Bore 40mm TWM TB to 46?

[Walt Fricke]

I bought a guy's long stroke 2.8. He used parts from his 2.2 in building it, including the TWM/Tec3 fuel injection, with 40mm TBs.



I think 40mm is too small for a 2.8 race motor with 40mm intake ports. I purchased a suitable manifold and am using my Weber 46s (taken from my 2.7 motor). Runs great. I haven't dynoed it with my exhaust and the Webers, but his setup dynoed at around 220 RWHP. Which is pretty much the same as my 2.7 did, though at a lower RPM. I think there is more HP to be had here with a larger injection stack bore.

In addition, these 40s don't line up with the 46mm air horns (though that could be solved with other horns I suppose). Somehow, having a sharp step down in the intake like this doesn't seem like a good (or even neutral) thing.



I'd really like to move to EFI, and the 2.8SS I am slowly assembling could use this as well and I could just swap it all over.

I posted on the parts for sale forum to see if anyone wanted to downsize from 46 to 40 and would trade, but haven't gotten a nibble. I don't want to sell the whole shebang and buy a new one. The old setup has all of the parts (except for a fuel pump), and his mapping will be a good baseline. Plus TWM 46s (with air horns I have and a fuel rail I have) go for $1,600. Etc.

I see that these TBs have a straight bore. It looks like I could have a machine shop bore them out to 46, and on down a bit into the manifold too. I can smooth out the manifold I think to blend it into its 40mm head end.

That would leave purchasing larger butterflies, and I suppose (I haven't disassembled anything) having a shop cut the slots in the throttle shafts wider on each side.

Anyone done this? Anyone see issues here? Know where one might purchase butterflies? Or would it be easier just to purchase some brass (or whatever it is) stock and make them?

Walt Fricke
(Happy camper as he got 4 seconds faster with this motor than his previous personal best and using last year's slicks last weekend.)

[jpnovak]

Walt, I suspect that there are several places that could bore your ITBs larger. I can think of a few off the top of my head.

Aaron Burnham at RennWorks

and a great shop for weber refurbishment
Performance Oriented

I also suspect Bieker Engineering (SP?) and Eurometrix could also do this work.

Congrats on the Personal Best! This is always a good feeling and that is quite a jump.

[1QuickS]

I make 46mm Webers from 40's so I understand the topic. Stock 46mm butterflies are available from PMO however there is an angle associated with them, they are ellipses; both Webers and PMOs use 78 degree ellipses. I suspect this isn't an issue with MFI but for carbs it is critical for the ellipse angle to be correct to make progression holes align with closed butterfly. The depth of the bore through your stacks appears to be quite long for my tooling but I think a good machine shop could help you both with the boring and the slot lengthening for the 46mm butterflies.

A step from smaller to larger is OK, a stem from larger to smaller is not good at all.

Paul Abbott
Performance Oriented

[Walt Fricke]

Paul

Thank you so much. As often as I have looked down the throats of my Webers, I guess I hadn't noticed that full closed was not at a 90 degree angle. I'll have to delve into my TWMs a bit more to see how they handle that.

There goes the plan to buy butterflies from PMO. Maybe TWM will sell me 6 of them for their 46s.

[stownsen914]

Since these butterflies would be going into EFI stacks, does it matter what the angles are? Even if they are different from what TWM uses, I would think that the only implication would be that you might have to adjust your throttle linkage and/or throttle stop.

Now that I think about it, I think I must have experienced something similar when I had my MFI stacks bored out years ago. I used throttle plates from some Webers, and I noticed a difference in the overall throttle angle/travel. In my case I had to adjust the throw for the throttle position lever on the MFI pump to keep it in synch with the bored out throttle bodies.

Scott

[304065]

Walt, I would be surprised if TWM used a different casting for the 44mm and 46mm sizes of the throttle bodies. They probably do something similar to PMO-- use a single large casting and then bore to fit. I bet they would tell you if you inquired. The real question becomes whether you can actually send them your 40's and get back 44 or 46s-- that would be a great way to proceed because they know all the tricks.

My only concern with boring them larger would be the risk of damaging them, but I suppose that is the risk with everything.

[moparrob]

If you change your mind and would consider selling the old setup please let me know as I am looking for 40mm TBI or carbs for my 3.0 engine. I'd appreciate it.

Thanks.

Rob

robrose426@att.net

[Walt Fricke]

Finally waking up, I e-mailed TWM. They have butterflies and shafts, but:

"I don't believe you can rebore the throttles. This is because the injector pocket is positioned differently on every bore size and boring the bodies will leave a long oval hole which will not seal at the injector."

I'm going to have to look more closely into this, but this is something which makes sense and which I hadn't thought about. If what he is saying is that the injector is located farther out on a larger bore TB, then maybe the fuel rail position also changes?

My machinist may have a way around it. Possibly even doing some welding and re-machining.

The TWM guy opined that I'd be money ahead just buying new ones. I'm going to ask if I can buy naked but machined castings. The TBs are around $1,600, and I can get a lot of machining done for that. The boring doesn't sound like it would take a whole lot of setup time: line it up on the table on your mill, crank first bore under the boring bar, get centered, bore, move table (now lined up) to next one, center, etc. But I am hardly a skilled machinist.

So, Rob, I'll keep you in mind. But you need the whole shebang from air filters down to manifolds. I only need larger TBs - and want to keep all of the rest, excepting maybe the manifolds.

[304065]

Hmm, does the injector seal in the throttle bore, or in its own bore that is diagonal to the throttle bore? There is probably an oval hole there already.

Of course you could ditch that injector placement entirely and move them up to the top of the stacks for more power in the style of the 3006 throttles. . .



Just call me Mephistopheles, I'll return to your shoulder now.

[Walt Fricke]

Aided by responses, I took a closer look at how the 40mm TWM TB is made:

1) It appears it uses a copy of the Weber butterfly. I measured some distances, got a tangent, and came up with an angle: 77.24 degrees. The butterflies all have "78" stamped on them. Since Paull Abbott let us know that Webers are 78 degrees, and my measurements weren't meticulous, this has to be the deal.

2) The TWM 40s are roughly 50mm OD for the tubular part. That gives a 5mm wall thickness (a bit more, as I measured their bore at only 39.41 with a digital caliper). A 46mm bore would leave the wall at 2mm. Would this be enough? I'd be willing to take that chance, all other things being equal.

3) But then there is injector placement. The injector entrance into the bore is an ellipse. I measured the angle as about 30 degrees.



The tip of the injector is roughly right at the edge of the TB bore.



Doing nothing but boring the TB would leave the injector tip protruding 3mm into the bore.

But just drawing it back would mess up the fuel rail attachment - the rail would have to come up.



On top of this, as the TWM rep warned, it looks like that 3mm would impinge on the lowest O ring seal on the injector.

Mephisto's plan might be an option. The stock fuel rail could attach to a fabricated holder and squirt down through the top of the air filter. TWM thought this risky. Faust did get his soul back in the end, though: Verweile doch, du bist so schoen.

All I want is 50 more HP.

[304065]

Walt, great research and photos. I'll be following this to see what you decide.

I don't know ANY racer who would ever say, "du bist so schoen" to his current setup. . . there is ALWAYS more power to be had, another tenth left on the table. Perhaps this striving will save us all?

[Walt Fricke]

You are right about there generally being something more out there. Examples: straight up manifolds and TBs for same, to avoid the blasted Weber based bends in two of the three. Just more $, or clever use of motorcycle TBs and a way to make tapered tubes, TIG aluminum, etc etc. Several discussions of that. If I didn't have a complete TWM/TEC3 as part of the price of a solid running motor purchase, I'd be futzing around. Probably for years and years.

Just splurged on some Buckley Racing exhausts.



Of course, I had to lighten the rear support.



If I can figure out how best to lighten square tubing with minimal reduction in its resistance to bending (compression and tension don't seem too important here), I'll be back at the drill press, saving grams and nanoseconds on laptimes.

One of the great East Coast engine builders is running a vacuum in his 911 motors, as Porsche does in its race motors these days.

But, alas, I have pretty much abandoned the idea of boring these TWM 40s. Switching to up top injectors calls for a lot of careful thought and work, coming up with a rather different air filter (though that would be a use for the K&N Watershields I retired in favor of ITGs). To say nothing of some more $ and time.

If I can work a swap (working on a 3 way deal), I just might be able to run EFI at the Miller PCA race in September.

Walt

Oh - speaking of lightening our cars, there is a thread about that out there on one of the Pelican fora. I'm even subscribed, but for the life of me I can't find it to post several more labor intensive savings of very little weight. Anyone know it? 911 or Autox/Racing, most likely.

[TimT]

My 911 on the scales....



And I have lost 20# in the last two years...