Carb info for the turbo design.

charlief1

RIP Charlie!
Sep 20, 2007
20,982
3,680
113
No Where But TEXAS!!!
Well I finally got the time to take the carb apart today. With the help of my bud Greg we did some tear down and research for you guys. The first pics of the two carbs are for reference. One is the 81 and the other is for a 74. Other than the electronics on the 81 there's not a lot of difference between how they really operate. 4M and an E4ME for reference.

The mistery of the boost referenced power valve is forever solved as well. There's very little difference between the NA cab and the boost reference one and you CAN make one if you have the right parts. I'll explain this as I go through the pics. Both Greg and I are suprised no one figured it out but since we have and now know how to do it you guys are gonna have a fit. And it's so simple even a newby could do it if you have a a drill press and basic metal working skills. I think this means you can do it as well Aj. LOL (you knew it was coming):biggrin:

The first pic is of the base plate. This is the most important part and without it you can't have a boost reference to the power valve. As you can see I used straws so you can see where the port is. This port is what Makes the carb so special. It doesn't alow vac to influence the power valve. It isolates it so you can use the boost for a reference to keep fuel flowing when you let off the throttle.
 

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charlief1

RIP Charlie!
Sep 20, 2007
20,982
3,680
113
No Where But TEXAS!!!
The next three are of the body of the carb and bases. This passage is for the power valve to get it's signal. Between the two of them is why the carb is so special. Well now for the best info. The base is the clue, not the body! With the proper base you can turn any of the 4M carbs into a boost referenced carb and all you need is the right sized tube and a couple of drill bits. No jokes, it's that simple!
 

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charlief1

RIP Charlie!
Sep 20, 2007
20,982
3,680
113
No Where But TEXAS!!!
I have a bunch of base plates so I started looking at them to see what is needed to do this. The 4 base plates I'm showing are all 4M but for different apps so you won't see much of a difference in them. I highlighted the areas you need to look at. This is normally where the power valve gets a vac signal from in an NA carb, but in the turbo carb this is blocked off. That's what the tube sticking out of the front is for. You have to look at the passeges really carefully to see what I'm talking about. I have two boost carb bases and I only have 1 carb so it will go on another body to make it work. If you want to build one you MUST have an NA 4M carb to do it. You will also need the base off of an E4ME because they don't have the passage for the vac signal to the power valve . Nothing's drilled there which is exactly what you want. The other benifit for this is the fact that the hole to the power valve is started but it's just dimpled so you don't have to worry about figuring out where it goes. You will need to measure where the tube goes on the front of the base and I'll give the measurements for that later as well as the size pipe you need and what drill bit to use. I'll also add the depth of the hole on top so you drill it out without going through the base plate.
 

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Freddie's Buick

Hell No! I'm not a junior
Jan 3, 2002
1,010
4
38
Miami, Florida
B4turbo,
Hey you should be able to modify one of those Q-jets of yours for turbo usage. That's right up your ally. Cool thing about that mod is that the carb can be used in turbo and non-turbo apllications. On a non-turbo application the vacuum line can be run straight into any vac port on the intake, (like the one on the transmission modulator line).
I see you're finally getting around to doing this as I suggested. You must have major time in your hands right now....:biggrin:






Modifying a non turbo Q-Jet for turbo usage​



1) To modify a Q-jet one would have to remove the throttle plate and seal the original power valve vacuum hole with epoxy.
2) Then, one would have to drill a 3/16" hole in the front of the throttle plate straight into were the sealed epoxied hole is at.

3) What one is doing here is essentially re-routing the hole.
4) Once drilled, insert a 3/16" metal tube/vacuum port into the throttle plate.

That's it! :eek:

After putting the throttle plate back on the carb and the carb on the intake, run a vacuum line from that newly made port to a one-way check valve. And from the check valve run a vacuum line that attaches to any port on the intake side of the turbo.

The check valve is a one way valve. It prevents boosting from 'popping-out' the power valve and allows vacuum to work the power valve.
This can be done with any non-electronic Q-jet carburator.
Now you have a functioning turbo carb.
 

Freddie's Buick

Hell No! I'm not a junior
Jan 3, 2002
1,010
4
38
Miami, Florida
I have a bunch of base plates so I started looking at them to see what is needed to do this. The 4 base plates I'm showing are all 4M but for different apps so you won't see much of a difference in them. I highlighted the areas you need to look at. This is normally where the power valve gets a vac signal from in an NA carb, but in the turbo carb this is blocked off. That's what the tube sticking out of the front is for. You have to look at the passeges really carefully to see what I'm talking about. I have two boost carb bases and I only have 1 carb so it will go on another body to make it work. If you want to build one you MUST have an NA 4M carb to do it. You will also need the base off of an E4ME because they don't have the passage for the vac signal to the power valve . Nothing's drilled there which is exactly what you want. The other benifit for this is the fact that the hole to the power valve is started but it's just dimpled so you don't have to worry about figuring out where it goes. You will need to measure where the tube goes on the front of the base and I'll give the measurements for that later as well as the size pipe you need and what drill bit to use. I'll also add the depth of the hole on top so you drill it out without going through the base plate.
Although this can be done successfully, (switching out TB plates), I'm old school and I rather drill baby drill!! The reason for this is that the vac ports from one plate to another may have slight differances enough to cause idle problems along with the TB gasket. You'll have to be as good as Charlie to catch 'em.

One major important step that should be mentioned here is that the TB gasket has to match. They can be very differant as well. The N/A type can function but one has to punch out the hole for vacuum on the gasket too, (or buy a turbo carb gasket kit if you like to spend more money). That is why I rather take the TB out of the original carb, epoxy, drill and re-route. ;)
 

charlief1

RIP Charlie!
Sep 20, 2007
20,982
3,680
113
No Where But TEXAS!!!
Although this can be done successfully, (switching out TB plates), I'm old school and I rather drill baby drill!! The reason for this is that the vac ports from one plate to another may have slight differances enough to cause idle problems along with the TB gasket. You'll have to be as good as Charlie to catch 'em.

One major important step that should be mentioned here is that the TB gasket has to match. They can be very differant as well. The N/A type can function but one has to punch out the hole for vacuum on the gasket too, (or buy a turbo carb gasket kit if you like to spend more money). That is why I rather take the TB out of the original carb, epoxy, drill and re-route. ;)
That was one thing we lookd at yesterday Freddie. Any of the 4M designs have the same base design and the only difference we noticed was the way the power valve vac hole is made. The E4ME doesn't use this feature since it's electronic. It has the same base but also has the extra ports you need just like the turbo specific carb. I guess I should take some more detailed pics so everyone can get a better idea of what I'm talking about.
 

MNcarbturbo

What's Fuel Injection?
Mar 9, 2009
1,058
61
48
Western MN
I unserstand what you mean about a mechanical version. I should check to make sure the PEVR I have actually works. Only problem is I have no idea where the other side hooks up to as my vacuume diagram makes no sense.
 

MNcarbturbo

What's Fuel Injection?
Mar 9, 2009
1,058
61
48
Western MN
Since this kinda relates to the whole system, is there a way to test the PEVR? I got all the vac lines rerouted but I kinda want to check the pevr to make sure that it works. This pevr/carb system isnt easy to peice together after 5 months of it being tore apart!!!
 

charlief1

RIP Charlie!
Sep 20, 2007
20,982
3,680
113
No Where But TEXAS!!!
Since this kinda relates to the whole system, is there a way to test the PEVR? I got all the vac lines rerouted but I kinda want to check the pevr to make sure that it works. This pevr/carb system isnt easy to peice together after 5 months of it being tore apart!!!
A basic understanding of it is that it removes vac when the turbo spools up. You'll need 2 people to check it. Suck on the line that's hooked up to the carb from the pevr and see if you get any resistance. If not you then need a small (8lbs) amount of pressure on the turbo side of the valve and see if it cuts off flow.
 

MNcarbturbo

What's Fuel Injection?
Mar 9, 2009
1,058
61
48
Western MN
When I sucked on the side that goes to the carb (the small spaghetti line if I remember right) i had no resistance. I will just stick a small force on the steel ported line then. Only thing that confuses me, does this hook to the turbo at all to sense boost? I thought it just hooked into some vacuume lines.

Thanks for the help so far!
 
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