Twisted Crank?
Thread Starter
Race Director
Joined: Feb 1999
Posts: 12,772
Likes: 17
From: Indianapolis IN
Twisted Crank?
Today I found out that the vibration that I had in my engine is a result of my crank being out of balance front to back by 30 grams 
This all began when I did one launch on a prepped track at our local drag strip, The crank was Scat 9000 crank good to 500 HP and 7000RPM but evidently,.,, not good to 385RWHP and 415RWTQ
The crank was an external balance crank with no weights in it so it did not throw a weight.
My question is, would a twist in the crank throw the crank off by 30-grams?
I plan on upgrading to a 4340 crank but I have the Eagle SIR rods that are good to 500hp
I am told that the rods are much less of an issue than a crank.
Also, I only spin to 6000-6300rpm at the the strip.
This all began when I did one launch on a prepped track at our local drag strip, The crank was Scat 9000 crank good to 500 HP and 7000RPM but evidently,.,, not good to 385RWHP and 415RWTQ
The crank was an external balance crank with no weights in it so it did not throw a weight.
My question is, would a twist in the crank throw the crank off by 30-grams?
I plan on upgrading to a 4340 crank but I have the Eagle SIR rods that are good to 500hp
I am told that the rods are much less of an issue than a crank.Also, I only spin to 6000-6300rpm at the the strip.
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
How long has the crank been in the car?
I have never heard of a crank twisting. A good balancing shop will remove weight from the crank to get it balanced. Since you know it is off by 30, I am assuming that you have it at a crank shop....what do they say?
Just because you have a "new" crank right off the shelf does not mean that it is balanced. Read my old post about my brand new ZZ4 block with new GM crank.....
http://forums.corvetteforum.com/engi...e-part-ii.html
My brand new crank was 28.5 grams out front to back.....and the engine had never been turned over! I bought it in short block from and when i received it, tore it down to have it balanced. So you might have received a new crank, but it may not have been balance the way you would want it.
-John
I have never heard of a crank twisting. A good balancing shop will remove weight from the crank to get it balanced. Since you know it is off by 30, I am assuming that you have it at a crank shop....what do they say?
Just because you have a "new" crank right off the shelf does not mean that it is balanced. Read my old post about my brand new ZZ4 block with new GM crank.....
http://forums.corvetteforum.com/engi...e-part-ii.html
My brand new crank was 28.5 grams out front to back.....and the engine had never been turned over! I bought it in short block from and when i received it, tore it down to have it balanced. So you might have received a new crank, but it may not have been balance the way you would want it.
-John
Last edited by John A. Marker; Feb 9, 2009 at 09:52 PM. Reason: add link
Thread Starter
Race Director
Joined: Feb 1999
Posts: 12,772
Likes: 17
From: Indianapolis IN
The crank was a brand new crank that I took to the shop and it was balanced by my shop for my engine ($150) and everything was fine, no mallory weight was added only weight taken off for the correct balance.
I went to the dyno and did some dyno runs and everything was running very nice and smooth, so at the dyno I had taken the engine up to 6300 rpm 4-5 times on that day.
I went to the track, did one drag launch and the vibration begun. I spent a year searching thru the heads and numerous other mechanical and electrical components, but the vibe would always begin at @ 2800 rpm and get worse as the rpms increased.
So last week I pulled the engine and took the crank back to the shop that balanced it, they said it was out 3-grams and they cannot explain why, it has the same flywheel, pressure plate etc.
They corrected the balance yesterday but I fear that it could be twisted etc because I can no come up withy a reason that the balance would be fine then go out by 30-grams.
I went to the dyno and did some dyno runs and everything was running very nice and smooth, so at the dyno I had taken the engine up to 6300 rpm 4-5 times on that day.
I went to the track, did one drag launch and the vibration begun. I spent a year searching thru the heads and numerous other mechanical and electrical components, but the vibe would always begin at @ 2800 rpm and get worse as the rpms increased.
So last week I pulled the engine and took the crank back to the shop that balanced it, they said it was out 3-grams and they cannot explain why, it has the same flywheel, pressure plate etc.
They corrected the balance yesterday but I fear that it could be twisted etc because I can no come up withy a reason that the balance would be fine then go out by 30-grams.
Intermediate
Joined: Jan 2009
Posts: 47
Likes: 0
From: Houston Texas
The crank was a brand new crank that I took to the shop and it was balanced by my shop for my engine ($150) and everything was fine, no mallory weight was added only weight taken off for the correct balance.
I went to the dyno and did some dyno runs and everything was running very nice and smooth, so at the dyno I had taken the engine up to 6300 rpm 4-5 times on that day.
I went to the track, did one drag launch and the vibration begun. I spent a year searching thru the heads and numerous other mechanical and electrical components, but the vibe would always begin at @ 2800 rpm and get worse as the rpms increased.
So last week I pulled the engine and took the crank back to the shop that balanced it, they said it was out 3-grams and they cannot explain why, it has the same flywheel, pressure plate etc.
They corrected the balance yesterday but I fear that it could be twisted etc because I can no come up withy a reason that the balance would be fine then go out by 30-grams.
I went to the dyno and did some dyno runs and everything was running very nice and smooth, so at the dyno I had taken the engine up to 6300 rpm 4-5 times on that day.
I went to the track, did one drag launch and the vibration begun. I spent a year searching thru the heads and numerous other mechanical and electrical components, but the vibe would always begin at @ 2800 rpm and get worse as the rpms increased.
So last week I pulled the engine and took the crank back to the shop that balanced it, they said it was out 3-grams and they cannot explain why, it has the same flywheel, pressure plate etc.
They corrected the balance yesterday but I fear that it could be twisted etc because I can no come up withy a reason that the balance would be fine then go out by 30-grams.
The point of balancing is to get the force the counterweight has equal to the force the piston/rod has when it changes direction. It's not really a "front-to-back" issue.
30 grams is close to the weight of a chevy rod bearing. Someone may have just forgotten to add it all up correctly to calculate a bobweight.
Thread Starter
Race Director
Joined: Feb 1999
Posts: 12,772
Likes: 17
From: Indianapolis IN
3 grams isn't a big deal, 30 grams is though.
The point of balancing is to get the force the counterweight has equal to the force the piston/rod has when it changes direction. It's not really a "front-to-back" issue.
30 grams is close to the weight of a chevy rod bearing. Someone may have just forgotten to add it all up correctly to calculate a bobweight.
The point of balancing is to get the force the counterweight has equal to the force the piston/rod has when it changes direction. It's not really a "front-to-back" issue.
30 grams is close to the weight of a chevy rod bearing. Someone may have just forgotten to add it all up correctly to calculate a bobweight.
Intermediate
Joined: Jan 2009
Posts: 47
Likes: 0
From: Houston Texas
You guys are missing the point, the entire rotating was balanced and everything was fine for 2-3 months, I went to the track and the vibration started, pulled the crank and the machine shop re-checked the balance and it was off by 30-grams,,, but it was perfect for months before that
How did the main bearings look when the crank came out?
Thread Starter
Race Director
Joined: Feb 1999
Posts: 12,772
Likes: 17
From: Indianapolis IN
I must not understand how they do this, the shop claims that the crank was balanced properly after weighing my pistons and rods when I bought the crank, but when I took it back on Monday they said if was off 30-g using the same bob weights.
My simple brain could only imagine that the crank was twisted causing an inbalance hence causing the vibration.
As i mentioned, the day before the vibe started, I had taken the car up to 62300rpm at the dyno 4-5 times and there was no hint of a problem, then after 1 drag launch at @ 2500rpm, the car would shake badly from 2800rpm and got worse as the rpms went up.
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Intermediate
Joined: Jan 2009
Posts: 47
Likes: 0
From: Houston Texas
The front main was a little worn and # 2 rod bearing looked worn.
I must not understand how they do this, the shop claims that the crank was balanced properly after weighing my pistons and rods when I bought the crank, but when I took it back on Monday they said if was off 30-g using the same bob weights.
My simple brain could only imagine that the crank was twisted causing an inbalance hence causing the vibration.
As i mentioned, the day before the vibe started, I had taken the car up to 62300rpm at the dyno 4-5 times and there was no hint of a problem, then after 1 drag launch at @ 2500rpm, the car would shake badly from 2800rpm and got worse as the rpms went up.
I must not understand how they do this, the shop claims that the crank was balanced properly after weighing my pistons and rods when I bought the crank, but when I took it back on Monday they said if was off 30-g using the same bob weights.
My simple brain could only imagine that the crank was twisted causing an inbalance hence causing the vibration.
As i mentioned, the day before the vibe started, I had taken the car up to 62300rpm at the dyno 4-5 times and there was no hint of a problem, then after 1 drag launch at @ 2500rpm, the car would shake badly from 2800rpm and got worse as the rpms went up.
But if the same exact bobweight was used, and it was off 30g, then something is off for sure. I have heard bad things about the 9000 cast cranks, maybe it did twist. If the keyway is worn, the damper may have moved. That is odd though.
Thread Starter
Race Director
Joined: Feb 1999
Posts: 12,772
Likes: 17
From: Indianapolis IN
Cranks are balanced by calculating a bobweight based off of the weights of not only the pistons and rods, but the rings and bearings as well. Some shops will add another 5g for oil, and sometimes even a certain percentage for "overbalance". Chevy rod bearings are generally about 30 grams, and are used twice in the bobweight, so I could see one being left out causing the problem.
But if the same exact bobweight was used, and it was off 30g, then something is off for sure. I have heard bad things about the 9000 cast cranks, maybe it did twist. If the keyway is worn, the damper may have moved. That is odd though.
But if the same exact bobweight was used, and it was off 30g, then something is off for sure. I have heard bad things about the 9000 cast cranks, maybe it did twist. If the keyway is worn, the damper may have moved. That is odd though.
I have had 2-buddies experience the vibe and they both said, something is out of balance,
AI told them to order me a 4340 crank and throw the Scat 9000 in the trash, my engine is only 385rwhp and 415rwtq but it must have been enough to mess it up some how or another, twisting just seems to me to be the only likley cause.
They offered to index the crank and check the throws to see if they are accurate at 90-degrees but since the crank was off so much, I just told them to buy me a new forged crank.
Burning Brakes
Joined: Nov 2001
Posts: 1,189
Likes: 2
From: Austin TX
If you're wanting to understand what the problem is I wouldn't pass up on that opportunity. I should think the shop would want to do that anyway so that they might learn something...
I will get the crank back from them and check it out myself, I have just been so frustrated by this episode that I wanted nothing to do with it any more.
Burning Brakes
Joined: Nov 2001
Posts: 1,189
Likes: 2
From: Austin TX
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,443
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I'm a firm believer in "You get what you pay for."
external balanced is crap. How much more was the cost of internally balanced 4340 rotating kit VS your junk and the time and money spent pulling it out again?
Just give that junk to your shop and start over with a 3.875 stroker forged kit from http://www.flatlanderracing.com
external balanced is crap. How much more was the cost of internally balanced 4340 rotating kit VS your junk and the time and money spent pulling it out again?
Just give that junk to your shop and start over with a 3.875 stroker forged kit from http://www.flatlanderracing.com
Last edited by gkull; Feb 11, 2009 at 09:14 AM.
Thread Starter
Race Director
Joined: Feb 1999
Posts: 12,772
Likes: 17
From: Indianapolis IN
I'm a firm believer in "You get what you pay for."
external balanced is crap. How much more was the cost of internally balanced 4340 rotating kit VS your junk and the time and money spent pulling it out again?
Just give that junk to your shop and start over with a 3.875 stroker forged kit from http://www.flatlanderracing.com
external balanced is crap. How much more was the cost of internally balanced 4340 rotating kit VS your junk and the time and money spent pulling it out again?
Just give that junk to your shop and start over with a 3.875 stroker forged kit from http://www.flatlanderracing.com
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
What was your total timing?
The front main was a little worn and # 2 rod bearing looked worn.
I must not understand how they do this, the shop claims that the crank was balanced properly after weighing my pistons and rods when I bought the crank, but when I took it back on Monday they said if was off 30-g using the same bob weights.
My simple brain could only imagine that the crank was twisted causing an inbalance hence causing the vibration.
As i mentioned, the day before the vibe started, I had taken the car up to 62300rpm at the dyno 4-5 times and there was no hint of a problem, then after 1 drag launch at @ 2500rpm, the car would shake badly from 2800rpm and got worse as the rpms went up.
I must not understand how they do this, the shop claims that the crank was balanced properly after weighing my pistons and rods when I bought the crank, but when I took it back on Monday they said if was off 30-g using the same bob weights.
My simple brain could only imagine that the crank was twisted causing an inbalance hence causing the vibration.
As i mentioned, the day before the vibe started, I had taken the car up to 62300rpm at the dyno 4-5 times and there was no hint of a problem, then after 1 drag launch at @ 2500rpm, the car would shake badly from 2800rpm and got worse as the rpms went up.
I have seen nearly new block ruined by to much adv. Will the dyno boys admit how much total timing they used to tune with?
cardo0
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
As for increasing to a richer mixture requires less total advance. This was hard for me to see also but it is easier to see it as the mixture becomes leaner there is a less dense mix in the cyl and the fuel burn becomes slower - needing less adv. We want our max cyl press to happen after TDC.
Compression is easier to see as an increase in c.r. puts the mix under higher press and this increases the combustion rate - again needs less adv with higher c.r..
Well good luck with your new rebuild. U may want to try a little less adv when u increase the mix next time on the dyno.
cardo0
