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

[Cadmaro]

Did you have your TB bored out to 46mm too?

No I haven't. Still stock bore with "ultimate tbi mods"

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.

I have remove the factory regulator and have an aftermarket adjustable regulator mounted on the firewall. I've played around with different pressures and this seems to be where it is happiest with the current tune.

Are you still running the 1227747 ECM?

Yes

Have you adjusted your timing tables at all?

Supposedly the guy I ordered a tune from did. Not totally convinced much was change based on how it runs. Plugs seem to indicate on the verge of lean at idle/part throttle and pig rich at WOT. Truck will also all but die if you wack the throttle open but doesn't hiccup if you roll into it.

 

[PlayingWithTBI]

I have remove the factory regulator and have an aftermarket adjustable regulator mounted on the firewall. I've played around with different pressures and this seems to be where it is happiest with the current tune.

Unfortunately it's all related - too small TB, too little injector DC, wrong timing maybe? If you want 375 HP, you need to start there.

I tried running with an 18LB spring set at 18LBS (in my 46mm TB) and was running ~106% duty cycle WOT @ 5400 RPM, which meant they were open all the time. Do you have a WBO2 or are you guessing at what "pig rich" is?

 

[Cadmaro]

Unfortunately it's all related - too small TB, too little injector DC, wrong timing maybe? If you want 375 HP, you need to start there.

I tried running with an 18LB spring set at 18LBS (in my 46mm TB) and was running ~106% duty cycle WOT @ 5400 RPM, which meant they were open all the time. Do you have a WBO2 or are you guessing at what "pig rich" is?

I do not have a wideband 02. Guessing by reading spark plugs. Tips light grey, end of porcelain light tan, base of porcelain jet black. From the research I've done after buying the tune, I know this setup really needs a bored throttle body and most likely would like to have 454 injectors along with a better ecu. At that level of money input a Holley sniper starts to look like a more attractive option but funds can't currently be allocated to that. I already have a brand new Holley carb and HEI distributor that were for another project that fell through. In theory, I just need to lower the fuel pressure and figure out the wiring to run that setup hence this post.

 

[PlayingWithTBI]

I stole it with pride from another site but, when my last laptop died, I lost the link in my "Bookmarks", ha ha.

I found the link - some good reading. At the bottom of post #3 is the link to the spreadsheet.
www.thirdgen.org/forums/diy-prom/761049-spark-advance-table-example.html#post6301288

I can't remember who said it but - instead of tuning your carburetor once a week, why not let EFI do it 100s of times a second?

 

[Erik the Awful]

Is your engine already together? I'm assuming you're using the 'Vette cam because you already have it. If not, call a Cam company and get a recommendation for a cam, you'll be happier. Do some cleanup work on those Vortec heads. I didn't do that on mine, and I regret it. Above 4000 rpm my engine really seems to run out of flow.

For switched power on my Sniper, I tapped one of the injector leads. It's constant 12v, and the factory computer grounded the injector to fire it.

My factory computer is still in place, and it runs the torque converter clutch just fine, even with the TBI gone.

Should the oil pressure switch be taken out of the circuit to ensure the bowls of the carb are always fed?

Good question. Which do you fear more: not noticing the oil pressure disappearing and nuking you motor, or getting stranded and having to troubleshoot your crapped out oil pressure switch on the side of the road?

 

[Cadmaro]

Is your engine already together? I'm assuming you're using the 'Vette cam because you already have it.

Yes it already together and used it because I was trying to use up some spare parts and still keep it TBI compatible

For switched power on my Sniper, I tapped one of the injector leads. It's constant 12v, and the factory computer grounded the injector to fire it.

Good idea. I'll check there.

My factory computer is still in place, and it runs the torque converter clutch just fine, even with the TBI gone.

It still runs lockup without a tps signal?

Good question. Which do you fear more: not noticing the oil pressure disappearing and nuking you motor, or getting stranded and having to troubleshoot your crapped out oil pressure switch on the side of the road?

Sounds like it's getting bypassed. Easy enough to keep an eye on the oil pressure gauge.

 

[Erik the Awful]

It still runs lockup without a tps signal?

Yes. It's the only reason I still have the factory computer and wiring. I think even the HEI will run without the computer - it just won't have spark advance.

 

[ccreddell]

On my 90, I built a 383, did the vrfpr, ran a 7.4L TBI with the 2" factory spacer that mounts to Edelbrock spreadbore mani. I think the cam is a comp 268 roller. Oh, and I run the FLASH ECM. I finally got it to run, but really badly. Could seem to get it tuned. So, I'm in the process of slapping on the Sniper. Going to run the Small cap HEI to save money.

 

[tayto]

i made 375FWHP on a 10:1 355 w/ vortec heads, 218/228 @ 113 LSA cam and 454 TBI unit with 67# injectors @ 32 psi. the L98 cam is more like 202/206 duration, at least the one I degreed was... you will be leaving some power on the table with that tiny cam. I would have gone with the LT4 hot cam here. stay away from comp xfi stuff

EDIT: keep the ECU and let it control lockup. you will hate driving around with the old school shift into 4th and lockup immediately IT SUCKS. you don't understand how the fuel pump circuit works. the oil pressure switch is there as a means to supply voltage to the fuel pump if the relay fails. it also IMO takes some of the load off the relay contacts once the engine is running