+1

I know this is pinned in the 420A section, but its useful for anyone who wants to eventaully turbo their car. Why not move it somewhere that more people can find it?

Also, do keep going, cause I'm sure there is much more to be explained.

we could add threads in other sections with links to this thread.

what else you would like explained?

The more that is explained, the more I find I didn't know/understand. Its gonna be a while before I actually get to turbo my car, and I still may decide to go the N/T route.

But the more you can explain, the more informed I'll be as to making my decision.

The engine itself has alot to do with turbo selection. For the first part of this we will assume that compression ratio is the same throughout each engine discussed (8.6:1) and that these are all inline 4 cyl engines. h-blocks and v-blocks are a different world.

a 2.4L engine will spool any turbo slightly quicker than a 2.0L. the main reason is that extra .4L.

There are 2 major things that help turbo's spool: 1. volume; 2. heat. Therefore (using previous A/R example) a .42 a/r turbo will spool super quick on a 2.4L and give tons of torque but will restrict the higher rpm horsepower much more than the same turbo on a 2.0L because the 2.4 is a larger displacement and will have a larger exhaust charge per revolution. The 2.4L engine itself will be a much more torquey engine just because of its different characteristics using a longer stroke. Therefore, turbo selection with regards to engine plays a big role.

with regards to 1g/2g dsm's engine and turbo selection all comes down to personal preference. The opinions i've shared in this entire thread can be argued with valid counter points. An example of what and why I am trying to build my talon to will help. Its a 96 tsi awd automatic. I want it to be a drag car and fun daily if necessary. So, i'm building it with either a 2.3 or 2.4L (haven't decided completely yet). The reason i'm going with the larger displacement is because it's awd so I can get good traction off the launch and its automatic. Automatics, not only rob more power, they also use torque more effectively from a start than 5-speeds.

The starter turbo i'm going with (depending on availability) will probably be a holset h1c or holset hx40. They spool quick and are happy at 30psi+. They (from what ive been able to find so far) can use a t3 housing or larger A/R mitsu flanged housing. I'm not very concerned with spool time because of the bigger displacement engine and the way an automatic trans uses the converter to launch. I don't want a housing that's too large because I want the torque early in the rpm range but I also don't want a housing that's too small causing me to run out of top end power. I plan to have my rev limit at about 8000rpm but that could change depending on where the power curve starts to drop.

For my compression ratio i'm going to use 9:1. I will get into why in the next post.

the exhaust manifold i'm going to use is a cast t3 manifold, an OEM 2g, or OEM evo III manifold (again, depends on availability). I don't want a tubular manifold because I don't want to worry about it cracking or my turbo being too heavy for it. Even though tubular does help decrease turbo lag I'm going to use the displacement, torque convert, and CR to help with that.

Alright, so if I'm understanding you, if I were to turbo my car, I'd be better off using a turbo that gave me more torque than top-end power because I have a small engine/low compression automatic transmission. This would give me the additional power off of the line and would in aid in acceleration without robbing me of too much top-end speed?

After you discuss compression ratios and such, would you mind going into maintaining boost, and such?

The problem there is for another topic called traction. I'm sure if you romp on it right now from a dead stop you will get the tires to light up.

If I floor it from a dead stop, my good ol' 4G94 will make a high pitched wine and accelerate without loosing traction. There is too much power loss to the wheels to loose traction.

it all depends on what kind of racing you want to do. You are getting the basic concepts but if you want a 1/4 mile monster you should pick the intermeditate turbo. with an automatic you have the advantage of "stall."

The basic explanation of "stall" is a certain RPM that your torque converter will reach it's maximum power of transfer and will start to "slip." If you go out to your car, start it up, let it warm up, put it in drive, hold the brake as hard as you can and then push the gas pedal to the floor your car will want to move. This is "stalling up, " or "getting on the converter." If you hold it there for about 15 seconds you will do 2 things:
1) build a ton of heat that could kill your trans
2) reach your "stall point"

The point at were the engine starts to build power and the rpm's rise is "stalling up." The point at where the rpms stop rising but the car is still wanting to go is the "stall point." Aftermarket torque converters can be calibrated to different stall points which can be essential to the type of racing you want to do. A high stall will allow the rpms to rise at the line to spool a larger turbo and give that great launch (1/4mile). A low stall will keep the rpm low to create the torque needed for a fast response and pull out of a corner (auto x). A moderate stall will give both for the best launch and shot out of the hole (1/8th mile).

this vid gives you a good idea of stalling at the line. you can see how the car sags as he holds the brakes but pushes the gas pedal down: http://www.youtube.com/watch?v=rSs0ydtPumM...feature=related

this launch ( my favorite dsm vid by the way) is an example of a 5 speed launch. they use whats called "anti lag" to spool the turbo instead of stalling: http://www.youtube.com/watch?v=Tix0SJl7Q7k...feature=related the basic concept of anti lag is when you push the throttle a certain percentage (80% of higher depending on user settings) the ECU will retard timing and dump fuel into the cylinder causing fire balls to be pushed through the turbo spooling it.

here is an example of why a bigger A/R turbo is better for 1/4 mile. you can hear how high this car revs: http://www.youtube.com/watch?v=eMrqleOmesM&NR=1

The car in question, my Lancer, is a daily driver and has to remain such. I just want to get more power out of said daily driver. I know that I can't go with more than 6psi before melting the stock pistons in my 4G94 (at least that's what I read somewhere on this forum). HOWEVER, what intrigued me about this thread is the fact that 6psi on one turbo is different from 6psi on another.

My car has 7:1 compression ration pistons stock, and about 120BHP. I want to get enough power going through the car so that I don't loose out on comfort on a daily drive to and from school/work/etc. But at the same time, I want to be able to move off of the line when some punk-bitch in a Honda pulls next to me and tries to look tough. And I want to be able to not have to floor the accelerator to keep up with my friend's when there is a group of us cruising to a meet.

Am I just talking pipe dreams, or is this actually plausible?

Factory 7:1 comp pistons? DUDE your car is being ROBBED of power! Throw some 8:1 or 8.5:1s in that bitch! Even doing that W/O the turbo will give you a nice gain.

Bro, I know. Problem is, new pistons and rods are some expensive shit...cause I would do both at once...and a new cam while the engine was open.

If you throw in a bigger cam you DEF gotta replace the valve springs too. If not your springs will wear faster than ricky bobby. Your right tho, with all this your lookin at a pretty hefty chunk of change.

7:1???? that doesn't sound right. i've never even heard of a turbo car having that low of a compression ratio. are you sure about that?

the biggest thing that will help you is to remove that speaker box, amp and anything else thats heavy in your car. extra weight is just slowing you down and with 120bhp you need all the help you can get

My car doesn't have a turbo. I want to add one. But again, I'm not interested in actual racing, just the BS off the line for shits and giggles racing that goes on near a community college...and having the power to keep up with my friends' cars.

This is a daily driver, I'm not about to go doing full weight reduction on it: I need to be able to drive it comfortably...

I think he meant hes never heard of a non turbo car with 7:1. Normally N/A cars (Non turbo) have higher compression pistons (Usually 9:1 or greater) and turb motors usually have lower compression. To be honest, 7:1 is really even sorta low for a turbo app... tho I have seen a guy with a 2g GSX (or GST I forget) that has 7:1 in it.
Rule of thumb is:

higher compression = more power, harder to tune
lower compresssion = less power, easier to tune

;perfect example of the compression is the 4g63. the NA model has a comp ratio of somewhere are 10.3 or something. (+/- .5 ish) the 4g63t has a comp ratio os 8.3:1 somewhere around there. its the same engine with diff pistons. kinda the way i understand it is the lower compression your engine has the more exhaust gases being pushed out to the turbo. is this correct or no? also the bigger pistons in the 4g63 NA will hit the bottom of the head and u will tear ur chit apart lol

n/a is 9:1 turbo is around 8.3, yes. I can't remember the exact compression for the turbo either. Also, the 1.8 compression is 9:1 as well.

yes, I meant i've never heard of an n/a car with 7:1 pistons. turbo cars, like you said, with 7:1 are also rare.

Higher compression isn't "harder to tune" necessarily. It just restricts the amount of changes you can make (IE timing, high boost, etc) without using race fuel, alcohol, or water inj



compression doesn't really have anything to do with the amount of exhaust gas. it has the biggest impact on how much heat is created during the compression stroke. thats why diesels have something rediculous like 15:1 because they don't use spark plugs, but use actual compression to ignite fuel.

all 4g63 pistons will hit valves if the timing belt breaks or slips far enough

That is what I mean. With higher compression, when trying to tune, your going to encounter alot sonner (assuming your leaning the mixture out a bit) than you would with a lower compression. Therefore, it becomes more of a challenge to get "just right" (I loose the term "just right VERY loosely.)

Since we are on the subject, let's talk about compression. Low Vs High in regards to turbo applications ONLY.

There are two most common types of pistons now a days. Hyperutectic (alloy) or Forged aluminum.

Hyperutectic pistons - These are cast pistons that are made primarily using aluminum, but also incorporate silicone to form an alloy. the amount of silicone determines whether a piston is simply utectic(an alloy), hyper utectic (more silicone) or hypo utectic (less silicone). the hyperutectic pistons are typically an upgrade from the stock utectic pistons because the added silicone makes them more flexibile and resistant against detonation. they are, however, still cast and therefore are weaker than forged. Typically, OEM 4g63 pistons are hyperutectic but to compensate for the amount of heat that is generated under boost oil squirters were installed into the block. They open at a certain oiil pressure and squirt oil on the bottom of the pistons to cool them.

Forged pistons - are cast into a slug still using an alloy but the slug is then pressed into shape while still hot (forging). The forging press smashes the alloy together which reduces the chance for weak spots (bubbles, etc) and also makes the molecular structure more densely packed further strengthening it.

Forged pistons are the ideal choice for turbo applications because they handle heat better and are stronger in case of detonation. Turbo cars build TONS of heat under boost and forged pistons need to have larger clearances from the cylinder walls because they expand more than a hyperutectic piston would. Lets say a typical piston to cylinder bore clearance for an OEM piston is .0035. Installing forged pistons one should have the block honed to a clearance of .0050 to compensate for the extra expansion of the forged piston under boost. Piston slap can be a by product of the larger clearances but that explanation can be found here: http://www.mitsubishi-forums.com/index.php...;hl=piston+slap

Now that you've figured out which pistons you want you need to figure out what compression ratio you want. Turbo cars can go anywhere from 8:1 up to 10:1. The 10:1 turbo cars are typically alcohol ONLY cars. 9:1 turbo cars can get away with high octane gasoline while 8:1 cars could use as low as 89.

Octane, simply put, tells you how easily a fuel is ignited. Octane can be changed by adding things, such as lead, to gasoline making it harder to ignite (higher octane). Low compression turbo engines can use lower octane fuel because the create less heat during the compression stroke. High compression turbo engines need alcohol or high octane fuel because they create more heat during the compression stroke. We will get into alcohol later, for now, i'm going to stick with gasoline.

When air is compressed the friction from the molecules getting packed tighter together causes heat. The more you compress air and the faster you compress it makes even more heat. Thats typically what causes detonation. Higher compression causes more heat when the air is being compressed on the compression stroke. If it exceeds the flash point of the fuel that's mixed with it the fuel is going to ignite. Read here for more on detonation: http://en.wikipedia.org/wiki/Engine_knock

Why do higher compression pistons limit what you can do with tuning? Every engine fires the spark plugs BEFORE the piston has reached top dead center (TDC). The reason for this is that there is a certain amount of time that it takes for the burning fuel to expand enough to force the piston back down the bore making useable power. If the fuel gets a head start on burniing before the piston is at the top then by the time the piston gets there the fuel is already burning enough to force it back down. The amount of advance on the ignition timing is a huge factor in detonation prevention and can play a big role in how much power the engine can make.

Physics lesson (IMG:style_emoticons/default/smile.gif) . You have to understand one VERY important part of this though. The piston does not move just because fuel is burning. 1 law of physics is that matter can not be created or destroyed. Therefore, it only changes state. In simple terms, when fuel burns it changes state from a liquid(gasoline) to gas(exhaust gas) creating heat (burning). Gas takes up more space than a liquid does. Hot air also takes up more space than colder air. Since the cooler gas/air mix that entered the cylinder became compressed in a small space, when it was ignited changing state from "liquid mixed in cool air" to "gasses and hot air" a large positive pressure was built up inside the cylinder and it wants to escape or at the very least make room for itself. This is what forces the piston down the cylinder.

**For example purposes only, these numbers are not actual. This also assumes that the A/F ratio is ideal for conditions**
Lets say that an engine has 8:1 pistons. the ignition fires the spark plug at 12* before TDC. Since the engine is using lower compression the air/fuel charge wasn't heated too much during the compression stroke causing detonation. Therefore, you could add some timing (advance it) maybe another 3*. Now, the ignition fires the spark plug at 15* before TDC, and since the fuel is getting a bigger head start on burning/expanding in the cylinder, it makes more power. You add some more timing +2* but this time when the spark plug fires at 17* before TDC it detonates and the engine knocks. You have a few choices here. 1) pull timing back -2* to where you know it was safe 2) use a higher octane fuel.

Now, lets assume that you have 8:1 compression pistons, your timing is unchanged but your boosting 30psi and knocking. You have a few choices here as well. 1) higher octane fuel 2) retard timing to a safe level 3) use some sort of knock suppression like water or alcohol injection. This example is different than the above example because in this case heat is the culprit, not timing adjustments. Thats why you can use the water injection to cool the intake charge and also why you may have to retard timing further than OEM. An upgrade to a FMIC, or larger FMIC, is also an option to cool the intake charge.

Finally, you have an engine with 9:1 compression pistons. You are at a comfortable level of boost, say 25psi, and you are running 93 octance already. You don't want to use race fuel because its not available, you don't want to keep changing O2 sensors, etc. BUT you want a little bit more power. In this case timing alone won't do it. You don't have near as much room to adjust timing because the high compression pistons are already creating more heat in the cylinder making it close to knocking already. You are using the highest octane pump gas you can find and don't want to get into alcohol fuel or race fuels. So, you add water/alcohol injection. It will cool the intake charge so you can either add some boost, or nudge timing up a little bit. In this case, IMO, its safer to add boost instead of adding timing, but, advancing the timing isn't out of the question if you do it safely.

Just to toss this out there so people don't become terrified of timing adjustments. If your car does knock the ECU "feels" that knock from the knock sensor and pull (retard) timing to compensate and protect itself. If it keeps "feeling" knocks it will continue to pull timing BUT if can only go so far. If you exceed the safe limits of the engine and the ECU can't pull enough timing you can blow your engine. So, you want to be extremely careful when doing any kind of timing adjustments but don't be afraid to add 1* here or there if you don't see any knocks and all your other parameters are good (air intake temps below 100*, coolant temps below 230*, etc)

In regards to compression and turbo performance, it's a "6 to one, half dozen to the other" type thing.

Higher compression will spool a turbo slightly quicker and will take advantage of power making ability without major timing adjustments. It also creates more heat and higher cylinder pressures, so you want to be more careful and run the highest octane fuels available.

lower compression will spool a turbo slower, but you can make more timing adjustments to get similar amounts of power. It also allows you more choices with fuel and doesn't create cylinder pressures as high.

Extremely high compression (10:1) typically is an alcohol only car.