Carb Advise

[Daviboy]

Hi guys I have been toying with the idea of swopping out my CIS for carbs for a while now and want some solid advise. First off let me list the motor specs:
- 3.0 euro SC with 39mm intake ports
- 964 cam profile
- Standard springs and retainers
- Standard 95mm 9.8:1 JE pistons
- Balanced conrods and crank
- Run 95 octane fuel
Motor was rebuilt around 1,500 miles ago. I am running at sea level. I know that I would need to have the dizzy recurved but what I would like to know is would weber 40 IDA's be a good choice? And if so a guesstimate at venturi, main and idle jet sizes. Any help on this would be greatly appreciated.

[charlie13]

talk to Richard at pmo carbs, hes a great guy...he will tell you weber jets etc

[timmy2]

For about the same $ you could install EFI with throttle bodies. You'd get the added performance and be able to retain some efficiency for gas mileage if that matters.

[snbush67]

If you have CIS pistons, then CIS is your optimum fuel and air system. Adding carbs is not going to give you any benefits over the CIS. You may get more snap and a raw visceral experience, the sound of carbs is also great. Carbs look sexy. But, the CIS is smoother and will get you better overal engine performance and longevity.

Yes, 40's will work and are probably the best option with the CIS shaped pistons.

[Targa Time]

+1 Shanes advise. A lot of money, ($4000+) with little benefit for your engine set up.

[Eagledriver]

Originally Posted by snbush67

If you have CIS pistons, then CIS is your optimum fuel and air system. Adding carbs is not going to give you any benefits over the CIS. You may get more snap and a raw visceral experience, the sound of carbs is also great. Carbs look sexy. But, the CIS is smoother and will get you better overal engine performance and longevity.

Yes, 40's will work and are probably the best option with the CIS shaped pistons.

While it's true that a well maintained CIS system runs better than carbs, the power differences are larger than indicated here. With 40mm carbs and a race exhaust you should easily get 245 HP from your motor. Mine dyno's at 230 to the wheels with Euro pistons and SC cams.

-Andy

[Eagledriver]

Forgot to mention that 46mm carbs are better and will give about 10HP more (that's why my motor is so high).

-Andy

[Targa Time]

Must have been a generous dyno.

[Eagledriver]

Originally Posted by Targa Time

Must have been a generous dyno.

Yeah, that's probably it. We only had 21 Spec911 cars at the last race, and dozens of dyno results from both chassis and engine dyno's.

The number's I quote are conservative. If you want an optimistic tuned to the max number, how does 257HP grab you!

-Andy

[snbush67]

I think the record shows that the factory was very conservative with HP numbers as well. If you got an SC that put out factory 204 than you got the minimum. Most engines were likely de tuned to run smoother with the emissions crap. Once you eliminate emissions stuff, add some timing, good fuel, and the ability to breath than it is proven to get much higher HP.

I haven't had my car on a dyno yet, I went way overboard on my build, it got away from me and was not at all what I needed. It's fun, but not something that I would like to be stuck in traffic with.

[VFR750]

Interesting. My CIS dynoed at 155 at the rear wheels, which works out to ~175-180 at the crank. Factory number was 172 on the sticker.

My 40mm carbs increased hp noticably. Have not measured it directly, but my 2nd gear accel rate suggets >180 at the wheel. Plus, DE instructors riding shotgun comment on the power and ask if it is a 3.2SS or cams. Nope, just carbs on a CIS pistons and cams, 34mm ports too.

The power gain is real.

[Joe Bob]

While carbs look beetchen, they are a PITA, cold blooded and your mpg goes into the toilet. Best to go with ITBs, stay with what you have or upgrade to a 3.6.

[Daviboy]

Well numbers wise I had mine dyno'd after putting about 1,000 break in miles on the rebuilt motor and I am putting out 209rwhp so ~240 at the crank. My CIS and timing was set up by a wonderful old German Bosch specialist I found hidden away in a small backstreet garage he also dyno'd the car whilst tweaking. So maybe I just stick with the CIS, really only wanted carbs for aesthetics and that beloved induction howl....

[moneymanager]

I'd stick with CIS, but if you change, consider PMO's vice Webers. Unless Webers are dirt cheap in your neighborhood, the apparently high cost of PMOs will be matched by the cost of the Webers plus necessary (pricey) rebuilds. More important, PMOs come with many refinements which improve Weber design and performance.

[Roger 911]

Is it possible to retrofit a 3.2 Motronic setup? I would think you may be able to pick one of these up for a reasonable cost. Someone like Steve Wong could probably fix you up with a custom chip. Just a thought.

Incidentally, I swapped MFI for Webers on a modded 2.4 many years ago (long before MFI was "in vogue", and Webers were still cheap!). I loved the Webers. Easy to tinker with too.

Roger