Scat Stroker Crank Balancing Act (6 cwt)
06-18-2007, 03:04 PM
#16
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It appears that these cranks are built for using after market product pistons "only" which are about 200g lighter.
you can get pins at the 136g or modify yours, I would not go lighter with 968 pistons. The mahle pins are harder and stronger than any after market us pin. I had a set made at 121 g that bent and wore poorly.
The pistons are minimally adjustable.
Ti rods are available. I have a set of 5.850 with 24mm pin for the smaller 2.0 chevy journal...will have to grind your crank to the smaller rod journal. These are not cheap rods and will handle the heavy 968 for high revs. Those pistons are indestructible!
External balance is a burden that you really don't want to take on.,,,but doable as last resort. Downside is that you are now limited to "the" flywheel as it is matched to the crank for life.
Might consider coating the block and running us pistons and 6 in Ti rods like the others...???
you can get pins at the 136g or modify yours, I would not go lighter with 968 pistons. The mahle pins are harder and stronger than any after market us pin. I had a set made at 121 g that bent and wore poorly.
The pistons are minimally adjustable.
Ti rods are available. I have a set of 5.850 with 24mm pin for the smaller 2.0 chevy journal...will have to grind your crank to the smaller rod journal. These are not cheap rods and will handle the heavy 968 for high revs. Those pistons are indestructible!
External balance is a burden that you really don't want to take on.,,,but doable as last resort. Downside is that you are now limited to "the" flywheel as it is matched to the crank for life.
Might consider coating the block and running us pistons and 6 in Ti rods like the others...???
06-18-2007, 03:25 PM
#17
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I don't see me discarding the costs already incurrect to change direction at this point.
I already had the block bored and lapped to these 968 pistons and had the pistons coated by HPC.
So my investment in this setup is too high to discard it. Plus I have second block bored and another set of 968 pistons for a second motor build (one for race car, one for street car).
I am definitely a budget guy, so changing directions at this point rather than externally balancing the motor (at worse case) isn't going to happen.
I already had the block bored and lapped to these 968 pistons and had the pistons coated by HPC.
So my investment in this setup is too high to discard it. Plus I have second block bored and another set of 968 pistons for a second motor build (one for race car, one for street car).
I am definitely a budget guy, so changing directions at this point rather than externally balancing the motor (at worse case) isn't going to happen.
Last edited by Stan.Shaw@Excell.Net; 06-18-2007 at 09:45 PM.
06-18-2007, 03:59 PM
#18
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Hopefully I don't run into same problem. Moldex 8 weight crank seems very heavy and they knew my piston and rod weights when they did it. Its not balanced by Moldex as I didn't know 100% sure what pistons I was going to run (968 or 944 S2) and how much rods were going to weight since they weren't yet modified to 24mm wrist pins at the time.
It could be that cheapest way around this is to sell 6 weight crank and get heavier 8 weight as replacement. Probably would take several months to solve the problem this way though.
It could be that cheapest way around this is to sell 6 weight crank and get heavier 8 weight as replacement. Probably would take several months to solve the problem this way though.
06-18-2007, 04:04 PM
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I didn't have the crank built to the right specs, but thought I did with the rods. I probably didn't know enough to communicate all the pertinent information when I ordered the rods.
I plan on using two stroker cranks, so I may just have to spec rods for this crank and another crank for these rods.
I plan on using two stroker cranks, so I may just have to spec rods for this crank and another crank for these rods.
06-18-2007, 08:49 PM
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Sounds like I can save about 25g per piston pin using some JE pins. My understanding is the impact may actually be greater than 25g due to the piston pin being at the end of the rod. Does anyone have a way of me calculating this?
06-19-2007, 03:54 AM
#23
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Carrillo makes an A-beam connecting rod that is very lightweight....we've been using them for a few years now. They are incredibly tough! The material is of very high quality...which is what you would expect from Carrillo. Grab ahold of a Carrillo rod and try and do some machining on it....you will instantly understand what I'm talking about. We have had problems with the Oliver rods getting "out of round" very quickly....might be another one of those famous "Southern California" only problems....so don't bother jumping on my butt! We've already established that Cometic head gaskets will not seal in Orange County! (Has to do with the relationship of Orange County to the position of Uranus!) Anyway, I know that the material is way, way different than the stuff that Carrillo uses. Grab ahold of an Oliver rod and machine on it....after you try the Carrillo rods...and you will instantly know what I'm talking about. Anyway, the Carrillo rod (we use a Chevy version and have the wrist pin diameter changed on the rod for the 968 piston pin....so that we don't have to wait 6 months for them to make us a custom rod) and the lighter pin will solve your Mallory problem. You want to use as little weight as possible in the crank...so that it revs quickly. That is the reason to use the 6 counterweight crankshaft in the first place.....in my humble opinion. The guys that like 8 counterweights can save their breath....we've heard it all before....it is only my opinion...but until someone drives a 928 past Mark Anderson on the track....I'll do it this way! Tougher to argue with reality! (BTW....we are redoing Joseph Fan's engine with the lighter crank and rods....he is tired of this problem....so we will have more data about this soon!) The guys that like the two extra counterweights and think I'm wrong can buy Joseph's 8 counterweight crank and rods from Mark Anderson....it is a good deal.
Try and remember that horsepower means very little.....unless you are at a steady state. If you are building an engine to go run the Silver State.....your requirements are completely different than if you are building an engine for the race track.....(hopefully). Torque and the abiltiy to change rpm's quickly is what makes cars accelerate better. The prime example of this is Joseph Fan's race engine and Mark Anderson's race engine. Joseph's makes way more horsepower, but Mark's car accelerates much quicker, due to the lighter internals. Mark's engine drives away from Joseph's engine on the race track. The Nascar guys know this too. They will use 6 counterweight cranks when they are at a track that requires more shifting and rpm changes. You could use a 20 counterweight crank at some of those long tracks, like Daytona....the more counterweights help store energy when you have to do a little "lift"....think of "extra" counterweights as the same as a heavy/light flywheel. You need to think about all of the factors when you are building/designing engines.
Porsche put the 8 counterweight crank in the GTS engine to try and take away some of the "shake" at idle.....that's the official story from the horse's mouth. They knew it would rev slower, because it was heavier, but thought that the trade off would be worth it.....the GT's have a pretty "lumpy" idle. Tougher to sell a $90,000 car that shakes at idle.....which was one of the bitches that Porsche got back from the GT's. They were trying to solve a marketing problem.....nothing else. What they forgot is the fact that it takes more energy/effort to accelerate those extra counterweights.....which is one of the reasons that the GTS engines have a tendency to "ping' when you try and accelerate quickly from idle....especially on the automatic cars with the torque tube and convertor essentially part of the flywheel.
gb
Try and remember that horsepower means very little.....unless you are at a steady state. If you are building an engine to go run the Silver State.....your requirements are completely different than if you are building an engine for the race track.....(hopefully). Torque and the abiltiy to change rpm's quickly is what makes cars accelerate better. The prime example of this is Joseph Fan's race engine and Mark Anderson's race engine. Joseph's makes way more horsepower, but Mark's car accelerates much quicker, due to the lighter internals. Mark's engine drives away from Joseph's engine on the race track. The Nascar guys know this too. They will use 6 counterweight cranks when they are at a track that requires more shifting and rpm changes. You could use a 20 counterweight crank at some of those long tracks, like Daytona....the more counterweights help store energy when you have to do a little "lift"....think of "extra" counterweights as the same as a heavy/light flywheel. You need to think about all of the factors when you are building/designing engines.
Porsche put the 8 counterweight crank in the GTS engine to try and take away some of the "shake" at idle.....that's the official story from the horse's mouth. They knew it would rev slower, because it was heavier, but thought that the trade off would be worth it.....the GT's have a pretty "lumpy" idle. Tougher to sell a $90,000 car that shakes at idle.....which was one of the bitches that Porsche got back from the GT's. They were trying to solve a marketing problem.....nothing else. What they forgot is the fact that it takes more energy/effort to accelerate those extra counterweights.....which is one of the reasons that the GTS engines have a tendency to "ping' when you try and accelerate quickly from idle....especially on the automatic cars with the torque tube and convertor essentially part of the flywheel.
gb
06-19-2007, 03:59 AM
#24
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Here is the crank, doesn't look like I will need to turn down the counter weights for clearance.
If you've got the Mahle full skirt pistons, you will have some clearance issues with the counterweights. When I did my first mock build up with the 968 Mahle's, the skirts had to be clearanced under the pin boss. It's not an issue on every cylinder, because on some the counterweight rotates inside the piston skirt. However, there are some cylinders that are offset in such a way that the counter weight spins right under the piston skirt, and when the piston approaches BDC and the counterweight is rotated in the inverted position, there is contact. It isn't much, but it is enough to lock up the works.
If you have the KB partial skirt pistons you won't have this issue because there is no skirting under the pin boss to come in contact with the counterweight.
06-19-2007, 04:53 AM
#25
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Originally Posted by atb
If you have the KB partial skirt pistons you won't have this issue because there is no skirting under the pin boss to come in contact with the counterweight.
06-19-2007, 07:38 AM
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Hi Greg,
Thanks for the input. It wasn't me, and won't be, that questions your knowledge. Besides someday I may come out to SoCal, and I wouldn't want my car to implode when that happens.
What would you want for a set of modified Carillo rods? Do you know their weight? The weight of the Oliver rods determined by the machine shop is listed in my first post.
Thanks for the input. It wasn't me, and won't be, that questions your knowledge. Besides someday I may come out to SoCal, and I wouldn't want my car to implode when that happens.
What would you want for a set of modified Carillo rods? Do you know their weight? The weight of the Oliver rods determined by the machine shop is listed in my first post.
06-19-2007, 07:40 AM
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Originally Posted by atb
Hey Stan,
If you've got the Mahle full skirt pistons, you will have some clearance issues with the counterweights. When I did my first mock build up with the 968 Mahle's, the skirts had to be clearanced under the pin boss. It's not an issue on every cylinder, because on some the counterweight rotates inside the piston skirt. However, there are some cylinders that are offset in such a way that the counter weight spins right under the piston skirt, and when the piston approaches BDC and the counterweight is rotated in the inverted position, there is contact. It isn't much, but it is enough to lock up the works.
If you have the KB partial skirt pistons you won't have this issue because there is no skirting under the pin boss to come in contact with the counterweight.
If you've got the Mahle full skirt pistons, you will have some clearance issues with the counterweights. When I did my first mock build up with the 968 Mahle's, the skirts had to be clearanced under the pin boss. It's not an issue on every cylinder, because on some the counterweight rotates inside the piston skirt. However, there are some cylinders that are offset in such a way that the counter weight spins right under the piston skirt, and when the piston approaches BDC and the counterweight is rotated in the inverted position, there is contact. It isn't much, but it is enough to lock up the works.
If you have the KB partial skirt pistons you won't have this issue because there is no skirting under the pin boss to come in contact with the counterweight.
06-19-2007, 09:54 AM
#29
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The saga continues for those following this....
I can get JE piston pins that are about 25g lighter and apparently that may make a difference of about 35g (when it is plugged in the formula). What I didn't understand, and think I do now, is that 499g needs to be added is per pair of cylinders, so 499g - @70g is still over 400g.
Seems like this crank was really designed for lightweight components, so it may require different rods too.
I can get JE piston pins that are about 25g lighter and apparently that may make a difference of about 35g (when it is plugged in the formula). What I didn't understand, and think I do now, is that 499g needs to be added is per pair of cylinders, so 499g - @70g is still over 400g.
Seems like this crank was really designed for lightweight components, so it may require different rods too.
06-19-2007, 11:54 AM
#30
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The mahles are used in the 944s2. I have not seen them in 968 engines...
Check with carrillo about the strength of the rods...give them your piston weights and they will tell you the safe rpms. The mahles are very heavy compared to the normal pistons used with those LW rods.
You can always cut down the weight of a crank to match components, it is much harder to add weight!
marc
Check with carrillo about the strength of the rods...give them your piston weights and they will tell you the safe rpms. The mahles are very heavy compared to the normal pistons used with those LW rods.
You can always cut down the weight of a crank to match components, it is much harder to add weight!
marc