Proper sized exhaust

[TJ's GMC]

Originally Posted by bigblockv6

I'm pretty happy with 2 1/2 inch duals and 50 series mufflers on my 68 478M, of course that's the same system when I had the 305E, In the future I may switch over to 3 inch pipes and 40 series mufflers

That 478 will love 3 inch. haha There's actually a guy who's done it. Not sure if you've seen the vid. https://www.youtube.com/watch?v=liKIlN9Z9Jg

I've been wanting to go back to the exhaust guy and try some other mufflers for sound reference.

[1972RedNeck]

Originally Posted by TJ's GMC

Turbobill. I fully agree with you on that Years ago I tested all 3 of these carbs on my 68 305E and the Holley 500 was still by far the top performer, it's all about the real world results

So if the stock Stromberg is in the 300 CFM range, math shows that it is too small. The normal old go to "(CID X RPM X VE)/3456 shows that we only need a 285 CFM carb at 3800 RPM with 85% VE. But, that is not the whole story. Do some looking around, and you will find that 4 barrel carb CFM ratings are at 1.5 inches of mercury while 2 barrel carb ratings are at 3 inches of mercury. At wide open throttle on an engine aimed at performance, we don't want 3 or even 1.5 inches of pressure drop. Eight to nine tenths of an inch is much better so we have to adjust our 285 CFM.

To do so, we need to divide the 285 CFM by the square root of the quotient of our desired vacuum (.8) over the rated vacuum (3). This shows us that we need a two barrel carburetor "rated" at 552 CFM.

Do the math with a 4 barrel carb and we would need one "rated" at 390 CFM - I think Holley even happens to make a 390 CFM 4 barrel...

So yes, you should see a large performance increase from your stock 300-ish CFM carb to a two barrel "rated" at 500 CFM.

[bobdylan]

Are you sure you are a Red neck farm hand? I would guess a math professor.

[TJ's GMC]

Least this is a man who actually Knows his math!