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SLANT DRAFT Opel GT Intake Manifold

10K views 101 replies 15 participants last post by  GThound 
#1 · (Edited)
I decided after much deliberation and experimentation to design and build a custom intake manifold that would maximize the advantages of the Holley Sniper 2300 (throttle response, tunability, clean install, etc) and minimize the drawbacks of the the stock Opel Intake manifold design. This project started almost a year ago. Sorry it took so long to get up the first post. This idea is inspired by and builds upon many of the ideas from other posts on OpelGt.Com. I read Rally Bobs article on intake head porting many times. Last summer I learned to TIG weld and modified a spare Opel GT intake manifold in the Rally Bob torquer style. I then spent the rest of the summer porting and proving out the principles in that articles with many iterations using a home built (think shop vac powered) flow bench. I was pleased with performance improvement and wanted more. However, I have come to the conclusion (as Bob did) that there is a law of diminishing returns with the hours put into modifying an intake manifold.

I was also captivated by the Rally Bob post that said
I proved that a few years back when I flow tested a scratch-built intake I made and found 30 CFM per runner compared to a modified stock intake. Across 4 cylinders that is 120 CFM. And each CFM has the potential to make .43 hp. So...51.6 hp higher potential. Shows how restrictive the stock intakes are!
The intent of this project is to go after that horsepower and free the engine from the bottleneck of the intake manifold, in a design that fits under the hood of an Opel GT. So the journey begins, and I decided to build my own intake manifold.

The basic idea is what I am calling a SLANT DRAFT Intake Manifold. The SLANT DRAFT design approach gets rid of the two 90 degrees bends from the stock manifold (one at the bottom of the plenum and one at the elbow of the arms) and replace those with a single long fluid line from the plenum to the intake flange. I also want it to fit under the hood of an Opel GT and retain the heater box. The Holley tech I talked to said that the Sniper could run at any angle, but said that the prime pump (when there is no vacuum, before the car is running) relies on gravity feed, so we agreed that some downward angle is required. My current design has an angle of about 30 degrees.

The idea with the slant draft intake is to to build the slant right into the intake and use the slant to minimize the bends required. The plenum shape I ended up looks kind of like an upside down bird house. That exit angle helps get the runner arms pointing in the right direction. This sort of looks reminiscent of Rally Bob's hammer form mold, but with the plenum tilted forward to keep the profile of the manifold low. So, it is sort of a cross between a down draft and side draft manifold

I went through countless designs in CAD and 3D prints over the course of many months (usually an interation a day). The early CAD drawing below shows roughly the angle of the plenum and the rough path of the runners.

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Below is a picture of some early 3D printed components glued together.

View attachment 443711

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Please let me know if you have other thoughts or design considerations.

Here is some of my other design inspiration.
Rally Bob's Hammer form - This one really got me thinking about smoothing out the constriction in the arms and improvements in plenum flow beyond the V shaped spacer in the bottom of the torquer manifold (which helped a lot).
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Another Rally Bob inspiration for this design is from his Project Boom. Here you have the upside down birdhouse inspiration.
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One of the Sci Fi Guy's ponderings helped open up the thinking about running at unconventional angles to keep it under the hood.

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#57 · (Edited)
Well, I took off my Weber carburetor and tented intake and tried to fit up of the SLANT DRAFT intake. For the most part, the new SLANT DRAFT INTAKE prototype fit, with exception of some interference with the webbing of the 75 Sprint exhaust manifold.
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But the Holley Sniper did not land in the right place. It cleared the fan box area just fine, but it was wrestling with the hood pivot / latch mechanism and it is too far to the passenger side so misses the center of the hood bubble.
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It seems that to align with the center of the hood bubble, the center of the intake needs to be about 8-9 cm inboard of the hood latch.
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Several possible paths forward come to mind…
  1. Shorten the runner arms by 30-40 mm (slice and dice approach)
  2. Refine angle of the arms to gain clearance from hood hatch (belt sander)
  3. Change to 45 degree SLANT (design work, 3D printing, cast new arms)
  4. Try assymetical arms (design work, 3D printing, and cast new arms)
  5. Drop engine height with different motor mounts (rallybob suggestion anticipating this problem earlier in this thread)
  6. Relocate / eliminate passenger side hood pivot / latch. Has anyone done this? I’ll have to search the forum.
  7. Trim the Holley Sniper air cleaner mount surface to prevent interference with hood latch
  8. Switch to the smaller single barrel Holley Sniper auto lite 1100 with centered air cleaner mount
  9. Use SLANT draft intake prototype but switch to port injectors (don't want to go down this path, as I like to drive my car frequently, and designing and getting a custom engine management system could take my Opel GT out of commission for some time)
  10. Make own custom cast aluminum throttle body air inlet hood
Maybe combination of the above will work, else...Install sniper 2300 on current tented intake manifold.

I ordered a few air intake options and will try those and work backwards with what the Sniper position has to be for the hood to close and see what the runners and angles would have to look like to make it work. This is the opposite of where I started (focus on good flow).
 
#71 · (Edited)
I am still scrounging around in the design space for a little more room in each direction.

7. trim the Holley Sniper air cleaner mount surface to prevent interference with hood latch

I was working on my Sniper today and notice that there is dado, or circumferential recess around the barrels at the intake plate. Would it be possible to trim the plate
Something I am contemplating is trimming the air filter base on the Holley Sniper 2300. It is a 5.125" circular base, and it extends too far towards the firewall. So, I am thinking about making a straight cut at the back of the circle (see red line in photo below) where there are no components underneath and then machining the routed edge to match routed edge of the circle. Below is a picture of where I propose to make the cut and the new routing. I am going to hold off on making the cut until final installation, which is a ways off yet. I still need to either refurbish my gas tank and add the in tank fuel pump, or possibly even weld up a fuel tank out of 5052 1/8" aluminum. I'll start a thread on that if I go in that direction.

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And the Sniper Air Intake Hat for the cold air intake would look something the this initial rough design. It is essentially a loft that goes from the circle with the flat cut line to the round forward facing air inlet pipe.

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I would first print it out of plastic to run through a few iterations to make it fit well, as I usually don't get it right out of the gate. Then, I would make a sand core and cast it out of aluminum in the backyard foundry. Pouring hot metal into the sand mold and ending up with a solid part in front of you minutes later is my favorite and most rewarding part of the design process. It is always fun to have the vision of what is in your head a tangible reality sitting in front of you.
 

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#58 ·
You definitely have a bunch of options to figure out where you want to go from here. This thread really makes me wonder how much demand there would be for a turn key EFI kit for the CIH? Especially if it controlled spark and included an intelligent fuel delivery system.
 
#59 · (Edited)
I decided to go with a combination of the 1 (trim runners), 2 (refine angles), 6 refine hood latch, and possibly 7 (trim Holley air cleaner base plate). First, I decided to cut at least an inch off of the runners on the head side. This makes more work in terms of porting / blending the flange with the runner, since I cut off the rectangular portion. but worth it, as it should help make the design fit in the Opel GT. So, I broke the tack welds and used painters tape to mark where to cut the runners with the metal bandsaw.
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The shortening of the runners moved the manifold much closer to the right location.
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Then, I re-tack-welded and committed to weld the runner arms to the plenum. I am not a great welder by any means, but made some progress. I had issues with contaminants in the plenum which was made with poor quality aluminum from my early foundry days. So, I really had to crank up the AC balance on the welder for more cleaning action. And I had issues with weird growths on my tungsten. I finally pre-balled my tungsten in advance and that helped. I may be using too small of tungsten (3/32) and could maybe use 1/8” for this high amperage work. And I had issues with the ground on my welder. The runners were made with a much better metal from alloy wheels and better foundry technique. So, that metal welds much better.

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After a little bit of forum research, it seems that the big hooks on the hood (right side in the picture below) are a safety mechanisms to prevent the hood coming through the windshield during a front collision. The latch on the left seems to be actuated by the hood release cable and the rod rotated to pop up the hood using the offset cam in the middle of the rod. I am thinking about removing / relocating the hooks on the passenger side to accommodate the slant draft air intake.
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#61 ·
#63 · (Edited)
So, you can see that I am trying to make the inlets sort of like a bowl / funnel leading into each port.
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I may sharpen the transition between each port for a cleaner cut of the air. But what should I do with the circled yellow portion in the middle? Should I build up metal there and make it a 4 pointed divider? Or should I keep it smooth to facilitate cross flow between ports?
 
#66 ·
So, you can see that I am trying to make the inlets sort of like a bowl / funnel leading into each port. View attachment 444082
I may sharpen the transition between each port for a cleaner cut of the air. But what should I do with the circled yellow portion in the middle? Should I build up metal there and make it a 4 pointed divider? Or should I keep it smooth to facilitate cross flow between ports?
Radiused leading edges work fine. I would not raise that center too high as it may disrupt air/fuel emulsion. It would be different if you had injectors in each runner.
 
#65 ·
I was working on my Sniper today and notice that there is dado, or circumferential recess around the barrels at the intake plate. Would it be possible to trim the plate, and produce a couple of velocity stacks, maybe in a twist formation (to look awesome), and eventually point the forward with a filter, or sponge filter at the intake side?
 
#67 · (Edited)
So, while reinstalling the tented torquer intake and Weber carb (so I can drive my Opel GT today!), I measured the thickness of the sprinter exhaust manifold flange ears. They seem to be about 11 mm thick. And the new intake flange was cut out of 1/2” thick aluminum at about 12.7 mm thick.

What is the best practice to deal with the difference?
  1. Machine the new flange down to 11 mm thick
  2. Use a spacer 1/2 washer so that the manifold bolts provide even force across both when torqued?
  3. Ignore the difference (this seems like a bad idea)

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#68 ·
The original flanges on intakes and exhaust manifolds should be 12 mm thick, or .472”.

1/2” thick is close enough that it will work, though admittedly the closer they are to being the same thickness, the better off you are.

I once made an intake flange from scratch (years before I had them water-jet), and I used the material I had laying around. It happened to be 5/8” thick aluminum, so this was my solution. I countersunk the flange at the bolt/washer junction.

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#69 ·
Deleted.
 
#70 ·
You definitely want those thicknesses to be as close to the same as possible, a mismatch, especially if the exhaust is thicker than the intake, will worsen the chronic Opel flaw of vacuum leaks at the manifolds. It's not so bad if the exhaust leaks a little, but it's very bad if the intake leaks. Considering that machining and milling to correct the mismatch is expensive and difficult to do, using "half washer" shims seems to be the easiest fix to try.
 
#73 ·
When I installed a header on my Gt the flange was way thinner that the intake so what my grandpa and I did was weld some small pieces of metal to the header and made a little indent for the bolts. I seems like it was worked pretty good and I have 19 vacuum and it runs amazing.
 
#74 ·
What are the advantages to this over the 1.9 FI intake? Is it just a matter of simplicity or did the stock set up just not flow for.... 🤔
Anyway, nice build on the intake. I love to see people cast their own parts. Have you considered lost foam casting? Seems perfect for creating rapid prototypes of intakes and such.
This guy does some crazy stuff with lost foam. Figure it's right up your alley 😀
 
#75 · (Edited)
What are the advantages to this over the 1.9 FI intake? Is it just a matter of simplicity or did the stock set up just not flow for.... 🤔
1) The stock Opel GT manifold has some serious flow limitations (you are correct) and would require extensive modifications to overcome.

2) I built a rally bob style tented torquer manifold last summer. Fabulous project (learned to TIG weld, port, flow principles, etc). The gains were great and I wanted more, but had reached point of diminishing returns for that approach.

3) back in the beginning of the thread you will see that Rally Bob had found a lot of airflow ( read horsepower potential) in one of his custom intake fabrications. The aim of this fabrication is to get as much of that flow as possible and fit under the hood of the Opel GT.

4) I want a snappy throttle response and improvements over range of RPMs that unlocks the Opel GTs potential without losing its agility, nimbleness, reliability, drivability.

5) The Slant Draft Intake approach has slow gentle minimal curves and fits under the Opel GT hood. Instead of 2 constrictive 90 degree turns (one at bottom of plenum and one mid runner) it has 1 gentle sweeping curve of maybe 70 degrees. plus, the plenum exits are at the bottom instead of the sides. It is delivering over 50% less pressure drop (flow potential) and can utilize Holley Sniper all in one fuel throttle body injection which enables me to run at an angle and get rid of some carburetor limitations.

6) not sure how the stock 1.9 Opel mechanical FI manifold flows (maybe others have experience and could comment). However, it would require a complex long ECU engine management system install and I like to drive the Opel and am not willing to take it out of commission.

Anyway, nice build on the intake. I love to see people cast their own parts. Have you considered lost foam casting?
I have seen some of this videos. Quite amazing designs, intricacies equipment and results. For me, 3D Printing technique for cope and drag style and casting is the way to go. It is more computer aided design work and I don’t have to hand machine the foam for each iteration (I never get it right first time). This, I have developed a technique that works for me with very limited equipment and expense that I really enjoy. I will be posting some of my approach in another thread abs some of my models here. It is one thing to come up with a cool design for a component, but it gets more interesting, challenging, and complex when the additional constraints of design for manufacturing are taken into consideration. What’s the point of a new design if you can’t make it? It is so rewarding and satisfying to dream something up and make it real!
 
#76 · (Edited)
Before I cast my own intake cap, I am going try to see if I can make this low profile air intake from Spectre work.
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In a SLANT DRAFT configuration, the long smooth curve on the edge goes up in the Opel GT’s tear drop shaped hood bubble. I think that the profile is actually a pretty decent match.
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#77 ·
The next steps on the SLANT draft include:
  1. Final fitting of the cast / machines manifold complements. This requires removing carburetor and intake and thus taking car out of commission. So, I will coordinate this with gas tank removal / refurb and installation of the custom in tank fuel pump module
  2. TIG welding the cast aluminum intake runner arms to the machines aluminum head flange
  3. TIG welding Holley 2300 flange to cast aluminum intake runner arms
  4. Iterative porting and flow testing
  5. Adjustment of hood hooks and latch assembly for adequate clearance
  6. Prepping for Holley Sniper EFI install (fuel tank removal and refurbishment, install new fuel supply line, complete in tank fuel pump fabrication and install, etc)
 
#78 ·
New potential twist to project and resolution to under hood tightness. Holley seems to have a 2 barrel Sniper Throttle Body Injection unit called the 2GC that is based on the Rochester 2GC footprint. I actually really Ike the Holley 2300 footprint, but I am struggling to get the air cleaner base to fit under the Opel GT hood.
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#83 · (Edited)
Did some initial porting after welding the head flange on the runners and conducted some flow bench testing. Seems to have significant variability across ports and had some weird pulsations on port 1. More work to do. When I first cast the arms, they went from round to square, but I cut off the squarish end to shorted the intake manifold design for better under hood fitting.

Here is a before rough porting look.
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Here is an after rough porting look.
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also, the whole intake surface does appear warped after closer inspection, despite being bolted down. I can only Omaha one how bad of a twisted mess it would be without bolting it down. It seems that the ends stayed down, but the flange pulled away from the steel beam in the middle.



The belt sander test shows ends being sanded while middle is not.

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#89 · (Edited)
I pulled the intake manifold today off of my GT today and tried to fit the SLANT draft intake manifold.
Several observations:
  1. The head flange plate warped enough so that it won’t quite slide on the alignment pins. I am hesitant to force it on. Maybe I can straighten it in my press. Worse case is very slightly enlarge alignment pins and flatten the flange surface with machining.
  2. There are issues with the hood clearance (see yellow circled area below)
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RallyBob posted this solution to trim the hood bar and relocate the hinge point on the DIY intake manifold thread.

The hood latch issue is not uncommon and has been done for 45-50 years in GT’s at least. No biggie there either. I’ll see if I can dig up pics. View attachment 444576
This solves my problem! (see yellow circled area)
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Thank you for the picture! I have been at an impasse and could not see a clear path on how to resolve the conflict of the SLANT draft with the hood latch mechanism.
 
#90 ·
There's no need to use that huge Holley 2300, other than that we can get flanges to mount to the 1.9 manifold a bit easier than the Rochester type. The 2GC and the BDD for Jeep CJ has a much more favorable air intake shape than would adapt nicely to common cold air intake ducting. Plus, you can mount them sideways and point the carb forward towards the cold air intake hole in the radiator wall.
 
#91 · (Edited)
There's no need to use that huge Holley 2300, other than that we can get flanges to mount to the 1.9 manifold a bit easier than the Rochester type. The 2GC and the BDD for Jeep CJ has a much more favorable air intake shape than would adapt nicely to common cold air intake ducting.
Thank you for the comment and perspective. I have had similar thoughts and, so this confirms the direction of my thinking to move away from the sniper 2300.

I actually have a return request already at Holley, but have not shipped it back yet as they don’t have a shiny or black 2GC in stock and will not get them in until September. But the Holley tech did confirm that the inner workings of the 2GC and 2300 are essentially the same. Although, Holley said the 2GC does not quite flow as much air, it is still rated at the same 350 HP max with the 2 x 100 lb/hr injectors.

Another bonus is that I have not welded on the carb flange to the slant draft yet! So, I still have flexibility in throttle body style.
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The other reason I have not yet pulled the trigger on the Holley 2GC throttle body injection is that I am pondering the Holley Terminator X to control a port injection application.
 
#92 ·
If I had a Holley Sniper 2GC available / R&D funding, I would proceed to weld on the correct adaptor plate to the slant draft manifold and evaluate the SLANT DRAFT on my Opel GT vs RallyBob style tent bottom ported down draft intake manifold.

but, since funds are limited, I am going to hold off and spend my ECU dollars on the Holley Terminator X ECU. This approach will give me more flexibility (ECU is separate vs built in on the Sniper) with trying different intake manifold approaches. The beauty of the sniper TBI though is the simplicity (fuel rail and injector built in, idle air control built in, pressure sensor built in, throttle position sensor built in, fuel pressure regulator built in). All of that gets rid of a lot of wires for a very clean looking install.

So, hard decisions. But, once have the Terminator X and support systems installed, I should be able to try the slant draft manifold if I can find a low cost 2 injector throttle body / fuel rail combination.
 
#93 ·
Uhhhh........here's an update on the 2 injector concept I was trying to pull off. I had a discussion with a tech dude at Holley and he absolutely would not give me any advice on how to make the Terminator X use 2 injectors to run a 4 cylinder engine. He said that the software could not be configured to run any less than 4 injectors. I've talked with him before about running a Sniper in a sideways config and he was getting downright impatient with me. He basically said they they would not provide any support to me if I try to use any of their products "in a way that they were not designed to be run".

Could it be done with some clever wiring swaps of the injectors? Maybe, maybe not, but I would be totally on my own if I tried to do anything like that. I don't want to be "on my own", I'm not into engine tinkering and I don't think that figuring out how to make 2 injectors run a 4 cyl engine would be "fun". I bought the Terminator X mainly because their 2 barrel Snipers only use 2 injectors, so......I thought......SURELY their Terminator X system would have the settings to run just 2 injectors. Nope, Terminator X will not normally run 2 injectors and even if I could "fool" the system into thinking that it is running 4 injectors, he wouldn't tell me how to do it.

Mannnnn, am I pissed. This has blown a big hole in my boat and I'm sinking fast.

Is there a solution? Yeah: Run 4 injectors. But, I am really adamant about trying to use my single side draft manifold to make this all work. But it's shape is all wrong for trying to add 2 more injectors somewhere in it's runners. The manifold has just 2 runners, but there is a divider going down the center of each one and it is shaped to give a bit more flow to one side or the other to take into account the unusual shape and length of the runners. This manifold was designed by one of the top manifold designers in the world, but it was designed for ALL of the air/fuel mix to enter at the opening of the manifold. I could easily F-up the whole balance of things if I just jammed injectors willy nilly somewhere in the manifold. The twin throttle bodies have 2 injectors just after the throttle plates I need to add 2 more.....somehow.....in a similar location. That's not going to be easy at all and I've got a lot of hard thinking and designing to do. Or I'm going to have to scrap the whole idea of the SSD manifold and go the traditional route of TWO twin throttle bodies set up the same way as dual side drafts. As I'm typing this, this may be the only logical way to go. The twin throttle body only cost about $100, I would need to buy another one, and 2 shorty side draft manifolds, and possibly a special thermo housing, and a throttle body unifying device to operate both pairs of TB's. I would then need to make an air box to fit and service all 4 throttle body barrels. But I bought the long, side draft-sized, twin throttle body, so that it would match my existing air box I used when I was running a single side draft carb. That would make things tight between the engine/manifold/TB and the heater box, I really should buy TWO shorty twin TB's to free up the space. Hoo boy, it starts getting complicated and expensive the more I think about it.

Aarrgghh!
 
#94 ·
Uhhhh........here's an update on the 2 injector concept I was trying to pull off. I had a discussion with a tech dude at Holley and he absolutely would not give me any advice on how to make the Terminator X use 2 injectors to run a 4 cylinder engine. He said that the software could not be configured to run any less than 4 injectors. I've talked with him before about running a Sniper in a sideways config and he was getting downright impatient with me. He basically said they they would not provide any support to me if I try to use any of their products "in a way that they were not designed to be run".

Could it be done with some clever wiring swaps of the injectors? Maybe, maybe not, but I would be totally on my own if I tried to do anything like that. I don't want to be "on my own", I'm not into engine tinkering and I don't think that figuring out how to make 2 injectors run a 4 cyl engine would be "fun". I bought the Terminator X mainly because their 2 barrel Snipers only use 2 injectors, so......I thought......SURELY their Terminator X system would have the settings to run just 2 injectors. Nope, Terminator X will not normally run 2 injectors and even if I could "fool" the system into thinking that it is running 4 injectors, he wouldn't tell me how to do it.

Mannnnn, am I pissed. This has blown a big hole in my boat and I'm sinking fast.

Is there a solution? Yeah: Run 4 injectors. But, I am really adamant about trying to use my single side draft manifold to make this all work. But it's shape is all wrong for trying to add 2 more injectors somewhere in it's runners. The manifold has just 2 runners, but there is a divider going down the center of each one and it is shaped to give a bit more flow to one side or the other to take into account the unusual shape and length of the runners. This manifold was designed by one of the top manifold designers in the world, but it was designed for ALL of the air/fuel mix to enter at the opening of the manifold. I could easily F-up the whole balance of things if I just jammed injectors willy nilly somewhere in the manifold. The twin throttle bodies have 2 injectors just after the throttle plates I need to add 2 more.....somehow.....in a similar location. That's not going to be easy at all and I've got a lot of hard thinking and designing to do. Or I'm going to have to scrap the whole idea of the SSD manifold and go the traditional route of TWO twin throttle bodies set up the same way as dual side drafts. As I'm typing this, this may be the only logical way to go. The twin throttle body only cost about $100, I would need to buy another one, and 2 shorty side draft manifolds, and possibly a special thermo housing, and a throttle body unifying device to operate both pairs of TB's. I would then need to make an air box to fit and service all 4 throttle body barrels. But I bought the long, side draft-sized, twin throttle body, so that it would match my existing air box I used when I was running a single side draft carb. That would make things tight between the engine/manifold/TB and the heater box, I really should buy TWO shorty twin TB's to free up the space. Hoo boy, it starts getting complicated and expensive the more I think about it.

Aarrgghh!

Your thinking too much... computers are stupid things and only know what they are told. Again. If the TX can be configured to fire 4 injectors in batch sequence then set it up that way and substitute resistors for 2 of the injectors.
Or switch back to the microsquirt......
 
#95 ·
I've talked with our guru, RallyBob, and he agrees that when you start modding injector positions and manifolds you don't know what your end result might be. If your end result works better than the tried and true methods, then great, but you would have to prove that on a flow bench and a dyno. If you find out that it works worse than tried and true, then you just spent a whole bunch of time and money on a failure and you're back to square one.

I'm waiting for info to come in, but I will probably go with short manifolds made for a 2.4 and two shorter twin throttle bodies. All the widgets might cost me $500-$700 and I've already got the operating system. This would be a tried and true set up proven by decades of use by racers in Europe and here and it's all off the shelf. At my age and lack of interest in engine tuning and experimentation, this would probably be the best route for me to follow.
 
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