compression rating on a piston

[samoo]

What does the compression rating on a piston mean?

Do the pistons have different designs between lo-comp and high comp that make it so when you run either it changes ur compression ratio a bit?

would there be anything bad about running hicomp forgies in a turbo setup if u gave it a thicker head gasket?

[BeRad]

A thicker hg would defeat the purpose of hi comp pistons...

Theres plenty of info out there that will give you a better knowledge of what piston ratios mean and how they effect things.

[oldcorollas]

compression rating is the static compression you get when you put those pistons in a relatively stock setup.
yes, the often have a different dish or dome volume. it depends on the engine.

thicker head gasket = smaller quench area and generally thought ot be worse for detonation.

how high compression are you thinking? if you have the tuning to back it up.. you can get away with fairly high compression (depends on boost level etc)

[celicaboy_2000]

Compression is the amount of pressure in the combustion chamber, having a lo comp piston means less compression in the combustion chamber and vise versa. Forced induction usually run a lower compression eg 8:1 and normal aspirated run are higher ration of 11:1 etc but you may also have a higher ration then 8:1 for forced induction when your using racing fuel etc.

Usually designs of piston vary but the main difference between lo comp and hi comp pistons is the dish or dome on the top of the piston, lo comp will have a deep dish and the hi comp will either have a dome or a dish. But they all vary between manufactures etc

I dont see there being any problem with the hi comp pistons and a thicker gasket as long as it is about the same ration at end as the original or you tune the computer to handle this setup with a higher octane fuel etc. People please correct me on any of the above if im wrong

[oldcorollas]

Compression is the amount of pressure in the combustion chamber, having a lo comp piston means less compression in the combustion chamber and vise versa. Forced induction usually run a lower compression eg 8:1 and normal aspirated run are higher ration of 11:1 etc but you may also have a higher ration then 8:1 for forced induction when your using racing fuel etc.

compression RATIO is the ratio of the swepth volume plus the combustion chamber volume, to the combustion chamber volume.....

forced induction... umm.. aren't 4AGZE's available with 9:1 pistons anyway? 8:1 seems rather low.
older cars had 9:1 for NA.. 10:1 is also ok for pulp normally... 11:1 seems fairly high

higher comp pistons and a thicker headgasket usually means less quench, and a poorer combustion chamber shape (for domed pistons).... seems a half arsed way to go about things...

[celicaboy_2000]

oldcorollas, im on here and alot of other people are on here to learn. As I stated thankyou for correcting my errors BUT the ratio's I said were just examples and also the way I described the meaning of ratio is correct and so was yours, mine was just in much simpler terns As for the hi comp pistons and headgasket I stand by what you said

higher comp pistons and a thicker headgasket usually means less quench, and a poorer combustion chamber shape (for domed pistons).... seems a half arsed way to go about things...

but for alot of people including myself we use what we can get our hands onto and in this case hicomp pistons might be the cheap option in the engine build, does that make sense?

[samoo]

hicomp pistons might be the cheap option in the engine build, does that make sense?

thats the main reason i was asking im a very long way from doing anything to my engine, but im just curious

[thu187]

umm.. aren't 4AGZE's available with 9:1 pistons anyway? 8:1 seems rather low.

Yup, from memory AE101 GZE's have a 8.9:1 and the earlier ones use 8.1.

[Talasas]

^ That's right.

[oldcorollas]

wasn't trying to have a go, but there is a big big difference between cylinder pressure, and compression ratio.
same as there is a big difference between static compression and dynamic compression ratios depending on cam profiles..

[Vios-GT_07]

hm

since this thread is up

okay i made a mess of calculations.. back to square one, i'm trying to figure out why my engine has the following compression results.. from 1 to 4.. (super cheap auto compression gauge isn't exactly 100% accurate but *meh* u get the idea)

215, 220,215,210..

the engine is a 4A-GE smallport and visually it's stock standard.. manual lists compression to be 140 to 180... and 180 being new.. test was done when engine was fairly warm.. (just cruised for 20 mins, stop for 10 mins and tested)..

not going to even discuss the mess.. but yea... what's the compression ratio i'm looking at and any reason why i'd get compression results almost 40 psi above normal?

[Steve M]

either bigger pistons in there, it's been stroked with high comp pistons or something in your cam timing has changed. For a rise of that much, I would think that more than one of those things has occured.
Changing my brothers 18RG cams upped the compression figures by a little over 10 points on a 9.7:1 compression ratio.

[Vios-GT_07]

it sure doesn't feel like bigger pistons... i've driven several 4A-GEs and i can tell it just feels like a very new engine.. it's still got its limitations, runs stock ecu and picks up like any other 4A-GE smallport... no faster or anything.. i'd think that if there was any modifications like bigger pistons, stroker kit etc etc there'd be a very big difference in performance? i know i've driven a bigport with oversized pistons, it's only got 200 psi comp on a standard head gasket and u'd know straight away something's different..

oh well i shouldn't think too much of it, the engine's pretty good.. it's cracked the oil cap though, so i'm still looking for another one and possibly a catch can just to make sure it's not doing anything silly..

thanks for that

[oldcorollas]

so your compression gauge is shit

[Vios-GT_07]

yea yea ... but if it's shit wouldn't it show a lower reading?