I'm currently in the process of putting together a massive SP 12A, and i was wondering which carb i should go with. the only differences that i have really seemed to hear are the differences between the mid-range and top end power due to different runner lengthes. i am going to be using this engine in the GTS challenge with NASA, so governing rules on what i use are not really a problem. so i could use some advise as to what you guys think about the pro's and con's about both carb's when using them on the track.

thanks in advance for the advice

 

Most of the guys that I raced with (SCCA EP) used the IDA. Some tried the side draft but soon switched over. I used the IDA and was very happy. I belive the runner length is the issue and also heat from the top of the motor with the side draft mounted on top.

 

Go with the bigger carb you can always choke it down and make way more power with a 55 DCO than a 48 IDA. People always say bigger is not better but, if your competing with a Rotary bigger carb is a good thing with N/A Rotary Engine. Turbocharging is a different story but, it sounds like you are building a N/A setup.
Correct.

Good Luck.

 

yes this will be used on an N/A set-up. I'm currently running 45 DCOE and my 4 port 13B and i really didn't notice to much heat soak with the carb being over the engine. no matter which carb i choose i will be building an air box to draw air from outside of the engine bay anyways, so i'm not to worried about the heat at this point. and other thoughts would awesome!

thanks again

 

On my streetport 12A, from 1990 thru 1994 I ran a side draft Solex Mikuni with a long runner manifold placing the carb on top of the motor. In late 1994, I switched to a downdraft Weber 48 IDA, and I started running track days along with street driving. In my opinion, the long runner manifold with the side draft carb made torque at least 500 rpm earlier than the short manifold down draft Weber.
Long manifold side draft pulled strong from 4500 rpm, better for street use.
Short manifold down draft pulled strong from 5000 rpm, better suited for track only, had more top end pull than the long manifold side draft setup.

A downdraft can be had as big as you want. Jaycee will open up a 48 IDA to 52mm, and Gene Berg makes his own even bigger IDA copies.

I now race (road course only) a 52 Jaycee modified Weber IDA down draft carb on my 13B pp motor, and it makes 300 whp with a flat torque curve. I strongly recommend having IDA mods done by Jaycee. http://www.jaycee-ent.com/carburatorparts.html

All the carbureted rotary E Prod race cars I see use the Weber IDA downdraft.

 

The heat issues with side drafts are when using the stubby manifolds. The carb hangs right over the header. On track use is not a problem, it's the hot soak and vapor lock after coming off track. You can get better horsepower than the long tube, but sacrifice torque