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Old 06-26-2002, 03:44 PM   #1
johnb
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Default Do exhaust and/or headers help reduce overly sensitive throttle ("lurching")?

One of the things I have always hated about my '01 RS is the overly sensitive throttle response. If you let off the gas pedal even a little bit, the car dives and slows down immediately, making it difficult to drive smoothly in traffic. While this makes the car more responsive, it's also a bit ridiculous.
Adding an intake reduced this "lurching" a bit, so I was curious if those of you with aftermarket mufflers and/or headers noticed a difference, or if something else is contributing to the problem.
Thanks!
John
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Old 06-26-2002, 03:58 PM   #2
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i think what you are actually feeling are the springs in the clutch disk that cause the unsmooth feeling. the only reason i say this is because it's MUCH worse with my ACT clutch and it hink what is happening is those springs are compressing then releaseing causeing that bounce...and yes it's VERY irritating sometimes, especially when you have a passenger.

jeremy
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Old 06-26-2002, 04:36 PM   #3
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I have the Greddy cat back and it doesn't have any effect at all. Still try's to break your neck at a slow pace.
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Old 06-26-2002, 04:51 PM   #4
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Hmmm...interesting. So I wonder if an STi transmission mount (or a full set of engine mounts) would help?
I test drove a WRX and it did the same thing, only it was much more tolerable.
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Old 06-26-2002, 05:11 PM   #5
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What you are feeling is slack in the driveline. This has nothing to do with throttle response or the engine whatsoever. It's more prominent in 4WD and AWD vehicles.

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Old 06-26-2002, 05:30 PM   #6
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I thought that this was mostly a tranny/engine mount problem...
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Old 06-26-2002, 06:11 PM   #7
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When I removed the silencers and constrictive air intake tubing, and installed a tube running from the fender to the airbox with a K&N filter inside that problem seemed to be reduced dramatically.
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Old 06-26-2002, 06:34 PM   #8
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That's exactly what I've done, and it did make an improvement. That's how I came to wonder if a freer flowing exhaust would help any further. The idea that it is driveline slack makes sense, especially with the extra driveshafts and rotating mass an AWD car has. Also, it's been my experience that cars with high final drive ratios have similar effects.
Ultimately, I'm looking for ways to reduce it to a reasonable level for all-around driving. Some responsiveness makes driving more enjoyable, but too much makes it a chore!
Thanks for the replys, and let us know if anyone has any more ideas!
John
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Old 06-26-2002, 06:40 PM   #9
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The thing that really helped my car, was engine tuning. I added an aftermarket ECU and I have a little more fuel on decel than stock. It seems like the ECU totally 100% cuts fuel as soon as you lift. My car felt like I was at the rev limiter when ever I lifted my foot.
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Old 06-26-2002, 06:47 PM   #10
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Quote:
Originally posted by Corey
What you are feeling is slack in the driveline. This has nothing to do with throttle response or the engine whatsoever. It's more prominent in 4WD and AWD vehicles.

Corey
What's slack in the driveline? And my Impreza is FWD, but still has that awful lurching. My GF thought I couldn't drive stick but I kept telling her it's the car.
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Old 06-26-2002, 07:58 PM   #11
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ENGINE BRAKING

when you let off the throttle your motor is still rotating. the compression inside the engine acts as a "brake" to slow down your car (when the clutch is engaged)

you all know this, so think it through.

Freeflowing exhaust=less backpressure=easier for exhaust to escape=less engine braking effect.

thats why a good exhaust lessens the lurch.
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Old 06-26-2002, 08:07 PM   #12
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Prez&SUS,

Regarding "Freeflowing exhaust = less backpressure = easier for exhaust to escape = less
engine braking effect.
thats why a good exhaust lessens the lurch."

I agree with this in theory, which is why I started this post. But I was hoping someone who has an aftermarket exhaust could tell me if it actually did help reduce the lurch before I spend my money. Which exhaust do you have?

Thanks!
John
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Old 06-26-2002, 09:31 PM   #13
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Yes! An aftermarket exhaust will reduce the "lurch" that you get when you lift off the throttle. I had a Ganzflow CAI alone, and that did not affect the lurching, but my Stromung axle-back has practically eliminated the bucking I used to get when I would lift off the throttle. When I saw this thread I went out for a test drive just for kicks, and without a doubt, the high flowing exhaust makes a huge difference.
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Old 06-26-2002, 11:29 PM   #14
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mods that will reduce torque at part throttle will help this. I have extensive mods that kill lowend torque and my car is incredibly smooth on and off the throttle at <3000 RPM. Headers, 2.5' exhaust cams, ect.

If you are talking about really slow ideling in gear, that is all the slack and sprung clutch working together to give you a really nice electric bull ride.

cheeRS,

Greg
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Old 06-27-2002, 02:36 AM   #15
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Quote:
Originally posted by johnb
Hmmm...interesting. So I wonder if an STi transmission mount (or a full set of engine mounts) would help?
I test drove a WRX and it did the same thing, only it was much more tolerable.
nope...i have them...doesn't correct it, makes it slightly better but it also makes it just as harsh sometimes. and i don't care what these guys say...you are still most likely going to have that lurch because it's not from exhaust pressure or intake vacuum, it's a mechanical movement...he's not talking about snapping the throttle shut at high RPM, he's talking about low RPM surging caused by the torsion springs in the clutch disk.

jeremy
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Old 06-27-2002, 10:39 AM   #16
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This is what I assume he was talking about...

He said driving in traffic. My car, when stock, cruising around 2-3000 RPM (at 5-20% throttle) was not always easy to be smooth. The torque was easy to make the car accelerate slightly if you are not paying attention, correcting for the increase was to let off the throttle a little bit. THis causes a very unpleasent experiance for the passengers.

The excessivly low RPM "bucking" (coming to a semi stop in 2nd gear and not haveing the RPMs to get going again) definitly contains clutch springs an driveline lash bouncing (there are ALOT of gears, and the diameter of the wheel/tire amplifly it, along with the front and rear differentials due to their gearing). Gear lash is not the primary cause, but it is certainly a part of it.

THe exhaust isn't the cause, its the loss of lowend torque from a bigger exhaust that eliminates the cars desire to jump everytime you lightly apply accelerator.


cheeRS,

Greg
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Old 06-27-2002, 06:38 PM   #17
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My 1.8L doesn't make much torque but still I have low RPM bucking. So you think it's the restrictive stock exhaust system that's causing this and not the engine and tranny moving when you accelerate? I was hoping to get the upgraded mounts to help this go away, but apparently that doesn't do much either. I guess I would trade some low-end power for smoothness and more high-end.
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Old 06-27-2002, 08:59 PM   #18
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Quote:
when you let off the throttle your motor is still rotating. the compression inside the engine acts as a "brake" to slow down your car (when the clutch is engaged)
Anybody ever heard of compression braking? If you've ever done any serious off-roading (i.e. rock crawling), you know the benefits of compression braking.
Quote:
you are still most likely going to have that lurch because it's not from exhaust pressure or intake vacuum, it's a mechanical movement...he's not talking about snapping the throttle shut at high RPM, he's talking about low RPM surging caused by the torsion springs in the clutch disk.
Yes, it is true that the bucking itself is due to mechanical movement of the drivetrain (drivetrain lash) - it's a simple spring-mass-damper system (basic physics). But when you lift off the throttle, even at moderate to low rpms, as fuel into the cylinder is reduced, the engine no longer has enough power to equal or overcome the losses due in part to compression (thus the drop in rpms).
It's a well known fact on this board (at least I thought it was) that our factory exhaust are very restrictive. So it's not a major leap to think that a less restrictive exhaust will reduce the resistance to flow of exhaust gases out of the cylinder, and therefore, reduce compression braking and the initial lurch the stock car has when lifting off of the throttle. But what would I know? I'm just a geeky engineer.
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Old 06-27-2002, 10:33 PM   #19
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I thought part of it had to do with not just the fuel cut but also the igniton timing.

Brad
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Old 06-28-2002, 12:18 PM   #20
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I reread the first post again. I had misread it initially, refering more to "throttle response" ie, both on and off, (and I spent my time on "on" because that was my original gripe ), instead of just "letting off" that the original poster had expressed concern over.

Quote:
Anybody ever heard of compression braking? If you've ever done any serious off-roading (i.e. rock crawling), you know the benefits of compression braking.
Compression breaking doesn't have much to do with the exhaust. The compression stroke happens with the valves closed, so there is no effect of the exhaust in the compression stroke.

Quote:
But what would I know? I'm just a geeky engineer.
Congrats. Alot of people change majors. Not to be a dick, but having that particular piece of paper doesn't mean that you know EVERYTHING.

What compression braking is, is the engine producing less power than the parisitic losses generated inside the engine, with little power from the other cylinder that is fireing at that moment with very little air and fuel in it due to the closed throttle that basically creates a vacuum. Imagine how much engine braking you can have if you managed to plug the intake instantaniously at high RPM. The pumping losses would be ENOURMOUS, you could probably suck some oil right out of your engine, if nothing colapsed.

The "lurching", if you are refering to the (primitive description) back and forth movement of the car when you quickly pick up your foot, it is the gear lash and the sprung clutch. Try lifting off the throttle in a more controlled mannor instead of just picking your foot up quick. This is not always possible, but it makes the trasition between accel and decel much smoother and the springs will not compress so much and reduce the "bounce" or eliminate it.

cheeRS,

Greg
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Old 06-28-2002, 12:28 PM   #21
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Oh, another thing.

While you can force diesel engines to brake by severly restricting exhaust (Jake brake), the difference between back pressures of a 1 7/8" exhaust and 2.25 when the throttle is CLOSED, are not much different (even with the difference in mufflers) to make a real difference in the bounce (what I am taking as 'lurch") when you let off the throttle quickly.

You probably would notice some difference in engine braking with a significantly larger exhaust, but not the extent of a diesel engine with a exhaust brake, where it would freewheel with out.

cheeRS,

Greg
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Old 06-28-2002, 05:25 PM   #22
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Quote:
Compression breaking doesn't have much to do with the exhaust. The compression stroke happens with the valves closed, so there is no effect of the exhaust in the compression stroke.
Granted my post was overly simplified regarding compression braking and the effects of backpressure on the engine's performance characteristics, but let me suggest this - why don't you go put your stock exhaust back on your car, drive it around for a while, carefully noting the vehicle's response when you lift off of the throttle. Then, put the aftermarket performance exhaust on the car, drive it around for a while again carefully noting the vehicle's response when you lift off of the throttle, and then tell me there is no reduction in the "lurch" the vehicle exhibits with an aftermarket exhaust. I just did this a week ago, and the difference is very noticable!

If you would read the first post, the question was the nosedive the car takes when you lift the throttle.
Quote:
If you let off the gas pedal even a little bit, the car dives and slows down immediately, making it difficult to drive smoothly in traffic.
Quote:
What compression braking is, is the engine producing less power than the parisitic losses generated inside the engine, with little power from the other cylinder that is fireing at that moment with very little air and fuel in it due to the closed throttle
Exactly. And one of the parasitic losses generated inside the engine, is resistance to flow of exhaust gases out of the cylinder. On the exhaust stroke, the piston acts like a pump. The higher the resistance to flow in the exhaust piping, the harder the engine is having to work to pump the exhaust gases out. Thus, the higher flowing exhaust reduces the resistance to flow out of the engine, reducing the parasitic losses, and (by your definition) reducing the effect of compression braking! Therefore, less lurching!
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Old 06-28-2002, 06:44 PM   #23
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There is less air and fuel in the cylinder when the intake is operating under vacuum. This causes less exhaust, making the difference in pipe diameter insignificent at low RPM. If you severly restrict the exhaust you can build pressure in the cylinder with exhaust that is unable to exit (ala exhaust brake) and gain higher pumping losses. While there is a difference, I cannot see it being smooth with a slightly larger pipe while it is "lurching" with the stock pipe. If you cruise at 4-6000 RPM I could see it making a difference.

My car never "nosedived" unless I was at 6000 in first gear and slammed the throttle shut during an Auto-X. Maybe the poster/you have brake dragging problems, or bad wheel bearings, or forgot to take the parking brake . I just don't know what the "lurch" is that you guys are talking about then, the car just bounces twice when I slammed the throttle shut at low RPM, it never nose dived.

My car won't go back to stock that easy to test it.

cheeRS,

Greg
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Old 06-28-2002, 10:42 PM   #24
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Quote:
There is less air and fuel in the cylinder when the intake is operating under vacuum. This causes less exhaust, making the difference in pipe diameter insignificent at low RPM.
If the difference in pipe diameter is insignifigant at low RPMs as you say, then why did you say this a few posts ago?
Quote:
its the loss of lowend torque from a bigger exhaust that eliminates the cars desire to jump everytime you lightly apply accelerator.
Oops! There seems to be a contradiction! In fact, some of us on this board may remember that SCC magazine tested some exhausts on an RS a couple of years ago, and If I remember correctly, the stromung showed an increase in performance pretty much across the powerband including low-end. I think the scoobysport muffler also shows low end gains (in addition to high end gains). Keep in mind, both of these exhaust are "bigger" than stock. But I'm sure you could still disagree with me.
Oh, and I think you could be right. I do frequently "forget my parking brake." Just last night, I left it lying on my couch as I went out for a drive.
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Old 06-29-2002, 02:09 AM   #25
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That was ealier when I thought the lurch was when he was applying the throttle. I didn't like that when my car was stock. How can you compare those two when the dynamics are totally different???

Applying throttle:

Larger volume of stagnent gas needing to be accelerated out of the now larger pipe. LESS instant torque. This is the only lurch that I have experianced other than the retard move of pulling the wrong gear at parking lot speed. Electric bull ride.

Lifting off throttle:

What we have recently been discussing.

Makes perfect sense.

While it appears to be the same it isn't. Much like a fuel pump is great at "pushing" fuel but is very poor at "pulling" it.


cheeRS,

Greg
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