Question about sintered rods
Thread Starter
Race Car
Joined: Nov 2012
Posts: 4,779
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From: Houston TX
Question about sintered rods
Hi guys, I don't own a 928 but I'm a huge admirer of all the water-cooled P-cars. Quick question about the sintered rods that some of your 928s came with. Is there a general "feel" for how they stack up against cast rods? I am sure they are more capable, but have no basis for how much more.
I'm familiar with the manufacturing process but as far as real-world translation into safe work rating, there doesn't seem to be a lot of information known on the 944 side. I understand the 928 parts are similar in their design.
Any insight appreciated! There seem to be more people who have boosted 928s than there are people who have built turbo 944 engines with NA internals (because normally, you'd just buy 951 parts).
I'm familiar with the manufacturing process but as far as real-world translation into safe work rating, there doesn't seem to be a lot of information known on the 944 side. I understand the 928 parts are similar in their design.
Any insight appreciated! There seem to be more people who have boosted 928s than there are people who have built turbo 944 engines with NA internals (because normally, you'd just buy 951 parts).
Thread Starter
Race Car
Joined: Nov 2012
Posts: 4,779
Likes: 74
From: Houston TX
I thought so too - but I was reading the WSM carefully, and it turns out that the '86 951 rods are drop-forged, while the '82-83 944 rods are sinter-forged (I'm building a turbo motor from a '82 block/crank/rods and leaving my '83 NA engine as a spare). The sintering a different forging process than what you get with 951 rods, technically...enough to make a difference.
User ptuomov just sent me a very helpful PM about sintered rods in boosted 928s... it looks like they're good for 700+ whp+ in a 5.0
exactly what I was looking for. This motor is probably going to be between 300-350 whp in a 2.5.
User ptuomov just sent me a very helpful PM about sintered rods in boosted 928s... it looks like they're good for 700+ whp+ in a 5.0
exactly what I was looking for. This motor is probably going to be between 300-350 whp in a 2.5.
Nordschleife Master
Joined: Jan 2009
Posts: 5,609
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From: MA
That's just my opinion. It's not a huge sample size that I've seen do that with turbos. Supercharger people may have much more data points.
Plus of course the reliability of all components will go down if you multiply the factory spec power by 2.5-3x. This includes rods. Something will probably break at some point.
It's however my humble opinion that the 928 PPF rods are very strong as long as one doesn't exceed the stock redline. The stock rods and pistons are heavy, high rpms are their Achilles heel.
I know nothing about 944 rods, what was forged and what was cast in which year etc.
Plus of course the reliability of all components will go down if you multiply the factory spec power by 2.5-3x. This includes rods. Something will probably break at some point.
It's however my humble opinion that the 928 PPF rods are very strong as long as one doesn't exceed the stock redline. The stock rods and pistons are heavy, high rpms are their Achilles heel.
I know nothing about 944 rods, what was forged and what was cast in which year etc.
Rennlist Member
Joined: Aug 2014
Posts: 2,480
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From: Adirondack Mountains, New York
I'm not an engine builder, or particularly knowledgeable about engine design practice. The metallurgy of jet engine parts was my thing. I'll blather on anyway....
I would think top-of-stroke acceleration produces the only major tensile stress, proportional to the square of RPM. The gas pressure-induced compressive stress is probably minor compared to bottom-of-stroke acceleration stress (guessing); a compressive failure seems unlikely.
So, going past redline is risky - the squared relationship is nasty. The rod's temperature might rise with power output; that might cause tempering and reduce strength; probably not.
What's not mentioned in these discussions is the alloy being used. "Steel" can vary from a strength of 20,000 psi to 300,000 psi. Fatigue strength varies proportionally, all else being equal. Cast versus forged versus powder means things are seldom equal, however. The specifics are critical within each approach - fatigue is defect sensitive. For example, are the parts shot-peened? A metallurgist would insist on cutting up a few rods and looking at polished sections under a microscope - and more - before venturing any opinions.
A Porsche drawing presumably references numerous formal specifications that govern the alloy chemistry, all processes involved, and all testing and inspection protocols. The word "Confidential" would be prominent on all drawings and specification, so these details are unknown to us. A supplier would be required to fix his process and disclose any deviations. Where I worked, deviant hardware was literally locked in a cage, pending review by a Material Review Board.
I would think top-of-stroke acceleration produces the only major tensile stress, proportional to the square of RPM. The gas pressure-induced compressive stress is probably minor compared to bottom-of-stroke acceleration stress (guessing); a compressive failure seems unlikely.
So, going past redline is risky - the squared relationship is nasty. The rod's temperature might rise with power output; that might cause tempering and reduce strength; probably not.
What's not mentioned in these discussions is the alloy being used. "Steel" can vary from a strength of 20,000 psi to 300,000 psi. Fatigue strength varies proportionally, all else being equal. Cast versus forged versus powder means things are seldom equal, however. The specifics are critical within each approach - fatigue is defect sensitive. For example, are the parts shot-peened? A metallurgist would insist on cutting up a few rods and looking at polished sections under a microscope - and more - before venturing any opinions.
A Porsche drawing presumably references numerous formal specifications that govern the alloy chemistry, all processes involved, and all testing and inspection protocols. The word "Confidential" would be prominent on all drawings and specification, so these details are unknown to us. A supplier would be required to fix his process and disclose any deviations. Where I worked, deviant hardware was literally locked in a cage, pending review by a Material Review Board.
Rennlist Member
Joined: May 2010
Posts: 950
Likes: 3
From: SW FL
I'm not an engine builder, or particularly knowledgeable about engine design practice. The metallurgy of jet engine parts was my thing. I'll blather on anyway....
I would think top-of-stroke acceleration produces the only major tensile stress, proportional to the square of RPM. The gas pressure-induced compressive stress is probably minor compared to bottom-of-stroke acceleration stress (guessing); a compressive failure seems unlikely.
So, going past redline is risky - the squared relationship is nasty. The rod's temperature might rise with power output; that might cause tempering and reduce strength; probably not.
What's not mentioned in these discussions is the alloy being used. "Steel" can vary from a strength of 20,000 psi to 300,000 psi. Fatigue strength varies proportionally, all else being equal. Cast versus forged versus powder means things are seldom equal, however. The specifics are critical within each approach - fatigue is defect sensitive. For example, are the parts shot-peened? A metallurgist would insist on cutting up a few rods and looking at polished sections under a microscope - and more - before venturing any opinions.
A Porsche drawing presumably references numerous formal specifications that govern the alloy chemistry, all processes involved, and all testing and inspection protocols. The word "Confidential" would be prominent on all drawings and specification, so these details are unknown to us. A supplier would be required to fix his process and disclose any deviations. Where I worked, deviant hardware was literally locked in a cage, pending review by a Material Review Board.
I would think top-of-stroke acceleration produces the only major tensile stress, proportional to the square of RPM. The gas pressure-induced compressive stress is probably minor compared to bottom-of-stroke acceleration stress (guessing); a compressive failure seems unlikely.
So, going past redline is risky - the squared relationship is nasty. The rod's temperature might rise with power output; that might cause tempering and reduce strength; probably not.
What's not mentioned in these discussions is the alloy being used. "Steel" can vary from a strength of 20,000 psi to 300,000 psi. Fatigue strength varies proportionally, all else being equal. Cast versus forged versus powder means things are seldom equal, however. The specifics are critical within each approach - fatigue is defect sensitive. For example, are the parts shot-peened? A metallurgist would insist on cutting up a few rods and looking at polished sections under a microscope - and more - before venturing any opinions.
A Porsche drawing presumably references numerous formal specifications that govern the alloy chemistry, all processes involved, and all testing and inspection protocols. The word "Confidential" would be prominent on all drawings and specification, so these details are unknown to us. A supplier would be required to fix his process and disclose any deviations. Where I worked, deviant hardware was literally locked in a cage, pending review by a Material Review Board.
+1 think Pratt & Whitney or General Electric, BTDT