Weber IDA jetting on 2.45 l
 

Hi,

I am planning to build the "Henry Schmidt" 2.45 l (see link)

http://forums.pelicanparts.com/911-engine-rebuilding-forum/932564-more-poop-options-2-0-69-t.html

Slowly collecting the right bits and pieces.
currently working on my Weber 40 IDA's and looking for the correct jetting/venturi info!

Have been searching on different forum.....without success (only someone who lives on 7000 ft. I live almost below sea level (The Lowlands ;-)

Thanks,
Bart

Bart,

To determine basic jetting, one needs to know (in addition to displacement), cams, CR, & exhaust configuration in order to get close. :) :)

The old formula that has been around since the start of time was:

Choke Size(mm) = 20 x Sq Root (V/1000 x N/1000)

V = Cylinder capacity in cm^3

N = RPM at Peak Power

A starting point for the main jet size is to use a jet 4 x the size of the Venturi and an Air Corrector which is Main Jet + 50

The choke to barrel ratio should be around 1.2:1 although on a high revving race engine 1.1 may be OK.

These starting points may not be perfect but should be within the ballpark.

hi Steve & Chris,

thanks for quick reply!

the 'Henry Schmidt' configuration seems to me a nice example of sporty but still down town drive-able ;-) for sure ther are more options....

the specs were I need the jetting for is as follows:

2 2450 liter based on 69T,& 70T (I haave small spigot 7R case) 70.4 stroke 86mm bore. 9.5:1 compression, 36 mm intake and 35mm exhaust ports with DC 40 (Mod S) cams.
40 mm Webers,SSI heater boxes and an M&K sport muffler.

hope that this helps?
could also sent Henry a PM but was hoping he would read thread.

best regards,
Bart

Jetting is a "black art" that requires a plethora of baseline decision.
Altitude
compression
cams
port size
exhaust (both primary size and muffler back pressure.)
ignition
desired red line and so on.
Everything matters. Including your driving habits
That said: here's where I would start.
32-34 Venturi (smaller is better for low end response)
135-145 Main
180 Air correction
60 Idle
F3 or F5 emulsion tube (F5 is generally better as engine approached 2.8)

Henry,

Thanks for the info!

I was planning to build it as close as possible to the original set-up / spec you described in the link above and in other threads! So wil start with your suggestion as that is probably what you use in your own 2.45 engines...?!
I am working in a tire research department so I know a bit about (other) 'black art' that needs once in a while some 'black magic' ;-)

Best!
Bart

Take the mystique out of the final jetting, buy a wideband o2 meter .;)

Or install an EGT and buy an air density meter (drag racers use these a lot). I used that for my jetting for several years, trying to keep the EGTs at about 1,300 F or a bit below. When finally I put the engine on a dyno, and swapped jets a few times, I found that the table I had constructed was within one jet size of max power, and the difference wasn't much anyway.

A wide band O2 meter should do the same thing for you, telling you where max power is, and where max cruise fuel efficiency is. I'm a bit sour on my Innovate LM2, because it gives me information which drives me nuts - at idle it says I a way lean when an exhaust gas analyzer says I am just a bit rich, and at WOT on the track it says I am quite rich (11s and 12s), and I manifestly am not from car behavior, plugs, and exhaust residue. But maybe something went wrong with mine. I liked the EGT approach better, as it never let me down.