Baron

Sorry, but it is a concern for me, as it's my daily driver...how will getting the GM truck TBI unit (summit/jegs) affect gas mileage? Thanks...and I can't find it in summit's catalog, only jegs...weird!

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'89 RS LO3- 140,000 miles
14x3 K&N Open Element
Ram Heavy Duty clutch
Edelbrock TES (Jet-hot coated)
Catco cat
Edelbrock 3" Cat-back
Pioneer DEH-P77
Dual .50 caliber machine guns just under ground effects

Coming:
KYB shocks
Edelbrock Strut Tower Brace
South Side Machine subframe connectors

When your buddy reaches for a 10-pound sledge and says "I think we're gonna have to modify the floor pan, okay?" the answer is NO.

JPrevost

Both sell it but it's not worth it unless you have more cubes and over 300hp. You'd be better off with taking the current TB and bringing it to a machine shop to get it cleaned up and ported. Mill off the top completely and just try and make it as clean as possible. The 670cfm unit is pretty big with 2" bores. Most high performance small blocks don't need more than 650cfm. That is my opinion and I know a LOT of guys that think the same way. As for Jegs only having it...Summit can get it too, just gotta ask and tell them the Holley part number. It's $320 though, spend the money on headers , just my $.02

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, Jon (350 TBI!)
91 Red My website

bbtaz97

iTrader: (3)

unless you have a 30 enigne with alot of work,you really wont need that unit. i agree with taking the stock one in and gettting it cleaned and ported. its cheaper

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91 camaro RS
- 305 TBI
Mods: Performer TBI intake, 1" TBI spacer, K+N, jet fan switch ,160 thermo,Accel control module, Jet Stage 2 chip, edelbrock TES headers, Compleate 3" Flowmaster exhaust, Edelbrock springs, KYB struts and shocks, 700R4 built by TSI, SLP tq converter, b+m trans cooler, white guage faces, megashifter..
next month: spohn LCA's and panhard rod, BMR relocation brackets, 323 gear

Baron

Thanks guys...been reading www.goingfaster.com/spo/mods.html too much I guess...so I guess I should go with the Edelbrock TBI intake then? Before I was going to go with the 670, the TBI adapter plate, and 4-bbl manifold...since you guys are some of the smarter ones around, please tell me what you'd think of this for a daily driver...especially gas mileage!

World 305 heads
LT1 cam
Edelbrock TBI intake

Thanks...note mods in sig...

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'89 RS LO3- 140,000 miles
14x3 K&N Open Element
Ram Heavy Duty clutch
Edelbrock TES (Jet-hot coated)
Catco cat
Edelbrock 3" Cat-back
Pioneer DEH-P77
Dual .50 caliber machine guns just under ground effects

Coming:
KYB shocks
Edelbrock Strut Tower Brace
South Side Machine subframe connectors

When your buddy reaches for a 10-pound sledge and says "I think we're gonna have to modify the floor pan, okay?" the answer is NO.

Dan W

It will not necessicarily cause a measurable gas mileage change. The Holley injectors are a different size than the stock injectors. To correct for this, you can either swap your old injector pod onto the Holley TBI, lower the Holleys fuel pressure or modify your chip to accommodate the larger injectors (easy mod).

If you dont do one of the above, you will run rich.

Some advice:
Some have complained that the stock pump shot is not large enough for the larger 2" TBI unit... the 670 flows about 65% - 70% more than the stock unit... this extra aiflow when the throttle blades are first opened needs extra fuel. Larger pump shots are needed to retain the same throttle response. This is accomplished in the EPROM.

Folks, when you look at Holleys rating on the 2bbl TBI units, keep in mind that they rate them at 3", not 1.5" like they and everyone else rates carbs (and 4bbl TBIs). 670 cfm with a 2bbl TBI is much less real airflow than 670 cfm with a 4bbl carb.

I've got to agree with JPREVOST in that early in the game, the larger TBI unit is not a requirement. Unfortunately, our TBI systems don’t have just a single component that needs to be swapped to un-tap all that hidden power. The whole package works very well to make low amounts of power... low displacement, horrible heads, cam, exhaust, intake, etc.

When you have an airflow requirement above 5000 rpm and or add some cubes, you will start seeing some measurable gains from the larger TBI unit.


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1988 T/A,
9" Ford, 3.50 gears, Auburn posi, 700r4 -w- 2100 converter, 360ci, 9.8:1 cr,
AFR 195 Heads, Weiand #7525 intake, Lunati roller 219/227, .479/.480, 112 LSA
Holley 700cfm 4bbl on TBI truck 7747 computer and chip by Howell-EFI

FastBroker

IMO, you WILL see a decrease in mileage for two reasons:

1) If/when you change the PROM to add more squirt for the transients, you will use more gas.

2) When you open the throttle in a larger bore situation (carb OR EFI) there is a corresponding larger change in MAP (decrease in engine vacuum!), which increases the engines need for fuel, all other things being equal. Ie, for a 10% change in TPS, you may change from a MAP of 30 at idle to 50 with the small TBI but for exacltly the same 10% change in TPS with the 2" unit, you may get a MAP of 70+. In laymans terms, if you put less throttle input into your driving with the 2" unit, you'll be good, but in practice (reality) this never happens, thus the complaints with larger TBI and carb units. EFI engines are better poised to make the engine run GOOD because it is automatically adjusting for the changes, short and long term (INT/BLM resp), so driveability problems sometimes associated with misadjusted large carbs are gone.


I'll agree with these guys that the 670 is probably too big for you... Unless you ave major changes or a 350cid+ down the road.

Dan W

Yes but this happens only a small percentage of the time when compared to overall time spent driving. I would apply the word measurable here. Also of note, this is dependant on whether a custom chip is used and if the driver wants to utilize the extra throttle response by applying the accelerator pedal quickly.

This can be true for carbs but is fundamentally incorrect for EFI.

With the larger TBI, his car will maintain speed with less TPS. It will use the exact same air and fuel at this speed. Any more TPS and the car will accelerate.

Acceleration is essentially (as you fastbroker would agree) unaltered because the engine is not consuming more air because a stock motor cannot create the demand for more air... not with its stock rpm range and VE%. The relationship between tps% and rate of acceleration changes. Harder acceleration is reached with lower TPS% with the larger TBI. But given the same rate of acceleration, both will occur at the same map and use the exact same fuel. Maximum acceleration rate does not change and is still found at 100% TPS. Again, fuel is unaltered. If this were not true, we would both be advocating a large TBI unit on a stock motor.

FastBroker

A agree with you in THEORY but in REALITY the car with the bigger TBI will have an overall higher MAP, more torque generated and more fuel consumption, ESPECIALLY if the PROM is mapped properly.

There is no way to tell your brain to "lay off" the small changes in TPS, even the ones that activate Transient fuel addition. You will not burn the same amount of fuel, it will be more. It certainly will not be easier to burn less, I am sure you'd admit.

snflupigus

you loose about 30 mpg per tank.

i had a 454 tbi on a stock l03. took it off a few months ago and am now getting 240+ miles per tank, instead of the constat 210 i was getting with the 454. the 454 was also on durring the winter when my foot is much much lighter. sooo. besides not gaining much, and it not being worth it unless you plan on doing much more to your engine, you will lose milage.

o, and my integrator #'s where nuts, after having the 454 on for about 7 months.

high in some parts and low in others. so, stick with your tbi till you are going for larger mods.

Baron

Thanks all, I'm convinced...appreciate all the info. I'll pay you back by replying to stupid newbie posts when I've got some knowledge myself.

FastBroker

yeah, baby. More air = more fuel.