ITB Sizing- Recommendation

[tx996tt]

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

[Classic 911]

Following


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[Joesmallwood]

Recommend calling William Knight.

[Bill Verburg]

Originally Posted by tx996tt

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

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

[pampadori]

distributor instead of COPs?

[stownsen914]

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

[Classic 911]

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:

[tx996tt]

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