>so what are you running for size on your boosted engines? I am using Comp Cams 1.6rr on 7/16 studs for both intake and exhaust.
does anyone think it would make more sense to use 1.5 on the exhaust side? But still use the 1.6 for the intake?

 

>just curious

 

Why do you think it would be beneficial? If anything, though it's hard to generalize, the opposite might be true.

Rich

 

>are you saying that 1.6 on the ext and 1.5 on the int? or are you saying stay the same for int and ext?
I'm confused... but hey thats not new

 

I think the idea that some of the good supercharger work ($$$) is being wasted by blowing out the exhaust before that valve closes is the wrong question. When it's time to fill that cylinder or empty it I want all the port and valve capacity I can afford. The real question is when to open and close those valves. Sooooooooo ... can super/turbo gains be enhanced with duration and overlap that are different than normally aspirated cams? I suspect Mr. Krause has knowledge the rest of us can benefit from. Will he choose to share it with us?

 

Typicallly, an NA setup works best with valves sized so that exhaust flow is ~80% of the intake flow. If this can't be achieved, a cam with longer exhaust duration (dual pattern) is used. This is pretty typical for SBC cams. With forced induction, the difficulty getting adequate exhaust flow is exacerbated. The SC will increase intake flow but have little effect on exhaust flow. It causes the motor to act somewhat as if the exhaust valve were way to small compared to the intake. One solution is to significantly increase the exhaust lift and duration. One way to achieve this is with increasing the rocker ratio on the exhaust side. Obviously, the cam can be designed so that this isn't needed. But it may help on a given combo. The only way to tell for sure is to try it.

We designed a cam for a buddies high boost SBF stroker. It worked pretty well with 1.7 rockers on both intake and exhaust but not quite what we expected. There was a gain of ~25hp going to 1.75's on the exhaust side. We would have tried 1.8's but that caused some clearance problems that we weren't prepared to deal with at the time. We never did try 1.75 on both sides and I wonder what would have happened?

Rich

 

Thanks for the engineering insights. Seems to explain why supecharging appears to be more effective than turbos on the sbc or most na engines that live on the street and need a relatively broad response range. The valve size ratios of the factory and mild after-market heads favor them. Thanks, again.