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The reason I ask is a friend has a modified Golf GTI (with the 1.8t engine, K03S turbo) running 320 lbft @ 3200 RPM. Somebody said this is borderline the limit of what the rods can take before they could bend. As the engine has a small turbo however, higher up the rev range this tails off. The poople who usually get this sort of torque usually have bigger turbos and produce it higher up the rev range.
So basically is it still the same stress producing all that torque at a lower rev range?
Formula One's fundamental ethos is about success coming to those with the most ingenious engineering and best ..............................organization, not to those with the biggest budget. (Dave Richards)
The torque curve is a result of the turbo been run to its maximum (it runs 24PSI at 3200) but not been able to flow enough air so as the revs rise, torque falls.
So as a visulisation - is it the forcefull push down on the psiton that bends it as apposed to how fast its moving up and down?
I may have misunderstood the question. If it relates to valve drives, connecting rod (which experience considerable innertial forces) rpm certainly matters. Also where roller bearings are involved. I thought the questio was about the power transmission over a rotating shaft due to the mentioning of torque and rpm.
Formula One's fundamental ethos is about success coming to those with the most ingenious engineering and best ..............................organization, not to those with the biggest budget. (Dave Richards)
You need some more parameters for this specific engine to start on some approximations for stress in the con-rod.
Firstly - mass of the piston, mass of the con-rod and the crank radius (or stroke). If you approximate the motion of the piston as a simple harmonic motion (with amplitude of the crank radius or 1/2 stroke length) then the maximum acceleration will be (crank radius) x (rotation speed)^2. So the inertial force will be (mass piston + some fraction of con rod mass) x (crank radius) x (rotational speed)^2.
Bear in mind that this force is maximum at top and bottom dead centre - for an SHM approximation it is zero when the piston is halfway through the stroke.
Secondly you can approximate the force on the con-rod during the power stroke. If you know the engine configuration (4 inline) ( Not sure if the Gti has a flat crank or 4 cranks evenly spaced) and the firing sequence then you can figure out how many cylinders are contributing to power at any given time and hence the torque per cylinder. If we neglect friction and the pumping losses at the cylinders not contributing to power, then the torque per cylinder divided by the crank radius gives the average force acting on the crank (and hence the con-rod). Note that for this approximation the con-rod force due to the power stroke is maximum some where between TDC and BDC. Also this approximation is a lower bound, as some power is needed to overcome friction and pumping losses, so the actual torque at the cylinders during a power stroke will be higher.
Everything I have suggested above is an approximation of some kind - no allowances are made for transients, where max cylinder pressure occurs, etc.
Inertial forces increase w.r.t speed^2. So for your example with double the engine speed, the inertial forces are 4 times greater. The absolute inertial force depends on the masses of the pistons and con-rods. The force due to torque does not depend on the speed at which the torque occurs, and is a linear function - so halving the torque as per your example halves the force.
Thank you for taking the time to write such an amazing reply. Unfortunatly a lot of it is too complicated for me
Its the 1.8t 20v engine. There were a few versions of it, all essentially the same but the higher output ones (this wasnt one of them) had a forged crank, this has a cast crank apparently rated to 350 lbft or 370 BHP. Standard power the engine is 180 BHP and 175 lbft.
Anyway...
The only information I can find is stroke which is 86.4mm. Obviously this on its own is pretty useless
Essentially though what your saying is the car is at the limit of the rods (and crank it now seems) even though that torque is at relatively low revs?
With thanks to Carrillo Industries, one of the leading aftermarket con-rod suppliers, taken from their website:
"Carrillo is often asked about general horsepower ratings for the various designs of its connecting rods. Unfortunately, this cannot be answered in a simple way. The most relevant rod design parameters are Inertia forces (a function of engine speed, crank train geometry and assembly weights) and Cylinder firing pressure (tuning, fuel, boost, etc.).
Through increased engine speed, displacement, firing pressure or a combination thereof, horsepower is gained. However, though the various changes in the cycle, the demand on the rods varies greatly. Moreover, extreme dynamic loads on the drive train such as intermittently free spinning wheels or propellers (Hill Climb races, Off-Shore boat races) should be considered the when making the right choice of rod.
Based on our experience combined with our analysis methods we like to provide you with our best suggestion for your specific application."
I happen to know through speaking to some of their tech guys (and they may not endorse this to you if you ask, and probably quite rightly) that a good quality lightweight forged rod (such as their A-beam design) should generally take 50hp per cylinder O.K.
If you are going more than that a chunkier design may be necessary (such as their H-beam) or at least that is how I remember it. Try giving a U.K. distributor a call.
Being a turbo your engine may have forged rods from the factory and they may be of similar quality as aftermarket ones such as Carrillo, but as you can tell already by the amount of 'may's' I have used in this post it is pretty hit and miss. Reuben has given an insight into the factors that need to be taken into account to make a more accurate calculation there is no real middle ground, your friend can either take the 'she'll be right approach' or try and really crunch the numbers, start weighing pistons etc.
Dedicated V.W. tuning forums may have a wealth of historical experience on this engine, so maybe someone from their could help?
I suppose if it is not a particularly rare or expensive engine and your friend is both a member of a recovery organisation and handy with the spanners and an engine hoist there is an argument to say that they might as well run it and if it blow's you can replace the con-rod's on the next one... maybe buy the one with the higher output next time and start from there?
If you decide to tear it down though it is probably wise to replace the crank shell bearings and fit uprated main cap bolts while your doing it. Also check the pistons and bores for wear, if it's done over 50-80,000 miles, particularly hard ones it will be worth getting a re-hone, and fitting fresh piston rings.
A ballpark figure for aftermarket con rod's is around £150/cylinder in batches of four, the re-hone could cost £60-£100, rings maybe a tenner a cylinder. Perhaps these are the number's you guys should crunch? Do you defiantly spend plenty of cash now, or maybe spend even more later???
this exact topic has been recently talked about by Mike Kojima, ex Nismo and TRD engineer, and Dave Coleman, SCC ex editor/writer on their new online mag/website.
I have that engine on my car, standard. The thing that "tunning" people should learn is called "FATIGUE".
This is a typical fatigue curve:
The element (in this case a rod) is designed with a method that contemplates the fact that it will repeat its job several times in its life cycle. If you watch the curve for ferrous alloys, you will guess that the more stress you put to the rod, the less cycles you can use it prior to the failure. Rods for street cars, unlike for racing cars, are generally designed to work below or at the fatigue limit. This is typically the stress where the element can be cycled 5*10^5 times before failure:
So, the stress the rod can handle is arround the double of what it is stressed in the standard version of the engine. Maybe it will last some use, but what I can not tell you is how much time it will last. Maybe a standard engine rod will last 1,000,000 kilometres using the engine like my grandma does... but "maybes" is not the field were I like to be
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti wrote:I have that engine on my car, standard. The thing that "tunning" people should learn is called "FATIGUE".
This is a typical fatigue curve (...)
fatigue curves (like S-N) usually show number of cracks till the crack, of defined size which needs to be checked, initiation (it's sometimes called Ni life). reaching the limit does not have to be an equivalent of failure as crack can grow up to the critical size (Np life). once the second limit is reached we can say full LCF capabilities were utilized. it's also possible crack will not grow at all, that may happened if compressive stress field dominates.
I am not an piston engine specialist but I think you may also need to check if HCF is an issue.
Its certainly giving me a lot to think about and look into.
That thread on another forum was also very interesting, although a little in conclusive.
So far the engine has been at this power level for around 20,000 miles. Its driven hard a lot of the time and has done approx 4 track days. The engine all together has done 82,000 miles.
The engine itself has been flawless, the same can't be said for the gearbox and flywheel though.
If a rod does go is it usually quite easy to fix or does it usually break lots of things? By that I mean is it worth changing them (given how well they are doing so far) or should we just wait and see, then replace if it goes?
