tbi to carb , vacum gage reading / perfomance help

[Busted Knuckle]

That is not true, there are plenty of carburetor equipped engines that had over drive and ran just fine. TBI was introduced because it is able to control air/fuel mix MUCH better than a carb.

That is not true. We had modified Rochester 2bbl, 4bbl with PWM controlled metering rods that were able to go closed loop for the 14.7:1 AFR. They were used from 1980 until TBI.

The directive from Engineering Staff and Milford Proving Ground was that carbs would not have adequate driveability with overdrive transmissions. Those with tall diff ratios would not work at all. Something like a 4.10 diff you could get away with it, but only with expert carb tuning and no closed loop. Couldn't meet emissions.

 

[Schurkey]

because in overdrive you will not have enough vacuum to pull fuel out of a carb. And before the flow stops completely, you will be operating in enrichment mode. It should run fine by NOT using overdrive.

We had modified Rochester 2bbl, 4bbl with PWM controlled metering rods that were able to go closed loop for the 14.7:1 AFR. They were used from 1980 until TBI.

I remember mixture-control solenoids in various Rochester carbs. (Not ONLY Rochester carbs.)

The directive from Engineering Staff and Milford Proving Ground was that carbs would not have adequate driveability with overdrive transmissions. Those with tall diff ratios would not work at all. Something like a 4.10 diff you could get away with it, but only with expert carb tuning and no closed loop. Couldn't meet emissions.

I'm thinking that the 700R-4 was introduced for the '82 model year, in mid-year '81. From then until the end of carbs; you're saying the driveability at highway speed was "inadequate"? Millions of cars and trucks made with overdrive and a carb, and they all had "inadequate" driveability at highway speed? Of course, prior to '87, the National Mandated Speed Limit (NMSL) was the dreaded "55"; so very low engine RPM as well-this according to you would make the fueling even worse. (The NMSL wasn't fully repealed until 1995.)

Please confirm that this is what you mean.

For the record, I took a TH350 out of a '75 Nova, bolted a TH700 in place. Even had a horrible vacuum switch from TCI to operate the converter clutch. That was my highway-driving work car for about a year. The only driveability issue I had with it was due to that piece-of-crap vacuum switch that had no hysteresis. As soon as the converter clutch engaged, vacuum dropped...so the converter unlocked. Which raised RPM and vacuum, allowing the converter clutch to re-engage. Cycle repeated.

 

[evilunclegrimace]

That is not true. We had modified Rochester 2bbl, 4bbl with PWM controlled metering rods that were able to go closed loop for the 14.7:1 AFR. They were used from 1980 until TBI.

The directive from Engineering Staff and Milford Proving Ground was that carbs would not have adequate driveability with overdrive transmissions. Those with tall diff ratios would not work at all. Something like a 4.10 diff you could get away with it, but only with expert carb tuning and no closed loop. Couldn't meet emissions.

That is funny, there were tens of thousands of Chevy's S-10's with 2.8 v6 engines that had carbs and 700r4 transmissions and they worked just fine.

 

[454cid]

Don't forget about the th200r4, too. I'm pretty sure that's what was in my grandmother's Buick Lesabre with a 4.1l V6. That had one of the electronic quadrajets in it, I think.

 

[Busted Knuckle]

That is funny, there were tens of thousands of Chevy's S-10's with 2.8 v6 engines that had carbs and 700r4 transmissions and they worked just fine.

Those are lightweight vehicles where the vacuum is not going near zero for expended periods of time. If its has a computer controlled carb, a lot of critical factors like TCC lockup and spark knock are already addressed.

This is exactly why 2500's have hydroboost.

 

[evilunclegrimace]

Those are lightweight vehicles where the vacuum is not going near zero for expended periods of time. If its has a computer controlled carb, a lot of critical factors like TCC lockup and spark knock are already addressed.

This is exactly why 2500's have hydroboost.

Ok, I suppose that a suburban with a 350 engine and a 700r4 with a 3.42 gear set is a light weight vehicle
There were more 2500's and 3500's built with out hydroboost than with it.

 

[evilunclegrimace]

And another thing, the only time the engine sees " near zero vacuum" is a WOT and when you go WOT the transmission down shifts out of OD into 3rd gear. Lock up in OD is for cruising at light to mid throttle .

 

[PlayingWithTBI]

when you go WOT the transmission down shifts out of OD into 3rd gear. Lock up in OD is for cruising at light to mid throttle .

^^^Yup
Plus, when you go to WOT the TV increases pressure which makes it drop down. Here's typical tables (from a 7747 ECM) in which it goes out of lock at ~50% TPS at higher speeds in 3rd or OD.

You must be registered for see images attach

 

[retorq]

Those are lightweight vehicles where the vacuum is not going near zero for expended periods of time. If its has a computer controlled carb, a lot of critical factors like TCC lockup and spark knock are already addressed.

This is exactly why 2500's have hydroboost.

Are you the same Busted Knuckle guy from NastyZ28?

 

[Trlrider]

That is not true. We had modified Rochester 2bbl, 4bbl with PWM controlled metering rods that were able to go closed loop for the 14.7:1 AFR. They were used from 1980 until TBI.

The directive from Engineering Staff and Milford Proving Ground was that carbs would not have adequate driveability with overdrive transmissions. Those with tall diff ratios would not work at all. Something like a 4.10 diff you could get away with it, but only with expert carb tuning and no closed loop. Couldn't meet emissions.

Hmm!
I have a 1994 350 Ex-TBI engine in a Suburban 4x4 , 670 Holley now sitting on a Performer intake, ignition switch to straight HEI. A 700R4 now resides behind the engine pulling 3.55 gears.
The only other change to the engine was a better cam. Ditched the crappy factory emissions cam to one with a little more lift and duration, not much. Still currently have the crappy heads on that project. It is pending a 6.0 LS swap!

One needs to remember that the early TBI truck engines were only rated at 210hp. In reality at the wheels comes out to maybe 165hp. in factory trim. The cams sucked and the cylinder heads sucked even more to meet emissions. We did find on a buddies 1990 that a 500 cfm 2bbl Holley did very well with good throttle response.

So there are many factors that come in to play and how much you want to play with the old emissions engine and what state you live in.
But if you have the 4L60E without a stand alone tcm with the elimination of the TBI system, you are on borrowed time.
Swap to a 700R4 (or non-E 4L60) like I did or get a stand alone TCM!

Greeting from South Central Texas.