The 3B's slightly-unusual butterfly valve and vacuum driven injector pump has been discussed many times on this board. The presence of the butterfly creates inefficiency in the form of pumping losses, however how much of a difference this makes is open for debate. I'm interested in finding out if there is any benefit (gas mileage?) to be had in bypassing the butterfly to reduce intake restriction and control the IP directly.
Now, I'd assumed, like many before me, that the IP vacuum system was simply proportional - high vacuum = idle, low vacuum = redline. However, its easy to see that this is not the case as obviously the vacuum developed in the venturi is a product of throttle position AND current engine RPM... oops. I took some measurements:
Steady-state
650rpm - 4"Hg
2500rpm - 5"Hg
Accelerating hard (any RPM)
0.5 - 1"Hg
Deccelerating hard (any RPM)
10"Hg
So basically IP holds maintains constant engine speed at 4-5"Hg, accelerates when the vacuum is less then this, and deccelerates when its higher. [Also note, the static port (hose that connects behind the diaphragm) plays a role only in decceleration; if you disconnect it you can drive just fine, but you'll notice the engine doesn't slow down very quickly when you release the pedal]
Now, I'll admit that I didn't take complete measurements initially, so I just assumed that 5"Hg was an idle "setting" and 0"Hg was a high speed setting. I wanted to just attach a cable directly to the IP rack, however I didn't want to wreck my housing so I decided so use the vacuum system, but bypass the butterfly. As you know, there is another convenient source of vacuum on the truck, from the vacuum pump and reservoir for the brake booster. If only there was a way to regulate this vacuum source without disturbing or dumping the vacuum for the brakes....
Some digging in the junk box produced this! A DIY vacuum regulator. Inside the black portion is a diaphragm of delrin for strength and rubber for for airtightness, and a spring forming a regulator. Attached to the diaphragm via the screw is a rack and two electric switches. The switches control solenoid valves that let vacuum into and out of the diaphragm until the vacuum level is equal to that being set by the rack position. This keeps the "dumping" of vacuum to a minimum so that the brake vacuum isn't depleted.
The idea was that I attached the throttle cable to the sliding rack, and then attached my regulated vacuum to the IP diaphragm. Obviously this didn't work, however, given the way the vacuum system actually functions (above). It did manage to throttle the engine, however you'd have to hold the pedal "halfway" down to maintain speed, let it up to slow down, and press it in to speed up. Also it wasn't really robust enough for me to want to drive around with for a full tank of gas, so I never left the garage.
Of course, I still have other ideas, but they'll have to wait for more funds. Anyway, just thought I'd share so no one else has to repeat this.
Now, I'd assumed, like many before me, that the IP vacuum system was simply proportional - high vacuum = idle, low vacuum = redline. However, its easy to see that this is not the case as obviously the vacuum developed in the venturi is a product of throttle position AND current engine RPM... oops. I took some measurements:
Steady-state
650rpm - 4"Hg
2500rpm - 5"Hg
Accelerating hard (any RPM)
0.5 - 1"Hg
Deccelerating hard (any RPM)
10"Hg
So basically IP holds maintains constant engine speed at 4-5"Hg, accelerates when the vacuum is less then this, and deccelerates when its higher. [Also note, the static port (hose that connects behind the diaphragm) plays a role only in decceleration; if you disconnect it you can drive just fine, but you'll notice the engine doesn't slow down very quickly when you release the pedal]
Now, I'll admit that I didn't take complete measurements initially, so I just assumed that 5"Hg was an idle "setting" and 0"Hg was a high speed setting. I wanted to just attach a cable directly to the IP rack, however I didn't want to wreck my housing so I decided so use the vacuum system, but bypass the butterfly. As you know, there is another convenient source of vacuum on the truck, from the vacuum pump and reservoir for the brake booster. If only there was a way to regulate this vacuum source without disturbing or dumping the vacuum for the brakes....
Some digging in the junk box produced this! A DIY vacuum regulator. Inside the black portion is a diaphragm of delrin for strength and rubber for for airtightness, and a spring forming a regulator. Attached to the diaphragm via the screw is a rack and two electric switches. The switches control solenoid valves that let vacuum into and out of the diaphragm until the vacuum level is equal to that being set by the rack position. This keeps the "dumping" of vacuum to a minimum so that the brake vacuum isn't depleted.
The idea was that I attached the throttle cable to the sliding rack, and then attached my regulated vacuum to the IP diaphragm. Obviously this didn't work, however, given the way the vacuum system actually functions (above). It did manage to throttle the engine, however you'd have to hold the pedal "halfway" down to maintain speed, let it up to slow down, and press it in to speed up. Also it wasn't really robust enough for me to want to drive around with for a full tank of gas, so I never left the garage.
Of course, I still have other ideas, but they'll have to wait for more funds. Anyway, just thought I'd share so no one else has to repeat this.
Silly question I guess - because I don't think you can get access without drilling a hole in the manifold!
