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Old 10-13-2001, 01:29 PM   #1
FuJi K
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Question Tuning hp and torque curve w/ low compression and high compression pistons

ok here's another question that some of you will say, "oh, there's another FuJi K post" hey guys, i'm looking for answers here not lecturing about 'quit dreaming stuff.' do i make myself clear?


if a RS was to be turboed, with its stock pistons, would it be easier to tuned the hp/torque curve than a RS w/ low compression pistons? Or would it be the same?

stock 10ish compression ratio=low PSI
after market 8ish compression ratio=higher PSI

IMO---i believe with low compression pistons, one could tune it with more precision due to the more PSI.

anyone wanna help out? any tuners out there?

FuJi K
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Last edited by FuJi K; 10-14-2001 at 12:25 AM.
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Old 10-13-2001, 01:36 PM   #2
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Default Re: Tuning hp and torque curve w/ low compression and high compression pistons

Quote:
Originally posted by FuJi K
ok here's another question that some of you will say, "oh, there's another FuJi K post" hey guys, i'm looking for answers here not lecturing about 'quit dreaming stuff.' do i make myself clear?


if a RS was to be turboed, with its stock pinstons, would it be easier to tuned the hp/torque curve than a RS w/ low compression pinstons? Or would it be the same?

stock 10ish compression ratio=low PSI
after market 8ish compression ratio=higher PSI

IMO---i beleive with low compression pinstons, one could tune it with more percision due to the more PSI.

anyone wanna help out? any tuners out there?

FuJi K
okay talk like that and I will lecture

"Pinstons"?, = Pistons
"Beleive", = Believe
"percision" = Precision

and... Tuning wasnt any easier with the stock CR compared to the lowered CR...
it will not be more precise, since you are tuning at various points of intake manifold vacuum/pressure. SO, you will infact have more tuning to do with a higher boost level. But it will not be more "accurate".
Good enough?
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Old 10-13-2001, 01:43 PM   #3
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driveability on a high compression engine is MUCH better, like cold starts. WRX guys are already complainging cause the car has to crank so long to fire. just a byproduct, not enough compression heat=bad cold starts.

jeremy
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Old 10-13-2001, 01:57 PM   #4
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so if one were to turbo his/her RS, staying with stock pistons would be a better idea? That means that staying with stock compression ratio, a big turbo is not necessary.

another question:
would a 10ish compression ratio piston work well if one were going to make 400hp?

ok

anymore input from anyone else?

FuJi K
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Old 10-13-2001, 02:01 PM   #5
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Quote:
Originally posted by FuJi K
so if one were to turbo his/her RS, staying with stock pistons would be a better idea? That means that staying with stock compression ratio, a big turbo is not necessary.

another question:
would a 10ish compression ratio piston work well if one were going to make 400hp?
FuJi K
stock pistons are not the strongest things you can have when turbocharging. Although I had no problems with mine (knocks on wood!)
You can keep your CR and have upgraded pistons, you know.
Ahh I'm going back to sleep
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Old 10-13-2001, 03:33 PM   #6
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This is a difficult question to answer because so many factors play into it.

Say a car maked 300hp with 8lbs. of boost with 9.5:1 compression. Now say that the person lowers the compression to 8.0:1 and runs 14lbs of boost and makes the same 300hp.

Boost and compression ratio combined is known as static compression ratio. On a street driven car on pump gas, your goal is to keep your static CR under 18.0:1 or you sill start getting detonation. Fort short periods of time, such as a full throttle 1/4 mile pass, you can go up to 22:1 but don't stay there long . And If you run a timing retard you can run a higher Static CR but I would not recomend going much higher. Here is a good chart to show what your static CR will be based on your CR and boost:



As you can see, an engine with a 9.0:1 CR running 4lbs. of boost nes you a static CR of 11.4:1. Similarly, an engine with CR of 8.0:1 running 6lbs. of boost has an almost identical static compression ratio! In theory (and it is what happens) these too engines should make the same hp. because all things else being equal, they are pushing the same amout of air through the engine.

The problem with a low compression motor is that it relies heavily on the turbo for its power. An 8:1 motor is definitely not going to to make monster power. Sure, you can throw 18 lbs of boost on it and get some real power, but why? A higher compression motor of 9.5:1 will have much more power without the turbo. Then, with less boost you could easily have the same overall power - only it would be much more usable. Both of the motors (8:1 with 4lbs boost and 9.0:1 with 6 lbs boost) will have almost the same effective compression and about the same overall power. The big difference will be where you see the power, and how much of a demand will be placed on the turbo. Obviously, the 9.0:1 motor is going to have far greater torque and low end power as the boost is only starting to come in.

The high compression engine will make more low end power, like stated, and have a flatter torque curve (As flat as one can be for a turbo car anyway) than a lower compression engine. Also, it will generally reach it's power peak at a lower rpm than a lower compression engine.

So, if you build the car right and it is for the street, go for the highest possible CR that is safe for the engine. Keep in mind also that the less boost you run, the cooler the charge will be and the less you will work your turbo. Not to mention, the car will be more drivable.

So why do automotive companies generally go with low compression and higher boost? One of the reasons is that the parts used in mass produced cars are not of the highest quality. The companies err on the safe side and use low compression and medium amounts of boost that are easily within range of a safe CR.

If you build the car right, in my mind the ideal compression ratio for our engine is 9.0:1. Most people will run from 8.0-8.5:1 to be safe I would guess though. But, for me, 9.0:1

Eric
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Old 10-14-2001, 12:03 AM   #7
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Default What the hell am I thinking...

I think that what your'e asking is how to tailor the Torque Curve to your liking using different Compression Ratios? Right?

Well, The stock 10 to 1 Ratio will be very torquey down low and will create problems for your tranny because of the hard pull off the start.

Lower it down to 8.5 to 1 and you will not get the fastest 60' time but I bet your tranny will be much more happier...

Ok, well the stock 10 to 1 Ratio will therefore make power now for now and the 8.5 to 1 will make power a little bit later.


Now, we have to consider turbo sizing too...

I for one will eventually be running Shiv's latest designed turbo with 8.5 to 1 Forged Pistons tuned by a Tec 2 Stand-Alone Engine Management System. I figure that this setup will give me the kind of driving experience I want to feel... Bottom line.

I don't really care about hitting a 1.6 60' time. I'm looking for power from 3500 to 7500 with enough power from 1000 to 3500 to get me moving as fast as I do Naturally Aspirated. My tranny will be ok, I hope, and I'll stay with these Supras around here. At least in theory.

I didn't really answer your question but your question seems really vague. Ok, How about this Answer:

Yes you can tune your HP and Torque Curve with Raising and Lowering your Compression Ratio. Did I Clear things up for you?

Zee
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Old 10-14-2001, 12:12 AM   #8
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It sounded like he was asking if it would be harder to tune. The answer is "probably not" since a 400hp EJ25 is going to take a lot of dyno time to get right, no matter what. However, you are not going to get 400hp out of an EJ25 with the stock compression on pump gas (see the last installment of project Impreza in SCC...~350hp on 100 octane gas and tuned right to the edge).


Ben
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Old 10-14-2001, 12:41 AM   #9
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So Eric,

Based on your calculations why are you suggesting a group buy for 8.5/1 CR
pistons. I suggested 9/1 and got shouted down. Not a criticism, just a question.
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Old 10-14-2001, 12:41 AM   #10
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Smile thankx for the replies

ok..........a bit more to clear things up.

tuning, i mean as in using turbo timer and boost controller. so i guess it wouldn't matter, right??

they'll both be tuned with the same amount of precision.................but the difference in power at the start will vary from low to high compression ratios? so this should be right, right?


FuJi K
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Old 10-14-2001, 12:44 AM   #11
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Tuning with a turbo timer? ARGH! Fuji K, a turbo timer does _nothing_ but let your car idle for a fixed period after you remove the key from the ignition. Additionally, you do not want to try to tune a 400hp EJ25 with piggybacks. A standalone engine management system handles the tasks of a boost controller, in addition to everything else it does.


Ben
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Old 10-14-2001, 12:51 AM   #12
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Quote:
Originally posted by bsquare
Tuning with a turbo timer? ARGH! Fuji K, a turbo timer does _nothing_ but let your car idle for a fixed period after you remove the key from the ignition. Additionally, you do not want to try to tune a 400hp EJ25 with piggybacks. A standalone engine management system handles the tasks of a boost controller, in addition to everything else it does.


Ben
so TEC II engine management would do most of all the jobs? would a boost controller come handy though or would that not be needed if TEC II was installed?
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Old 10-14-2001, 01:21 AM   #13
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I was suggesting 8.5:1 because the weak link in our cars is the trannies. A lower CR will net less torque off the line and be easier on the car.

I have a 2.2L engine in my car which gives less torque than the 2.5, so by going to a 9.0:1 setup in my car, I will make some of that low end torque back

Eric
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Old 10-14-2001, 01:40 AM   #14
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With a TEC-II you do not need a boost controller. It would serve no purpose.


Ben
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Old 10-14-2001, 03:34 AM   #15
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Old 10-14-2001, 03:43 AM   #16
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Default Re: thankx for the replies

Quote:
Originally posted by FuJi K
ok..........a bit more to clear things up.

tuning, i mean as in using turbo timer and boost controller. so i guess it wouldn't matter, right??
FuJi K
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Old 10-14-2001, 04:40 AM   #17
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Thumbs up do not take turbo timers lightly people.

turbo timers are the best way to properly tune your engine aside from a fuel pump computer or a 40mm engine stabilizer. setting the turbo timer to 2:00 should clear all compression questions up. you could even run 13.54psi wiht a 12.6:1 CR with proper turbo timing. just becareful at a gas station. if you dont time that turbo properly, the whole thing could send gas right back to the gas tank and be pressurized to the point where it will shoot out when you stick in the gas nozzle. also when tuning with a turbo timer please keep your windows clean. you need a clear signal to the satellite to make sure everything is running ok. and beware of BOVs. if the vacuum line isnt setup properly (or you use cheapo line that collapses at 20 vac) to the timer it wont know when to vent the air!! this would be very bad and cause the timer to go goofy and cause compressor surge within the engine.

one more thing. if some punk comes up and revs at you. turn the timing of that turbo up!! you can gain like 20hp by increasing the timing 4:00. its critical that you dont over time the turbo. this would cause the turbine to actually go in reverse and send air back through the downpipe up the headers! i've seen this happen before and it is NOT pretty! the car pretty much spits everything out. rods shoot out the exhaust, the cats fly through the passenger seat...OH that reminds me! the new Apexi-Turbo Timer Hyper XKR can help with those cats. relocate them on your uppipe and your car becomes smog legal. the cops cant do anything about it!! i dont know exactly how it works but that turbo timer is freakin awesome! way better than that TEC II.

also beware of the so called "good gas." when you upgrade your engine with a turbo timer and boost controller, you change how it uses gas. you can now make huge amounts of power with Diesel gas, yes...DIESEL gas. just put a CB radio on your car and ask the trucker's where they fuel up. thats where you want to go. they have the best gas at truck stops.

turbo timers are not to be taken lightly. you should read up on them. i think there is a couple books at amazon.com. i cant remember the titles, they were something like "Tuning by Timing" or something like "Time your turbo right!"

Andrew
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Old 10-14-2001, 12:32 PM   #18
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Exclamation GEEZZZ..

Damn Drew you spent all that time to say NOTHING....

This kid's 15 and he's like I-Club's Youngest so give him the BENEFIT OF THE DOUBT....

ZEE
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Old 10-14-2001, 12:37 PM   #19
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a turbo timer does NOTHING AT ALL for tuning. all it does is keeps your car running for the set amount of time after you take the key out. this helps preserve the life of the turbo.
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Old 10-14-2001, 01:10 PM   #20
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Quote:
Originally posted by bsquare
With a TEC-II you do not need a boost controller. It would serve no purpose.


Ben
kool...........thankx!!!!


Andrew
thanks for the info!!!

i guess i'll find some 8.5ish-9ish compression pistons then right?


subachad
i been there, and knew how a turbo worked before i even visited that site. Thanks for the link!!

i guess this is it..........thanks for all the info

FuJi K
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Old 10-14-2001, 06:12 PM   #21
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Arrow

still
very
skeptical
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Old 10-14-2001, 06:33 PM   #22
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Default Re: GEEZZZ..

Quote:
Originally posted by Sunrise City Rider
Damn Drew you spent all that time to say NOTHING....
ZEE
Good Laugh, though...
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Old 10-14-2001, 07:25 PM   #23
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i got bored

dont worry fugi, i used to be a newbie too and didnt know jack about turbos. just read as much as you can and you will slowly learn.
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