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Parts list for TBI to Carb

33K views 33 replies 10 participants last post by  BogiesAnnex1 
#1 ·
Can you all help me out with what I will need to do a TBI to Carb conversion. Its on my chevy truck. I know I need: Braided fuel line, Fuel pressure regulator, HEI distributor, and I cant think of anything else besides carb and intake. As far a a regulator I have seen alot of different ones but I dont know which one to get. Can anyone help me out with a link to one.

Thanks....SS
 
#2 ·
If you check out diyautotune.com site you'll find there is a section with directions on how this guy converted his carbed 350 to tbi. He is using a megasquirt ecm, which you do not have to use, but if you read and watch the 1st and 2nd parts of the conversion, then you'll find all of the parts needed.

In terms of parts, a short list is intake and coolant temp sensors (same sensor), IAC and TPS that is on the GM TBI, and MAP or MAF sensor depending on ecm being used. Fuel pump rated for the TBI you have. If you are using a stock GM TBI, then I believe the fuel pressure regulator is built into the throttle body. Also, if using the stock TBI, then you'll need both lines (steel braided) that attach to the rear, properly rated fuel pump, and a fuel filter rated for the TBI unit (micron size). Now that I've typed all of this, are you planning on using an aftermarket setup or stock setup?

What are you planning on using for an ecm?

And also, if you are running something like Megasquirt, you don't have to swap distributor's, but is highly recommended. Otherwise you have to know exactly what is going on with your distributor in terms of vacuum and centrifugal advance at varying MAP readings. Basically you have to have your old distributor tuned - MUCH easier to bolt on the HEI unit!
 
#4 ·
Can you all help me out with what I will need to do a TBI to Carb conversion. Its on my chevy truck. I know I need: Braided fuel line, Fuel pressure regulator, HEI distributor, and I cant think of anything else besides carb and intake. As far a a regulator I have seen alot of different ones but I dont know which one to get. Can anyone help me out with a link to one.

Thanks....SS
you didn't say what year truck... but I'll guess some

you'll need what you listed..
the fuel reg. can't be what they call a deadhead. you have to use one that uses a return line.
if your trans is computer controlled it will not like this at all and you'll be forced into a stand allone controller
if the tranks is a 7004r you'll need the correct t.v. cable and brackets . if factory intake you can use the factory brackets, if aftermarket you'll need to get the brackets that work with that intake.. edel/etc makes/sells them..
you'll need a hei thats not computer controlled..

your check engine light will be on. if they plug into the service port for state inspection. you'll fail..
your cruise will not work..
and your speedo might not also (not knowing the year/model)
good luck
 
#7 ·
Depends on year and transmission. For a stick no problem, for a 700R4/4L60 analog 4 speed automatic no problem, for the 4L60E (E for electronic) digital 4 speed automatic, 1993 in some cases and 1994 up in all cases it's a PIA to keep needed sensor data flowing to the tranny.

For the manual gear box or 700R4/4L60 automatic you will need an adjustable bypassing regulator to drop the fuel pressure to 4-6 psi. I know these regulators are expensive but by the time you fiddle the existing fuel system to accept a low pressure pump you'll spend this much or more and have to do a lot of plumbing and wiring work to boot, so the expensive regulator is the easy way through the problem of fuel pressure. by p****ing regulators prevent the pump from dead heading when the enigne is idleing preventing the electric pump from overheating Other than that you'll need a 4 barrel intake that bolts to the Swirl Port heads, the difference between these and older heads is just the angle the bolts make to the head's intake mounting surface pre 1987 use a 90 degree angle 1987 up use a 72 degree angle. Open the instrument cluster and remove the bulb from the check engine light. Hardwire the fuel pump relay to the ignition switch as the computer is looking for a tach signal to keep the fuel pump running, if you change the distributor to an older HEI (and who wouldn't in this case) there is no point to keeping the computer in the distributor loop so there will be no tach signal to the computer, so the hard wire gets around all that.

Note, this is NOT LEGAL to do anywhere in the U.S. while your state may not enforce Federal Law on emissions nor have emissions laws itself; it still is a Federal Law; so be aware of that.

Bogie
 
#8 ·
Well I am not 100% sure of what my transmission is. I thought it is the 4L60E but How do I tell. Looks like that is my only issue Im going to have to deal with. If it is a 4L60E could I Use a computer out of a truck with a manual trans and take care of it. First tho how do I tell what trans it is? It is a 94 so Im guessing it is computer controlled. If it is what year trucks had the 4L60 in them so I can get on or is the 700r4 the same length.
 
#13 ·
The 700R4/4L60 will have a mechanical cable link coming from the TBI throttle linkage which is the modulator control; the 4L60E will not have any mechanical linkage or cable connection with the throttle linkage. Underneath the 4L60E will have a big black box on the driver's side that the shift lever connects to and a very large wire bundle that enters the case on the passenger side.

If this has the 4L60E, my advice would be to fix the TBI. Not knowing why you want to replace the TBI if it's malfunctioning or you want more performance. Either-way it's probably less expensive to repair or hot rod the TBI. The 4L60E can be operated from a standalone controller but this isn't very cheap. The other way would be to put in a 700R4/4L60. There is no cheap solution to the fully electronic vehicles that start with the 93-95 late OBD I production and it only gets worse with 96 up OBD II production. The levels of Systems Integration just keep increasing making it ever-more expensive and difficult to simple these later model vehicles.

Bogie
 
#16 ·
So are you sayin the none computer trans 4L60 has two cables hooked to the throttle body.
The 700R4/4L60 will have a mechanical cable connection from the throttle bell crank that runs down to the transmission. This is the modulator that controls shift points and kick down in relation to throttle position.

The 4L60E will not have this feature connected to the throttle; it gets modulation as digital data from the computer which gets it data from the throttle position sensor mounted on the TBI body and is connected to the computer through the electrical wire harness.

If you have a 4L60E this is the program to follow for the lowest cost solution:

- The existing computer needs the Coolant Temperature Signal (CTS), Throttle Position Signal (TPS), Vehicle Speed Signal (VSS), and the RPM (tach) signal in order to operate the transmission.

- While you can fabricate brackets and a connection to the throttle shaft for your existing TPS, Holley sells a kit that makes this way simpler but it needs to be a Holley with an electric choke. >>> Holley Performance Products Throttle Position Kit for Electric Choke Carburetors 534-202 <<<

- The VSS signal comes from the tranny, there is some variation in frequency year to year so keeping your tranny and computer together for your year of truck keeps the speedo tracking with reality and the tranny shifting correctly including the TSS converter lock up.

- The Coolant Temp Signal (CTS) is not the same as what drives the engine temp gauge on the panel, you'll need to use an intake manifold that provides space for two temp sensors unless you move the gauge sender to the cylinder head fitting. This mostly controls TSS converter lock up and prevents 4th overdrive if the engine is running hot and working hard, this protects the tranny from going up in smoke and trust me it will if overloaded in TSS and or 4th gear.

- Since going to a carb would indicate perhaps a performance application then you should also use a stand alone distributor so the computer isn't running the engine on its emissions profile, so an earlier model HEI is needed. Connect its tach lead to the purple and white wire that goes into the old distributor connector of the TBI distributor.

- The tach signal is used for shift points which are modualted by the TPS, it also contributes to converter lock up TSS and 4th gear application.

- Follow what I said for an adjustable fuel pressure regulator with a return. The return is needed to keep the pump from dead heading which will cause it to overheat.

Keep in mind that none of this is legal to do.

Bogie
 
#18 ·
Here is some pics of the throttle body

One of the cables run to the black box by my brake booster.

Also can anyone tell me what that connector plugs into? Its been unplugged since I got the truck and there IS NO WHERE for it to plug into. It only reaches about 8 inches so I can see everywhere it could go but nothing there to plug into.
 

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#21 ·
the 700R4 and 4L60 (no E) are the same transmission. The TH350 with a lock up converter was modified back in the early 1980's into a 4 speed overdrive with a lower ratio first gear. This provided more torque to be delivered to the rear axle from a stop with less throttle opening, thus better stop and go fuel mileage. For cruise they put in an overdrive 4th for better milage. Then they tended to chenage rear axle ratios from the 3 speed TH350 which tended to be in the 2.7-2.8 to 1 range to 3.08. This gave brighter acceleration during the mid gears of 2nd and 3rd, while the 80 something% overdrive dropped freeway cruise RPMs to or even less than the 2.73 ratio axle.

The ealy 700R4 had it's share of problems which were worked on year to year becoming pretty well solved by the 1987 production cycle. In 1990 they renamed their transmissions into a formate that explaned the number of gears (4), the positioning longitudinal (front to rear became L), 4L60 and a gross vehicle weight application of 6 times 1000 pounds or 6000 pounds designated as (6). These continued to be used in production till 1993 and 1994 where by model and time period in thoae years they were replaced by a redesigned version that is electronically contolled rather than using the classic differential modulated pressures that controlled automatics since their inception back in the late 1930's.

So the new version of the 4L60 acquired the suffix (E) to show that it is the same transmisison structurally as the 700R4/4L60 but is now controlled electronically by the vehicle's computer. So we end up with the 4L60E which has continued on to become the 4L65E which uses a seperated bolt-on bell housing and other changes that uprate its use by 500 pounds, so it is rated for a GVW of 6500 pounds.

The transmission you have gives a lot of flexability to make engine and rear axle changes that are more difficult and expensive to do the "E" electronically contolled versions.

All right, I have an appontment at 4:30 PST so I'm out of here.

Bogie
 
#25 ·
What i'm trying to say is that unless the engine is built enough that the fuel injection wont work, there is no benefit to a carb. It will be worse in fact. You couldnt pay me to take a carburetor on a vehicle anymore. Its bad enough you have to put up with them on lawn mowers and such.

It will probably be easier and cheaper to modify the TBI to work with what you have, assuming it wont work as is. Alot of times you can just change the programming in the ECM and youre done. Other times you need bigger fuel pumps, bigger injectors, adjustable fuel pressure regulators etc on top of changing the programming. In that case a carb might be easier and cheaper but either way youd be looking at alot of work.

The way I understand it you are wanting to use the stock tbi fuel pump and regulate it down to carb fuel pressure (which if i recall right isnt a big drop - i dont remember TBI having much more than carb fuel pressure anyways) which tells me the engine is atleast close to stock. Otherwise that pump couldnt supply enough fuel. I would think you could get away with changing the programming in the ECM and be done.
 
#32 · (Edited)
What i'm trying to say is that unless the engine is built enough that the fuel injection wont work, there is no benefit to a carb. It will be worse in fact. You couldnt pay me to take a carburetor on a vehicle anymore. Its bad enough you have to put up with them on lawn mowers and such.

It will probably be easier and cheaper to modify the TBI to work with what you have, assuming it wont work as is. Alot of times you can just change the programming in the ECM and youre done. Other times you need bigger fuel pumps, bigger injectors, adjustable fuel pressure regulators etc on top of changing the programming. In that case a carb might be easier and cheaper but either way youd be looking at alot of work.

The way I understand it you are wanting to use the stock tbi fuel pump and regulate it down to carb fuel pressure (which if i recall right isnt a big drop - i dont remember TBI having much more than carb fuel pressure anyways) which tells me the engine is atleast close to stock. Otherwise that pump couldnt supply enough fuel. I would think you could get away with changing the programming in the ECM and be done.
Politely disagree. Modifying a TBI is labor-intensive and doesn't really allow for many gains given its diminutive size and capability. You have to do a bunch of trial-and-error PROM chip burning. It's not a "plug in your laptop" tuning, it's finding someone with the talent, ability, and hardware to burn multiple chips. The TBI was not designed as an improvement over the Qjet it replaced, it was a marketing choice. Everyone else was going EFI and GM just couldn't be left behind. In truth, the last Qjet that rolled off the line was in 1989, and it was still perfectly fine meeting emissions standards. The truth is, the TBI was forced on Rochester, and my buddy was one of the engineers behind it. They struggled for almost two years to get it to pass emissions, and the resulting 180hp from a 350 was the best they could really do in 1988. They had to neuter the cam and use tiny ports in the heads to compensate for the TBI's wet flow of only about 390 cfm. The TBI is also not really fuel injection. It has large nozzles that squirt liquid fuel at about 25 psi. It's about as sophisticated as having your friend under the hood with a Windex bottle full of gasoline.

That aside, hundreds of thousands of Qjet conversions have been done, and the results on an otherwise stock motor are usually about 10-15hp gains and 2 mpg gains by going back to the carb.

Not to mention... the OP has given multiple reasons why he is doing this, not the least of which is "hairy cam and 383." He won't be tuning a factory TBI to handle that. It's not my truck, so rock on and have fun with your farm truck!
 
#26 ·
Im building a 383 vortec stroker for this With big cam. I dont want a custom chip burned. Once I have all the parts it will take no time at all to do this swap. I already have an intake. The stock fuel pump will be fine but need to know what regulator to use. It would cost more to stay with TBI and get an upgraded intake and throttle body. That right ther could cost me up to 600-700 bucks. Im old school and this is what I want to do. Its not a daily driver and Ill keep all the wiring and computer in the truck in case one day I want to go back to TBI. If I wanted to stay with TBI and had the money I would go the e-street route but I dont and Im not gonna go to a junk yard and get used TBI parts. I can get awsome power and tourque by doing this and not spend alot of money in the process. I dont care about cold start issues. Gas mileage isn a concern.

Thanks everyone for all the input
 
#27 ·
This would make sense, TBI isn't really friendly toward big cams, it gets very difficult after about 210 degrees of duration to keep enough vacuum in total and enough sensitivity between the relationship of vacuum changes to throttle opening to RPM to keep it fueling correctly. Additionally, the 2 barrel injector body has a real problem in supplying enough fuel for power at and certainly above 330 horses, at this point two 90 pound injectors are working themselves to death just to keep up. This why Holley went to the 750 and 900 CFM four barrel, four injector configuration, this was to get enough injector flow capacity while still retaining reasonable injector life more than an effort to get more air flow.

Bogie
 
#29 ·
This is not a daily driver. This is a farm truck for hauling wood and a horse trailer and anything else. If you take care of an engine u can get 300,000 miles easy and that's with stock parts (bearings rings or whatever). I'm not putting a s h i t load of money in this. Now I might consider e-street efi with my roller engine I talked about in other posts. I seen the guys use that on horsepower tv and I was impressed but $ is an issue and I want my money going towards my big build. I appreciate your advise but that's not what I want on this build. People are entitled to their own opinions so I respect what you are saying but I'm not spending money on a upgraded or used TBI or EFI system as we all no they aren't cheap. Put it this way the most expensive thing I'm buying for this is the 383 crank.

Thanks.......SS. :drool:
 
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