View Poll Results: 2-6 Rod Bearing Failures, which 928 engine-trans?
32V 2.727 gearbox (89+)
8
53.33%
32V 2.20 gearbox (85-88)
3
20.00%
16V any gearbox
5
33.33%
Multiple Choice Poll. Voters: 15. You may not vote on this poll
2-6 Rod bearing failure....a study!
04-17-2009, 03:08 PM
#91
Race Director
Well,
In the 944 motors I have never seen a rod bearing fail from detonation. I have seen blown head gaskets from detonation, but never spun bearings. I have seen guys running octane too low and pop head gaskets, but never ever spin a bearing. Also deontation can occur on all cylinder and yet only the #2 spins. On some occasions the #3 is close behind. My take is that at least in the 944 a denontaing motor will fail in the headgasket long before the rod bearings. If an engien is knocking the headgasket is the first to go.
As for the cross drilling it is ONLY #2 and #3. Not all 4. I have hearf of all 4 for a long time, but never just #2 and #3 until Greg came with it. According to him all 4 was better than none, but just #2 and #3 was the best since it balanced the flow to all 4 journals.
Now the larger clearnaces is really odd to me for two reaons. One is the physial fact that the larger clearnances require more oil flow to maiting clearance. So if you have more clearance you need more oil flow to prevent contact. If you open the clearnace and don't add more oil you are waiting for disaster. Now what evidence I have to suport this? Old motors have worn bearings due to the normal start up cycles. These slowly increase clearance over the years as metal is worn away. Now the number of engines that fail with old worn bearings as compared to new fresh bearings clearly shows that old worn bearings are much more likely to fail than an engine with fresh bearings. So that to me sounds like opening up clearnaces without providing more oil flow.
In the 944 motors I have never seen a rod bearing fail from detonation. I have seen blown head gaskets from detonation, but never spun bearings. I have seen guys running octane too low and pop head gaskets, but never ever spin a bearing. Also deontation can occur on all cylinder and yet only the #2 spins. On some occasions the #3 is close behind. My take is that at least in the 944 a denontaing motor will fail in the headgasket long before the rod bearings. If an engien is knocking the headgasket is the first to go.
As for the cross drilling it is ONLY #2 and #3. Not all 4. I have hearf of all 4 for a long time, but never just #2 and #3 until Greg came with it. According to him all 4 was better than none, but just #2 and #3 was the best since it balanced the flow to all 4 journals.
Now the larger clearnaces is really odd to me for two reaons. One is the physial fact that the larger clearnances require more oil flow to maiting clearance. So if you have more clearance you need more oil flow to prevent contact. If you open the clearnace and don't add more oil you are waiting for disaster. Now what evidence I have to suport this? Old motors have worn bearings due to the normal start up cycles. These slowly increase clearance over the years as metal is worn away. Now the number of engines that fail with old worn bearings as compared to new fresh bearings clearly shows that old worn bearings are much more likely to fail than an engine with fresh bearings. So that to me sounds like opening up clearnaces without providing more oil flow.
04-17-2009, 03:10 PM
#92
Race Director
No I believe the vibes caused it. I also had a fuel rail crack due to vibes. Vibration can cause all kinds of probelms and the engines was designed to have those secondary harmonics cancelled not amplified. So I can easly see how if amplifed they can cause all kinds of parts to break off.
04-17-2009, 03:32 PM
#93
Administrator - "Tyson"
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Wait a minute, I thought the spun bearings were from rod caps installed backwards.....
+1 I'm very surprised this isn't talked about more.
+1 I'm very surprised this isn't talked about more.
04-17-2009, 03:53 PM
#94
Quote:
Originally Posted by GregBBRD
I just have one thing that confuses me and it has nothing to do with the oiling of these engines....I keep two of the fastest 928 race engines in the world together. Please get Mark Kibort to explain how a car, (with ABS) can pull more lateral g's than braking g's with the same tires....that's the real mystery!
Originally Posted by GregBBRD
I just have one thing that confuses me and it has nothing to do with the oiling of these engines....I keep two of the fastest 928 race engines in the world together. Please get Mark Kibort to explain how a car, (with ABS) can pull more lateral g's than braking g's with the same tires....that's the real mystery!
04-17-2009, 03:56 PM
#95
I had a machinist tell me the 928 cranks were some really hard metal. If they flexed much don't you think their would be reports of broken cranks? Lots of american cranks develope stress cracks with racing.
04-17-2009, 05:17 PM
#96
Former Sponsor
There's a great book by Herb Adams called Chassis Engineering that sums this up nice. Basicly it talks about a tire's circle of traction and sums up that optimum grip in a given situation occurs when their is equal load on all four tires. With the weight up somewhat high in the front I imagine it has quite a bit of weight transfer to the front reducing overall traction a bit. When cornering laterally the front/rear distribution can be kept decent. If the front of the car is a little heavier it can be balanced with very mild acceleration thru an apex if a car is set up right. If their is much negative camber in the front it can reduce maximum breaking a touch as well. Any racers here cooberate this?
04-17-2009, 05:36 PM
#97
Former Sponsor
Well,
In the 944 motors I have never seen a rod bearing fail from detonation. I have seen blown head gaskets from detonation, but never spun bearings. I have seen guys running octane too low and pop head gaskets, but never ever spin a bearing. Also deontation can occur on all cylinder and yet only the #2 spins. On some occasions the #3 is close behind. My take is that at least in the 944 a denontaing motor will fail in the headgasket long before the rod bearings. If an engien is knocking the headgasket is the first to go.
As for the cross drilling it is ONLY #2 and #3. Not all 4. I have hearf of all 4 for a long time, but never just #2 and #3 until Greg came with it. According to him all 4 was better than none, but just #2 and #3 was the best since it balanced the flow to all 4 journals.
Now the larger clearnaces is really odd to me for two reaons. One is the physial fact that the larger clearnances require more oil flow to maiting clearance. So if you have more clearance you need more oil flow to prevent contact. If you open the clearnace and don't add more oil you are waiting for disaster. Now what evidence I have to suport this? Old motors have worn bearings due to the normal start up cycles. These slowly increase clearance over the years as metal is worn away. Now the number of engines that fail with old worn bearings as compared to new fresh bearings clearly shows that old worn bearings are much more likely to fail than an engine with fresh bearings. So that to me sounds like opening up clearnaces without providing more oil flow.
In the 944 motors I have never seen a rod bearing fail from detonation. I have seen blown head gaskets from detonation, but never spun bearings. I have seen guys running octane too low and pop head gaskets, but never ever spin a bearing. Also deontation can occur on all cylinder and yet only the #2 spins. On some occasions the #3 is close behind. My take is that at least in the 944 a denontaing motor will fail in the headgasket long before the rod bearings. If an engien is knocking the headgasket is the first to go.
As for the cross drilling it is ONLY #2 and #3. Not all 4. I have hearf of all 4 for a long time, but never just #2 and #3 until Greg came with it. According to him all 4 was better than none, but just #2 and #3 was the best since it balanced the flow to all 4 journals.
Now the larger clearnaces is really odd to me for two reaons. One is the physial fact that the larger clearnances require more oil flow to maiting clearance. So if you have more clearance you need more oil flow to prevent contact. If you open the clearnace and don't add more oil you are waiting for disaster. Now what evidence I have to suport this? Old motors have worn bearings due to the normal start up cycles. These slowly increase clearance over the years as metal is worn away. Now the number of engines that fail with old worn bearings as compared to new fresh bearings clearly shows that old worn bearings are much more likely to fail than an engine with fresh bearings. So that to me sounds like opening up clearnaces without providing more oil flow.
Yes, the problem has always been the oiling to the rods on both sides of the thrust bearing. It doesn't take a big scientific test to see that these rods don't get oil from the center main and only get fed from #2 and #4 main bearings and thus are sharing oil that goes to the front and rear rod bearings. However, most people are both cross drilling the crank and drilling to get the passages near the edges of the main bearings and away from the center. This is to provide oil at higher RPM's, which is another big problem, with the oiling design. In order to accomplish this, you need to drill all the mains to all the rods.
In terms of rod bearings, I think it might be easier to talk about this, if we separate "old bearings" from "new bearings" and not confuse wear with clearance. As I said, the Porsche rod bearings are uber soft and tend to pound out very easily. Long before the bearings fail, they loose their "crush" in the rods, from pounding. This can happen, no matter how much oil you push through them. This happens on dry sumped engines...which is why they need bearings replaced, too.
04-17-2009, 05:46 PM
#98
Rennlist Member
This might be the hidden solution. rich mixtures and stock ecu's. Maybe that is where the luck is. I almost bought dennis' car, but after seeing the A/F ratios, i was a little leary. it had AFRs in the 14:1 range. mine did too originally, until i put a regulator on there and cranked it u to 70psi! I also think a lot of the dynos might not have their systems calabrated so its often times good to go to multiple dynos just to make sure you are getting the ratios you think you are getting. Plus, the tail pipe readings are usually off initially, but end up near correct, but the wide band o2 sensor in the header is probably the best way in my opinion.
mk
mk
Uhh....
We started cross drilling these cranks, in 1987....Greg just came along and did what others had been doing, for years.
And we opened up the rod bearing clearance on the journals that are effected to get more oil between the bearings and the crank...not less.
And you missed the most important thing of all....the absolute most important thing to do to keep the bearings alive is to keep the engine from ever pushing backwards on Porsche's uber soft rod bearings. If you have over "50 knocks per 10,000 cycle", the rod is going to come out the side of the engine, before 8 hours of track time....guaranteed! Quicker, if there isn't an adequate oil cushion.
We started cross drilling these cranks, in 1987....Greg just came along and did what others had been doing, for years.
And we opened up the rod bearing clearance on the journals that are effected to get more oil between the bearings and the crank...not less.
And you missed the most important thing of all....the absolute most important thing to do to keep the bearings alive is to keep the engine from ever pushing backwards on Porsche's uber soft rod bearings. If you have over "50 knocks per 10,000 cycle", the rod is going to come out the side of the engine, before 8 hours of track time....guaranteed! Quicker, if there isn't an adequate oil cushion.
04-17-2009, 05:52 PM
#99
Rennlist Member
Are you talking about wear in terms of the bearings being uneven if they are worn, or "pounded out" vs new? So, by design, you make the new bearing clearances wider and get a little more oil in the bearing. the old bearing will have high spots and uneven areas that wear and come apart faster? Is that what you are saying?
mk
In terms of rod bearings, I think it might be easier to talk about this, if we separate "old bearings" from "new bearings" and not confuse wear with clearance. As I said, the Porsche rod bearings are uber soft and tend to pound out very easily. Long before the bearings fail, they loose their "crush" in the rods, from pounding. This can happen, no matter how much oil you push through them. This happens on dry sumped engines...which is why they need bearings replaced, too.
mk
In terms of rod bearings, I think it might be easier to talk about this, if we separate "old bearings" from "new bearings" and not confuse wear with clearance. As I said, the Porsche rod bearings are uber soft and tend to pound out very easily. Long before the bearings fail, they loose their "crush" in the rods, from pounding. This can happen, no matter how much oil you push through them. This happens on dry sumped engines...which is why they need bearings replaced, too.
04-17-2009, 06:07 PM
#100
Rennlist Member
When stopping, you have weight transfer. this is a fact of physics, and we all know this. The amount of weight transfer will be a function of the wheel base, CG and stopping g's. So, with 1.5g of stopping force, almost 80% of the weight of the car will be on the front tires, and there aint nothing you can do about it. Even if you stop, ONLY with rear brakes, you will have weight transfer to the limit of the tires on the pavement. (which will go down with more stopping g forces) You have a contact patch, as we all know as well as "slip". To get maximum traction, you need some slip, that manifests itself in tread deformation for both stopping and cornering. The contact patch forward on the front two tires might actually be different than the tire acting on the pavement laterally. In a 2g lateral turn there is also weight transfer. this is why some cars lift one or even both inside tires. This is also based on CG, track width, and weight. The tire does its job in both directions if operated in its "slip" range , both laterally and longitudinally.
Negative camber allows for better contact patch on the tires that need it. The better the geometry alows for this, but also doesnt create a need for so much that straight line braking is compromised, the better handling and braking will be. The 928s are unique in that they dont require much camber so that straight line braking is not compromised. Plus, being wider than most platforms and putting huge tires up front doesnt hurt any either!
So, in the end, its pretty easy to see that lateral acceleration will be pretty close to longitudinal decelleration if you are using the full capability of the tires.
mk
Negative camber allows for better contact patch on the tires that need it. The better the geometry alows for this, but also doesnt create a need for so much that straight line braking is compromised, the better handling and braking will be. The 928s are unique in that they dont require much camber so that straight line braking is not compromised. Plus, being wider than most platforms and putting huge tires up front doesnt hurt any either!
So, in the end, its pretty easy to see that lateral acceleration will be pretty close to longitudinal decelleration if you are using the full capability of the tires.
mk
There's a great book by Herb Adams called Chassis Engineering that sums this up nice. Basicly it talks about a tire's circle of traction and sums up that optimum grip in a given situation occurs when their is equal load on all four tires. With the weight up somewhat high in the front I imagine it has quite a bit of weight transfer to the front reducing overall traction a bit. When cornering laterally the front/rear distribution can be kept decent. If the front of the car is a little heavier it can be balanced with very mild acceleration thru an apex if a car is set up right. If their is much negative camber in the front it can reduce maximum breaking a touch as well. Any racers here cooberate this?
04-17-2009, 06:12 PM
#101
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Plastigauge will leave a small mark in the bearings they are so soft.
The old timers used to tune drag race motors by looking at the rod bearings, that is a skill that has went away and we are all better for it.
Folks now have the ability to dial in as much ignition lead as they want, and the bearings and head gaskets are going to pay a price for that in some cases.
The old timers used to tune drag race motors by looking at the rod bearings, that is a skill that has went away and we are all better for it.
Folks now have the ability to dial in as much ignition lead as they want, and the bearings and head gaskets are going to pay a price for that in some cases.
Luckily, the head gaskets are going to fail first, when someone is really "stupid" and has one detonate that bad. That's the "fuse" in the overloaded circuit. I'm talking about "marginal" or "gentle" knocking....thus the more than "50 knocks per 10,000" mention. If you push the head gasket out, there is 500 per 10,000, or more. That's a separate problem. However, if you push out the head gasket, you should probably replace the rod bearings, also. They are going to be pounded...can't get one, without the other.
Yes, the problem has always been the oiling to the rods on both sides of the thrust bearing. It doesn't take a big scientific test to see that these rods don't get oil from the center main and only get fed from #2 and #4 main bearings and thus are sharing oil that goes to the front and rear rod bearings. However, most people are both cross drilling the crank and drilling to get the passages near the edges of the main bearings and away from the center. This is to provide oil at higher RPM's, which is another big problem, with the oiling design. In order to accomplish this, you need to drill all the mains to all the rods.
In terms of rod bearings, I think it might be easier to talk about this, if we separate "old bearings" from "new bearings" and not confuse wear with clearance. As I said, the Porsche rod bearings are uber soft and tend to pound out very easily. Long before the bearings fail, they loose their "crush" in the rods, from pounding. This can happen, no matter how much oil you push through them. This happens on dry sumped engines...which is why they need bearings replaced, too.
Yes, the problem has always been the oiling to the rods on both sides of the thrust bearing. It doesn't take a big scientific test to see that these rods don't get oil from the center main and only get fed from #2 and #4 main bearings and thus are sharing oil that goes to the front and rear rod bearings. However, most people are both cross drilling the crank and drilling to get the passages near the edges of the main bearings and away from the center. This is to provide oil at higher RPM's, which is another big problem, with the oiling design. In order to accomplish this, you need to drill all the mains to all the rods.
In terms of rod bearings, I think it might be easier to talk about this, if we separate "old bearings" from "new bearings" and not confuse wear with clearance. As I said, the Porsche rod bearings are uber soft and tend to pound out very easily. Long before the bearings fail, they loose their "crush" in the rods, from pounding. This can happen, no matter how much oil you push through them. This happens on dry sumped engines...which is why they need bearings replaced, too.
04-17-2009, 06:29 PM
#102
Race Car
This might be the hidden solution. rich mixtures and stock ecu's. Maybe that is where the luck is. I almost bought dennis' car, but after seeing the A/F ratios, i was a little leary. it had AFRs in the 14:1 range. mine did too originally, until i put a regulator on there and cranked it u to 70psi! I also think a lot of the dynos might not have their systems calabrated so its often times good to go to multiple dynos just to make sure you are getting the ratios you think you are getting. Plus, the tail pipe readings are usually off initially, but end up near correct, but the wide band o2 sensor in the header is probably the best way in my opinion.
mk
mk
Forced induction may also help better equalize the amount of air going into each runner, avoiding some cylinders running leaner than others.
Dan
'91 928GT S/C
475hp/460lb.ft
04-17-2009, 08:43 PM
#103
Former Sponsor
Are you talking about wear in terms of the bearings being uneven if they are worn, or "pounded out" vs new? So, by design, you make the new bearing clearances wider and get a little more oil in the bearing. the old bearing will have high spots and uneven areas that wear and come apart faster? Is that what you are saying?
mk
mk
Yes, by design, I want the new bearings to have wider clearances and get more oil between the bearing and the crank.
gb
04-17-2009, 10:43 PM
#104
Rennlist Member
So maybe the clearances of the motor I built. (measured with plasticguage ) had the wider bearing clearances.
By the way, did some research with the other racers. It seems everyone gets more lateral acceleration vs braking acceleration. I suspect this has soemthing to do with the contact patch acting in a straigth line, vs laterally. If you notice a tires contact patch is wider than long, so it might provide more leverage to great greater lateral g's vs stoping g's.
In the end, its what all the racers see. I would be interested to see why.
mark
By the way, did some research with the other racers. It seems everyone gets more lateral acceleration vs braking acceleration. I suspect this has soemthing to do with the contact patch acting in a straigth line, vs laterally. If you notice a tires contact patch is wider than long, so it might provide more leverage to great greater lateral g's vs stoping g's.
In the end, its what all the racers see. I would be interested to see why.
mark
Pounded. You can drop one of these shells and ruin the crush in the cap. Once the crush goes away...it's over, very quickly. I could care less about the first layer that is designed for "shelf life". I sometimes remove this layer, depending on what I need or want.
Yes, by design, I want the new bearings to have wider clearances and get more oil between the bearing and the crank.
gb
Yes, by design, I want the new bearings to have wider clearances and get more oil between the bearing and the crank.
gb
04-18-2009, 12:27 PM
#105
Hey a couple more thoughts...I don't think detonation is too big an issue because it usually occurs at lower rpm. For that reason many stock cars have detonated just as much as race cars probably. The older 928's have pretty fragile looking pistons. All engines, particularly the older 16 valves 928s, that detonate much are going to get cracked rings and broken ring lands in conjunction with failed head gaskets and rod bearings. Evo's are notorious when built (and some stock) for spinning rod bearings because the rod bolts stretch. New bolts fix it. That's not the case here because the patterned failure of 2/6 only.
I'd be cautious about wanting to open up bearing clearances in a motor. Yes a "looser" motor will have more oil between the bearing and journal ideally, but in reality the oil system as a whole (and I mean down to diameters of oil holes in bearings and crank) has to be able to supply sufficient volume to maintain the clearance. In other words, if you have "x" pressure oiling system and are happy with it, increasing clearances will effectively lower oil pressure unless oil supply has sufficient spare capacity to maintain the new increase in volume requirement. Worse yet, if...say a certain passage...like the antfarm in the bedplate should become the point of lowest flow capacity for instance oil pressure can read normal where measured and be signifigantly lower after the point of a flow restriction. Ideally the multiple "ends of the line" if you will should be the point of lowest flow i.e. the bearings, lifters, etc. One of the easiest ways to maintain pressure is to use a proportionally thicker oil. Ford went thru this years ago with FE big blocks. Turning up oil pressure would not cure high rpm failures no matter how they tried. The side oiler block cured all their ills to the point of eventually winning lemans. This might sound familier I put something about this in another old post. I'm not saying tight motors are good they always fail under hard use. It's just important to have a good oil supply to match a looser motor.
BTW, I haven't heard many 16 valvers here talk about using the 944 oil flow restricters to reduce flow to the heads.
I'd be cautious about wanting to open up bearing clearances in a motor. Yes a "looser" motor will have more oil between the bearing and journal ideally, but in reality the oil system as a whole (and I mean down to diameters of oil holes in bearings and crank) has to be able to supply sufficient volume to maintain the clearance. In other words, if you have "x" pressure oiling system and are happy with it, increasing clearances will effectively lower oil pressure unless oil supply has sufficient spare capacity to maintain the new increase in volume requirement. Worse yet, if...say a certain passage...like the antfarm in the bedplate should become the point of lowest flow capacity for instance oil pressure can read normal where measured and be signifigantly lower after the point of a flow restriction. Ideally the multiple "ends of the line" if you will should be the point of lowest flow i.e. the bearings, lifters, etc. One of the easiest ways to maintain pressure is to use a proportionally thicker oil. Ford went thru this years ago with FE big blocks. Turning up oil pressure would not cure high rpm failures no matter how they tried. The side oiler block cured all their ills to the point of eventually winning lemans. This might sound familier I put something about this in another old post. I'm not saying tight motors are good they always fail under hard use. It's just important to have a good oil supply to match a looser motor.
BTW, I haven't heard many 16 valvers here talk about using the 944 oil flow restricters to reduce flow to the heads.