87tpi

iTrader: (5)

Is there a gas mileage difference between tpi and tbi?I heard that tpi offers about 30% more mpg's but am curious to know for sure.

BigWhiteGTP

I highly doubt 30% better mpg. That would mean your getting less than 170 miles to the tank. I have to granny drive it to get 240. And I have 2.73's and the car runs great and passed emissions and the afr was perfect according to the dyno.

How many miles are you getting to the tank?

87TPI350KID

I wouldn't know, I don't baby my car

I get 19 mpg if I drive it right, usually average 15-16.

GOY

Maybe not 30%, but it is a huge improvement... I'd say 15-20% wouldn't be unreasonable. I'm fed up with my TBI's "Economy" or lack there of... while my LB9 Formula is much more fuel friendly.

83 Crossfire TA

iTrader: (2)

I seriously doubt that a properly tuned TPI is even in the same legue as a properly tuned TBI setup… it’s not uncommon for 305 TBI cars to get >30mpg on the highway, (my Crossfire car got 35-37 with it was “right”), where most TPI cars get in the low/mid 20’s, some as high as high 20’s. My ’87 Formula 350 is lucky to pull a low 20 on the highway. They all get crappy mileage around town unless you’ve got a lot of gear.

3.8TransAM

TBI cars ran the highway mode in the code from the factory(i know on the manual cars for sure, believe the autos as well)

Where most of the additional mileage came from.

Guy I went to school with got almost 30mpg with a 305 TBI manual and I believe Prevost was over 25 consistent with an auto 305 TBI.


I have 2 350 TPI cars and could avg 18ish around town and high of 25 and low of 21.6 on road trips. I usually do drive nice except for when I'm in a mood or someone wants to go.

The low was the result of 02 sensor going south.

A properly running 350 car should net more than 20 on the highway.

I avg'ed 20 wth mixed 50/50 city/highway driving on a hotcammed vortec tpi setup also.

At 21.7 and climbing with the latest 383 on the highway, should know more on that one Sat when I head to Iowa.

later

Jim85IROC

iTrader: (2)

I think it's hard to do a direct comparison unless you can find 2 identical cars with identical gearing. Most of the TBI cars had 2.73, but most TPI cars had 3.08s or 3.42s. Your driving style, and your local area will also provide significant contribution.

If you were to swap TPI and TBI on and off the same motor in the same car, I think you might find that the milage didn't vary that much, though I would suspect that TPI would fare a bit better partly because the added low rpm torque may allow you to remain in a higher gear more often.

GOY

My lo5 - 15 MPG average - 2.73 rear gears
My lb9 - 23 MPG average - 3.45 rear gears

Sure, one is a 350 and one is a 305, but we are talking about an 8 MPG difference, and the 350 has the "Highway" gears, while the 305 has the second most aggressive set of gears ever put into a thirdgen (I'm pretty sure some L69 thirdgens had 3.73's from the factory. Some - meaning maybe 25-30% of them, not all)

8 MPG is pretty much is the average fuel consumption for a TBI 454 - 409 inches greater than the 45 cubic inch difference between my two comparison motors. TBI is garbage for economy in my book. GREAT for hassle free maintainence, but not the fuel efficient run around that some make it out to be.

406TPI

iTrader: (1)

Tune for tune considering identical cars, they should be REALLY close.

91 camaro racer

iTrader: (2)

i get about 25-27 mpg in my 305 tbi.

dimented24x7

iTrader: (2)

The engine setup and tune have more to do with it then anything else IMO.

cdartz

I agree with some of the others in that you have to do an apples to apples comparison. The reason the TBI cars got better mileage from the factory is a matter of gearing, and the fact that the smaller cam and crummy flowing heads couldn't move that much air. It's not that the TBI is better for fuel mileage, it's more a function of what it was attached to. With the right gearing, and the ability of the driver to keep his foot out of it you can achieve better mileage with TPI since it's a more precise metering system. If you have a heavy foot however and choose to use the hp then you won't get the mileage. Sure you can floor a TBI car and still get good mileage, but that's cause even floored you aren't moving the volume of air an L98 can move. By the way, my 350 TPI I swapped into my 90 RS stick shift car gets better fuel mileage than my old V6 Camaro got.

AC398

damn im surprised that you all are getting that much mileage, with my 89 GTA w/ 350w ith babying and driving it hella slow im lucky to pull 13.5 mpg and barely manage 180 miles out of a full tank. when i first got the car i was doing 9.5 mpg but i put in a new iac, fpr, stock computer and prom. Next im thinkin it may neeed an 02 sensor.

mcwpns

My new 1991 TPI formula just clocked 245miles on 12 gallons of go go juice 2:73's So around 20-21mpg ----- My 89 bird with tbi got about 18mpg with 2:73's

ShiftyCapone

iTrader: (7)

When my T5 LO3 had a cat-back and an open element I used to get 31mpg all day long on the highway. This was with the stock 3.08 gears and 15x7 wheels. I believe you can achieve the same numbers with a TPI car as well. There are almost too many variables to even have this duscussion.

WinnipegGTA

I get almost 370miles to a full tank of gas on the hwy cruising at 2000RPM with the cruise set the whole way. Thats with the car loaded with stuff including 450lbs worth of passengers and that being curteous. and atleast an extra 200lbs of crap IE suitcases, Subbox and subs and what not. Not to mention a fully loaded Leather interior. I did a full tuneup the day before. New fuel filter, heated 02sensor, plugs, cap&rotor, ran throttlebody cleaner through the plenum, and cleaned and reset the IAC & TPS. The car has no mod other then the ghetto CAI.

kyles88bird

ttops off, doing bout 70mph, 5th gear, with 3.42s and a cammed 305 i was gettin 30mpg on the highway. i only get bout 17-19 city tho

srdynamics1

iTrader: (10)

Agreed.
My T5 L03 with msd wires, high flow cat and dynomax exhaust has achieved over 31 mph highway cruising between 70 and 80 mph.
I have yet to make a road trip with the new tpi cold air cleaner and tbi mods. I'll would venture to say 35 + mpg is possible if I drive 55.

weberflorida

iTrader: (2)

Just for stats I am getting an average range of 26- 28 driving highway and just a tad under 20 on the city streets. That is all assuming staying out of the gas. Which doesn't happen much.

83 Crossfire TA

iTrader: (2)

I doubt that was it… I’ve turned highway mode on as it’s set in the code and tried with more aggressive values on my ’87 L98 and I didn’t measure ANY increase in mpg. If I had any documentable change I would have left it but eventually turned it off.

My ’83 Crossfire car that got such killer mileage did not have highway mode in the code at all.

That’s about what my L98 gets, but I don’t think that I’ve ever seen more then mid/low 20’s no matter what I tried with it.

Again, that’s why I used my 2 cars as a comparison. Weighed within 50lbs of each other, aerodynamically were almost identical (the 83 TA did not have the optional “aero package” so with the exception of the wheel spats they were the same shape), both have had the same set of wheels and tires on them, both were 700R4 cars and the 83TA had 3.23 gears (when I eventually broke them they got replaced with 3.42’s from my ’97 which killed highway mileage about 1.5mpg), and the formula has 3.27’s. They even had the same exhaust (as in the same actual one, I swapped it when the tree fell on the ’83) on them with the exception of the headers.

Pat Hall

iTrader: (31)

I think there's some truth to what cdartz said about it not mattering if you have your foot in it a lot with a tbi car. If I granny drive mine around town, it gets about 230-240 miles to a tank, if I drive it normally (i.e. foot in it most of the time), it gets about 215-220 per tank. On a long freeway trip, it knocks down an impressive 340-350 miles before I get nervous enough to start looking for the next exit with a gas station. Mine is a 305 tbi auto car, and the only real mods to it are 3.23 gears, gutted cat, and tpi exhaust manifolds and 3 inch catback. I gotta admit, the exhaust mods and better gears made a huge difference in the power and pickup. The other cool thing is the milage didn't really change that much from when it had 2.73's. I even ran 3.42's for a while, and didn't notice a big milage drop with those either.

Fast355

iTrader: (2)

I normally get 22 MPG highway in a 240 RWHP TBI 305 fullsize van. I just drove 760 miles on the last tank. 32.8 gallons for the curious. That equals 23.17 with the cruise set at 75 for most of the trip and the A/C on.

Edit-that is with a 2000 rpm stall in a 700r4, 3.08 gears, >5,700 lbs, and on the way to San Antonio and back. For reference my 280 RWHP 350 got 19 mpg on the highway.

GOY

The force is strong with you.... (because I have no clue what else could have got you there, lol.)

8Mike9

Problem with the comparison is no one has swap results from TBI to TPI (intake only) on the same car/etc...

Heck, different cam profiles and heads between TPI 305's and TBI 305's, make just more guessing than anything.


That said, I'd hedge towards TPI for the following reason...

DynoDon swapped a stock TPI onto a TBI car, he dyno'ed it...don't recall the HP, but do recall he picked up 30 or so FT/lbs of torque over the TBI intake.

Since we all know that the longrunner TPI intake most likely did not move the curve up, I'd guesstimate that that particular car has more freed up torque at highway/cruise than it did before...which *should* yeild better fuel economy?? Just my guess.

87tpi

iTrader: (5)

Ok,If you had an lo3 and swapped a tpi setup on it do you think the mpg's would go up or down?I think it would go up.

ShiftyCapone

iTrader: (7)

According to the old fuel milage model I created as an under grad IC eingines project the fuel comsumption difference between the two inductions on an LO3 is negligable. They are basically the same. However, a stock TPI car with different gearing (3.42 rear and .73OD gears) and the true TPI set-up should theoretically be 7% to 8% worse than an LO3 car with the 3.08 rear and .63OD gear). This is assuming the T5 230hp/320lb ft TPI LB9 and T5 170hp/255lb ft LO3.

tom3

I think theory would give the TPI the edge, port injection should be more precise that the wet manifold TBI, especially with the MAF TPI vs the speed density TBI. But in real world comparisons the old throttle body cars sure seem to get super gas mileage. Not unusual to get near 30 mpg, regular gas, with a V8 car and that is pretty darn good in any book.

ShiftyCapone

iTrader: (7)

I agree. The gearing takes away that advantage though.

83 Crossfire TA

iTrader: (2)

I supect that it wouldn’t, well not unless the TPI was in a some combination of a much heavier vehicle and much fewer cubic inches below it and it was geared to cruise in the mid 3K rpm range and something that actually operated in a fairly narrow rpm band.

Maybe in a suburban without OD and a 305 in it…

The thing is that the lowest rpm useful harmonic with the stock TPI runner length is somewhere in the 3500rpm range, and the typical 2500rpm cruising rpm that a lot of cars have would fall right in one of the low spots WRT to harmonic tuning.

Real world, if you expect it to be efficient over a reasonable range you’re probably better off without a tuned manifold, with just some mild ram tuning.

In this example, I can’t see anything that the TPI gives you an advantage over a TBI with a properly designed manifold (with good a/f distribution…). I can see advantages to the TBI, the fuel will be better atomized/vaporized by the time it gets in to the cylinders and it will cool the intake charge some allowing you to run a marginally higher compression ratio/more timing without detonation, increasing combustion efficiency

For that matter, with reversion and other good stuff going on, I’d be pretty surprised if batch fire port injection really does provide a metering accuracy advantage over even a mediocre TBI manifold, and the rest of your arguments are strictly based on how good the tuning really is for either rather then an advantage or disadvantage of one manifold setup over the other.

whoaru99

Around 25 MPG highway for mine. 3.23 gears...

Kevin91Z

iTrader: (1)

Yes we do have swap results from swapping only the TBI to TPI on the same engine. My dad's old car (DynoDon) went from 158 RWHP and 220 TQ on a 305 TBI, and got 28 MPG going from SoCal to Phoenix, AZ and back. Then a year later, he swapped to a stock TPI intake, went to 176 RWHP and 290 TQ, and got 31 MPG going from SoCal to Reno, NV and back.

8Mike9

Ya, I knew my memory was weak with numbers, but the gist of it was correct.

I think Kevins reply pretty much should put this thread to bed, huh?

83 Crossfire TA

iTrader: (2)

what kind of a car was it? what ecms?

Kevin91Z

iTrader: (1)

'92 Camaro RS, white in color, with a T5, 3.08 gears, open rear, single cat, stock 2.75" exhaust from a 350 '88 IROC, used a stock 8747 TBI ecm and stock 5-speed TBI prom, and a 7730 stock TPI speed density ECM and a stock AXXD TPI 5-speed 305 prom.

Any other questions, or was I specific enough for you this time?

EDIT: Oh, forgot the engine had over 200k miles on it at the time.

srdynamics1

iTrader: (10)

Kevin91Z- What sizes wheels and tires?

8Mike9

I also have a question, was the car waxed before or after the swap?

3.8TransAM

U laugh at the whole wax thing, but they have done studies and found from .25 to .5 more mpg with a clean car vs a filthy one. But again we keep our cars clean so it doenst matter, dont think anyone is mudding a thirdgen lol

Also.

If comparing factory TPI to factory TBI, the TPI would lose. (although I have never owned a TPI 305 car, we might have some luck there) Everyone I know with a TBI car gets better mileage highway than me. (again I am talking stock well functioning and tuned cars)

When it comes to well a TPI on a TBI with a TBI on a TPI and on and on the results get too skewed to know anymore.

later
Jeremy

8Mike9

Jeremy,

Did you read Kevins reply? And you still think TBI is better for fuel economy?

Ya, wax thing was a tounge-in-cheek joke... .25-.5MPg doesn't really effect the MPG outcome...that's the difference in taking a good wizz before the drive...or holding it in until you get there

83 Crossfire TA

iTrader: (2)

Crap… Ok, I’ll wash my cars…

that’s exactly the deal… I can’t think of anyone knocking down mpg’s in the range of some of the better TBI cars which is why I was so interested in Kevin’s dad’s results, but of course, I can’t ever seem to get a serious answer from him about his or his dad’s car…

Kevin91Z

iTrader: (1)

Stock 16x8" wheels with 245/50-16 Pep Boys Furtura tires.

The car was washed before each trip.

What do you mean I dont give you a serious answer?

You know, I dont really care what gets better mileage. If I did, I would drive a four-cylinder. I have about 400 HP, get 15-20 MPG on the highway, and I enjoy every minute of it. No TBI car is going to get that. The only thing better is an LS1.

dimented24x7

iTrader: (2)

Ben73's CFI car with a late model TBI PCM seems to do pretty well on the HP side. I think he may even be trapping faster as well. Dont know what his fuel economy is but its a pretty mild 406 so Id imagine its not half bad.

Fast355

iTrader: (2)

I was making roughly 330 HP at the crank with a super mild TBI 355. Think ported 193 swirl ports, LT1 roller cam, stock intake, stock TBI, prom tuning, 90 lb/hr injectors, and headers. It was getting 18-21 MPG on the highway in an overweight brick. Think 1983 G20 Conversion Van. It also ran a 15.6 @ 87 into a 20+ MPH head wind. I only gained 10 mph the last 1/8 mile! Better heads, more fuel pressure under load, less at idle, and more cam would have put me at 400 HP without a doubt while retaining good fuel mileage.

My current 305 TBI is making near 300 FWHP and got almost 24 mpg on a long highway trip.

GOY

Hummm, my truck's LO5 get's an average of 15 MPG, 60% city 40% highway - and barely makes the advertised power with the few mods I have.....


Kevin's post put the final stamp on my feelings about going to TPI. I'm sorry, wet flow manifolds are not precise fuel metering devices, nor should they be - but the fuel still has to go through them. Even if I could get 16 MPG going to TPI, I'd save several hundred dallors a year and have a lot more mid range torque for pulling my TPI Formula around. I'll still have to deal with TBI until I get the swap together (maybe next spring) but either way... TBI's gotta go for fuel economy alone. My Formula gets MUCH better fuel economy.... almost 10 MPG difference, and I always have my foot in the throttle with the 'bird.

dimented24x7

iTrader: (2)

No matter what you have if you dont tune it, it aint gonna run good. Its that simple. There was a guy here awhile back who switched to SD TPI because he was only running 15s with tbi. Guess what? He was running just as slow with the TPI if not slower because it wasnt tuned. Oh, theres also the fact that your truck is a heavy *** overweight brick wall driving down the road . The formula is much more aerodynamic.

I dont see how fuel metering cant be precise with a wetflow manifold. The fuel that goes in has to go out during steady state. If the ecm is metering the fuel well, then itll be precise. Now, there are effects from having fuel vapors present in the manifold that can skew a SD based algo., but it remains to be seen just how much these really matter.

dimented24x7

iTrader: (2)

Oh, for reference, my full sized sherman tank, er, I mean blazer with tbi got around 10 mpg in the city.

MrDude_1

Oh, for reference, my 91 with the LS1 in it gets a avg of 19-20 MPG... almost all city driving.. and i beat the **** out of it.

its somewhere in the upper 20s on the hwy.. i dunno exactly since i havent burned a whole tank on the hwy yet.


fact is, neither TPI nor TBI cars are setup for good gas mileage... they have low BSFC.. they dont have 6spds... and while a highway mode KICKS ***, there's still alot to be done in the tuning to help both mileage and power....

Chrome

Yeah, I don't know why all V8 third gens didn't come with the TPI installed. For the cars that were RS cars with the (L03 305) they could have just tuned it for the small cam/crappy heads. Then at least the kids might have gotten alittle more torque out of them - and have alot more tuning potential from the factory. But somebody I guess missed that opportunity.

GOY

TBI fires heavy liquid (fuel) right at the base of the intake manifold at 13-15 PSI, and hopes that air can "Catch" it in time and change it's direction within a few inches. I'm sorry, but some of that fuel ends up on the manifold floor. It has to, especially at low engine speeds - and if it doesn't end up there, then it ends up dripping off the throttle blades. Secondly, on deceleration when high vacuum causes fuel absorbed by the walls of the intake to boil out and enrich the mixture, making for CO and HG emissions - and obviously points to fuel not going to where it should have in the first place!

CPI was created to allow the reliability of TBI to live on, but make the manifold "Dry" to eliminate these obvious problems. Of course - the spider and valves became their own maintainence problem as time has gone on, but that's another story.

83 Crossfire TA

iTrader: (2)

Your formula would get that much better mpg if it had a sorta tuned carb, the formula has a much lower CD and frontal area and probably weighs at least 1000lbs less

GOY

I understand the effects that a vehicle can have on it's fuel economy - but bare none, MPFI has fewer inherent drawbacks than TBI in terms of preformance, economy, and emissions. If someone cares to debate that, they have a multi-trillion dollar automotive industry that has turned it's back on TBI to fight and try to convince. Typing it here for us to see is basically meaningless. The proof is on the dealership lots.

This thread has become purely hypothetical at this point. Kevin91Z posting the facts that people had been debating over and that TBI guys were standing on, MPFI came out on top.