# 350 CU with after market fuel injection added. Is this a gas hog?



## 82elcamino (Jul 29, 2012)

If a 350 has an after market fuel injection added is it a gas hog or if I drive sainly will it be realitivly easy on gas in town?
ed


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## danhr (Jul 26, 2012)

depends on the vehicle its in and the curb weight of the vehicle

the type of transmission

the rear end axle ratio

driving habits

and i'd assume it's a gen i sbc... since it used to be carb-retarded......

what kind of mileage do you consider a "gas hog" and what do you consider "relatively easy"


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## D-rock (May 23, 2011)

All what he said.....

also a quality fuel injection system will be more efficient then a carb set up, thus why they are all gone.


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## Mort (Nov 26, 2008)

All else being equal, if you take off a carburetor and put on, say, Holley Pro-Jection, your mileage will go up. But, if you bump the compression, add a wild cam, and a bunch of other stuff, your mileage will suffer.


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## bbo (Feb 28, 2010)

if its an 82 el camino, I suspect the mileage gain would not be great.

whats your definition of gas hog?


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## danhr (Jul 26, 2012)

Mort said:


> All else being equal, if you take off a carburetor and put on, say, Holley Pro-Jection, your mileage will go up. But, if you bump the compression, add a wild cam, and a bunch of other stuff, your mileage will suffer.


Incorrect.

Your mileage will only go up, all things staying the same, by a marginal amount by switching from carb-retarded to fuel injection. Stoich is stoich. You will need X amount of fuel for the grams/cylinder. Fuel injection shines in driveability, compared to the carbs. You don't have to constantly adjust your idle screw and fuel jets to maintain stoich with fuel injection, as you do with a carb. Not only that but it gives you a lot more control and account for a lot more variables with such things as the IAT sensor, MAF, PE tables, VE tables, long term and short term fuel trims, etc. etc. etc.

Not only that but increasing compression will actually do the vice versa... it will INCREASE your mileage. You are giving the combustion chamber a more efficient burn and getting more BTU's out of the fuel that is atomized in the cylinder. The only problem is that if you increase it too much, you can risk problems with detonation or knock, which will take your mpg down to 0 because your motor will be blown up. This could easily be done by getting a "bad batch of gas". This is though another part where EFI will shine, because it can detect the detonation with knock sensors and switch the timing to a low octane VE table and compensate for it.


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## D-rock (May 23, 2011)

danhr said:


> Incorrect.
> 
> Your mileage will only go up, all things staying the same, by a marginal amount by switching from carb-retarded to fuel injection. Stoich is stoich. You will need X amount of fuel for the grams/cylinder. Fuel injection shines in driveability, compared to the carbs. You don't have to constantly adjust your idle screw and fuel jets to maintain stoich with fuel injection, as you do with a carb. Not only that but it gives you a lot more control and account for a lot more variables with such things as the IAT sensor, MAF, PE tables, VE tables, long term and short term fuel trims, etc. etc. etc.
> 
> Not only that but increasing compression will actually do the vice versa... it will INCREASE your mileage. You are giving the combustion chamber a more efficient burn and getting more BTU's out of the fuel that is atomized in the cylinder. The only problem is that if you increase it too much, you can risk problems with detonation or knock, which will take your mpg down to 0 because your motor will be blown up. This could easily be done by getting a "bad batch of gas". This is though another part where EFI will shine, because it can detect the detonation with knock sensors and switch the timing to a low octane VE table and compensate for it.




ok.......so you say incorrect but your first sentence agrees with him.

He was just telling him that if you remove a carb, install a fuel injection kit along with a bunch of high performance parts then gas mileage will go down.


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## danhr (Jul 26, 2012)

D-rock said:


> ok.......so you say incorrect but your first sentence agrees with him.
> 
> He was just telling him that if you remove a carb, install a fuel injection kit along with a bunch of high performance parts then gas mileage will go down.


Driveability is not the same as mileage.

and some high performance parts increase effieciency. like increasing compression ratio, which is what the poster said would decrease it.

On my personal daily driver I have increased power from 300 rwhp to 423 rwhp (confirmed on my friends' 224x dynojet dyno), and have increased my average mileage from 14 mpg to 20 mpg. So I've added 100+ horsepower while still gaining 6 mpg.


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## Mort (Nov 26, 2008)

danhr said:


> Incorrect.
> 
> Your mileage will only go up, all things staying the same, by a marginal amount by switching from carb-retarded to fuel injection.


I didn't say it'll turn into a Prius, so what I said was correct. 



> Stoich is stoich. You will need X amount of fuel for the grams/cylinder. Fuel injection shines in driveability, compared to the carbs. You don't have to constantly adjust your idle screw and fuel jets to maintain stoich with fuel injection, as you do with a carb. Not only that but it gives you a lot more control and account for a lot more variables with such things as the IAT sensor, MAF, PE tables, VE tables, long term and short term fuel trims, etc. etc. etc.


You're preaching to the choir about driveability. I hate screwing around with carburetors. They're simple, but FI allows you to plug in a scanner and diagnose it instantly. What could be easier than that?



> Not only that but increasing compression will actually do the vice versa... it will INCREASE your mileage. You are giving the combustion chamber a more efficient burn and getting more BTU's out of the fuel that is atomized in the cylinder. The only problem is that if you increase it too much, you can risk problems with detonation or knock, which will take your mpg down to 0 because your motor will be blown up.


You're correct, in theory. Diesel engines operate on this principle, often using compression ratios of 21:1 or more. But you noticed the flaw in your argument, detonation. This requires increasing the octane of the fuel (negating your savings, because you have to buy either premium or race fuel), or increasing the fuel flow, which again negates your gain.


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## danhr (Jul 26, 2012)

Mort said:


> You're correct, in theory. Diesel engines operate on this principle, often using compression ratios of 21:1 or more. But you noticed the flaw in your argument, detonation. This requires increasing the octane of the fuel (negating your savings, because you have to buy either premium or race fuel), or increasing the fuel flow, which again negates your gain.


Really? because on my last vehicle I cut my heads .030", which increased my static compression ratio by over a full point (can't remember how much it increased my dynamic compression ratio by off the top of my head). and I did not see any knock, running off of the same octane as it was before.  In fact, I ended up adding 5* of timing into it on top of milling the heads, and still no knock.

and why would you have to increase "fuel flow" for a higher compression ratio? You are not adding more grams/cylinder of oxygen, so if you add more fuel, you will no longer be maintaining stoich and will just be running rich and well... be wasting fuel. lol. Increasing compression ratio increases how efficient the air/fuel in the cylinder gets burnt. it doesn't increase how much air is in there. iirc, the general rule of thumb was for every point you increase, you see a 8-10% increase in efficiency.


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