Con Rods - How much Torque is too much ?
11 December 2002, 11:53 PM
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This was in another thread and was not obvious from the title, may be of interest to some ?
On the topic of high engine torque potentially causing con rod failures.
In conjunction with Katana for verification, we looked at the two examples below.
Engine A > 280 bhp revving to 8250rpm
(std, very safe factory spec Sti)
Engine B > 350lb-ft torque at 5000 rpm
(recklessly modded car allegedly about to bend its con rods)
Assuming a piston&pin mass of 0.5kg and 50% of conrod weighing 0.28kg
Stroke = 75mm Bore = 92mm and disregarding the effect of conrod length on piston acceleration for this excercise
The standard Sti is putting 2.7x the tensile loading on its rods.
Although the combustion forces are double on the high torque motor, when they are added to the inertia loads on the downstroke, the std STI still has 14% more 'buckling' compressive forces acting on the rod than the 350lb-ft engine.
When you compare the sum change in forces between tensile and compressive, the Sti is subjecting the rods (and the big ends & crank) to an extra 50% cyclic loading stress.
The main point being that Subaru consider the rod is designed for use up to 8250 rpm on the Sti. At 350 lb-ft torque we have still not exceeded the standard loading in tension or compression.
At a 5000 rpm peak torque it would take 415lb-ft of torque to load the rod to the same level as a bog standard Sti running to 8250 rpm.
If your peak torque Is produced at lower than 5000rpm then your rods/bearings/crank are even safer !
I’m more worried about the standard head gaskets and bolts ! If they go, lets water into the oil, runs a big end bearing, siezes on to the rod, snaps the rod, punches a hole out the side of the block…………………..
Someone looks in and says “Aye, running too much torque bent the rods”
On the topic of high engine torque potentially causing con rod failures.
In conjunction with Katana for verification, we looked at the two examples below.
Engine A > 280 bhp revving to 8250rpm
(std, very safe factory spec Sti)
Engine B > 350lb-ft torque at 5000 rpm
(recklessly modded car allegedly about to bend its con rods)
Assuming a piston&pin mass of 0.5kg and 50% of conrod weighing 0.28kg
Stroke = 75mm Bore = 92mm and disregarding the effect of conrod length on piston acceleration for this excercise
The standard Sti is putting 2.7x the tensile loading on its rods.
Although the combustion forces are double on the high torque motor, when they are added to the inertia loads on the downstroke, the std STI still has 14% more 'buckling' compressive forces acting on the rod than the 350lb-ft engine.
When you compare the sum change in forces between tensile and compressive, the Sti is subjecting the rods (and the big ends & crank) to an extra 50% cyclic loading stress.
The main point being that Subaru consider the rod is designed for use up to 8250 rpm on the Sti. At 350 lb-ft torque we have still not exceeded the standard loading in tension or compression.
At a 5000 rpm peak torque it would take 415lb-ft of torque to load the rod to the same level as a bog standard Sti running to 8250 rpm.
If your peak torque Is produced at lower than 5000rpm then your rods/bearings/crank are even safer !
I’m more worried about the standard head gaskets and bolts ! If they go, lets water into the oil, runs a big end bearing, siezes on to the rod, snaps the rod, punches a hole out the side of the block…………………..
Someone looks in and says “Aye, running too much torque bent the rods”
12 December 2002, 12:48 AM
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Andy,
It was the head gasket that finally let go on my STi engine.
It's all very well comparing the forces exerted on a rod at 280bhp/8250rpm, and 350ftlbs/5000rpm, but these are just two figures, and not very representative.
I think it's fair to say, that most people who tune their engine to 350ftlbs, are also getting far higher than 280bhp. So this also needs to be taken into account.
So, most STi's will actually be producing their max power at closer to 7000rpm, and if you were to rev to the same, like most people who tune their cars do, how safe would you feel then
Mark.
It was the head gasket that finally let go on my STi engine.
It's all very well comparing the forces exerted on a rod at 280bhp/8250rpm, and 350ftlbs/5000rpm, but these are just two figures, and not very representative.
I think it's fair to say, that most people who tune their engine to 350ftlbs, are also getting far higher than 280bhp. So this also needs to be taken into account.
So, most STi's will actually be producing their max power at closer to 7000rpm, and if you were to rev to the same, like most people who tune their cars do, how safe would you feel then
Mark.
12 December 2002, 02:04 AM
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Interesting post Andy,
I'm a firm believer in mechanical sympathy
Revs and heat will always be the main engine breakers. Dodgy mapping aside.
Just out of interest maximum piston speed in a scoob at 8250rpm is around 110 feet per second, 65ft/sec at 5000rpm.
0 to 75mph and back to 0 again in 6 inches
I'm a firm believer in mechanical sympathy
Revs and heat will always be the main engine breakers. Dodgy mapping aside.Just out of interest maximum piston speed in a scoob at 8250rpm is around 110 feet per second, 65ft/sec at 5000rpm.
0 to 75mph and back to 0 again in 6 inches
12 December 2002, 10:07 AM
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I think it's fair to say, that most people who tune their engine to 350ftlbs, are also getting far higher than 280bhp. So this also needs to be taken into account.
RPM kills rods. My 360bhp at 6000 rpm is still safer than running to 8250 rpm at standard power (even on Euro pistons, Richard
)
12 December 2002, 10:08 AM
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Andy,
great post have to say.
I have always been a firm believer in theory, and you have calculated something I hadnt considered before.
To be honest, I only worried about the rods under compression when running serious boost, and attempting to extract 400lbft. I always thought it was tensile stress which caused failures, and this comes from excessively heavy pistons and too high a rev limit.
I was tempted to push my new rev limit up to 7400rpm, even with the longer stroke, but what you are saying is making me consider pulling it down even further, perhaps to below 7000.
It certainly makes a lot of sense, even with uprated rods, mechanical sympathy would suggest 7000rpm to be the absolute maximum.
With regard to the question you pose, on the facts alone, I cant see any flaw in what you are saying. Am inclined to believe it is a combination of torque and high rpm in the uprated engine that might tend to kill them.
We are within the "standard envelope" at peak power rev point on the 350bhp according to what you have calculated, but then I dont actually know of many cars which have failed due to excessive loads on the rods caused by a power hike.
most people get nervous and then go internal. If it is mapped properly, I cannot actually see why you might be wrong.
Would appear that rev limit is the crucial factor. Also explaining why the Uk cars seem to be fine at 350bhp.
great post have to say.
I have always been a firm believer in theory, and you have calculated something I hadnt considered before.
To be honest, I only worried about the rods under compression when running serious boost, and attempting to extract 400lbft. I always thought it was tensile stress which caused failures, and this comes from excessively heavy pistons and too high a rev limit.
I was tempted to push my new rev limit up to 7400rpm, even with the longer stroke, but what you are saying is making me consider pulling it down even further, perhaps to below 7000.
It certainly makes a lot of sense, even with uprated rods, mechanical sympathy would suggest 7000rpm to be the absolute maximum.
With regard to the question you pose, on the facts alone, I cant see any flaw in what you are saying. Am inclined to believe it is a combination of torque and high rpm in the uprated engine that might tend to kill them.
We are within the "standard envelope" at peak power rev point on the 350bhp according to what you have calculated, but then I dont actually know of many cars which have failed due to excessive loads on the rods caused by a power hike.
most people get nervous and then go internal. If it is mapped properly, I cannot actually see why you might be wrong.
Would appear that rev limit is the crucial factor. Also explaining why the Uk cars seem to be fine at 350bhp.
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12 December 2002, 10:14 AM
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What's the weight difference between and STi and Euro model piston? And how does this affect the outcome assuming the same 350lb/ft torque?
Interested to see, as a ratio, how much more dangerous on the conrods it is to be thwapping this much air through a Euro car......
Richard
Interested to see, as a ratio, how much more dangerous on the conrods it is to be thwapping this much air through a Euro car......
Richard
12 December 2002, 10:21 AM
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Cheers Adam/Richard I can put some relative figures together for different stroke engines and different piston weights if someone can supply the exact weights of piston/pin/rod and crank stroke, rod length would also be helpfull as it can have a big effect on peak piston acceleration.
I'm off to play in my garage at the moment but will do it at work this evening
I can put some relative figures together for different stroke engines and different piston weights if someone can supply the exact weights of piston/pin/rod and crank stroke, rod length would also be helpfull as it can have a big effect on peak piston acceleration.I'm off to play in my garage at the moment but will do it at work this evening
12 December 2002, 12:37 PM
#11
cool
what happens for the people getting a flat torque curve though?
That would be my worry, with your engine say, if you were getting 400ftlbs at 6000rpm (450hp).
Not saying it (the torque and rpm) would happen, but it's getting away from high torque peak at mid range dropping at high rpm.
In some sense would it not be good to rate rods on HP as a general spec?
Subaru rod in paticular is quite a narrow section.
Paul
what happens for the people getting a flat torque curve though?
That would be my worry, with your engine say, if you were getting 400ftlbs at 6000rpm (450hp).
Not saying it (the torque and rpm) would happen, but it's getting away from high torque peak at mid range dropping at high rpm.
In some sense would it not be good to rate rods on HP as a general spec?
Subaru rod in paticular is quite a narrow section.
Paul
12 December 2002, 12:38 PM
#12
(recklessly modded car allegedly about to bend its con rods)
There have been a number of claims on this BBS that running with torque over 300lb-ft can bend standard rods
If the bearing picks up and the rod ends up bent, the bearing damage will be considerable. A simple bent rod will leave the bearing more or less intact, providing the crank stops, and doesn't just rip the block to pieces.
Sti also has 50grams less piston mass in its favour
paul
[Edited by Pavlo - 12/12/2002 12:46:36 PM]
12 December 2002, 12:43 PM
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I don't think any results could deemed to be "clinically proven" - for me it's more hypothetical interest....should I buy your short motor or not Paul?!
Richard
Richard
12 December 2002, 01:06 PM
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Paul
Not aimed at you !! Can't even remember who it was that posted the 300lb-ft warning ?
Agree a bhp/rpm limit is appropriate for rods, not mid range torque
ps How do you get 5000lb-ft from a scoob anyway
Not aimed at you !! Can't even remember who it was that posted the 300lb-ft warning ?
Agree a bhp/rpm limit is appropriate for rods, not mid range torque
ps How do you get 5000lb-ft from a scoob anyway
12 December 2002, 02:01 PM
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...scampers off wondering if he shouldn't rev it to 7000 RPM any more 
I do drop the boost from 6000-7000 RPM (from 20 to 18 PSI) but it is a heck of a lot of fun from 6000-7000 RPM.
I do drop the boost from 6000-7000 RPM (from 20 to 18 PSI) but it is a heck of a lot of fun from 6000-7000 RPM.
12 December 2002, 07:38 PM
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Don't read too much into this guys. The calculations that we did are sorta "Mickey Mouse" like and is just as a guide. I'm sure the materials used are strong enough and also has a bit extra safety factor (something like 9000 rpm). Engineers aren't stupid. But on the other hand all you're doing is shortening the life of the materials.
Here's something for you guys, a single M8 bolt can take the weight of a small car easily.
Here's something for you guys, a single M8 bolt can take the weight of a small car easily.
12 December 2002, 07:54 PM
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The post was not meant to scare anyone
The opposite actually, as long as you are not way over 400lb-ft torque and remain within the manufacturers rpm limit the rods should not pose a problem.
Assuming here that you don't still have 400lb-ft at the rpm limit ! (this would equate to 533bhp @7k or 609bhp @8k )
The opposite actually, as long as you are not way over 400lb-ft torque and remain within the manufacturers rpm limit the rods should not pose a problem.
Assuming here that you don't still have 400lb-ft at the rpm limit ! (this would equate to 533bhp @7k or 609bhp @8k
)
12 December 2002, 10:59 PM
#20
fatigue is the big problem here.
On the bolts, which are arguably more stressed than the rest, since they have to deal with nearly all the mass of the rod. However, the shape, surface finish and heat treament are more repeatable for the bolt, and can be heat treated to a higher ultimate strength due to size and shape, and it doesn't need to be worked (any cutting or forging process) so much, so can be made from a harder base material.
Generally you need to take the UTS of the steel to be that of the fatigue limit. Steel does have a fatigue limit (unlike most Al alloys), that is to say a stress which is lower than yield, but if not exceeded, will take an infinite number of cycles before fracture.
Manufacturers may choose to move further up the fatigue curve to a point where Xmillion cycles relates to 200,000 projected miles of use, or whatever.
Usual caveats apply, fatigue limit is heat treatment, surface treatment and operational temperature dependant.
Paul
On the bolts, which are arguably more stressed than the rest, since they have to deal with nearly all the mass of the rod. However, the shape, surface finish and heat treament are more repeatable for the bolt, and can be heat treated to a higher ultimate strength due to size and shape, and it doesn't need to be worked (any cutting or forging process) so much, so can be made from a harder base material.
Generally you need to take the UTS of the steel to be that of the fatigue limit. Steel does have a fatigue limit (unlike most Al alloys), that is to say a stress which is lower than yield, but if not exceeded, will take an infinite number of cycles before fracture.
Manufacturers may choose to move further up the fatigue curve to a point where Xmillion cycles relates to 200,000 projected miles of use, or whatever.
Usual caveats apply, fatigue limit is heat treatment, surface treatment and operational temperature dependant.
Paul
12 December 2002, 11:45 PM
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Andy,
So are you telling me that I could have got away with stock rods in my engine Hmmmmmmm............
Does anyone know what the stock rod bolts are good for ?
Just as a note: I realise that you guys have worked this all out, but could you be missing something ?
I've spoken to several of the rally guys, and engine builders, and they say that they will only use the stock rods in the GrpN engines for circa 3 events, before they scrap them. This is because they have a habit of breaking.
Now, we know that the GrpN's aren't running much more than 280bhp, and that's a well below 8000rpm. We also know that GrpN's will be lucky to see 400ftlbs, although some may.
Whilst it's obviously against the rules, most of the guys I speak to, tend to replace the rods with up rated ones (tut tut
It just seems that wilst the numbers add up, the reality doesn't.
Any thoughts ?
Mark.
So are you telling me that I could have got away with stock rods in my engine
Hmmmmmmm............Does anyone know what the stock rod bolts are good for ?
Just as a note: I realise that you guys have worked this all out, but could you be missing something ?
I've spoken to several of the rally guys, and engine builders, and they say that they will only use the stock rods in the GrpN engines for circa 3 events, before they scrap them. This is because they have a habit of breaking.
Now, we know that the GrpN's aren't running much more than 280bhp, and that's a well below 8000rpm. We also know that GrpN's will be lucky to see 400ftlbs, although some may.
Whilst it's obviously against the rules, most of the guys I speak to, tend to replace the rods with up rated ones (tut tut
It just seems that wilst the numbers add up, the reality doesn't.
Any thoughts ?
Mark.
13 December 2002, 08:05 AM
#24
I have said this before (and been told i was wrong ) but i have an article from Racetech magazine when they profiled the MY96 Group A Impreza as driven by Colin and Carlos. In this they state that the group A cars MUST run standard rods as dictated by the rules (only the pistons can be changed). I don't know this for a fact but im just going off what the mag said.... which in the past, and currently, has some very good factual articles.
I may try to scan in the section on what can and was done to the group a engines next week if anybody is interested.
Tony.
) but i have an article from Racetech magazine when they profiled the MY96 Group A Impreza as driven by Colin and Carlos. In this they state that the group A cars MUST run standard rods as dictated by the rules (only the pistons can be changed). I don't know this for a fact but im just going off what the mag said.... which in the past, and currently, has some very good factual articles.I may try to scan in the section on what can and was done to the group a engines next week if anybody is interested.
Tony.
13 December 2002, 10:57 AM
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Tony,
I'm not questioning the rules. What I'm saying, is those people who stick to them, tend to change the rods on a very regular basis.
However, I know for a FACT, that many of the engines are built with parts that BREAK the rules.
Mark.
I'm not questioning the rules. What I'm saying, is those people who stick to them, tend to change the rods on a very regular basis.
However, I know for a FACT, that many of the engines are built with parts that BREAK the rules.
Mark.
13 December 2002, 11:11 AM
#26
Also worth remembering, that 2 rods can look the same, but may be vastly different in terms of material heat treatment and surface treament, leaving it stronger (or weaker) as a result.
paul
paul
13 December 2002, 11:24 AM
#27
Mark,
I'd agree that just cos the rally cars used to use them (369lb/ft) doesn't mean that's a good indication that if you run 370lb/ft you'll be ok. The rally cars only need them to last for three days.... not so good on a road car .
What i was actually questioning was that i was told that these were not the rules and that uprated rods could be used (don't think it was you that said this tho?). I guess that people competing under group a rules nowadays could try to get away with using uprated rods that 'look' standard but would Subaru/Prodrive really have tried to get away with this under the FIAs nose (1995,96)????
Tony.
I'd agree that just cos the rally cars used to use them (369lb/ft) doesn't mean that's a good indication that if you run 370lb/ft you'll be ok. The rally cars only need them to last for three days.... not so good on a road car
.What i was actually questioning was that i was told that these were not the rules and that uprated rods could be used (don't think it was you that said this tho?). I guess that people competing under group a rules nowadays could try to get away with using uprated rods that 'look' standard but would Subaru/Prodrive really have tried to get away with this under the FIAs nose (1995,96)????
Tony.
13 December 2002, 11:29 AM
#28
2 rods could be made from the same forging tool, from different matierals, using different bolts, and higher spec heat treatment that costs more money, takes more time etc etc.
afterwards, shot-peening, cryo treatment
Still a standard rod, but stronger and lasts many times longer.
I'm sure that Subaru would have sorted out some special rods for pro-drive to use if they asked nicely.
Paul
afterwards, shot-peening, cryo treatment
Still a standard rod, but stronger and lasts many times longer.
I'm sure that Subaru would have sorted out some special rods for pro-drive to use if they asked nicely.
Paul
13 December 2002, 11:47 AM
#29
The article says this:
'The two-bolt I section steel cond rod is another mandatory production part but it is fitted with competition big end bolts and a competition steel gudgeon pin which runs in a bronze bushed small end, retained by circlips.'
Taking into account that the engine was rarely run past 6500rpm, due to the restrictor meaning the power drops off from there, do you think it would have been worth Prodrive breaking the rules when the standard rod could probably take this abuse for 3 days? I really couldn't see it.... even using cleverly 'disguised' standard rods.
Tony.
'The two-bolt I section steel cond rod is another mandatory production part but it is fitted with competition big end bolts and a competition steel gudgeon pin which runs in a bronze bushed small end, retained by circlips.'
Taking into account that the engine was rarely run past 6500rpm, due to the restrictor meaning the power drops off from there, do you think it would have been worth Prodrive breaking the rules when the standard rod could probably take this abuse for 3 days? I really couldn't see it.... even using cleverly 'disguised' standard rods.
Tony.
13 December 2002, 12:03 PM
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So are you telling me that I could have got away with stock rods in my engine
Which one of your engines are you meaning Mark? Any increase in stroke or bore is likely to increase the rod loading at the same rpm as a 2.0 so uprated rods would be required.
I would consider that if you are rebuilding an engine with a 2.5 crank or a target power over 400bhp then uprated rods are essential. I for one would fit uprated rods if I was going internal.
I think comparing road cars with rally cars is not ideal, rally cars spend their whole life at high revs on 0 or 100% throttle, the two worst conditions for rod life. They also leave the road and bounce off the limiter frequently.
I would estimate that rally cars also generate a lot of exhaust backpressure to work that compressor into pulling through a small restrictor. If the EGBP is high then the BMEP (force on piston) will be higher for the same power/torque output as a car with a bigger, more efficient turbo.