How did EGR work with a Qjet?

Supercharged111

Royal Smart Person
Oct 25, 2019
2,170
113
Colorado Springs, CO
I'm familiar with what the EGR system does, and I know on an EFI motor fuel delivery is reduced (and ignition timing advanced) when EGR is flowing, but a carb doesn't know when an EGR valve is open or closed. I'm curious how the EGR was managed on a carb'd engine? It seems to me the thing would have been jetted for the EGR to be open, i.e. lean? Was it open at an idle too? Maybe a temp controlled vacuum switching valve to open it warm? Was it still open WOT?
 
Oct 14, 2008
7,086
113
Melville,Saskatchewan
Good question. I know a Qjet has a specially timed vacuum port to run EGR. It is the second largest one, is ported but not the same as a ported vacuum source so can't be ran as one.
 

Clone TIE Pilot

Comic Book Super Hero
Aug 14, 2011
2,596
113
Galaxy far far away
The EGR runs off a ported vacuum source on the Qjet plus a TV switch to keep it from operating when the engine is cold. Both the port on the EGR valve and its return spring were calibrated for the engine setup. Later on with CCC, a ECM controlled vacuum valve was installed in the vacuum supply line for finer EGR control. Low end CCC setups its just a simple on / off switch while higher CCC setups used on the performance engines used fancier Pulse Width control.
 
  • Like
Reactions: 83MonteCarloSS

Bonnewagon

Geezer
Sep 18, 2009
7,925
113
Queens, NY

Bonnewagon

Geezer
Sep 18, 2009
7,925
113
Queens, NY
You started out with advanced basic timing. Instead of 6 or 8 degrees crank timing on a pre-EGR engine you might see 12 to 16 degrees. They got away with that because the compression ratios were so low to begin with. Then your centrifugal and ported vacuum timing came in, more-so then pre-EGR. But by then the EGR was on and diluted the air/fuel mixture so much there was no detonation. Early problems were the EGR operating at idle causing horrible idle roughness. Or failing and causing detonation because of the advanced timing with no dilution. I was delivering the mail at a Ford stealership around 1977 and overheard a service manager explaining why they could not fix a customer's crappy running car because no one really understood EGR yet. I knew it was operating when it shouldn't but I kept quiet.
 

Supercharged111

Royal Smart Person
Thread starter
Oct 25, 2019
2,170
113
Colorado Springs, CO
Well that make sense. It's just timed in anticipation of EGR from the get go. When the engine is cold and EGR not yet flowing it won't detonate because it's cold. When you're WOT they probably never gave it enough timing to hurt itself, and as you pointed out the compression was so low in other scenarios they simply didn't hurt themselves.
 

Bonnewagon

Geezer
Sep 18, 2009
7,925
113
Queens, NY
One last thing. It never really affected gas mileage. It may prevent maximum power output on a non-computer engine, but I have had many engines both with and without EGR and mileage was similar if not identical. Modern computerized engines only enable the EGR within particular parameters and they get incredible power and mileage at the same time.
 

Supercharged111

Royal Smart Person
Thread starter
Oct 25, 2019
2,170
113
Colorado Springs, CO
One last thing. It never really affected gas mileage. It may prevent maximum power output on a non-computer engine, but I have had many engines both with and without EGR and mileage was similar if not identical. Modern computerized engines only enable the EGR within particular parameters and they get incredible power and mileage at the same time.

My understanding is it exists to reduce nox. Anything else is 2nd or 3rd order.
 
  • Agree
Reactions: oldsmobile joe

Similar threads

G-Body Performance Upgrades

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck ConsolesDixie Restoration DepotMike's MontesP-S-TSouthside Machine PerformanceUMI Performance

ContactAdmin@GBodyForum.comfor info on becoming a sponsor