just wondering what diffrence i'd be seeing in a NA kind of way. how much loss would i be seeing if went from the 10.5:1 now to a 9.5:1 or 9.0:1 ratio?

what hinders would i see besides slight power power loss? any bads on the mileage?

 

why would you want to lower your compression? don't you usually only do that if you are going turbo?

 

well ya, thats a no brainer, i'm just wondering what the diffrence is on the mileage and NA power so i know what i'm loosing in order to gain.

 

okay...why don't you spell out your master plan here. are you asking if it is tolerable to lower the compression on your engine now in advance for your anticipated turbocharging of your box?

 

in a way that is what i'm asking, but at the same time not for anticipation...

i'm wondering how much the compression change is changing the barebone engine power going from 10.5:1 to 9.0:1 and what changes it would do to the mileage. already knowing that the turbo will drop mileage a little by a few miles but when is the compression ratio going to do as far as barebone changes of power and mileage as well.

 

i don't think anybody has ever lowered their compression without going turbo. and there are very few people who turbo their xa/xb's to begin with and even fewer of them tend to go so far as to change the compression.

but it will probably be very very bad since your ems will still be set as if it is running at normal compression. you could have some serious problems. probably alot of stalling. or run too rich.

i can only say seriously don't drop your compression until you are turbo'ed AND tuned or you might be looking for a new engine.

 

thats my whole point! i'm going turbo, but i want to figure out what power as a barebone i would be loosing and what it would do to my mileage. i'm not doing it ahead of time, i'm wanting to know what it's draw backs are for doing it period with the turbo. what's going to do to the mileage ontop of what the turbo will be doing, what diffrence in barebone power UNDER the turbo for a estimated expected power range when it's actualy changed and turbo'ed. and also, why change the compression for more boost when if you kept the same compression, ya youhave less boost but still would be about the same power right? if you add more boost on a lower compression then it would just seem like the same compression all togethere if it was left alone and used a little less boost. i'm wondering why have the need to change the compression at all when an internals kit, rods and crank with a 10.0:1 or the stock 10.5:1 given the new internals fit with the stock pistons with what... 15psi boost with the GT2554R from TT or ZPI would probably be the same power gain as if you were to change the compression on the pistons to handle more boost when to me my physics teaches say it would be basicly the same thing in gain but just has a more "aww factor" effect when you say "oh i've got 25psi" or "i've got 30psi" like them WRX boys do...

is my point comming clearer at all?
i don't mean to be difficult at all and i'm sorry if i'm making it hard to understand =/

 

you lower your compression to make more power with a turbo kit that will spool up faster. lowering your compression ratio won't negatively effect your mpg and power with a turbo so long as it is tuned right. what changes mpg is how much gas your engine uses, and how efficiently it increases power output.

alot of tc drivers report better mpg on stock engines with turbochargers because of the increased power effciency.

anyway, i wouldn't get too boost happy with an xb, they seem to break their drive shafts before hurting the engine. zpi seems to have established 230 whp as the limit of the xa/xb drivetrain.

 

thats totaly fine with me. my goal right now is to get 15psi running safely, replacing the internals, rods and crank plus new front axles should alow me to do that safely.

 

If you are putting that much $ into the motor, you should sleeve the block also. putting rods and pistons wont allow that much more power input from what i've discovered.. the block will blow out.

 

the block has a very thin wall. with pistons and rods only, you might get away with 9psi, maybe up to 11psi with a very careful tune

 

Reducing the compression ratio of any car will lower you power quite a bit, along with running too rich because all of the fuel will not be combusted, which lowers mpg. The turbo doesn't only produce pressure, but effectively raises compression when it's in boost. That's why cars with huge turbos suck when they take off, they have no power due to low compression.

Karl

 

so basicly speaking it would be better to just keep to normal 10.5:1 compression an boost from there being that it would give more power in a way but to a point.

the reason i ask is it seems pointless... for example...

10.5:1 with 10psi boost would give... lets say 20.0:1 resulting compression

lower the stock compression to say 9.0:1 and give 15psi boost to get that 20.0:1 again for the same gain in power as the 10.5:1 + 10psi boost...

number ofcourse probably not correct but thats why it seems pointless to me. am i not correct? seems to be more of an aww factor and a suprise blow during the race, start off slow but get REAL quick afterwords. i'd rather beef up the internals and drivetran and boost 10-15psi.

i really don't see sleeving as an option unless i was going for more fuel efficiency, it wouold only cut down my power gain. if the block wont take 15psi, i'll go and spend the extra $$ to have a titanium block made. freind of mine found a company that will make his SR20DET block out of full titanium for him for about $1,500 and for weight gain, i wouldn't be seeing more then 100lbs of extra weight, that being if they use pure titanium or an alloy, and alloy being slightly lighter and less then 100lbs of extra weight.

 

"10.5:1 with 10psi boost would give... lets say 20.0:1 resulting compression

lower the stock compression to say 9.0:1 and give 15psi boost to get that 20.0:1 again for the same gain in power as the 10.5:1 + 10psi boost... "



You're right with that statement. If you leave the stock pistons in, your boost pressure should/will be lower and you'll make the same amount of power as lowering compression and boosting higher. Not too many people run say 10.5:1 and boost, as you may require race fuel for the domed pistons. Running 9.0:1 will also increase the amount of space inside the combustion chamber, also effectively rising your displacement. With raising the amount of space, you make more power by boosting higher because more fuel to burn, more air to burn, more BTU's of power. With that being said, lowering compression is usually better in the long run, especially if you spike alot more boost than the controller is saying it's keeping it at.

Karl

 

i figured as much.

will lowering the compression also result not having to use such high octane fuels or no?

 

if you lower the comp while staying na you will lose mad power and mad mileage cause your power won't be there.


high octane fuels? what? our cars only need 87.....

 

That's not the question he asked. He asked if lowing compression will result in using a lower octane gas with a turbo, and that answer is no. The effective compression ratio of the vehicle will be the same running 9.0:1 at 20 psi as it is running 10.5:1 at 15 psi, if the previous scenario was true. Either way, it would equal that rough estimate of 20:1compression. Higher compression needs higher octane to burn slower, which cools the cylinder while maintaining high cylinder pressure.

Karl

 

good job trying to burn me but he doesn't make his questions very precise....

 

I wasn't trying to burn you, and I completely understand where you're coming from, but somehow I did catch it on what he meant. I'm new, but I don't flame people, and I've been on other boards for 5+ years, so yeah.

Karl

 

some on here just don't read my question well enough to understand them *hint hint*.

the first time i asked i mentioned i wanted to know what barebone power i was loosing BECAUSE it wouldn't make sense to get lower compression pistons to handle more boost when leaving them they way they are with just a little less boost would be exactly the same thing. i never wanted to know for NA reason, anyone who lowers thier compression for NA use is an idiot. i knew that you would loose power, i just wanted to know how much power was being lost and why someone would want to do that when it's a waste of time unless they want that "aww factor" with having more boost then others even though they have the same amount of power.

i never said i WANTED to do that for an NA application, i wanted to know what i would be dealing with in looses. and thats been answered, i don't need to be informing why i want to know and what i'm doing because i'm not talking to my parents like i'm 10 years old. stalling, loose of power, more boost capability that has no point on the power side but gives for good show and also answered the side question of fuel octane. Thankyou Karl, thats all i really needed to know