The next big thing....3.0 EFI/ITB w/Rasant engine management

[al lkosmal]

I find that most people oversize their intake and exhaust, as if they are designing engines strictly for providing power at the top end of the RPM range....like for racing, rather than for the street. Or they size their ITBs, based on carburetor throttle body sizes. Remember that carbs have a main venturi in the throttle body....so, for example... a 46mm carb will have a 38mm main venturi in it....I.E. increasing velocity (a good thing), but reducing max airflow capacity due to the reduction from 46mm to 38mm.

but the EFI/ITB does not have/need that main vent....so a 46mm EFI/ITB is really 46mm and a 40mm EFI/ITB is really 40mm.

Over the years, I've collected some reference info...this may have been off of the Jenvey site, but there are several sources and they all offer similar information regarding EFI/ITB sizing.......as follows:

This is a basic reference for ITB sizing, based on HP per cylinder

-Up to 30 hp - 30mm
-Up to 33 hp - 32mm
-Up to 39 hp - 35mm
-Up to 46 hp - 38mm
-Up to 51 hp - 40mm
-Up to 56 hp - 42mm
-Up to 65 hp - 45mm
-Up to 74 hp - 48mm
-Up to 80 hp - 50mm
-Up to 87 hp - 52mm
-Up to 93 hp - 54mm.

I hope that this helps you with your project.

regards,
al

[onboost]

Al,

Thanks for the quick response and yes this does help..

I just realized that I had a fresh set of twin-plug 3.0 Carrera heads, and 98mm Mahle 10:1 P/C set stashed in "that closet." Will have to check intake size on the heads as they may have been opened a bit. I've had this stuff for so long I've forgotten the details though that's probably in one of the boxes as well.

Also, have access to a nice set of 40mm TWMs.. so it looks like my big decision will be with the cams as I think my decision is made with regard to engine management.

['76 911S 3.0]

Originally Posted by onboost

The motor you reference above is 3.0/3.2SS with Carrera intake? Just curious.

I've looked through the thread with regard to TB size/sizing and either have missed it, or there is not much talk around this. So are you typically running 40s or 46s TB with these builds.. lets say on 3.2 SS specifically?
Also, with regard to cam choice, the 964 cam seems to come-up often. Have you built using other grinds thus far, and again with what size TB, compression etc..

Thanks guys.. and looking forward to picking-up one of these systems in the near future

Yes, the engine I built is a 3.2SS based on a '79 3.0 using a 3.2 intake manifold (using adapter spacers from Clewett Engineering for the reduction from 41mm to 39mm). I am also using Hemi 5.7 injectors that have a longer reach pintle on them that puts the injector tip right into the intake stream. These injectors also have a very nice atomization pattern and excellent idle characteristics. Max injector duty I have seen is 75%, so well within the allowable duty limits.

I will be converting to E85 compatibility as well as turbocharging in the near future so larger injectors will be needed in order to cope with the added fueling requirements. One thing to note with the Infinity is that since it is a VE based fueling equation, injector characterization is very important for the fueling model. Using an injector that is in the Wizard database is advantageous as if, for example, you were to change injectors for added fueling requirements, you would not need to adjust VE values of areas of the map that have already been calibrated.

If you were not using an injector found in the wizard database, and you entered your own values for offset, dead time, flow rate, etc., you may end up having to recalibrate your entire VE table as that table was built off "unknown" data. The wizard is there to help and to mitigate time required on the dyno for changes like this. All injectors in the wizard have been characterized in house and are known to work well in the fuel model and allow injector changes without much or any change to the VE table.