Engine and Transmission Tech

Ive been looking around a lot, and seems like gm used the same tbi with different injectors for the small blocks and V6s. For a 2.8, with some headwork and a larger cam, should I use a tbi with 4.3 injectors or 350 injectors, since that was only a little over 200 hp in that era? If i get the 350 version, could i just use megasquirt and have it deliver a little less fuel if its too much?

Hi Rob

Like you, I've tried to do some research/study in this area. Based on what I know about Megasquirt and the GM TBI you could go with the stock 350 injectors and let the computer adjust the fuel flow. Having said that I found some info showing there are injectors that are a better match for your 2.8. I found the following on another web site:

These injectors are rated at the pressure used by the factory, about 13 PSI Application Throttle bore size Injector color code Flow rate Part #

2.8l v6 1-3/8" Black/White ??? GM 5235430
4.3L 1-13/16" Yellow/Blue ??? GM 5235203
5.0L 1-13/16" Green/White 40 lb/hr GM 5235279
5.7L pickup 1-13/16" Orange/Black 55 lb/hr GM 5235206
5.7L Cop Car
1-13/16" Yellow/Brown 65 lb/hr 17084327-ACR
Late big block pickup 2" Blue/Black 80 lb/hr GM 1708430 big block pickup 2" Dark Red/Dark Blue 90 lb/hr GM 5235231

later,

Paul

A few places ive read said not to match the injectors to engine displacement, but to horsepower rating. Get the injectors that are equal to or a little above what your hp is. Ive read so much, its hard to know whats really right or wrong. In a few weeks ill try to get over to the u pull it yard and get some good measurements. Im trying to keep the cost low as possible, so ill get as mch as i can off a donor vehicle. But first, i just got my cam in, and lightened lifters are on the way, had my spare heads magnafluxed, and getting ready to send them off for bigger valves and port/polish/mill. Hopefully at least this part will be complete by summer, then next will be efi. Head work is very expensie these days.

Yesterday I ended up passing a u pull it yard on the interstate and pulled in.. these gm tbi's were missing off most cars. The few V8 cars that had the tbi's on them still were all missing the injectors. Maybe this swap is very common? I finally found one on a 4.3 astro van that just got there a week before after looking in about 40 different cars. In a couple months when i get around to doing this, ill see if these injectors will work, if not ill swap them for 305 injectors. The collar that goes between the tbi body and intake is very tall, but if you find a blazer 1990ish, it had a slightly different looking fi system, i think it said mpi on it or something like that, but it had the same collar as the tbi, but was only about half as tall. The tbi with that collar is about 3-7/16 inches tall to the top of the collar. I also read a lot of people use an econoline van frame mounted fuel pump with the gm tbi. There were only 7 or 8 vans there, and only 1 had the fuel pump in it, all others were taken already. I was also having trouble finding the IAT sensors in the air cleaners, those were all taken too.

my friend mark has an 88 ford t bird.302 injected,3.73 gears and an overdrive.he gets 28MPG! he is a fuel efficiency nut(read that cheap) and knows the mpg of all his vehicles .he has an aod trans in the car.if a motor is lugging at low rpm it can take more fuel to run than an engine at a higher rpm and running efficiently/smoothly.your best bet is an aod trans and a taller tire if you can.the lower first gear in the trans will get you out of the hole quickly and highway cruising will be very EASY on the wallet.my 2008 mustang is at 2000 rpm highway speeds and gets 30 mpg canadian gallon.now stock they have a 3.31 gear and get 35mpg but i put a gt rearend in it with 3.55 gears.keep in mind it is also supercharged and makes 350rwhp ,that's well over 400hp at the motor without the nitrous.get yourself an air/fuel gauge and wideband o2 sensor to keep an eye on things.you should be getting more mpg but your cam may be hurting you. what are the cam specs (all of them) and what is the vacuum at idle and cruising?if you go sequential you tb can go just about anywhere and the injectors won't interfere at all with hood clearence.hear is a pic of an intake i made for my stang ,notice the tb is lower down at the front.you could make a bonnet or plenum on top of the manifold and mount the tb forward or off to the side.because the sequential efi system only passes air and not fuel through the tb it can be just about anywhere.tb injectors with the injectors in the tb still must deal with fuel distribution problems to all cylinders such as fuel suspension dropout and fuel perculation on hot days especially if you race it or are a spirited driver.mutiport efi is the ONLY way to go in my opinion if you are going to switch to injection.just my 2 cents.good luck.notice the box between the runners on the manifold.it is filled with dry ice and chills the manifold to -38degrees.cold air=hp.it was worth 2 tenths of a second in back to back testing in the 1/4 mile.yes that is frost on the manifold.i just sit a six pack on it for a few minutes if i get beat out and drown my sorrows.lol





Edited 1 time(s). Last edit at 03/20/2011 02:52AM by rampant racing.

IMHO, NOT being an expert, I feel tbi's do just about everything better than carbs, especially for street use. Things like a correct air-fuel ratio, regardless of atmospheric conditions, are a big. Used/rebuilt oem systems are real cheap and adaptable to our motors. Fuel distribution ? Say (2) 2bbl/tbi's on an Offy/ 2bbl manifold, should have very good distribution.I feel tbi's also "atomize" better. Why not use Rover injection ? Good Luck, roverman.

A carburetor, if properly set up will do at least as well as most TBI set-ups, and not much worse than a port injection set up once the engine is warmed up. Cold starts are a different matter.

High mileage is not really that dependent on a perfect air-fuel ratio; the real key is ignition timing. Any carburetor (if properly set up) can provide 16:1 A/F ratio at cruise. The limit to using a lean ratio is burning it. All modern fuel injected cars have an integrated engine management system of which the fuel injection is just a part. Computer control of ignition allows the use of 10:1 compression and regular gas, impossible with “old school" technology. It also allows ignition advance levels of 50 degrees or more at cruise conditions. High compression and high ignition advance contribute greatly to fuel economy, but need computerized ignition control to be practical.

A lean mixture burns slowly, a nearly closed throttle lowers actual compression to as low as 4:1; a very advanced ignition timing is needed to get the most from these conditions. With a knock sensor, timing will be advanced just short of detonation on the most sensitive cylinder (impossible with a conventional system). When the throttle is snapped open from cruise conditions, compression may go right up to 10:1 and the A/F ratio may go to 12:1. If the timing stays at 55 degrees there will be severe detonation. Computerized ignition systems can handle this easily, conventional systems cannot. Note that most of the carbureted conversion V8s run just a mechanical advance distributor, not even a vacuum advance. This gives, at most a 20 degree range to vary ignition timing, and has no provision for advancing timing at cruise conditions. No wonder carburetors give much worse economy than fuel injection...

To get the best economy, add an integrated engine management system, not just fuel injection.

Thanks Larry, that's some pretty advanced thinking. :-)

There are a few of our cars capable of this, and our numbers are growing.

JB