Vortec headed tbi to carb swap - wiring and 700r4 lockup

[Cadmaro]

Sorry if this isn't the right place but I've been searching for weeks and haven't found the answers I'm looking for. I have a 1990 scsb z71 that I bought with a knocking engine so I put together a bunch of spare sbc parts I have accumulated over the years with the goal of 375hp.

Engine is bored 0.040, KB pistons, forged rods, stock crank, 9.67:1 compression with around .038 quench, 062 vortec heads, 88 Corvette roller cam (207/213 @.050, .415/.430 lift on 117 lsa) jegs carb intake, ultimate TBI mods, 18psi on stock 350 injectors, Spectre 14" air cleaner, 1 5/8 mid length headers, 3" into 3.5" y pipe, dynomax muffler and a tube from someone we will leave unnamed for now.

Basically, the truck runs, but nowhere near like I feel it should and driveability isn't even as good as several similar carb combos I've built in the past. I think I want to carb swap it as fitech or Holley sniper don't fit the budget for now. Just a couple small questions remain and I haven't been able to find the answers I need.

1:Where can I find a good switched power source for the electric choke. Would really prefer in the engine bay somewhere besides the distributor and fuel pump.

2: Torque converter lock up wiring : is it really as simple as sending switched 12v power to pin A on the 4 prong plug for 4th gear only lockup? Lots of contradictory info out there

3: fuel pump wiring. Should the oil pressure switch be taken out of the circuit to ensure the bowls of the carb are always fed?

Sorry again for the long winded post but thanks in advance for any help

 

[kennythewelder]

Things all need to work together. So, what gear ratio do you have, and what size tires. I once worked with a mechanic offshore that would put 411 gears in any of his old street trucks. He said, 411s will make a I-6 engine run good. This was a long time ago, so I know he had 275-60-15 tires. I have 373s in my 97 with a Eaton trutrac in the rear end, and 275-60/15s all around. I am at about 325 HP. My truck runs about 14.90 quarter mile. This is an extended cab SWB 2-WD.

 

[Schurkey]

Perhaps a moderator will move this thread to LSx + Carb Swaps sub-section.

LSX + Carb Swaps

This section is for info on LSX or carburetor swaps into the OBS trucks.

www.gmt400.com

 

[Cadmaro]

Truck has 3.73 gears and currently has 285/70r17 tires. Has had 265/75r16 and 265/70r17 on it as well since the engine was installed. Probably a little under geared for the 285s but the performance has been fairly underwhelming with all 3 tire sizes. I feel like my issue is based more in the airflow restrictions of the throttle body and the tune. Would be surprised if this thing would even break into the 15s.

 

[Cadmaro]

My bad. Wasn't sure if this should go there as it isn't LSX related.

 

[kennythewelder]

My bad. Wasn't sure if this should go there as it isn't LSX related.

No problem, I moved it.

 

[kennythewelder]

Truck has 3.73 gears and currently has 285/70r17 tires. Has had 265/75r16 and 265/70r17 on it as well since the engine was installed. Probably a little under geared for the 285s but the performance has been fairly underwhelming with all 3 tire sizes. I feel like my issue is based more in the airflow restrictions of the throttle body and the tune. Would be surprised if this thing would even break into the 15s.

Man, that's is slow. Sound like you gear ratio is not off much if at all. Perhaps @PlayingWithTBI can shed more light on this for you.

 

[Schurkey]

Wild Guess: The last thing holding you back is "airflow restrictions of the throttle body".

I'm mighty suspicious of your "tune".

I think you're creating needless problems for yourself by carb-swapping instead of figuring out a better "tune"--or locating an actual problem with the engine or computer system.

What is your cranking compression pressure?
What is the fuel pressure UNDER LOAD?
What does the scan tool show for sensor data, and output response?
Initial timing? Does the timing advance?
Is the magnet by the pickup coil in the distributor broken?

 

[Cadmaro]

I'm mighty suspicious of your "tune".

What is your cranking compression pressure?
What is the fuel pressure UNDER LOAD?
What does the scan tool show for sensor data, and output response?
Initial timing? Does the timing advance?
Is the magnet by the pickup coil in the distributor broken?

I am suspicious of the tune as well. But what do you do when most of the tuners you contact about a tune tell you your combo is pushing the limits and that they you'd be better off with aftermarket stuff?

Would need to go rent the tool to check cranking compression. Might try to this afternoon.

Not really sure how to check the fuel pressure under load because no Schrader valve in the fuel line and I can't see the guage on the fpr with the hood closed.

From what I can tell, the scan tool doesn't show any abnormalities in sensor data and the tuner hasn't pointed any out either.

Initial timing is at 0° , tuner said total timing is "about 3 degrees more than stock"

Entire distributor is brand new so I assume the pickup coil is working correctly

 

[PlayingWithTBI]

Perhaps @PlayingWithTBI can shed more light on this for you.

I'll try

ultimate TBI mods, 18psi on stock 350 injectors, Spectre 14" air cleaner,

Did you have your TB bored out to 46mm too? 46mm will flow 650 CFM which is good for a 5.7L up to 6K RPM. Or you can go with a 50mm from a 7.4L. An 18LB spring for your set up isn't enough, closer to a 28 LB spring from a 94 - 95 7.4L would be better. Here's a calculator, look on the right side "Approximate HP Supported" based on 0.45 Brake Specific Fuel Consumption and 85% Injector Duty Cycle. Or go with bigger injectors but, you may have issues with idle unless you have a VRFPR.

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Are you still running the 1227747 ECM? The problem with it is it's too slow for a performance aspect. It runs at a 160 Baud Rate. Think about going with the EBL Flash-II, it runs @ 8192 Baud and has automatic "Learn VE" where you can tune with a WBO2 all the way up to WOT. Plus it has expanded tables up over 6K RPM.

EBL_Flash

Have you adjusted your timing tables at all? There's a lot left on the table with the stock GM tune. I have an Excel spreadsheet for that but can't post it here - this site won't accept .xls(t) extensions. I stole it with pride from another site but, when my last laptop died, I lost the link in my "Bookmarks", ha ha. I need to take the time to find it again or, you can PM me your Email address and I'll send it to you. Here's what mine looks like right now (note low end MAP and RPMs are flattened out to smooth out the idle) with "Fast Burn" design aluminum heads. YMMV

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Enjoy!