Forged engine: rework counterweights?
03-16-2017, 02:09 AM
#21
Facts: bjh/spm pulley isnt perfectly balanced. we didnt drill into once because we didnt want it to fail. it was a small amount and the risk wasnt worth the reward.
general consensus is dont lighten a miata crank. best thing to do would be to simply smooth out any extra material that was left on the counterweights for windage reasons. I'm currently running a lightened crank and i cant say it was worth it. it was drilled at the pins.probably lost about a pound and half total. reward vs time not worth it again.
My next crankshaft will not be lightened, it was ground 10 thou slightly offset (mini stroker
?) with a big radius. its going to be streamlined as i mentioned above and then to be nitrided and finally balanced with the damper/fw/pp
Also from experience think that hines balance machines provide some of the better balance jobs iv had done especially if your machinist is willing to spend a little time and is capable in trying to reduce some of the rocking couple forces at play.
general consensus is dont lighten a miata crank. best thing to do would be to simply smooth out any extra material that was left on the counterweights for windage reasons. I'm currently running a lightened crank and i cant say it was worth it. it was drilled at the pins.probably lost about a pound and half total. reward vs time not worth it again.
My next crankshaft will not be lightened, it was ground 10 thou slightly offset (mini stroker
?) with a big radius. its going to be streamlined as i mentioned above and then to be nitrided and finally balanced with the damper/fw/ppAlso from experience think that hines balance machines provide some of the better balance jobs iv had done especially if your machinist is willing to spend a little time and is capable in trying to reduce some of the rocking couple forces at play.
Last edited by Twibs415; 03-16-2017 at 12:27 PM.
Reply
0
0
03-16-2017, 03:44 AM
#22
ʎpunq qoq
Join Date: Jan 2014
Location: Western Australia
Posts: 604
Total Cats: 201
I would have thought the main key criteria for a harmonic damper was that it was balanced! Anyone know how good the ATI dampers are out of the box? (I don't run a special damper... just a cut down stock one)
Reply
0
0
05-05-2017, 09:16 AM
#24
My understanding is that you only need to 'balance' a L4 crank for centripetal forces so that the center of mass is inline with the centerline of the bearings. You don't need to factor in the weight of rods, pistons etc because there are 2 on each side at equal spacings from the centerline. An out of balance crank will put increasing sideways loading via vibration on the bearings as revs increase.
So this means you can remove the counterweights entirely right? Well you can if the forces acting on the crank are equal, but the pistons apply load at different points along the length of the crank at different times which causes harmonic flex. This is basically causing the crank to bend like a guitar string at multiple orders of harmonics at once all combining on top of each other like waves in an ocean. At certain frequencies they all add up to cause really high stresses. It's these frequencies that the harmonic balancer is attempting to dampen. Harmonic flex causes the crank to twist, and potentially fail outright... however more likely is that the main bearings fail first due to rotational misalignment between the crank and the bearing surfaces (causing contact or leaking oil pressure and then contact).
The counterweights are there to try and put mass on the opposite side of crank to the piston forces to try and slow the amount of deflection with time.... but the more counterweight mass the more the crank can deflect once it gets into a harmonic frequency. So it's an engineering challenge to balance all the various forces so that the crank doesn't outright fail or the cranks don't deflect enough to cause bearing failure.
You can't really balance out harmonics as it depends on lots of factors, like crank weight distribution, piston and rod weight, horsepower, revs etc etc. Plus you really need to spin the crank at ludicrous speeds to test it. F1 teams run sensors on the crank bearings to measure deflection at really high data rates which is really the best way to test for flex.
My experimental lightweight crank was made because I know in a L4 design that counterweights 2 & 3 and 6 & 7 are less critical than the others for harmonic balance. Plus there are lots of racing cranks like Cosworth and even the SOHC Protege crank that have these counterweights removed and survive high revs. There is a reason that I haven't run it yet in my race engine as I'm not sure on the outcome. I have to build up enough courage to drop it in first.
So this means you can remove the counterweights entirely right? Well you can if the forces acting on the crank are equal, but the pistons apply load at different points along the length of the crank at different times which causes harmonic flex. This is basically causing the crank to bend like a guitar string at multiple orders of harmonics at once all combining on top of each other like waves in an ocean. At certain frequencies they all add up to cause really high stresses. It's these frequencies that the harmonic balancer is attempting to dampen. Harmonic flex causes the crank to twist, and potentially fail outright... however more likely is that the main bearings fail first due to rotational misalignment between the crank and the bearing surfaces (causing contact or leaking oil pressure and then contact).
The counterweights are there to try and put mass on the opposite side of crank to the piston forces to try and slow the amount of deflection with time.... but the more counterweight mass the more the crank can deflect once it gets into a harmonic frequency. So it's an engineering challenge to balance all the various forces so that the crank doesn't outright fail or the cranks don't deflect enough to cause bearing failure.
You can't really balance out harmonics as it depends on lots of factors, like crank weight distribution, piston and rod weight, horsepower, revs etc etc. Plus you really need to spin the crank at ludicrous speeds to test it. F1 teams run sensors on the crank bearings to measure deflection at really high data rates which is really the best way to test for flex.
My experimental lightweight crank was made because I know in a L4 design that counterweights 2 & 3 and 6 & 7 are less critical than the others for harmonic balance. Plus there are lots of racing cranks like Cosworth and even the SOHC Protege crank that have these counterweights removed and survive high revs. There is a reason that I haven't run it yet in my race engine as I'm not sure on the outcome. I have to build up enough courage to drop it in first.
Reply
0
0
05-05-2017, 09:44 AM
#25
ʎpunq qoq
Join Date: Jan 2014
Location: Western Australia
Posts: 604
Total Cats: 201
The reason I cut down a forged crank rather than use a cast SOHC crank is that hopefully my crank might take some power and revs (Rotrex)
Reply
0
0
05-05-2017, 10:24 AM
#29
You don't suppose there was a reason why Mazda went to a forged steel crank when they went to the BP engine @ 7000 redline vs the Protege @ 6000 redline? There's a very real premium in manufacturing a forged steel crank vs a cast steel crank especially in part & tooling costs.
I guess if you start out with a magnafluxed crank so you know the base point.
I guess all the parts houses are selling the SOHC crank now as replacements for stock BP cranks. FWIW
I guess if you start out with a magnafluxed crank so you know the base point.
I guess all the parts houses are selling the SOHC crank now as replacements for stock BP cranks. FWIW
Reply
0
0
05-05-2017, 10:27 AM
#30
You don't suppose there was a reason why Mazda went to a forged steel crank when they went to the BP engine @ 7000 redline vs the Protege @ 6000 redline? There's a very real premium in manufacturing a forged steel crank vs a cast steel crank especially in part & tooling costs.
I guess if you start out with a magnafluxed crank so you know the base point.
I guess all the parts houses are selling the SOHC crank now as replacements for stock BP cranks. FWIW
I guess if you start out with a magnafluxed crank so you know the base point.
I guess all the parts houses are selling the SOHC crank now as replacements for stock BP cranks. FWIW
I've never actually seen a broken B8 crank, and i've seen them turbo'd. But... i also haven't seen anyone spin them before, so that's really the unknown.
Somewhat related: Late KLs had cast cranks that were quite a bit lighter. They didn't have any lower of a redline, and people to this day spin them over 8000rpm with no issues.
Reply
0
0
05-05-2017, 10:49 AM
#31
They didn't move "to" a BP. Both engines were made at the same time. There weren't boosted B8s, but there were boosted BPs.
I've never actually seen a broken B8 crank, and i've seen them turbo'd. But... i also haven't seen anyone spin them before, so that's really the unknown.
Somewhat related: Late KLs had cast cranks that were quite a bit lighter. They didn't have any lower of a redline, and people to this day spin them over 8000rpm with no issues.
I've never actually seen a broken B8 crank, and i've seen them turbo'd. But... i also haven't seen anyone spin them before, so that's really the unknown.
Somewhat related: Late KLs had cast cranks that were quite a bit lighter. They didn't have any lower of a redline, and people to this day spin them over 8000rpm with no issues.
I realize some of the lower redline came because of the SOHC design of the B8 engine and likely not specifically the crank. It could also be that considering the lower redline, possibly because of the SOHC design, it was decided a cast crank was sufficient. Who knows... Automotive engineers are very much cost sensitive or as they call it "value driven".
And like you, a quick google doesn't bring up much of any problems with the cast crank.
Reply
0
0
05-05-2017, 10:51 AM
#32
ʎpunq qoq
Join Date: Jan 2014
Location: Western Australia
Posts: 604
Total Cats: 201
I think the cast crank is certainly weaker than the forged crank.... that is why I cut down a forged crank for my Rotrex build. I think the SOHC crank is usable for a N/A engine as it will be power that will make it fail especially given stock miata cranks survive in 500hp engines. High revs put deflection and harmonic loads on the crank... the cast crank is less balanced but on the flip its also far lighter so the loads are less.
The sample size isn't large but I've never heard of them failing in race engines. If you are looking for that last extra bit on a build and willing to participate in the experiment then its an option.
The sample size isn't large but I've never heard of them failing in race engines. If you are looking for that last extra bit on a build and willing to participate in the experiment then its an option.
Reply
0
0
05-05-2017, 10:58 AM
#33
I realize some of the lower redline came because of the SOHC design of the B8 engine and likely not specifically the crank. It could also be that considering the lower redline, possibly because of the SOHC design, it was decided a cast crank was sufficient. Who knows... Automotive engineers are very much cost sensitive or as they call it "value driven".
And like you, a quick google doesn't bring up much of any problems with the cast crank.
And like you, a quick google doesn't bring up much of any problems with the cast crank.
Reply
0
0
05-05-2017, 11:48 AM
#34
figured I would add this since it is now being discussed.
I am currently putting a cheap motor together with parts that I have accumulated from various projects over the years. I plan to run the sohc crank with a msm turbo setup, b6t rods, 9.0:1 pistons, and a bp26 head. Hoping it makes a decent street motor. It has to be better than a 200k mile 1.6...right?
As concealer said, I have yet to see a sohc crank fail due to being over powered or rev'ed. Seems the dohc forged crank was one of the few pieces that mazda overbuilt.
Reply
0
0
05-05-2017, 01:35 PM
#35
Junior Member
Thread Starter
Join Date: Jan 2016
Location: Munich
Posts: 362
Total Cats: 93
I'm currently building a turbo engine with forged rods and supertech pistons. The parts AR weight-matched within 0.3 g.
however, the forged rods are 50 g each lighter than the stock ones. So should I have the machinist remove 4x50 g from the counterweights on the crank? Or should I just have the crank fine-balanced and call it a day?
however, the forged rods are 50 g each lighter than the stock ones. So should I have the machinist remove 4x50 g from the counterweights on the crank? Or should I just have the crank fine-balanced and call it a day?
now that I've done 1600 km I must say the engine runs and idles smoother than before the rebuild. There is noticeably less vibration felt in the chassis although I'm now running stiffer engine mounts.
Reply
0
0
