383 problems
Thread Starter
4th Gear
Joined: Oct 2003
Posts: 4
Likes: 0
From: San Jose CA
383 problems
Installed a new 383 motor in my ’92 coupe. Attached a recent dyno run chart. Redline in the chip is set to 6500rpm, but the car starts to stumble big time at about 5300rpm like it's running into a rev limiter. Any ideas?
Heads are stock but 5-axis porting from CNC Heads, slightly bigger valves. Cam is Lingenfelter 219/219 .530/.560. Headers, ported intake, 58mm throttle body, etc. Got a new eprom from TPIS for this specific motor combo.
Hate to start chasing after things without listening to you experts first. Suggestions? Wires are stock GM, but new. Fuel pump is stock, fuel filter only has about 10K miles on it.
Mark
<img src="
file:///D:/data/Downloads/DYNORUN.JPG
" border=0>
Heads are stock but 5-axis porting from CNC Heads, slightly bigger valves. Cam is Lingenfelter 219/219 .530/.560. Headers, ported intake, 58mm throttle body, etc. Got a new eprom from TPIS for this specific motor combo.
Hate to start chasing after things without listening to you experts first. Suggestions? Wires are stock GM, but new. Fuel pump is stock, fuel filter only has about 10K miles on it.
Mark
<img src="
file:///D:/data/Downloads/DYNORUN.JPG
" border=0>
Race Director
Joined: Nov 2000
Posts: 17,103
Likes: 39
From: stop the fun suckers
Re: 383 problems (Mark383)
if your using stock injectors that is probably your problem.. my stock injectors are at about 99.3% DC at 5700rpm.. after that they jsut cant deliver any more fuel.. and that would cause your bog (and a super lean condition). hook up a scanner and check your blms!
Thread Starter
4th Gear
Joined: Oct 2003
Posts: 4
Likes: 0
From: San Jose CA
Re: 383 problems (Mark383)
Thanks for the feedback.
CFI-EFI: The valve springs are dual springs, they're the heavy duty ones recommended by CNC Heads who did the head work. I'll check the fuel pressure.
BluByU: The injectors are 27 lbs, up from the stock 22 or 24 lbs ones. If I could have pasted the dyno run, it also has an air/fuel chart with it. Goes from about 12:1 at 2500rpm up to about 13:1 at 5500rpm. I think that means it's not too lean, right?
Mark
CFI-EFI: The valve springs are dual springs, they're the heavy duty ones recommended by CNC Heads who did the head work. I'll check the fuel pressure.
BluByU: The injectors are 27 lbs, up from the stock 22 or 24 lbs ones. If I could have pasted the dyno run, it also has an air/fuel chart with it. Goes from about 12:1 at 2500rpm up to about 13:1 at 5500rpm. I think that means it's not too lean, right?
Mark
Race Director
Joined: Nov 2000
Posts: 17,103
Likes: 39
From: stop the fun suckers
Re: 383 problems (Mark383)
ok.. now that i have some a/f ratio info.. let me revise my earlier psot.. a ratio of 14.7:1 is ideal! if your @ 11:1 - 12:1 you are too RICH! it sounds as though you might want to turn your fuel pressure down a tad.. however, it appears the condition get better as your rpm increase.. did you use a winde band sensor to get the a/f ratios?
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Thread Starter
4th Gear
Joined: Oct 2003
Posts: 4
Likes: 0
From: San Jose CA
Re: 383 problems (Mark383)
Not sure what kind of sensor was used to determine A/F ratio. The (Dynojet) dyno shop put the sensor in the tailpipe during all the runs.
It starts out at 14:1 @ 2200rpm then drops quickly to 12:1 @ 2400rpm. It's flat at 12:1 up to 4500rpm then rises slowly to 13:1 @ 5400rpm and that's about where the problems start.
Torque and HP curves are smooth and relatively straight until 5400rpms where they have large reverse spikes and the power then falls off dramatically.
I've replaced the optispark about 10K miles ago, but I realize that can also be troublesome for the older models like mine.
I'm inclined to think this is fuel related. Also had a problem where the car ran out of gas with three bars showing on the fuel gauge. Could this be a fuel pump problem? I can try messing with the adjustable fuel pressure regulator.
Mark
<img src="
file:///D:/data/Downloads/DYNORUN.JPG
" border=0>
[Modified by Mark383, 2:40 AM 11/10/2003]
[Modified by Mark383, 2:43 AM 11/10/2003]
It starts out at 14:1 @ 2200rpm then drops quickly to 12:1 @ 2400rpm. It's flat at 12:1 up to 4500rpm then rises slowly to 13:1 @ 5400rpm and that's about where the problems start.
Torque and HP curves are smooth and relatively straight until 5400rpms where they have large reverse spikes and the power then falls off dramatically.
I've replaced the optispark about 10K miles ago, but I realize that can also be troublesome for the older models like mine.
I'm inclined to think this is fuel related. Also had a problem where the car ran out of gas with three bars showing on the fuel gauge. Could this be a fuel pump problem? I can try messing with the adjustable fuel pressure regulator.
Mark
<img src="
file:///D:/data/Downloads/DYNORUN.JPG
" border=0>
[Modified by Mark383, 2:40 AM 11/10/2003]
[Modified by Mark383, 2:43 AM 11/10/2003]
Race Director
Joined: Nov 2000
Posts: 17,103
Likes: 39
From: stop the fun suckers
Re: 383 problems (Mark383)
three bars and running out of gas is a common problem with C4's.. it has happened to mine as well.. make sure your EVAP system is functioning and your fuel filter isnt plugged.. also, you might want to hook up a fuel pressure guage when doing your next run to see if it falls off in the upper rpms..
One fellow here on the forum had a similar problem.. i forgot who but i know its a yellow C4 (LT1) and he spun a bearing so he decided to stroke it.. after stroking it it fell on its face @ 5500.. ended up being a fuel pump
One fellow here on the forum had a similar problem.. i forgot who but i know its a yellow C4 (LT1) and he spun a bearing so he decided to stroke it.. after stroking it it fell on its face @ 5500.. ended up being a fuel pump
