ZCar Forum banner
Status
Not open for further replies.
1 - 3 of 3 Posts

·
Registered
Joined
·
107,695 Posts
Discussion Starter · #1 ·
Does anyone know what each S.U. carb on a 240 flows, when compared to, say, a 240 with triple 40 dcoe with 32mm chokes?

Since those Nascar boys can get big HP from little carbs and restricter plates, at what point are the S.U.s hurting my performance? (I think I might have said a bad word. Sorry.)

I've also heard a bit about someone that makes a kit for triple S.U.s. Anybody know anything about them? Good? Bad? Out of business?

Any input would be appreciated.

Thanks, Willie and his '73 240
 

·
Registered
Joined
·
482 Posts
Well, a single SU has a 38mm choke per 3 cylinders, while a Weber/Mikuni has a 32mm choke per 1 cylinder. A racing rule of thumb is that the carb choke should be < or = to Intake valve diameter.

For example, I'm running triple 45 Webers with 36mm chokes on a L28 with E88 head. My intake valve size is 42mm. So there is only a 6mm difference (or restriction)between the valve size and choke size. Since this is an individual runner setup, a choke that is slightly smaller then the intake valve provides enough velocity to make the engine somewhat streetable. Of course my version of streetable differs from others.

The difference being that the velocity of air at low engine speeds will be very high with the SU, since 3 cylinders are drawing from it, but will fall short when all 3 cylinders are screaming for more air at high rpms. This is a more streetable setup, since the carbs are operating at efficiency more of the time (due to the high airflow), and only fall short when you are really in high rpm (5000+), where the restricted amount of airflow hurts performance.

Whereas the velocity of air through a single 32mm choke on a single cylinder at low engine speeds will be very low, hence the use of rich idle jets. This basically just dumps fuel into the manifold, making up for the lack of airflow needed to draw fuel from the primary jets. But, when you hit the right rpm for the main jets to 'come on' these carbs really begin to flow. They are fully capable of running up to 8,000+rpm, never running out of fuel or air. Of course you pay for this all out performance with a lack in low end power.

I've also heard a bit about someone that makes a kit for triple S.U.s. Anybody know anything about them? Good? Bad? Out of business?

This setup is inbetween the dual SU and triple Weber/Mikuni setup. Now you have 1 carb/2 cylinders. Performance and driveability should fall between the dual SU and triple Webers. The guy making this was Preston, he sometimes visits this forum, but I think he has his own webpage. Maybe someone else has the link.

As far as NASCAR is concerned, the whole engine has been matched to the carb. This makes the engine operate at or above 100% Volumetric Efficiency. This means the engine is injesting the maximum amount of air/fuel per revolution possible. Hence the most power available. Keep in mind though, these cars are tuned to run at sustained high rpm, and the low end/low speed driveability really sucks. They also run extremely high compression ratios and wildly high lift/long duration cams.
 
1 - 3 of 3 Posts
Status
Not open for further replies.
Top