ITB Sizing- Recommendation
 

Hi All,

Looking for recommendations as I am rethinking carbs vs ITBs. Initially the plan was to build a 3.0L running 46mm PMO carbs. Following are build specs:

Stock crank 70.4mm
JE 10.5:1 Pistons
Twin Plug 39mm heads
Web Cams 120/104 cams. Close to GE 60 specs but on 100 degree lobe center vs 102
JBR Twin Plug Distributor
Twin MSD ignition boxes/ coils
ARP Head / Rod bolts
1 5/8" headers
Dansk 2-2 Sport

All parts purchased except carbs. PMO has now released their line of ITBs which look great. Couple questions / concerns:

1. ITB sizing 40mm vs 46? Plan to spin the engine around 7500. Concerned 40s too small and 46s too large especially if forced to tune Alpha N
2. Tight LCA cam and lack of manifold pressure for tuning. Options: tune hybrid Alpha N (throttle position) with manifold pressure reference?
3. Option 3 - stay with carbs

Appreciate any input /feedback

Following


Sent from my iPhone using Tapatalk

Recommend calling William Knight.

conventional wisdom is to go 40mm for 3l @7500

You want to keep the velocity up for throttle response, 46s will drop the flow velocity by 32%

even for 3.2s the 40s are the better way to go

distributor instead of COPs?

Keep in mind that it's common with ITBs to have the throttles larger than the base/port diameter, with taper in between.

Hi,

My engine is almost exactly the same as yours with the same camshaft profile only it’s a 3.2 short stroke with RSR Mahle p/c’s.
It has 46mm PMO ITB’s and I am very happy with the way it performs.

Here’s data from the hub dyno:

https://uploads.tapatalk-cdn.com/202...de8463c059.jpg

Thanks all - very much appreciate the input.
Classic 911 - nice numbers :-) - thanks for posting the dyno results