383 tuning help

[C2500]

First thank for all the earlier posts I have asked question about!! I have a 383 that was built and I’m thinking its not the power I was expecting. Here’s the build info..
9.5 compression..The shop that built it is a well known shop in this area and builds a lot of race motors from drag to dirt motors and built may motors for me also..Engine was plate honed,block squared,heads milled flat and rings file fit..
447/471 213/219 @ 50 112 hyd roller
083 heads that are pocket ported and runners cleaned up
3074 eddy intake stock(I noticed that I should have had it bored as there is a lip that’s in the flow path)
Bored 46mm TBI unit
stock 61ppm injectors
14 psi regulator spring (fuel pressure checked with gauge and at 13.5)
Fast chips piggy back chip from Ed Wright

Truck runs good but I think it should run better..I was asking about the intake, boring it to 2” and put on a BB TBI that I have.. I’m thinking that its running on a 2bbl with that TBI and just up to a BB one should help it..timing is set at 0* with plug unplugged..Any thoughts that you have will be great..
I removed the Y pipe and put another bung in it to see what the AFR is using a wide band with an LM1..
Thanks again!!!!

[notime2d8]

So, what issues are you having exactly? Sounds like you need to do some real tuning and data-logging along with a wide-band before throwing more money at it. It could be fueling, timing, Power enrichment, or Acceleration enrichment giving you issues. Adding more air with a bigger throttle body may make it run even worse if it's lean now.

[Kitch]

If you aren't already you need to be data logging your current setup so you can check out whats going on e.g. spark, VE tables.

A seperate wide band O2 sensor will allow you to get an idea of your W.O.T Air fuel ratio is.
13.5 PSI with stock 61 lb/hr injectors sounds a bit on the light side to supply enough fuel for a 383 at W.O.T. (a healthy stock TBI 350 would push that)
Assuming you have everything like spark and VE table close to where they need to be have a look at your Injector DC% in tunerPro, if it get's to 50% then you will be running out of fuel because the injectors will be working too hard.
Have a read of this: http://www.gearhead-efi.com/Fuel-Injection/showthread.php?4846-TBI-Injector-DC&highlight=Kitch
I use an Innovate LM1 and I mounted the gauge so I can see it while I drive and I eventually managed to get it connected to my Moates APU1 which allows it to stream the wideband O2 data into TunerPro so I can log the data rather than just look at the dash mounted gauge.

The factory air cleaner is very restrictive above 3500RPM and if your still using that it will be holding you back. If you are using it at the very least look at flipping the lid on it but only once you've checked the Injector DC% and wideband AFR as more airflow may only make your problem worse. Assuming everything else is okay you should see a change in the W.O.T MAP values at high RPM with the lid flip on a stock air cleaner.

Are you still running EGR with the 3074?

[C2500]

Kitch, I had Ed at fast chips do a chip for me that’s piggybacks the original. I sent it back 2 times already as it was rich at cold start and hot restart and cleaned up when it went into closed loop..
Yes egr is active on the 3704 eddy intake..
So I guess I need to get turner pro..
I’m really green on this so I need as much advise as you guys can give me..
I read threw that post you listed and it really looks complicated to understand.
I have a snap on scanner so I have some data on there but I’m sure I may need more..
Thanks

[C2500]

What do I need as far as cable to use tuner pro..and I would guess get the RT version?

Also it has a red/blue 6395 ecm as its got a 4l60E trans

[Kitch]

Even the most basic setup which would be a laptop with TunerPro installed and connected to your trucks ALDL port with an ALDL to USB cable would allow you to at least see whats going on in real time with things like spark, throttle position, RPM, MPH, BLM's and what the INT is trying to do during closed loop.

You won't be able to make any changes with that basic setup but it would be a good start as far as gathering information on how things are running.

It is a steep part of the learning curve you're on at the moment but given time and help from the guys here it gets easier.

[C2500]

Where to purchase that ALDL to usb cable?? Don’t see it on tuner pro web sight.

[Kitch]

Try a google search but Moates has what you need if you can't find it else where.
http://www.moates.net/gm-19851995-c-64.html

[Kitch]

If no one has pointed it out to you yet, until your motor is up to a certain temperature it will only operate in open loop and when in open loop the engine basically runs of the preset values in your PCM's VE table.

If the values are set on the high side it will generally run rich, if they are on the low side it will run lean.

That's one of the things we change when we tune, to try get the values in the VE tables close to what the engine likes best so to speak.
In theory once you get the VE table set up correctly you could effectively drive around in open loop all the time but I certainly wouldn't recommend that.
During W.O.T. the PCM defaults to open loop and ignores the O2 sensor, thats why it's important to get the values in the VE table correct.

When the engine eventually goes into closed loop the narrow band O2 sensor takes over and tries to trim the fuel to near stoichiometric.
So if your truck seems to be rich when in open loop but better when warmed up and in closed loop, that's just the O2 sensor doing it's job.

Tuner Pro can actually be set to display the point that it changes from open loop to closed loop.

Any time you change your fuel pressure you will need to potentially redo your VE tables.
More fuel pressure = richer across the board
Less fuel pressure = leaner across the board
You need to make sure you have enough fuel pressure with your stock injectors so that are not maxing out their duty cycle at the top end. Then you tune from there.

A bit of a crude description but hopefully it gives you a bit of an idea.

Some of the more knowledgeable guys here might ask you to upload a TunerPro log from your truck and they can then comment on what they see is going on in your log.

[C2500]

Thanks for that last post.. Ill go over to moats and order a cable and get tuner pro.
I’m not sure what chip ed at fast chips uses but can I with the proper equipment reprogram that chip?

[C2500]

I think I should get the aup1 auto prom package??

[myburb]

The apu has always worked well for me. Moates has always been helpfull when needed. Might be an idea to have them ship a few extra chips. If you are going to seriously get into tuning it yourself a zif socket comes in handy and the ( off memory ) g1 adapter

[Kitch]

I'm not sure what Ed at fast chips uses, did the piggy back chip he sent you look anything like the Moates G1 memory adaptor?

I use an APU1 and have been happy with it, I've used it with my 94 Chevy van and my 95 GMC Yukon.
I did buy a GP1 package so I could fit the programable chip to my PCM but if you don't need the quick change ZIF socket and a spare chip then you could just buy the G1 memory adaptor.
I also later bought the HDR1 memecal header only because with the APU1 it allows you to read the contents of the stock chip.

[C2500]

Kitch, myburb, So if I get the apu and a GP1 package I should be set to tune this then? Do I also need the ZIF ? What does that do for me? I want to order what I need to do this..
Here’s a pic of Ed’s piggy back adapter. 12461

[myburb]

The prongs on the chips are very thin and I tended to get one bent trying to put it in. The zif allowes you to just drop the chip in and flip the lever and you are done. If you want to get the tune really good you will tweak and data log a LOTA LOTA times. The only problem with the zif is it makes the chip sit up higher. Moates does have a low profile zif. What some do is use the zif while you are tuning and when you get the tune done then you can carefully insert the chip without the zif and then the cover on the case will fit ok. That adapter you have looks like it does a similar job to the g1. With the g1 you retain your limp. Not sure if yours does.

[dktool]

The prongs on the chips are very thin and I tended to get one bent trying to put it in. The zif allowes you to just drop the chip in and flip the lever and you are done. If you want to get the tune really good you will tweak and data log a LOTA LOTA times. The only problem with the zif is it makes the chip sit up higher. Moates does have a low profile zif. What some do is use the zif while you are tuning and when you get the tune done then you can carefully insert the chip without the zif and then the cover on the case will fit ok. That adapter you have looks like it does a similar job to the g1. With the g1 you retain your limp. Not sure if yours does.

This ^^^

I originally started with the low profile zif for the sake of the low profile and cover clearance, what I found was that I was R&R'ing the chip so much that the lesser quality and more difficult lever operation was causing some grief so I went with the higher profile / quality zif with the metal lever, sooooooo much better !
With the taller zif the chip guides into place very easy and the large lever is easy to lock even with my gigantic fingers.
I can reach down and remove the chip without leaving the captain's chair.

I will someday put the low profile one back in and the cover on but I still find I like the ease of pulling / installing the chip and still play with the tune even after getting it REALLY good. (I never leave thing's "well enough", got to have tinker-time)

1 year ago I didn't even know this tuning was possible, now I am comfortable with what I do. I will say that based on the steep learning curve I am glad I went simple and didn't go the APU1 route. For me, stopping and pulling the chip and re-burning a change is close enough to "on the fly real time tuning".

[C2500]

Thanks for that experience with the zif guys. Dktool so what did you use for doing the re-burning?? I was going with APU1 cause it looks like its all there to do what I need to..
I’m still trying to figure out turner pro..I don’t have it yet but doing a lot of reading and don’t understand all the symbols are with the $B or whatever $ that is....Thanks

[myburb]

The apu doesn't have to be used emulating. I just data log with mine, decide what I need to change and burn a chip with the apu.

[myburb]

In ref to tuner pro and the process, it is a litte steep on the learning curve at first. Then it is kind of "bingo" and it all starts to fall in place. With the $, they are the mask which is relevant to your vehicle. In your case it will be $OE or $OD depending on what transmission you have.

[dktool]

Thanks for that experience with the zif guys. Dktool so what did you use for doing the re-burning??

I went cheaper (my nature) bought the following from Moates;

http://www.moates.net/aldu1-and-cabl2-combo-p-128.html?cPath=64

http://www.moates.net/burn2-chip-programmer-p-197.html

http://www.moates.net/g1-memory-adapter-tpi-etc-p-32.html

http://www.moates.net/hdr1-memcal-header-p-52.html

(http://www.moates.net/hdr1-memcal-header-p-52.html)http://www.moates.net/s2-aries-28pin-zif-socket-p-44.html

http://www.moates.net/c2-sst-27sf512-chip-p-39.html?cPath=26

http://www.moates.net/s4-aries-28pin-lowprofile-zif-p-46.html?cPath=26

(http://www.moates.net/s4-aries-28pin-lowprofile-zif-p-46.html?cPath=26)

[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]

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.


450 HP Hour requires 9.16 Gallons per hour of Gasohol.
Mechanical efficiency of stock motor is 49% with new pistons rings increased compression and roller cam 76-80%

At 6lbs per gallon thats 54.96 PPH

The injectors will provide at stock fuel pressure 336HP no problem.
At 18psi like I recommend they will flow reliably at 74-75PPH




Simply advancing the base timing will also not work very well.


I agree base timing is really not that important but the 0degrees as stated would cause the exhaust vales and pipes to over heat at idle.
At 3000 RPM GM HEI will reach no knock retard maximum advance after 30 seconds.
That has been the normal for all SBC since the beginning.
Setting the timing to 38 degrees at this point sets the distributor phasing and the maximum no knock advance possible.
At 4200 and up knock will retard the timing 4-5 degrees no problem.

At 4200 shift point the roller rockers will make noise this is not knock but will retard the timing an additional 2-3 degrees.



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.

On this one I can authoritatively say that since I wrote the GM service bulletins on Corvette and Camero TPI ignition timing with aluminum heads.
That the timing retard for these heads was solved in 1991 with the new knock sensors and EST modules.
Yes there is a different module for the corvette and the sensors are different.
This was not for the cast iron vortex heads only the aluminum.
Same device was mirrored in 1996 with the different EST and Knock sensors on the LT-4 engines because of the roller rockers.

As for the power available and RPM capabilities of the GEN1 and GEN1.5 TBI engines with stock injectors and HEI
Here is a snap shot of a 1984 Corvette I am tuning tuning at @7400 RPM with no Knock and only a 4ms Pulse width on the Crossfire TBI injectors.
There is no load attached to the motor so no HP rating.
12484

[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.

[Fast355]

450 HP Hour requires 9.16 Gallons per hour of Gasohol.
Mechanical efficiency of stock motor is 49% with new pistons rings increased compression and roller cam 76-80%

At 6lbs per gallon thats 54.96 PPH

The injectors will provide at stock fuel pressure 336HP no problem.
At 18psi like I recommend they will flow reliably at 74-75PPH


My experience and wideband both disagree with your HP potential of the TBI injectors.

350 with TBI heads, stock TBI cam, 1.6 roller rockers, Edelbrock 3704 intake manifold bored to 2", 454 TBI unit with 350 injectors, doug thorley tri-y headers, dual 2.5" exhaust into a X-pipe in my 1983 G20 van put down right at 200 RWHP and 300 RWTQ. Stock injectors at 14 psi were insufficient over 4,000 rpm to feed the stock long block. They went static, duty cycle above 100% and the engine misfired terribly and fell flat on its face before 5,000 rpm. I swapped to a Marine regulator and injectors from a 405 HP Mercruiser 502. Those are high pressure 61s that deliver 105 lb/hr @ 32 psi. I could turn the engine 5,500 rpm after with no fuel issues. I put a Weiand 177 on the same engine and used a rising rate FMU. At 10 psi boost it took 50 PSI of fuel pressure to feed the engine. I went 13.7 @ 101 mph in a 5,300 lbs G20 van with the Weiand 177 on the stock longblock, 700r4 with stock 1,600 rpm converter, and 3.08 gears turning P295/50R17s.

61# injectors are good for about 72 lb/hr @ 18 psi. I have once flowed one on an injector bench in 4 psi increments from 10 psi through 62 psi. The high pressure injectors will reliable open up to about 70 psi, however above about 50 psi the increased pressure does very little to increase flow.

[C2500]

Sorry for the confusion guys I sent that post on timing with my phone..Any way I set 0* with the spout disconnected..shut it off and reconnected the spout then started and timing is in the 20* mark at idle..

[C2500]

Ed just sent me back my chip with an adjustment and told me that the bored TB I have may be junk. Any way here’s his chip anyone recognize it??12485

[daveosx]

The calculation for HP to pounds of fuel is universal not a short cut it's just the physics.
1 HP = 2545 BTU’s per hour

Gasoline is 5.92 lbs per gallon
Gasohol is 6.1 Lbs per gallon

Flat head ford is rated at 49% Therodynamic Efficiency and BTU per pound is based on that .
Modern engines run from the factory at @76%.
Theoretical 18000 BTU per pound of gasoline is based on 49% conversion ratio.
Stock 1979 and up engine is @76% or 28000 BTU per pound of gasoline.
Rings valve seats and throttle plate sealing provides 80% on typical modified engine or 29000BTU per pound.
1 LBS of fuel on a modern modified engine should produce 11.4 HP
Parasytic Loses valve springs, piston skirts, oil pump timing chain ETC lower the net power.

pumping 130 Gallons of air and 9.16 Gallons of fuel on that motor will yield 335 HP


C2500s engine build is the Super Chevy magazines Test mule for carboration and the same build is typically 450 HP.
These motors with carb are routinely dynode with 650 and 700 CFM at 450 HP

As far as your results I understand you are commanding the injector to open and close.
Have you put a scope on the injectors them selves while running and confirmed that you are getting the actual commanded on period.
I alter the ignition with CDI after the module and give a solid 5v pp signal to the reference line.
I get @7400 rpm no load at 4ms on 65/68 TBI injectors
I have seen refurb ECM with cheap slow Mosfet installed could be a slew issue rather than injector flow.

In example I had a 1995 Impala/caprice with 1996 LT-4 Corvette, Radically ported iron heads 280cc intake runners, 194/160 with orange ZZX Beehive springs, 1.6 roller rocker crane rev kit lifter springs, .575 intake .560 exhaust 117lsa cam, SPO timing chain on steel gears, Low RPM water pump drive, HyperUtetic Pistons with tungsten Disulfide coated skirts, Rods drilled with piston squirters, copper shim head gaskets final compression ration 10.8 : 1, HMV55HV millings with 5W-50 oil, oil cooler trans cooler, 4L60E with 4.3L convertor, truck low gear spray, Steel drive shaft GM 10bolt 3.73 with Eaton Posi.
TH ignition system was/ is triggered using opti spark pick up CDI box and twin coil MSD coils. Ford 4cylinder ecoboost injectors in sequential mode rather than batch.
Routinely ran at 7500RPM and even peeked it a few times at 8500rpm.
According to iPhone pocket Dyno and Cats Tuner dyno hit 600HP on regular basis.
Never drag raced the car just street racing.
Have driven the car at Taledega at over 180MPH and at Daytona over 160

[lionelhutz]

The i on that memory chip appears to be an Intel mark.

[C2500]

I spoke to Ed and he said to set timing at 0* with spout disconnected.. Can’t help but all this is confusing what some are telling me and what others are saying. I’m sure there are way to do this and all get to where it should be..I need someone to help me figure this out..
Also tuner pro web help never seems to help..lots of unanswered questions there...

[myburb]

There is lots of differing opinions and reasons for them. The initial timing is used only on the start and limp. The initial is added to the computer timing to get the final advance. If you just use the gm 0 degrees the spark table should be useable as it is. It you use some other initial you will have to modify the table to suit that. My two bits would be use 0 degrees and at a later date when you feel you understand it all, experiment as much as you like.

[lionelhutz]

There is a setting in the calibration called spark bias where you set the static advance of the distributor. So, if Ed says to use 0* that is because he knows the tune is set to work with 0* at the distributor. Basically, you just match the advance you set to the setting in the tune.

When you want to adjust the operating timing you don't the distributor or the static timing setting. You adjust the 3-D timing table.

[C2500]

[QUOTE=Fast355;68671]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.


What injectors would you recommend for my set up?? Thanks for any help

[Fast355]

[QUOTE=Fast355;68671]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.


What injectors would you recommend for my set up?? Thanks for any help

Keep the same injectors, get a fuel pump for a 1993-1995 Astro/Safari van with a 4.3 "W" vin code CPI engine and get your hands on one of the 28-32 psi marine TBI regulators. That will boost your 61 lb/hr injectors up to about 104 lb/hr each. At that point you are good to over 350 HP at 0.50 bsfc and 85% duty cycle. I am sure you can get a TBI to run to 7,200 rpm without load at 4 msec, but when you go to full load that same rpm would require a good 4-5x the fuel. Because the TBI injector fires twice per crank rotation, 4x per complete engine cycle, at 5,000 rpm you have something like 6 msec between injector firings. At 6,000 rpm you have 5 msec between firings injector. During that time you need to stay under 85% duty cycle and factor in both intake and closing times of the injectors. If it takes the injector 300 usec to open, its going to take roughly the same time to close. So that 5 msec cuts down to 4.25 msec to stay under 85% duty cycle and finally allowing opening and closing times it drops to 3.65 msec to spray the necessary fuel accurately without running the injectors static. What will happen if you get to 100%+ duty cycle is the the engine will develop a high rpm misfire and if you get into the 110% range it will quit pulling all together under load. Running an asynch fueling strategy will help this condition but you should closely watch the air/fuel mixture distribution if you run that.

[C2500]

Fast, Thanks for the above post. I have a new Delco EP376 pump Not sure what it is but I’ll look it up. I would think you want a AFR in the 13’s at wide open correct? I got a wide band hooked up and an LM1 on the seat next to me and went for a drive..Runs fairly well driving it normal but it has a lean heasation if you run at 25mph and lightly step on the throttle.. Also I went WO up a hill and in no time the AFR went into the 17’s. Not good
So I’m trying to tunerPro up and running as that’s another challenge with my limited skills with a laptop..

Delco EP 376 pump. Will this do?
http://www.gmtuners.com/tech/fuel_pumps.htm

[Fast355]

Fast, Thanks for the above post. I have a new Delco EP376 pump Not sure what it is but I’ll look it up. I would think you want a AFR in the 13’s at wide open correct? I got a wide band hooked up and an LM1 on the seat next to me and went for a drive..Runs fairly well driving it normal but it has a lean heasation if you run at 25mph and lightly step on the throttle.. Also I went WO up a hill and in no time the AFR went into the 17’s. Not good
So I’m trying to tunerPro up and running as that’s another challenge with my limited skills with a laptop..

Delco EP 376 pump. Will this do?
http://www.gmtuners.com/tech/fuel_pumps.htm

You want a cast iron head small block around 12:1 under and near peak tq and about 12.6:1 at peak hp. Those heads usually like a pretty quick advance curve and about 30-34 total advance at WOT. Cruise timing can be upwards of 42 at 3,000 and no load. If you set the initial timing at the distributor to 10* and set the Initial timing in the .BIN to match you can experiment up to about 52* of timing to see what works best at cruising speeds without load. Generally to get the no load timing right, I will use my Autoprom, hold the engine at a steady 1,200 rpm. Look at the timing cell the engine is running in KPA wise, holding the throttle steady, I will then slowly advance the timing until the RPM stops rising. I will repeat this process at every RPM value of the timing table up to about 5,000 rpm. For a good starting point you can then interpolate the values down to your WOT timing. Works well as a starting point for me.

[Fast355]

Fast, Thanks for the above post. I have a new Delco EP376 pump Not sure what it is but I’ll look it up. I would think you want a AFR in the 13’s at wide open correct? I got a wide band hooked up and an LM1 on the seat next to me and went for a drive..Runs fairly well driving it normal but it has a lean heasation if you run at 25mph and lightly step on the throttle.. Also I went WO up a hill and in no time the AFR went into the 17’s. Not good
So I’m trying to tunerPro up and running as that’s another challenge with my limited skills with a laptop..

Delco EP 376 pump. Will this do?
http://www.gmtuners.com/tech/fuel_pumps.htm

Looking at that chart that pump should be fine, good for 550 hp @ 32 psi. I did not realize you had already upgraded the pump.

[C2500]

Fast pumps not in yet I have it here new inn the box and was asking if that one will do and it will.. Thanks