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Discussion Starter #1 (Edited)
Hi guys, I wanna make my stock 2015 STi premium run S M O O T H, I've driven it a bit over a year and I'm finding certain aspects of the car annoying. Shifter feel, engine bucking in stop/starts (I feel it in 2nd too, if I'm slowly accelerating for the comfort of a passenger, and hit a bump, the engine pulls hard immediately after I back off the throttle), and wandering clutch engagement.

From my googling and browsing the forums, I've come up with a to-do list of mods for my STi. I just wanna know if I'm on the right track with the brands/quality.
  • Cobb double-adjustable Short Shifter + bushings
  • Group N Transmission Mount
  • Cobb/Torque Solutions Pitch Stop Mount
  • Group N Engine Mount? Not entirely decided on whether I need it or not.
Could I also get some suggestions on what to do with the clutch? The OEM clutch doesn't feel too bad, but sometimes it bites ridiculously high (a mere cm from full release) and other times it's right in the middle, I can keep the shifts as smooth as butter if I pretend I'm doing a lazy Sunday drive, but I don't always have to time to baby the clutch and guess where it feels like engaging... e.g. darting across a main road quickly and accidentally dunking my revs into the ground because it's suddenly decided to engage right at the very top (This situation is kinda rare, but sometimes the clutch just radically changes on me and I get it wrong.)
 

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A carbon fiber driveshaft will help with a lot of the jerkiness from a stop. Group N transmission and engine mounts firm up the driveline but they increase NVH, not make it smoother. I installed something as simple as urethane exhaust hangars last week and it made the car feel harsher.

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^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

I liked, but the above needs reinforcement. Pretty much nothing geared toward performance will make to your car smoother except the eliminating any stumble if you experience it, and a CFDS.
 

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Discussion Starter #4 (Edited)
A carbon fiber driveshaft will help with a lot of the jerkiness from a stop. Group N transmission and engine mounts firm up the driveline but they increase NVH, not make it smoother. I installed something as simple as urethane exhaust hangars last week and it made the car feel harsher.
Ah, I see, thanks.
So I'm better off with the carbon fibre driveshaft over the transmission/engine mounts for ride quality improvements. I'm not a huge fan of NVH, but I was prepared to get a little more of it from the bushings and short shifter. How about the pitch stop mount?

^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

I liked, but the above needs reinforcement. Pretty much nothing geared toward performance will make to your car smoother except the eliminating any stumble if you experience i, and a CFDS.
Thanks.
What do you mean by reinforcing the driveshaft(also CFDS)?
 

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Carbon fiber driveshaft will help with the bucking. I love my group N transmission mount as it makes the shifter feel amazing and prevents it from moving around under acceleration. It does increase NVH though so I'm not sure what your definition of smooth is. As for the wandering clutch engagement, I believe this is due to the clutch delay valve. I think in our cars it is built into the master cylinder and is tough to remove but I haven't really gone down that path yet.
 

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Our Firewalls flex like crazy and I was having to deal with inconsistent clutch feel similar to what you are describing. After installing a clutch master cylinder brace. engagment was much more smother and consistent
 

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Discussion Starter #7
Carbon fiber driveshaft will help with the bucking. I love my group N transmission mount as it makes the shifter feel amazing and prevents it from moving around under acceleration. It does increase NVH though so I'm not sure what your definition of smooth is. As for the wandering clutch engagement, I believe this is due to the clutch delay valve. I think in our cars it is built into the master cylinder and is tough to remove but I haven't really gone down that path yet.
My shifter doesn't really move around too much, but I struggle with stop starts occasionally due to a rotator cuff injury (RHD; 1st and 2nd can be a bit of a literal pain when the gearbox decides it's gunna be overly stiff and notchy, it's like shifting a truck sometimes). My hope is that I can make the gearbox feel more effortless, being able to flick from gear to gear with 2 fingers instead having to drive them in. Every now and then I can't even put the car into 3rd without double clutching, and this is just driving at normal speed.

As for the clutch, I did see some stuff on the clutch delay valve causing issues, but people have also suggested a clutch stopper mod. I'm not sure which way to go on this.
 

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Honestly, you need a different car. You are going to spend thousands of dollars you'll never get back for minimal improvement while there are other cars which will meet your needs much better.
 

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Discussion Starter #9
Honestly, you need a different car. You are going to spend thousands of dollars you'll never get back for minimal improvement while there are other cars which will meet your needs much better.
Oof.
I had a think about my epic rant and took a drive around, to be honest I was being a little sensitive. The transmission isn't as bad as I make it out to be, and when the clutch is behaving itself I'm damn good at shifting, I feel I'm painting a masterpiece with my feet. I just need to clutch to stay consistent, and maybe smooth out the notchiness of the gearbox.
 

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what about a tune for your car? your tuner should tune the bogging out by making it richer/leaner at light throttle


keep everything stock except anything fails... try Tyres with a soft sidewall, and or a higher sidewall. like 17s instead of 18s
 

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Discussion Starter #11 (Edited)
Our Firewalls flex like crazy and I was having to deal with inconsistent clutch feel similar to what you are describing. After installing a clutch master cylinder brace. engagment was much more smother and consistent
It's possible this is the issue I'm facing. The previous owners of the car might have stomped on the clutch a little hard or driven it like maniacs. Maybe the firewall is a bit munted.


what about a tune for your car? your tuner should tune the bogging out by making it richer/leaner at light throttle


keep everything stock except anything fails... try Tyres with a soft sidewall, and or a higher sidewall. like 17s instead of 18s
I had it retuned by my local subaru guys at one point, it helped quite a bit. I'm not sure if I wanna do a performance tune, I'm pretty happy with the stock power, and I don't want to increase the wear and tear. I think the majority of my bogging issues are the clutch being inconsistent.

Sometimes the biting point is right in the middle and the car drives perfectly, I can clutch up to the biting point, add revs as I ease it through and then continue to add revs as I release the clutch, and it's very smooth, other times I bring the clutch to the same point, add revs, and as I release the clutch and I'll feel the transmission get a small jolt and bog a little, because the biting point was suddenly way higher, and I didn't give it the time it needed to get to the right speed. I've since learned to baby the clutch ALL the time, but after driving some of my friends cars around (s2000, evo 8, s15 etc.) I kinda just want the clutch to act normal and stay normal.


Would all my clutch troubles go away if I just replaced it with a stage 2/3 and lowered the pedal height? I was told by a subaru guy that sometimes the clutch changes engagement height due to the nature of the hydraulics.
 

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OP I think you’re on the right track. And I have to disagree with those that said Grp.N mounts will increase NVH. I just got GrpN motor and tranny mounts, pitch stop, and tranny bushings done a week ago and I don’t notice any increased NVH at all. In my old GD I did feel some NVH making that switch, but the extra chassis rigidity VAs already have make any increased NVH unnoticeable.

I also got motylgear put in the tranny and shifting is super smooth. With that new oil and GrpN mounts, it’s really way more smooth than stock. Shifts are more precise, smooth, with way less shifter play and jerkiness when letting the clutch go. And power transfer is smoother since it doesnt have to rock the engine/tranny before power gets the wheels moving and it has less rebounding happening too.

Now if you’re looking at full metal mounts, I’m pretty sure those will have noticeable NVH.
 

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you can do everything you want, but these cars will never be super smooth.

Driveshaft does help but still bad between 1-2800k
 

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Strange how the clutch is described as inconsistent.

Maybe need to check if there's bubble inside the clutch fluid line?

Braided clutch line is also available and that is a good way to keep things feels more consistent.

STI jerks at low speed, but at high speed this is one very smooth car to drive fast.
 

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the driveshaft helped in the 2800-4500 rpm, but I didn't notice that much difference in the lower rpm regarding smoothness.

I further added Group N Tranny and Engine Mounts and sill the same, so...

It is what it is. I feel it's the worse when you let off the gas...
 

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Discussion Starter #18
OP I think you’re on the right track. And I have to disagree with those that said Grp.N mounts will increase NVH. I just got GrpN motor and tranny mounts, pitch stop, and tranny bushings done a week ago and I don’t notice any increased NVH at all. In my old GD I did feel some NVH making that switch, but the extra chassis rigidity VAs already have make any increased NVH unnoticeable.

I also got motylgear put in the tranny and shifting is super smooth. With that new oil and GrpN mounts, it’s really way more smooth than stock. Shifts are more precise, smooth, with way less shifter play and jerkiness when letting the clutch go. And power transfer is smoother since it doesnt have to rock the engine/tranny before power gets the wheels moving and it has less rebounding happening too.

Now if you’re looking at full metal mounts, I’m pretty sure those will have noticeable NVH.
Good to hear, I figured the group N stuff wouldn't be too bad since it uses rubber as well, just not as soft as stock.

you can do everything you want, but these cars will never be super smooth.

Driveshaft does help but still bad between 1-2800k
That's ok, I know it's a rally car, but I want the jerkiness to be a result of my own poor driving and take away the external factors.

Strange how the clutch is described as inconsistent.

Maybe need to check if there's bubble inside the clutch fluid line?

Braided clutch line is also available and that is a good way to keep things feels more consistent.

STI jerks at low speed, but at high speed this is one very smooth car to drive fast.
That's a really good idea. Frankly everything will be a million times better if the clutch just stays at the same spot.
I find when the car is cold (Australia cold, so like 8-14 degrees C) and on a flat surface the clutch is quite consistent for a while, but after driving for a while and parking, the clutch engagement height seems to change. It could be a bubble.

the driveshaft helped in the 2800-4500 rpm, but I didn't notice that much difference in the lower rpm regarding smoothness.

I further added Group N Tranny and Engine Mounts and sill the same, so...

It is what it is. I feel it's the worse when you let off the gas...
I thought the pitch stop mount was the important one for early smoothness. Either way, I'll be happy if I can shift a little easier.
 

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Good to hear, I figured the group N stuff wouldn't be too bad since it uses rubber as well,
Most all offer rubber isolated mounts, Cusco, Torque Solution, Perrin, Beatrush . . . and in every case if you can feel the difference in stiffness, you can feel it in NVH.


That's ok, I know it's a rally car,
Why I'm writing. No, its a sporty family car.


everything will be a million times better if the clutch just stays at the same spot.
Eliminate the clutch delay valve. Already suggested and not that difficult - and there is no downside except possibly warranty. Not likely to be noticed if If you are gentle with your tools.


I thought the pitch stop mount was the important one for early smoothness. Either way, I'll be happy if I can shift a little easier.
Stiffer mounts will create NVH. N and V come with H.

ADDED: This is simple physics - people can claim otherwise if they like.
 

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Most all offer rubber isolated mounts, Cusco, Torque Solution, Perrin, Beatrush . . . and in every case if you can feel the difference in stiffness, you can feel it in NVH.
All those use either polyeurethane and/or much much less rubber than the OE design which is why they have noticeable NVH. They’re definitely harder than group N because of it. Now with group n you can feel the rubber is stiffer than stock, but on the VAs it doesn’t affect NVH like it did on the GD.
 
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