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Thinking about FAB installation with Rotec TBI

B

bifft

Well Known Member
I decided to go with a Rotec TBI for the occasional inverted excursion. Getting to the point of hooking up my Van's FAB to the engine, and I see that the TBI is about half the size of the carb it replaces. This means the FAB ends up about 3 inches too high. So, I have to decide what to do about that. These are the ideas I came up with:

FAB.png


Option 1: bend the front of the FAB down to meet the air scoop. Probably the least amount of work, but would force the air to make two big bends.

Option 2: build a tube to extend the whole thing down a bit. Pretty much what was done here. This gives some extra expansion space, and will help straighten the flow into the TBI. I know that this area moves a lot, and worry about fatigue on a aluminum box, I am not an aircraft quality welder to make a steel one. I suppose if it was thick enough.

Option 3: rebuild the cowl to move the air scoop up. This would probably be the lightest, and the lowest drag. However it is also the most work, and almost all of it fiberglass. Yuck.

Pretty much any of these require some fiberglass work to bend the supplied air filter box, except maybe #2, if I drop it a little more than 3 inches. Mostly just thinking out loud here.

Working through these options has made me mostly lean #2, thought I would pass it by the group for comments.
 
Did option #1

I recently had the same issue and used option 1, and here are the reasons why. I eliminated option 3 for two reasons. First, if I have a problem with the rotec TBI (if they go under and I can't get parts for instance) then I'd have to redo the cowl,,, again. Secondly, it's a much more visible part than the FAB and I'd need to spend a lot more time on cosmetics. I eliminated option 2 mainly to save weight. I added very little material to the FAB. I'll take a pic on Monday and email it to you if you want.

Lance
 
N941WR said:
Why not have a spacer made that lowere the TBI so the FAB lines up properly?
Click to expand...

That would put more weight on the end of the extension, so more vibration loads. Also, the mixture control line is close to the cowling as is, dropping it more probably wouldn't fit.
 
judoka5051 said:
I recently had the same issue and used option 1, and here are the reasons why. I eliminated option 3 for two reasons. First, if I have a problem with the rotec TBI (if they go under and I can't get parts for instance) then I'd have to redo the cowl,,, again. Secondly, it's a much more visible part than the FAB and I'd need to spend a lot more time on cosmetics. I eliminated option 2 mainly to save weight. I added very little material to the FAB. I'll take a pic on Monday and email it to you if you want.

Lance
Click to expand...

Thanks, a picture would be great.
 
My recollection is that the Rotec is designed to be mounted to the engine at the same point as the carb... In a retrofit installation (replacing a carb) one installs a spacer between the Rotec and the air box. It's been a while since I looked into the Rotec, but I thought Rotec even sold a spacer, that was either cast or machined and mated nicely with the TBI... Maybe I saw photos of something a builder made for himself.

Still, it shouldn't be difficult for someone who can build an RV, to come up with a structurally sound, vibration safe, spacer to go between the TBI and the FAB. Isn't the TBI significantly lighter than a carb to start with? The extra weight, even if it came up to the weight of a carb installation, shouldn't matter much...
 
# 1

You may want to look at "SCEET", as it is not to hard to make two flanges. The other way would be to farm a plug of wax or foam and over lay it with say 2oz. "S-cloth" then melt the plug out and you would have a custom fit snorkel. this is what is done on the IO-360 M1B. The old rule is anything under 7* with a bend is going to not amount to much drag, but a smooth radius like that in the snorkel will give the best flow with the least loss. This will allow you to re-use the lower cowl if you can't live with your set-up down the road some time. Yours as always
 
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