Lighter Pistons, should I rebalance the crank?
04-25-2003, 06:56 PM
#1
User
Thread Starter
Join Date: Oct 2001
Location: Cincinnati, Ohio
Posts: 380
Likes: 0
Received 0 Likes
on
0 Posts
Lighter Pistons, should I rebalance the crank?
I found out today my JE pistons will be about 3.5 oz. lighter than stock.
Anybody have an opinion on removing some weight from the crank to minimize loading?
I found a local shop that can do it. $170.00 to balance the crank.
Anybody have an opinion on removing some weight from the crank to minimize loading?
I found a local shop that can do it. $170.00 to balance the crank.
04-25-2003, 08:43 PM
#4
Instructor
Join Date: Jan 2003
Posts: 183
Likes: 0
Received 0 Likes
on
0 Posts
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Jim bailey - 928 International:
<strong>Balancing is money well spent not so much for any better performance but just feels so much better .</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Actually I'd say it feels better if it's not balanced. Very similar to those vibrating beds you put quarters in. In the long run it would be much cheaper to just put quarters in the bed though. Get everything balanced.
<strong>Balancing is money well spent not so much for any better performance but just feels so much better .</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Actually I'd say it feels better if it's not balanced. Very similar to those vibrating beds you put quarters in. In the long run it would be much cheaper to just put quarters in the bed though. Get everything balanced.
04-26-2003, 08:23 AM
#5
Burning Brakes
Join Date: Apr 2002
Location: Denmark
Posts: 881
Likes: 0
Received 0 Likes
on
0 Posts
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by John Veninger:
<strong>I suggest you get the entire rotating assembly balanced.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">John,
Do you mean with or without piston?
I cannot see how to do with piston, and if you do it without, then it is already done from the factory.
For me to see you have the rotating mass - This has to be balanced, and is already from the factory
Then you have the moving mass - This will balance each other - I.e all the piston and con-rods to have equal weight
Is it me who is wrong? - Please explain
<strong>I suggest you get the entire rotating assembly balanced.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">John,
Do you mean with or without piston?
I cannot see how to do with piston, and if you do it without, then it is already done from the factory.
For me to see you have the rotating mass - This has to be balanced, and is already from the factory
Then you have the moving mass - This will balance each other - I.e all the piston and con-rods to have equal weight
Is it me who is wrong? - Please explain
04-26-2003, 09:51 AM
#6
User
Thread Starter
Join Date: Oct 2001
Location: Cincinnati, Ohio
Posts: 380
Likes: 0
Received 0 Likes
on
0 Posts
The issue is that the crank has a countermass which offsets the stock piston mass. When you go to a lighter piston, now the crank has excess weight which is not required because the piston mass is reduced. In this case, we are talking about 1/4 lb per piston. Damn, that is a lot of reduction, this thing should reve much faster now. I would have to really study the crank and do some calculating to put some numbers on this.
My guess is if I do nothing, then the crank in and of iteslf will exert more force on the bearings, because it is essentially over compensating for the piston mass. It may be insignificant.
I'm sure the crank is balanced well right now, as I can see a lot of drilling in the counterweights.
The process is done by simulating the weight of the piston and rod assembly with some sort of a bob that gets connected to the journals. They then spin the crank and the machine tells them where to remove or add weight. As I see it it is much like balancing a tire.
The local shop I found is supposed to be very good, but I may have to take a ride out there to check it all out with my crank. I hate carrying that thing around in the trunk. It would become a huge missile in an accident.
My guess is if I do nothing, then the crank in and of iteslf will exert more force on the bearings, because it is essentially over compensating for the piston mass. It may be insignificant.
I'm sure the crank is balanced well right now, as I can see a lot of drilling in the counterweights.
The process is done by simulating the weight of the piston and rod assembly with some sort of a bob that gets connected to the journals. They then spin the crank and the machine tells them where to remove or add weight. As I see it it is much like balancing a tire.
The local shop I found is supposed to be very good, but I may have to take a ride out there to check it all out with my crank. I hate carrying that thing around in the trunk. It would become a huge missile in an accident.
04-26-2003, 09:55 AM
#7
User
Thread Starter
Join Date: Oct 2001
Location: Cincinnati, Ohio
Posts: 380
Likes: 0
Received 0 Likes
on
0 Posts
Also, I have a scale at work that I can use to measure all of the pistons, rods, rings, etc., etc., etc. I think it is good down to .01 oz. I figure I can do that portion of the balancing job pretty easily. The crank is where I need help. The JE pistons are supposed to be withing +/- 2 grams. The factory spec on the entire piston assembly is +/- 4 grams.
Trending Topics
04-26-2003, 11:40 AM
#9
Ah, Er....John,
I hope I'm the only one who cringed when you compared the crank balancing to tire balancig...
what with all the gipes about botched work and worn tires...
A question: If you are removing that much weight will the engine actually rev faster?
If you remove weight from the journals do you have to tinker with the flywheel as well?
Just curious.
I hope I'm the only one who cringed when you compared the crank balancing to tire balancig...
what with all the gipes about botched work and worn tires...
A question: If you are removing that much weight will the engine actually rev faster?
If you remove weight from the journals do you have to tinker with the flywheel as well?
Just curious.
04-26-2003, 12:55 PM
#10
User
Thread Starter
Join Date: Oct 2001
Location: Cincinnati, Ohio
Posts: 380
Likes: 0
Received 0 Likes
on
0 Posts
I believe the engine will be more responsive, yes. I mean if I give you something weighing 2 lbs and tell you to shake it back and forth as fast as you can and I measure your speed....then remove .25 lbs and have you do the same thing, I'd expect you'd be able to do it faster than with the heavier weight. It should respond somewhat more quickly.
True, it is not like balancing a tire. They have to do a lot ot figure out exactly where to drill the crank.
On the flywheel, well I'm not sure on that one? It is balanced by itself, i.e. no offset weights. I would assume it can be left as is.
True, it is not like balancing a tire. They have to do a lot ot figure out exactly where to drill the crank.
On the flywheel, well I'm not sure on that one? It is balanced by itself, i.e. no offset weights. I would assume it can be left as is.
04-26-2003, 02:25 PM
#11
Rennlist Member
Join Date: Dec 2002
Posts: 1,051
Likes: 0
Received 0 Likes
on
0 Posts
<a href="http://www.panhandleperformance.com/balancer.html" target="_blank">http://www.panhandleperformance.com/balancer.html</a>
<a href="http://www.442.com/tech/balancing.html" target="_blank">http://www.442.com/tech/balancing.html</a>
<a href="http://www.442.com/tech/balancing.html" target="_blank">http://www.442.com/tech/balancing.html</a>
04-26-2003, 09:28 PM
#12
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Help me out here...
The counterweights on the crank are there to cut down on crank flexing as the rotating mass changes? My vision of this is that the crank is balaced to itself, maybe with some consideration for the big-end weight on the rods. Not changing the rod weights, are you?
The piston and the small end of the rod do not rotate, so the effect on the rotating balance changes at each degree of rotation. Add in compression and combustion forces at continuously varying thrust angles on the rotating crank, and my guess is that a weight change on the piston wouldn't affect the balance of the crank at all.
I changed out of ME to EE, then to CS as the dollars were shifting. Any ME's want to take a shot at this?
So, balance the crank by itself, balance/match the rods based on total weight and end-to-end weights, and match the weight of the pistons. Do NOT try to adjust the counterweight mass to make up for a difference in piston weight. My recommendation.
By the way, 3.5 OUNCES per piston? You are getting shorted on your metal purchase...
The counterweights on the crank are there to cut down on crank flexing as the rotating mass changes? My vision of this is that the crank is balaced to itself, maybe with some consideration for the big-end weight on the rods. Not changing the rod weights, are you?
The piston and the small end of the rod do not rotate, so the effect on the rotating balance changes at each degree of rotation. Add in compression and combustion forces at continuously varying thrust angles on the rotating crank, and my guess is that a weight change on the piston wouldn't affect the balance of the crank at all.
I changed out of ME to EE, then to CS as the dollars were shifting. Any ME's want to take a shot at this?
So, balance the crank by itself, balance/match the rods based on total weight and end-to-end weights, and match the weight of the pistons. Do NOT try to adjust the counterweight mass to make up for a difference in piston weight. My recommendation.
By the way, 3.5 OUNCES per piston? You are getting shorted on your metal purchase...
04-26-2003, 10:29 PM
#13
Racer
Join Date: Jun 2001
Location: Gautier Ms
Posts: 270
Likes: 0
Received 0 Likes
on
0 Posts
Quickly, If you didn't do anything to your crankshaft like welding, stroking new flywheel, or harmonic balancer your crank should still be balanced per the factory (pretty darn good).
the closer you get the pistons in weight the better. stock rods are sintered metal and fairly closely matched.
If you do have the extra money (send it to me) or have the entire rotating mass done as stated previously. when the crank is done the machinist will attach "bob" weights to make up for the absence of the rods and pistons.
Then there is the while you are in theres: cc match the combustion chambers probably pretty close again. Not and ME or EE so for the rest of us "The closer you get each piston in compression, compression ratio, weight, fuel charge, air charge, to the other seven the smoother the motor will run the HP and the longer it will live...
the closer you get the pistons in weight the better. stock rods are sintered metal and fairly closely matched.
If you do have the extra money (send it to me) or have the entire rotating mass done as stated previously. when the crank is done the machinist will attach "bob" weights to make up for the absence of the rods and pistons.
Then there is the while you are in theres: cc match the combustion chambers probably pretty close again. Not and ME or EE so for the rest of us "The closer you get each piston in compression, compression ratio, weight, fuel charge, air charge, to the other seven the smoother the motor will run the HP and the longer it will live...
04-26-2003, 10:53 PM
#14
Three Wheelin'
Join Date: May 2001
Location: Hudson Valley NY
Posts: 1,641
Likes: 0
Received 0 Likes
on
0 Posts
I agree with Dr Bob
Balancing and blue printing or rotating and reciprocal parts are done individually.
The crank is balanced by itself; the connecting rods should be within 0.1grams or each other in weight and balanced end to end.
The most critical parts are the pistons. The weight should match. Weight all the pistons the lightest one is the bench mark remove small portions of metal from the other 7 to match the lightest one. It is done by drilling the pistons. The rods are done the same way. Any engine machine shop can do the job.
Balancing and blue printing or rotating and reciprocal parts are done individually.
The crank is balanced by itself; the connecting rods should be within 0.1grams or each other in weight and balanced end to end.
The most critical parts are the pistons. The weight should match. Weight all the pistons the lightest one is the bench mark remove small portions of metal from the other 7 to match the lightest one. It is done by drilling the pistons. The rods are done the same way. Any engine machine shop can do the job.
04-27-2003, 10:35 AM
#15
User
Thread Starter
Join Date: Oct 2001
Location: Cincinnati, Ohio
Posts: 380
Likes: 0
Received 0 Likes
on
0 Posts
I know people have put lighter pistons in without doing any work to the crank and have had no issues. I had heard from some other sources that weight should be removed from the crank...this the reason for my post. Afterall, the factory crank is balance matched to the pistons and rods in the engine. I'll be removing significant weight from the piston.
I figure I'll balance the piston assemblies myself, probably to within 2 grams or so. I have to devise a way to weigh the end of each rod. I should be able to come up with something. Plus I am going on the assumption that the rods are pretty closely matched out of the factory already. The factory tolerance on the pistons is +/- 4 grams. The JE stuff is supposed to be within 2 grams. So, I'll pair match piston to pins to rin sets for the best match. If they are within a few grams, I'll call it a wash and move on. I'll only be spinning this thing to 6200 rpm anyway.
The big what if in my mind is leaving the crank counterweights as is. The post z put on here is a good one and confirms what I have been asking for the last few days.
I figure I'll balance the piston assemblies myself, probably to within 2 grams or so. I have to devise a way to weigh the end of each rod. I should be able to come up with something. Plus I am going on the assumption that the rods are pretty closely matched out of the factory already. The factory tolerance on the pistons is +/- 4 grams. The JE stuff is supposed to be within 2 grams. So, I'll pair match piston to pins to rin sets for the best match. If they are within a few grams, I'll call it a wash and move on. I'll only be spinning this thing to 6200 rpm anyway.
The big what if in my mind is leaving the crank counterweights as is. The post z put on here is a good one and confirms what I have been asking for the last few days.