This may be a stupid question, but if you bore an eco, say .040 over, that added space in the cylinder, wouldn't it cause compression to drop slightly
2002 Cavalier "LS Sport"
I'm planning an L61 engine build, an this popped into my head. I'm building the engine to run NA, but eventually plan to go boost. My plans are forged bottom end, an overbore (not sure how far I can go with this), 10:1 pistons, port and polished heads, aftermarket valvetrain to handle ~7400rpm and a nice cam, cam will be a turbo grind, LE5 Intake manifold, bigger TB, and an LSJ exhaust manifold and LSJ Injectors until the time for boost comes along.
For tuning, I'm planning on using HPT. Can the fuel maps from an LSJ be used for the L61 build to get a baseline tune? I'd just like a rough tune to get the car running and moveable to get it to a dyno or to someone who can take it from there and give it a proper tune.
2002 Cavalier "LS Sport"
i cant remember why eactly but compression goes UP when boring the cylinders.
i actually got into a huge argument with brian speed racer z about this awhile ago on here...
Familiar Taste of Poison.
z yaaaa wrote:i cant remember why eactly but compression goes UP when boring the cylinders.
i actually got into a huge argument with brian speed racer z about this awhile ago on here...
I think it would depend on the condition of the walls/rings before hand. It should only go up if blow-by was getting bad before the rebuild. If you had no blow-by before you bore, then I think it should go down slightly, unless something else changes.
FYI - The stock sleeves in an eco are already thin and i wouldnt trusted them bored with boost thrown at them.
Tinkles
2003 Cavalier 1SV
Bagged and Blown
Tinkles: I talked to Mr. Hahn the other day...he said he was running over 500hp with a 0.020" overbore before he had any issues (fully built motor, but still stock sleeves). So if you're gonna bore and boost, I wouldn't do more than 0.020 unless you're gonna drop the money on some Darton's.
there are guys on here running a 0.030" overbore with boost, and i'm sure i've read lately that there are a couple going as far as 0.040" over. Can anyone say for sure how far is too far with boost in the future?
2002 Cavalier "LS Sport"
Well i'll have a full built motor, and a nice flowing top end, but i'm not looking to set records here, i'm looking for ~320 at the wheels. I wouldn't go so far with an engine build but it's gonna be a while before I can go boost, and i'd like to do a mild NA built in the meantime. I figured it's just as well to go with a forged bottom end too seeing as \i'm tearing into the engine anyway, rather than go into another engine when I can go boost...
2002 Cavalier "LS Sport"
z yaaaa wrote:i cant remember why eactly but compression goes UP when boring the cylinders.
i actually got into a huge argument with brian speed racer z about this awhile ago on here...
When you over bore a cylinder you increase the displacement, slightly but it does go up. When it is compressed into the same space as a stock engine would be (stock head) you are increasing the compression ratio.
Just random numbers for comparasion.
25cc cylinder displacement into a 5 cc head results in a 5 to 1 ratio.
30cc cylinder displacement into a 5 cc head results in a 6 to 1 ratio.
and so on and so forth, boring changes a lil bit but changin the stroke adds a whole new world of fun to the math.
^^^ yea there we go.
thanks buddy.
Familiar Taste of Poison.
Antler, just a quick note as to what you're shooting for...a 0.020" over fully built motor with a Hahn stage V kit + meth, with a dis-2 and some other tweaks will hit your mark dead on (on the stock crank). There's a couple guys that I know of (and one who I've seen his set up) that have done pretty much exactly this. The guy that I've met that's done this was pushing 328whp (iirc, may be off by a couple), and has since then refined it a little...on pump gas I should add.
Not really sure If I want to use meth, but it's a possibility. I haven't done much reading on the meth.
I am gonna be running pump gas, the highest we can get around here at the pumps is 91. I don't think I should have much trouble reaching my goals, hahn made over 400 with just rods and valves IIRC.
2002 Cavalier "LS Sport"
Apparently the logic of a piston squeezing air from from the cylinder into the head space temporarily escaped me, lol. Sorry for posting an incorrect guess. I think the medicine I've been taking for back pain is starting to have side effects
So whats the final word here, does it go up or down?
My way of thinking would lead me to believe that pushing more air/fuel it into the stock head would raise the CR, but the piston doesn't go the whole way to the top of the cylinder wally(I think this is called the deck?), so there is actually more space in the top of the cylinder where the mix is being compressed so there really shouldn't be much of an increase on a .10 or .20 overbore right? maybe .5 at .20 tops?
I'm not pretending to answer the question and know everything lol I am just interested and would like somebody to explain this better so I can understand it.
It was already explained. The head volume does not change (unless you modify the chamber or use flat vs undercut valves). Static compression ratio increases with increased displacement.
So, scr increases.
I have no signiture
ur right, it doesnt increase it a bunch but it still does. doing this in conjunction with a decked head and or decked block and or thinner head gasket can yield a very nice bump in compression, though.
Whalesac wrote:It was already explained. The head volume does not change (unless you modify the chamber or use flat vs undercut valves). Static compression ratio increases with increased displacement.
So, scr increases.
This was the part I didn't understand. 'SCR increases with displacement.' I could see how this would explain an increase via adding stroke. Im still not understanding why its going up with a bore. If the top of the piston went clear to the deck and was only compressing the mix into the head, I could easily understand why it would go up.
But its not. You are GAINING space in the top of the cylinder by boreing it, and the engine can only take in as much air/fuel as the valves/cam allow.
So here your probably going to get a stronger pulse of fuel via the computer. The volume of the mix shouldn't change because the car can't ADD air, only fuel right?
like I said, i'm not one of those know it all people. This is just the way i'm thinking and if its wrong i'd like to know why, and details on how.
How this subject applies to me: eventually i'd like to bore a 2.4LD9. I noticed awhile back that when Quad 4 went to twin cam in 96, the bore went from 3.6in to 3.5in(stroke went up).
I'm still doing research, but I can't find anything that says the oil passages or water jackets were changed before 2000. So in theory that should leave a .10 overbore possibility right? Which I know is insanely high. Ive never heard of anything going over .045 without needing sleeved. But if the factory had it out that far why can't I?
blah blah I'm just trying to make sure my CR stays at or under 10 thats all. The pre-99 LD9s already had dished pistons, so if it does go way up, I'm going to need head work. >.< just planning ahead.
Why are you talking about cams and junk when talking about static compression? Those things only effect dynamic compression.
1994 Saturn SL2 Home Coming Edition: backup car
2002 Chevy Cavalier LS Sport Coupe: In a Junk Yard
1995 Mazda Miata R-package Class=STR
Sponsored by:
Kronos Performance
WPI Class of '12 Mechanical Engineering
WPI SAE Risk and Sustainability Management Officer
my 2000 LD9 had flat top pistons that met the deck evenly... my current pistons, go ABOVE the deck... (domed w/ valve pockets FTMFW)
Leafy (Club Jeffie FEA man) wrote:Why are you talking about cams and junk when talking about static compression? Those things only effect dynamic compression.
sorry? I don't know the difference between the 2...
I'm trying to learn. thats why i'm asking questions... I was hoping I was on the right track...
The size of the cylinder dome vol doesnt change. Youve only increased the cylinder size. So your squeezing more air into the cylinder and against the head. This is why it increases. I hope thats simple enough.
Zack Welsh wrote:Leafy (Club Jeffie FEA man) wrote:Why are you talking about cams and junk when talking about static compression? Those things only effect dynamic compression.
sorry? I don't know the difference between the 2... I'm trying to learn. thats why i'm asking questions... I was hoping I was on the right track...
Yes, static is literally the volume of the cylinder + head at the bottom of the stroke divided by the volume of the cylinder at the top of the stroke + head.
Dynamic uses the static compression and cam timing events. Its dynamic that really matters, but its much harder to use. Like I cant take an engine that knocks, no matter what you do in the tune, and adjust the cam timing and it wont knock anymore.
Edited 1 time(s). Last edited Sunday, September 04, 2011 5:45 PM
1994 Saturn SL2 Home Coming Edition: backup car
2002 Chevy Cavalier LS Sport Coupe: In a Junk Yard
1995 Mazda Miata R-package Class=STR
Sponsored by:
Kronos Performance
WPI Class of '12 Mechanical Engineering
WPI SAE Risk and Sustainability Management Officer
Mystic02VA (GME Chat) wrote:my 2000 LD9 had flat top pistons that met the deck evenly... my current pistons, go ABOVE the deck... (domed w/ valve pockets FTMFW)
brodycog wrote:The size of the cylinder dome vol doesnt change. Youve only increased the cylinder size. So your squeezing more air into the cylinder and against the head. This is why it increases. I hope thats simple enough.
Ok, now I understand the physical aspect of this. I really appreciate you guys taking the time to explain this to me.
I was under the assumption that the top of the piston wouldn't meet the deck because quite a few of the older Engines i've worked on don't and even cause almost like a small lip to form near the top of the cylinder walls after extensive use. I was thinking that subtracting that material(the lip) with the bore, that area would be included in the compression zone when the piston reaches TDC creating more room for more air/gas to be compressed into.
Looking at it from this stand point, it completely makes sense AND explains why there isn't a legitimate stroker crank for the LD9. Its already at its max stroke.
Now I just have to find somebody to teach me about SCR and DCR. and fuel injection... Any takers? lol
