six question, again hypotheticly speaking, what size carbs would one by if....... Options

[pete-stevers]

one was building a 2.5?
or
if one was building a 2.7........
or.....
if one was building a 3.2
or........
a 3.4.....
of course my next question is
what are they worth???

[Thorshammer]

Found my long post:

Eric,

In the Porsche world it is very common to run a fairly small venturi in the carb for good throttle response, the most given reason I have heard is: you need a good signal at the carb for atomization and this is what the factory has done.

Problem, is most tuners use the idle air bleed to balance and not really to tune. It has some nice effects to the main circuit and with the right emusion tubes the "strong" signal is not as important as it once was. Also, camshaft technology has changed significantly which leads to a different intake pulse.

Also remember, with a strong pulse stated at the inlet port, and the proper valve overlap the signal at the carb is very strong.

My contention is, along the way, people stopped developing these engines, oh about 25 years ago. Only a very small few have continued, and the result is some new thinking. When I started, I was a cookbook thinking type of guy. ie... 906 cam, 46intx40m exhaust 35mm intake ports, tall manifolds, 30-32 inch flat collector exhaust, with megaphones?????? I can tell you that after alot of development, not of the above mentioned stuff is in ANY 6 engine I am involved.
BTW the traditional Weber velocity stacks that everyone has, are really not ideal, the sharp lip near the top of the stack causes some major turbulence.

Also consider this, Motorcycle engines are making almost 120bhp per 600ccs and use 36-38 mm throttle plates with no venturi. So if we consider they are displacing 150cc per cylinder, and the standard 2.4 liter porsche engine 400cc, what do they have that we don't..... Alot, but suffice to say our heads flow alot less due to having 2 less valves and we don't spin our engines to 15,000 rpm. But the major improvements in motorcycle engines started coming with increasing inlet port velocities to the extreme, and then focusing on inlet flow at low lifts, this may clue a few people into what I do with my race motors.

In my testing, the combinations I have indicated in my previous post only perform well as a package. The porting of the heads is critical. The cross sectional changes are absolutely everything. They must be correct.


Erik Madsen