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?

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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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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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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.

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

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.

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

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.

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?


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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

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 explain:thumbsup



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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.

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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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.

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.

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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