I have a question reguarding the stock 57 and up 4bbl intakes. they flow better than the pre-57 intakes but do they respond well to porting and polishing??????

The iron 4bbl manifold is capable of sustaining over 325 hp naturally asparated with modifications that can be performed at home.  The plenum under the carb requires almost all the attention.  What is done is to join the front and back holes on each side.  The fore and aft plenum divider is not removed. Then radius the holes as they drop into the passages so that there are no sharp edges to disrupt the flow into the horizontal passages.  There is a obvious narrow top to bottom place where the passage drops down to the lower runners that connect the left and right Cyl Head. You will see this easily once you get the carb base holes joined together.

Doug T

The Highlands, Louisville, Ky.

I did some work on my B manifold.  Mostly on the carb end.  I kinda ovaled it out.  Oh, let's see if I can get a picture...

There we go.  So I ovaled it out and radiused the transition into the runners a little.  This work was done with a dremel.  If I had a regular die grinder to use at the time I would have completely ovaled it.  The dremel is doable but it's slow going.  Carbide burs are available from many industrial supplies for 1/8" tools, such as a dremel.  This helps a lot with material removal.  I think my tool list was carbide burs, a  grinding stone, 60 grit sanding rolls and fiberglass reinforced cut off wheels.

I noticed an increase in power, particularly in the higher revs.  Before doing this, the holes were stock size, and the Edelbrock 600 was restricted and then there's the turbulance because of the step, etc.  I never had it on a dyno or a flow bench but it definitely felt stronger.

I did a clean up on the runners.  I got them to about a 60 grit finish but there was no real porting in that area.

There is potential.  I seem to remember seeing flow figures in Y block magazine at some time for professionally ported vs. stock.

Lawrenceville, GA

has anybody ever just made the four holes bigger in diameter to get the better flow but not get rid of the holes. I got a truck so i would hate to get rid of the holes for the low end torque. 

I did not notice a loss in low end after performing this mod. It felt stronger throughout but mostly on the higher end.

Some truck guys here may be running the blue thunder intake and may be able to speak on how it performs compared to the iron intake for a truck. I'm running the blue thunder but the engine is now significantly altered from the time I was using that iron manifold. Thus it can't be an apples to apples comparison.

Just start by opening up the holes. See how you like it and if you dare, take it a little farther.


Lawrenceville, GA

64,

      Read what doug said, this is the classically accepted method, and it seems to work pretty well. The Blue Thunder is of course, lighter and flows somewhat better. But the iron intake works pretty good. Maybe better on the street.

Frank/Rebop

Bristol, In ( by Elkhart) 

That is a very good picture from Charlie and shows a couple points that I mentioned.  One is that area in the cordory bottom of the runner where is starts to bend downward.  This is a real pinch point and can be radiused for smoother flow. Also Charlie polished the bores as they go down from the carb base but didn't radius them into the runner.  This radius would help flow turn the corner from downward to horizontal.

A 1/8" Dremil tool must be awfully slow there is quite a bit of cast iron to chew through.  I like a 1/4" electric die grinder with carbide tools. 

 There is a case to be made against polishing which is that a rougher surface provides more area to evaporate fuel and therefore less liquid fuel gets into the cyl.  I subscribe to this idea and do not polish intake systems.  My guess is that a sandblast rough is about optimal.

Doug T

The Highlands, Louisville, Ky.

 

              Also, laminar flow is aided by the rougher surface. Makes the air against the walls act as a bearing for the main air column. On the ehaust side of the Head (subject change) we do polish for the opposite reason, less surface to conduct heat back into the cylinder head.

Frank/Rebop

Bristol, In ( by Elkhart) 

alright. thanks for all the tips and suggestions. Now i just have to find a 4bbl intake and a autolite 4100 from a 289 or 302 and buy it.lol. thanks again.

Frank,



Your comments re: intake manifold laminar flow/rough surface/air bearing, makes me wonder how the oil introduced by a carb-spacer PCV might eventually affect performance (especially in a motor with excessive blow-by)? Could the oil build up and streamline the rough surface (helping velocity?), but also cause overly rich running?

6 VOLTS/POS. GRD. NW INDIANA