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Discussion Starter · #1 ·
Guys, I'm looking for some advice and true facts about carb vs efi mpg differences...my case is now my daily driver F%&d pick up with edelbrock headed, cammed etc 390fe...It had speed demon750 and it did run really good but fuel consumption left much to desire....then I changed demon to 600 edelbrock which I jetted quite alot "fatter". The performance is worse than with demon and mileage is only marginally better.
Could Fast ez efi or some similar 'tbi' style system improve my mpg numbers...what kind of improvement should I expect?
Last but not least...what brand is best value? Professional products, holley, fast???
Thanks in advance!
 

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There are no absolute answers to your questions.

A carb can be tuned to be really good at a few engine speeds and loads. What you get for the rest depends on the carb itself. EFI as a general statement can be tuned to be really good at a larger number of speeds and loads. That's where it offers advantages. I'd say that EFI is usually going to be worth 2-3 mpg just because it's better under more driving conditions. If you were to switch to EFI and get more increase than that, what you had for a carb setup was probably poor.

What brand is the best value? Really hard to say. It's a matter of your needs, wants, and budget. The FAST EZ and some of the others that compete with it don't have electronic control of ignition timing. To me that control is just as important for driveability, mileage, and power as the fuel control. So if you want that feature, you need to step up to Holley HP, a more advanced FAST system, etc. Then the most advanced EFI systems have added things like nitrous control, timing retard relative to boost or nitrous, etc. Of course the more features, the more knowledge and tuning required. For some engines, the OEM EFI is a good value when combined with an aftermarket tuning package.
 

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Discussion Starter · #3 ·
Thanks Rick, you kinda summed what I had in mind.
I know that I could propaply better my current mileage by leaning the mixture, but I really don't want to sacrifice my performance. EFI is more like 'having your cake and eat it too'
I'm also wondering, how important is to match injector size to combo and power range. I see that for example FAST put different lbs injectors depending hp's.
 

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Thanks Rick, you kinda summed what I had in mind.
I know that I could propaply better my current mileage by leaning the mixture, but I really don't want to sacrifice my performance. EFI is more like 'having your cake and eat it too'
I'm also wondering, how important is to match injector size to combo and power range. I see that for example FAST put different lbs injectors depending hp's.
I got more bang for my buck by leaning out the primaries of the Quadrajet and at the same time made the secondaries richer. I went from 8mpg to 10 mpg which doesn't sound like much but it's a 25% increase! So I get better mileage when I don't get on it too much, and more HP when I want it.
 

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timing control is a must for efi to get better milage. Carb selection is important too. a demon will usually get the worst because the tend to run fat, a holley is a better choice because they are usually better on the air bleeds etc. as far as injectors yes it is important to match injector size to engine/ hp. too big injector will pulse slower leading to puddling and poor atomization. too small injector and you will run the risk of overworking them and potentially running out of fuel. injectors should be sized to run @ around 75% duty cycle.
 

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Ideally a TBI is not going to net best MPG for EFI MPFI is better. As TBI injects fuel into the intake throat unlike port injection which is right at the intake valve. So it metering a certain amount of fuel to account for this. granted its not much difference but it is there.

Now all that said its more a function of VE. Than anything. The 390 I assume has poor VE. Much like the 392 in my IH truck. It has very poor VE too. I swapped from a untuned carb to a GM TBI and only neted 2 mpg avg increase in MPG. The motor will only physically make some much power and MPG. Better VE the engine is more efficient as well.

You can buy alot of gas vs converting to EFI as well so figure that as it will take alot of savings to recoupe the cost. And the net gain of MPG is only going to be 2-5mpg at most anyway.
Retune carb for you conditions. Then I would also look at another DD if your worried about MPG.

Or since your in a diesel friendly country I would look at swapping a diesel into you truck and getting great power and MPG as well.
 

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An overdrive would give you better bang for the buck than EFI will. The new engines are more efficient yes, but a large part of that is still the fact that they run overdrive tranny's now.
 

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An overdrive would give you better bang for the buck than EFI will. The new engines are more efficient yes, but a large part of that is still the fact that they run overdrive tranny's now.
Perhaps if driving on the hwy alot and in a car an OD would make a difference. My aforementioned 392 is in a truck with an OD. But typically those motors get 10mpg empty, loaded, in OD, unloaded, downhill, uphill or towing alot.:sign0020:
 

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Oh I missed the part about the truck...apologies.

In a car though yeah, the OD makes a huge difference. In an auto OD, the lockup converter makes it even better.

Just fyi for you fellas...FAST has a new EZ-EFI system simply called EZ-EFI 2.0. It's really new so there's not a lot of info out there yet, but their tech specs say that it will control timing unlike the original EZ-EFI.

http://www.fuelairspark.com/fas/efi-systems/ez-efi-2-0-self-tuning-fuel-injection

I'm waiting to hear some reviews of it...might give their retrofit kit (for MPFI) a try on my 355 (gonna put a Stealthram MPFI on it).

EDIT: It looks like the EZ-EFI 2.0 will control timing to an extent, but it's not a true timing table based system. It'll control the curve and mimic vacuum advance, but the ability to adjust timing based on RPM and load isn't quite there. Still...looks like an improvement over the old system.

Look here at the manual if interested: http://www.fastmanefi.com/images/EZ-EFI_2_0/EZ 2.0 Instructions 1.pdf
 

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Discussion Starter · #10 ·
Great info guys! Maybe should invest to A/F meter first and see what kind of ratios i'm using in my drive rpm range...which is around 2000-2500. I'm only driving around 50-60 miles per hour, mostly around 50. Not much or any highway driving at all.

I'm not so sure about fe's bad v/e, since I have good heads, 10.5:1 compression, headers, true dual exhaust etc. That's for sure that fe bbc isn't the greatest gas mileage mill there is but I cant see anything terribly wrong with it either.

Fast ez 2.0 looks interesting...as well as new Holley terminator, lots of choises now adays it seems. I just want to learn more about timing control, WHY it is so crusial to get full advantage from efi? I have mallory ignition with vacuum advance and plenty of spark power...i thought that's good enough?

Thanks guys much appreciated!
 

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You should be able to make an improvement by tuning your carb's A/F ratio with a wideband. You still probably won't match the EFI, but you'll get much closer.

VE is affected by just about every engine parameter you can think of. The two you have easier control over are timing and a/f ratio. So you are improving VE when tuning. VE is highest at peak torque no matter what you do.

The reason that electronic spark control works better is that you can tune it past the limits of the mechanical and vacuum advance of your distributor.

Let's look at a common example. You have a performance camshaft. Usually the engine will operate better at idle and low speed with a lot of ignition advance. So you start increasing the initial timing and it responds to it. At some point it either kicks back against the starter when it's warm, or it pings at an intermediate rpm. With electronic timing, you can back off the timing while cranking but have a lot once it's running. Or if it pings in a narrow rpm/load range, you can have less timing at that spot but you don't have to back off on the whole timing curve.

Glad to hear FAST has something for ignition timing, even if it's still not a full up deal.
 

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my Dad had a 70 ford PU

with 390, auto.
we did an edelbrock intake, 600 holley, and headers and dual exh,
my dad was shocked at the improvement in power and gas mileage.
then recurved the dist, and changed to edelbrock 600 carb, and dialed in the
metering rods and jetting,
his mileage on open road was 17mpg, running 70 mph, I'm sure it would have improved if he slowed down some.
why did you change the jets in the edelbrock carb? I'll bet they were pretty close before you messed with them!
 

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Discussion Starter · #13 ·
the 390 was running way too lean...and hot! I have edelbrock RPM heads, RPM intake, Crower solid cam with .600 lift 245/250 at 050, headers, 10.5:1 compression. That's why I messed with carb :) Btw my truck is also '70 but with manual trans.
 

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Discussion Starter · #15 ·
Actually the engine is running surprisingly cool now, but before jetting the carb, header pipes were glowing. I'm running as much advance as i can...a tad more and it wount run over when hot and I have powermaster high torque starter. I may try to lean main circuit some but before that i'm going to weld lambda to collector...just in case.

Btw Bo185...I have been in that dieseling route before...as strange as it might sound, i put 6.5 turbo diesel GM to '79 Bonneville coupe...:) That thing was "interesting" to drive on slippery roads as the engine pulled like grazy from the idle.
 
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