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Whoa this went way further in depth than I was expecting.
I will add that I don’t think removing weight from the balance shafts is really necessary for a lightened build. If you look at the force graph and were to add the opposing piston forces and a sum it all together. The total sum of the forces of a lighter piston setup should still remain below any damaging level. It wouldn’t be perfect but any I4 engine without balance shafts isn’t perfect. In reality a little imbalance of this nature is just lived with, it’s only once you reach a damaging level that it becomes a problem.
Whoa this went way further in depth than I was expecting.
I will add that I don’t think removing weight from the balance shafts is really necessary for a lightened build. If you look at the force graph and were to add the opposing piston forces and a sum it all together. The total sum of the forces of a lighter piston setup should still remain below any damaging level. It wouldn’t be perfect but any I4 engine without balance shafts isn’t perfect. In reality a little imbalance of this nature is just lived with, it’s only once you reach a damaging level that it becomes a problem.
only needed if you have an OCD issue!!! Most engine builders do.
Ok, so luckily the engine was supplied with B/shaft covers which means we can look to retro-fit some. Am I reading it correctly that with lightened components we should consider lightening the b/shafts? I'll also have to check with my shop to see if we can make these work now that the front of the motor is quite different. Should be ok I hope.
That looks like a tough fit. I wouldn't worry about spending a lot of time to retrofit them, I am assuming you went with pistons and rods that are a lot lighter than stock - that will help out a bunch.
Usually the big concern in balance shaft delete engines is the oil pick up - it have a long leverage moment so vibrating that as high frequency is a problem and failure of that unseen part is always catastrophic. Dry sump is a good thing!! Use lots of locktite on fasteners!
Originally Posted by 333pg333
Ok, so luckily the engine was supplied with B/shaft covers which means we can look to retro-fit some. Am I reading it correctly that with lightened components we should consider lightening the b/shafts? I'll also have to check with my shop to see if we can make these work now that the front of the motor is quite different. Should be ok I hope.
That looks like a tough fit. I wouldn't worry about spending a lot of time to retrofit them, I am assuming you went with pistons and rods that are a lot lighter than stock - that will help out a bunch.
Usually the big concern in balance shaft delete engines is the oil pick up - it have a long leverage moment so vibrating that as high frequency is a problem and failure of that unseen part is always catastrophic. Dry sump is a good thing!! Use lots of locktite on fasteners!
Yes, custom Carrillo rods / CP pistons. Saved a fair bit of weight. I guess we'll have a look to see how feasible it might be to retro fit b/shafts. If too hard we will have to deal with it. Car is built for very short duration bursts rather than enduros so that should help too. But, yes, spanner checks are a constant.
are you running a Davies Craig water pump? I have one on my 968 turbo street car, works great
Originally Posted by 333pg333
Yes, custom Carrillo rods / CP pistons. Saved a fair bit of weight. I guess we'll have a look to see how feasible it might be to retro fit b/shafts. If too hard we will have to deal with it. Car is built for very short duration bursts rather than enduros so that should help too. But, yes, spanner checks are a constant.
The crank whip I was referring to earlier is derived from the piston orientation, as 1 and 4 are at TDC when 2 and 3 are at BDC at high rpm a rocking motion can be induced.
What the harmonics can do is loosen the main studs.
The crank whip I was referring to earlier is derived from the piston orientation, as 1 and 4 are at TDC when 2 and 3 are at BDC at high rpm a rocking motion can be induced.
What the harmonics can do is loosen the main studs.
Thanks Luke. My position is that I either can or can't fit the b/shafts. What I'm trying to narrow down is what actually can happen to the motor with these undesirable but unavoidable i4 harmonics. It may come down to a decision based on how difficult and costly it will be to fit them vs taking care with spanner checks and knowing that we've eliminated many of the usual suspects that fail without them.
Appreciate your input.
the usual suspects that fail with BS delete are oil pickup and the stock fuel rail - they both crack at the welds
you have neither of those parts on your engine now...
plenty of other 4-cyl engines do without B-shafts, even ones with a stroke similar to or longer than a 3.0l 944.
a good crank damper is paramount, B-shafts or not.
Thanks Luke. My position is that I either can or can't fit the b/shafts. What I'm trying to narrow down is what actually can happen to the motor with these undesirable but unavoidable i4 harmonics. It may come down to a decision based on how difficult and costly it will be to fit them vs taking care with spanner checks and knowing that we've eliminated many of the usual suspects that fail without them.
Appreciate your input.
I’ve been listening to the High Performance Academy podcasts religiously recently, there are some very knowledgeable guys sharing information that is usually held very tightly. One of the more recent one’s regarding the 4g63 mentioned they were having issues just like your last failure. Block cracking under the mains. They eventually put it down to the main studs loosening off. The remedy was to upgrade the mains to ARP and use a higher torque.
It may or may not translate to our platform but certainly sounds familiar enough to think would. With the hardware you’re using at higher clamp loads you may just be fine. My biggest concern with your engine is how much higher your max rpm is.
the inherent vibrations of a 4 cyl is much higher frequency than the crank twisting harmonics. The only thing I believe it could effect on the bottom end is nuts backing off. Invest in lots of locktite! I might be tempted to use some on the crank hardware. V2 is right on - the main issues are the pick up and fuel rail and you have taken care of those.
the usual suspects that fail with BS delete are oil pickup and the stock fuel rail - they both crack at the welds
you have neither of those parts on your engine now...
plenty of other 4-cyl engines do without B-shafts, even ones with a stroke similar to or longer than a 3.0l 944.
a good crank damper is paramount, B-shafts or not.
Thanks Spencer. I figured the same.
Originally Posted by Chris White
the inherent vibrations of a 4 cyl is much higher frequency than the crank twisting harmonics. The only thing I believe it could effect on the bottom end is nuts backing off. Invest in lots of locktite! I might be tempted to use some on the crank hardware. V2 is right on - the main issues are the pick up and fuel rail and you have taken care of those.
Ok, so it would seem that we've rounded out the list and should be able to deal with it without the b/shafts. I'll still look into it but not if it means redesigning the front of the motor.
Originally Posted by 951TurboRS
I’ve been listening to the High Performance Academy podcasts religiously recently, there are some very knowledgeable guys sharing information that is usually held very tightly. One of the more recent one’s regarding the 4g63 mentioned they were having issues just like your last failure. Block cracking under the mains. They eventually put it down to the main studs loosening off. The remedy was to upgrade the mains to ARP and use a higher torque.
It may or may not translate to our platform but certainly sounds familiar enough to think would. With the hardware you’re using at higher clamp loads you may just be fine. My biggest concern with your engine is how much higher your max rpm is.
The builder said he designed this motor to rev to 8800rpm. I don't imagine we'll go too near that in reality. It had a pretty broad tq range and they even pulled up 1000rpm short of that on the dyno. For a big turbo (EFR 9180)that wants to be pushed to 25+ psi it spools up remarkably early at lower boost. Are you saying this from the pov of stroker config?
Nice torque curve on that one!
btw, something to keep in mind with balance shafts - they spin at 2x crank speed, so a 8800 rpm engine would be spinning the balance shafts at 17,600 rpm!!
That really is a nice torque curve and the exact reason I bought a 968 engine to build up.
I mentioned the rpm because 2nd order vibrations increase exponentially with rpm and you stated 8800 which is considerably higher than stock. The rpm’s on that dyno graph look a lot more reassuring.
The 2nd order vibrations are very minor at lower rpm’s and I predict balance shafts were only necessary for the last thousand rpm or less in the stock motor and rpm limit. By lightening the pistons and rods it could be possible to shave close to 30% in piston weight as the stock items are so heavy. But because of the exponential factor this is only going to increase the vibration threshold rpm limit by a fraction of 30%. So a balance shaft delete engine is very feasible but how high you set the rev limiter is the factor of wether or not it will survive.
You also have the factory nanny level of safety margin that a race engine can do without. So there’s likely a few hundred rpm to be had there. It’s just important to know a race engine built to the limit needs more care and attention and will ultimately have less reliability. Checking the main stud torque after a couple of sessions would be high on my list. If they are all good you won’t need to check them again all season. Also keeping the rev limit down and only using those additional rpm’s when absolutely needed will be key. At a 6000rpm rev limit I would consider your engine and level of build as an endurance engine. Raise that past 8000rpm and it’s now a qualifying engine.
02-07-2023, 09:57 AM
