To continue with Kentetsu's post - Even if you have something "OMG huge" like the judge Ito port there is still PLENTY of carb left over for more flow at WOT.

 

Here's something I've always wondered. Is there and equation to figure out what my carb will flow? Or, do I have to throw it on a flow bench? 48IDA with 43mm chokes and 36mm chokes.

 

thats why webers,dellortos, oers and mikuni carbs are not rated in cfm's since they are variable cfm carbs simply change the venturies to larger ones and your cfm rating goes up

 

Flow bench, although since they are relatively popular carbs my bet is that the info could be found online somewhere. Not sure where exactly though. Never researched em much.

 

It's hard to find flow ratings for Webers. According to this thread, the 48 IDA with 44mm chokes is 622 CFM at 1.5" Hg! I remember I found somewhere that said my 45 DCOE with 40mm chokes was 444 CFM at 1.5" Hg.

 

I suppose flowing the carb main body without the venturis would give a good base to compare for efficiency.
But if you were to just forget about the efficiency descrepency for a basic round-about comparison, just calculate pi (r^2) to get area.
The Sterling is bored out to 22mm primary & 30mm secondary.
The total of all four bores is 2174 square mm.
The total of the two 44mm Weber bores is 3042 square mm.
This is at the apex, so it's pretty loose a comparison, but it should be pretty obvious that the Weber's got oodles of flow.

 

Could you give me a brief explanation on the physics claiming a 4bbl works better low end than the 2bbl?? I am trying to do the research here before just slapping something on. I plan to house the carburetor in an enclosure for a low 6-8psi turbo application.

On one hand it makes sense the 4bbl is better because the primary bores are smaller which should increase velocity, however using a similarly sized over all 2bbl.... the blades just aren't open as far, combined with a RB intake I would think should suffice well enough to get the ball rolling for the turbo?


Am I correct in thinking this or am I way off base?

 

Wow so my 48 IDa with 43mm chokes should flow just under 622cfm, say 607 for he hell of it sice it's 97% the size of a 44.
So if 36mm is 81% of 43mm then my rough guess is that my 48 ida w/36mm chokes flowed around 497cfm... then why did switching from 36mm chokes to 43 mm chokes net me 12whp? I though the engine didn't need the extra cfm.

 

Well, the valves are basically stopping air flow. The velocity "business" part happens far up from the valves.
But once you enter boost into the equasion, it's a whole new ball game. You can use undersized carbs with boost because the carbs are measured for flow at 14.7 psi. You add 6 or 7 psi to that, and now the max cfm is going to be higher.It doesn't. Was this peak HP up top? Maybe your jetting was too rich / lean for the 36mm set-up.
Did you have A/F meter readings or A/F readings from the dyno to compare?
This happened to me a few times when I was trying out new venturi cuts. Even subtle changes sometimes made a big enough difference that I could tell without a dyno.
Also, keep in mind that I was just illustrating that the Webers are actually a pretty big breather. I would never suggest cfm is directly proportional to bore area. There are just too many variables.

 

I am SO stealing that. It's going in my sig right after I post this.

 

you are 100 % correct!

 

Agreed there are way too many veriables to put a finger on it because all the jetting was changed as well. But afr's were targeted at 12.5 on both runs. So arf changes weren't the issue it made more power everywhere, but above 5500 it makes tuns more power. Once I get back on the dyno I'll post more resluts in my weber issues threads I have going.

 

If the stock Nikki is 313 CFM

What would be the better choice for boost and why? Here is what I'm up against:

350 CFM 2bbl flows like a 247CFM (4bbl)
500 CFM 2bbl flows like a 354CFM (4bbl)

Now if I used the smaller 350 cfm carb, it wouldn't have near the performance on the top end N/A, but should have much better drivability characteristics down low. The smaller carb COULD be harder to tune with boost 6-8psi, due to the small size air bleed modifications may be necessary.

The 500CFM carb should perform well up top, but be a bit lazy down low. Tuning with the turbo should be much simpler because the total air flow will be closer to the realm the carb is designed to work with.

This is my daily driver, just looking for a little more go from the Turbo. So I'm leaning more towards the smaller carb, seems like it would be much more efficient.

Thoughts?

 

Guys?

 

Sorry, no experience with boost....

 

Between the two you'd definitely want the 500. The 350 is just going to choke you and the car won't make power anywhere. You haven't given us any details about your setup, but the 500 is already smaller than what most people use for boost, even blow-through, which doesn't require as big a carb as draw-through.

 

when you look at flow numbers, you NEED to look at the pressure drop used.

we know a 4 bbl holley is rated at 1.5 in

a 2 bbl holley is rated at 3

the webers from that linked thread were flowed at various numbers, i see 20" and 25"

it makes a huge difference weber 48 w 44 venturies @25" is 348cfm per venturi @20" its 311.

 

Right. And if you see up top I did... the 2bbl 500CFM is like a 354CFM 4bbl. Etc.

The issue I'm trying to hash is maintaining enough velocity for low end (under boost) performance for everyday driving. Everyone says you can use a smaller carb for Boost than the unboosted NA setup. If the stock nikki is 313 then the 2bbl choices are equivilently 264/354 however since they aren't a 4bbl the primaries are the effictive secondaries as well.

And above in the post we've deamed the 70CI Wankel to only need 340CFM at a high VE and an RPM well above the redline my tach says anyways.

 

for n/a, just slap on a weber 48 IDA and thats it. once you feel the IDA, use the carb as a door stop. spend your cash wisely

 

Give Rob at rotary shack a shout. www.rotaryshack.com He can definitly help you out with your boost questions.

 

you can use a smaller carb with a turbo, because you have a turbo! instead of atmospheric pressure on top of the carb, you have atmospheric + whatever boost you're running, so the carb will flow more.

if the stock nikki is 313 (at what pressure drop?) and the holley is 354, why would you bother buying the holley when you can run more boost?

 

Use the bigger carb.

 

You seem to be the carb guru, so I'll give it a shot!