any downsides to offset ground cranks?
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any downsides to offset ground cranks? I am buying forged rods and pistons and was thinking of buying a cast iron offset ground crank to increase my stroke a little... I have seen cranks advertised with a 1.88" journal... I think stock is around 2.1" or something like that?? Anyway, are there any serious downsides to this? The only think I can think of is the reduced rod bearing surface area, but I don't know how much of an effect this would have. I am trying to build an FI 600hp daily-driver setup.
-Dave
-Dave
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Originally Posted by FastKat
I am buying forged rods and pistons and was thinking of buying a cast iron offset ground crank to increase my stroke a little... I have seen cranks advertised with a 1.88" journal... I think stock is around 2.1" or something like that?? Anyway, are there any serious downsides to this? The only think I can think of is the reduced rod bearing surface area, but I don't know how much of an effect this would have. I am trying to build an FI 600hp daily-driver setup.
-Dave
-Dave
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Thanks for the input... know anywhere that does a quality job?
Originally Posted by J-Rod
Bearing speed goes down. NASCAR runs 1.888 The only downside is making sure the crank grinder knows what he is doing. The most critical part is the fillet (the curve in the corner of the journal). If that isn't right, the crank will develop stress in that area. That is where cranks break. That is where all the strength is the crank is.
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Originally Posted by J-Rod
Bearing speed goes down. NASCAR runs 1.888 The only downside is making sure the crank grinder knows what he is doing. The most critical part is the fillet (the curve in the corner of the journal). If that isn't right, the crank will develop stress in that area. That is where cranks break. That is where all the strength is the crank is.
The stock LS cranks have an undercut fillet, which may also be "rolled" to increase it's fatigue strength. That would be very difficult to duplicate, and grinding it away wouldn't be very helpful.
The other downside to reducing rod journal diameter and increasing stroke is that you reduce the "overlap" between the main and rod journals. This overlap adds strength to the crank. Look along the axis of the crank: the rod journal surface overlaps the main journal surface about .61 inches stock. When you offset grind and reduce the rod journal diameter about .200, that amount comes out of the overlap. If you draw this to scale, you'll see that the overlap area is reduced about 1/3 or a little more.
If it was an NA engine I'd not be as concerned as I would be about an FI engine.
You are going to need very special (expensive) rods, and you are only gaining about 19 cubes. Personally I'd go .010 (or .25 mm) under on the rods and mains, keep the stock fillets and add another 1/4 psi boost
and turn it 5% faster to make up for the lack of 19 cubes. The 5% keeps the piston speed and g's pretty much the same vs. the longer stroke at a lower rpm.Just curious, FastKat, how much FI fwhp and how many revs are you considering? over/under 750 and 7500?
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Putting it all in plain ol english, I wouldnt go that route. The little bit you do gain in stroke is not worth the loss in journal diameter and strength. Especially if your looking to go boost or juice. I know I wouldnt want any sleepless nights or stressful days thinking about if its gonna blow to pieces during a race or just driving. GoodLuck 
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Thanks for the info, this confirms my suspicions...
I am shooting for 650rwhp which is probably around 750hp at the flywheel? I want something torquey because my car is going to weigh around 4,200lbs and with the high rear end gear ratio (2.88's stock, considering 3.07's or 3.31's) I need something to move it off the line... that's why I'm interested in more stroke. I initially didn't plan on reving over the stock 6200 redline if I went with any kind of stoker motor due to increased piston speeds, rod angles and rod bearing loads.... however I will cross that bridge when I come to it.
I can't justify spending the money on a forged crank and not going with a more stroke! Something inside me says "there's not replacement for displacement!" After balancing the stock crank and other prep work, it seems like it would only be an extra $500 for a forged steel crank... do you think the 3.9" stroke forged Eagle crank would require the block to be clearanced? I was considering the offset ground crank because I am trying not to let this project get rediculously expensive
Thanks, -Dave
I am shooting for 650rwhp which is probably around 750hp at the flywheel? I want something torquey because my car is going to weigh around 4,200lbs and with the high rear end gear ratio (2.88's stock, considering 3.07's or 3.31's) I need something to move it off the line... that's why I'm interested in more stroke. I initially didn't plan on reving over the stock 6200 redline if I went with any kind of stoker motor due to increased piston speeds, rod angles and rod bearing loads.... however I will cross that bridge when I come to it.
I can't justify spending the money on a forged crank and not going with a more stroke! Something inside me says "there's not replacement for displacement!" After balancing the stock crank and other prep work, it seems like it would only be an extra $500 for a forged steel crank... do you think the 3.9" stroke forged Eagle crank would require the block to be clearanced? I was considering the offset ground crank because I am trying not to let this project get rediculously expensive
Thanks, -Dave
Originally Posted by Old SStroker
The stock LS cranks have an undercut fillet, which may also be "rolled" to increase it's fatigue strength. That would be very difficult to duplicate, and grinding it away wouldn't be very helpful.
The other downside to reducing rod journal diameter and increasing stroke is that you reduce the "overlap" between the main and rod journals. This overlap adds strength to the crank. Look along the axis of the crank: the rod journal surface overlaps the main journal surface about .61 inches stock. When you offset grind and reduce the rod journal diameter about .200, that amount comes out of the overlap. If you draw this to scale, you'll see that the overlap area is reduced about 1/3 or a little more.
If it was an NA engine I'd not be as concerned as I would be about an FI engine.
You are going to need very special (expensive) rods, and you are only gaining about 19 cubes. Personally I'd go .010 (or .25 mm) under on the rods and mains, keep the stock fillets and add another 1/4 psi boost and turn it 5% faster to make up for the lack of 19 cubes. The 5% keeps the piston speed and g's pretty much the same vs. the longer stroke at a lower rpm.
Just curious, FastKat, how much FI fwhp and how many revs are you considering? over/under 750 and 7500?
The other downside to reducing rod journal diameter and increasing stroke is that you reduce the "overlap" between the main and rod journals. This overlap adds strength to the crank. Look along the axis of the crank: the rod journal surface overlaps the main journal surface about .61 inches stock. When you offset grind and reduce the rod journal diameter about .200, that amount comes out of the overlap. If you draw this to scale, you'll see that the overlap area is reduced about 1/3 or a little more.
If it was an NA engine I'd not be as concerned as I would be about an FI engine.
You are going to need very special (expensive) rods, and you are only gaining about 19 cubes. Personally I'd go .010 (or .25 mm) under on the rods and mains, keep the stock fillets and add another 1/4 psi boost
and turn it 5% faster to make up for the lack of 19 cubes. The 5% keeps the piston speed and g's pretty much the same vs. the longer stroke at a lower rpm.Just curious, FastKat, how much FI fwhp and how many revs are you considering? over/under 750 and 7500?
