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  #1 (permalink)  
Old 03 Apr 2011, 02:40 pm
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Default PT Pistons

There was a post made recently, commenting on the design of our 2.4 pistons. The following is a condensed version of the reasoning behind their design.

The first step was to design new pistons - something deemed necessary since peak combustion pressures are 50 percent higher than in the NA engine. Further strength was required because the engine was to be able to operate at stoichiometric (14.7:1) air/fuel ratios, even at wide-open throttle! This is required for US-06 government test procedures. Since turbo engines normally run relatively rich mixtures (eg: 12:1) at full throttle, such a requirement meant that the pistons had to be able to withstand much higher than normal in-cylinder temperatures. Finally, a full floating pin design was used in the new design. The resulting pistons are cast - rather than forged - from Mahle 124 eutectic alloy, with their skirts coated in Mahle Grafal to protect against scuffing and improve Noise, Vibration, Harshness (NVH). The new pistons lower the engine's compression from 9.4:1 to 8.1:1.

The pistons used in the NA engine have a 'ski ramp' crown shape - the top of the piston projects upwards in an asymmetric wedge. This shape was retained for the turbo engine - on both it and the NA engine, computational fluid dynamics (CFD) modeling showed improvements to the air/fuel mixture with this shape. These modelled gains were proved on the engine dyno, with the piston crown shape showing improved wide-open throttle spark and idle stability.

The top land (space between the upper ring and the crown of the piston) was set at 4mm - the smaller this distance, the less crevice volume remains where incomplete burns of the air/fuel mix can occur. However, a short land also results in the ring being subjected to higher temperatures and so hard anodizing was added to the top ring groove to prevent the compression ring from welding itself to the piston. To help cool the pistons, oil squirters were mounted in the block. These spray onto the underside of the pistons for the full length of their stroke. Interestingly, during testing thermistors were mounted on the pistons so that their real-time temperatures could be measured.

The rings for the turbo engine are also new. A 1.2mm barrel shaped steel upper compression ring is used; it has a molybdenum face coating to reduce wear. The second ring is wider at 1.5mm. It is made from grey iron and doesn't use chrome plating, as is commony the case. A conventional 3-piece oil control ring assembly is also used.

I hope this sheds a little light on our pistons design process. Personally, I think the engineers did a great job.

Jerry
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  #2 (permalink)  
Old 03 Apr 2011, 03:38 pm
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Default Re: PT Pistons

Jerry,

We sure don't see problems with the internals of the engine block so I would go along with what you said about doing a good job.
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Old 03 Apr 2011, 09:04 pm
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Default Re: PT Pistons

Quote:
Originally Posted by wwpptc View Post
pistons used in the NA engine have a 'ski ramp' crown shape - the top of the piston projects upwards in an asymmetric wedge. This shape was retained for the turbo engine - on both it and the NA engine, computational fluid dynamics (CFD) modeling showed improvements to the air/fuel mixture with this shape. These modelled gains were proved on the engine dyno, with the piston crown shape showing improved wide-open throttle spark and idle stability.

I hope this sheds a little light on our pistons design process. Personally, I think the engineers did a great job.

Jerry
ROFL-

They just put out in some big words- "Wide open throttle Spark ... stability"
WTF does that mean? Ignition spark?! Or engineer lighting up during break?
Name an engine that's had WOT spark 'stability' issues! Certainly not the
sister 2.0 DOHC engine with the same head!!!!!

I'd guess they had no choice. The engine has a small bore (same as 2.0 and
2.2) but a long stroke, so a 'flattop' would have to be severely dished to
get 9.4:1 CR.

I wonder if dome was needed for 'tumble' (Their "Air/Fuel mixture") with low
deck height.
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Old 03 Apr 2011, 09:37 pm
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Default Re: PT Pistons

If you're inferring that Chrysler wrote this discription, then you couldn't be farther from the truth. This was written by a well respected third party, with no ties to Chrysler, namely "Auto Speed". But of course we all know that they don't know there butt from a whole in th --- Oh wait, actually we all know that they do.

Yea, I knew when I included thier big words it would be over your head, But I posted it anyway for everybody else.

If you like I can hook you up with the engineer that designed the Turbo version of the 2.4 and has been a contributing member over a The Crew for years.

Jerry
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Old 03 Apr 2011, 11:05 pm
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Default Re: PT Pistons

I know lots of PT owners with high mileage on their cars and none of them have ever had problems with the engine internals so I would say it's a good design.I would like to have a little more power and better mileage also but with that aside the PT has a great power plant when properly maintained.Thats just my 2 cents for what it's worth.
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Old 04 Apr 2011, 10:49 am
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Default Re: PT Pistons

I'm thinking the 2.4 is a proven little work horse. You don't hear any problems about the internals unless someone sticks a turbo on a non turbo engine. I have no clue if the piston design is optimal or not and at this point I'm not changing them if they aren't. They do seem to working well for 70k in my ride.
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Old 04 Apr 2011, 11:12 am
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Default Re: PT Pistons

Quote:
Originally Posted by SCBluePT View Post
I would like to have a little more power and better mileage
Exactly.

Other're confusing durability with efficiency.

I've read the description well before your listing. It's just blather.
The point is it's a poor combustion chamber because of the piston design.

That there's some engineer or other party 'in the know' that says otherwise is silly.

Chamber design has been established for decades; 2.4's is on the inefficient
(read: BAD) end.
It's an "A" while the same head in a 2.0 DOHC is a "J".



Again, don't confuse the longevity of the engine with it's efficiency, two
different things.
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Old 04 Apr 2011, 04:23 pm
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Default Re: PT Pistons

Wow .... it sounds like the engineers took the same step Ford did with the 2.3L N/A engine when they turbocharged it to put in the Capri and the Turbo TBird 20+ years ago. They took the N/A from an 8.sumthin CR to a 7.sumthin CR for the turbo.

If I recall, even my 80 2.3L N/A Mustang did not have an ECU, the one in my Brothers 84 TTBird might only have had as much an ECU as a non smart phone has today. I would think the Chrysler engineers could have kept the CR much higher and the output much higher if they programmed the Modern ECU to handle it.

My 1.8T VW engine is 10 years old now and has a CR above 10:1. It can be done.
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  #9 (permalink)  
Old 04 Apr 2011, 06:18 pm
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Default Re: PT Pistons

I'm trying not to migrate to 'turbo' discussion, OP started thread in response
to comments about N/A pistons- Turbo pistons are similar.

There was a comment (again, not to g turbo) about turbo having to run
WOT at 14.7. Even I know that's bad news!

The more I think about it, the more it seems a necessary evil-
Bore, headbolt pattern stayed the same as 2.2/2.5 (and of course, the 'Neon'
2.0). They couldn't change the DOHC head, it's nice and compact ,
they needed displacement for large 'cloud cars' & c-vans so stroked the 2.0.
With even a flattop, this 'stroking' lead to immense compression ratio's with
the DOHC head, unlike the 2.5's 'bathtub' design.

They had to drop piston and do away with any hope for quench, next best
was to try a ski-ramp for tumble. Inefficient, HUGE combustion chamber
(see flathead) surface area but it got the job done, and cheap.
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  #10 (permalink)  
Old 04 Apr 2011, 09:58 pm
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Default Re: PT Pistons

Sorry if I strayed .. I did not know the original post was a response and all of what I said above was turbo related based upon when I read "The new pistons lower the engine's compression from 9.4:1 to 8.1:1." I thought that could only have been done to an engine with an external factor, like a turbo, that can be tweaked and adjusted to increase the serious loss of power from that reduction of compression ratio. Please tell me they did not do this to the N/A engine!
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