Too much?
 

I have been planning on running a TWM 50mm (IDA type) throttle body on a downdraft intake on a large street ported 13B. I would prefer to not do a bridge port, since I would like some bottom end. For the most part, I generally just autocross the car.

The challange is that most folks use 42 or 44 mm chokes in the IDA. This really drops the actual size of the port. a 50 mm throttle body will flow a good deal more than a 48 IDA (even a 51 IDA).
That is why I ask if it is too much...

I believe usually the 50mm is used by the turbo folks and even then it can be a bit excessive. From what research I have done when I was looking into a weber style setup 50mm is going to be too much carb for streetported NA. Even 48mm can be too much on a ported NA. Weber gurus chime in and correct me if I am incorrect.

Bump

I ran a 48IDA on a stockport 12A, no problems. 40mm venturis.


I also ran the same carb on a streetported 13B from a 76 Cosmo, again, with no problems. It had 42mm venturis in it.



My J-Bridge 13B that I have sitting in my garage (which is for sale, BTW, *shameless plug here*) HAD a Gene Berg 58mm IDA on it, and it made 288.9rwhp, N/A. With a Racing Beat prepped 51 IDA, it made 266rwhp, and with a 48 IDA, it made 229. You see the obvious difference. Currently, it has no carb on it, but it has a intake manifold for a IDA carb/throttle body bolted to it.

I am far from an expert here, but...
I believe the 50mm throttle bodies will be too large, and will kill throttle response, and lower end power. Throttle response is a great benefit on the AX course, as is mid-range power. Most AX courses are setup in a manner to cause the use of only one or two gears thru the tighter areas. Having good throttle response and mid-range power is key to getting thru these areas quickly.
The 50mm TB setup will allow for great power in the upper RPM, at the cost of low-mid RPM power and TR.

I'm using a Sterling for AX and love it. Great power throughout the entire power band. A big two barrel like you mentioned, well, you might be able to tune it for good power at the top end but it is definitely going to have some drawbacks to it. Just my 2 cents...

I spoke with the guys at TWM and they said it should be fine. The size is less critical since there are no venturis ( becuase it is a throttle body), stop thinking carbs was the prime thought.

EFI is able to provide far more precise fuel metering under low vacuum/rpm conditions than a carby. Therefore it won't suffer from the same driveability problems as a large carb on a ported engine. I think the 50mm throttle bodies will work great.

Perfect, that is just what I needed to hear!

Ah, sorry man. Thought you were talking carbies here...

Well, I kind of was...
More thinking like carbs, but begining to work with FI.

I don't know if FI needs the velocity of a carb, to produce the best results. If it does, 50mm may not provide enough velocity for the engine to achieve maximum power. This likely has a lot to do with the factory FI system using 3 throttle plates. One for low to mid range, and the other 2 for the mid to upper range. Much like the 4bbl carb on the 12a and older 13b.
Swapping from the 4bbl to a large 2bbl on those engines helps the upper end, but hurts low to mid range power and throttle response. This is due mainly to the lower velocity of air flowing thru the carb.
I am certainly interested in the results of such a system.