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View Full Version : Question About Max RPM's N/A and Boost


SuperG
12-21-2005, 10:02 PM
After attempting to really learn the characteristics of my Cobra, especially, in this case, the acceleration aspects, I discovered that my car begins dying at about 6000 rpm's, rather violently, too...it acts as if there is no more fuel, or air, or it's having an anxiety attack. Redline shows to be about 6800 rpm's on the tac.

For those of you with N/A or supercharged Cobra's...is this a common ocurrence? I hope not and there is a culprit needing chased.

Thanks for any advice!

Bartman01
12-22-2005, 12:09 AM
My 01 pulls all the way to the redline (when the computer shuts things down and just kills the power). Almost sounds like your redline has been lowered. It would be worth having your engine management program looked at, because if it is cutting fuel at that point - and you are still in boost - bad things can happen. Some GT's have blown their engines after supercharging them and the speed limiter kicked in and pulled fuel (thanks to Ford putting lower speed limits on the cars that didn't come from the factory with high speed rated tires).

SuperG
12-22-2005, 07:31 AM
That could explain the sudden loss of power and the jerky feeling, not to mention the smoke following me!

SNAKEYE
12-22-2005, 01:11 PM
After attempting to really learn the characteristics of my Cobra, especially, in this case, the acceleration aspects, I discovered that my car begins dying at about 6000 rpm's, rather violently, too...it acts as if there is no more fuel, or air, or it's having an anxiety attack. Redline shows to be about 6800 rpm's on the tac.

For those of you with N/A or supercharged Cobra's...is this a common ocurrence? I hope not and there is a culprit needing chased.

Thanks for any advice!Of course, what you describe is not normal. Redline, indeed, is 6800 RPM.
What you don't make mention of is what your fuel pressure gauge is indicating, perhaps because you don't have one. Lack of sufficient pressure, due to a bad fuel filter, a bad FMU (if so equipped) or fuel pump, will cause the condition you describe. If the fuel filter has more than 20K miles on it, change it. If you do have a fuel pressure gauge, when in boost you ought to be seeing 60-70 psi as opposed to the normal 35-40 psi. I see by your bio that you are running a Vortech S/C which should be equipped with the T-Rex in-line fuel pump. Make sure that the ground wire to the T-Rex is properly grounded. You should find this ground connection in the spare tire well along with the auxiliary relay for the T-Rex. Heck, while you're at it, make sure the power source wiring to and from the relay are in proper order.
It's the little stuff that'll mess you up!

SuperG
12-22-2005, 02:10 PM
Of course, what you describe is not normal. Redline, indeed, is 6800 RPM.
What you don't make mention of is what your fuel pressure gauge is indicating, perhaps because you don't have one. Lack of sufficient pressure, due to a bad fuel filter, a bad FMU (if so equipped) or fuel pump, will cause the condition you describe. If the fuel filter has more than 20K miles on it, change it. If you do have a fuel pressure gauge, when in boost you ought to be seeing 60-70 psi as opposed to the normal 35-40 psi. I see by your bio that you are running a Vortech S/C which should be equipped with the T-Rex in-line fuel pump. Make sure that the ground wire to the T-Rex is properly grounded. You should find this ground connection in the spare tire well along with the auxiliary relay for the T-Rex. Heck, while you're at it, make sure the power source wiring to and from the relay are in proper order.
It's the little stuff that'll mess you up!
Tomorrow the fuel pressure and boost gauges get installed for that very reason...thanks for the great advice! I will also go over the entire setup per the instructions. The Vortech was installed by someone else for the previous owner, so I have no idea as to the reliablity of that person. But as you said, something is definitely wrong. Oh and yes, the T-rex is installed...can't wait to see the running numbers on this...

SNAKEYE
12-22-2005, 09:51 PM
Tomorrow the fuel pressure and boost gauges get installed for that very reason...thanks for the great advice! I will also go over the entire setup per the instructions. The Vortech was installed by someone else for the previous owner, so I have no idea as to the reliablity of that person. But as you said, something is definitely wrong. Oh and yes, the T-rex is installed...can't wait to see the running numbers on this...
Take your time with the gauge installation. Fuel pressure can be picked up at the front end of the right (passenger) side fuel rail where there is a Shrader valve. Be careful as gas may be under pressure and squirt out as you hookup to it. Vacuum for the boost gauge is available from the vacuum line going to the fuel pressure regulator located at the back end of the same fuel rail.

I have a Vortech too, and the instructions which I haven't looked at in years. Main power for the T-Rex comes from up front somewhere. Without pulling out the instructions I'd assume that there is a fuse up there somewhere. Just go over all of the electrical stuff. Both the in-tank and the T-Rex should run for a few seconds when you turn the ignition switch to the RUN position. You should be able to here them humming for those few seconds. If you get under the car you should be able to hear them, and feel the T-Rex run for those few seconds with you fingers as someone switches the ignition to RUN.
If the T-Rex doesn't feel like its running it may be fried or just isn't getting power from the relay in the trunk. Stick your ear in the spare tire well and listen for the relay to click as someone turns the ignition to RUN. Hold it with your fingers to feel it click. No click may mean a fried relay (equivalent replacement available at Radio Shack). Try jumpering the relay to apply power direct to the T-Rex to see if it runs.

I have deleted my T-Rex (which is something like a 188 lph pump) and a Ford Racing 190 lph in-tank pump in favor of just one pump - an in-tank 255 lph Walbro. It eliminates all of the extra wiring and relaying, as in KISS (keep it simple stupid). Just a whole lot less to go wrong.

While your poking around the T-Rex, the fuel filter is right there. You can replace it with normal tools. It just takes an understanding of the connections to the fuel lines. If you replace it be prepared for the loss of some gas from the filter and hoses. A towel will do, and NO FLAMES(like your house heater if you're in your garage) nearby. Then put the towel outside to eliminate the vapors.

smashedheadcat
12-24-2005, 12:31 AM
Another thing to look at is your spark plugs. Pull them out and have a look at them. They can tell you a lot.

If it feels like you hit a rev limiter down low like that the most likely thing going on is spark blow out. I would change the plugs to Autolite 764's or NGK TR6's and gap them down to .032 or so. This fixed my blow out issue on my vortech ride.

SuperG
12-24-2005, 08:01 AM
Another thing to look at is your spark plugs. Pull them out and have a look at them. They can tell you a lot.

If it feels like you hit a rev limiter down low like that the most likely thing going on is spark blow out. I would change the plugs to Autolite 764's or NGK TR6's and gap them down to .032 or so. This fixed my blow out issue on my vortech ride.
Yep, that's another thing getting pulled. I ordered the NGK's already, knowing that I will either need them now, or sometime soon, anyway. It'll be interesting to see what the current plugs look like!

ausie
12-24-2005, 10:30 AM
If you are still running the stock coil packs (not quite sure if you have the coil on plug or the dual 4 pack deal ) it may matter how cold your spark plug is. I know that the coil on plug are sensitive to plug heat range and if too cold it will not fire properly. If you are getting blow out, you may need to add a spark booster.

As far as the N/A RPM goes, my 01 would scream all the way up to 7000 RPM before shutting off the fuel. I believe the 04 shuts the fuel off at about 6800 RPM but dumps the boost around 5200 RPM.

Has the car been dyno tuned? It is possible you may be running out of room with the MAF depending on how much boost you are running. I would at least have it checked to make sure you are getting the right amount of fuel for the applied boost. Fuel pressure is one thing but if the signal to the fuel injector is not sufficient you will run into problems at the upper RPM range if the MAF was not calibrated properly for the application. If it feels like you are hitting a brick wall when all goes into havoc mode, it may be detonation instead of fuel shut off. Hopefully that is not the case here.

ausie
12-24-2005, 10:31 AM
By the way Gary, nice wheels :thumbsup:

SuperG
12-28-2005, 10:08 PM
By the way Gary, nice wheels :thumbsup:
Ha! Thanks!