Long Term Fuel trim v. Short term - idealogy - Page 2 - Dodge SRT Forum
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post #16 of 74 (permalink) Old 07-23-2006, 11:00 PM
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So Lag, so how are you adjusting your SAFC? I mean, how do you determine which RPM increment to adjust? Just by watching your Scan Gauge and where your RPM's are at at that moment?

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post #17 of 74 (permalink) Old 07-23-2006, 11:22 PM Thread Starter
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i have my rpm points set evenly

first 1000 then 1600

with a fp of 50 (its high because im tuning wtih out the boost source)

im pulling i think around 33% then at 1600 its like 30 the rest i set at -25

ill watch my LTFT and my STFT my LTFT as soon as i put the scan gauge on was like -17.5 my afc was -26% i just watched the ltft and stft and adjusted the safc accordingly

manifold pressure and timing daily driving stayed the same

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post #18 of 74 (permalink) Old 07-24-2006, 10:02 AM
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^Makes me very mad that I didn't buy the aeroforce gauge. The price and good looks of my current gauge sucked me in.

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post #19 of 74 (permalink) Old 07-24-2006, 10:05 AM Thread Starter
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your telling me!

i have no idea why this gauge isnt as marketed to every one running fuel kits!

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post #20 of 74 (permalink) Old 07-25-2006, 03:05 AM
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Quote:
Originally Posted by blackbird_R/T
The fuel trims aren't something I'd necessarily be using as much as A/F, but it is something to keep in mind and watch to see how the car is behaving to your modifications. This becomes a lot more important if you start having drivability or tuning issues. Optimally and the best-case scenario on a modified car is you want them at 0% (no) correction if everything else is running well. But even in a stock or "Stage" car they will shift around a little bit.

If you're using fuel pressure and/or bigger injectors along with a piggyback system then it's something to keep an eye on more so than a stock/Stage car. I wouldn't sacrifice a safe A/F ratio but if I saw something like where adding fuel pressure and slightly altering the piggyback might get them down a little I'd probably try it (also taking into consideration how those changes in the piggyback would affect the PCM and consequently timing, etc.). The more you try to work with the NGC, the less the NGC will fight you back.


Quote:
Originally Posted by boost junkie
Fuel trims are ignored in open loop
That is incorrect. It cannot use short-term correction in open loop since the stock narrowband O2 sensor can't be used in cases like when going to WOT (since it can only accurately measure 14.7:1 AFR and tell if you're richer or leaner of that point), and while the long-term variables are not adjusted in open loop they are still applied.

The NGC uses the short-term corrections in conjunction with O2 sensor feedback to adjust the injector pulse width to keep the A/F where it should be (which would be 0% correction on the fuel trims). The long-term mirrors this and the NGC will alter the stored "cell" that you're currently in (which depends on throttle position/idle/decel and MAP voltage) when it sees the short-term continually making a correction of a certain value for a certain period of time in order to get the short-term back towards zero.

The long-term correction is always used including at start-up and open loop (WOT). The long-term correction amount itself will not adjust or change/update though until the engine is fully warmed up, it's been running for two minutes, and it's in closed loop operation at the time (where it's also using the short-term correction). That comes straight from the FSM and is something to keep in mind when tuning.




There's probably some other posts you can dig up if you do searches for "short term fuel", "STFT", "LTFT", etc. I'll look around later and see if I can find the rest of them.

black you say its applying LTFT to WOT witch LTFT is it applying idle? one of the other cells?

GM computers + LTFT to WOT this is why it is almost always better to have a -3 then a + 3 forwhatever reason

FORD on the other hand does not apply LTFT to WOT non that i have ever seen

the reason open loop does not care about fuel trims is because its exspecting a set standard with a set outcome

like X+Y=1 This is not adding LTFT
not (Z+Y)+X=???? this would be adding LTFT

if it adds LTFT witch i am pretty sure it does not because i have had my trims + and - and far in each direction and it always has Zero effect on the A?F ratio i have tuned for

have you seen different?

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post #21 of 74 (permalink) Old 07-25-2006, 10:04 AM Thread Starter
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Quote:
Originally Posted by Vector
black you say its applying LTFT to WOT witch LTFT is it applying idle? one of the other cells?

GM computers + LTFT to WOT this is why it is almost always better to have a -3 then a + 3 forwhatever reason

FORD on the other hand does not apply LTFT to WOT non that i have ever seen

the reason open loop does not care about fuel trims is because its exspecting a set standard with a set outcome

like X+Y=1 This is not adding LTFT
not (Z+Y)+X=???? this would be adding LTFT

if it adds LTFT witch i am pretty sure it does not because i have had my trims + and - and far in each direction and it always has Zero effect on the A?F ratio i have tuned for

have you seen different?

the long term fuel trims are applied at wot vec we tested this out yesterday on a stock SRT

27 deg no KR 14.3 max boost 13.3 at redline

LTFT at idle range'd from -10 - -1.7

@ WOT or open loop LTFT stuck at -4.7 , my guess mb an avrg of the LTFT

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post #22 of 74 (permalink) Old 07-25-2006, 11:09 AM
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hmm i could be wrong but as long as your lt adaptive is below 30% i dont think that there is any difference in timing.i never bother tuning for driveablilty unless im using bigger than 750 injectors.there have been instances though where its pretty close and if im pulling 30% at wot i have to drop the fp because timing gets low.the long term is a broad learned value .i try to pay more attention to short term.i havent really noticed any performance differences in driveability by getting the fuel trim close to 0.but i havent spent all that much time tuning for driveability.i think i am gonna try that on the dyno and see if it really does make a difference.either way the adaptive memory has nothing to do with the big turbo stumbles.i spent tons and tons of time when the stumbles first started showing up trying to get rid of them.i thought the same thing at first,and i had noticed that when i log it with a drb it was always when the pcm switched from one cell to another we were getting a lean spike.still got it even when driving it and forcing both cells to match in numbers.vector and aaron were the ones that realized it was the fuel system restriction that was causing the stumbles.since then any of them that have the stumbles ive been removing the bottleneck and the stumbles are gone.we had a 7 million post discussion about it

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post #23 of 74 (permalink) Old 07-25-2006, 12:05 PM Thread Starter
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Quote:
Originally Posted by never2muchboost
hmm i could be wrong but as long as your lt adaptive is below 30% i dont think that there is any difference in timing.i never bother tuning for driveablilty unless im using bigger than 750 injectors.there have been instances though where its pretty close and if im pulling 30% at wot i have to drop the fp because timing gets low.the long term is a broad learned value .i try to pay more attention to short term.i havent really noticed any performance differences in driveability by getting the fuel trim close to 0.but i havent spent all that much time tuning for driveability. i think i am gonna try that on the dyno and see if it really does make a difference.either way the adaptive memory has nothing to do with the big turbo stumbles.i spent tons and tons of time when the stumbles first started showing up trying to get rid of them.i thought the same thing at first,and i had noticed that when i log it with a drb it was always when the pcm switched from one cell to another we were getting a lean spike.still got it even when driving it and forcing both cells to match in numbers.vector and aaron were the ones that realized it was the fuel system restriction that was causing the stumbles.since then any of them that have the stumbles ive been removing the bottleneck and the stumbles are gone.we had a 7 million post discussion about it

why would u pay attention to short term fuel trims? they make no differences because they are based on the long term fuel trims

for example -3.0 on short term with a ltft of -9 is -3 on top of the -9
notice that the short term fuel trims like to stay around zero? while long term will jump out into double digits?

the reason id want to get it as close to zero as possible is because it allows the computer to have the leway it was designed to have for daily driving.

doing this will eliminate the stumbles and bogs those that run fuel kits get while daily driving because the car is running the "fuel talbes" in a way that it was designd.

in my experience the range of change in LTFT between teh cells is what causes the stumble, switching in a great ammount from say -17.9 cell one to -4 cell 2 causes the quik glitch

just an experiment, mb not related to it at all.

because the factory fuel table is setup to run a bit richer at idle, one cant just pull a certain map voltage across the board.

doing so will force the computer to over compensaite LTFT therefor affecting cells and transition

(from my experience) in one instance


if im wrong some one point me in the write direction, these are just conclusions i have come to while monitoring LTFT v. STFT

note: i also did the bottle neck fix stumbles were not "all gone" still had an idle stumble when the rpms would de-celerate and come to in idle rpm, the car still had a bog stumble.

erik's(wowguysrt) car does not have the bottle neck fix and does not have "the stumble"

you say you havent spent any time tuning daily driving or seeing what affects LTFT had on teh dyno yet you go on to say you spent so much time trying to figure out what the stumble was caused by. well how can u even say that its not caused by that if u never tuned your long term fuel trims or daily driving???


another point to your post - the more map you pull timing wont go lower heh that would be great, in reality timing gets higher because the timing equation the pcm uses is based on map volage, less boost = more timing. sooo if u pull too much your just going to get more timing (BAD! on big turbo cars) if your timing is being lowerd its the cause of Kock Retard(the ammount pulled from the total timing in degree's to eliminate knock)

that happens when your tune is too aggresive and causing knock

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post #24 of 74 (permalink) Old 07-25-2006, 01:48 PM
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Quote:
Originally Posted by Vector
black you say its applying LTFT to WOT witch LTFT is it applying idle? one of the other cells?
I know it varies by manufacturer and cars. Some do not use their adaptive learned values at WOT/open loop and some will use them only if they add fuel. The short term fuel trim shouldn't be utilized at WOT on the SRT-4 because it can't use the stock 02 sensor, but I have seen them used on the long term. I'll post the exact cell structure it uses in a minute.



A little project I've been try to work on is compile some videos I took of my scan tool at a few SoCal dyno days. I've monitored a ton of cars with different mods but only took videos a few times, but I have quite a bit of them. The two below are of an SRT-4 with a 60-1 running on an otherwise stock car with only 12 psi of boost. The timing stays pretty high, but considering when the boost finally starts to build and the maximum amount it's about right (but a stock turbo car and the big turbo cars running higher boost will be different).

The short term (STFT) and long term (LTFT) are in the lower right corner. Video was taken on a digi cam and the resolution isn't great. Also the sampling rate what I data log to a computer is much faster, but this should still give a little bit of an idea showing how they can change and being applied at WOT.

Run 1 and run 2.
(Encoded in QuickTime as MP4. Approximately 1-2 MB. Right or Control-click and save/download.)

When "tuning" a car and working with fuel pressure and piggybacks I also wouldn't worry too much about the fuel trims as long as the values aren't getting too large in one direction. The NGC is very complex and it's going to be hard to figure out exactly why it's adjusting them in the first place. But they are a good indicator of your fueling system and any changes you've made that alter how the PCM will respond.

When you watch some people run out and buy 750 cc injectors along with a fuel kit that can alter pressure, it makes me wonder sometimes why go so big of an injector if you have to run a very low fuel pressure, pull a ton of fuel with the piggyback (and get even more timing with your big turbo), as well as seeing the PCM freak out and still try to take fuel away (i.e. fuel trims). Using the fuel trims for that type of guidance is what they're most useful. They should not affect timing but they can vary fuel somewhat. You might notice it more after a PCM reset when the adaptives are cleared and haven't re-learned yet.

And the long term fuel is (primarily) based on the short term. The only time I'd be looking at short term is when working on drivability issues out of boost and/or still in closed loop operation. That's where it's the most helpful. On issues of stumble I initially was leaning towards a fueling or control issue but it may very well be a combination of factors. I think the fuel system is important, but also if you're essentially relying on a stock/Stage PCM for engine management you want to keep it working as much as you can like it would be working with a stock turbo. You might be able to alter it to work better with a big turbo but you'd need to be careful that it's still within what it considers normal operating parameters.

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post #25 of 74 (permalink) Old 07-25-2006, 02:41 PM
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Quote:
Originally Posted by Lagzilla
Quote:
Originally Posted by never2muchboost
hmm i could be wrong but as long as your lt adaptive is below 30% i dont think that there is any difference in timing.i never bother tuning for driveablilty unless im using bigger than 750 injectors.there have been instances though where its pretty close and if im pulling 30% at wot i have to drop the fp because timing gets low.the long term is a broad learned value .i try to pay more attention to short term.i havent really noticed any performance differences in driveability by getting the fuel trim close to 0.but i havent spent all that much time tuning for driveability. i think i am gonna try that on the dyno and see if it really does make a difference.either way the adaptive memory has nothing to do with the big turbo stumbles.i spent tons and tons of time when the stumbles first started showing up trying to get rid of them.i thought the same thing at first,and i had noticed that when i log it with a drb it was always when the pcm switched from one cell to another we were getting a lean spike.still got it even when driving it and forcing both cells to match in numbers.vector and aaron were the ones that realized it was the fuel system restriction that was causing the stumbles.since then any of them that have the stumbles ive been removing the bottleneck and the stumbles are gone.we had a 7 million post discussion about it

why would u pay attention to short term fuel trims? they make no differences because they are based on the long term fuel trims

for example -3.0 on short term with a ltft of -9 is -3 on top of the -9
notice that the short term fuel trims like to stay around zero? while long term will jump out into double digits?

the reason id want to get it as close to zero as possible is because it allows the computer to have the leway it was designed to have for daily driving.

doing this will eliminate the stumbles and bogs those that run fuel kits get while daily driving because the car is running the "fuel talbes" in a way that it was designd.

in my experience the range of change in LTFT between teh cells is what causes the stumble, switching in a great ammount from say -17.9 cell one to -4 cell 2 causes the quik glitch

just an experiment, mb not related to it at all.

because the factory fuel table is setup to run a bit richer at idle, one cant just pull a certain map voltage across the board.

doing so will force the computer to over compensaite LTFT therefor affecting cells and transition

(from my experience) in one instance


if im wrong some one point me in the write direction, these are just conclusions i have come to while monitoring LTFT v. STFT

note: i also did the bottle neck fix stumbles were not "all gone" still had an idle stumble when the rpms would de-celerate and come to in idle rpm, the car still had a bog stumble.

erik's(wowguysrt) car does not have the bottle neck fix and does not have "the stumble"

you say you havent spent any time tuning daily driving or seeing what affects LTFT had on teh dyno yet you go on to say you spent so much time trying to figure out what the stumble was caused by. well how can u even say that its not caused by that if u never tuned your long term fuel trims or daily driving???


another point to your post - the more map you pull timing wont go lower heh that would be great, in reality timing gets higher because the timing equation the pcm uses is based on map volage, less boost = more timing. sooo if u pull too much your just going to get more timing (BAD! on big turbo cars) if your timing is being lowerd its the cause of Kock Retard(the ammount pulled from the total timing in degree's to eliminate knock)

that happens when your tune is too aggresive and causing knock
when i say i pay attention to short term i reffering to when im trying to find a driveability issue and the reason why i am doing it is because that is what the pcm is doing at the moment and thats what im concerned about.when i said i spent all that time trying to figure out the stumble i didnt mean i was on a dyno,that was on the street and yes it is when there is a switch between cells.i said that..but like i said even when i forced those cells to have equal values i still got the stumbles.the bottleneck fix has gotten rid of all the stumbles ive come across.the one you are explaining sounds like a different issue.we spent a ton of time trying to figure out the stumble initially we werent tuning driveability with a purpose,trying to find a problem rather.when i say that corrections that high are pulling timing thats the reason why because its too aggressive and pulling it back.it seems like youre picking apart what i say but im on your side dude and just sharing what ive seen

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post #26 of 74 (permalink) Old 07-25-2006, 03:00 PM Thread Starter
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ya but the short term isnt for example -3 off zero directly, its -3 off the ltft thats what makes no sense u really have no idea what its doing because your basing stft off a guess that your ltft is zero when it more than likley is not

equal short terms arent really equal is what im saying


because -3 on stft on zero is different from -3 on a -15 ltft

so basing your tune on stft is erroneous


your not really on my side (no offense) your tuning solly based on your stft while not paying attention to ltft thats blind tuning because your stft are based on ltft that is the mistake

what im saying is that oen must get long term fuel trims as close to stock as possible

stock ltft are close to zero and do not range in change between cells as much as those that get the "2400-2600 rpm stumble" have their ltft dialed at

again, stft's have no affect even if equal because they are not equal they are set upon the long term fuel trim

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post #27 of 74 (permalink) Old 07-25-2006, 03:11 PM
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Here are the different "cells" that the NGC uses for the fuel trims. You can think of a cell as a range of values, in this case based on a certain range between rpm's and load (MAP voltage). There are a total of 16 cells, but only 12 used when the throttle is open. They are as follows:

Idle
Two cells, based on TPS.


Decel (throttle closed)
Two cells, based on TPS, rpm's, and speed.


Open Throttle
Twelve cells, six for high rpm and six for low. Based on MAP voltage range.

Below 1,984 rpm's
(1.) 0 - 1.3 volt MAP
(2.) 1.4 - 1.9 volt MAP
(3.) 2.0 - 2.5 volt MAP
(4.) 2.6 - 3.2 volt MAP
(5.) 3.3 - 3.8 volt MAP
(6.) 3.9 - volt MAP

Above 1,984 rpm's
(1.) 0 - 1.3 volt MAP
(2.) 1.4 - 1.9 volt MAP
(3.) 2.0 - 2.5 volt MAP
(4.) 2.6 - 3.2 volt MAP
(5.) 3.3 - 3.8 volt MAP
(6.) 3.9 - volt MAP

Note that according to the FSM these are typical values for the NGC. They might have been tweaked or slightly different in the 2.4L turbo. That could also be the case with the "Stage" PCM's, but if you're monitoring on a DRB-III or other aftermarket scan tool with a fast refresh you should be able to catch when the change occurs. If you're really concerned about it then I'd look into that method for verification.

Also another thing I was reminded about when checking the info was that the PCM will momentarily go to open loop and shut down the fuel injectors on harsh deceleration (such as when the throttle snaps shut and MAP voltage drops) and shut off the fuel injectors. If you see a lean spike that might be a possible cause as it's done on a stock turbo car for emission reasons.

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post #28 of 74 (permalink) Old 07-25-2006, 03:21 PM
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Originally Posted by Lagzilla
ya but the short term isnt for example -3 off zero directly, its -3 off the ltft thats what makes no sense u really have no idea what its doing because your basing stft off a guess that your ltft is zero when it more than likley is not

equal short terms arent really equal is what im saying


because -3 on stft on zero is different from -3 on a -15 ltft

so basing your tune on stft is erroneous


your not really on my side (no offense) your tuning solly based on your stft while not paying attention to ltft thats blind tuning because your stft are based on ltft that is the mistake

what im saying is that oen must get long term fuel trims as close to stock as possible

stock ltft are close to zero and do not range in change between cells as much as those that get the "2400-2600 rpm stumble" have their ltft dialed at

again, stft's have no affect even if equal because they are not equal they are set upon the long term fuel trim
read what im saying........im not tuning off stft, I USE STFT TO DIAGNOSE A DRIVEABILITY PROBLEM THAT IS ALLWHEN I TUNE ,IM LOOKING AT TIMING ,INTAKE TEMPS AND FUEL.and a few other things

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post #29 of 74 (permalink) Old 07-25-2006, 03:35 PM
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Okay guys heres a simple question LOL.....I have a wideband, therefore I took the stock o2 sensor off and plugged thw wideband in its place....does this mean that my STFT and LTFT are going to be off...

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post #30 of 74 (permalink) Old 07-25-2006, 04:03 PM
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Quote:
Originally Posted by Lagzilla
equal short terms arent really equal is what im saying

because -3 on stft on zero is different from -3 on a -15 ltft
...
again, stft's have no affect even if equal because they are not equal they are set upon the long term fuel trim
I've read it a few times and I'm still not following what you're saying. The long-term is adjusted by the PCM to keep short-term closer to zero/no correction. The PCM adjusts long-term based on the short-term adjustments, and the short-term is based on the O2 feedback. And just because you have +5% fuel in short-term doesn't mean it will automatically change long-term to +5% either and short-term back to zero every time. You can use short -term for drivability issues and light acceleration/boost to see what your fuel system is doing. And if you wanted to you could take a look at WOT in open loop to see if the long-term values are ever changing based on adjustments made by the PCM in closed loop to the long-term.


The short-term and long-term can both add and remove up to a total of 25% of the pulse-width. That's what the numbers you're seeing on a scan tool mean if it is displaying a percentage value. A positive number is the PCM adding extra pulse-width (add fuel/richen) and a negative number is removing pulse-width (remove fuel/lean out). Pulse-width is the time in milliseconds the injector is on and can also be converted to duty cycle.

After startup the PCM uses short-term correction (which means O2 feedback) in open loop warm-up when the engine coolant reaches ~30-35F. When the engine goes into a closed loop mode and is above ~170-190F it will adjust or allow the long-term adaptive correction values to be updated based on the short-term direction of movement/correction. It's important to note again that the long-term adaptive memory and cells are used in all operating conditions (including open and closed loop modes) but only change when above that temp, in closed loop, and when the engine has been running for over two minutes.

The long-term values are adjusted by the PCM in order to keep the short-term as close to zero (no correction) as possible. It probably has an algorithm in the programming that alters the long-term if it continually sees short-term in a certain cell having to add/remove fuel, how much it add/removes, and how long the short-term has been making corrections. The long-term can add/remove up to 25% pulse-width, so if the LTFT max's out, the short-term can still alter an additional 25% when in closed loop (O2 feedback) for a total of 50%. But when you got to WOT (open loop/no O2 feedback) it will only be using LTFT and subsequently a max of 25% change in pulse-width.

During acceleration the car is still in closed loop mode with the O2 feedback being used (which means short-term values are adjusted). This might occur when you roll onto the throttle while cruising and start building boost but aren't at WOT. That's a likely scenario of how the short-term and eventually the long-term get adjusted under boost. When you go to WOT it reverts to open loop and only reads (not modifies) the LTFT. Since the last cell range is for 3.9 volts and above, that correlates to about 12 psi and above on a stock/S1 car.

Also the short-term memory is lost when you shut the car off. The long-term adaptive memory is held as long as you don't disconnect power NGC.

Eric H. - '04 Neon SRT-4 (eBlue/S2) - '04 SRT-4 (blk/stock)
'92 Dodge Daytona IROC R/T --- -- '91 Dodge Spirit R/T
plus a couple other turbo Dodges, a Cobalt, a couple Focus, a V-wagon, and some other stuff...
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