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Old 09-09-2010, 02:18 PM   #1
GTAHVIT
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3" Exhaust, Stock Tunnel Brace and Knock Sensors..... Lesson Learned.

I'm sure a lot of you probably know where I'm going with this. But as this is the first car I've had with this much engine work, I'm learning a lot along the way.

So here's what happened.

Pretty much since I got the car from THR, the exhaust bumped the tunnel brace when going over bumps... Not Tom's fault, as I planned on a new tunnel brace, and told Tom not to sweat it. Well I never got a new brace and the more I drove the car the more often it would happen...

No big deal I knew what it was so I just need to order the brace and move on...

Well a couple weeks ago my car just wasn't as snappy as it used to be. I kinda chalked it up to heat or the AC being on... but didn't give it much thought. A few days later, I was messing around on some back roads, and went WOT in 3rd gear and heard a noise, I'd never heard before, and the car was hesitating a little. To me it sounded like a header leak. Not metallic but like an old chevy truck with an exhaust leak. So, I'm thinking I blew a header gasket or, a header bolt came lose. So I check all my bolts, go over the car completely trying to find it.... no luck....

I'm talking to everyone I know... Tom, Speedster, Spike, PQ, Tim@TandT Performance, asking them if it is serious or what.

Pretty much to a man, they all said, put it on a dyno, and chase it down.

So, on the advice of some local Camaro owners here in Jax, I was told to go see Doc, at EFI Alchemy. Doc invited me over and went for a test drive. we hooked up his computer to the OBDII port and off we went. immediately he heard my exhaust bump and asked what it was, I told him about the tunnel brace and he looked at me a little funny, but didn't say anything.... (more on that later).

So, we go WOT, and I thought his computer caught fire... he jumped a little and said.... yeah you've got some serious timing retard.... If I remember, he said almost 18 degrees. I believe he was showing -4 degrees of timing when I got on it.

So, I'm sure the color drained out of my face, and he said.. "well the good news is the computer is doing its job keeping you from blowing up the motor, so you're safe to drive it."

HUGE SIGH OF RELIEF....

Then he says, with that same funny look he gave me earlier, "So... how long has the exhaust been hitting the brace??" I explain it to him... He kinda smiles like he's thinking... silly newb... but in a way that's funny not rude. and says "well, do you know how the knock sensors work?"

I of course blab out, "yeah it checks for detonation!" (probably a little louder than I should have)... He very politely answers.... "uhm yeah. but not exactly..." Doc, then takes the time to educate me and explains that Knock sensors are really just microphones that are tuned to listen for metallic vibrations that could be detonation. Hence why they are called "knock" sensors.. duh .... two pieces of metal knocking together....

So, I say, "cool, that makes sense, I didn't know that, why does it matter?"

He answers, "well you hear that exhaust bump...." and in my head it's starting to dawn on me.... "if your exhaust is constantly bumping the brace, it could very easily be triggering the knock sensor."

Me, "REALLY???"

Him, "really."



So, he gives me some homework and says, "I can't know for sure until I get it on the dyno, but, go to lowes pick up some washers and put them on your car tonight to drop the tunnel brace down for clearance, come back tomorrow and I'll put the car on the dyno and tell you for sure if it's exhaust rattle or if you are really detonating... once I can look at the AFR I'll have a better idea.

Cool, so I'm off, pick up 12 3/8" washers, 3 for each brace bolt, and the next day go for a ride....



THR 22 is instantly back to normal...

I would have never guessed it... NEVER.



So anyway, short story longer.....

Since I've been itching to redyno the car I head over there to get a baseline of how my car is performing since it was tuned in PA in the winter and I'd like to see the difference driving in FL in the summer.... Well, the difference was a lot more than I thought it would be.

So we go ahead and retune THR22 and wow, what a difference.

See the vid below.

So, I thought I'd share my learning experience, after updating everyone I spoke with, on what the problem was, most of them kinda had that, "yep that makes sense" reply.

So a big thanks to everyone who helped me out. And now I know how this works...

So, anyone else out there who has 3" exhaust that is hitting the tunnel brace? you may want to find someone who can read the knock sensor to see if it is impacting performance.... I learned that even a little knock can cause enough of a timing adjustment to affect performance.

Hope this helps!!!!

DYNO VID!!!!!!!!

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Old 09-09-2010, 02:38 PM   #2
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First running out of gas, and now messing up the knock sensors cus you were procrastinating on the tunnel brace.

Bad Chris Bad!

J/k buddy.

Glad you got that figured out. When I had my 2.5" exhaust put on, I was getting some knocking against the brace too. Took it back to the shop, 30 secs later I was A-ok.

But I would have never thought such an issue would screw up the sensors.
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Old 09-09-2010, 02:43 PM   #3
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First running out of gas, and now messing up the knock sensors cus you were procrastinating on the tunnel brace.

Bad Chris Bad!

J/k buddy.

Glad you got that figured out. When I had my 2.5" exhaust put on, I was getting some knocking against the brace too. Took it back to the shop, 30 secs later I was A-ok.

But I would have never thought such an issue would screw up the sensors.


Luckily it only triggered the sensor and didn't damage it, not sure if you can damage it.

After talking to Tim, he said you can "soften" the sesitivity of the knock sensor. Apparently some of the more noisy blowers can tirgger the knock sensor...
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Old 09-09-2010, 03:16 PM   #4
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People always seem shocked when they find out that exhaust rattle they had was causing so many problems.
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Old 09-09-2010, 03:19 PM   #5
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I have been waiting to hear what you found, originally I thought it was one of the oranges stuck in the exhaust. Glad to hear it is fixed.
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Old 09-09-2010, 03:22 PM   #6
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Wow, you learn something every day. now i know how the knock sensors work.

Glad to hear you got it straighten out.
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Old 09-09-2010, 03:23 PM   #7
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I told you that from the beggining.

In fact, I remember going to THR back in early April telling you on the way home that you need to drop that tunnel brace because if it vibrates it could retard timing via knock sensors. But you didn't listen.






Ok. .............. Fine............. maybe not.



BUT, is that the WHOLE story???

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Old 09-09-2010, 03:24 PM   #8
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WOW!!!
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Old 09-09-2010, 03:27 PM   #9
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a good read, and a good video.

if only more people on C5 had the literary skills to compete with yours.
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Old 09-09-2010, 03:47 PM   #10
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Quote:
Originally Posted by DSteck View Post
People always seem shocked when they find out that exhaust rattle they had was causing so many problems.


Quote:
Originally Posted by Tom Henry Racing View Post
I have been waiting to hear what you found, originally I thought it was one of the oranges stuck in the exhaust. Glad to hear it is fixed.
People always comment on how good my car smells.....

I guess I haven't found them all .....



Seriously, Thanks for staying involved even though I'm 1300 miles away.



Quote:
Originally Posted by christianmotox View Post
Wow, you learn something every day. now i know how the knock sensors work.

Glad to hear you got it straighten out.


Quote:
Originally Posted by PQ View Post
I told you that from the beggining.

In fact, I remember going to THR back in early April telling you on the way home that you need to drop that tunnel brace because if it vibrates it could retard timing via knock sensors. But you didn't listen.






Ok. .............. Fine............. maybe not.



BUT, is that the WHOLE story???



Quote:
Originally Posted by KeepItGreasey View Post
a good read, and a good video.

if only more people on C5 had the literary skills to compete with yours.
Thanks brother.
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Old 09-09-2010, 03:53 PM   #11
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Quote:
Originally Posted by Tom Henry Racing View Post
I have been waiting to hear what you found, originally I thought it was one of the oranges stuck in the exhaust. Glad to hear it is fixed.
...........

Quote:
Originally Posted by GTAHVIT View Post
Hey. I'm just sayin'

BUT, this info is huge. NO WAY in hell I thought a center frame rattle, or even an engine compartment rattle, externaly could cause this. I didn't know how the knock sensors work either.

This is front page stuff imo. Very nice.
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Old 09-09-2010, 04:13 PM   #12
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Quote:
Originally Posted by PQ View Post
I told you that from the beggining.

In fact, I remember going to THR back in early April telling you on the way home that you need to drop that tunnel brace because if it vibrates it could retard timing via knock sensors. But you didn't listen.





Ok. .............. Fine............. maybe not.

Quote:
Originally Posted by PQ View Post
BUT, is that the WHOLE story???
mostly

Quote:
Originally Posted by PQ View Post
...........

Hey. I'm just sayin'

BUT, this info is huge. NO WAY in hell I thought a center frame rattle, or even an engine compartment rattle, externaly could cause this. I didn't know how the knock sensors work either.
you never asked either....



BTW GTA, congrats on getting it figured out.

you never called me back with the results from the "testing"

told you it was the exhaust....
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Old 09-09-2010, 04:15 PM   #13
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Originally Posted by GTAHVIT View Post


Luckily it only triggered the sensor and didn't damage it, not sure if you can damage it.

After talking to Tim, he said you can "soften" the sesitivity of the knock sensor. Apparently some of the more noisy blowers can tirgger the knock sensor...
Wow,
This might be my problem. I have a 2SSRS A6 here in Vegas and it ran 14.10 at 102 stone stock with street tires. Since we're at 2300 feet and it was 105, I wasn't too worried, but wanted more. After a couple bracket races I decided to put on JBA LTH/w H-Flow Cats with 3" Exhaust and K&N CAI and head for the track. I had some real clearence problems with the CATs and the 3" exhaust and it took me a couple of days to get the raddles out, which were really bad. The driver side collector is real close to the heatshield.

Anyway, off to the track with dreams of the high 12s running through my head, the best run was a lowly 14.3 at 100mph. Slower than stock. I figured it was the tune, so off to the Dyno. First pull is around 315rwhp. After 8 or 9 pulls we see 383.5rwhp with 392tq, whoopi we fixed it.

Out to the track. First pass, 14.38 at 99MPH:( and nothing much better. 1/8 mile times around 9.30? 60 feet is consistant at 2.2 to 2.3, even with drag radials. Basiclly the car is slower now, than it was totally stock.

Could I have damaged the sensors, or maybe the driverside header is hitting the frame at launch??? So far, nobody seems to come up with anything other than, that's weird.
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Old 09-09-2010, 04:30 PM   #14
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Wow,
This might be my problem. I have a 2SSRS A6 here in Vegas and it ran 14.10 at 102 stone stock with street tires. Since we're at 2300 feet and it was 105, I wasn't too worried, but wanted more. After a couple bracket races I decided to put on JBA LTH/w H-Flow Cats with 3" Exhaust and K&N CAI and head for the track. I had some real clearence problems with the CATs and the 3" exhaust and it took me a couple of days to get the raddles out, which were really bad. The driver side collector is real close to the heatshield.

Anyway, off to the track with dreams of the high 12s running through my head, the best run was a lowly 14.3 at 100mph. Slower than stock. I figured it was the tune, so off to the Dyno. First pull is around 315rwhp. After 8 or 9 pulls we see 383.5rwhp with 392tq, whoopi we fixed it.

Out to the track. First pass, 14.38 at 99MPH:( and nothing much better. 1/8 mile times around 9.30? 60 feet is consistant at 2.2 to 2.3, even with drag radials. Basiclly the car is slower now, than it was totally stock.

Could I have damaged the sensors, or maybe the driverside header is hitting the frame at launch??? So far, nobody seems to come up with anything other than, that's weird.
I'm certainly not an expert, but, I'm guessing it could be.

Here's something to consider. On the dyno, they should be able to see the timing retard due to "knocking"

If they aren't seeing it on the dyno, have them ride with you pluged into the ecm and do some runs on the road to see if they pick up any knock or timing issues that they don't see on the dyno.

Maybe that will help resolve it.
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