Questions about forged pistons, cold start, and piston slap

[charley3]

How cold of a day causes noticeable piston slap at cold start? The coldest it ever gets in my climate is 8F, though usuall6 it doesn't go below 20F.

On days that are cold enough to cause slap at cold start, how much does the engine need to warm-up for the slap to stop? For example, when dash guage reads 100F? or 1 min after starting? Or what?

What is recommended warm-up procedure?

[SilverXJ]

Loudness will vary depending on clearance. I usually begin to hear slap around 40*F. Not very much though. 8*F and it will really be noticeable. Depending on the temp, with in 2 minutes of easy driving it is typically not noticeable. I don't let the engine sit/idle and warm up much more than 1 minute in the coldest weather. The cam is splash lubricated and at idle it isn't getting much splash. Plus there is also very little work being done by the engine so it won't build heat as quickly as driving it. I have a remote start, but I usually activate it when I exit my apartment door to leave. I'm just not fond of long idling periods.

Coolant temp is a poor indicator of much as far as if the engine is up to temp. The real indicator is oil temp. It takes quite a bit of time for the oil to come up to temp after the coolant does. I've typically found 20 minutes after coolant of continuous driving. Have a look here for some data: http://www.jeepstrokers.com/forum/viewt ... f=5&t=4543

Lake Speed, Jr. at Joe Gibbs recommended the oil be up to 180*F at least prior to really getting on the skinny pedal. I pay more attention to the oil temp gauge than I do to the coolant temp gauge.

[charley3]

My bottom line question/concern is: Would forged pistons cause me problems in the Winter? Does piston slap cause signifigantly increased wear?

[Russ Pottenger]

Because of many factors forged pistons will have a greater growth rate than a cast/hypereutectic piston.
At a cold start they will roughly have a additional .001 to .002 clearance. After a minute of warm up it should
go away.

Because of the strength of a forged piston your "less" likely of any piston failures.

[akadeutsch]

This is just my humble opinion but unless you are going to boost your engine why do you want/need expensive forged pistons? Everyone here seems to think that forged pistons will make your engine last longer or run better. But every one negates the advantages of hypereutectics = tighter tolerances= no slap on cold start = less wear. I just dont see why everyone here wants to use Forged slugs with an 8to1 CR on 87 octane gas. There may be better places to spend your cash. Again...just my opinion

[charley3]

I would love to build a stroker using hyperatic pistons, but AFAIK there are no hyper pistons that would fit with a stroker

[charley3]

With regard to cold start forged piston slap, is a, 4.6 better or worse than 4.7, or does it make any difference?

[charley3]

In my 4L with stock hyperautic pistons, I patiently wait for oil pressure to come up plus an additional 20 seconds before putting it in gear. After that I put the hammer down as needed to merge into traffic from where I live. The stock hyper pistons are great for that.

With a stroker with forged pistons I'd be worried about putting hammer down to merge into traffic 1 or 2 minutes after cold start, especially on a 20F to 30 F Winter day. That's my concern. Is that a valid concern?

[SilverXJ]

that .010" would have no difference.

Put the hammer down right after a cold start? That is just a bad idea. And not just because of the piston materials.

[Cheromaniac]

This is just my humble opinion but unless you are going to boost your engine why do you want/need expensive forged pistons? Everyone here seems to think that forged pistons will make your engine last longer or run better. But every one negates the advantages of hypereutectics = tighter tolerances= no slap on cold start = less wear. I just dont see why everyone here wants to use Forged slugs with an 8to1 CR on 87 octane gas. There may be better places to spend your cash. Again...just my opinion

The main reason for using forged pistons is that there aren't any hypereutectic pistons that are tall enough to keep the quench height below 0.050" in a stroker without having to shave at least 0.030" from the block.
Basically you can only use off-the-shelf hyper pistons with the shorter 4.2L rods.

[IH 392]

They need to make a hyperutectic, short pin height, BIG dish, metric ring piston, that's what we all need!, the forged is the only option for use with the long rods.

[Russ Pottenger]

This is just my humble opinion but unless you are going to boost your engine why do you want/need expensive forged pistons? Everyone here seems to think that forged pistons will make your engine last longer or run better. But every one negates the advantages of hypereutectics = tighter tolerances= no slap on cold start = less wear. I just dont see why everyone here wants to use Forged slugs with an 8to1 CR on 87 octane gas. There may be better places to spend your cash. Again...just my opinion

The main reason for using forged pistons is that there aren't any hypereutectic pistons that are tall enough to keep the quench height below 0.050" in a stroker without having to shave at least 0.030" from the block.
Basically you can only use off-the-shelf hyper pistons with the shorter 4.2L rods.

This is correct.

That's why some of us that use Hypereutectic in some stroker applications have to increase the existing depth of the
dish. If not done properly, piston failure can and will happen. Recognizing the the thickness of the top of the piston
especially the critical area behind the back of the ring land is essential. Also as important is utilizing a cutter with a generous radius. Typically I'm able to increase the volume to 28cc.


Russ

[charley3]

When I said "hammer down" to merge into traffic around 1 to 2 min after cold start, I really meant about 1/3 hammer down.