Twin Carb Guru's

[Porkpie]

Ok Guys, You know best so here goes. What is the difference between 36 and 40 idf's? I understand that they both have 40mm throttles so why can't I fit 32mm vents and use them on a bigger engine?

Regards, Si

 

[Trikky2]

36's will work fine unless your going to build a monster engine.

 

[Porkpie]

Thanks for the reply Trikky2, was thinking of something between 1.9 - 2.2. What do you think? I know Sparkywig was asking similar questions about 40's on a bigger engine too!

Regards, Si

 

[sparkywig]

Depends what heads and cam you want to run. I've got stock heads that have been slightly ported and cleaned up with 35.5mm inlet and 32mm exhaust valves and have IDF 40's fitted. These are fine as it's not build to rev much over 5k.
If you're going to fit heads with bigger ports and valves and higher lift cams, you'll need to go up to 44's or even 48's to get the airflow.

 

[Trikky2]

Thanks for the reply Trikky2, was thinking of something between 1.9 - 2.2. What do you think? I know Sparkywig was asking similar questions about 40's on a bigger engine too!

Regards, Si

If your building a typical bus engine for torque you will be using a fairly soft cam to give a wide spread, some ratio rockers and stock lightly ported/polished heads, stock size valves maybe slightly radiused with a triple cut seat to give you the higher gas velocity associated with this type of engine build. In such an engine the 36's will work ok.

If on the other hand you go for larger valves, wilder cam and higher revs then, as already posted, you would probably need bigger carbs.

 

[Porkpie]

Thanks Guys, I'm too old to be revving things now so looks like I can keep the carbs and replace the 30mm vents with something larger. Thanks again.

Regards, Si