When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Greg, Thank you! i know this is everyday stuff, but for us that dont build engines everyday, but have experience in engineering (or engineering 101) we sometimes just try and look at problems to solve them by removing variables. since both engines used this new tapered rod, i was just asking if this could possibly be an issue. my POINT was that you were saying at the beginning, the rod was "exactly the same", and then you corrected yourself. so, you think its possible, but not probable that the new rods are an issue. OK.. lets move on....
As far as offset, YES, i understand the increased side loads and was also thinking that the rod might be "bending" because of the misalignment, even as subtle as it is. i also thought there was enough room on the wrist pin side to side, to accomodate some variance, but can see if there was, how the side loading and bearing wear might be accelerated due to misalignment. I didnt know that the Devek motors blew because of this . i know the first motor, Gregory's, is still running today, 30 years later. (100,000mile street motor with some track days)
So, what is your take in the Chevy sized bearing accommodation vs the stock smaller 928 bearing? toss up, or not a big deal?
again, thanks for the info... i appreciate your insight . things like figuring out how to optimize the rebuild in the 5 liter 2 valve by optimizing the piston offset, was awesome.
Let us know when you pull this motor apart.. i would be expecting that piston to show sign of detonation, correct?
Mk
Originally Posted by GregBBRD
Of course, the new rods that Carrillo designed for this application are the newer tapered beam design.....but you knew this.....you have a picture of the current design 928 rod that you showed last week.....why are you being so stupid?
What, exactly, is your point?
Rod offset is engine design 101......day one. Cylinder offset on opposite sides of a V-8 (or any V design) engine vary from one engine type to the next engine type. This requires an offset that is built into the connecting rod to center the rod on the piston pin. Study up. Look up the big words in the dictionary and try to understand.
The "side" loading on the pistons and rods in an engine with the incorrect offset rod (rod not centered on the pin) is incredibly high.....and was largely responsible for the terrible cylinder damage in all of the early Alusil bore stroker engines that Mark Thomas built. Nicosil engines, because their bores are tougher fair better....but the loads are terrible and wear always occurs on the top of one side of the piston and at the bottom side of the piston on the opposite side.....the piston literally "rocks" in the cylinder every single time combustion occurs.
Chevy rods in a 928 engine is a terrible lapse in judgement/knowledge.
Does that plug look that unusual? it certainly is not "normal" but the insulation area doesnt look too much different than when i first drove my stroker for a few 100 miles and dyno runs before its first race
see below plug over concrete. but it does appear that the insulation is a little overheated. i dont think there is an issue with the steel portions being "clean" isnt that a function of additives, and race gas tends to make plugs look "cleaner". but those "little *****" on the electrode are odd and could be the result of detonation. could that cause a rod to snap? and if it was detonating, wouldnt you think the rods would be pounded flat?????
I think your logic is spot on. if the other plugs dont look like this, that wasnt the issue (engine tune) and maybe that single cylinder was the cause due to an air leak as you pointed to.
I wonder if Mark has the pictures of what was left of the plug on the MAJOR air lead cylinder when the intake failed
at Road ameica.
Ive seen plugs from engines that were pinging and they looked a little worse than this one. but , like greg, i dont have a huge amount of experience reading plugs.
Mk
Originally Posted by SwayBar
That spark plug pretty much tells the entire story why that cylinder blew, and had nothing to do with rod choice.
Let's say the rest of the spark plugs look good, therefore it's safe to assume that there was some sort vacuum-leak leaning out that one cylinder causing the detonation and destroying the engine. That would also imply the fuel was good, as well as the tune, because the other 7 plugs/cylinders are good.
On the other hand, if the rest of the plugs all exhibit some form of detonation, then it is reasonable to assume that the tune and/or fuel was the root cause of the failure.
Being able to view the rest of the plugs would provide valuable forensic clues, and are a lot easier to pull versus an entire engine.
Originally Posted by GregBBRD
I'm not sure I know how to read this spark plug....unlike any spark plug I've ever seen. I've seen spark plugs with melted center electrodes, caused from excessive timing....but I've never seen anything like this.
The "steel" portion of the plug is bare steel, which looks like something I just removed from my bead blaster. There's absolutely no trace of anything....just bare steel all the way to the base.
It appears like there are tiny "*****" attached to the "strap" portion of the electrode. If you blow up the picture, you can see them.
The porcelain appears to be burned on the left side (in the picture). The rest of the porcelain seems normal.
That spark plug pretty much tells the entire story why that cylinder blew, and had nothing to do with rod choice.
Let's say the rest of the spark plugs look good, therefore it's safe to assume that there was some sort vacuum-leak leaning out that one cylinder causing the detonation and destroying the engine. That would also imply the fuel was good, as well as the tune, because the other 7 plugs/cylinders are good.
On the other hand, if the rest of the plugs all exhibit some form of detonation, then it is reasonable to assume that the tune and/or fuel was the root cause of the failure.
Being able to view the rest of the plugs would provide valuable forensic clues, and are a lot easier to pull versus an entire engine.
I'd be happy to post pictures of all the spark plugs.
All spark plugs, are almost identical, with the exception of the cylinder that failed. It is covered with a white color on the steel portion and has redish clear spherical ***** down in the gap between the porcelain and the steel. This initially was thought to be rust, however the perfectly round "*****" are not typical of any rust I've seem. These red spheres are very shiny and appear to be clear....like tiny ***** of red glass. I'm not sure this spark plug can be read, or what it can tell us, because the injector continued to spray fuel and the spark plug continued to fire, without the piston moving for many revolutions after the failure.
One other spark plug has these same redish clear spherical *****, I have no idea what these are or represent.
Rob Edwards looked at these ***** and said' "Finally, alien life has been found.:"
Greg, Thank you! i know this is everyday stuff, but for us that dont build engines everyday, but have experience in engineering (or engineering 101) we sometimes just try and look at problems to solve them by removing variables. since both engines used this new tapered rod, i was just asking if this could possibly be an issue. my POINT was that you were saying at the beginning, the rod was "exactly the same", and then you corrected yourself. so, you think its possible, but not probable that the new rods are an issue. OK.. lets move on....
As far as offset, YES, i understand the increased side loads and was also thinking that the rod might be "bending" because of the misalignment, even as subtle as it is. i also thought there was enough room on the wrist pin side to side, to accomodate some variance, but can see if there was, how the side loading and bearing wear might be accelerated due to misalignment. I didnt know that the Devek motors blew because of this . i know the first motor, Gregory's, is still running today, 30 years later. (100,000mile street motor with some track days)
So, what is your take in the Chevy sized bearing accommodation vs the stock smaller 928 bearing? toss up, or not a big deal?
again, thanks for the info... i appreciate your insight . things like figuring out how to optimize the rebuild in the 5 liter 2 valve by optimizing the piston offset, was awesome.
Let us know when you pull this motor apart.. i would be expecting that piston to show sign of detonation, correct?
Mk
I have way too much experience with Turbo engines getting overboosted and melting small holes on the edges of the pistons. I'm not sure of the "fine line" between things being just a bit lean (and making too much heat) and detonation.
Unlike overheating or overboosting I'm not sure the piston will melt or distort with detonation. I know the piston gets "pushed" back in the direction it was coming from....but the piston has to follow the bore. The connecting rod is still at an angle (and can be at a rather severe angle,depending on what position the piston is when detonation occurs) and the loading on the connecting rod gets very high.
Although I'm not sure if it's detonation, overheating, or just wear, but Porsche Motorsports paid particular attention to the "lathe" marks on the outside of the piston, above the 1st ring. The marks will get "mushed" or completely disappear, depending on the manufacturer of the piston.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
It doesnt sound like the rod would fail based on any detonation factor alone , but it could be the starting /initiating factor. A bad detonation would or could crack a piston, in an area that could jam it in the bore, and then the weak link (the rod) could break.
Again, you have to look at the variables of these two engine failures. the ONLY change was the rods (that you selected and were different, and corillo designed with 928 offsets) Same track, same driver, same engine tune , settings, redline, shift points, etc, If it was detonation, what caused it, especially if mark has been running 100octane, all along (with a mixture 50/50 of 110 and 91octane)? I'e been with mark enough to know at the track , he is pretty methodical about his gas mixture and refueling. There is little chance that a mis rev downshift could develope the forces nessary to destroy a rod like that. i brought this topic up a while ago, as to a possible cause to damage to rod bearings(as the force can increase over engine output with a mismatch RPM clutch released downshift from 4th to 3rd, or much higher force, 5th to 4th, IF and only if you get near a rear tire "chirp" or near slip. but that would require a pretty fast clutch release on that clutch release to increase those forces to the engine to do damage.)
also, i would imagine that max force detonation would be caused at near TDC, so rod angles wouldnt be as sever.
so, what is left, high RPM? compare the rod ratios to other engines using this rod design. if the tapered H design was sub-par in strength, wouldn't it break near the small end? corvettes, BMWs, Aston martins, and many others use this kind of stroke length, and operate at near 7000-8,000rpm all day long with no issues.
.
Originally Posted by GregBBRD
I have way too much experience with Turbo engines getting overboosted and melting small holes on the edges of the pistons. I'm not sure of the "fine line" between things being just a bit lean (and making too much heat) and detonation.
Unlike overheating or overboosting I'm not sure the piston will melt or distort with detonation. I know the piston gets "pushed" back in the direction it was coming from....but the piston has to follow the bore. The connecting rod is still at an angle (and can be at a rather severe angle,depending on what position the piston is when detonation occurs) and the loading on the connecting rod gets very high.
Although I'm not sure if it's detonation, overheating, or just wear, but Porsche Motorsports paid particular attention to the "lathe" marks on the outside of the piston, above the 1st ring. The marks will get "mushed" or completely disappear, depending on the manufacturer of the piston.
Chevy rods in a 928 engine is a terrible lapse in judgement/knowledge.
Have you ever shared a photo showing the difference in offset compared to the "off the shelf" Chevy design? I understand the difference, but I wonder if for those playing along at home, a visual wouldn't help explain things 1,000x's better.
Originally Posted by Catorce
Erik,
This is an unusual amount of vitriol for you. Quite frankly, it sounds like Greg took over your keyboard. What gives?
Sorry, wrong thread. One of those situations where one too many things were discovered and said by someone you thought you trusted that makes you question things.
Carl knows what I'm talking about, but I don't expect him to admit it. We've been discussing this via PM but I had to stop responding, I couldn't roll my eyes any further into the back of my head without risk of permanent injury.
so, is a chevy offset almost symmetrical? i would think all the 928 rods share the same offset, correct?and if there was not room in the piston to move to one side, it would put a side load on the rod which would be a problem. I also wonder why the A -rods were pulled out of mark and joes engine ...or , why they were even selected. why were they better?? just lighter weight being the goal?
Originally Posted by Hacker-Pschorr
Have you ever shared a photo showing the difference in offset compared to the "off the shelf" Chevy design? I understand the difference, but I wonder if for those playing along at home, a visual wouldn't help explain things 1,000x's better.
so, is a chevy offset almost symmetrical? i would think all the 928 rods share the same offset, correct?and if there was not room in the piston to move to one side, it would put a side load on the rod which would be a problem. I also wonder why the A -rods were pulled out of mark and joes engine ...or , why they were even selected. why were they better?? just lighter weight being the goal?
Hacker:
That's a good drawing of the problem. It's been a few years, but as I recall the Chevy rod in the 928 application is .065" off of the centerline. Truthfully, I didn't think much about the problem when I first started building these engines and just did "Monkey See, Monkey Do" in the beginning.....following what everyone else had always done. That changed, one day, when Carillo got involved in a review of a broken rod......they had a **** fit! They now have notes in their computers regarding never selling Chevy rods for a 928 application, because of the incorrect offset.
The Chevy offset problem is so bad in a 928 engine that the piston boss generally has to be ground on, so the rod doesn't rub on the boss it gets moved towards!
That "tipping motion" and resulting load on the rod is insanely high. I've had a computer virus since Carrillo sent me the printout with the actual forces involved.....I'm trying to see if any of the people I sent that to still have a copy of that information.
so, is a chevy offset almost symmetrical? i would think all the 928 rods share the same offset, correct?and if there was not room in the piston to move to one side, it would put a side load on the rod which would be a problem. I also wonder why the A -rods were pulled out of mark and joes engine ...or , why they were even selected. why were they better?? just lighter weight being the goal?
I broke an "A" rod with Chevy offset in testing, sent it to Carrillo for failure analysis, when they discovered the "offset problem. I think I learned more about engine design during this phase than I ever knew before. They've got some really bright people in their engineering department!
Joseph's engine never had a set of "A" rods installed. I changed the connecting rods in Joseph's engine to this rod, when we changed the crankshaft design. Mark only had the "A" beam rod in for an event or two, when I removed them.
[Try to keep in mind that I'm using absolutely zero parts with the same dimensions that the "original stroker" (Devek generation). These people (and still some people, today) were trying to use "off the shelf" items to save money and be able to obtain pieces without a custom design and special order (more $$$$). Every single part that I currently use has been re-designed for use in the 928 application....only.]
If the acceptable crank end float is 0.4mm [approx 16 thou] one presumes that an acceptable centreline is maintained by the stock rods providing the crank spins within a window that is plus or minus 8 thou wide- a motor in good condition typically demonstrates a crank end float of 0.2mm.
The diagrams Jim attached in his mail are pretty much self explanatory. Assuming rods are made from 4340 or some equivalent material, a rod correctly centred on the piston will see a max stress of 94 ksi which given the yield stress is about 104 ksi is just about where one would want it [90% of yield]. The diagram for the 65 thou offset rod is rather startling indicating max stress would be 175 ksi so no wonder failure occurs given a defined UTS of 161 ksi.
If an incorrectly offset rod is 65 thou away from the centreline it should sit on can impart such stress, how could anyone install a rod that far out? It seems so obvious I have to wonder what it is that I fail to understand - surely I am missing something here?
If the acceptable crank end float is 0.4mm [approx 16 thou] one presumes that an acceptable centreline is maintained by the stock rods providing the crank spins within a window that is plus or minus 8 thou wide- a motor in good condition typically demonstrates a crank end float of 0.2mm.
The diagrams Jim attached in his mail are pretty much self explanatory. Assuming rods are made from 4340 or some equivalent material, a rod correctly centred on the piston will see a max stress of 94 ksi which given the yield stress is about 104 ksi is just about where one would want it [90% of yield]. The diagram for the 65 thou offset rod is rather startling indicating max stress would be 175 ksi so no wonder failure occurs given a defined UTS of 161 ksi.
If an incorrectly offset rod is 65 thou away from the centreline it should sit on can impart such stress, how could anyone install a rod that far out? It seems so obvious I have to wonder what it is that I fail to understand - surely I am missing something here?
I agree.. which makes me wonder why Greg , having installed the A-rods why were they not just pulled to be replaced with another same set with the correct offset. why the change and why not use the rods that had been used successfully? seems those were the most robust. (the non tapered H beam) . maybe it was in the interest in weight savings of the rotating assembly
05-15-2018, 12:55 PM
