TB, Carb, FI...

[94_camaro_z28]

I kinda got the feeling that this might stir up some arguing, but how does a throttle body on my LT1 Z differ from the one on on an old 89 C1500 also on a 5.7L? Is there really a difference? and lastly how are these any different from a carburator or true fuel injection?

[89IROC&RS]

the one on your LT1 is just a body with butterfly valves in it. because in both that and TPI systems, its a dry flow system, fuel does not enter the intake manifolds till right before the valves. in the application of the throttle bodies on TBI vehicles, including the trucks, its a wet flow system, where the injectors are in the throttle body much like the ventury of a carb, and the fuel travels the whole lentgh of the intake runners. also, the two designs are very physically different, although the LT1 and TPI throttle bodies are very close in design, if not interchangeable, im not sure about that though.

[94_camaro_z28]

I see. and do the LS1's have throttle bodies on them aswell? I haven't seen one to know...

[89IROC&RS]

yes, they have a similar setup to a TPI or LT1, but instead of two butterflys, it has a single blade.

[Hypsi87]

Originally Posted by 94_camaro_z28

I see. and do the LS1's have throttle bodies on them aswell? I haven't seen one to know...

all MPI cars have a throttle body, or at least something similiar. Have to controll airflow somehow.

[94_camaro_z28]

..and what does the MPI stand for??

[Hypsi87]

Originally Posted by 94_camaro_z28

..and what does the MPI stand for??

Multi Port Fuel Injecton


anything that has one injector per cylinder

[94_camaro_z28]

how are the different types of fuel injection different from one another? Like how is Tuned Port different from Sequential, and how exactly does sequential work in the first place? Is one system (including TB and Carb) really any better than the others? Which produce more power?

[89IROC&RS]

well, i wouldnt say that any of them produce more power than another, so long as the fuel and air gets where it needs to go its all the same. the difference comes in controlability, which has more of an impact on fuel economy, driveability, and emissions. the crudest form of fuel injections (FI) is TBI or Throttle body injection, as it is really just a step up from a carb, but is computer controled to spray the propor ammount of fuel baised on imput from sensors rather than having engine vacume suck out fuel from the venturi. But this is a wet flow system, where fuel flows through the whole intake track, which can cause larger fuel droplets, puddling of fuel in the intake track, and impropor fuel distribution. all of these lead to a less efficcient burn, and poor combustion of the air fuel mixture. the next step was EFI, or electronic fuel injection, for chevy, and particularly the camaros and vettes, this was TPI, or tuned port injection, this was a dry flow, batch fire system. meaning that only air flowed through the intake runners, eliminating puddling, and large fuel droplets, and the fuel was sprayed directly into the chamber off the back of the intake valve, retaining the best possible fuel dispersion, for optimal burn characteristics, but it was a batch fire system meaning that every time an injector on the driver side fired, all four of them fired, and likewise on the passenger side, it was far better than TBI, but still not perfect. the SFI used on LT1 and LS1 engines is a sequential multiport fuel injection system, that only fires the injector that needs to be fired at that time. for the best control of fuel distribution, and control. hope that helps.

[4onFloor]

i wouldn't say that tbi is less efficient than tpi. you still get unburned fuel seeing as how injectors are still spraying when valves are closed. the only difference is that the un-used fuel burns off the intake chamber and the face of the valve rather than "puddling" on the intake track. the only thing that really sets them apart is the tuned runners.

tbi is not really like a carb. carb's have internal flow where the fuel is drawn out of the bowl by low pressure(not to be confused with vacuum) created by the venturi. tbi does not have a venturi, it has a chamfered edge on the top of the bores to make air flow more smooth. a venturi is a design inside a carbureter that decreases the size of the throttle bore to speed up air flow, you dont get more air into the engine, it just speeds up the incoming air which then passes by a tube sitting at an angle going into the the float bowl with the top of the tube above the fuel level. when air passes by this tube with enough speed, it creates low pressure inside that tube, fuel then replaces the displaced pressure and rises out of the tube which then goes into the engine. the fuel goes through a serious of metered jets first, but you get the idea.

tbi has injectors outside the throttle body and is electronically controlled, therefor you need no venturi, metering rods, metering jets, accelorator pump, needle valves etc. tbi is no different than port injection other than injector placement. and....tbi came out after tpi, so i really dont see why some people refer to it as "the bastard child of a carburetor"

[89IROC&RS]

um, not to correct, but TBI came out before TPI, it was a crossover, went from carbs, to computer controled carbs, to TBI, to TPI. thats why the crossfire injection came out before the TPI engines on vettes and Z28s.

and yeah, didnt mean to sound like i was implying TBI was similar to carbs in operation. just that they are both wet flow systems.

[4onFloor]

your right tbi came out before tpi, but it was crossfire injection and it sucked. the tbi i'm refering to is the 2bbl throttle body. tpi rolled around bout 85-86, tbi was 87 or 88