Piston slap explaination by GM Engineer
08-29-2005, 12:06 PM
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Piston slap explaination by GM Engineer I pulled this from www.bobistheoilguy.com. This is what a GM engineer had to say about piston slap. It does make me feel better.
Here is what a GM engineer told me on this subject..
quote:
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Are you talking about piston slap during the first few minutes of warmup..??
If so...that is what it is. Nothing to fear or nothing really wrong...just the piston noise until the piston warms up to fill the clearance to the bore.
Pistons rock as they cross TDC where the thrust load from the rod angularity shifts from one side to the other. This makes noise. Harmless..but it is the source of piston slap in most all situations.
To prevent this, pistons are designed with long, tapered, flexible skirts so that they can be fitted very tight in the bores when the bore and piston is cold. The taper of the skirt and flexibility of the skirt then prevents scuffing when the piston is hot. Also, the piston pin in OEM production pistons is always offset to one side....it is NOT in the middle of the piston. By offsetting the pin in the piston, artificial thrust load is created to control the piston "rocking" as it crosses over TDC.
Unfortunately, all of the above control techniques, common in past model engines to the extreme, create excess piston mass, cause friction and cost power and fuel economy. With the desire to build in as much power and free-revving capability and to improve fuel economy as much as possible thru friction reduction these design features are pushed in the other direction on modern engines.
Piston pin offset has been reduced over the years to a bare minimum today to reduce the thrust load generated and reduce friction. Pistons have been lightened up considerably by shortening the skirts. This creates less rotating/reciprocating mass which is good for power, free revving capability and fuel economy. Light weight pistons are great but the skirts, by necessity, are short making it hard to make them both strong and flexible and the shorter ckirts make them more prone to rocking.
Unfortunately, when the performance and fuel economy oriented pistons are run cold they are very prone to "slap" until they warm up to operating temperature.
The piston designers and development engineers are always treading the fine line between piston slap cold and friction and power/fuel economy loss when the engine is warm.
It is possible that you are hearing piston noise from an engine that is on the "high limit" for piston clearance so that it makes some noise cold. The good news is that the condition is harmless and that engine is probably a little more powerful (due to less friction) than a "quiet" counterpart. The bad news is that...it makes noise cold.
As an example of what the piston pin offset can do, it was common back in the early 70's to turn the pistons around "backwards" in the large displacement Chrysler engines to gain power. Those engines had large piston pin offsets to create thrust load to control the piston slap. So much thrust load and friction was created that just turning the pistons around in the bores was often good for 10 HP. The engines were very quiet with the pistons in correctly and they slapped like crazy, especially when cold, with the pistons reversed so as to reverse the pin offset.
Racing engines do not have the pin offset and thus the pistons slap like mad...but no one hears them over the open headers...LOL.
The noise is an annoyance but will not hurt anything and the engine is fine.
I don't remember if that engine had full floating pins or not but they can also cause some ticking just after start. Same sort of deal...the "correct" clearances in floating pins will cause clicking after startup for 30 seconds or so. To prevent the start up noise, the floating pin clearance must be kept so tight that the pins are almost press fit again.
Here is what a GM engineer told me on this subject..
quote:
--------------------------------------------------------------------------------
Are you talking about piston slap during the first few minutes of warmup..??
If so...that is what it is. Nothing to fear or nothing really wrong...just the piston noise until the piston warms up to fill the clearance to the bore.
Pistons rock as they cross TDC where the thrust load from the rod angularity shifts from one side to the other. This makes noise. Harmless..but it is the source of piston slap in most all situations.
To prevent this, pistons are designed with long, tapered, flexible skirts so that they can be fitted very tight in the bores when the bore and piston is cold. The taper of the skirt and flexibility of the skirt then prevents scuffing when the piston is hot. Also, the piston pin in OEM production pistons is always offset to one side....it is NOT in the middle of the piston. By offsetting the pin in the piston, artificial thrust load is created to control the piston "rocking" as it crosses over TDC.
Unfortunately, all of the above control techniques, common in past model engines to the extreme, create excess piston mass, cause friction and cost power and fuel economy. With the desire to build in as much power and free-revving capability and to improve fuel economy as much as possible thru friction reduction these design features are pushed in the other direction on modern engines.
Piston pin offset has been reduced over the years to a bare minimum today to reduce the thrust load generated and reduce friction. Pistons have been lightened up considerably by shortening the skirts. This creates less rotating/reciprocating mass which is good for power, free revving capability and fuel economy. Light weight pistons are great but the skirts, by necessity, are short making it hard to make them both strong and flexible and the shorter ckirts make them more prone to rocking.
Unfortunately, when the performance and fuel economy oriented pistons are run cold they are very prone to "slap" until they warm up to operating temperature.
The piston designers and development engineers are always treading the fine line between piston slap cold and friction and power/fuel economy loss when the engine is warm.
It is possible that you are hearing piston noise from an engine that is on the "high limit" for piston clearance so that it makes some noise cold. The good news is that the condition is harmless and that engine is probably a little more powerful (due to less friction) than a "quiet" counterpart. The bad news is that...it makes noise cold.
As an example of what the piston pin offset can do, it was common back in the early 70's to turn the pistons around "backwards" in the large displacement Chrysler engines to gain power. Those engines had large piston pin offsets to create thrust load to control the piston slap. So much thrust load and friction was created that just turning the pistons around in the bores was often good for 10 HP. The engines were very quiet with the pistons in correctly and they slapped like crazy, especially when cold, with the pistons reversed so as to reverse the pin offset.
Racing engines do not have the pin offset and thus the pistons slap like mad...but no one hears them over the open headers...LOL.
The noise is an annoyance but will not hurt anything and the engine is fine.
I don't remember if that engine had full floating pins or not but they can also cause some ticking just after start. Same sort of deal...the "correct" clearances in floating pins will cause clicking after startup for 30 seconds or so. To prevent the start up noise, the floating pin clearance must be kept so tight that the pins are almost press fit again.
08-29-2005, 05:02 PM
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This explaination is total dog dung.
Keep returning the car and document all returns. Under the lemon law they have to fix it or you can demand a full refund.
If pistom slap were normal, all cars would have it. Since GM is the only one and only certain GM cars from any given model have slap, it IS NOT NORMAL. This is a factory defect that GM has ignored for many years.
They tried to fix it by changing the design of the piston rings. There are several GM service bulletins outlining the issues.
My 99 F-body does not have it but plenty of 99's do. I'm just lucky!
Keep returning the car and document all returns. Under the lemon law they have to fix it or you can demand a full refund.
If pistom slap were normal, all cars would have it. Since GM is the only one and only certain GM cars from any given model have slap, it IS NOT NORMAL. This is a factory defect that GM has ignored for many years.
They tried to fix it by changing the design of the piston rings. There are several GM service bulletins outlining the issues.
My 99 F-body does not have it but plenty of 99's do. I'm just lucky!
08-29-2005, 07:09 PM
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narc, what you are probably talking about is spark knocking
LS1s are so noisy becuase of the short piston skirts, the valve train and the fact that they are all aluminum. my car sounds like a diesel truck when it first starts up, that or a spun bearing. its horrible. it use to bother me, but it dosent bother me at all now. LS1s are noisy for many reasons, but its not an issue on reliablity or performance (otherwise my A4 car with 140k on it wouldnt run 13.4s)
LS1s are so noisy becuase of the short piston skirts, the valve train and the fact that they are all aluminum. my car sounds like a diesel truck when it first starts up, that or a spun bearing. its horrible. it use to bother me, but it dosent bother me at all now. LS1s are noisy for many reasons, but its not an issue on reliablity or performance (otherwise my A4 car with 140k on it wouldnt run 13.4s)
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08-29-2005, 09:48 PM
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Originally Posted by robertbartsch
This explaination is total dog dung.
Keep returning the car and document all returns. Under the lemon law they have to fix it or you can demand a full refund.
If pistom slap were normal, all cars would have it. Since GM is the only one and only certain GM cars from any given model have slap, it IS NOT NORMAL. This is a factory defect that GM has ignored for many years.
They tried to fix it by changing the design of the piston rings. There are several GM service bulletins outlining the issues.
My 99 F-body does not have it but plenty of 99's do. I'm just lucky!
Keep returning the car and document all returns. Under the lemon law they have to fix it or you can demand a full refund.
If pistom slap were normal, all cars would have it. Since GM is the only one and only certain GM cars from any given model have slap, it IS NOT NORMAL. This is a factory defect that GM has ignored for many years.
They tried to fix it by changing the design of the piston rings. There are several GM service bulletins outlining the issues.
My 99 F-body does not have it but plenty of 99's do. I'm just lucky!
08-30-2005, 01:54 AM
#10
Originally Posted by robertbartsch
This explaination is total dog dung.
Seriously piston slap complaints are silly. Do you guys pee standing up? I mean seriously.
What do I know I just grew up with a real GM automotive engineer for my whole life, hell I'm sure we could call up a few people who work in GM Powertrain and get the same answer from all of them, but in reality "do you guys pee standing up?" gets about the same point across. It just makes me sick to see guys complain about something as minimal as piston slap.... forget that you have a motor that with a set of headers on it can out RWHP a 1969 427 COPO ZL1 car, deliever 2-3 times the fuel economy and last for well over 100,000 miles. I'd like to see the guys who complain about piston slap adjust the valves on the original ZL1 cars and drive them to work everyday 35 years ago.
Bret
08-30-2005, 03:17 AM
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Originally Posted by SStrokerAce
Actually it's a GREAT explaination to what is going on and that it is 100% correct.
Seriously piston slap complaints are silly. Do you guys pee standing up? I mean seriously.
What do I know I just grew up with a real GM automotive engineer for my whole life, hell I'm sure we could call up a few people who work in GM Powertrain and get the same answer from all of them, but in reality "do you guys pee standing up?" gets about the same point across. It just makes me sick to see guys complain about something as minimal as piston slap.... forget that you have a motor that with a set of headers on it can out RWHP a 1969 427 COPO ZL1 car, deliever 2-3 times the fuel economy and last for well over 100,000 miles. I'd like to see the guys who complain about piston slap adjust the valves on the original ZL1 cars and drive them to work everyday 35 years ago.
Bret
Seriously piston slap complaints are silly. Do you guys pee standing up? I mean seriously.
What do I know I just grew up with a real GM automotive engineer for my whole life, hell I'm sure we could call up a few people who work in GM Powertrain and get the same answer from all of them, but in reality "do you guys pee standing up?" gets about the same point across. It just makes me sick to see guys complain about something as minimal as piston slap.... forget that you have a motor that with a set of headers on it can out RWHP a 1969 427 COPO ZL1 car, deliever 2-3 times the fuel economy and last for well over 100,000 miles. I'd like to see the guys who complain about piston slap adjust the valves on the original ZL1 cars and drive them to work everyday 35 years ago.
Bret
Could it be new technology.
08-30-2005, 03:39 AM
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Originally Posted by robertbartsch
This explaination is total dog dung.
Keep returning the car and document all returns. Under the lemon law they have to fix it or you can demand a full refund.
If pistom slap were normal, all cars would have it. Since GM is the only one and only certain GM cars from any given model have slap, it IS NOT NORMAL. This is a factory defect that GM has ignored for many years.
They tried to fix it by changing the design of the piston rings. There are several GM service bulletins outlining the issues.
My 99 F-body does not have it but plenty of 99's do. I'm just lucky!
Keep returning the car and document all returns. Under the lemon law they have to fix it or you can demand a full refund.
If pistom slap were normal, all cars would have it. Since GM is the only one and only certain GM cars from any given model have slap, it IS NOT NORMAL. This is a factory defect that GM has ignored for many years.
They tried to fix it by changing the design of the piston rings. There are several GM service bulletins outlining the issues.
My 99 F-body does not have it but plenty of 99's do. I'm just lucky!
I accept mine not so much as normal but as the nature of the beast, and I sure don't want a mechanic I don't know and trust poking around inside my block. I live with it for the 2 minutes it lasts during warm up and then accelerate smoothly past all of those quiet engines.
08-30-2005, 07:34 AM
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Do a search on some of robert bartsch's posts. You'll see he has some "interesting" and "unique" opinions on things 
I guess I've gotten used to piston slap when cold over the years - it doesn't bother me at all, and in fact I didn't even notice until people started talking about it that my brand new LS1's did it. There's no harm done as evidenced by the thousands of tear downs performed by our members.
I guess I've gotten used to piston slap when cold over the years - it doesn't bother me at all, and in fact I didn't even notice until people started talking about it that my brand new LS1's did it. There's no harm done as evidenced by the thousands of tear downs performed by our members.
08-30-2005, 01:02 PM
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All I am saying is that if I paid a ton of money for a new car and it had slapping pistons I would demand a refund. I really don't care that the noise goes away after a few miles. So I'm picky!
After you own this car for a few years you may want to sell it to someone else. I don't think I would buy a used car with piston slap; would you?
There was some law firm that posted here a year ago asking for the names of people who owned piston slappers. They were planning to file a class action suit to force GM to deal with this issue.
After you own this car for a few years you may want to sell it to someone else. I don't think I would buy a used car with piston slap; would you?
There was some law firm that posted here a year ago asking for the names of people who owned piston slappers. They were planning to file a class action suit to force GM to deal with this issue.
08-30-2005, 01:37 PM
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After tearing down a motor from my Formula @ 72k, that I bought brand new in 1998 with 9 miles on the clock, I absolutely would buy a used car with cold start piston slap as commonly experienced in the LS1's. I wouldn't give it a second thought - just like I didn't give it a second thought when I bought my brand new 2004 GTO, which is quieter than the Formula but still will rattle a bit on cold mornings.
Heck, my forged 370cid motor rattles a LOT more than a stock LS1 does when its cold, does that mean its a piece of crap and I should sue the engine builder?
bartsch, it really sounds like this platform isn't what you want. Maybe you would be happier in something a little more "refined" (read: slower, heavier, less efficient, built with Chinese tools -- but nice and quiet)
Heck, my forged 370cid motor rattles a LOT more than a stock LS1 does when its cold, does that mean its a piece of crap and I should sue the engine builder?
bartsch, it really sounds like this platform isn't what you want. Maybe you would be happier in something a little more "refined" (read: slower, heavier, less efficient, built with Chinese tools -- but nice and quiet)
08-30-2005, 01:58 PM
#17
Relatively we might be talking about a lot of money, but in the grand sceme of things it's not a Benz or a BMW... I rather like noisy pushrod power plants that have a good design, light weight, low center of mass and are cheap to mass produce.
I agree tear one of these piston slapping motors down and your not going to see any odd signs of wear.
Bret
I agree tear one of these piston slapping motors down and your not going to see any odd signs of wear.
Bret
08-30-2005, 02:17 PM
#18
On two LS1's that I've had so far, the so called "piston slap" has gone away when I changed my lifters to the comp stock style lifters. Right off the top of my head I believe they are the 850-16 I think. I highly dought this Is the cure for eveyones problem or else I't would have been a popular fix by now.
Last edited by Oatmeal; 08-30-2005 at 02:24 PM.
08-30-2005, 02:36 PM
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I like the platform but don't care much for GM's lack of customer support. This is one of many reasons that GM is now selling cars for less than it costs them to build them.
As a friend used to say, ..."they lose money on every car they sell but they make it up on volume!"
As a friend used to say, ..."they lose money on every car they sell but they make it up on volume!"
