ITB's What Size??

[1972_911T]

Im wanting to order some ITB's for my engine project, I will probably go with TWM 3003 or the new PMO's. Only thing I need to decide is what size? My setup is 3.0SC base with 9.8:1 ROW pistons, DC20 Cams and twin plugged with SSI's for exhaust. Should I stick with 40's or 46's for future performance upgrades.

Pro's and cons of each?

Steve

[Eagledriver]

Given that choice I'd go with the 46mm size. I think some brands are available in 44mm and that would be optimum. You'd be leaving about 10 HP on the table if you use the 40's.

-Andy

[sww914]

Bigger= potential for more maximum HP at high RPM's, maybe balky at lower RPM's.
Smaller = better driveability, maybe lower max HP.
It depends on how you're planning to drive it.
Lots of people here in the US buy 4X4's that they plan to use, but never actually use off road at all.

[jpnovak]

What is your intended usage? If its mostly track I would suggest the 46mm. If it will be street legal with the occasional track outing then go with the 40mm. Those cams will not produce power at a high enough rpm band to take true advantage of the extra cross sectional area that a 46mm intake will give you. Besides, your port sizes will not likely be any larger than 40mm. Even with a normal 1 degree taper the 46mm would be too large.

btw, I have 46mm on my otherwise stock 3.0. I did this size since I would be building a full track motor in the future. getting the off-idle response tuned with EFI gave me some trouble. the issue is with the large swept area that a 46mm butterfly gives. Any small change in throttle position causes HUGE changes in air volume when at small throttle openings. This means you have to over compensate for the over-lean condition brought on by the sudden, extra airflow. I also think the low-speed throttle response would be better with the 40mm.

[safe]

Originally Posted by jpnovak

Any small change in throttle position causes HUGE changes in air volume when at small throttle openings.

That could be fixed with a progressive throttle, as the often do on motor bikes.

[1972_911T]

Originally Posted by jpnovak

What is your intended usage? If its mostly track I would suggest the 46mm. If it will be street legal with the occasional track outing then go with the 40mm. Those cams will not produce power at a high enough rpm band to take true advantage of the extra cross sectional area that a 46mm intake will give you. Besides, your port sizes will not likely be any larger than 40mm.

My car will be prodominantly used on the road, it may see a small amount of track time. At this stage I would defiantly go with tuneability over top end HP if the 40's will be easier to tune.

Steve