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I’m slowly improving my tune on the 17058553 quadrajet. A couple of days ago I swapped in a slightly lighter power piston spring to see if It would improve part throttle economy. Results aren’t conclusive yet, but it felt better.
Along the way, I took this picture to show the different lengths and windings of GM’s power piston springs. This is a big part of how they made the qjet work on everything from 250-500 CID engines:
[img]blob:https://classicoldsmobile.com/68efcf34-ff97-4c3c-adb1-13d95278ff45[/img]
Thought you all might enjoy the shot.
I believe the left hand ones to be front the later M4MC design and the longer ones on the right to be from the earlier 170... series. Can anyone confirm or correct me? It’e be great to know conclusively.
I've seen a variety of lengths in the 70 series carbs, and the aftermarket assortment kits have some very short springs in them. Don't know about what precisely Rochester did in the factory though.
The fun part is they just have to be tested. I had my wife sit in the passenger seat with a bare carb bowl holding a vacuum line to the power piston bottom hole while I drove in circles in a parking lot. Most accurate way I could come up with to find the release point of each spring. You can't just use a vacuum pump sSince it's a pretty sizable controlled leak.
Not sure what you are calling a"release point". I do recall the springs were to operate in vacuum ranges. Example: 8-14" vacuum. The springs with smaller spring wire size and fewer coils per inch, would be used on engines with lower vacuum (longer duration camshafts). I am sure there is some relationship to the length of the taper on the primary metering rods.
Vacuum pulls the primary power piston down to lean out the mixture. When the vacuum decreases, the spring pressure starts to raise the power piston and richens up the A/F mixture. At WOT, the primary power piston is at the top of its travel and maximum richness. This is when the smallest diameter of the primary metering rods flows the most fuel.
......Just my two cents worth.
Chris is correct. The stiffer the spring (for a given length), the earlier the metering rods transition to the small end and thus the sooner the mixture richens up. The AFB/E-brock carbs work the same way, though there are two springs and two power pistons, one on each side. Same principle, however.
"release point" as when it starts to move. It was reasonable to try to catch that in my cockamamie setup, but a full extension vs vacuum mapping wasn't. Especially since the movement with the air hon installed is pretty short (1/8"?)
It just so happens that the first movement point was dramatically different for all 10 or so springs that I tested.
Thanks for the clarification. I was under the impression that mixture richness (rise of the power piston) was transitional. There is a fair amount of mystery around enrichment by primary and secondary metering rods. I had something at one time about three different primary power piston springs and the vacuum range.
Thanks for the clarification. I was under the impression that mixture richness (rise of the power piston) was transitional. There is a fair amount of mystery around enrichment by primary and secondary metering rods. I had something at one time about three different primary power piston springs and the vacuum range.
The power piston is transitional. Each spring is different, but essentially for a given manifold vacuum, a given spring will hold the power piston at a certain height. In reality the movement is very brief - only a few inches of mercury make the difference between fully seated and fully extended, and I don't think it's common to be driving in that range very often. You'll either be cruising with the power piston fully seated, or accelerating with it fully extended, with a few moments somewhere in between.
The "seating vacuum" - the vacuum reading at which the power piston is fully seated with a spring - varies quite a bit. I think there are some springs as low as 4" and some as high as 12" (or more!). A spring with a higher "seating vacuum" can be thought of as a "leaner" spring. That's not quite precise but close enough. That just changes the point at which the primary system richens - earlier or later? Of course, performance engines tend to have low idle vacuum, so you need a spring with a low seating vacuum rating so the rods stay buried for a chance at a decent idle, but that means the power piston stays seated *longer* when accelerating, which means the fuel curve is actually *leaner*. Funny, right?
Of course this is just one factor of MANY in a carb.
IMO there's two points to tune on the primaries - cruise AFR, which is easiest set with an adjustable part throttle modification, and WOT. Of course the secondaries are much more important at WOT, so really getting a good cruise AFR then general good driving manners is, IMO, good enough on the primary side.
The secondaries are a bit more straight forward. Maybe. The rods are lifted by the turning of the air flaps. The air rushing past the flaps tries to pull them open. The tension of the secondary air flap spring resists this and essentially creates the fuel curve. The more air rushing past the air flaps cause a greater force trying to open them. So a tighter spring setting keeps the flaps more closed, which means lower rods which means less fuel. The vacuum break on the side of the carb is really to prevent the secondaries from getting much air while there's lots of manifold vacuum. In some setups that vacuum can also controls the rate at which the secondary air flap can move. And in some setups the can moves very fast so it's not a factor.
Home tuning specific secondary rods and especially hangers can be quite tough. Easier on the drag strip. I use a wideband O2 to make sure it's running in a decent range then just focus on driveability.
July 30th, 2019, 06:07 AM
