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Discussion Starter · #1 ·
Here's the problem:

When I am doing a WOT run (I do a quick lift during shifting.....ok..you can stop laughing now and read the rest) I get a bad stumble immediately after shifting. The T/C is OFF (isn't it always supposed to be off). It's almost like I get a split second of boost dump as I jump back in the throttle. The shift is fast and smooth but the car acts like it loses power for a split second.

It goes kind of like this: 1st gear WOT.....6K....clutch - breath throttle- WOT as clutch is released.....cars just starts to pull - quick stumble (as if you lost power for a spilt second) - then keeps on pulling. It does exactly the same thing on the 2-3 but have not tried it WOT on the 3-4 or 5-6.

My mods are:
9" K & N
FM cat back
2.93 Pulley
74" gatorback belt
stock plugs (AWSF22FM1) gapped at .038
Dyno tuned Autologic chip
434 RWHP SAE
433 RWTQ SAE

I don't have the boost bypass mod done.

I have driven many STOCK 03' Cobras and have never had a problem like this after fast shifting on WOT runs.

I need the wealth of knowledge available on this board to show me the light. :)

I have thought about disconnecting the boost bypass solenoid to see if this ia a PCM issue but have not had (I really mean taken) the time.
 

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Sounds like your plugs may be missing. You may be experiencing blowout or they may load up during the shift. Try the denso plugs. I had a misfire problem before but now with the IT20s the power is always there, even more responsive.
 

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Discussion Starter · #3 ·
My plugs were gapped at .044 from the factory and I don't know if this was happening then. I originaly had the stock belt on with the 2.93 pulley and stock gap plugs. When I changed the belt to the 74 I re-gapped the plugs to .038 figuring that it may hold a little more boost and the smaller gap may be required to eliminate the spark blow out.

I did re-dyno the car after changing the belt and plugs to check a/f and see if there was anything suspect on the graph that may indicate misfires. Everything looked fine....I gained 6 peak RWHP but lost 6 RWTQ with the new plug gap and belt. I put a vaccum gauge on to check the boost and it held about about 1/2 lb more at 6K.
 

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mine did the same thing today just after startup, 1st to 2nd and it fell flat on its face. Let me know if theres something wrong. I wouldn't doubt this has happenes to others also. It might be a cold start up thing pulling timeing?:( :confused:
 

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I put on the new pulley and belt and re-gapped the stock plugs too, but everytime it still showed misfires on the dyno chart between 5600-6100 rpm. If you see little spots on the curve where it goes from smooth to a little choppy they are misfires. The denso IT20s gapped at .038 have made the car more responsive and ended the misfires.
 

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Discussion Starter · #6 ·
brad65 said:
mine did the same thing today just after startup, 1st to 2nd and it fell flat on its face. Let me know if theres something wrong. I wouldn't doubt this has happenes to others also. It might be a cold start up thing pulling timeing?:( :confused:
I don't do WOT runs on a cold engine so cold engine is not a factor in this one.
 

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Mine are gapped at .038
 

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Ok either one of two things
1. your just touching the rev limiter wich is what i used to do.
2. you lifting to much off the throttle when you shift and your starving the blower of air. What your actually feeling is the eaton trying to build boost back up. This I also went through ,it takes split second to build boost back up cause your at a higher rpm. Just try to stay on the gas more when you shift and youll be fine
 

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Discussion Starter · #10 ·
FINICKY said:
Ok either one of two things
1. your just touching the rev limiter wich is what i used to do.
2. you lifting to much off the throttle when you shift and your starving the blower of air. What your actually feeling is the eaton trying to build boost back up. This I also went through ,it takes split second to build boost back up cause your at a higher rpm. Just try to stay on the gas more when you shift and youll be fine
Well FINICKY......I think you hit it on # 2. It definately was not the rev limiter. I did put denso's in on Friday but only started the car to make sure it ran. The idle did seem a little smoother than stock plugs. I didn't run the car until Monday when I took it to a road course track day. That's where I experimented with shifting techniques and got around the stumble. The stumble was not the s/c building boost back up because it had power for a split second after the shift but then fealt like the boost bypass just dumps some boost for a second.
 

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My car did this alot last fall i described it as a double grab coming from a heat soaked clutch.I put a new clutch in and it went away for about 3 months and now it is stumbling again as you described it.I really do not think it is loss of boost because it feels like a jolt just for a split second during the shift than instant boost is there.I still think it has something to do with the clutch.
 

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Discussion Starter · #12 ·
HIGH ROLLER said:
My car did this alot last fall i described it as a double grab coming from a heat soaked clutch.I put a new clutch in and it went away for about 3 months and now it is stumbling again as you described it.I really do not think it is loss of boost because it feels like a jolt just for a split second during the shift than instant boost is there.I still think it has something to do with the clutch.
I don't think it is a clutch issue. I have not done any hard launches that would heat the clutch up before the stumble happens. I ran the car Monday for almost 100 miles (4 track sessions) on the road course and was able to avoid the stumble by changing shifting techniques.

Oh yeah...I averaged 4.75 mpg during the track runs. I got 66 miles out of a tank of fuel and the refill was 13.9 gallons....ouch.
 

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WhaT Did you change in your shifting technique???
 

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Discussion Starter · #14 ·
When I shifted before I did a quick stab of the clutch / fast lift of the throttle.......like a power shift but with a quick lift of the throttle. Now instead of a quick stab of the throttle I leave it in for a split second before releasing it. Still a very fast shift but just leaving the clutch down briefly before releasing it.
 

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Why not powershift? There is almost no way to do a lift shift of any kind without causing the boost bypass to actuate. Here's why. When the throttle is closed any significant amount, the engine transitions from boost to vacuum. When the boost bypass valve is subjected to vacuum, it opens the boost bypass butterfly. When this happens, all boost is lost in a split second. Jumping back into the throttle after the shift causes a momentary time lag until full boost is achieved again. Going from 12 or 13 psi boost to 0 psi and then back to peak boost might be the hesitation that you are describing.
 

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Discussion Starter · #16 ·
larryc7777 said:
Why not powershift? There is almost no way to do a lift shift of any kind without causing the boost bypass to actuate. Here's why. When the throttle is closed any significant amount, the engine transitions from boost to vacuum. When the boost bypass valve is subjected to vacuum, it opens the boost bypass butterfly. When this happens, all boost is lost in a split second. Jumping back into the throttle after the shift causes a momentary time lag until full boost is achieved again. Going from 12 or 13 psi boost to 0 psi and then back to peak boost might be the hesitation that you are describing.
I don't power shift (yet) for a couple of reasons; trying to avoid wheel spin and wheel hop, trying to reduce the shock to the driveline being it seems to have so much play in it and the car is still to new for me to break down and start power shifting.
 

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Forgive my ignorance, but how can you powershift an '03 without hitting the rev limiter?

If you are shifting at say 6200 rpm, how can you stab the clutch, pull the gear and release the clutch pedal so quick that the rpm's don't hit 6500?

Please go easy on me. :)
 

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mosconiac said:
Forgive my ignorance, but how can you powershift an '03 without hitting the rev limiter?

If you are shifting at say 6200 rpm, how can you stab the clutch, pull the gear and release the clutch pedal so quick that the rpm's don't hit 6500?

Please go easy on me. :)
You have to do it REALLY fast :D I used to have a problem hitting the limiter powershifting in third, I've gotten the hang of it now though. The clutch adjustment proceedure helps too.
 

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Ray hit it right on the money. If you can keep at least 1/2 to 3/4 throttle on shifts you will not lose as much boost everytime you pull a gear. One thing is gap the plugs down to .035-.038. What we do is set the shift light to 6k and you actually shift around 6.2k. If my customer powershifts there car I always bump the rev limiter up to 6.6k on my race tunes.

Rick
 

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Discussion Starter · #20 ·
I just changed my plugs last Friday from stock AGSF22FM1's gapped at .038 to Denso's gapped at .038. The only difference I noticed was a smoother idle. The plugs I took out had about 800 miles on them and appeared to be burning perfect. My car has 2100 miles on it but I had put new plugs in when I re-gapped them from the factory .044.
 
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