800 cfm Stage II SMI Quadrajet Carb
October 26th, 2010, 11:02 PM
#1
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Join Date: Oct 2009
Location: Lansing, MI
Posts: 267
800 cfm Stage II SMI Quadrajet Carb
I just got this carb two weeks ago, and although it runs fine, it just doesn't feel like it performs as well as the Holley that was on my 455 previously. I bought this Stage II carb from Sean Murphy Induction (SMI) because I had no desire to become a Q-jet rebuilder. The list of things done to improve the performance of these Q-Jets is impressive. Here is some information about them from my ebay ad:
Features
Front Fuel Inlet
Intended for moderately modified street performance vehicles.
All circuits will be modified and tuned to match the specifications of your engine combination, Including:
Idle down tube(idle jet) and idle air bleed resized for smooth idle quality with performance camshafts.
CRO(channel restriction orifice) resized for crisp off-idle and part throttle response.
Main jet and metering rods selected for clean cruise mixture and strong part throttle response and acceleration.
Proper metering rod power piston spring selected to match engine vacuum characteristics
Secondary metering rods and hanger selected and high speed air bleeds resized for maxium full throttle horsepower and torque.
Select highest quality cores for best performance and reliability
Strip to bare metal, inspect for defects, then apply dichromate coating for brilliant gold finish.
Strip and cadmium plate all hardware, including throttle shafts, linkage, levers, spring and screws.
Install bushings in baseplate to eliminate any potential vacuum leaks at primary throttle shaft.
Install APT (ADUSTABLE PART THROTTLE) to allow adjustment of part throttle and cruise speed (steady throttle, 1800-4000 rpm) air/fuel mixture in seconds without any disassembly, tuning can be done while engine is running!
Apply epoxy to main body casting plug to eliminate common "leak down" problem associated with Quadrajets.
Idle, transition, main, and high speed circuits modified to provided optimum fuel curve for YOUR engine
Assembled using the highest quality components, including:
New jets and metering rods.
High capacity accelerator pump featuring materials that are resistant to alcohol and other additive commonly found in today's fuel
Nitrophyl solid composite float that can't crack or sink.
Premium quality gaskets
High flow .130" needle and seat assembly.
New electric choke for cold starting and driveability.
New choke pull-off for clean, smooth warm-ups.
Although this carb works quite well. It does not give me as good of a "seat-of-the-pants" feeling when I floor it as the Holley it replaced. I am by no means a Q-Jet expert, that is why I bought a modified unit to begin with. I have no desire to make any changes to the calibration of this carb. I hate to sound ignorant, but there is just to much going on in a Q-Jet for me to mess with it.
I paid over $400 for this carb with the option of a -08 AN fuel inlet fitting. I will include a copy of Cliff Rugles Q-Jet book along with the carb.
I have this listed on ebay, item number ( 230543396252 ). The reserve is set at $250
Features
Front Fuel Inlet
Intended for moderately modified street performance vehicles.
All circuits will be modified and tuned to match the specifications of your engine combination, Including:
Idle down tube(idle jet) and idle air bleed resized for smooth idle quality with performance camshafts.
CRO(channel restriction orifice) resized for crisp off-idle and part throttle response.
Main jet and metering rods selected for clean cruise mixture and strong part throttle response and acceleration.
Proper metering rod power piston spring selected to match engine vacuum characteristics
Secondary metering rods and hanger selected and high speed air bleeds resized for maxium full throttle horsepower and torque.
Select highest quality cores for best performance and reliability
Strip to bare metal, inspect for defects, then apply dichromate coating for brilliant gold finish.
Strip and cadmium plate all hardware, including throttle shafts, linkage, levers, spring and screws.
Install bushings in baseplate to eliminate any potential vacuum leaks at primary throttle shaft.
Install APT (ADUSTABLE PART THROTTLE) to allow adjustment of part throttle and cruise speed (steady throttle, 1800-4000 rpm) air/fuel mixture in seconds without any disassembly, tuning can be done while engine is running!
Apply epoxy to main body casting plug to eliminate common "leak down" problem associated with Quadrajets.
Idle, transition, main, and high speed circuits modified to provided optimum fuel curve for YOUR engine
Assembled using the highest quality components, including:
New jets and metering rods.
High capacity accelerator pump featuring materials that are resistant to alcohol and other additive commonly found in today's fuel
Nitrophyl solid composite float that can't crack or sink.
Premium quality gaskets
High flow .130" needle and seat assembly.
New electric choke for cold starting and driveability.
New choke pull-off for clean, smooth warm-ups.
Although this carb works quite well. It does not give me as good of a "seat-of-the-pants" feeling when I floor it as the Holley it replaced. I am by no means a Q-Jet expert, that is why I bought a modified unit to begin with. I have no desire to make any changes to the calibration of this carb. I hate to sound ignorant, but there is just to much going on in a Q-Jet for me to mess with it.
I paid over $400 for this carb with the option of a -08 AN fuel inlet fitting. I will include a copy of Cliff Rugles Q-Jet book along with the carb.
I have this listed on ebay, item number ( 230543396252 ). The reserve is set at $250
October 27th, 2010, 02:21 AM
#2
Registered User
Join Date: Nov 2006
Location: Leasburg, NC
Posts: 1,190
It probably doesn't perform because it sounds like a generic build. How does he know for sure you need a .130 needle and seat? YOUR application may require something different, and that could be all the difference in your power.
Anyway, regardless of who snags it, they may need to make some adjustments to get it to work on thier application. You may be letting a perfectly good carb go for no reason.
John
Anyway, regardless of who snags it, they may need to make some adjustments to get it to work on thier application. You may be letting a perfectly good carb go for no reason.
John
October 27th, 2010, 02:53 AM
#3
Registered User
Thread Starter
Join Date: Oct 2009
Location: Lansing, MI
Posts: 267
It is not a generic build. I had to fill out a tech sheet with information about my engine/car combo. Comp Ratio, camshaft type and specs, intake style, cylinder heads, weight of the car, trans, rear end gear, etc. SMI was quite thorough as far asking the important questions in order to know what they needed to do for this particular carb.
When I think about how much they needed to know, I am surprised that this Q-Jet doesn't out-perform the previous carb. But the proof is in the pudding as they say, and to me, when I floor it, this carb just doesn't get up and go as fast as I would like it too. I know that shouldn't be the only thing to base your carb choice on, the mpg would most certainly be better with the Q-Jet. If mpg was my main concern, I wouldn't have a 455 Olds to begin with!
When I think about how much they needed to know, I am surprised that this Q-Jet doesn't out-perform the previous carb. But the proof is in the pudding as they say, and to me, when I floor it, this carb just doesn't get up and go as fast as I would like it too. I know that shouldn't be the only thing to base your carb choice on, the mpg would most certainly be better with the Q-Jet. If mpg was my main concern, I wouldn't have a 455 Olds to begin with!
October 27th, 2010, 04:06 AM
#4
Registered User
Join Date: Sep 2010
Location: Shorewood, Il.
Posts: 3,057
Could be the secondary air door is a little tight.
If you're using a double pumper Holley, that 'all-in' SOP feeling is considerable!
A 2 minute adjustment may change your attitude about the Q-Jet, and you may feel a lot better about the price you paid, too!
I went through a stock Q-Jet on a friend's '71 W-30, and it'd smoke the stock type tires when you got into the secondaries!
If you're using a double pumper Holley, that 'all-in' SOP feeling is considerable!
A 2 minute adjustment may change your attitude about the Q-Jet, and you may feel a lot better about the price you paid, too!
I went through a stock Q-Jet on a friend's '71 W-30, and it'd smoke the stock type tires when you got into the secondaries!
October 27th, 2010, 04:18 AM
#5
I got mine to work ok took me a week of playing with the door and mix and so on. It too is a smi carb I have performer rpm on it and I need a straight performer (Its a pontiac 400) with tall gears and low compretion ( its a driver). I was tempted to go back to my old carb but the SMI carb looked so nice I just kept at it some one will get a nice carb.
Last edited by firefrost gold; November 1st, 2010 at 04:03 AM.
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