hartsmike

Out of curiosity, and I know this has probably been asked hundreds of times before but what is the main difference between TBI and TPI? Why is TPI better and how much can either 350 be built using both fuel injection types?

novaderrik

the TBI setup runs on low pressure (15 psi or so) and has a pair of centrally mounted injectors right above the throttle body- it's essentially a fancy version of a carburetor... properly maintained, it's a great system that offers good fuel economy and about the same power as an equivalent carburetor. it's also a very simple system that is easy to work on with cheap service parts.

TBI was sort of a transitional thing as the oem went from old school carburetors to full on fuel injection..

TPI (Tuned Port Injection) is GM's marketing name for it's version of MPFI (Multi Port Fuel Injection) which has a fuel injector for each cylinder, mounted in the intake manifold so that it shoots atomized fuel at the back of the intake valve just before it opens... it runs at a higher pressure (42psi, if memory serves).. it's better because the intake manifold is "dry" and doesn't have to deal with keeping the fuel suspended in the air and having an injector at each cylinder allows the computer to "tune" each injector firing to what that particular cylinder needs to do it's job at any particular time.
i can't think of a performance oriented vechicle built since the late 80's that has had anything besides some sort of MPFI setup on it, and i can't think of a single production vehicle built since the mid 90's that hasn't had it..

but now even MPFI is becoming old news as the new generation of direct injection (DI) engines make their way into production.. these ditch the intake manifold mounted injectors altogether and replace them with ultra high pressure (2500+ psi) injectors that shoot the gas directly into the combustion chamber as the piston travels up to and back down away from top dead center... it's relatively new tech for production gasoline engines, but it's been in regular use on diesels for a couple of decades...

AlkyIROC

Injectors are rated at pounds of fuel per hour. Depending on how much HP the engine can possibly make, the injector size can be calculated. You want an injected that can flow enough fuel for the engine demand and not be maxed out on the duty cycle.

As mentioned above, a TBI system is a glorified carb system since it functions similar to the way a carb work but with 2 electronic injectors pulsating fuel over the butterfly valve of the throttle body. The system works but isn't very efficient. The biggest drawback to TBI is that you can only flow so much fuel through 2 injectors and power limits are very low.

With a TPI system, the throttle body is dry and each cylinder gets it's own injector over the intake port. The computer can control each injector separately if required to fine tune the amount of fuel going into each cylinder. With 8 injectors, you can also pump a lot more fuel into the engine to produce more power than what a TBI system could produce and larger TPI injectors are easier to swap to.

Even so, a TPI system still has it's limitation especially with very high HP cars. It's not uncommon for very high HP cars to have a second set of injectors installed to provide even more fuel when required like under high turbo boost situations. That's something a TBI system just can't do or can't do easily.

The modern fuel systems now use a common rail high pressure fuel system. These operate at very high pressures and can produce more power by pulsating the injector at different times during the power and combustion stroke instead of once like the older EFI systems did. Not only can this make more power but it can also lower emissions.

hartsmike

Reason for asking is due to me seeing/talking to a guy yesterday that had a 91 305 RS.. Pretty sharp car and he was interested in selling so my mind began to wonder if I bought it and swapped out to a 350 if the TBI could handle a lopey cam and built motor?

AlkyIROC

Like I said, there's a limit to how much fuel can be pushed through 2 injectors. I don't have the formulas handy but I believe the limits of two factory SBC TBI injectors is far less than 300hp. I know my 91 454SS which has larger TBI injectors has a limit of around 310hp.

Before TPI became available, it was common to convert to a dual TBI system with a low pro manifold just to get more fuel into an engine that needed it.

hartsmike

Are there aftermarket injectors that woukd allow for more or is the casing in the fuel injection the limiting factor?

novaderrik

i've heard of mythical TBI engines that claimed 400hp, but i can't tell you what you'd have to do to support that much power...

unless emissions are something you need to worry about, it's easy enough to swap to a carburetor- even stock quadrajets can be tweaked to happily feed close to 500hp, which is probably 150 more than you will make with a mild 350 that is saddled by the crappy stock exhaust and what not that these cars have..

hartsmike

To put a carb in would you have change out gas tanks?

NagleMac

iTrader: (1)

No, you'll just need a fuel pressure regulator. Carbs need between 6-8 psi

novaderrik

i just dropped the tank, replaced the fuel pump with a chunk of hard line, and ran a block mounted mechanical fuel pump..

AlkyIROC

To my knowledge, there are very few if any larger TBI injectors. Short production run of only a few years and very little performance potential means there's very little aftermarket demand since a TPI system offers so much more potential.

The BBC throttle body is larger but there is an adapter plate to use it on a SBC manifold. The larger BBC injectors can provide more fuel that the SBC injectors.

hartsmike

If I wasnt wanting some big 500 HP engine, say 300 HP with maybe a LT4 lopy cam would the TBI be able to handle that?

thomas1976

I would like to point out, a pair of 80#/hr TBI injectors @ 30 psi of fuel pressure flow 243#/hr, should be good for 450 fwhp.
(I run 80 lb/hr injectors @ 19 psi trough a VRFPR, emissions around 28 ppm HC and 12.7 CO2. trough a 3" high-flow catalytic converter)

Also the TPI system is not capable of controling each injector separately to fine tune the amount of fuel going into each cylinder. The injectors spray the same amount of fuel all at the same time.

For 300 fwhp you need around 160#/hr of fuel, that is not to complicated with a TBI system, even the stock TBI intake manifold and TB with minor work will work well.

thomas1976

Evidence https://www.thirdgen.org/forums/tbi/...imple-fun.html