearing as well. Is the supercharger bearing so strong that it'll last longer than a Porsche crank bearing when subject to the same belt tension? If so, maybe we should be talking to the supercharger manufacturers about making us some bearings to fix the 2/6 rod bearing issue that a number of people have experienced.

My guess is that the bearing damage may very well have more to do with the guy who installed it rather than the belt tension.

fst951

Hi Guys.

Marc made an interesting observation. Look, he pushes the stroker units because they work and he sells them. This is no surprise. He has done a lot of engineering on this engine and made a quite a few interesting pieces of info available to us.

With that said I think that it is interesting that he would notice bearing wear in such a short life engine. The supercharger certainly does apply heavier loads on the nose of the crank and pulls it up against the front bearings. Once the engine is running, however, the oil pressure is able to easily carry the loads that the supercharger provides.

With regards to Mark Kibort's auxillary tensioning devices, I am exactly the same way. I HATE to see grossly overtensioned belts. I always leave them a little under spec probably, but it makes me feel better. Also, that is a balanced system with a little more force down than up, but all in all a net zero effect on the crank.

I read Wally's points and find them interesting. I hope that you know if your car only generated 360psi in a cylinder you wouldn't be going anywhere fast. Did you know that mechanical pumps in diesel engines have to generate 15,000-20,000psi just to properly atomize the fuel and overcome the flame front pressures? Performance diesle engines operate with cylinder pressures in excess of 4000psi. In a dynamic high performance non-direct injection gasoline engines you will see cylinder pressures of 800psi with regularity.

Well even though it makes sense that a supercharger will accelerate the wear on the front of the crank an bearing surface, will it affect most people? Probably now. I suspect that your rings and piston ring grooves will wear out at before or about the same time that the engine needs to be overhauled.

In summation, enjoy the low cost performance of boost. If you want that 200,000 mile stroked engine, cool, it just costs a little more.

Good luck!

GoRideSno

Curtis,
Coged belt = very hard on bearings. Some SC mfgrs will not waranty after cog belt use.

Marc,
What type of SC was it?
What was the position of the belt on the crank pulley?
What is the diameter of the crank pulley?
How is the crank pulley shaped?
What was the tension reading on the belt?
What other accesories were run?

ALL of these must be known before any type of information on this can be called useful.


Andy K

Bill Ball

Really?! I was assuming with no personal knowledge that a toothed belt would be better by allowing lower tension. The Vortech factory kit I saw on a 350Z was toothed. What does a toothed belt do to wreck bearings? Thanks.

hacker-pschorr

This is so funny. The day after a thread starts about Turbo vs SC in the 944 forum, the 928 room jumps to bearing wear from SC belts.

Well, at least we are past the SC vs turbo argument - or until spring when they dyno sheets start flying around again.

Andy - jump over to the 944 forum - a few turbo guys are trying to say Superchargers don't give full bust until redline. That is more your area than anyone's.

Pete's going to have fun with that thread - Randy, you might need to school this young admin.

James-man

My read is that Marc has made an observation, has asked a legitimate question on tension, and has opened an interesting thread topic for which there have been several good responses. Whether or not Marc's post was truly in good faith is absolutely subordinate to the questions and issues raised. I encourage you to open another thread if you want to talk about history and integrity of individuals.

I am interested in how one would approach optimization between minimum of slippage and a minimum of tension. Wouldn't it be to place the tensioner very close to the pully - and how close?

Echoing Bill's question: Cogged belts? Yes/No & why.

Why not chain-driven superchargers? Tension could be low. Is RPM the problem or is it keeping the chain clean/lubricated? Maybe sound?

Wally's analysis is interesting, perhaps WAY over my head, however aren't the stresses applied to the crankshaft from within the engine shared between bearings on both ends whereas SC stress/tension is really only on one?


Z - good point on SC bearing. Used SC buyers have something else to consider? Could the same be said for a lousy/wobbly SC pulley? On the other hand, who would let somthing that bad remain on their car?

Maybe Marc can post some pics for you guys to look at in addition to a few of Andy's questions. Although, Andy, it is not very likely that the tension was measured before the engine was taken apart. Good thing to put on a to-do list though!

Party on!

heinrich

Hold your horses. What you're hearing Me, Z and Andy say is .... we have only one sample. Then Blown Beast adds, he has sample TWO and that has shown NO wear from the supercharger. And Z adds, he has seen sample number THREE with no such wear. Andy adds, OK then we need more data.

heinrich

James, note no-one is doubting Marc's findings. We in fact do accept that he has found this and we are discussing it.

James-man

The minute I saw Marc's post, I assumed that this would turn into another "Marc trying to sell strokers" melee. People are actually behaving themselves pretty well.

James-man

Heinrich - you are right, we now have several cases where there is no evidence of bad wear. Encouraging. Would be nice for others to chime in with theirs as well as it influences buyers. If we get 10 good experiences to 1 bad, then we should feel even better about our odds with this and it will offer more empirical evidence of success.

Absolutely, you are right in that we should not jump off the deep end, but I think that the tensioning issues have been shared by enough folks that the "wrappage" suggestion may be applicable under multiple contexts. I think most SCers agree that they do not want extreme tension for several reasons. Lower tension / higher contact patch sounds pretty good to me, um, if it works.

John..

The key on the pulley is to minimize the overhung loads. I like the fact that I can leave every belt in position, including the VC fan with the twin turbos. Every boosted solution comes with its set of issues along the way and there are no free lunches (yes, Jim I know lunch was free at Sharktoberfest). There will always be more wear here, or more stress there with a given solution.

James-man

What do you mean, in layman's terms, by overhung loads?