Emery writes;
>The amount of air/fuel mixture going through a carb is totally controlled by
>the RPM of the engine. Unless someone plans to run the GMC 455 at 5000 RPM
>or so, you have no need whatsoever for a larger carb. The higher cfm carbs
>are for automobile engines -- not motorhomes. Big is not always better. A
>properly tuned stock carb should give better mileage and performance than a
>much larger one.
Thats very true- In haste to get a post out I see my original one would
leave a person with the impression that bigger was better- bigger is just
big, and QJets are big suckas!
I think the basic idea is Quadrajets are very very large carbs indeed, and
the full potential of them is seldom exploited. Also, QJets with different
CFM ratings vary in the primary bores- they all have the same size
secondary bores AFAIK. GM was using the super large versions of the QJet in
many seventies engines that in no way needed the larger carb, because they
were essentially using them as two barrels, and the larger size primaries
were useful for some engine combinations. They would effectively reduce the
opening of the secondary tract bu limiting travel mechanically (either in
the carb, or like Olds 307 in the eighties by just using a too short
throttle cable) or by delaying opening to a point it seldom occured.
In the early seventies there were a number of 455 2 bbl combinations used
in several GM lines as well, and despite low HP ratings man they were torquey.
There is a great book about QuadraJets published by HP Books availible that
has all the information you could ever want, beleive its by Doug Roe if I
recall correctly, most libraries should have a copy. Its called "Rochester
QuadraJets" or some similar title thats pretty straightforward. I would
definitely recommend it as reading for anyone.
I certainly feel on the GMC that a QuadraJet offers the best overall
performance for most people if FI is out of reach. A local GMC'er here in
town who is not on the net is planning to use GM TBI in his coach, using
eighties era parts, which arent terribly expensive as most items are
readily availible used. I'll keep everyone posted on his progress;-)
Brent Covey
Vancouver BC
>The amount of air/fuel mixture going through a carb is totally controlled by
>the RPM of the engine. Unless someone plans to run the GMC 455 at 5000 RPM
>or so, you have no need whatsoever for a larger carb. The higher cfm carbs
>are for automobile engines -- not motorhomes. Big is not always better. A
>properly tuned stock carb should give better mileage and performance than a
>much larger one.
Thats very true- In haste to get a post out I see my original one would
leave a person with the impression that bigger was better- bigger is just
big, and QJets are big suckas!
I think the basic idea is Quadrajets are very very large carbs indeed, and
the full potential of them is seldom exploited. Also, QJets with different
CFM ratings vary in the primary bores- they all have the same size
secondary bores AFAIK. GM was using the super large versions of the QJet in
many seventies engines that in no way needed the larger carb, because they
were essentially using them as two barrels, and the larger size primaries
were useful for some engine combinations. They would effectively reduce the
opening of the secondary tract bu limiting travel mechanically (either in
the carb, or like Olds 307 in the eighties by just using a too short
throttle cable) or by delaying opening to a point it seldom occured.
In the early seventies there were a number of 455 2 bbl combinations used
in several GM lines as well, and despite low HP ratings man they were torquey.
There is a great book about QuadraJets published by HP Books availible that
has all the information you could ever want, beleive its by Doug Roe if I
recall correctly, most libraries should have a copy. Its called "Rochester
QuadraJets" or some similar title thats pretty straightforward. I would
definitely recommend it as reading for anyone.
I certainly feel on the GMC that a QuadraJet offers the best overall
performance for most people if FI is out of reach. A local GMC'er here in
town who is not on the net is planning to use GM TBI in his coach, using
eighties era parts, which arent terribly expensive as most items are
readily availible used. I'll keep everyone posted on his progress;-)
Brent Covey
Vancouver BC
