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Piston Design for Efficient Burn

 
Old 06-11-2019, 09:13 AM
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Default Piston Design for Efficient Burn

If I were aiming for 12:1 CR on a 4.185" bore... would a 2.8cc reverse dome top piston and 70cc combustion chamber or an 8cc flat top piston and milled 65cc chamber provide a more efficient burn? Assuming the same head and piston manufacturer...

The 8cc FTs look like this FYI.


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Old 06-11-2019, 09:31 AM
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Should be flat tops for the better quench
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Old 06-11-2019, 09:53 AM
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I am going to go w/ the flat tops. Another reason is you have far less sharp edges to start pre-ignition hot spots. Even blended, it's better to have none. I have seen some pistols with a shallow complete depression be very efficient, though. Picture the top of a baseball bat pushed into a soft material. Usually that's common in a 4 valve motor application. My machinist is putting together a Dart block stroker LSA right now with similar pistols to what I am talking about. Will try to snap a pic for you later.
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Old 06-11-2019, 11:02 AM
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That sounds good.
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Old 06-11-2019, 11:16 AM
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Aren't you meant to use flat top pistons for NA engines?
Dome pistons reduce compression.
Dome Pistons may be suitable if you have milled heads and you need to get your compression down or building a stroker which naturally increases compression.
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Old 06-11-2019, 11:20 AM
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The pistons in question are both NA pistons. One is a flat top and the other is a reverse dome... which is a dish piston with a quench pad (like a FT).

The question I have is between a Wiseco -8cc FT and -2.8cc R/Dome, which would be better at the same compression ratio for flame travel, efficient burn, etc.
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Old 06-11-2019, 11:24 AM
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Originally Posted by JakeFusion View Post
The pistons in question are both NA pistons. One is a flat top and the other is a reverse dome... which is a dish piston with a quench pad (like a FT).

The question I have is between a Wiseco -8cc FT and -2.8cc R/Dome, which would be better at the same compression ratio for flame travel, efficient burn, etc.
Oh,
I have no idea.....
Maybe contact Wiseco and ask them.
Interesting question.
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Old 06-11-2019, 02:16 PM
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There's no easy answer. There are way too many variables to make an educated decision - such as port flow milled vs. unmilled, squish/quench with dished vs flat, size/shape of the quench pads milled vs. unmilled, etc etc.

Most likely, without a test cell outfitted with laboratory grade instruments, you wouldn't be able to tell a difference - other than maybe one design would take an extra degree of spark over the other. Especially considering these designs aren't wildly different from one another.
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Old 06-11-2019, 02:40 PM
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That's what I figured. At the end, it's not a big difference.

If I were at 16:1 it might make a difference...
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Old 06-11-2019, 04:08 PM
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Jake,
Have you started a build thread? From what I have pieced from several different
threads is that you are using SPS LS3 Heads when/if they show, FAST W/Mid-Length
Runners. LLSR (Specs?) 440". I would encourage you to test the Short Race-Runners
as well, reason being is that the INCREASED PLENUM AREA will be beneficial
from 6500-7500+ RPM and your stall, gears, and Displacement will cover any
percieved loss of low-end which may not be useable anyway.

Just saw list at bottom of sig, hadn't noticed before and didn't want
to go through 6 years of posts lol.

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Old 06-11-2019, 05:00 PM
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Originally Posted by JakeFusion View Post
That's what I figured. At the end, it's not a big difference.

If I were at 16:1 it might make a difference...
Exactly, simple is always better. At least w/ a flattop you really can't go wrong. A dish may/may not work, it's a roll of the dice. I have seen some crazy designs on BBC heavy NOS pistons which have channels in them for flame front propagation. You look at all that and then it's like, why not a flat top...
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Old 06-11-2019, 08:42 PM
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Originally Posted by NAVYBLUE210 View Post
Jake,
Have you started a build thread? From what I have pieced from several different
threads is that you are using SPS LS3 Heads when/if they show, FAST W/Mid-Length
Runners. LLSR (Specs?) 440". I would encourage you to test the Short Race-Runners
as well, reason being is that the INCREASED PLENUM AREA will be beneficial
from 6500-7500+ RPM and your stall, gears, and Displacement will cover any
percieved loss of low-end which may not be useable anyway.

Just saw list at bottom of sig, hadn't noticed before and didn't want
to go through 6 years of posts lol.
Build thread is coming... if I ever get parts.
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Old 06-12-2019, 09:27 AM
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Heavy subject. Depends on injection type, spark plug location, cylinder head geometry, induction air swirl / tumble, quench (as mentioned), fuel type, flame front propagation, etc... Flat top may not be the most efficient type, actually. One that contours to the head may be. But, you would have to do a ton of combustion analysis to get any type of real answer. So, I guess my writing here is pointless....
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Old 06-12-2019, 10:57 AM
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Originally Posted by FormulaBoat View Post
Heavy subject. Depends on injection type, spark plug location, cylinder head geometry, induction air swirl / tumble, quench (as mentioned), fuel type, flame front propagation, etc... Flat top may not be the most efficient type, actually. One that contours to the head may be. But, you would have to do a ton of combustion analysis to get any type of real answer. So, I guess my writing here is pointless....
It's fun to muse about, but in reality it makes very little difference - provided you aren't taking it to a wild extreme. The huge domes on the Chrysler 426s definitely hurt combustion efficiency, as do super dishes (or similarly, having the piston .150" deep in the hole @ TDC). But even then, it's almost always workable.

The manufacturers have all tested every thinkable combination, myriad times, over the course of the last 150 years of engine development and yet what we see in OEM engines is whatever it takes to get the compression where desired after the combustion chamber/port design is nailed down. Where the rubber meets the road, the differences between a little dome, a flat top or a little dish are pretty insignificant.

Final thought: Most of what I'm saying applies to 4-stroke, spark ignition, port-fuel engines. 2 strokes, direct injection, diesel, stratified charge and now HCCI are a different animal - and piston crown shape matters much more for those.
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Old 06-13-2019, 11:36 AM
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I’d opt for the lighter piston if your undecided and haven’t laid out any money for a set, whether they’re dished or flat. JE, Mahle and Manley tend to be lighter, Icon has a new line up of coated skirt 2618 alloy 4cc flat tops that come in a 4.155” (IC902.30) that weigh just 400 grams, this is a shelf piston for a 434ci build, I bought a set of these for an LSX block build in the future, came with Hastings rings (4125BD8.035) . The provided wrist pins weigh just 88 grams, the double cut valve reliefs appear to be machined for 15 or 12* heads. Wiseco tend to be heavy IMO.
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Old 06-13-2019, 01:35 PM
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Wiseco are heavy...
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Old 06-13-2019, 06:10 PM
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I really like the Mahle Powerpak pistons I got. Low price, low expansion aluminum, 1/1/2 mm ring pack, OEM quality, and light weight. The valve reliefs are also forged in and not cut in. No sharp edges.

8 pistons are a full pound lighter than stock on an ls7.
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Old 06-14-2019, 08:47 AM
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Originally Posted by spanks13 View Post
I really like the Mahle Powerpak pistons I got. Low price, low expansion aluminum, 1/1/2 mm ring pack, OEM quality, and light weight. The valve reliefs are also forged in and not cut in. No sharp edges.

8 pistons are a full pound lighter than stock on an ls7.
One pound collectively? Per set? Could you explain?
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Old 06-14-2019, 09:42 AM
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Originally Posted by 64post View Post
One pound collectively? Per set? Could you explain?
Stock ls7 piston is 490g + 133g pin

Mahle 4.130" piston is 451g + 117g pin

440 gram total savings, .97 pounds.
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Old 06-14-2019, 09:56 AM
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The JE Assymetric pistons are lighter than that... 410g or so.
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