86 GMC PU with new crate 350 getting 8 MPG. - Page 2 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

gchemist · 09-09-2014, 12:19 PM

Wow, without a bigger cam the carb is too big. On my ZZ4 with 355 hp a 1406 works great. The 1407 may work with the smallest jets possible or better tuning.

Desert1957 · 09-09-2014, 12:36 PM

Quote:

Originally Posted by dyno

Thanks. I'll head over to Harbor Freight today.

Forgot to mention the Edelbrock is a 750 CFM. It was a newer carb that came on the old motor. I had 650 CFM Edelbrocks on 350's before. If metered right will there be a mileage difference between the 750 and a 650?

Yes and YES....

rich weyand · 09-09-2014, 01:11 PM

A normally aspirated 350 can only use about 500 cfm at 5000 rpm, even with a volumetric efficiency of 1 (which it doesn't have). It won't suck more air than that, ever, regardless of the carb capacity. The issue is that the bigger carb will have a lower stack velocity at all flow rates, which will reduce the efficiency of atomization of the fuel.

With a square carb like the Edelbrock, you have 375 cfm in front and 375 in back, which means that you are almost never into the secondaries, and even when you are, they won't pull the air door much. You basically have a big two-barrel on there, and one of the advantages of a four-barrel over a big deuce is that you get better mileage running off of smaller barrels until you actually want the extra and pop the secondaries.

So yeah, it's gonna hurt mileage a bit compared to the 650, no matter how you jet it. That being said, your best jetting/metering/idle settings will be obtained using an A/FR meter. The meter doesn't lie, and can't be fooled. I think you should still be able to get it to 9 or 9.5 in town, and 11-12 on the highway.

If you decide to change carbs to get better mileage, the Qjet with its smaller primaries is the way to go. Higher stack velocities at all flow rates. Once again, it needs to be tuned, and once again the best way to do that is with an A/FR meter.

dyno · 09-09-2014, 10:54 PM

Well, I'll be looking for a 650CFM then. As mentioned, I put a Q-jet on it initially thinking I would get good mileage with those smaller primarys, but it was still around 8mpg. Isn't a Q-jet a 750CFM carb?

rich weyand · 09-09-2014, 11:01 PM

What really matters is the size of the primaries. Gas mileage isn't one's first consideration when dumping the secondaries.

If you still have the Qjet, why not put that back on and get the meter?

09-09-2014, 12:19 PM	#26
gchemist BAD BOW-Silverado XST Join Date: May 2000 Location: Senior Member from Austin, TX Posts: 6,431	Re: 86 GMC PU with new crate 350 getting 8 MPG. Wow, without a bigger cam the carb is too big. On my ZZ4 with 355 hp a 1406 works great. The 1407 may work with the smallest jets possible or better tuning. __________________ Gerardo a.k.a. Mad Chemist Silverado XST videos

09-09-2014, 01:11 PM	#28
rich weyand Registered User Join Date: Apr 2014 Location: Bloomington Indiana Posts: 1,041	Re: 86 GMC PU with new crate 350 getting 8 MPG. A normally aspirated 350 can only use about 500 cfm at 5000 rpm, even with a volumetric efficiency of 1 (which it doesn't have). It won't suck more air than that, ever, regardless of the carb capacity. The issue is that the bigger carb will have a lower stack velocity at all flow rates, which will reduce the efficiency of atomization of the fuel. With a square carb like the Edelbrock, you have 375 cfm in front and 375 in back, which means that you are almost never into the secondaries, and even when you are, they won't pull the air door much. You basically have a big two-barrel on there, and one of the advantages of a four-barrel over a big deuce is that you get better mileage running off of smaller barrels until you actually want the extra and pop the secondaries. So yeah, it's gonna hurt mileage a bit compared to the 650, no matter how you jet it. That being said, your best jetting/metering/idle settings will be obtained using an A/FR meter. The meter doesn't lie, and can't be fooled. I think you should still be able to get it to 9 or 9.5 in town, and 11-12 on the highway. If you decide to change carbs to get better mileage, the Qjet with its smaller primaries is the way to go. Higher stack velocities at all flow rates. Once again, it needs to be tuned, and once again the best way to do that is with an A/FR meter. __________________ Rich Weyand 1978 K10 RCSB DD.

09-09-2014, 10:54 PM	#29
dyno Registered User Join Date: Sep 2008 Location: Middleton, Idaho Posts: 352	Re: 86 GMC PU with new crate 350 getting 8 MPG. Well, I'll be looking for a 650CFM then. As mentioned, I put a Q-jet on it initially thinking I would get good mileage with those smaller primarys, but it was still around 8mpg. Isn't a Q-jet a 750CFM carb? __________________ "Built- not bought" 1958 GMC 150 4x4 in progress 1980 K20- Restored 1985 K3500 Flatbed 58 GMC 4x4 build thread: http://http://67-72chevytrucks.com/vboard/showthread.php?t=557675

09-09-2014, 11:01 PM	#30
rich weyand Registered User Join Date: Apr 2014 Location: Bloomington Indiana Posts: 1,041	Re: 86 GMC PU with new crate 350 getting 8 MPG. What really matters is the size of the primaries. Gas mileage isn't one's first consideration when dumping the secondaries. If you still have the Qjet, why not put that back on and get the meter? __________________ Rich Weyand 1978 K10 RCSB DD.



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