Cadillac Owners Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
27 Posts
Discussion Starter #1
Hello everyone,this is my first post so i hope i get it right.
I would like to know if anyone has installed an electronic q-jet,dist and computer on 472\500 if so please explain it to me if no do you think it would work,pros and cons please
 

·
Banned
Joined
·
7,971 Posts
terrible one said:
What's an electronic Quadrajet?
It's computer controlled... they were used in the 80s 307 Fleetwood Broughams. The computer can adjust the carb based on O2 sensor feedback. I don't see any reason it wouldn't work on a 500, and you could net slightly better fuel economy. You'll have to tune the carb for a 500 since they were never used on one, but that shouldn't be too bad. Go for it...
 

·
Registered
Joined
·
595 Posts
I think it would be cool, but for one problem...
Our big-inch Caddies got 800cfm Q-jet's. The electronic Q-jets used the 750cfm body (the secondaries were also limited so the carb only got 650cfm, but that can be fixed in a jif). So, unless you go through and carefully port the carb (David Vizard talks about this in one of his books, and claims to be abe to get 1000cfm out of a Q-jet) you will be losing some airflow.
Will that make a difference though? I'm not sure... For a cruiser you will probably not notice it 99% of the time, but will likely lose something off the topend.
Of course, the difference is only in the primaries, the 800cfm models bores are a little bigger. The secondaries are the same on ALL Q-jets. So, if you can get a little more airflow out of the primaries, you should be set.

I have thought about doing this on other cars though, and it does seem like a great idea. You'll just need a manifold pressure sensor, coolant temp sensor, and O2 sensor, all of which would be easy enough to add in. The throttle position sensor is integral with the carb, so that's already done.
There would be very little tuning needed as compared to a regular carb. I'd just re-jet the primaries (if you even can on the electronic ones) and use the same secondary rods and hangar as your Caddy came with. The beauty of the simple programming for the electronic Q-jets is that they basically do all the tuning for you. It either works to keep the fuel mixture at stoich, or it holds the metering rods full-open when the manifold vacuum falls low enough that the computer says you are near WOT.
The computer has no idea what the size of the engine is, and doesn't care. All it sees is the O2 sensor and uses that to keep the mixture ideal whenever you are at partial throttle. The secondaries on the electronic models are the same as on the mechanical ones, so there's nothing different there.

Oh, other little bits to be aware of. I think all the electronic carbs had an electric choke. So, you can either go to electric choke (hook up a switched +12V wire), or swap over the hardware for the divorced choke from the stock Caddy setup.
I do not know if there are differences in the throttle linkage, and the fuel line connection (different angle), there may be, so there might be a few other little bits to work out.

All in all, it shouldn't require too much work, and we have plenty of room under the dash to hide the computer once you get past the maze of vacuum lines for the climate controll. I would try and have a light for the trouble codes to help in case any of your sensors go bad.

Hmmm, you've got me thinking...
 

·
Registered
Joined
·
27 Posts
Discussion Starter #7
THANK YOU ALL,With your imput I think that I'am moving in the right direction.
The car is a 1984 chevrolet caprice 2dr.as you may know it already has the electronic q-jet,sensors,computer controlled Hei and the computer.Now its just the matter easing
out the 305 and easing in the 472 without the computer knowing it

THANKS
 

·
Registered
Joined
·
595 Posts
Ditch the electronically controlled HEI totally. Non-adjustable timing curve, and way too little advance, and no chips available to fix that. A mechanical HEI will let you set a far better timing curve.
 

·
Banned
Joined
·
7,971 Posts
750cfm is definately enough for a 472 anyway. The smaller primaries should help part throttle economy as long as the you aren't always opening up the secondaries.
 

·
Registered
Joined
·
595 Posts
terrible one said:
The lower cfm will help throttle response too.
Theoretically yes, but I think the q-jet already has so much throttle response you won't notice. The primaries are super-tiny for a carb that flows so much air. Seriously, I rebuilt the 2-barrel carb on a 1978 Toyota Corolla with a 1.6L motor. The size of the butterflies on that carb are comparable to the primaries on a Q-jet!

What I wonder about is if the 750cfm model might force you into the secondaries more often. The extra 50cfm in the 800cfm model is all on the primary side, so you should be able to get more from that carb before having to open up the two gas pipelines on the back!
Honestly, I'd still do it though. At very worst it won't work and you can switch an 800cfm carb on there. Just leave the wiring to the carb hanging and ignore the check engine light!
 

·
Banned
Joined
·
1,631 Posts
BluEyes said:
Theoretically yes, but I think the q-jet already has so much throttle response you won't notice. The primaries are super-tiny for a carb that flows so much air. Seriously, I rebuilt the 2-barrel carb on a 1978 Toyota Corolla with a 1.6L motor. The size of the butterflies on that carb are comparable to the primaries on a Q-jet!

What I wonder about is if the 750cfm model might force you into the secondaries more often. The extra 50cfm in the 800cfm model is all on the primary side, so you should be able to get more from that carb before having to open up the two gas pipelines on the back!
Honestly, I'd still do it though. At very worst it won't work and you can switch an 800cfm carb on there. Just leave the wiring to the carb hanging and ignore the check engine light!
That is really interesting. I did not know that.
 

·
Registered
Joined
·
27 Posts
Discussion Starter #14
If I understand this.I think the m/c solenoids controls the metering rods in the primarys which is in turn controlled by the ecm,by leaving the wires hanging,I don't think the primary side of the carb.would work
 

·
Registered
Joined
·
595 Posts
Correct about the solenoid. It recieves pulses from the ECM. The ECM regulates the frequency of the pulses at part throttle to keep you at the ideal A/F ratio.
But, IIRC, the default position for the metering rods is all the way out, so leaving the wires hanging results in a car running pig-rich (untill WOT). I'm pretty sure the spring on the metering rods pushes them up and the solenoid pushes them down.
Of course, if I'm wrong and the metering rods would be all the way down with the wires hanging and you'd still be able to idle just fine or use *light* throttle, but heavy throttle application would cause you to lean out.
 
1 - 15 of 15 Posts
About this Discussion
14 Replies
6 Participants
BluEyes
Top