change of tune causing HP loss?
Thread Starter
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From: Oklahoma
change of tune causing HP loss?
Long story short - my 996TT was purchased as a MAFLESS setup with an unknown tune and 1000cc Cobb injectors. I repeatedly got CEL with codes 1145 and 1155 for O2 sensors.
My local independent Porsche mechanic (who I've used for 20 years) recommended I reinstall a MAF, reinstall stock injectors and stock FPR, and place a GIAC tune to eliminate several possible reasons for the CEL codes.
Supposedly the GIAC tune was to give me 100hp over stock. After receiving the car back, I took it for multiple full throttle pulls in 1,2,3 gears. I saw only 0.6-0.7bar boost on the stock gauge rather than the 1.1-1.2 I previously saw. The car also feels noticeably slower - no longer a big rush after 3000rpm. So I'm wondering if my car is simply back to stock performance. At least no CELs appeared.
my question is - wouldn't I be getting 1.0-1.1 bar of boost if i truly had a tune giving me 100 more HP than stock? what factors would influence the perceived drop in performance?
My local independent Porsche mechanic (who I've used for 20 years) recommended I reinstall a MAF, reinstall stock injectors and stock FPR, and place a GIAC tune to eliminate several possible reasons for the CEL codes.
Supposedly the GIAC tune was to give me 100hp over stock. After receiving the car back, I took it for multiple full throttle pulls in 1,2,3 gears. I saw only 0.6-0.7bar boost on the stock gauge rather than the 1.1-1.2 I previously saw. The car also feels noticeably slower - no longer a big rush after 3000rpm. So I'm wondering if my car is simply back to stock performance. At least no CELs appeared.
my question is - wouldn't I be getting 1.0-1.1 bar of boost if i truly had a tune giving me 100 more HP than stock? what factors would influence the perceived drop in performance?
Last edited by joseph mitro; May 29, 2026 at 12:16 AM.
Racer
Joined: Apr 2019
Posts: 400
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.6 to .7 is less than stock boost. Could be happening for many reasons. I would look for a boost leak first. Check for obvious places first like the big boost tube connections at the intercoolers, the Y pipe, etc.
Last edited by yvvvz; May 29, 2026 at 12:59 AM.
Drifting
Joined: Jul 2021
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From: New Jersey
If this was my car I would return it to stock and make sure it is running right before going with bigger injectors, tunes, etc but I get that not everyone can work on their own cars or have time constraints (the injectors aren't exactly easy to change like in some other cars)..so..he could return the rest of the car to stock and reach out to a tuner who can comptently scale the stock map with the new injectors to make sure everything is running right..then go from there.
Thread Starter
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Joined: Apr 2002
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From: Oklahoma
@docwyte yes the stock injectors were installed to replace the cobb units
@vtec_ yes thats exactly the train of thought. I did a lot of work to pin the coolant lines but when it comes to diagnosis i leave that to the professionals. I don’t know much about tuning.
I dont see anything that would suggest a boost leak; I just need to talk to my mechanic when he’s available to try and sort it out.
thanks for the input
@vtec_ yes thats exactly the train of thought. I did a lot of work to pin the coolant lines but when it comes to diagnosis i leave that to the professionals. I don’t know much about tuning.
I dont see anything that would suggest a boost leak; I just need to talk to my mechanic when he’s available to try and sort it out.
thanks for the input
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Drifting
Joined: Jul 2021
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From: New Jersey
@vtec_ what is involved with getting a custom tune as opposed to off the shelf tune?
kevins (umw tuning) datalogger and the cobb accessport are the only tuning solutions that i know of that provide the ability to log knock sums by cylinder
there are a handful of cobb certified tuners that deal with porsches
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@docwyte yes the stock injectors were installed to replace the cobb units
@vtec_ yes thats exactly the train of thought. I did a lot of work to pin the coolant lines but when it comes to diagnosis i leave that to the professionals. I don’t know much about tuning.
I dont see anything that would suggest a boost leak; I just need to talk to my mechanic when he’s available to try and sort it out.
thanks for the input
@vtec_ yes thats exactly the train of thought. I did a lot of work to pin the coolant lines but when it comes to diagnosis i leave that to the professionals. I don’t know much about tuning.
I dont see anything that would suggest a boost leak; I just need to talk to my mechanic when he’s available to try and sort it out.
thanks for the input
Not sure I would have swapped the injectors before knowing more.
Thread Starter
Rennlist Member
Joined: Apr 2002
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From: Oklahoma
Ive had the car 4 years. It came with absolutely no records. It ran like a raped ape until now, but with the CEL always on. It was mafless and had straight pipe exhaust (loud) and k&n air filter on the turbos. The turbos and wastegates appear stock. I discovered the cobb injectors when i dropped the engine to pin coolant lines.
my mechanic’s rationale to make the CEL go away was to put it back to “stock” configuration and then use a GIAC tune that would only work with stock injectors and MAF.
my mechanic’s rationale to make the CEL go away was to put it back to “stock” configuration and then use a GIAC tune that would only work with stock injectors and MAF.
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From: denver, co
I'm not a huge fan of the GIAC tune, but it should still be making 1 bar of boost, so you definitely need to leak test the car. The "Big 3" for tuners for these cars are Kevin@Ultimate Motorwerks, Markski and Sam@By Design. Do you still have the original tune? If you put that back in and immediately boost goes back up, then it's the GIAC tune and nothing else.
What was the CEL on for?
What was the CEL on for?
Thread Starter
Rennlist Member
Joined: Apr 2002
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From: Oklahoma
