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Old 04-15-2004, 04:43 AM   #1
elstevo
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Legacy Higher C/R rate and a smaller turbo for quicker acceleration???

I want to eventually turbo my 2.5 and right now I am just trying to do my homework. I don't car about top end. I rarely go above 80mph anyway. I'm basically looking for the best way for quick acceleration. I have been reading several posts and one idea was the use of a small turbo with a high C/R engine. Something like 10:1 compression. It was said that the high C/R would get the small turbo to spool up extremely fast, thus getting power to the road quickly. I did run at my local 1/8th mile track back in my V8 days. I would like to return to some drag racing with the Suby. (nothing major just basic street car night shoot outs.)

Thoughts and opinions are encouraged!!!!!!
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Old 04-15-2004, 05:06 AM   #2
HndaTch627
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you know the EJ25 has a compression ratio of 10.0:1 already right?? and they melt ring lands at that ratio


i'd reccomend you do a lot more research
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Old 04-15-2004, 10:19 AM   #3
elstevo
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The Idea itself makes sense, but still even I am kinda wondering about it. Back when I had my V8 I was wanting to put a roots blower on it because they put out the best amount of bottom end torque and I was going to have to go with a 8:1 compression ratio.

I know there are turbo kits that will go on a realitivly stock 2.5 but I want to upgrade my internals before any type of forced induction is used. I was simply asking if the idea is feasable. I know most turbos make their power in the higher rpm range, and I am just trying to figure out my best route to get my power happening in the mid to lower rpm range.
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Old 04-15-2004, 11:55 AM   #4
Matt Monson
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There are some folks running 8.8 or 9.0:1 CR pistons, which is considered high for a turbo by modern standards. I don't know that going with a really small turbo would matter that much as long as you get one with good spooling characteristics. My EJ22T w/EJ20 heads build is going to be 9.0:1 and run with a VF-29 that spools up below 3000rpm. It maxes out at 7000rpm @17 psi, but with my compression and discplacement I expect great driveability and around 350hp at the crank...
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Old 04-15-2004, 02:35 PM   #5
no-coast-punk
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for all practical purposes compression ratio will NOT affect your turbo spool up times, it seems a bit counter intuitive but it's true. The thing that spools turbos the fastest is very short intense pulses of exhaust. A very large oversquare motor (short stroke wide bore) like our boxter motors have will cause these short and intense pulses. If exhaust valve snapped open when the piston was at top dead center and vented all the exhaust then you would see a shorter and more intense pulse from the exhaust, however that's not what happens. High compression pistons have a slightly shorter distance to move when pushing the exhaust pulse however when compared to the time this takes it is extremely negligible and the increased gas volume from the increased combustion chamber volume will totally offset this. HOWEVER your motor will be completely gutless when off boost when you go to low compression, this is what leads to the common misconception that high compression leads to faster spool up time because you notice the car move more when you first hit the gas and the transition from NA power to FI power is much smoother so it feels like the turbo is spooling in a faster more linear manner. Low compression WILL allow you to run more boost than a high compression motor translating into more power, and the increased combustion chamber volume due to a slightly lower piston face at top dead center will also lead to more power. However the lower your compression gets the more violent the power delivery will be. If you don't believe me all you have to do is look back to the days when turbos were allowed in formula 1 competition, they were running anywhere from 5.5:1 -> 6:1 compression, and if you don't believe that you want a fast spooling turbo in an F1 car, go back to your carbeurated oval track racers. Side note, sometime go look up the bore/stroke ratio and the rod/stroke ratio on our motors, both of those ratios are EERILY similar to F1 motors of the turbo era

Last edited by no-coast-punk; 04-15-2004 at 02:40 PM.
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Old 04-15-2004, 02:48 PM   #6
leondal
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Also, if I may make a comment.

When you are talking TOP-END, it is not necessarily indicative of HOW FAST you're going. TOP-END would mean (to me, anyways) high RPMs.

The main issue I have with smaller turbos that spool up immediately is that you're done with the powerband faster, because the smaller turbo can't flow enough to keep up as well with the engine's need for air at higher RPMs. You get the initial burst of power, then it feels pointless to stay in gear any longer and you end up shifting at like 5000 RPMs.

I feel that having a longer powerband is much better. I like the feeling of stomping on the gas and having the power build and build and build the longer I hold down. It's the feeling of knowing that there's more to be had if I just keep going, instead of feeling like your peak has come and gone.

I personally like a CR in the 9:1 area. It's safer, without sacrificing all the "off-the-line" grunt that I like to feel when driving around town.

I'm not a technician or an expert, but I know what I like....
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Old 04-15-2004, 07:11 PM   #7
no-coast-punk
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Quote:
Originally posted by leondal
Also, if I may make a comment.

When you are talking TOP-END, it is not necessarily indicative of HOW FAST you're going. TOP-END would mean (to me, anyways) high RPMs.


Sorry for the bad terminology, by top end I meant exactly what you desire in a turbo, one that builds and holds it's power all the way to redline, a systems overall maximum wrung out power

Quote:
I personally like a CR in the 9:1 area. It's safer, without sacrificing all the "off-the-line" grunt that I like to feel when driving around town.

I'm not a technician or an expert, but I know what I like....
Most people really like this range, that's why alot of the aryan deathwagon manufacturers use that exact CR in all their OEM turbo motors.... personally, not what I like, although I'm not too concerned with off the line grunt (road racer), when I'm on the gas it's usually in a situation where I'm out to keep the tach above 4k at all times... in those situations i'm usually from vacuum to full boost within 300 RPM or so, other than the off the line grunt thing.... I totally agree with you, what's the point of building a motor that falls on it's face at 5k?
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Old 04-16-2004, 12:11 AM   #8
squirrelmasta
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with a lower CR engine would you be able to launch at higher RPM's?
-Eli
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Old 04-16-2004, 12:21 AM   #9
no-coast-punk
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depends on what your gearbox can take... the ideal drag launch gives you a 10% co-efficient of wheel spin (this is where tires get the most traction) for as long as you can hold the wheel spin. If your gearbox can take a launch where your motor has built 3/4 of full boost sitting still then by all means launch at whatever RPM you want as long as you don't go over the 10% wheelspin thing. On a weaker gearbox if you have a boost controller that you can set to build no boost while the car is still, yes you will be able to launch at a higher speed because the power being transmitted through the gears during the shockloading will be much less... however your motor will bog BADLY, if you have a drivetrain that can handle a big clutch drop and wheel spin all the way through first gear, go ahead and do it, if not gently laying on the clutch with a low RPM launch is the way to do it (no bogging)
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Old 04-17-2004, 02:13 PM   #10
Unsung Boxer
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Can i point a misconception out?

I am using a Turbo Legacy block w/ EJ22na heads. Which means im at around 8:1 compression (maybe less?), and for a while i ran without a turbo. Losing compression does not make your car rediculously slow...

In fact, if you go from 10:1 to 12:1 compression, its not gonna be as much of a power gain as going from 10lbs of boost to 12lbs.

To me, its worth the little bit of low end power loss, in order to gain more boost, and more importantly, more reliablity.

How many WRX guys do you hear complaining about not having enough pre-boost power? Please.

-Jake
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