Guest Nick Harris Posted July 26, 2002 Share Posted July 26, 2002 I ask because in straight sixes, a cast unit will be superior to a forged unit, machined all round and prepared correctly it is less likely to break than a forged unit, fours and V8's are better with forged. Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 26, 2002 Share Posted July 26, 2002 Originally posted by Nick Harris I ask because in straight sixes, a cast unit will be superior to a forged unit, machined all round and prepared correctly it is less likely to break than a forged unit, fours and V8's are better with forged. I'd be interested in seeing the justification for this. Firstly, design for design a forged crank will always be stronger than a cast one. The choice of material or process is more to do with the requirements from the crank and the crank design. IMHO you can't make a sweeping statement that I4s are better forged, I6s are better cast, V8s are better forged. It depends entirely on the geometry of the crank and the requirements from the engine. Casting is a lot better than forging for production. The tooling is cheaper for any crank layout, cycle times are faster, and the process is more accurate so you can minimise the amount of material that needs to be removed by machining. Machining a crank all over is something that is avoided like the plague on production cranks because it piles on cost. So with a cast crank you end up with the cheapest part, which is why in my experience it is always the first choice. Forged cranks are stronger and will turn up where the crank has an extra-long throw and hence very little bearing overlap, or has fewer main bearings than is strictly desirable, or where the engine layout makes for an inherently weak crank (like a four-bearing 60degree V6 with even firing or an I6). If the crank is not "flat" then the forging tooling will be complex and multi-staged instead of a roughpress / finepress / cliping process. Anyone who has been inside a drop-forging shop will know that the process is brutal to say the least. The tolerances are much larger so machining all over is more common, and the cycle times and part cost are proportionally higher. In return for all this extra ball-ache the crank is, however, stronger than the same design as a cast part. Typical applications of forged cranks may be where an existing engine is being enhanced, or the stroke is being increased without the crankpin or main bearings being enlarged, or as mentioned above when the crank layout is inherently weak in relation to the engine output. Quote Link to comment Share on other sites More sharing options...
Guest Nick Harris Posted July 26, 2002 Share Posted July 26, 2002 >I'd be interested in seeing the justification for this. Firstly, design for design a forged crank will always be stronger than a cast one. The choice of material or process is more to do with the requirements from the crank and the crank design. Strength is only part of the equation, forged cranks lack ductility, they are very harsh on the bearings and caps, there rigidity it not compatible with a long block, all 6s flex longitudinally to a far greater extent than the 4/V8, our 6s are extended 2.5s the added piece at the top of our blocks is the only potential weak point, so it is vital that the flexure is not made worse by having a crank that opposes this flexure, in doing that the flex will transfer up the block to the weakest point. >IMHO you can't make a sweeping statement ............It depends entirely on the geometry of the crank and the requirements from the engine I'll think about that. Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 26, 2002 Share Posted July 26, 2002 Originally posted by Nick Harris Strength is only part of the equation, forged cranks lack ductility, they are very harsh on the bearings and caps, there rigidity it not compatible with a long block, all 6s flex longitudinally to a far greater extent than the 4/V8, our 6s are extended 2.5s the added piece at the top of our blocks is the only potential weak point, so it is vital that the flexure is not made worse by having a crank that opposes this flexure, in doing that the flex will transfer up the block to the weakest point. [/b] I think we may have to agree to disagree on this. Ductility is a measure of a material's ability to be elongated or drawn without fracture. I suspect you may mean "resilience" which is the ability to absorb impact. In a crank this would be the gas load on the power stroke. The inertia load of the piston at TDC on the exhaust stroke, which is the largest tensile load, requires good tensile properties. Definitions aside, whatever the case a stiff crank is a good thing. You don't want it bending around and going out of control under load. You want it to be as stiff as possible. Considering a single bay, the gas load is going to get transferred to the main bearings no matter what happens. The crank just has to be resilient enough to do it again and again and again. Because the gas and inertia loads act in opposite directions, a crank is subject to fatigue. Fatigue life is a function of the ulitmate tensile strength (UTS) and yeild point of the material. Again, these are tensile properties (althouh their compressive couterparts do exists). This brings us rather nicely back to shot peening, who's purpose it is to extend fatigue life in the loaded parts of the crank. Considering two adjacent bays, there will be loads acting in opposite directions at the same time to create a bending force, but again, bending strength is a function of UTS and crank geometry. Cranks tend to crack across a straight diagonal line joining the main bearing and crankpin fillets (again, the areas that are usually shot peened or fillet rolled). The bigger the overlap between the main journal and the crankpin, the stronger this region will be. Considering the whole crank, yes a long crank will flex more than a short one, but the predominant flexure in a well supported crank (like the Supra's) is torsional, or twsiting. There is an overall bending load in the Supra crank because the vibration damper in the crank pulley has both torsional and bending masses, but the main component in long cranks is torsional. Check out the size of that dual mass flywheel! Again, this is why a stiff crank is a good thing. Undamped torsional vibration causes all sorts of problems from misfire diagnosis, to the crank positon sensor not working properly to, of course, breaking crankshafts. The ability to resist all of the above require high tensile strength and good fatigue resistance. I don't have my materials references with me but the UTS of steels used in cranks must be in the region of 150% to 200% of that of a good cast iron - and that's for billet parts. The forging process will make the part even stronger. Check out a billet crank for a V12 F1 engine - it will almost certainly have an ultra short stroke and be made from nitrided EN40B or something similar. As stiff as hell, and with exactly the same layout as an I6, just twice as many cylinders. There are crank configurations that tend to impart larger loads on the cylinder block than others - a crossplane V8 for example. This will put large loads into the structure around the main bearings, while externally the engine is almost perfectly balanced (the engine mounts will see very little vibration). I think that these cranks may tend to be forged because of their strange layout and because they tend to go into high-output engines - not because they bend less than longer engines. In fact they may tend to bend as much if not more. If any excessive bending gets transferred to the cylinder walls across the crankpin, it will trash the cylinder, the pin and the piston. I doubt the block structure itself would be damaged - especially in a cast iron block like the Supra has. Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 26, 2002 Share Posted July 26, 2002 B*gger. Forgot to add the pic. Quote Link to comment Share on other sites More sharing options...
Guest Nick Harris Posted July 26, 2002 Share Posted July 26, 2002 >I think we may have to agree to disagree on this. You're right, this is not the forum for this type of debate, too technical and dry. Fair enough. Quote Link to comment Share on other sites More sharing options...
Alex Posted July 26, 2002 Share Posted July 26, 2002 Damn I was loving that....I think you'll find the Forum really likes in depth technical discussion. Well, as long as it comes to an amicable conclusion. Quote Link to comment Share on other sites More sharing options...
Terminator Posted July 26, 2002 Share Posted July 26, 2002 Top post Darren. "Oil give it foive" Don't stop now it is just getting good, nothing dry about it. An excellent technical post for all of us sad gits out in cyber space. Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted July 26, 2002 Share Posted July 26, 2002 Originally posted by Darren Blake B*gger. Forgot to add the pic. Fabulous posts Darren. Great info, well put. Gotta throw this in for discusion though. You show "weakest link" as between main bearing 7 and rod pin 6. I feel it MAY be between rod pin 6 and crank journal 6. Why? The oil feed hole is an internal stress raiser. Could be debated for years, only destructive testing might prove one way or another, but despiet the main loads culminationg at the very end of the crank the weakest link may be one step further forward. Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 26, 2002 Share Posted July 26, 2002 Cheers for the praise guys. Sorry if I make it all sound as interesting as watching paint dry, but it pays the mortgage, y'know? I only attached the pic to illustrate what journal overlap was, and where cranks tend to fail (across the crankpin / main fillets). In actuality it can happen on any crank web at all, it just depends on where the loading is worst - or where there is a stress raiser like an oil drilling as Chris points out. Quote Link to comment Share on other sites More sharing options...
Branners Posted July 29, 2002 Share Posted July 29, 2002 I would still like to see Nicks reply to this as he obviously has a different viewpoint which needs to be debated, and if you cant discuss it in the technical section where can you discuss it. JB Quote Link to comment Share on other sites More sharing options...
Guest Terry S Posted July 29, 2002 Share Posted July 29, 2002 Reading other posts I feel Nick may have got pissed off for some reason which is a shame as It was a very interesting debate. If Nick reads this I would just like to reassure him that we only want to know how to make our cars faster & stronger. There are no personalities involved. Knowing a little of your background will only help validate your posts. No one will come knocking;) Quote Link to comment Share on other sites More sharing options...
Guest Martin F Posted July 29, 2002 Share Posted July 29, 2002 I have just caught up with this thread after a couple of days and i must say it is the best thread for months. It's amazing that you can learn more during your lunch hour(+) than the rest of the working day. Quote Link to comment Share on other sites More sharing options...
pedrosixfour Posted October 24, 2015 Share Posted October 24, 2015 So is it safe to ask did anyone ever establish whether the crankshafts are cast or forged, or does this topic just result in an automatic ban for everyone without an engineering degree?? And do I win anything for the oldest thread revival? Quote Link to comment Share on other sites More sharing options...
Abz Posted October 24, 2015 Share Posted October 24, 2015 So is it safe to ask did anyone ever establish whether the crankshafts are cast or forged, or does this topic just result in an automatic ban for everyone without an engineering degree?? And do I win anything for the oldest thread revival? Blimey, you went digging deep for this! An a lot of banned folks here... the days of the trigger happy mods Quote Link to comment Share on other sites More sharing options...
pedrosixfour Posted October 24, 2015 Share Posted October 24, 2015 Blimey, you went digging deep for this! An a lot of banned folks here... the days of the trigger happy mods Just went looking for the answer to what the stock crankshafts are made of and ended up in a bloody minefield! Quote Link to comment Share on other sites More sharing options...
a98pmalcolm Posted October 24, 2015 Share Posted October 24, 2015 Just went looking for the answer to what the stock crankshafts are made of and ended up in a bloody minefield! haha how big was the shovel to get to this.. Everyone is banned.. Tread careful.... lol Quote Link to comment Share on other sites More sharing options...
V8KILR Posted October 24, 2015 Share Posted October 24, 2015 (edited) The stock crank has been used up to 2000 bhp in the NZ Rayglass Celica drag car (low 6 sec 1/4 mile) so it is extremely strong, whatever it is made from. Edit: I'm pretty sure the crank is forged. Edited October 24, 2015 by V8KILR (see edit history) Quote Link to comment Share on other sites More sharing options...
Ric Posted October 24, 2015 Share Posted October 24, 2015 Forged in the fire's of Mount doom. Quote Link to comment Share on other sites More sharing options...
David P Posted October 24, 2015 Share Posted October 24, 2015 Here's a link with information to add to the confusion. http://www.jdmspecengines.com/toyota-engines/jz-series/2jz-gte.html Quote Link to comment Share on other sites More sharing options...
ashloys Posted October 24, 2015 Share Posted October 24, 2015 It would be a shame in Pedro got banned for bumping such an old thread.. Quote Link to comment Share on other sites More sharing options...
Alex Posted October 24, 2015 Share Posted October 24, 2015 They didn't get banned at the same time or for this thread...if everyone had stuck to technical discussions no banning would have happened. But we still have the "winner" Digsy Quote Link to comment Share on other sites More sharing options...
Dnk Posted October 24, 2015 Share Posted October 24, 2015 So is it cast or forged ? Quote Link to comment Share on other sites More sharing options...
Fitz Posted October 24, 2015 Share Posted October 24, 2015 Wow 2002 Excellent revival. First thing I thought, seems to be the first thing that everyone else thought "F*ck theres a lot of banned people in this thread ) Quote Link to comment Share on other sites More sharing options...
Digsy Posted October 24, 2015 Share Posted October 24, 2015 Given the stick that you goddam boy racers have given that poor crank over the years there's a good bet that it was machined out of the heavy end of Thor's hammer Quote Link to comment Share on other sites More sharing options...
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