Venturi sizing, planning for my winter project Options

[jr91472]

ok, so Britt's crazy thread about the mysterious "two-throat webers" got me going on my upcoming winter project: Addition of Dual Carbs.

First to all you FI guys....I know, I know (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif)

Now to my question: My 1.8L engine (actually 1.7 heads w/ 1.8 p/c) already has a "carb friendly cam" and a progressive weber. Once again..... I know, I know (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif). Unfortunately I have no specs on the cam other than it was installed to worked with the progressive.

Now I want to install a set of dual carbs. It sounds like most recommendations are of the 40mm variety. My question is about vent sizing. An article on Aircooled.net states that Vent size should be 3-5mm smaller than the Intake valve. According to the manual, the intakes on 1.7L are 39mm, so looks like I need 34-36mm venturi's. Yes / No? (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) Seems too simple.

It also troubles me because I have read several 2.0L guys using 32mm vents (with 42mm intakes)(IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

thoughts anyone? Standby for future questions about jet sizes (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

sorry for the long post

[ArtechnikA]

what do you want to do?

smaller venturiis improve port velocity and give you really good throttle response because of the good metering signal. but they tend to starve the engine at high rpm/power levels.

large venturiis give you more top end power. on the track you spend a lot of time at WOT and midrange modulation isn't -as- important.

for a street/AX car, pick smaller rather than larger - it will be much more driveable and deliver better economy.

you, at least, only need to buy 4 at a time if you guess wrong.