Tensile and compression in Ti conrods.
Discussion
I'm considering some forged (not billet)Ti conrods for a blown application. I was told that they tend to fail earlier under forced application usage as they aren't as strong as forged steel in tensile loadings. Personally I think this is inaccurate but I suppose it does depend on the mix going into the forging. I'd have thought a Ti rod was superior in many ways.
Also, a blown engine will make more tq so more rotation on the crank but that's compressive and I accept there will be more activity during the exhaust pulse but won't that to some extent be damped by the backpressure due to the bug tubbies I plan to use. I'm not planning on increasing rpm's over stock usage for these rods although reciprocating masses will be a tad heavier.
The Ti rods are used in a factory supercharger installation. Anybody got some engineering based comments to share? Thanks.
Also, a blown engine will make more tq so more rotation on the crank but that's compressive and I accept there will be more activity during the exhaust pulse but won't that to some extent be damped by the backpressure due to the bug tubbies I plan to use. I'm not planning on increasing rpm's over stock usage for these rods although reciprocating masses will be a tad heavier.
The Ti rods are used in a factory supercharger installation. Anybody got some engineering based comments to share? Thanks.
GavinPearson said:
It depends on the cross sectional area of the two parts you are comparing, material properties and at what point they will yield at.
Thanks. Hmmm, I have to do an apples to apples comparison as far as size and section is concerned. Any techie data won't be available to me but I can't see GM moving to forged Ti if it's inferior to forged steel especially in a s/c application. I won't be using an s/c though.manufacturers do lots of strange things.
Why fit Titanium rods, then fit cast pistons ? ( LS7 )
I think they just throw in some bling bits for good PR. Personally, Id rather a nice set of Carillos.
Unless spinning serious rpm's, is there really any need or benefit from using titanium ??
And even then..SixSpeedsInc run a n/a LSx motor, that spins to 9600rpm. Im fairly sure they are using steel rods.
If that isnt extreme, what is ? !!!!!! lol
Why fit Titanium rods, then fit cast pistons ? ( LS7 )
I think they just throw in some bling bits for good PR. Personally, Id rather a nice set of Carillos.
Unless spinning serious rpm's, is there really any need or benefit from using titanium ??
And even then..SixSpeedsInc run a n/a LSx motor, that spins to 9600rpm. Im fairly sure they are using steel rods.
If that isnt extreme, what is ? !!!!!! lol
Evening chap, I knew you'd turn up sooner or later. I'm just wondering if ls9 rods would be better then a set of regular steelies in a 427. The Ti's look really nice in the web pics and I could polish them, surely they aren't going to be crap. Or I could go with that crude old tat, used everywhere, forged steel. Mind you, billet steel may be cheaper then forged tits
I'd like to know how turbo induction effects rods though, I suspect it's by not all that much if using sensible rpm's?
Boosted.
I'd like to know how turbo induction effects rods though, I suspect it's by not all that much if using sensible rpm's?
Boosted.
alright boosted. what conrods are you going to be using? LS7 items or custom? i cant see there being that much of a problem with the Ti over steel. having said that steel rods in LS application are making some BIG power right now. have you latest dyno championship event in austrailia?i will try and find a link.
thanks Chris.
thanks Chris.
I think that Titanium, in a strangth sense, is no better than steel. In some cases (various alloys, and working processes) steel can have better mechanical performance than titanium.
The significant advantage with titanium is that it produces consistent performance at elevated temperature, and achieves that with almost half the weight.
Unlike steel titanium work hardens very easily. Manufacturing is much more difficult, and without annealing, and a high temperature operating environment, it can become prone to stress cracking.
Titanium is much more like extra high temperature aluminium, to my mind. If you're considering steel, titanium is probably not a good alternative. If it's made of aluminium, and you want to make significant gains in thermal performance then it becomes a good choice.
Think about the geometry and composition of steel rods before considering titanium ones.
The significant advantage with titanium is that it produces consistent performance at elevated temperature, and achieves that with almost half the weight.
Unlike steel titanium work hardens very easily. Manufacturing is much more difficult, and without annealing, and a high temperature operating environment, it can become prone to stress cracking.
Titanium is much more like extra high temperature aluminium, to my mind. If you're considering steel, titanium is probably not a good alternative. If it's made of aluminium, and you want to make significant gains in thermal performance then it becomes a good choice.
Think about the geometry and composition of steel rods before considering titanium ones.
Edited by dilbert on Thursday 17th January 09:22
Boosted LS1 said:
Thanks Dilbert, there's food for thought there. I am aware about the scratching issue.
Chris, I'm looking at Ls9 rods. They look quite chunky with strength where it's needed but it's impossible to say how much if anything they have in reserve
ohhh sounds intresting! have you considered the LS7 rods?? i have't heard of anyone breaking one, but then people dont really push them that hard. Chris, I'm looking at Ls9 rods. They look quite chunky with strength where it's needed but it's impossible to say how much if anything they have in reserve
im guessing ally rods are out of the question??????
heard on EvolutionM.net that they are much more forgiving on the big end bearings. but that was on a 2.0l motor reving to 10K with 50+psi of boosT!
Chris.
Vhris, I don't fancy the ls7 rod as it's got a wedged little end and a larger pin size then sbc. This narrows my custom piston options somewhat. I've also heard thet it's not an over engineered piece. The billet Ti rods I had a while back had a thick diameter little end which restricted piston bowl depth so it looks like I'll have to check out the 'steelies'. I'll start with Callies, they make nice rods and stuff
Lets take a very simplistic approach.
My Callies rods cost me $450. they are about $550 now though I think. They just look like Eagles etc, which are also fine. Not the best looking rod in the world, but they both seem to get the job done.
I havent heard of anyone having issues with them.
Although next time around I'll get Howards rods....they look nice, and arent that much more expensive. Although I would like Carillos. They are big bucks though.
Howards....oooo nice.
Titanium is nice. But really...what benefits is it going to have ? It isnt going to make more power, they wont save a crank from breaking...because the cranks dont break.
So really, what benefit is there ? for that huge cost ?
My Callies rods cost me $450. they are about $550 now though I think. They just look like Eagles etc, which are also fine. Not the best looking rod in the world, but they both seem to get the job done.
I havent heard of anyone having issues with them.
Although next time around I'll get Howards rods....they look nice, and arent that much more expensive. Although I would like Carillos. They are big bucks though.
Howards....oooo nice.
Titanium is nice. But really...what benefits is it going to have ? It isnt going to make more power, they wont save a crank from breaking...because the cranks dont break.
So really, what benefit is there ? for that huge cost ?
Titanium is used in conrods solely for the significant weight advantage. As a conrod has to accelerate fast, stop then accelerate fast back the other way hundreds of times a second at high rpm the reduction in inertia is significant. Your engine won't rev higher with titanium rods but will get to the redline a little bit quicker.
As for tensile strength, this is not a factor as a rod is stressed nowhere near its UTS. The limiting factor is fatigue life and forged titanium alloys with good surface finishing have excellent fatigue life.
The normal failure mode for a conrod is catastrophic failure when the big end seizes due to lack of lubrication, followed by it sawing through the crankcase. That's the only time it's going to matter what they look like.
As for tensile strength, this is not a factor as a rod is stressed nowhere near its UTS. The limiting factor is fatigue life and forged titanium alloys with good surface finishing have excellent fatigue life.
The normal failure mode for a conrod is catastrophic failure when the big end seizes due to lack of lubrication, followed by it sawing through the crankcase. That's the only time it's going to matter what they look like.
Forged rods aren't as strong (like for like) as billet rods because the forging process folds and bends the metal, altering it's internal structure and creating high/low stress areas. If you're going for something reputable I wouldn't expect much difference in the real world though.
Forged roads will be used by a manufacturer over billet as they are cheaper to make - as noted Ti is a bugger to machine as it oxidises readily (at the surface) to produce Titanium Oxide... the stuff you find in a lot of cutting/grinding disks... because it's very hard.
If you're keeping the RPM the same then blowing the engine won't make any real difference to the stresses on the rods. The highest stress on the rods is tensile, on the exhaust stroke when the piston is trying to carry on up the boar, out the top of the engine and the rod has to stop it and pull it the other way. On the compressions stoke you have a mass in the cylinder pushing back against the piston, reducing the tensile stress on the rod.
You will be developing more power (hopefully) which equals more pressure when the charge goes bang and an increase in the compressive stress on the rod. This isn't much though as it can just move down the boar to get away from it.
Forged roads will be used by a manufacturer over billet as they are cheaper to make - as noted Ti is a bugger to machine as it oxidises readily (at the surface) to produce Titanium Oxide... the stuff you find in a lot of cutting/grinding disks... because it's very hard.
If you're keeping the RPM the same then blowing the engine won't make any real difference to the stresses on the rods. The highest stress on the rods is tensile, on the exhaust stroke when the piston is trying to carry on up the boar, out the top of the engine and the rod has to stop it and pull it the other way. On the compressions stoke you have a mass in the cylinder pushing back against the piston, reducing the tensile stress on the rod.
You will be developing more power (hopefully) which equals more pressure when the charge goes bang and an increase in the compressive stress on the rod. This isn't much though as it can just move down the boar to get away from it.
//j17 said:
Forged rods aren't as strong (like for like) as billet rods because the forging process folds and bends the metal, altering it's internal structure and creating high/low stress areas. If you're going for something reputable I wouldn't expect much difference in the real world though.
Forged roads will be used by a manufacturer over billet as they are cheaper to make - as noted Ti is a bugger to machine as it oxidises readily (at the surface) to produce Titanium Oxide... the stuff you find in a lot of cutting/grinding disks... because it's very hard.
If you're keeping the RPM the same then blowing the engine won't make any real difference to the stresses on the rods. The highest stress on the rods is tensile, on the exhaust stroke when the piston is trying to carry on up the boar, out the top of the engine and the rod has to stop it and pull it the other way. On the compressions stoke you have a mass in the cylinder pushing back against the piston, reducing the tensile stress on the rod.
You will be developing more power (hopefully) which equals more pressure when the charge goes bang and an increase in the compressive stress on the rod. This isn't much though as it can just move down the boar to get away from it.
Sorry, I don't agree with some of your statements.Forged roads will be used by a manufacturer over billet as they are cheaper to make - as noted Ti is a bugger to machine as it oxidises readily (at the surface) to produce Titanium Oxide... the stuff you find in a lot of cutting/grinding disks... because it's very hard.
If you're keeping the RPM the same then blowing the engine won't make any real difference to the stresses on the rods. The highest stress on the rods is tensile, on the exhaust stroke when the piston is trying to carry on up the boar, out the top of the engine and the rod has to stop it and pull it the other way. On the compressions stoke you have a mass in the cylinder pushing back against the piston, reducing the tensile stress on the rod.
You will be developing more power (hopefully) which equals more pressure when the charge goes bang and an increase in the compressive stress on the rod. This isn't much though as it can just move down the boar to get away from it.
Forgings optimise grain path. While they cost less to machine they cost way more to tool, and you need to produce a lot to pay off the investment.
J17 said "Forged rods aren't as strong (like for like) as billet rods because the FORGING process folds and bends the metal, altering it's internal structure and creating high/low stress areas. If you're going for something reputable I wouldn't expect much difference in the real world though".
I thinke he made a typo, billet is where the metal is bent and folded etc which I would have though made for a denser stronger material.
Boosted.
I thinke he made a typo, billet is where the metal is bent and folded etc which I would have though made for a denser stronger material.
Boosted.
I thought the difference was that forged components were forged into (approximately) the right shape so the grain structure followed the shape and tended to concentrate the strength where it was needed. Billet is just a uniform slab of material that is cut down to make the component, so the component is more or less uniform (apart from any machining stresses or subsequent treatment).
Not sure what happens with the grain structure in a rod but the metal is just compressed into the finished shape so I'd have thought the grain was pretty random throughout the part. If it were a billet item the grain tends to run lengthways and be uniform which makes the part more stable. Not a very techie reply but I think you'll know what I mean.
guys, all billet rods are machined from a rough rod-shaped forging, so they are the best of both worlds, machined and shot peened, with good grain flow.....they are NOT just machined form a square slab - thats bum info above..... this is certainly the way Arrow, Carillo and Farndons do it......also ensure the threads are rolled and NOT cut using a conventional tap, as this makes the threads 10 times more fatigue resistant, as the grain flow follows the thread form.......yes 10 times, I have done the tests........also Rods from Arrow are aluminium oxide coated prior to shot peening, this has a big effect to fatigue resistance too, thats why arrow rods look like they are chrome plated.....Arrows rods are the best in my experience, you want a good billet steel rod thats got good fatigue resistant properties......Arrow are the boys.
I previusly designed a set of steel rods for the Ford S1600 rally engine, it revved to about 9300rpm, and we never had a problem......a steel rod can actually be designed to be very light for a NA motor, as the highest loads are tensile on the exhaust stroke, this is due to the very high RPM...... as the compressive loads during firing are equaled out by the inertia forces......sounds wierd, but its true......these ware a carillo style H-beam design, and the ribs were VERY thin........for the Ford YB and Zetec and Hyundai WRC engines we would always use an I-beam design which could better handle the much higher firing forces of a high pressure turbo engine........just like the one in mateys pica above......they were beefy rods compared to the NA rods
forget titanium rods, they are a waste of money for the sort of engines we play with....... to my knowledge they are not even used in race spec motors like BTCC/WTCC, WRC or F3, its really F1 territory only.......exotic materials are banned in most race series in an effort to cap cost......
the one thing I'd really pay good money for is a decent set of bolts, then buy a stretch gauge and learn how to tighten them properly, as torque is actually not that accurate, as its highly dependant on friction........any decent bolt supplier (like ARP) will provide the distance that the bolt needs to be stretched by, its normally about 5 thou.......hope that helps!
I previusly designed a set of steel rods for the Ford S1600 rally engine, it revved to about 9300rpm, and we never had a problem......a steel rod can actually be designed to be very light for a NA motor, as the highest loads are tensile on the exhaust stroke, this is due to the very high RPM...... as the compressive loads during firing are equaled out by the inertia forces......sounds wierd, but its true......these ware a carillo style H-beam design, and the ribs were VERY thin........for the Ford YB and Zetec and Hyundai WRC engines we would always use an I-beam design which could better handle the much higher firing forces of a high pressure turbo engine........just like the one in mateys pica above......they were beefy rods compared to the NA rods
forget titanium rods, they are a waste of money for the sort of engines we play with....... to my knowledge they are not even used in race spec motors like BTCC/WTCC, WRC or F3, its really F1 territory only.......exotic materials are banned in most race series in an effort to cap cost......
the one thing I'd really pay good money for is a decent set of bolts, then buy a stretch gauge and learn how to tighten them properly, as torque is actually not that accurate, as its highly dependant on friction........any decent bolt supplier (like ARP) will provide the distance that the bolt needs to be stretched by, its normally about 5 thou.......hope that helps!
Edited by knighty on Saturday 26th January 14:13
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff