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Old 01-08-2021, 08:27 AM   #1
Zak6182
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Wink WTF?!? Another Piston Slap Thread!?!?!!

Before you roast me...Yes I know about piston slap and its completely normal in built motors during start up, especially when cold. This isn't my first engine with forged pistons and I'm fairly used to the noise. I'm not so concerned with the slap but more about WHEN the noise becomes audible. In all my previous engines the slap was there right from the start and went away when warm like expected.

My issue is sometimes you start hearing the slap almost immediately and other times it can take over a minute to start hearing it, yes I timed it. I have watched ambient temps and it doesn't seem to have any correlation to when the slap starts.

So my question to the people out there with forged pistons, do you always hear slap immediately or does it take a minute to become audible? Does it change, like what I'm experiencing, or does it start a consistent time?
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Old 01-08-2021, 09:12 AM   #2
rtv900
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just my opinion but I could see variations like that when starting cold and the engine is running sluggish, cold oil, etc, all kinds of variables

IMO the only thing that matters is that is goes away consistently once it gets to a real world operating temp
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Old 01-08-2021, 09:38 AM   #3
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Slap varies by RPM and temp. If you're hearing it after a minute pay attention to where the RPM is. it might not slap when cold at high idle, but then as idle settles down it might start.
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Old 01-08-2021, 10:22 AM   #4
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RPM is pretty consistent because of the normal warm up of 1500 RPM. Slap slowly becomes audible while RPM stays right around 1500 (-/+ 200). It takes at least 2 minutes for the RPM to start to drop with the weather I've had.
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Old 01-08-2021, 02:18 PM   #5
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I wouldn't be bothered by that. My STi has CP pistons in it and it kind of does the same thing. In the "winter" here in SC it'll slap a bit, but come summer time it's pretty quiet.
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Old 01-08-2021, 03:46 PM   #6
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Good to know. Off topic but do you know a good subie shop that handles aftermarket stuff around us?
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Old 01-08-2021, 04:15 PM   #7
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Quote:
Originally Posted by Zak6182 View Post
Good to know. Off topic but do you know a good subie shop that handles aftermarket stuff around us?
Depends on what you want done. The good one I used to use closed 3 years ago now, so I do all my own work including engine/trans building

The only thing I farm out now is dyno tuning and that's to a friend that used to work at the old shop. The other shops I'm aware of have hit or miss reputations, and are all up around the Charlotte area.
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Old 01-10-2021, 12:20 AM   #8
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I'm running JE Pistons 9.5/1 FSRs. No slap or knock at all with Darton dry iron sleeves in the block.
My Perrin fuel pressure regulator hydraulic locking sounds like the engine will disassemble itself into it's component parts at cold start and rattles at idle.
Well, it just goes along with the rest of the farts and whistles when you punch the throttle and stir the gas.
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Old 01-11-2021, 10:22 AM   #9
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JE usually has offset piston pins which negates a lot of slap. CP's don't have that same offset and slap a lot more. It's not really much to worry about. My DD has CP's with .003ish PTW clearance while the autox car has JE's with .004ish PTW and it's much quieter on cold start.
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Old 01-11-2021, 10:34 AM   #10
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Quote:
Originally Posted by subydude View Post
JE usually has offset piston pins which negates a lot of slap. CP's don't have that same offset and slap a lot more. It's not really much to worry about. My DD has CP's with .003ish PTW clearance while the autox car has JE's with .004ish PTW and it's much quieter on cold start.
Offset pins? I never knew that was possible. Is the offset negligible? Is the skirt wear greater on one side?
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Old 01-11-2021, 12:02 PM   #11
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Offsetting pins is interesting to see how it works on the EJ platform. But how Wiseco explains it, it makes sense.

"As engines evolved, engineers determined that offsetting the piston (wrist) pin yields two major benefits. First, it improves the noise characteristics of the engine due to piston slap at top dead center (TDC). This is a major NVH (noise vibration and harshness) concern to production engineers who want to eliminate alarming noises anywhere they can. The second reason is to improve engine output by reducing internal friction.

Offsetting the cylinder axis from the crankshaft axis, minimizes rod angularity when cylinder pressure is at its highest. Reduced angularity leads to lower piston thrust forces, hence lower frictional losses during the period of maximum cylinder pressure, especially at the beginning of the power stroke when pressure rises sharply about 12-15 degrees after TDC. Rather than revising a complete engine design, the easy way to do this is to offset the pin bore in the piston. This allows the same crank/rod pin axis geometry as an engine designed with offset cylinder axes, but within existing conventional engine architecture."
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Old 01-11-2021, 12:33 PM   #12
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Offset pins? I never knew that was possible. Is the offset negligible? Is the skirt wear greater on one side?
I know JE's pistons are asymmetric and are marked for which bank they go in. The skirts are sized appropriately to accept the forces.

The autox engine I built was the first time I'd used JE's, but seeing that I'd probably use their pistons going forward vs the CP's just because of how much quieter they are.
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Old 01-11-2021, 05:24 PM   #13
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I have manley with offset pins and just under .004" ptw it's very quiet.

Skirt wear is inevitable. The load side of the piston always will wear. Especially without oil squirters for the pistons. Some good wpc coating might help prolong that. But it will still wear out especially pushing the limits.
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Old 01-13-2021, 01:05 PM   #14
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I had a block built by a local builder. Super loud slap on cold. It blew up so I now have an IAG stage 2 Tuff block. No slap at all after 18K miles.

Both say they use JE pistons, and the same 99.75 bore. I suspect the IAG block has much tighter PTW, but I don't have numbers for either block.

Also my builder (for the first block) said he used stroker pistons, but longer rods to cancel that out. That's the only real material difference between the two blocks.
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Old 01-13-2021, 01:58 PM   #15
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I had a GM 5.3 liter which are known to have tons of piston slap at high miles- it was a lot like you describe- never the same, different days different sounds- but it always went away when warm.
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Old 01-13-2021, 02:48 PM   #16
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Quote:
Originally Posted by Zak6182 View Post
Good to know. Off topic but do you know a good subie shop that handles aftermarket stuff around us?
Shift Fast in Rock Hill, SC built my engine and has a pretty good reputation. JR does their tuning and comes in about once a month. They do a lot of the Subarus in the Charlotte area and are an IAG preferred installer or whatever the term IAG uses is. I haven't personally used them for much beyond engine work.

I think I know which shop subydude is referring to (42?) and it was sad to see them go.

Tim,
I'm the guy with the blue bugeye that raced against you in XSA at the CCR event when you had the mud brown STI.
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Old 01-13-2021, 03:36 PM   #17
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Quote:
Originally Posted by billyboy999 View Post
I had a block built by a local builder. Super loud slap on cold. It blew up so I now have an IAG stage 2 Tuff block. No slap at all after 18K miles.

Both say they use JE pistons, and the same 99.75 bore. I suspect the IAG block has much tighter PTW, but I don't have numbers for either block.

Also my builder (for the first block) said he used stroker pistons, but longer rods to cancel that out. That's the only real material difference between the two blocks.
I'm not too worried about how loud it is, just thought maybe it was weird as to when it starts. Its louder then others but hard to compare since I'm running Perring motor mounts with the new motor. My last block was iag stage 2 tuff. My new one is stage 2.5 (same but closed deck). I'll have to email iag to get the build sheet for my current one but I know the last block was .004 PTW. It never made much (if any) noise but I also only drove it for 2 months during the summer on stock mounts.


Quote:
Originally Posted by whozatguy
Shift Fast in Rock Hill, SC built my engine and has a pretty good reputation. JR does their tuning and comes in about once a month. They do a lot of the Subarus in the Charlotte area and are an IAG preferred installer or whatever the term IAG uses is. I haven't personally used them for much beyond engine work.
Thanks for the info. I found them when looking up JR tune locations. Good to hear they have a decent reputation. I do most of my own work (15 yr diesel mech) but when it comes to motor work I'd rather farm most of it out. Biggest reason, it's my DD and it would be hard to be without it for a while. I have a old XJ on 35's I can drive but its more of a buggy and I have to drive to Savannah one weekend a month.

The last 2 motors were done by the dealer. The master tech is a friend of mine and a great tech, but after the last go-round the service manager told me I had exceeded their abilities and needed to find a "speed shop"
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Old 01-13-2021, 05:20 PM   #18
subydude
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Quote:
Originally Posted by whozatguy View Post
Shift Fast in Rock Hill, SC built my engine and has a pretty good reputation. JR does their tuning and comes in about once a month. They do a lot of the Subarus in the Charlotte area and are an IAG preferred installer or whatever the term IAG uses is. I haven't personally used them for much beyond engine work.

I think I know which shop subydude is referring to (42?) and it was sad to see them go.

Tim,
I'm the guy with the blue bugeye that raced against you in XSA at the CCR event when you had the mud brown STI.
I haven't talked to Shift Fast, but they're at the top of the list in Charlotte from people I've heard. I'm hoping to rent their dyno for my autox car soon actually. Not that I don't trust JR, but having someone who understands autox tune makes a big difference in part throttle response.

I was referring to 42. I still work with Adam from there on tuning actually. He helped me with the dirty STi's tune enough that I can do basic stuff now.

I remember you as well. That STi is my daily. I've fired up the actual autox car now though so, pending dyno session, it should be out in March to do it's first shake down.
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Old 01-14-2021, 10:01 AM   #19
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Quote:
Originally Posted by Zak6182 View Post
Thanks for the info. I found them when looking up JR tune locations. Good to hear they have a decent reputation. I do most of my own work (15 yr diesel mech) but when it comes to motor work I'd rather farm most of it out. Biggest reason, it's my DD and it would be hard to be without it for a while. I have a old XJ on 35's I can drive but its more of a buggy and I have to drive to Savannah one weekend a month.

The last 2 motors were done by the dealer. The master tech is a friend of mine and a great tech, but after the last go-round the service manager told me I had exceeded their abilities and needed to find a "speed shop"
If you are looking for motor work, they are a good shop, most of the high HP Subarus in the area come from them. The owners are good guys. Usually pretty booked up, so plan in advanced, especially if looking for a tune.
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