vac. on a turbo

[TCIC 300ci superbeast]

i have a turboed 300 with vac. stuff on it. i noticed that it would'nt advance under boost so I think that moving the vac. manifold in front if the turbo would work. i have'nt tryed this because i shot my rings but does any one know if this would work?

 

[Does10s]

I assume you mean the vacuum advance for the distributor?
Put a one-way check valve in the vacuum line for the distributor. Under vacuum (idle/cruise) it'll allow full vacuum adv. but under boost it just use mechanical adv.
Also you don't want to have to much timing advance anyway under boost. Nothing more than around 24-25* total for 91 octane pump gas.

And there's no "vacuum" in front of the turbo inlet.

Later,
Will

 

[66Sprint6]

Yeah, Does10s is right, i only use the turbo source for my boost controller, and MSD 6BTM timing controller.

Not to hijack, but I have a question too...my new carb doesnt have any vac. sources, so I used the manifold tap...but Im gettin fuel in the vac lines...is that ok?

Matt

 

[XPC66]

I assume you mean the vacuum advance for the distributor?
Put a one-way check valve in the vacuum line for the distributor. Under vacuum (idle/cruise) it'll allow full vacuum adv. but under boost it just use mechanical adv.
Also you don't want to have to much timing advance anyway under boost. Nothing more than around 24-25* total for 91 octane pump gas.

And there's no "vacuum" in front of the turbo inlet.

Later,
Will

Advance/retard is a better solution if a suitable diaphragm can be sourced. I tend to run -7° on boost for jap engines..e.g 10° static + 13° centrif + 10° = 33° when off boost at 3krpm and 10° + 13° - 7° = 16° on boost.

Putting a non return in means the vacuum will hold on the diaphragm and keep the advance up, which is not what you want:- maybe a VTV in line that allows slow vacuum build, but rapid release is a better solution, in the absence of a retard diaphragm. Similarly the secondary barrel on the carb works well if the release is stepped via a VTV between the carby horn and release dashpot in parallel with a check&jet from an inlet runner.

 

[66Sprint6]

that sounds like common sense and all...but then why would the Jeeps use the distributor check valve? Thats the valve ive got on my car now.

 

[XPC66]

Dunno, maybe it isn't a check valve? Most of the moulded plastic fittings I've seen in the dizzy lines are jets & filters.

 

[Anonymous]

. . . And there's no "vacuum" in front of the turbo inlet.

Later,
Will

That all depends on how the system was setup Will, i have plans about 80% complete for a turbo build on a 300(lacking final cam, turbo, ignition decision) . Just not sure if i will go that route or not and the turbo will run in vacuum when the throttle blade is closed.

will be plumbing in brake booster between the TB and turbo
setup in order will be Head, lower EFI intake, Custom upper intake, Turbo, TB, Air filter. running the Turbo after the TB allows me to use any TB i want, and also will help with intake temp some.

So it all Depends on how His system is plumbed up.

 

[wallaka]

Nope, no vacuum before the turbo inlet. There can still be (and is!) vacuum in the intake tract when the throttle is partly open and the engine isn't under a load (the turbo is basically freewheeling). But it is after the throttle enters the picture.

 

[66Sprint6]

nah, it IS a check valve, its in the part description and name.

 

[TCIC 300ci superbeast]

well i was think like if you had 2 windows open in you house. one has a vac and one had boost. thats how i was thinking about it

 

[Does10s]

The only way there would be "vacuum" before the turbo inlet would be in a "draw-thru" system where the carb is before the compressor.

The "venturi effect" may apply here though. The air that's being sucked into the inlet will have greater velocity thereby creating a slightly reduced atmosphereic pressure at that point. But I don't think it would be enough to advance the timing. And if it did, it that is the opposite direection you want to go under boost.
Will

 

[Bort62]

I do have to say that a check valve on the vac advance line really seems like the wrong way to go.

Unless there is a leak in the system, it will hold vacuum after the vac source is removed.

This means that your going to get your maximum vac advance, all the time.

 

[XPC66]

well i was think like if you had 2 windows open in you house. one has a vac and one had boost. thats how i was thinking about it

I wouldn't get too cute with the sampling point. Afterall the inlet manifold/plenum pressure is a pretty good indicator of engine load.

 

[Does10s]

I do have to say that a check valve on the vac advance line really seems like the wrong way to go.

Unless there is a leak in the system, it will hold vacuum after the vac source is removed.

This means that your going to get your maximum vac advance, all the time.

You guys have got me thinking about that check valve. I've been using it for just a year or so, but I also have a BTM in there. I'll have to run some tests to see what happens in that vacuum tube between the check valve and the dizzy diaphram. It makes total sense that it would hold the timing at full vacuum adv. when the vacuum transitions to boost.
In our case I'm OK becasue I time the motor at total timing, then subtract whatever I have set in the BTM.
But the guy that doesn't have the BTM might have a problem. I don't want to give out any bad info. So I'll have to test this.
Hmmmmm....more work to be done!
Thanks!!
Will