High end missfire?!?!?
 

I just recently got my car back from the engine shop with a fresh 306. Everything I have is new except for the igniton and an aftermarket chip...I havn't installed them yet. Here's the problem. I can get to about 3500 rpm before it starts to break up and even backfire horribly. Is this because I havn't changed the stock ignition, or is a new chip really that important on a modified engine? I'm running roughly 50 lbs of fuel pressure on 24lb injectors. Please help...i'm frustrated.:confused:

Not sure of the _tune_ of your motor (whether it needs a custom chip or not)... but
Try gapping your plugs .005 closer. So if you are running stock .052, close it up to like .045-ish. Try gain.
Then save your money and get an aftermarket ignition like MSD and new "spiral core" wires. That outta do it.

I have a higher compression engine 10:1 to 10.5:1 I have an MSD Digital 6 ignition that I have not installed yet. As for my motor...this is what I have: 306, B cam, 1.72 roller rockers, Ported and polished GT40 aluminums with bigger intake and exhaust valves. I'm running 24lb injectors with a 77mm MAF sensor and 65mm throttle body. I have GT40 intake with ported lower. 1 5/8 long tube headers, offroad h-pipe, and 2 chamber Flowmasters out back. I have the fuel pressure set at about 50 lbs which is about where it should be for what I have. Do you think that it is the spark plug gap...my engine builder said it really shouldn't matter from 0.35 to about 0.65...is he full of crap or what?

hmmmmm,
 

I'd check the ignition., maybe a bad injector. Any codes going off(check engine light). Problem in the distributor itself.????/ hard to say.

Checked the plugs...all were gapped acceptibly at 0.40 and put in a new fuel filter...still no change. What I did notice however, was that when I put octane booster in my car it ran a whole lot better. It still gave me the rough bullsh*t after 3500 rpm, but it pulled a lot cleaner. I'm pulling and testing the distributer on Saturday...and I will also try to install my MSD Digital 6 and coil. If none of this works I guess I can start testing injectors. What else should I look at ?:confused:

After thought... no check engine light through any of this.

In case anyone is confused...this is my work profile. I am also 306 Blue GT:D

50 psi is too high. You should always stay at 38-40 psi @ WOT. At 50 psi the computer will just try to shorten the pusle width after a while anyway.

I'm with you...
 

Blue,
I have almost the exact setup on my 88 GT, except 73mm MAF., equal shorties, TFS heads, and a Mallory ignition.
I have the EXACT same problem now. If I gun it, at 3500 ish I get a terrible stutter and then it catches and goes on. It is very disconcerting. My 306 is only 3000 miles old. I have checked/swapped plugs three times, checked distributor, new coil, wires, checked TPS, IAC, new 255lph fuel pump , filter, Billet FPR @43 PSI, new O2's, temp sensor,etc, etc.
I do know that my TB (65mm BBK) plate is sticking. At least around idle, and I am thinking of upgrading to a 70mm with a new TPS and a Pro-M 77mm MAF. I ahve also been chasing the "wandering idle" problem too, especially at warmup. After the TB upgrade, I guess it's time to voltage check injectors and the computer.
By the way, my 88 doesn't have a check engine light. So it's even more fun.:mad:

exgmguy,

Are you sure about the fuel pressure...because everybody I talk to says roughly 50lbs is what I need w/ 24lb injectors and with the power I'm making... roughly 380 HP. If you are telling me that they (engine builder and dyno guy) are wrong I need a better explaination than what you gave me.

thanks,

Kevin

50 PSI...
 

Blue,
I will set mine to 50 lbs. from 43 PSI and see if that makes a difference.

Hozer,

Have you put in a chip or anything? ...Or have you taken your car to a dyno and had your stock computer adjusted? I'm getting a chip soon, and I'm trying to make an appointment with a dyno guy through my engine builder. Not sure what to expect out of either though. Let me know if the increased fuel pressure helps or hurts, and I will let you know what happens with the chip and the dyno. Keep me posted.

Mods...
 

Blue,
No I haven't gotten a chip. I really don't know much about them. All I have ever heard is that they bump the timing for you, which you can do yourself. Like I said though, I admit I don't know much about them. I would gladly do it if it was worth it though. I had to convert to Mass Air using the Ford kit about 4 years ago, so that's how old my computer is. It is really frustrating because I am a detail freak and this is kicking my butt.
I am doing the fuel pressure change right now to 50 PSI at idle with vacuum line disconnected. Film at eleven...

I think a chip is supposed to control more than just timing...I'm pretty sure it also controls air/fuel ratio so your 5.0 won't run too lean or too rich. I think it has a different map to help the stock computer better utilize your modified setup. I'm also talking about a custom chip based on your particular setup. I'll try to find out more than that for you.

Let me know if your car runs any better...;)

BTW...I too am a seriously detail oriented anal retentive bastard....and this whole thing is making me absolutely nuts.

Blueball,
sounds like a physical layer issue to me. might want to check the OSI model.

Right now, 50 PSI looks good...
 

Blue,
After multiple test runs, albeit when the engine was relatively warm, the car behaved perfectly.
I am about to go fire it up now after sitting for about 6 hours. I will report back asap.
I called JET chips and I looked up info on Superchips. Both were compelling. I'd like some feedback from users of chips with setups like ours. They both claim horsepower increases even with our mods of approximately 10 percent.
If I continue to get a response like I anticipate, I plan on having a programmed chip by the end of the week.

That's awesome...hope it runs as good cold as it did warm. Keep me posted on all of that.

The same at 50 PSI
 

I am becoming more and more convinced that my computer just can't keep up with my mods. I had the same usual result this morning after the car sat all night. PSI is indifferent...more to follow on the chip info.

Disconcerting...but very helpful. I'm checking out the distributor and installing my ignition Saturday...I'll tell you if I find anything. I guess the more things we systematically eliminate the closer we will get to a solution. Don't lose heart...:rolleyes:

Ignition...
 

If it helps, I just put in a new cap and rotor on my billet Mallory ignition and my MSD wires are only 2 months old. MSD Balster coil is 2 months old too. I bumped the timing all the way to 18* for a while , then back to 14* now. Calling Superchips in a minute. Good Luck. By the way, what code is on the side of your EEC-IV (i.e. A9L)?

I haven't checked my code out yet...not even near my car at the moment. I'll let you know tonight or tomorrow. I have brand new 9mm FMS wires, and I 'm installing the MSD Digital 6 Plus ignition and coresponding Blaster HVC coil on Saturday. My timing is set at the factory 10 BTDC...when I bumped the timing..it ran even worse than it is now. Let you know later though...about the code.:D

38 PSI...
 

Blue,
Tried 35- 43 PSI at 1 PSI intervals and car runs perfectly at 38. Could be that the 255 lph pump shoves so much fuel, that an apparent decrease in pressure (due to increased velocity) at the schrader valve still means plenty of fuel is reaching the injectors. I mean, this is the best my motor has run to date. Still getting a chip though