kevinoneill

I was trying to search around the forumn for an answer to that one and came up with a bunch of nothing.

So thats the question. 1.6L (1595) B16a with 9:1 pistons, starting with 10.2:1 compression. I'd think the displacement would go up to 1.7-1.8...anyone know for sure?

Irishweird00

Your displacement will not change..... your compression will drop, thats it... the only way to get more displacement it to lenthen your stroke, or bore out your cyclinder walls to a bigger bore.

r7

yea, disp. will not change

JSMC

btw the displacement of an engine is calculated with the bore and the stroke of your crankshaft, you don't seem to get what those thing means.

the bore is the diameter of your compression chamber in the block, so your piston are have a little les to let some clearance.

the stroke is the travel of your piston, calculated with the crankshaft.

remember the formula for a circle surface??
2 x pi x r² or
pi x d²
-which "d" is the diameter of the circle, so in our case the bore of the engine.

so now to have the volume of the inside you need to know how high your surface is going, this is the stroke, so the final formula would be

pi x d² x height or
pi x bore² x stroke=volume

you don't need the compression ratio to determine your volume. BUT, of course the formula isn't that precise, since you don't always have a flat top for piston, and their's always clearance at the top plus the head.

the compression ratio is juste telling us how much the mixture is being compressed, but I think you know that, and changing the piston for higher or lower compression ratio won't change the volume in the formula, but may change it a little bit, but not that much!! not enough to say your displacement has changed

don't know if i'm clear enough, I may have not well expressed myself

kevinoneill

Maybe i'm mis understanding what 9:1 pistons are...

I thought the pistons changed size, in the 9:1 case they would be shorter? so when they are at the bottom of the stroke, there would be more room...more displacement...and wouldn't travel as high up, there-fore less compression...hence the 9:1?

I dunno that makes sense to me. If thats wrong, how does the piston change the compression?

Thanks for be patient and not calling me a num-nut

kevinoneill

Yeah that makes perfect sense. Its good you say it changes a bit, but nothing significant, if you read my last post that's why i thought it would change. just not by what i was thinking it would.

So how is it that the new pistons would change the compression? does that very small amount of added/subtracted height change the compression that much? just not the displacement?

zer0s0n5

I think he was listening to the old-logic arguement that low compression gives more room for fuel and air mixture. Hence the confusion. More Mis-information.


Yes that small amount does change displacement, but unless you really drop compression your dis-placement won't increase a significant amount. Lower dis-placement is to make up for bad-gas, bad tuning, lack of good fuel management, and a variety of others variables.

JSMC

higher compression piston are usually domed, so they almost touch the valve, of course they let the engine eat less mixture since their's less place, but when it compress it goes higher, so the mixture get closer, heat up (if you don't have enough anti-detonant will kaboom before the spark plug does it job)

well yes, some guys are just planning the head, or take thicker headgasket, that change the cr too, so of course a higer/lower height for a piston will change de CR

tegasaurus

if the pistons are shorter, yes they may go down farther, but they also don't go up as high. the volume the piston compresses is displacement, the height of completion of the compression is the compression ratio. Here's a pic, pics help me out cuz im a visual learner hehe. Here are 2 different motors. First is low compression, 2nd is High compression. The piston of motor 1 is shorter, thus lower compression (because it doesn't end up as close to the head as motor B's hig hcompression). However they both have the same displacement here's why.




Modified by tegasaurus at 3:13 PM 1/15/2004

JSMC

nice pic, it goes with my explanation.

btw their's a little mistake in your picture don't wanna be an @ss, but you have different rod length, not different piston but it's okay, we got the picture

tegasaurus

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JSMC &raquo;</TD></TR><TR><TD CLASS="quote">nice pic, it goes with my explanation.

btw their's a little mistake in your picture don't wanna be an @ss, but you have different rod length, not different piston but it's okay, we got the picture </TD></TR></TABLE>

thanks! i thought something looked wierd about the picture... i fixed it now

JSMC

perfect

SiRkid

the compression changes on the top of the piston..
higher comp usually are domed and lowerd comp pistons are dished.

the piston size doesnt change
higher comp



lower comp
http://public.fotki.com/drlazy....html

tegasaurus

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SiRkid &raquo;</TD></TR><TR><TD CLASS="quote">the compression changes on the top of the piston..
higher comp usually are domed and lowerd comp pistons are dished.

the piston size doesnt change
higher comp

lower comp
</TD></TR></TABLE>

i think this guy needs to take one step at a time before he learns about dished vs domed If you have any questions about what goes on inside the motor regarding displacement, or compression or whatever, let me know. I'll be happy to answer any and all questions i can

-Ryan

kevinoneill

Ah Ha!, good stuff guys. Makes perfect sense. Thats one big *** picture thought.

Now that we got the piston ideas squared away (and dished and domed makes sense too ). We change displacement from either changing rod length, head gasket width, or...actually i think thats it. Oh! i lie. Re-sleeving and boring. Milling the head.

Changing the rod length would also change the compression aswell as displacement, correct?

Hey, appreciate the good info. You guys could of easily flamed me out and called me an idiot. Good stuff. I was mainly wondering all this b/c i'm getting my b16a crx in just a few weeks. I actually wanted a b18, so i'd like to up the disp. on the 1.6L and lower compression for my future turbo.

Dinner time! again guys, Gooood stuff.

SiRkid

HEAD GASKET and MILLING THE HEAD

DO NOT change displacement.

your over complicating things..


boring, chanign rod length(although you wouldnt really do this by it self) and changing the crack/stroke..those things change displacement

Sonny

Changing rod length does not change displacement. No matter how long or short the rod is, the distance moved by the piston is influenced by only one thing: the crankshaft

Sonny

kevinoneill

Alright, so we're basically down to boring as being the only means to changing displacement. And it would make sense that longer/shorter rods wouldn't change disp.

Now things that affect compression would be: head gasket thickness, piston type (domed/dished), rod length, milling the head.

What i don't like is that displacement is measured from bottom of the stroke to the top of the stroke. Are'nt you pretty much ignoring the fact that there's room between the top of the piston (top of stoke), where gas and fuel exist? and if your rod/piston is shorter that area would increase...although not calculated in, in the final displacement of the engine.

So if that above is true, although "displacement" itself by definition wouldn't increase there would be more room for air/fuel in the chamber with shorter rods.

Thanks for the good pictures tegasauras, and the offer!

JSMC

well that's the problem with math, it's almost never precise enough to get what's the real situation is. just think about the number "pi" we haven't even found the end of that number and we have calculators that can't get that far, so just there you loos precision, after that, you got what 2 variables in the formula for the displacement of the engine, when you may need about 5-6 more to get really close to the real life displacement (head cc, compression height, piston to block clearance,...)

and btw, domed or dished or flat piston don't always tell you the compression of the piston, since the compression height is really important, when you got domed piston it's mostly because you can't get higher compression height, so to get higher static CR they most be as close as they can to the valves, so they need to domed the pistons.

btw why would we be flaming you?? I never get it when someone here get really mad on some questions, before knowing something you ignore it!!! and because some of the questions you got may not be well answered on this board since a lot of mis-informed person are trying to get you wrong infos!! anyway you get the point