Which Carb makes more power?
 

I have a question for folks with experience with RE DCD set up AND 48 IDA


I have a streetported 13 B four port and I am running the Rotary Engineering Dual DCD set up. Running the Racing Beat streeport exhaust.

Primary Venturies are 28mm
Secondaries are 34mm (I know that they dont come this big, I made these on a lathe)

I have learned how to tune webbers so they are dead on AFM wise and I love the way these carbs run (No starvation issues except maybe a little on hard braking) and twins are cool, but I have been doing more track days lately and I am wondering if I could get more power by going with a 48 IDA setup.

3 questions:

Which setup would give the most power? ( Seems I remember an Rotary Engineering add that claims the DCDs would give more, but don't know if that is on stock ports or not)

Which would have more CFM? I have tried to determine this but I am not sure of the math.

If the IDA gives more power, would it be enough to justify the expense?

I don't have personal experience with the IDA, so I would like opinions.

Thanks

If it's any help, I put a 48IDA on my streetported 12A last year. It's still running in and was a gnat's cock over 200bhp (bearing in mind our European power figures usually come out lower than in the U.S.). That's good enough for me on an open diff :eek:

It's daily driven and once I got the hang of the starting ritual I don't even miss a choke... been going to work 3 a.m. in sub zero temps and it's always fired up. Runs like a three-legged dog till it warms up, mind :( Bang the throttle open before it's up to temp and it'll suck in a massive lungful of cold air and flap around like a prick in a shirtsleeve. But that's more my fat right foot than the carb's fault

i don't know if its worth the $ to swap over but the IDA is nice for track use, there is only the one, so its simpler to setup. it does well with corners too.

we know how much an IDA flows, as JGRewe did some testing. we don't have the same info for the DCD's but it looks like the IDA flows more. the IDA can do about 660cfm with big venturi's in it, which is enough for 300hp at the flywheel. so the question is, how much air flow does your engine really want?

I can lend you a 13-b 4port manifold and an IDA to try but i need a $1,000.00 deposit to ensure I get them back

Ok maybe I'm making this too simple but the RE DCDs is 2 x 36 and the IDA is 1 x 48.

(36 + 36) > 48 or to put it another way, that 48 is more like a dual 24.

I bet the DCDs are more CFMs and a better driving experience.

Of course a hogged out nikki jetted right might beat them all!

so the back of the envelope says, the dual DCD's flow about 440cfm total. the IDA is about 660cfm total (this is an actual tested number). a hogged out nikki is in the ballpark, the rotary engineering version is 465cfm. however the IDA and DCD's are on better manifolds, so they, all things being equal* will make more power.

*they aren't :)

actually if you want some REALLY rough math, a stock 12A makes about 1hp for every 2.75 cfm (330cfm/120hp). mod the nikki, and you gain hp, but hp/cfm drops (465cfm/145hp), about 3.2. why? at 330cfm the carb is restrictive, so you get big gains, at 465cfm, the carb isn't restrictive, so gains are pretty small. its a really rough number as it depends on too much stuff, like the tune you have, condition of the engine, etc.

Thanks for the response guys, this is just the sort of info I was looking for.

This answers my question. If the IDA flows enough for 300 hp, then my DCDs are surely flowing enough to support the probably less than 200 hp I am making (never dynoed it so I'm guessing),

that makes me pretty confident that there are no big gains to be had.

Maybe a bridge port is in my future.

Thanks again for the input.

Im not sure this is quite true, but I cannot remember exactly how it goes,

something to do with Pi r squared or something like that. I do know that you do not have to double the area of the tube to double the cfm, but that is about as far as my grasp on physics goes.

I've owned both. The progressive nature of the dual DCD's is very nice, I think it has a wider usable powerband especially down low. And i do miss having a cold start assist. The IDA is very good at moving lots of air and fuel, its hard to beat on full throttle.

But on to your power question. You can always just have the DCD venturis enlarged. Say bump the primary 28mm to 29 or 30.

:facepalm:

I never thought of that! but your are absolutely right!