Based on different responses to fuel economy questions that I have seen on here, it appears as though TBI cars get better mileage than TPI cars. Why is this? TPI seems like the kind of FI that would be most efficient...dry manifold, fuel sprayed directly into the chamber.

But, I have seen 305 TBI 5 speed owners saying they are getting close to 30mpg with their cars!

 

I hope this post stays pretty clean. You don't have to look very far back to find the now locked one. This post will likely turn into an all out war between TBI and TPI guys and gals.

The TBI cars have alot of benifits for part-throttle operation that the TPI engines don't.

A.) TBI engines have tiny cams with nearly no overlap
B.) The swirl ports burn faster, requiring less advance for proper burn. This is known as minimum best timing (MBT)
C.)TBI induction is efficient at part throttle for 3 main reasons. The fuel is sprayed across the TBI throttle plate, the throttle plate shears the fuel mist into a highly atomized state. The intake manifold then heats this highly atomized fuel (which is in a vacuum) into vapor form. This results in an extremly even a/f ratio between cylinders. TPI injectors can flow quite a bit different, especially with alot of miles on them. TBI injectors aren't too badly balanced and each one feeds 4 cylinders.
D.) Torque converters are typically tighter on TBI cars
E.) Gearing is taller on TBI cars.
F.) The TPI setup actually gives LESS TQ between idle and 2,000 RPM or so.

 

Double

 

All thirdgen V8's have the same 1600 RPM stall converter stock, so D isnt the answer.

My dad got 28 MPG with a 305 TBI going from Los Angeles to Reno and back. After converting the car to TPI, he got 30 MPG going from Los Angeles to Phoenix and back. Its not a perfect test, just my two cents.

 

They might not be the stock converter, but I have seen atleast 3 that are 1,250 in some earlier TBI cars.

I am not disclaiming your dads gains, but his TPI setup had more advance than his TBI setup did. I am going off of both setups being set at factory specs with stock chips respectfully. I almost always cheat on TBI timing by 4-6* (TBI timing setup in the chip is way too lame) and have a feeling that the TBI would have hit 30 MPG too, not positively, but likely.

Just for public knowledge, were your dads fuel mileage gains measured after the swap to the L98 twin cat exhaust? Did he ever check the fuel mileage with the TBI installed with the L98 manifolds, cats, etc. I would imagine that this improved it somewhat as well.

 

As I've said before, the only change was the intake manifold and computer. The exhaust was not touched until much later.

 

I get under 20mpg
I have yet to see 300miles/tank in my car. I can't imagine a 3rd gen V8 getting 450miles/tank.