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Old 05-08-2009, 12:05 AM   #1
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Default Spun bearing? Replace pitted pistons?

So,
Before going FI, I rebuilt the motor ~8 months ago (new rings, bearings, honed and decked the block), but left in the "slightly" pitted pistons in.

Two days ago I overboosted and hit 15psi during a tuning run, the motor developed a clunking noise right after. I pulled the plugs and the pistons and con rods appear intact. However, the pistons are really pitted (I can see that w/ my endoscope.)

I'm running 12psi, should I take the time to pull the pistons and replace them? If so, the rings have only a few thousand miles on them, can I reuse them?

I don't know what the clunking is, spun bearing? Who knows??
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Old 05-08-2009, 12:09 AM   #2
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Depends on your goals. Do you want pictures of a catastrophic explosion and a $750-2000 replacement bill? Keep on driving it. Do you want to deal with the issue and make the car reliable again? Rebuild it.
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Old 05-08-2009, 12:53 AM   #3
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were u by any chance logging durring this run? What were ur AFRS

Ik its kinda too late now anyway, but just curious.

I say rebuild it / replace them before something big gets damaged.
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Old 05-08-2009, 12:57 AM   #4
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I don't understand what the issue is...if the motor developed an internal noise after overboosting it...it is obvious that you are going to have to pull it apart and fix the damaged components. Are you going to wait for the whole thing to grenade and then have to spend money on an engine core?

It's a no-brainer.
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Old 05-08-2009, 01:54 AM   #5
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If you got the extra cash, forged internals, you won't regret it. I promise lol.

Definately rebuild it, I was in your shoes, some what, 2 weeks ago. Its worth it in the end. I'm saying this and Im not even into boost yet hah.
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Old 05-08-2009, 08:57 AM   #6
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Quote:
Originally Posted by 240_to_miata View Post
were u by any chance logging durring this run? What were ur AFRS

Ik its kinda too late now anyway, but just curious.

I say rebuild it / replace them before something big gets damaged.
It did go lean under extremely light load.
Attached Images
File Type: bmp overboost 15 psi top plot.bmp (282.7 KB, 191 views)
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Old 05-08-2009, 09:04 AM   #7
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Originally Posted by mrtonyg View Post
I don't understand what the issue is...if the motor developed an internal noise after overboosting it...it is obvious that you are going to have to pull it apart and fix the damaged components. Are you going to wait for the whole thing to grenade and then have to spend money on an engine core?

It's a no-brainer.
I'm not certain that it's from the bottom end, but if I need to replace the pistons since they're pitted, then I'll be rebuilding it.

What I don't know is: do pitted pistons warrant a rebuild? Assuming the noise isn't from the bottom end.
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Old 05-08-2009, 10:40 AM   #8
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I don't know how a "clunking" noise wouldn't be from the bottom end.
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Old 05-08-2009, 11:08 AM   #9
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How did you achieve 15psi under a "light load?" Are you suffering from flow restrictions or just understating how you blew it up?

Your engine could certainly run with a bent rod, it just might not run well and it might make abnormal noises while doing so.

BTW - Pitted piston tops will create hot spots and combined with FI is a recipe for acute depression. Out of concern for your mental health I would recommend that you change them.
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Old 05-08-2009, 11:57 AM   #10
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How did you achieve 15psi under a "light load?" Are you suffering from flow restrictions or just understating how you blew it up?

Your engine could certainly run with a bent rod, it just might not run well and it might make abnormal noises while doing so.

BTW - Pitted piston tops will create hot spots and combined with FI is a recipe for acute depression. Out of concern for your mental health I would recommend that you change them.
There's two lean spots on the plot, this was under part throttle. From ~4psi - 15psi was WOT, but was running rich there.

Ok, so the advice so far is to replace the pistons and rods. I guess, based on what I've read, that I'll go ETD rods and possibly OEM pistons.

I've heard of people reusing rings if they're still pretty new. Is this a **** idea?
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Old 05-08-2009, 02:08 PM   #11
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Quote:
Originally Posted by flier129 View Post
If you got the extra cash, forged internals, you won't regret it. I promise lol.

Definately rebuild it, I was in your shoes, some what, 2 weeks ago. Its worth it in the end. I'm saying this and Im not even into boost yet hah.
My "target boost" is 15psi.

I had a bottle of wine a few nights ago and on the drive home it was cold out and the car felt insano fast but kept popping and bucking...because I was overboosting to 19psi/230kpa. lol.

Oh well, just pull over and twist the boost controller and smile because I spent the money wisely on a built motor with overboost protection. If I had a stock motor I would have blown it up 3-times by now. So you have an idea of the amount of headroom I have with 8.6:1 pistons, on a dynodynamics I've had 252wtq/4000rpm/15psi with 30* of advance with no detonation. Buy the low-comp pistons because its $400 well spent.
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Old 05-08-2009, 03:13 PM   #12
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Quote:
Originally Posted by fluke View Post
I've heard of people reusing rings if they're still pretty new. Is this a **** idea?

It's a great idea if you really like the teardown and assembly process.
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Old 05-08-2009, 03:53 PM   #13
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Quote:
Originally Posted by fluke View Post
I've heard of people reusing rings if they're still pretty new. Is this a **** idea?
You want them to mate to the cylinder walls to achieve a good seal. When you take them out and put them back in, it will be impossible to keep them in the exact orientation that they were already in and mated. Replace the rings and check the tolerances well to make sure you don't need an overbore. Be sure to re hone either way.
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Old 05-08-2009, 03:59 PM   #14
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Originally Posted by hustler View Post
My "target boost" is 15psi.

I had a bottle of wine a few nights ago and on the drive home it was cold out and the car felt insano fast but kept popping and bucking...because I was overboosting to 19psi/230kpa. lol.

Oh well, just pull over and twist the boost controller and smile because I spent the money wisely on a built motor with overboost protection. If I had a stock motor I would have blown it up 3-times by now. So you have an idea of the amount of headroom I have with 8.6:1 pistons, on a dynodynamics I've had 252wtq/4000rpm/15psi with 30* of advance with no detonation. Buy the low-comp pistons because its $400 well spent.
Looks like the Belfab rod/piston combo is pretty reasonable, still not sure what compression ratio to go w/ though. I've read a lot of posts on the subject, guess there's no definitive answer.

Hustler, did you notice a significant loss of power when off boost w/ 8.6:1? Would you recommend 8.6 over 9:1?

Thanks.
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Old 05-08-2009, 04:00 PM   #15
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Originally Posted by AlexandertheOk View Post
You want them to mate to the cylinder walls to achieve a good seal. When you take them out and put them back in, it will be impossible to keep them in the exact orientation that they were already in and mated. Replace the rings and check the tolerances well to make sure you don't need an overbore. Be sure to re hone either way.
That makes sense, I'll do that.

Thanks.
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Old 05-08-2009, 04:22 PM   #16
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Quote:
Originally Posted by fluke View Post
Hustler, did you notice a significant loss of power when off boost w/ 8.6:1? Would you recommend 8.6 over 9:1? Thanks.
I came from a seriously tired 1.6 so no, it felt significantly stronger than my 1.6. You can advance spark MUCH further with lowered compression and I bet you can up the octane and get close to the stock off-boost output. FM seems to churn out tons and tons of 9.0:1 compression motors that make crazy power, but I think I'm making more torque/psi, especially when you compare dyno measurement differences.

Read through my dyno thread and you'll probably agree too. I have lots and lots of headroom.

if you're really worried about losing power off boost, then go with bigger pistons.
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Old 05-08-2009, 07:52 PM   #17
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Originally Posted by hustler View Post
I came from a seriously tired 1.6 so no, it felt significantly stronger than my 1.6. You can advance spark MUCH further with lowered compression and I bet you can up the octane and get close to the stock off-boost output. FM seems to churn out tons and tons of 9.0:1 compression motors that make crazy power, but I think I'm making more torque/psi, especially when you compare dyno measurement differences.

Read through my dyno thread and you'll probably agree too. I have lots and lots of headroom.

if you're really worried about losing power off boost, then go with bigger pistons.
I didn't realize you have a 1.6, 9:1 was stock for the 1.6 no? I'd like to make 15 psi safely.
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Old 05-08-2009, 08:33 PM   #18
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Quote:
Originally Posted by hustler View Post
You can advance spark MUCH further with lowered compression and I bet you can up the octane and get close to the stock off-boost output.
Seems like every thread i come into Hustler is talking about how much better low comp is.. btw i jumped on the bandwagon
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Old 05-08-2009, 08:58 PM   #19
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Hustler isn't the only one that understands that lower dynamic compression will allow you to run more boost and more timing and ultimately make significantly more power on pump gas. It is just a little science and math that happens to be indisputable. What you choose to do with this information is up to you. I wish there were more true believers like him on this forum. I guess it will take time for more to choose this route. You won't be disappointed.
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Old 05-08-2009, 09:50 PM   #20
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Originally Posted by sixshooter View Post
Hustler isn't the only one that understands that lower dynamic compression will allow you to run more boost and more timing and ultimately make significantly more power on pump gas. It is just a little science and math that happens to be indisputable. What you choose to do with this information is up to you. I wish there were more true believers like him on this forum. I guess it will take time for more to choose this route. You won't be disappointed.
Well, it makes sense to me. I'm going w/ 9:1 Belfab pistons.
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