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Gas Milage/ Carb question

Old 08-18-2012, 05:10 PM
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Default Gas Milage/ Carb question

So I have this motor on my Blazer...
http://www.chevroletperformance.com/...case/index.jsp

It's the smaller 350 with 290HP 326TQ

My blazer has TH400, 35" tires on20"s, and is geared to 4:10. I also have a mild cam, headers, to flows (no cats), a weiland intake and a Holley Street Avenger 670.

My question is about my carb setup. My gas milage is awful! Should it really be that bad with that setup? Also is my carb too big? Also when I first step on the gas the truck falls on it's face. Do I need to re-gear it? I'm towing a boat now too and it kinda struggles a bit as well. Since I have the 4 speed should I re-gear to 4:88?

Interesting, I just pulled the motor specs here...
http://www.jegs.com/p/Chevrolet-Perf...29887/10002/-1

and did the calculations here...
http://www.gtsparkplugs.com/CarbCFMCalc.html

and it shows me in the mid 400 range. Also the engine specs are for 87 octane. I run 89.

Last edited by evan matthews; 08-18-2012 at 05:17 PM.
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Old 08-18-2012, 05:37 PM
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Default Re: Gas Milage/ Carb question

That's prob too much carb on there now. What sort of mileage are you getting? Those calculators work off ideal conditions. Truth is, cheaper carbs won't be as efficient as higher end ones. I know 2 barrel 400 cfm carbs that power 500hp late model engines where 600cfm street carbs couldn't cut it. The stumbling is probably due to the mods. Do you have a single or dual plane manifold? From the sounds of it, with the overly big carb, you're dumping fuel in (accelerator pumps) with next to no intake velocity; hence the stumbling.

Also, whens the last you tuned your carb? (ie set the floats, adjusted your mixture, etc). How much timing are you adding? Adding a taller gear on it will only add more R's and further kill your fuel mileage. 4.10 Should be fine with the 700r4 and 35s. I'd say a 550 - 600cfm (max) carb is where you should be aiming.
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Old 08-19-2012, 07:02 AM
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Default Re: Gas Milage/ Carb question

Thanks for the reply! I've never tuned the carb. I didn't think you could tune it with the Avenger series. I figured I was dumping too much fuel. Even though I'm running no cats, It's SUPER stinky and rich smelling. Ok, just looked up the manual for the carb. It seems that maybe the floats are the only think I need to look at. It also says that 5-7PSI is recommended. I guess I should check that as well. I'm getting about 10-13MPG MAX.
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Old 08-19-2012, 07:18 AM
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Default Re: Gas Milage/ Carb question

A cam and no cats is going to mean strinky exhaust, though the difference between rich and lean exhaust should be detectable (but either way will smell a lot). Also you have to run richer with e10 than you did with the old gas. You need to get your hands on a wideband O2 sensor. Until then you're just guessing.
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Old 08-19-2012, 07:33 AM
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Default Re: Gas Milage/ Carb question

4:10 gears certainly aren't helping your gas mileage.
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Old 08-19-2012, 07:42 AM
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Default Re: Gas Milage/ Carb question

Originally Posted by Lay Zee H View Post
4:10 gears certainly aren't helping your gas mileage.
35" Tires and a 4speed tranny. I also want to put cats back on it as well.
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Old 08-19-2012, 08:05 AM
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Default Re: Gas Milage/ Carb question

Originally Posted by evan matthews View Post
35" Tires and a 4speed tranny. I also want to put cats back on it as well.
I'm aware. Its just one of the obvious things that's holding gas mileage back. But you should honestly expect the mileage to be pretty bad with the current set up.
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Old 08-19-2012, 08:27 AM
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Default Re: Gas Milage/ Carb question

Originally Posted by evan matthews View Post
35" Tires and a 4speed tranny. I also want to put cats back on it as well.
More rolling friction, more air drag, and much greater rotational inertia.
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Old 08-19-2012, 08:29 AM
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Default Re: Gas Milage/ Carb question

Get a wideband O2 and see what's going on. This is the modern era, there is no reason to rely on things like your nose.

The octane of the gas doesn't matter at all unless you're having knock problems.
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Old 08-19-2012, 10:03 AM
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Default Re: Gas Milage/ Carb question

So what does the wideband 02 do? Hook into the exhaust so it can be plugged into a CPU for proper adjustment and tuning?

Edit, google is my friend. Yup

Last edited by evan matthews; 08-19-2012 at 10:20 AM.
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