Best Carb Choice

Daviboy · Daviboy's Garage

Just wondering what the best carb choice is for the following:
3.0l SC engine with custom JE 9.8:1 pistons, 964 camshafts, exhaust set up equal length custom headers with 2 in 2 out muffler. Engine just rebuilt all new bearings, timing chains, new cylinders, heads rebuilt etc.

I have the CIS back on the engine but think I may save a bit of cash and go the carb route.

silverc4s · silverc4s's Garage

Davy,
Here is Colin Campbell's formula; as reprinted by Bruce Anderson in the Porsche High Performance Handbook:

Venturi size in mm = sqrt( [V/1000]*[N/1000])

Where V = cylinder volume of one cylinder in cc

and N = rpm where engine reaches peak horsepower

Throttle bores should be 10 to 25% larger than the selected venturi

source: B. Anderson; Porsche 911 High Performance Handbook

lindy 911 · lindy 911's Garage

Can of worms on this site; if you don't plan to change anything further (sure) 40 mm Webers or PMO depending on budget will work fine. IF you might move to a bigger cam (probable) in the future to see what this motor is all about you'll need 46s. I have Weber 46 and like them a lot. Some here will say they're junk; I find this not to be the case. My 3.0 @ 10.5:1 flat rips.

Now if I could just get it to quit bleeding!

Eagledriver · 10-14-2010, 07:05 PM

40's for the street. 46's for the track. To get the most power you need the 46's but they are harder to tune for part throttle. 40's are easier to tune and will get better milage. You'll lose about 5-10 HP at 6500 and above. Power will be about equal at 6k and below.

-Andy

Daviboy · Daviboy's Garage

Lindy your engine looks very nice, where is it bleeding from? Eagledriver this car will only ever be a street driver so the 40's would fit the profile nicely. I like the look of the PMO but are they expensive or what. I know there is a lot of talk on here about Weber vs PMO. I grew up with Weber and know them to be reliable if set up and maintained properly. Thanks for the info.
Bill that venturi calculation looks all well and good but I have no clue how to put it into practice.

1QuickS · 10-14-2010, 08:38 PM

My 42mm Weber conversions fill the gap between 40's and 46's.

Paul Abbott
Performance Oriented

lindy 911 · lindy 911's Garage

Davy,

I have a nose bearing seeping oil at the O-ring; the third leak so far after my build. I'm learning these motors and there is a curve as with most things new. The 46 Webers I got were bought from this site in the classified section and cost me $2500 complete with linkage, manifolds and air cleaners. They are in very good condition and I like them a lot.

Lindy

1QuickS · 12:54 PM

Hi Lindy,

Would you please provide engine spec's (cam) and Weber jetting, especially main venturi size. Is power delivery continuous or do you have any flat spots? Is it a street car or do you use it for track or a mixture of both with emphasis on track performance and accepting whatever you get for the street?

Paul Abbott
Performance Oriented

lindy 911 · lindy 911's Garage

I'll get you specs in the morning when I go to the garage. I think I have 36 vents with 160 mains. There is a little flat spot going from about 1500 to 2000 rpm noticeable in first and second gear. After that it goes away because I'm in the pedal more I guess. I'm using GE60 cams from Jerry Woods and twin plug from Arron. The cams are set at .21" and I'm running 28 degree timing. Pistons are JE 10.5:1 off the shelf new version with the JE ring stack. No porting but stock '78 SC heads with new valves and guides.

I don't have a lot of time in it yet but I think a re-stack in the gear box would help a little although I can't get my head around the cost of that at this point; maybe later. As is, it runs really well especially 2nd, 3rd and 4th gear and pulls out to 7700 easily. It's a street car but I'm an ex-motorcycle racer and drive it so.

Lindy

1QuickS · 10-15-2010, 02:48 PM

Do your 46's have the fourth hole in the progression or are they the earlier three hole version? The fourth hole will help fatten up the progression if that's the issue. Your air corrector is probably 80 like 46's should have vs. the 110 for IDAs. You could tweek this air corrector a little smaller to enrich the progression without changing to a larger idle jet.

Paul Abbott
Performance Oriented