Stuttering @ AutoX around 4-5000rpm in 2nd

Subie Roo · 02-28-2005, 05:08 PM

Yesterday I took the RS out to Autocross for the first time - one of my friends codrove with me, for a total of 8 runs (4 each) - there was a 180 degree turn at the far end of the course, and immediately following this turn, there was a fast straight - on the last three runs we made with the car, we encountered a shuddering/stuttering right around 4500rpm. The car's driven perfectly fine otherwise, and drove back with no hitches, despite driving it a little hard on the backroads on our way home.

The car is at 65k has the following engine mods - intake snorkel & airbox removed and a catback exhaust w/ magnaflow muffler.

Can anyone help me diagnose this? It probably added about half a second to our last runs, so I'd like to be able to work this out before the next event. Thanks!!

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munkis · 02-28-2005, 07:34 PM

Probably fuel starvation, how much gas did you have?

Jay

Subie Roo · 02-28-2005, 07:46 PM

Plenty, about half a tank.

I've had the stuttering issue happen to me a couple times coming from a stop light too, but never as pronounced as at the autocross. It hasn't happened since, either.

Could it possibly be a grounding issue? I noticed a loose ground behind the front bumper.

munkis · 02-28-2005, 07:53 PM

even though you may think half a tank is plenty, its not. I get fuel starvation over half a tank during auto-x, unfortunatly I have to auto-x with a full tank.

Since it was your first auto-x, next time fill up before you run and see if it goes away, you will probably find that it does.

I lost 1st place once because of fuel starvation, it was my first auto-x with my current RS and I didnt think it would happen because I had over a half tank.

Next auto-x i filled up and it was fine.

the 180 sloshed all the fuel to one side starving the pump, when you went to accelerate onto the strait you felt the affects of air in the fuel lines.

Jay

Subie Roo · 02-28-2005, 08:26 PM

If it matters, it was a right-hand turn - a vid of the course can be found here, showcasing the FTD '04 STI - but you can see the final hard right before entering the straight and slalom at around 0:35 or so.

I will try that though - what was interesting was for me, the stuttering didn't happen until I'd already been accelerating for a bit into the straight, probably a few hundred feet halfway to the slalom, while for my codriver it happened coming right out of the turn.

deuce.five · 02-28-2005, 08:28 PM

Quote:

Originally Posted by McDade

even though you may think half a tank is plenty, its not. I get fuel starvation over half a tank during auto-x, unfortunatly I have to auto-x with a full tank.

Since it was your first auto-x, next time fill up before you run and see if it goes away, you will probably find that it does.

I lost 1st place once because of fuel starvation, it was my first auto-x with my current RS and I didnt think it would happen because I had over a half tank.

Next auto-x i filled up and it was fine.

the 180 sloshed all the fuel to one side starving the pump, when you went to accelerate onto the strait you felt the affects of air in the fuel lines.

Jay

This happened to me also, fuel starvation with between 1/2 & 3/4 tank. I never let it get below 3/4 tank at the auto-x. It seemed to happen after long right-handers.

Subie Roo · 02-28-2005, 08:36 PM

Well two first-hand accounts is enough for me to believe it, consider the video above as a token of gratitude

mofugga · 02-28-2005, 09:04 PM

definately a fuel starve issue. i need atleast a half tank tho i've found i can run with less if i drive extra smooth. it's just easier to find that right amount of gas so you don't have to deal with the fuel starve. some RS's it's worse on than others

Storm · 02-28-2005, 10:55 PM

Note that the more suspension tuning you dial into the car, the more gas you have to run. If our needle is anywhere near 3/4, we'd get starvation.

The thing about this situation though, is the fact that it didn't happen until around 4500RPM. How much time elapsed from the tight right until you felt the car shudder? Usually, it'll happen pretty quickly....as you exit the corner. Or in some cases midcorner as you roll into it.....

Jay Storm

Subie Roo · 03-01-2005, 11:21 AM

Well it wasn't a pivot, more like a sweeping right. The course was pretty quick, so I don't think we slowed down to less than 20mph while coming through that turn.

My codriver took the turn a little slower & tighter, and felt the shudder about 500rpm sooner than I did, directly out of the corner - while I tried to be a little smoother with it and took it a little faster, so I figure that would be the reason for the discrepancy.

-Cliff

Storm · 03-01-2005, 06:08 PM

Quote:

Originally Posted by Subie Roo

Well it wasn't a pivot, more like a sweeping right. The course was pretty quick, so I don't think we slowed down to less than 20mph while coming through that turn.

My codriver took the turn a little slower & tighter, and felt the shudder about 500rpm sooner than I did, directly out of the corner - while I tried to be a little smoother with it and took it a little faster, so I figure that would be the reason for the discrepancy.

-Cliff

Well it would appear that starvation is your answer. Keep the tank full when you run and curse Subaru for such a pisspoor tank design.

Jay Storm

Evegan · 03-11-2005, 09:20 AM

Cliff, it's fuel starvation. I had this problem at every auto-x I did until I filled the tank up. Believe it or not the car would rotate better on a full tank.
I remember one time I was loosing to Tony by a second or so and I was getting the shuttering/jerking after a tight right hander, during lunch I went and filled up the tank and came back and beat him by a full second or so.
Hope you're enjoying the car.
Evan

02-28-2005, 05:08 PM	#1
Subie Roo Scooby Newbie Member#: 79011 Join Date: Jan 2005 Location: Christiansburg, VA Vehicle: 2000 Impreza 2.5RS Blue Ridge Pearl	Stuttering @ AutoX around 4-5000rpm in 2nd Yesterday I took the RS out to Autocross for the first time - one of my friends codrove with me, for a total of 8 runs (4 each) - there was a 180 degree turn at the far end of the course, and immediately following this turn, there was a fast straight - on the last three runs we made with the car, we encountered a shuddering/stuttering right around 4500rpm. The car's driven perfectly fine otherwise, and drove back with no hitches, despite driving it a little hard on the backroads on our way home. The car is at 65k has the following engine mods - intake snorkel & airbox removed and a catback exhaust w/ magnaflow muffler. Can anyone help me diagnose this? It probably added about half a second to our last runs, so I'd like to be able to work this out before the next event. Thanks!! * Registered users of the site do not see these ads.

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02-28-2005, 07:34 PM	#2
munkis Scooby Guru Member#: 1847 Join Date: Jul 2000 Chapter/Region: Tri-State Location: brokenmotorsports.com Vehicle: Team Broken: V6 GF rallyarmor.com	Probably fuel starvation, how much gas did you have? Jay

02-28-2005, 07:46 PM	#3
Subie Roo Scooby Newbie Member#: 79011 Join Date: Jan 2005 Location: Christiansburg, VA Vehicle: 2000 Impreza 2.5RS Blue Ridge Pearl	Plenty, about half a tank. I've had the stuttering issue happen to me a couple times coming from a stop light too, but never as pronounced as at the autocross. It hasn't happened since, either. Could it possibly be a grounding issue? I noticed a loose ground behind the front bumper.

02-28-2005, 07:53 PM	#4
munkis Scooby Guru Member#: 1847 Join Date: Jul 2000 Chapter/Region: Tri-State Location: brokenmotorsports.com Vehicle: Team Broken: V6 GF rallyarmor.com	even though you may think half a tank is plenty, its not. I get fuel starvation over half a tank during auto-x, unfortunatly I have to auto-x with a full tank. Since it was your first auto-x, next time fill up before you run and see if it goes away, you will probably find that it does. I lost 1st place once because of fuel starvation, it was my first auto-x with my current RS and I didnt think it would happen because I had over a half tank. Next auto-x i filled up and it was fine. the 180 sloshed all the fuel to one side starving the pump, when you went to accelerate onto the strait you felt the affects of air in the fuel lines. Jay

02-28-2005, 08:26 PM	#5
Subie Roo Scooby Newbie Member#: 79011 Join Date: Jan 2005 Location: Christiansburg, VA Vehicle: 2000 Impreza 2.5RS Blue Ridge Pearl	If it matters, it was a right-hand turn - a vid of the course can be found here, showcasing the FTD '04 STI - but you can see the final hard right before entering the straight and slalom at around 0:35 or so. I will try that though - what was interesting was for me, the stuttering didn't happen until I'd already been accelerating for a bit into the straight, probably a few hundred feet halfway to the slalom, while for my codriver it happened coming right out of the turn.

02-28-2005, 08:36 PM	#7
Subie Roo Scooby Newbie Member#: 79011 Join Date: Jan 2005 Location: Christiansburg, VA Vehicle: 2000 Impreza 2.5RS Blue Ridge Pearl	Well two first-hand accounts is enough for me to believe it, consider the video above as a token of gratitude

02-28-2005, 09:04 PM	#8
mofugga Scooby Specialist Member#: 19540 Join Date: Jun 2002 Chapter/Region: South East Location: Clearwater, FL Vehicle: 2000 old skool RS BRP	definately a fuel starve issue. i need atleast a half tank tho i've found i can run with less if i drive extra smooth. it's just easier to find that right amount of gas so you don't have to deal with the fuel starve. some RS's it's worse on than others

02-28-2005, 10:55 PM	#9
Storm Scooby Guru Member#: 5218 Join Date: Mar 2001 Chapter/Region: MWSOC Location: SAUL'S Motorsports Vehicle: 96L Most Over- Developed Beater	Note that the more suspension tuning you dial into the car, the more gas you have to run. If our needle is anywhere near 3/4, we'd get starvation. The thing about this situation though, is the fact that it didn't happen until around 4500RPM. How much time elapsed from the tight right until you felt the car shudder? Usually, it'll happen pretty quickly....as you exit the corner. Or in some cases midcorner as you roll into it..... Jay Storm

03-01-2005, 11:21 AM	#10
Subie Roo Scooby Newbie Member#: 79011 Join Date: Jan 2005 Location: Christiansburg, VA Vehicle: 2000 Impreza 2.5RS Blue Ridge Pearl	Well it wasn't a pivot, more like a sweeping right. The course was pretty quick, so I don't think we slowed down to less than 20mph while coming through that turn. My codriver took the turn a little slower & tighter, and felt the shudder about 500rpm sooner than I did, directly out of the corner - while I tried to be a little smoother with it and took it a little faster, so I figure that would be the reason for the discrepancy. -Cliff

03-11-2005, 09:20 AM	#12
Evegan Scooby Newbie Member#: 16578 Join Date: Mar 2002 Location: Greensboro, NC Vehicle: 2000 Outback - 89E30 2003 NCAC DSP Champ	Cliff, it's fuel starvation. I had this problem at every auto-x I did until I filled the tank up. Believe it or not the car would rotate better on a full tank. I remember one time I was loosing to Tony by a second or so and I was getting the shuttering/jerking after a tight right hander, during lunch I went and filled up the tank and came back and beat him by a full second or so. Hope you're enjoying the car. Evan