Webers vs. CIS on a '77 911S

[eddie914]

How much performance increase could be expected by replacing the stock CIS with 40IDA Webers and the stock exhaust with Bursch Muffler and headers/HEs with 1 5/8" primaries?

[jgparker]

The Weber part of this question was covered recently. I know Wayne's answer would be the same 2.7 or 3.0 CIS. Not worth it unless you change pistons, cams, etc...

Conversion-CIS to Webers

The exhaust changes should be a good improvement.

Good luck,

JP

[Alan Cottrill]

would you like that in round figures?

how 'bout the roundest one of all "0"

seriously, I haven't done this so I have no experience to speak from, I was just feeling froggy. but it seems like this is a hotly debated topic around here that leans toward no real power improvment. I'm sure someone who's done this will chime in.

[Wayne 962]

People say that the carbs have better "throttle response." Frankly, I'm not sure if I've ever really known exactly what they mean. The CIS cars I've driven have always responded on a dime, and so have the carbureted cars. There's theoretically a small delay as the CIS sensor plate opens from increased airflow, but I've never really been able to tell much of a difference, if any.

That said, some people swear they can. The bottomline is that you need a hotter camshaft (and pistons to match) to really take advantage of using carbs. Simply bolting them onto a CIS motor isn't going to buy you much except for a cool vintage-looking engine compartment, and horrific gas mileage.

-Wayne

[camgrinder]

I have an idea for a custom ground camshaft that should clear the cis-s pistons and make the webers work better. If someone is seriously thinking about doing the carb swap PM me.

[Wayne 962]

Just machine down the tops of the CIS pistons...

-Wayne

[jpnovak]

I converted to webers on my project for future expansion of the engine. This will likely happen next winter after the budget recovers.

How much of a pocket can you cut in a CIS piston? I guess I would like to know if I can notch the CIS pistons for a hot cam. I would like a high compression engine but the though of running on pump gas is tempting. As is the thought of not shelling out large sums of money for new pistons.

I don't have a 3.0 CIS piston to measure dome thickness. I think there needs to be 5mm left in the piston crown to prevent burn through. Any thoughts on this?

[Wayne 962]

Yes, minimum of 5mm. You can machine pockets - I can't remember how thick the domes are though. The piston looks really goofy when you're done, but the engine doesn't seem to care...

-Wayne

[Alan Cottrill]

where can I send pistons to be machined?