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Subject: "What makes a piston into a lower or higher compression?"

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jsupetran	Feb-22-04 01:53 PM
Member since May 04th 2002 4359 posts,

#65725, "What makes a piston into a lower or higher compression?"
Feb-22-04 01:54 PM by jsupetran

Just wondering...
Is it the material they use, or is it a clearance type of modification on the piston that makes it into a lower compression or high compression?

-------jezreel
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Replies to this topic

RE: What makes a piston into a lower or higher compression?, python6586, Feb-22-04 02:06 PM, #1

RE: What makes a piston into a lower or higher compression?, 2g99RSNC, Feb-22-04 02:14 PM, #2

RE: What makes a piston into a lower or higher compression?, thedawg, Feb-23-04 12:20 PM, #3

RE: What makes a piston into a lower or higher compression?, Moderator

Moderator

992gnt, Feb-23-04 02:41 PM, #4

RE: What makes a piston into a lower or higher compression?, Erik, Feb-23-04 03:01 PM, #5

RE: What makes a piston into a lower or higher compression?, thedawg, Feb-23-04 06:58 PM, #6

RE: What makes a piston into a lower or higher compression?, Moderator

Moderator

micyek, Feb-24-04 02:02 AM, #7

RE: What makes a piston into a lower or higher compression?, thegame, Feb-24-04 02:49 AM, #8

RE: What makes a piston into a lower or higher compression?, ZadensDSM, Feb-24-04 03:08 AM, #9

RE: What makes a piston into a lower or higher compression?, Moderator

Moderator

micyek, Feb-24-04 04:08 AM, #10

RE: What makes a piston into a lower or higher compression?, WickedESi, Feb-24-04 08:56 AM, #11

RE: What makes a piston into a lower or higher compression?, customrsturbo, Feb-24-04 03:11 AM, #12

RE: What makes a piston into a lower or higher compression?, Moderator

Moderator

992gnt, Feb-24-04 05:35 AM, #13

RE: What makes a piston into a lower or higher compression?, Erik, Feb-24-04 07:18 AM, #14

RE: What makes a piston into a lower or higher compression?, Team

Team

Michael_97RS, Feb-25-04 05:18 AM, #15

python6586	Feb-22-04 02:06 PM
Member since Oct 27th 2002 656 posts,

#65726, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 0

i believe it is the top surface of the piston, whethers its domed, dished, has ridges or notches, also placement of the wristpin too

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2g99RSNC	Feb-22-04 02:14 PM
Member since Jun 04th 2003 894 posts,

#65727, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 1

Originally posted by python6586
i believe it is the top surface of the piston, whethers its domed, dished, has ridges or notches, also placement of the wristpin too

Mods: CAI, OBX exhaust, Grillcraft grills, altezzas, ard ryver g6 17's wrapped w/ falken's, VIS racing c.f hood, 2 12" kicker comp's w/pioneer h.u, streetglow blue undercars, red dash kit, indiglo guages, n other shit

yeah its the shape on the head of the piston but i think the wrist pin location is changed to change the amount of pressure on the main thrust side of the piston

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thedawg	Feb-23-04 12:20 PM
Donating 2GNT member 6287 posts,

#65778, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 2

Can't really tell from the pics, but the 8.8:1 shiny pistons are slightly shorter than the 9.6?:1 oem pistons. Also the shiny ones are dished on top, the stock ones are pretty much flat on top.

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992gnt	Feb-23-04 02:41 PM
Donating 2GNT member 2245 posts,

#65790, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 3

What does being shorter have to do with it? This is a test.

Greg

'19 Genesis G70 3.3t AWD
'88 Chrysler Conquest TSI

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Erik	Feb-23-04 03:01 PM
Donating 2GNT member 1536 posts,

#65794, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 4

efficient stroke. Actually all the above make lower comp pistons. Each manufacturer chooses their own way... Also depends greatly on the nature of the motor... Stroking a motor that naturally makes good TQ to make more power on top end would probably sacrifice the stroke.

Shit is all apart. Typical...

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thedawg	Feb-23-04 06:58 PM
Donating 2GNT member 6287 posts,

#65820, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 5
Feb-23-04 06:58 PM by thedawg

Um... I'm gonna let motofool have a swing at this first emoji

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micyek	Feb-24-04 02:02 AM
Donating 2GNT member 5070 posts,

#65836, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 4
Feb-24-04 02:04 AM by micyek

Originally posted by 992gnt
What does being shorter have to do with it? This is a test.

well if you have a shorter piston, the piston is taking up less space in the cylinder..i.e. you'll have more of a gap between the head and the top of the piston. Thus give you a lower compression ratio. Kind of like if you have a full bucket of water...I mean right up to the top, on the verge of spilling over...if you put a put a bowling ball in there your going to force more water out then if you would if you were to put a baseball in there. That analogy brings up a question of my own. Are you increasing displacement with a lower compression piston? I would think yes...but on a very insignificant scale.

getty up

-kent-

'98 Talon...gone, but not forgotten

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thegame	Feb-24-04 02:49 AM
Member since Oct 08th 2002 1311 posts,

#65837, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 7

no clue. ill just ask this.... on 8:5.1, 8:8.1, 9:6.1 .... what do each of the number represent/stand for?

New/More Mods Comming... New 1/4 Times In March/April

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ZadensDSM	Feb-24-04 03:08 AM
Member since Aug 18th 2003 175 posts,

#65839, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 8

i was thinking about saying my thoughts in noob terms (seeing as how those are the only thoughts i have...)

but after carefully reading the thread, i'm just flat out confused.
here's what i've come up with...
the only reason why i can see that a piston would be shorter is to save weight...
now this is what i'm confused about...
let's say you have 2 pistons 1 stock 1 aftermarket.
the stock piston is flat on top and aftermarket domed...
both have same wrist pin location and are both the same length and bore, etc...

wouldn't the aftermarket piston give you an INCREASE in compression?
go ahead and flame if i'm way off, but it makes sense to me...

now changing wrist pin location is a whole other ball game. if you were to take those same 2 pistons, and move the location of the wrist pin on the aftermarket piston closer to the top wouldn't it also increase compression? and moving it lower would decrease compression?

fuck if i know, i'm straight confused now...
can someone please explain this... dino? mototool? motofool? anyone?
i'm a quick learner i promise lmao

reppin the nasty 'nati

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micyek	Feb-24-04 04:08 AM
Donating 2GNT member 5070 posts,

#65842, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 9
Feb-24-04 04:29 AM by micyek

Originally posted by ZadensDSM
let's say you have 2 pistons 1 stock 1 aftermarket. the stock piston is flat on top and aftermarket domed... both have same wrist pin location and are both the same length and bore, etc... wouldn't the aftermarket piston give you an INCREASE in compression?

yes...the aftermarket piston has a high compression ratio because you closed the gap between the head and and the top of the piston.

Originally posted by ZadensDSMnow changing wrist pin location is a whole other ball game. if you were to take those same 2 pistons, and move the location of the wrist pin on the aftermarket piston closer to the top wouldn't it also increase compression? and moving it lower would decrease compression?

you're backwards here...moving the wrist pin closer to the top of the piston will lower the compression because you would be lowering the overall TDC..that's kinda hard to explain if you can't visulize it...maybe I whip up a picture quick.

*edit*

here you g...this is exagerated greatly but it's just to help ppl visulize what I was rying to explai

thanks www.kaptainmyke.com for hosting this purdy pic.

getty up

-kent-

'98 Talon...gone, but not forgotten

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WickedESi	Feb-24-04 08:56 AM
Donating 2GNT member 3090 posts,

#65867, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 9

Originally posted by ZadensDSM
now this is what i'm confused about... let's say you have 2 pistons 1 stock 1 aftermarket. the stock piston is flat on top and aftermarket domed... both have same wrist pin location and are both the same length and bore, etc...

He ment dished (concave, going into the piston) when he said domed. Which decreases compression (more space at top dead center).

It's hard to see because such a small change makes a big difference, that aftermarket piston's wrist pin was also closer to the top of the piston (towards the face) than the stock one.

Joel Baldridge, ASE Certified Master Technician, Audi Certified Expert Technician

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customrsturbo	Feb-24-04 03:11 AM
Member since Oct 06th 2003 249 posts,

#65840, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 8

Really is 8.5:1 or 8.5 to 1, 8.6:1 or 8.6 to 1. Basically is the volume of the cylinder when the piston is at the lowest position in relation to the volume when the piston is at TDC(top dead center or higher position). The firts number 8.5 is the vol. at lower position. The number 1 refers to the volume at TDC. Refer to this web site for more info and calculations.

http://www.e30m3performance.com/myths/more_myths1/comp_ratio/comp_ratio.htm

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992gnt	Feb-24-04 05:35 AM
Donating 2GNT member 2245 posts,

#65848, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 12

Awesome. The only one close to being right so far is the self-admitted noob. C'mon guys, try harder. I know you can figure this one out.

Greg

'19 Genesis G70 3.3t AWD
'88 Chrysler Conquest TSI

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Erik	Feb-24-04 07:18 AM
Donating 2GNT member 1536 posts,

#65857, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 13

Efficient stroke is the answer. That is really all that is REALISTICALLY needed to change compression ratio... Yes that does include the area within the combustion chamber (no matter where it may lie) I've had this question asked to me in advanced engine building classes.

Shit is all apart. Typical...

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Michael_97RS	Feb-25-04 05:18 AM
Donating 2GNT member 5866 posts,

#65934, "RE: What makes a piston into a lower or higher compression?"
In response to Reply # 13

Originally posted by 992gnt
Awesome. The only one close to being right so far is the self-admitted noob. C'mon guys, try harder. I know you can figure this one out.

Wow, reason out the definition of compression, and think about shape of the top of the piston and you can get this.

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It's holding down the garage floor right now.

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