Boring out mfi throttles

[724doorE]

I know there have been lots of threads on this, I got a spare set and set about cleaning them up.

I think its common knowledge that the thottles are 38mm at the top, and at the butterfly and necked down to 36, 32, or 29mm for the S,E, andT engines. My engine has 2.7RS pistons and I want to eventually get to those specs or a little better. I originaaly decided to open up to 38mm ports on the heads instead of the 36mm for a stock RS. Thought about a lot of options, one of which was bore the throttles out for a larger butterfly and then taper to my 38mm ( I have a HAAS VF-1 CNC mill at my disposal). Of course being cheap, I wanted to find a cheap set of butterflys to use. Holley makes and sells a set of 4 1 11/16" bflys for very cheap with r without holes predrilled. that is about 42.86mm or so.... a little bigger than the factory racing sizes of 41 mm. I am worried about wall thickness of the throttle body. So does anyone have any hard info on how large a buttlerfly anyone has succesfully used in an early rebuilt remachined mfi throttle body???

BTW, they look great bead blasted and clearcoated!!

[Tim Walsh]

why would you want to do that when 36mm ports give you the proper gas speed for an S camed 2.7?

[CamB]

I apparently have 40mm ones. I never measured, but I trust the person who sold me them. I have no idea what the provess was.

Note: they were done like that for a 3.0 engine with GE80 cams I think. They are on a 3.2 with S cams (270ish hp). I think Tim's question is a good one - why?

[Tim Walsh]

cam,
I would have to do the math but I'd expect if you had left your engine at 38mm you would have had problems. GE80 cams 3.2 liters of displacement makes for alot more gas flowing compared to a 2.7 with S cams. To put it in persective a 2.4L with GE60(I belive) and 40 mm ports run BARELY within the optimal gas speed range.


OHH and the stock 2.7RS had 36mm ports on it's t-bodies and it's intake ports.

[CamB]

I only have S cams with the 98mm P&C - the t-bodies were originally matched to GE80s with 95mm P&C.

I am close enough - I also run only SSIs and a 2 in, 1 out muffler. The top end probably is a bit lacking (max power is at 6340rpm). I don't really need more power though!!

[Rot 911]

Originally posted by CamB
I don't really need more power though!!

Words rarely heard in the Porsche world.

[Tim Walsh]

except in connection with the 917/30

[Paul Thomas]

Dennis,
I realize that this is my standard response to everything throttle body, BUT, you should call Matt Blast at Eurometrix. He knows more about this stuff than anyone and is also willing to talk freely and advise even if it doesnt get him any money.
www.eurometrix.ws

Paul

[724doorE]

May try to talk to Matt, thanks Paul.