Mark Pietz
Member
And these.
Last edited by a moderator:
Limited disassembly of a 1980 IC 196
- Thread starter Mark Pietz
- Start date
Mark Pietz
Member
And a shot of how the pipe passes by the solenoid. Once I install the rest of the downstream piping, this pipe will adjust much closer to the starter. I originally had plenty of clearance but when I did my final tack-and-weld, the down pipe moved down and I didn't notice until it was too late and all the other pieces had been "set in cement", so to speak.
Attachments
Robert Kenney
Super Moderator
A quick pie cut and weld will fix her up. Even a cut off wheel cut and weld.
Mark Pietz
Member
Perhaps. But then that changes the location of the next pipe in line, and so on. Maybe this weekend I'll have time to bolt it all up and do one last look-see. There May be sufficient clearance and I do have a piece of embossed shield used on a volvo turbo diesel I picked up that will fit around the solenoid. (they call it the "turkey pan" or some such and it goes between the turbo and firewall) and I've used most of it to emulate the original shield that went around the turbine housing. That still to be mounted. I had planned on lining it with some left over fiberglass cloth, but there wasn't any left over…good thing I ordered the 25' roll! If this stuff doesn't work out I'll get the same stuff, but covered in vermiculite, rated to 1,500 degrees f.
Slightly shifting gears here, I obtained a first edition of mcginnis' turbo book off amazon. It was the first version I ever saw and in it they have a picture of the original Scout 80 used by trw to prototype the turbo setup, and a close-up of the engine compartment. What's interesting is that you can tell from the orientation of the turbo's center section (cbra) that it was mounted transversely over the engine. In my picture you see the final production iteration ended up being about 17 degrees off the truck's fore-aft axis.
Slightly shifting gears here, I obtained a first edition of mcginnis' turbo book off amazon. It was the first version I ever saw and in it they have a picture of the original Scout 80 used by trw to prototype the turbo setup, and a close-up of the engine compartment. What's interesting is that you can tell from the orientation of the turbo's center section (cbra) that it was mounted transversely over the engine. In my picture you see the final production iteration ended up being about 17 degrees off the truck's fore-aft axis.
Mark Pietz
Member
My goal is to run a max of 7 lbs. Boost and mostly to maintain near-stock performance at high altitudes.
Tonight I was perusing the 1st edition of mcinnes' book (1973). I have his 1978 edition that I bought when it came out. The second edition is far more polished, but at the expense of a lot of really cool, rare pics of early prototyping and the deletion of several "rules of thumb" and helpful tips. For example, I really should try to put my smog air pump back on as it is actually beneficial in adding about 25 extra "free" cfm for spinning the turbo; specifics on building a water injection system, and a better explanation of the relationship between carburetor (or exhaust) restriction and flow across the cylinder wrt limiting boost. And so on. Reading both really rounds out my understanding of the whole subject.
Aside from the earlier mention of the pics of the first Scout turbo prototype done by trw in 1963 (my guess as to year, as it went into production in late 1964), there is a pic of the prototype turbo corvair engine in 1961, same engine with rochester f.I. Adapted to it, details on the 1962 olds jetfire turbo setup, etc.
Tonight I was perusing the 1st edition of mcinnes' book (1973). I have his 1978 edition that I bought when it came out. The second edition is far more polished, but at the expense of a lot of really cool, rare pics of early prototyping and the deletion of several "rules of thumb" and helpful tips. For example, I really should try to put my smog air pump back on as it is actually beneficial in adding about 25 extra "free" cfm for spinning the turbo; specifics on building a water injection system, and a better explanation of the relationship between carburetor (or exhaust) restriction and flow across the cylinder wrt limiting boost. And so on. Reading both really rounds out my understanding of the whole subject.
Aside from the earlier mention of the pics of the first Scout turbo prototype done by trw in 1963 (my guess as to year, as it went into production in late 1964), there is a pic of the prototype turbo corvair engine in 1961, same engine with rochester f.I. Adapted to it, details on the 1962 olds jetfire turbo setup, etc.
Mark Pietz
Member
Probably getting tired of the pics, but here are more! I finished the drain line. Note that it passes right over where the flame arrestor fitting on the valve mounts. I believe there is enough room to put an "ell" in there and mount it horizontally.
Also see that I've almost completed the turbo's heat shield. This is fashioned from a '90s volvo turbo-diesel turbo-to-firewall heat shield. They call it the turkey pan because it is not unlike one of those aluminum pans you bake a turkey in. It's thin metal with a gazillion dimples embossed into it and an aluminum foil type liner. I plan on lining it with more of that fiberglass cloth. It emulates the oem heat shield, which are now all gone. It will clear the underside of the hood.
Also see that I've almost completed the turbo's heat shield. This is fashioned from a '90s volvo turbo-diesel turbo-to-firewall heat shield. They call it the turkey pan because it is not unlike one of those aluminum pans you bake a turkey in. It's thin metal with a gazillion dimples embossed into it and an aluminum foil type liner. I plan on lining it with more of that fiberglass cloth. It emulates the oem heat shield, which are now all gone. It will clear the underside of the hood.
Last edited by a moderator:
Mark Pietz
Member
And these. I bolted up the complete exhaust system. Right past the marman clamp is another section of piping that contorts itself around the drive shaft. It just clears the slanted part of the floor pan. If needed, I can take it out and flatten it for more clearance. Also, there is still adequate clearance to the starter solenoid. I doubled wrapped the pipe in this area, plus, there is room for a section of that dimpled heat shield material. So I think the solenoid will be fine.
On a separate note, I finished building a "hot wire" foam cutter today and have begun cutting foam for a pattern for that turbo-to-carb adaptor I need to cast.
On a separate note, I finished building a "hot wire" foam cutter today and have begun cutting foam for a pattern for that turbo-to-carb adaptor I need to cast.
Last edited by a moderator:
Mark Pietz
Member
A downside to unique or oddball projects is that often the tools you need are equally unique or oddball. In this case I need to make my own castings, which means I need to construct the tools to do that - and so on. I need an adaptor to bolt the carter yh side draft carb to the turbo inlet. If I had the end pieces in steel, I could possibly weld a short, stubby, bent piece of exhaust tubing to bridge the gap. In fact, I have a pic of such a prototype piece used to adapt a weber carb to the same turbo (corvair spyder) to prove concept and then production pieces were cast from aluminum. Or I can jump right to a casting…which involves a compound curve - a 17 degree bend so the carb can mount transversely, and at the same time drop about 1 1/2" inches so I can attach the air cleaner designed for it and clear the hood. Alternately, making just the 17 degree turn with it all level requires I then attach a 90 degree elbow to the carb's inlet and run the air cleaner in a vertical position. The original carb stand achieves similar bends - plus a cavity for water to warm things up on a cold day. Making such a pattern would be a daunting task for someone with my limited experience in casting, but now that I have discovered "lost foam" casting, I'm seeing real possibilities in fashioning a complex pattern. Hence, I built a hot wire foam cutter and am now cutting foam for my pattern. See attached photos of how that's coming along.
Last edited by a moderator:
Mark Pietz
Member
And this. The triangular flange will mount the carb and the bottom extension will be part of a heavier cast portion that I will drill through to serve as the warming chamber.
To carve the 2" passageway in the pattern for which I would normally have to create "core" for if doing it the normal green-sand/pattern way, I will build a hot wire "wand", which will be a 1" wide piece of wood and fashion a semi-circular loop on the end to cut each half of the channel with. Nifty! Once the pattern is built from foam and hot-glued up, I dip it in a slurry of thin drywall mud to make a "slip". Then it is placed into a large can and filled with sand for support. Then you pour in the 1,300 degree f molten aluminum and the pattern goes poof in a flash of fire!
There are a zillion plans on the net for building a hot wire foam cutter. I had a variac on hand and control a radio shack transformer (120 v to 25.2 v @ 2 amp) and a length of 0.020" stainless steel safety wire. You don't need nichrome wire for this.
Mental illness is not a liability when following through on a project like this.
To carve the 2" passageway in the pattern for which I would normally have to create "core" for if doing it the normal green-sand/pattern way, I will build a hot wire "wand", which will be a 1" wide piece of wood and fashion a semi-circular loop on the end to cut each half of the channel with. Nifty! Once the pattern is built from foam and hot-glued up, I dip it in a slurry of thin drywall mud to make a "slip". Then it is placed into a large can and filled with sand for support. Then you pour in the 1,300 degree f molten aluminum and the pattern goes poof in a flash of fire!
There are a zillion plans on the net for building a hot wire foam cutter. I had a variac on hand and control a radio shack transformer (120 v to 25.2 v @ 2 amp) and a length of 0.020" stainless steel safety wire. You don't need nichrome wire for this.
Mental illness is not a liability when following through on a project like this.
Attachments
Mark Pietz
Member
Making the pattern to cast an aluminum adaptor. My hot wire foam cutting technique leaves a lot to be desired, but fortunately there are tricks to minimize crappy surfaces. Here you see me using hot glue to put together sections for contouring. I built up horizontal sections and then cut the inlet passage layer by layer. Rough, but it won't matter. This saves me the difficulty of making a traditional form from wood and then making a core to form the interior passage. This is the most difficult part.
One one side you see a circular ring. This bolts to the turbo's circular inlet. The other side is triangular, which is to mate to the carter yh carb. Beneath this flange I casting in a heavy section which will serve two purposes: 1) I can bolt a brace to its bottom and run it down to the rearmost intake manifold bolts and 2) the section is large enough to drill through and tap both ends for pipe fittings to run coolant through. I plan on regulating this flow for warm-up only and then have it shut off when engine temps reach about 135 degrees (the opening temp for the tvs that runs the egr). Note that in a close up of the foam pattern I've cut little grooves to trap and heat fuel dribbles that drop out when running the choke. These grooves are found in our regular intake manifolds right beneath the carb.
I need to verify where the mounting studs will come through the circular flange. On the oem carb stand, 5/16" bolts passed through three bosses 120 degrees apart. Two are no issue as they come through on the outside curve. The stubby needed for the inside curve May be problematic.
Masking tape can be used to make fillets and cover gaps. The entire thing will now get dipped in thinned drywall compound to create a hard shell. Then it will be buried in sand and a suitable sprue attached. Hopefully this will be cast next week end.
important note: I am making this adaptor "horizontal", I.e., the centerline of the turbo inlet is horizontal to the carb's outlet centerline. A 90 degree elbow will need to come off the carb's inlet and the air cleaner will will mount on end. It is possible that I could construct a short piece of tubing with a jog in it (a drop) so the air cleaner would mount horizontally, but offset at a lower level than the carb. This is due to the fact that the air cleaner's body is 7" in diameter and would hit the underside of the hood.
One one side you see a circular ring. This bolts to the turbo's circular inlet. The other side is triangular, which is to mate to the carter yh carb. Beneath this flange I casting in a heavy section which will serve two purposes: 1) I can bolt a brace to its bottom and run it down to the rearmost intake manifold bolts and 2) the section is large enough to drill through and tap both ends for pipe fittings to run coolant through. I plan on regulating this flow for warm-up only and then have it shut off when engine temps reach about 135 degrees (the opening temp for the tvs that runs the egr). Note that in a close up of the foam pattern I've cut little grooves to trap and heat fuel dribbles that drop out when running the choke. These grooves are found in our regular intake manifolds right beneath the carb.
I need to verify where the mounting studs will come through the circular flange. On the oem carb stand, 5/16" bolts passed through three bosses 120 degrees apart. Two are no issue as they come through on the outside curve. The stubby needed for the inside curve May be problematic.
Masking tape can be used to make fillets and cover gaps. The entire thing will now get dipped in thinned drywall compound to create a hard shell. Then it will be buried in sand and a suitable sprue attached. Hopefully this will be cast next week end.
important note: I am making this adaptor "horizontal", I.e., the centerline of the turbo inlet is horizontal to the carb's outlet centerline. A 90 degree elbow will need to come off the carb's inlet and the air cleaner will will mount on end. It is possible that I could construct a short piece of tubing with a jog in it (a drop) so the air cleaner would mount horizontally, but offset at a lower level than the carb. This is due to the fact that the air cleaner's body is 7" in diameter and would hit the underside of the hood.
Last edited by a moderator:
Mark Pietz
Member
And these.
Last edited by a moderator:
Mark Pietz
Member
Mark Pietz
Member
rome wasn't built in a day. I just poured my adaptor, and the casting failed - but not by very much. I didn't have enough aluminum. Also, I could have cut off the portion that was shorted, but it was the defect in the circular flange that was the deal breaker. Close, but no cigar. 
in any case, this is a fairly short learning curve. Before I melt this chunk back into oblivion for the next attempt, I am going to practice drilling and tapping the mounting holes (there's a long 5/16" stud that projects from the compressor housing that passes along the adaptor's outer curve and out) and my filing skills in surfacing the flanges, since I don't have a mill. So all is not lost.
I didn't get the "geyser of fire" from the vaporizing foam, that I expected.
Last edited:
Mark Pietz
Member
Two additional pics. Here I have drilled a couple of holes for the compressor housing studs and bolted it to a spare housing (that particular turbo had been underwater so it's unlikely I'd ever use this for anything other than mocking up).
Although the casting looks funky, the next one will have a much better surface. This mockup proves my adaptor's shape and most importantly, that it will work.
What I have learned is that for accurately drilling the three equally spaced holes, I will need to make a jig, since I don't have fancy stuff like a mill and an indexing head. I think I will take the compressor housing and drill the holes through without mangling the threads. If the next casting is good, I'll drill at least one accurate hole on the drill press, and then bolt it up to the housing. Thus positioned, I can drill, from behind, and locate the correct spots for the other two.
Although the casting looks funky, the next one will have a much better surface. This mockup proves my adaptor's shape and most importantly, that it will work.
What I have learned is that for accurately drilling the three equally spaced holes, I will need to make a jig, since I don't have fancy stuff like a mill and an indexing head. I think I will take the compressor housing and drill the holes through without mangling the threads. If the next casting is good, I'll drill at least one accurate hole on the drill press, and then bolt it up to the housing. Thus positioned, I can drill, from behind, and locate the correct spots for the other two.
Last edited:
Mark Pietz
Member
the adaptor isn't useable for its intended purpose but is useful for mocking up the carb, etc. So that's what I've done. I made a jig from an unusable compressor housing to drill the required three holes and pulled that off. There is plenty of meat to drill/tap for studs or bolts for the carb. I was pleased to discover that the alignment planes are correct, I.e., the carb sits level and is oriented transversely. I was also pleased to discover that the air cleaner will mount as in its original application (corvair) and clear the hood, so one of my initial measurements was off. The only thing is that I'll need to point the snorkel backwards and have the hot air inlet pointing up. The snorkel will have to be shortened as it gets close to the firewall, but that's no problem. I will fab a support for the air cleaner housing that will mount to the side of the block, a similar thing done on the corvair. Thus the air cleaner will serve as an outboard support for the carburetor.I need to fab bits and pieces to adapt the throttle cable to the carter yh.
Bending up fuel line from the fuel filter (in the normal position) to go to the carb should be easy. I will need to drill/tap either the compressor housing or adaptor for the pvc valve. I have a useable one from the 152t - the early style with the threaded end that you could take apart to clean.
Getting close!!!
so….what do you think?
Last edited:
Mark Pietz
Member
and these.
Last edited:
Mark Pietz
Member
While it is not as perfect as I had hoped, I believe this raw casting will work! I hope to "face" the surfaces on my lathe. I cast a small boss on the underside for an inlet for the pcv valve. The casting is sufficiently thick in other places to tap for various vacuum needs (charcoal canister, flapper for the snorkel), and even water injection.
Last edited by a moderator:
Mark Pietz
Member
Three views of the casting.
Last edited by a moderator:
Mark Pietz
Member
That huge winter storm is blowing outside and I took the day off to keep an eye on things as I live in the sierra foothills where they expect horrendous winds and 3 or 4 inches of rain. Since the Scout is outside, my tinkering efforts were accomplished in the garage.
I finished the adaptor, at least for now. I May cast a lug onto its bottom and drill it for a water passage, but we'll see. In the meantime, it is finished. Aside from some pits and other surface defects, it actually polished up well!
I have a spare turbo exhaust housing and yh carb for the purposes of mocking up. That makes life a lot easier! I've begun fabbing linkage at the carb, using bits of a corvair spyder system. I've got the geometry correct, now just need to get more drill rod and figure out how to connect the carb portion to the existing throttle cable. Also, notice that I am retaining the hot air choke for the carter yh. The spyder used this type of tubing and it was a straight sideways shot from the carb to the turbine housing. Here, the carb turns a corner, and so must the tubing. All it is is 1/4" brake line, and I scrounged some of that insulating sleeve material used on hot choke tubes - the length was perfect. Picnpull came through again.
Fingers crossed that this weekend I can actually bolt this up and puzzle out the pcv tubing and other vacuum fittings to run the snorkel "servo motor" and the provide draw for the charcoal canister. Since I May need water injection down the road, I'll go ahead and drill and tap a hole on the adaptor, probably in one of the mounting flanges, or straight onto one of the sides. They're pretty thick.
I finished the adaptor, at least for now. I May cast a lug onto its bottom and drill it for a water passage, but we'll see. In the meantime, it is finished. Aside from some pits and other surface defects, it actually polished up well!
I have a spare turbo exhaust housing and yh carb for the purposes of mocking up. That makes life a lot easier! I've begun fabbing linkage at the carb, using bits of a corvair spyder system. I've got the geometry correct, now just need to get more drill rod and figure out how to connect the carb portion to the existing throttle cable. Also, notice that I am retaining the hot air choke for the carter yh. The spyder used this type of tubing and it was a straight sideways shot from the carb to the turbine housing. Here, the carb turns a corner, and so must the tubing. All it is is 1/4" brake line, and I scrounged some of that insulating sleeve material used on hot choke tubes - the length was perfect. Picnpull came through again.
Fingers crossed that this weekend I can actually bolt this up and puzzle out the pcv tubing and other vacuum fittings to run the snorkel "servo motor" and the provide draw for the charcoal canister. Since I May need water injection down the road, I'll go ahead and drill and tap a hole on the adaptor, probably in one of the mounting flanges, or straight onto one of the sides. They're pretty thick.
Robert Kenney
Super Moderator
Good job Mark!
Is there any way to plumb manifold pressure to the power enrichment valve (if the carb has one)? That will guarantee the mixture is better once it's above the preset pressure. Aka boost referencing the power valve.
Is there any way to plumb manifold pressure to the power enrichment valve (if the carb has one)? That will guarantee the mixture is better once it's above the preset pressure. Aka boost referencing the power valve.
Mark Pietz
Member
Getting so close it's distracting! But it May be early January before I get it on the road due to imminent holiday plans!
There is no power valve in the Holley sense. The carter yh is of the metering rod/jet style, and rod position does it all. Manifold pressure (vacuum) pulls the rod down against spring pressure. Power enrichment occurs when vacuum drops, causing the rod to rise, from whatever running position it May be in at the moment. Until conditions stabilize to a new running condition (like a qj or tq). All things considered it wasn't a bad setup for its day, as a rod could be tapered to go over-Rich and prevent detonation under maximum boost. It would, however, run a bit Rich under full throttle, non-boost conditions. Not ideal, but tuneable for non-boost conditions and a h*ll of a lot better than IH just putting a big Rich jet in the Holley 1904 and calling it good. The 152t ran Rich just about everywhere where it wasn't needed. And untuneable. So in this regard, GM got it right.
By way of comparison, once I put the crankfire setup on my corsa and imposed the normal, n/a ignition curve and timing, I picked up 4 mpg and got the same as a n/a model if I kept my foot out of it and avoided boost. The rod would then spend minimal time with its thinnest portion "plugging" the jet unless I nailed it and let the boost come on.
In this respect, I should be thankful I can use the carter and not screw around with a Holley.
There is no power valve in the Holley sense. The carter yh is of the metering rod/jet style, and rod position does it all. Manifold pressure (vacuum) pulls the rod down against spring pressure. Power enrichment occurs when vacuum drops, causing the rod to rise, from whatever running position it May be in at the moment. Until conditions stabilize to a new running condition (like a qj or tq). All things considered it wasn't a bad setup for its day, as a rod could be tapered to go over-Rich and prevent detonation under maximum boost. It would, however, run a bit Rich under full throttle, non-boost conditions. Not ideal, but tuneable for non-boost conditions and a h*ll of a lot better than IH just putting a big Rich jet in the Holley 1904 and calling it good. The 152t ran Rich just about everywhere where it wasn't needed. And untuneable. So in this regard, GM got it right.
By way of comparison, once I put the crankfire setup on my corsa and imposed the normal, n/a ignition curve and timing, I picked up 4 mpg and got the same as a n/a model if I kept my foot out of it and avoided boost. The rod would then spend minimal time with its thinnest portion "plugging" the jet unless I nailed it and let the boost come on.
In this respect, I should be thankful I can use the carter and not screw around with a Holley.
Robert Kenney
Super Moderator
Yes the enrichment system the carter has is for all intents and purposes a power valve like the afb. It can be boost referenced.
Mark Pietz
Member
I have the carter yh spec sheet and description of the function of each circuit. There is also a separate description for how it operates under boost, so it appears the "pv" aspect of its operation has already been worked out for this additional operating condition. Have to go back and read up on all that.
This basic carb design was also used on the 53-56 vettes and the 50s nash, but they are not interchangeable. Those carbs reached the end of their development for obvious reasons, but after the design was again tapped for the spyder, a few more modifications occurred. I am aware of an extra passageway to fix some vacuum stability issue that pulls down the rod - maybe that's boost related, I don't know - and they changed the check valve for "accelerator pump" action. It's a fairly simple carb and actually quite elegant for what it does.
Edit: today I got the carb back together with a few tweaks - a special inlet valve that uses ball bearings instead of the traditional needle valve. This greatly reduces - if not cures - a long-standing problem of the float not shutting off consistently, which occurred when the aftermarket increased the size of the inlet passage which caused a loss of leverage for the brass float to close it off. I also drilled out the body's mounting flange hole for the upper mounting bolt to press in a spacer, which cures a problem of the upper bolt's compressing and distorting the thick insulating gasket between the carb's zinc body and cast iron throttle body, creating vacuum leaks. It's a yh thing. Anyway, it's all mounted up and moving on to finishing up the linkage and tweaking a gasoline feed line into the carb. With just those things I can almost fire it up, just to see….
I'll have some pics of this later.
This basic carb design was also used on the 53-56 vettes and the 50s nash, but they are not interchangeable. Those carbs reached the end of their development for obvious reasons, but after the design was again tapped for the spyder, a few more modifications occurred. I am aware of an extra passageway to fix some vacuum stability issue that pulls down the rod - maybe that's boost related, I don't know - and they changed the check valve for "accelerator pump" action. It's a fairly simple carb and actually quite elegant for what it does.
Edit: today I got the carb back together with a few tweaks - a special inlet valve that uses ball bearings instead of the traditional needle valve. This greatly reduces - if not cures - a long-standing problem of the float not shutting off consistently, which occurred when the aftermarket increased the size of the inlet passage which caused a loss of leverage for the brass float to close it off. I also drilled out the body's mounting flange hole for the upper mounting bolt to press in a spacer, which cures a problem of the upper bolt's compressing and distorting the thick insulating gasket between the carb's zinc body and cast iron throttle body, creating vacuum leaks. It's a yh thing. Anyway, it's all mounted up and moving on to finishing up the linkage and tweaking a gasoline feed line into the carb. With just those things I can almost fire it up, just to see….
I'll have some pics of this later.
Last edited:
Mark Pietz
Member
Robert,
your post about referencing boost on that carb kind of stuck in my head. I won't do this, not at first, anyway, because this basic setup had a history of working. Not optimal (only efi is optimal), but reliable and "safe". So I am starting out very Rich and will go from there according to what afr readings come from a wide-band sensor.
Recapping the issue: you can be running half-throttle and pulling a good vacuum on the carb side, but the turbo is turning that vacuum into boost, and will run lean. The pv circuit isn't getting the right signal (assuming it would enrich enough even if it did). Running a line from the manifold side gives a proper signal to the pv, which then must be jetted properly to not only give proper enrichment during n/a running, but under boost as well.
Back to the yh. When vacuum goes away when you run, say, full throttle, assuming n/a only, then the rod needle is tapered so that full up the engine is enriched accordingly. But the rod needle can only go so far; with a turbo, when manifold pressure is tipping over into positive territory, that rod needle taper/jet combo cannot provide additional enrichment. Hence tapering the needle to run Rich enough under boost, but it will run over-Rich under a max throttle position and not generating boost.
I have a spare yh apart here and had a duh! Moment. The vacuum chamber that pulls down the rod is on the bottom of the bowl. It should be easy to simply drill and tap a tiny hole dead-center in the bottom of this chamber, which would then open to the outside, on the underside of the bowl. The passage feeding this chamber could be closed off with a teeny ball bearing or equivalent. Run that line to the manifold side. Would be ridiculously simple to do. Now, what would the advantage be? While the signal would reflect true operating conditions wrt rod position, I see the same problem in that full up, the needle needs to be the same diameter or taper in that area. I think any advantage would be in the step above that might be thicker and leaned out slightly? Again, as I'm perceiving this, the problem area is the transition zone at wot when manifold pressure goes to zero - not leaning out there. These are the physical limits of this particular configuration.
Is my logic right?
your post about referencing boost on that carb kind of stuck in my head. I won't do this, not at first, anyway, because this basic setup had a history of working. Not optimal (only efi is optimal), but reliable and "safe". So I am starting out very Rich and will go from there according to what afr readings come from a wide-band sensor.
Recapping the issue: you can be running half-throttle and pulling a good vacuum on the carb side, but the turbo is turning that vacuum into boost, and will run lean. The pv circuit isn't getting the right signal (assuming it would enrich enough even if it did). Running a line from the manifold side gives a proper signal to the pv, which then must be jetted properly to not only give proper enrichment during n/a running, but under boost as well.
Back to the yh. When vacuum goes away when you run, say, full throttle, assuming n/a only, then the rod needle is tapered so that full up the engine is enriched accordingly. But the rod needle can only go so far; with a turbo, when manifold pressure is tipping over into positive territory, that rod needle taper/jet combo cannot provide additional enrichment. Hence tapering the needle to run Rich enough under boost, but it will run over-Rich under a max throttle position and not generating boost.
I have a spare yh apart here and had a duh! Moment. The vacuum chamber that pulls down the rod is on the bottom of the bowl. It should be easy to simply drill and tap a tiny hole dead-center in the bottom of this chamber, which would then open to the outside, on the underside of the bowl. The passage feeding this chamber could be closed off with a teeny ball bearing or equivalent. Run that line to the manifold side. Would be ridiculously simple to do. Now, what would the advantage be? While the signal would reflect true operating conditions wrt rod position, I see the same problem in that full up, the needle needs to be the same diameter or taper in that area. I think any advantage would be in the step above that might be thicker and leaned out slightly? Again, as I'm perceiving this, the problem area is the transition zone at wot when manifold pressure goes to zero - not leaning out there. These are the physical limits of this particular configuration.
Is my logic right?