383 tuning help

[myburb]

while we are passing info to get you going, here is a pigtail extension I made so I could mount the pcm under the front seat but leaving the truck wiring intact.

[C2500]

Looks like a good idea thanks!!

[daveosx]

C2500
The computer tuner guys I think will have you covered.
However lets look at the basics first.

You are not getting what you expected to.
First lets check the timing
Are you running pump gas or REC90 race gas
On modern pump gas up to 10% ethanol you can set your total advance to 38 manually by bringing the motor up to 3000 rpm for 30 seconds and then set timing to 38 degrees. Then disconnect the timing bypass under the dash and check timing at idle should be about 10-16 degrees. On race gas the same, on REC90 set it down to 35 and 10.
Then check your vacuum on a 383 even with a medium cam the vacuum should be greater than 26 inch HG.
If lower increase fuel pressure bit and check again you want to get the highest vacuum you can obtain at idle.
Now if you put a vacuum pump on the EGR valve and pump it up the idle should get funky and vacuum go down release the vacuum pump and it should recover.
If not then yank the EGR and replace, on the 2500 you need the EGR to protect your top ring lands.

If the old motor was puking oil your cat and even the resonator can be clogged if the same pull the O2 sensor and put a adapter in with a 0-10psi gauge with a long hose take the truck out run it normal and look at the pressure gauge it should not exceed 2 psi if so replace exhaust before you ruin your new motor.

Another issue that a lot of truck owners have is the lame fuel pump on the trucks since the stick motors were only 200 HP the bean counters put the cheap fuel pump in 36 dollars at GM dealers.
I changed mine out to an AC Delco EP376 and set the fuel pressure at 18PSI.
Now the truck is crisp and pulls all of its 200 HP working ;-)
Before complete failure mine had no symptoms and the previous owner did not notice.
When I changed mine the fuel pressure was below 1 psi I measured on a digital gauge so swap out the pump.

Once all good then look into the tune.

I have played with the 7.4L 2inch TBI unit no good for less than 400 CI it tends to dribble instead of squirt at idle and low rpm and you get a bad BOG even if you raise the enrich table all the way. The bigger TBI was designed for the BBC not the SBC and kind of sucks on a small block.
The stock TBI unit with all of the Mods is good for 670CFM the 383 naturally aspirated is good for 450HP under 6000rpm.
With 6lbs of boost easy 600hp.

To get the 450HP hour available the intake and fuel system has to support 9.16Gallons of Gasohol per hour at 14.2:1 and 130 gallons of air per hour In and out.
Full rich 110 Gallons per hour full lean 156.6 Gallons per hour.
You need 55.62 PPH of gasohol to make 450HP.
The stock injectors new flow 55PPH.
Lots of guys are putting the 7.4L injectors (92PPH) in and trying to get an acceptable idle they dribble at low pulse width and actually are limited to about 5300 RPM until full open.
They are quite a bit slower reacting and the minimum injector off time is relatively longer so poor economy slower throttle response.

It reminds me of when the dominator carbs came out and a bunch of guys upgraded their 650 double pumpers on stock SBC to 1000 cfm Dominators because more was better. I actually got a couple of those pink slip titles back in the day.

Remember Nascar makes 900 to 1100 HP on 104 octane pump gasoline from a mandated 348 cubic inches through a 13/16 throttle restrictor plate.

[C2500]

Looks like I have some work to do..
Old motor was good no oil issues it was just tired exhaust and converter are good as I had the Y pipe off and converter is clean..
My Timing is set at 0* at idle with plug unplugged..
It’s got the stock deco pump in it but it was replaced a year ago but I do have a New EP241 Delco pump. What’s the difference between that pump compared to the EP376 Pump?
Other members said that the injectors are to small..
So what should I change the stock 61PPH injectors to ?

[daveosx]

First the base timing is way to low that will make exhaust manifolds glow.
The manual says 0 and it will run like crap like I said set your timing at 3000 to 38 degrees the base timing will fall in about 10-16.
With timing bypass connected.
This is a big difference in power.


If your single injectors are 61PPH thats well over the required 90 Gallons per hour to make 450hp.
If thats the combined even its enough.
I think you need a better fuel pump

On PFI many guys fall into the bigger is better when often its just the access pump curve rather than the injector providing the gain.
On the TBI it is much more like a carb
Cold enrichment should be 10-20% higher than stock
Accel should be 50-100% higher

[C2500]

On the Timing when it’s running idling with spout connected with timing set on zero disconnected it’s running about 20° advanced at idle

[Fast355]

A pair of 61 lb/hr TBI injectors are good for 205-210 hp @ 85% duty cycle at the stock 13 psi and if they were static, meaning on all the time, good for about 250 hp at most. Most engines run around 1/2 lb/per hr/per hp. 61 lb/hr x 2 injectors /.5 bsfc = 244 hp. Even if you were running a Vortec head and had a very good BSFC around .45 that is still only fuel for 270 hp.

Simply advancing the base timing will also not work very well. The TPI heads you are running like about 34* of timing at WOT not the 38* mentioned above. 38* is for the older slow burning smog heads. With 20* of idle timing when you advance the timing to 12-16 as mentioned above you end up with 32-36* of idle timing which is too much for the mild cam you have. The advance curve is also slower in the ECM than what the TPI heads need. The TPI heads like about 16-20* of timing from 1,000 rpm through about 2,400, then up to about 30* from 3,000-4,600 and finally on to 34* above 4,600. The TBI heads have a substantially different timing advance requirement.

I have been through converting a 305 HO with 14022601 heads to TBI and had to make massive changes to the chip in both timing advance and fueling to make it run correctly. To give the little 305 with the mild 204/214 @ .050 cam and headers enough to drink I had to run 350 injectors at 14 psi.

Finally....I am not even going to get started on why someone would pull the 191/193/810 swirl port heads off and put even worse TPI heads on.

[daveosx]

On the Timing when it’s running idling with spout connected with timing set on zero disconnected it’s running about 20° advanced at idle

Ok so with the timing bypass connector connected it is reading 0 degrees
And with the connector disconnected it is reading 20 degrees
If so than you have the timing set 20 Degrees AFTER TDC instead of before.

To check disconnect the bypass connector under the dash.
Start and idle the engine
Check the timing with a dial light set on 0 the mark should be at the largest notch for 0 degrees and towards the water pump edge of the timing tab for 14-16 degrees.
If it is at Zero or towards the water pump
Shut off the engine and reconnect the timing bypass connector.
Restart the engine
Have a friend slowly raise RPM to 3000 RPM
After 30 seconds have passed use the dial on the back of the timing light to align the timing mark to the largest notch.
Read the dial indicator on the back of the timing light.
Turn the distributor slowly checking again after each move.
When complete the reading should be 38 degrees.

Harbor freight has a dial timing light for 29USD that will work ok.
Also I know it is scary with the stock cooling fan raging and laying over the fender to reach the distributor. A 3 foot by 3 foot piece of 3/4 ply wood laying on the air filter to the front grill makes setting the timing on a suburban actually comfortable. You can lay head towards the drivers side and reach around the edge of the plywood to see the timing marks and use your left hand to move the distributor.
Thats KentMore tool 3x3x3/4-1 for medium duty service vehicles.