Designing my own ITBs?

Good morning,

So I’ve taken delivery of my pre-HE V12 engine, the idea is this is a chance for me to have a play with the engine, clean it up and replace all the worn items. I’m an engineer by trade and have access and knowledge to use CNCs so I’d like to design my own ITBs for the engine, for some fun, as I’m unable to get to the engine for a week or two, does anyone have the dimensions of the intake mounting holes?
And intake bores?


I’ll be using a Syvecs ECU to control the engine that was spare from my previous car project. If it proves affective I’ll upload of the files and drawings to grabcad.

Many thanks.


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Also helpful:

From my collected data (take with a grain of salt).

Spacing on the centre inlet ports is 3.5"
Bore offset side to side is 19.05mm (0.75").
The injectors are offset 0.1875" from the centre of the port. and is directed at about 12 degrees to the back of the inlet valve
Intake Center to Center:
Outer Pair: 107mm
Middle Pair: 105mm
Inner Pair: 90mm
Intake Port (manifold side): 34mm
Intake Stud Spacing: 67mm / 2.65" (9mm hole)

Note, I believe that a group A manifold exit is minimally 36mm. You’ notice most OEM heads have a very very poor heat to manifold transition w/ the port in the head having a sharp chamfer that needs to be addressed.

You might want to consider adapting (for cost reasons) some flavor of the relatively inexpensive modular 40+mm bike ITBs regularly available on eBay, or of some of the early BMW ITBs with round port exits.

~Paul K.

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I would suggest BMW K1200 GT ITB’s like I am using. They will fit factory injectors with no modifications only a simple custom bracket to hold them down. Check out my build thread for pics and stuff. the linkage can also be used it just needs to be extended. they are also very cheap on eBay roughly 60 for a set of 4. If you need them I have a CAD file for the injector bracket that I can send you.

build thread link: V12 XJ6 Conversion Build

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Just got 3 sets of these ITB’s so woulndt mind getting the CAD file.


i kinda like this setup, if nothing else should would be great for conversation!

slide throttle plates. Magnesiem castings!

Here is a link to the CAD file. Keep in mind that this is still a draft that was made for 3d printing and will probably need some small adjustments to work perfectly in metal. The link will always take you to the latest version of the file.

Just an fyi if anyone stumbles upon this in the future this link will always take you to the latest version of the file. And this was modeled using 1990 XJs injectors to fit 2006-2008 BMW K1200 ITB’s



Thanks for sharing your work

i use HONDA injectors, 12 new ones for $220. usd, O ring top and bottom, custom rails!
seen to many burned up Jag v12s hoses leaking/rotted, in junk yard!


Hi Gents,

Thanks for the details, I’ve started designing a set of ITBs with the sliding mechanism, I’ve split into banks of 3 as the CNC will only be able to fit on the machine bed easily. I can do two banks of 6 but I’ll need to shuffle and reprobe.

It works by rotating to arms from the centre of each bank, opening the slide outward from the centre. I’m still a bit confused on injector angle and position to the bottom of intake flange.

The reason for sliding throttles is I’m worried about galling of the butterflies if they rub the walls, on the plus side there is no obstructions in the intake.


The injectors on the stock manifold are roughly pointed away from the cam covers by 14 degrees.

For a brief period in time, it was popular for EFI setups in racing cars to have the injectors pointing upstream into the manifolds. I think the idea was they would get better atomization, but I don’t know for sure.

If I were designing my own setup here, I’d probably try to arrange the injectors so they would spray the max possible distance before the fuel hits metal. I suppose you’d have to know the spray pattern of the injector to calculate that. Whatever, if one edge of the spray cone hit a wall or a valve first, I’d move the alignment a bit away from that side.

The data I had was:

“The injectors are offset 0.1875” from the center of the port. and is directed at about 12 degrees to the back of the inlet valve" So that seems close.

On some race cars you’ll see injectors several CM above the bell mouth of the stack, but for street cars it seems most of the time the injector is pointed at the back of the intake valve so that when fuel hits the back of the hot valve, it will both cool the valve, and vaporize.

These days you can get disk style injectors (instead pf pintle injectors) with upwards of 12 disk holes for supposedly massively better atomization (higher fuel pressure helps too). The actual real world results/difference between the two seems to be debatable though. Some reports are that a modern pintle injector does better at atomization and not leaning-out at idle than disk injectors - but that could be tuning too.

~Paul K.

while you guys are redesigning the V12 ?
why not move up to the new world engine designs?

Direct Chamber injection , you could run 11/13-1 comp. ratios , meet Emission regs, better MPGs and most improtant ( MORE power/torque). Atomized fuel is difficult light, vaporized fuel in chamber, not port, and only at time of igniton, so little time for Detonation!
if you gonna do it ,go deep and all the way! (that what she said)
and you talk about tuning , and you dont even have an engine running??
i must be missin somethin?

Who knows, maybe someday the aftermarket world will come up with combined sparkplug/injectors that can be plumbed and wired to an aftermarket system. Barring that I don’t see any way classic engines (unless they’re in the vein of the ubiquitous SBC) are going to benefit from GDI.

~Paul K

Paul already been done 1944, B29 Superfortress, DI in the spark plug, i have write up about it , dated 1949!

That’s nifty! I’ll have to look that up. It still blows some folks minds that the Gull-wing 300SL was mechanical GDI. Now we need to get aftermarket parts makers to supply hobbiest GDI_Spark_Plugs in standard spark plug sizes/heat ranges and with electronic control over the DI part. :slight_smile:

~Paul K.

YAH , probably cost $ 500. per plug?
multipled by 12 ,YIKES!

ideas are simple , costs in this new world would make easy a 100K engine!
not realistic!
Paul, those 300SL s were not very fast, me 1957 Corvette twin 4 barrels, race one a 1955 Gullwing!
everybody was sayin stuff about fast!
we lined up on Highland ave,3 miles straight divided hyway, .
beat him bad , just maybe his engine had enough power , but the transmission was the slowest shifting thing ever seen, i would pull ahead 15/20ft on each shift , so by time we reached 4th gear i was gone , down the road, he never even started to close on top end, my speedo read 140mph!
thru the years i have read all kinds of BS about forigen cars being fast , if so i never seen one!
have you read about the New 2020 corvette , YUP its fast and drives fantastic! reviews!

also about building any hi-performance engine , it always comes down to the TUNING that seprates this and them!
but i only have 70yrs of performance engine building??? !!

The Corvette’s engine outclassed the 183ci straight six in the MB by 100ci, 50-75 ftlbs of torque, and 5-30hp depending on configuration so there’s little wonder it won… but consider wringing near contemporary Corvette HP and torque out of a little 2.9L straight 6 with 2 fewer cylinders and 100 fewer ci, and still get 20mpg or so cruising! You gotta respect both of them for what they are!

Yeah I don’t see generic combination GDI plugs anytime soon. Besides, you’d still need an additional set of TPI injectors just to keep the engine clean. The SBC guys will have GDI heads they can swap onto their engines though…

~Paul K

All well and good, but that would require drilling some holes in the head for installing the injectors.