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Old 06-26-2011, 10:45 AM   #21 (permalink)
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While Q- jets are generally good , due to small primaries I have had good luck with some smaller eco crabs . damn, its been long time but I think Holley made them in 450 and 600 cfm which is much smaller . not sure they make them anymore ,they were made to replace Q-jet .
Whatever you do don't use holley performance carbs, there terrible for mpg and need tunning all the time .

Edit: forget my post as i don't see the carb made anymore ,could not find it .


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Old 06-26-2011, 12:15 PM   #22 (permalink)
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I didn't mention that one of the reasons I'm thinking of the Performer carb is that I have a buddy who is very experienced with them and can help me tune it. It's what he recommends in lieu of the Q-jet.
The Performer 1406 is a 600 cfm carb factory tuned for fuel economy (as opposed to the 1405 which is the same carb but tuned for performance).
There's also the 500 cfm 1403/1404 which are intended for 305 cu. in. and smaller engines. They are calibrated for performance, so I might need to replace jets and rods to tune for economy. By my own calculations, I would have to be running at more than 5,000 rpm before running out of air with a 500 cfm carb, but I suppose it might not be any more efficient if the secondaries are opening sooner than on a larger carb.
I've thought about 2-barrel carbs too, but I need to understand the theory behind why 2-barrel would be more efficient. At part throttle, isn't a 4-barrel effectively a 2-barrel with even smaller barrels? Would a 2-barrel be more efficient at WOT, but not necessarily otherwise?
I also want the ability to tow with this truck, so there may be times when I need four barrels.
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Old 06-26-2011, 01:57 PM   #23 (permalink)
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Quote:
Originally Posted by northboundtrain View Post
I didn't mention that one of the reasons I'm thinking of the Performer carb is that I have a buddy who is very experienced with them and can help me tune it. It's what he recommends in lieu of the Q-jet.
The Performer 1406 is a 600 cfm carb factory tuned for fuel economy (as opposed to the 1405 which is the same carb but tuned for performance).
There's also the 500 cfm 1403/1404 which are intended for 305 cu. in. and smaller engines. They are calibrated for performance, so I might need to replace jets and rods to tune for economy. By my own calculations, I would have to be running at more than 5,000 rpm before running out of air with a 500 cfm carb, but I suppose it might not be any more efficient if the secondaries are opening sooner than on a larger carb.
I've thought about 2-barrel carbs too, but I need to understand the theory behind why 2-barrel would be more efficient. At part throttle, isn't a 4-barrel effectively a 2-barrel with even smaller barrels? Would a 2-barrel be more efficient at WOT, but not necessarily otherwise?
I also want the ability to tow with this truck, so there may be times when I need four barrels.
Stick with 4 barrel carb , the venturi will be smaller in general and that helps with tunning an low speeds .
While I have worked on those and there very tuneable I am not sure for eco purposes but probably can help .

I had but a 450 cfm 4 barrel on a old 75 buick 350 ci , this did give better mpg a bit and improved low-mid range response . for eco driving you don't need hi cfm at all . though depending on gearing of your truck you might need 600 if revs are a lot higher than a car .
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Old 06-26-2011, 02:35 PM   #24 (permalink)
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What is the maximum rpm the engine is going to spin and where do you want the torque peak to be at?
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Old 06-26-2011, 04:37 PM   #25 (permalink)
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Quote:
Originally Posted by cleanspeed1 View Post
What is the maximum rpm the engine is going to spin and where do you want the torque peak to be at?
Max rpm should not exceed 4,000. With the gearing I currently have, 4 speed, no overdrive, 4.10 differentials, I'd like peak torque in the 2,500 to 3,000 range if possible.
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Old 06-26-2011, 04:41 PM   #26 (permalink)
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Quote:
Originally Posted by cleanspeed1 View Post
What is the maximum rpm the engine is going to spin and where do you want the torque peak to be at?
I wasn't going to get this technical, but I'd bet 400-500cfm is all the motor can use. That's why I suggested the little two barrel carb. I doubt he'll be spinning more than 4500rpms and the motor probably has a torque peak around 2200-2400rpms. A typical chebby 350.

LOL, beat me to the post.
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Old 06-26-2011, 04:58 PM   #27 (permalink)
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Quote:
Originally Posted by northboundtrain View Post
Max rpm should not exceed 4,000. With the gearing I currently have, 4 speed, no overdrive, 4.10 differentials, I'd like peak torque in the 2,500 to 3,000 range if possible.
I hang out a hotrodders.com and one of the moderators can DynoSim what you have to at least see what the hp and torque curves would look like.

I looked at the cam and my first thought was a tighter lobe seperation angle, so that the engine will build up more pressure down low. Like go from 114 degrees to 108-110 degrees, maybe even tighter. The motor will be more responsive.

Just a thought.
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Old 06-26-2011, 08:39 PM   #28 (permalink)
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Quote:
Originally Posted by cleanspeed1 View Post
I hang out a hotrodders.com and one of the moderators can DynoSim what you have to at least see what the hp and torque curves would look like.

I looked at the cam and my first thought was a tighter lobe seperation angle, so that the engine will build up more pressure down low. Like go from 114 degrees to 108-110 degrees, maybe even tighter. The motor will be more responsive.

Just a thought.
I believe you have that backwards , wider separation = less overlap =better low end and narrow separation =more overlap = higher peak efficiency rpm

OP : 500cfm is all you need for low -mid range on 350 .
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Old 06-26-2011, 09:25 PM   #29 (permalink)
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Quote:
Originally Posted by EdKiefer View Post
I believe you have that backwards , wider separation = less overlap =better low end and narrow separation =more overlap = higher peak efficiency rpm

OP : 500cfm is all you need for low -mid range on 350 .
I'll look further into this, thanks for the correction.
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Old 06-27-2011, 05:46 PM   #30 (permalink)
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Has anyone looked at the effect of carb spacers on FE? A regular old spacer(not the tornado crap) can improve throttle response and power. Many magazines have done dyno tests, but none of them ever looked at the effect on part throttle cruising.

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