2wankel

Can anyone offer some wisdom and experience on these two types of fuel delivery systems. I dont see many aftermarket Multi-Port Fuel Injection projects, but I do see alot of TBI with the popular eastcoast TB for a 4150 Manifold. What are the neg. and pos. to each. While we are on the subject...What are the neg. and pos. to a 4-barrel throttle plate vs. single throttle plate 95mm, 100mm, working with MPI?



thanks in advance for your input

C. Ludwig

Most setups are port EFI which is what the stock setup would be considered. Going to the TBI style injection with the big 4-barrel throttle bodies allows you to easily mount more injectors and to flow more air than a stock manifold. You usually end up with a shorter runner also to compliment high rpm power. I can't think of a downside to port EFI. There are some advantages to having an injector at the port and another at the open end of the runner but an injector close to the port will provide better atomization and less chance for fuel puddling than a TBI injector. A 4-barrel throttle body could provide more progressive throttle response than one large throttle body. Four small runners will allow velocity to stay higher when the throttle is quickly opened versus one large opening.

ColinRX7

Well, most places I see TBI are V8s. Most places I see MPI is anything that doesn't have as much power as a V8.



V8s make so much torque naturally, with tons of mechanical advantage at the crank versus most other types of tuner engines out there. When they've got torque to spare, I don't think the most efficient low end power design was considered. If anything, they were concerned with optimizing top end since the abundancy of torque took over until high RPM.



When it comes to something like a 13B, torque isn't in surplus by any means, and mazda utilizes long runners with port injection in the LIM. They couldn't run TBI because the fuel would never make it all the way down to the combustion chamber efficiently. Why didn't they just use short runners then? Because they had to tune the intake design for more low end juice. Otherwise it would have been a dog until high RPM and then take off like a rocket. How do you market that to the average car buyer? The 6 port design itself was to optimize low end power without compromising top end, right?



If you take an early 90s GM econobox, and compare it to a same year Mazda, GM was using TBI on a 2.0L 4 cylinder. No choke, started everytime, lacked power, not bad on fuel. It sold just fine during it's time. Cheap car. Mazda however, was making a car with slightly more horsepower with 1.8 displacement, a much fatter powerband, and superior fuel economy. How? MPI, variable length intake runners, dynamic chamber, and single throttle body. Tuned intake. Mazda had one injector per intake port. More complex, yet no less reliable than GMs. Think about what that means for accurate injection and atomization. Sure the GM had one huge fuel injector, plenty to cover the needs of the motor, but it's a sub par design. Just an electronic carb. How much faster was a 1991 protege GT versus a 1991 chevy cavalier? The protege had the jam, and just another reason why the imports became more popular.



All just IMO anyways, a penny on the ground for you.

2wankel

Great read, thanks.



How close to the port we talking? Is there such a thing as to close to the port? Can you put an injector in each port, right near the flange where the manifold bolts to the engine block, lets say about an inch or so away?



thanks again.

C. Ludwig

Well the primary injector is in the "block" maybe less than an inch from the port opening. So I guess it can't be too close.

j9fd3s

if you do a quick search, theres an sae papre about the 787b engine, and they did a study of injector placements. this is on a peripheral port engine, so results prolly arent exactly the same as a side port, but they found that overall power was about the same, but closer injector placement to the engine resulted in slightly better fuel mileage.