G'Day guys,

I've just bought a spare FD TB and hoping to enlarge it in preperation for a new setup i'll be converting to down the track and i've got a couple quick questions regarding TB porting i'd like to ask you guys:

1. Do most people enlarge all throttles? Ie: bigger butterflies in both primary and secondary throttles? If enlarging both passageways is not common practice, why?

2. How big do most people go with the throttles?
Stock measures: pri=45.5mm sec=50.5mm
I'd think you could safely increase those to: primary-52.5mm secondary-53.5mm

3. Are There any companies that exist that will custom make new butterflies for me to save having a local shop make a jig to machine them for me? This is just an easy time saver for me really.

4. What sort of gains do most people making reasonable power (400-500whp) see?

Thanks in advance for all replies guys

 

hi Brad,

i think the intake tract from the intercooler to the shortblock needs to be considered when evaluating a throttle body change since flow is mostly determined by the smallest cross section.

the typical intercooler to elbow tube is 2.75 inches in diameter.

the FD throttle body features two 50 mm secondaries and a 45 mm primary

the runners at the junction between the UIM and LIM are 30 primary and 38 secondary


area in sq inches

intercooler to elbow tube 5.94 sq inches (based on O D)

throttle body butterflies 8.546 sq inches

runners 5.7 sq inches

90 MM throttle body 9.85 sq inches (+ 15.3% to the stock TB)

based on the data i don't see where enlarging the stock TB butterflies would accomplish much since they feed into 5.7 sq inches of runner area.

i do think any porting, smoothing of the TB could help along w closing the monster hole that exists for the air bleed/idle.

good luck and thanks for your help w my V88. i am up and running and expect to be on the dyno within 10 days.

howard

 

if you ever do, do it. max sizes should be 55mm for the secondaries and 50mm for the primary, this will work fine and you have enough to grind the gasket and reuse. NOTE: A 55mm primary wont work, the UIM can not be bored out the that large of a size.

 

The trick is to port match the complete system and to improve all points of turbulance as much as posible. Similar to blue printing an engine.

If you really look at the throttle body, keep the plates stock but fix the rest of the problems it has. There is one very bad design with the FD TB, along with a few others.
Can you identify at lease the worse?

Compare to a very good carb like a Weber 45DCOE.