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Any type of piston that has a flat section,such as SRP or JE,etc that is designed to promote the quench effect.New pistons by almost every maker that are well designed are really starting to be introduced for both 5.7" and 6" rods.I think that we are heading for the best time ever for building 400's.
Of course the need for a proper quench applies to all engines.
Mike
 

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Even a full dished piston still has limited amount of of flat surface around the edges,it's just not as effective as a flat top or an inverted dome piston.
MIke
 

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DriveWFO said:
Piston rock? Is that the movement of the piston about the wrist pin?
You have to have clearances so Pistons can move in the bore. This "slop" allows it to rock as it changes direction at TDC and BDC. Some pistons rock more than others. This is why CH is an important variable. Some stroker pistons have short CH and skirts and combined with loose forged clearances they can rock a great deal in the bores.
Remember when Jeff was showing the dual dial indicator deck height checker?
That makes it easier to measure the piston rock at TDC.

Don't assume anything when building an engine. Measure everything....at least twice.

You might be surprised what you find. Desk top dyno's and internet calculators are fun to play with but... your engine is in the reality zone.

I'll take and post a picture of a piston I kept as a souvenier of too little dynamic clearance.
 

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Discussion Starter #25
Paul Wright said:
You have to have clearances so Pistons can move in the bore. This "slop" allows it to rock as it changes direction at TDC and BDC. Some pistons rock more than others. This is why CH is an important variable. Some stroker pistons have short CH and skirts and combined with loose forged clearances they can rock a great deal in the bores.
Remember when Jeff was showing the dual dial indicator deck height checker?
That makes it easier to measure the piston rock at TDC.
You might be surprised what you find. Desk top dyno's and internet calculators are fun to play with...BUT your engine is in the reality zone.

Don't assume anything when building an engine. Measure everything.

I'll take and post a picture of a piston I kept as a souvenier of too little dynamic clearance.
How would I know what is acceptable piston rock for a particular piston? I can see why you have to "fit" each piston to a particular cylinder. How does piston rock affect the rings?



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DriveWFO said:
How would I know what is acceptable piston rock for a particular piston? I can see why you have to "fit" each piston to a particular cylinder. How does piston rock affect the rings?

Well, every piston should have a clearance spec range. The problem is the recommended clearance is tighter for street and looser for racing.
You want to minimize rock without running the risk of seizing the piston.
This is something that requires good judgement and experience. Duttweiler obviously needs more clearance to prevent scuffing and seizing the pistons so he must run more clearance to the head to prevent collision. There is no ONE right answer for every engine.

If you have building a minimum squish engine you must first fit the pistons to the bore. This requires a good machinist who can hold ten thousandth tolerance. Then mock up every piston and rod assembly in it's assigned bore then measure each deck clearance. This is tedious but you might also be surprised that they'll probably vary and don't match your computer guesstimates.

To answer your last question
Yes, ring seal is negatively affected by excessive piston rock. Pistons with very short CH often have ring seal gremlins. This is the downfall to the "longest rod that will fit makes the most power " theory.
While longer rods generally help on some combinations it's not a linear equation. At some point the CH gets so short the piston rocks excessively in the bore. The rings lose seal and power drops off.
 

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DriveWFO said:
I'm curious as to which general piston shape (top of the piston) best promotes swirl/quench?
The only book I have that talks about quench is "how to build big inch small block chevys" They have about half a page on the subject. Paul seemed to be covering it pretty well though. One quote from the book, "The best dished pistons utilize what JE calls a reverse dome. The flat portion of the piston creates an excellent quench area that can actually allow dialing in more static compression and yet reduce the engine's total ignition requirements."

This is what the piston looks like in this quote.
http://store.summitracing.com/partdetail.asp?part=TRW-LW2606F30&N=4294925232+4294891096+4294908216+4294840135&autoview=sku
I know its not a JE, I was too lazy to find them. :D
 

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Hi guys,

As far as the piston rock is concerned in my opinion it kills ring seal and the rings as well. Thats why I try and fit the bore clearance to the specific piston and just as important the specific application of the engine.

On the plus side, if the pistons you are using are manufactured correctly (most seem to be) and you have a handle on what bore clearance your combination needs, and most importantly the driver drives it properly, I see very little problem with piston rock.

Even a piston with a lot of rock cold (like you would measure with the gage Paul refered to) will stabilize at the temp. generated under load.

THe load is the tricky part as well as the driving. I have seen numerous blocks with cracked cyl walls that in my opinion were caused by loading the engine before the pistons could expand. This "first" expansion takes a little while and a usually a little load as cyl temp. (and piston temp) generally increase drastically with the load. Once the piston is "stabilized" in the cyl., this is where knowing the combination to properly determine the piston clearance is critical. If the clearance is about right there will be much less cyl wall loading and or scuffing, as well as no piston rock problems. The applications that Im refering to here are racing type with a forged piston with an alloy and/or design that requires a higher cskirt clearance.

The Hyperutectec pistons dont need much clearance but are usually not light enough to be used as a "serious" race piston. The tigher clearance is a plus as the drving style is almost eliminated as a problem with most engines. These piston rock very little.


Some of the coated skirt pistons are actually a little bigger at bottom of the skirt than the bore! Talk about no rock, and they seem to work well.

As far as the 6in rods in the 400, I use them all the time. Im putting 2 togther this week. Both use import parts, one an assembly the other just rods. I have even built a few 4in stroke 400s with 6in rods in them. Seems to work just fine.

Another thing you should disscuss along with piston rock is wrist pin offset.


Remember these are just my opinions, probably wrong.

Sorry to ramble

Jeff
 

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I think this is what he's talking about? I'd like to hear more also.

"Long strokes and rods combine to create very tight compression heights that push the wrist pin up into the oil ring. This requires oil ring supports to ensure the oil ring works properly when spanning the gap".

Or maybe not. :)
 

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offset

He's talking about moving the center line of the rod pin bore.
 

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Discussion Starter #32
Some questions:

1. Generally speaking, how much is to be gained from tighening up a quench distance of 0.053" to 0.040"? Assume identical pistons and heads.

2. With regards to squish, what gains could be expected by switching from a dished piston to a true flattop piston? Again, generally speaking.

3. What effect will both 1& 2 have on preventing detonation?



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DriveWFO said:
Some questions:

1. Generally speaking, how much is to be gained from tighening up a quench distance of 0.053" to 0.040"? Assume identical pistons and heads.

2. With regards to squish, what gains could be expected by switching from a dished piston to a true flattop piston? Again, generally speaking.

3. What effect will both 1& 2 have on preventing detonation?
Well, (I'm still groggy from this temporary change to night shift)
Squish isn't an additive selection. Meaning "how much HP do I get for the trouble?"
It's not something from an ala carte menu.

Minimum Squish is an advanced technique for people that trying to maximize compression parameters. For example it might make the difference on being able to boost 10.5:1 to 11:1 for more power and maybe allow different cam timing which gets more power, and maybe reduces octane sensitivity which adds a margin of durability (which is certainly worth something). Basically you are sneaking up to the "edge" of the limits.

It has dangers because .040" is a near collision with the head, but it has significant performance benefits.

It's helpful to think of the piston top is the floor of the combustion chamber.
Chamber shape helps or hurts flame travel. There are lot's of variations and there is arguably no one right answer. As a result a dished piston may or may not be superior to a flat top. Flame travel, engine CR requirements, usage, Octane, Plug position all factor in.

Detonation has many root causes, but you have to consider them as a whole interactive system not just a sum of parts.

You can make an excellent cake with the right combination of flour, eggs, butter, water, etc.

Adding more eggs won't make it better, it will make it worse..unless you change to other ingredients in the proper proportions.

You might like steak but adding it to a cake probably won't make it good. Adding chocolate might be good or bad depending if you are allergic. Changes aren't necessarilly good or bad. They have to be considered for the end use.

Think of what you want the engine to do and build it accordingly. Just remember that the closer you get to the "edge", the better the chances are of falling off.
 
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