Zoomie size?

[cad500justin]

I need to build new zoomies for the new motor. Because the new heads have a different bolt pattern.
529” injected alky/nitro 7000rpm limit
1.88 exhaust valve

Anyone ever try step tube zoomies? My current ones are 1-7/8-2” and work fine, but no dyno numbers to compare to.

I’m leaning toward 2” step to 2-1/8”, roughly 18” oal? (Old ones are 25”oal) But could go bigger? Could just do one size tubing without a step?

Anyone ever play with stainless for zoomies? 

[JrFuel Hayden]

I have there is no advange of step headers on alky/nitro, but on gas it helps

[THE FRENCHTOWN FLYER]

I never did buy in to the advantages of stepped headers. From a thermodynamic standpoint as the gas flows down the pipe it would get cooler, if anything, necessitating a transition to a smaller pipe size, not a larger one, to maintain gas velocity in the pipe. I get that you might suggest a megaphone effect by going larger but I don't think it would be measurable. From a fabrication standpoint I see where it would make building headers much easier because now you don't have to make perfect butt joints - just slip the next section into the previous one. I SUSPECT THIS IS THE REAL REASON WHY THEY ARE TOUTED BY HEADER FABRICATORS.

In automotive engineering circles we used to have a tongue-in-cheek saying," If you can't fix it, feature it."

As far as zoomies, a singular tube size just looks best to me, with the possible addition of a reinforcement sleeve near the flange, like T/F designs.

[Roger]

I agree, a stepped header just slows down the exhaust flow as it moves past the step into a larger area and that slows flow a little bit. With as short in length as most zoomies are, there probably isn’t much if any, difference in performance between straight and stepped. Now if you move in the other direction and make a set of “zoomies” that have a pipe length of about 32” in your case, then you could get back the inertial scavenging effect of equal length headers, less the collectors. Could be a little known speed secret from the past:)

[rooman]

 I can't see any advantage to stepped diameter zoomies but I don't think that the teams that do extensive dyno testing (pro stock) etc would be running them unless some advantage (power/torque etc) showed up on the dyno. I do know that my friend Mike from the Fab Shop here in Indy is in favor of them (along with big radius bends) and that his step headers have seen lots of dyno time.

Roo

[glofria]

When it comes to air flow, the smartest guy I know (my dad) told me once there is no advantage to stepping the tubes and velocity will increase with bends.

cad500justin he's seen you car and thinks it cool. If you call him and tell him what your doing he would gladly give you some free advice.

Larry @ Valley Head Service (818) 993-7000

[wideopen231]

With zoomies no advantage. With collector headers there is some power gain. Not opinion but actual dyno. We did a 565 and header change from 2" to 2 1/8 motor gained 8 hp. Changed to  2 1/8,2 1/4 to2 3/8 step headers motor gained almost 20 hp. Then richened motor and picked up few more. Now was 1000 hp engine. May not be worth as much on lower power engine.

[THE FRENCHTOWN FLYER]

On a related question: Has there ever been a R&D dyno effort to see if "D-shaped" bends help on zoomies, similar to the D-port configuration on many exhaust ports in cylinder heads? I remember a text book I used many years ago to teach an auto shop class that extoled and explained the virtues of a D-shaped exhaust port. It is not a new concept. How about applying D bends in zoomies, or any headers for that matter?

[glofria]

Air will always flow with more velocity on the outside of a bend. The inside is more of a dead air space.

My dad (proud son I am) worked with Smokey Yunick back in the 60's and did a lot of testing on the flow bench. One of the concepts that came about is the "D" shaped exhaust ports and why you see them as cocked on big block Chevy heads.

NOTE: after rereading what I typed, some corrections made.

[THE FRENCHTOWN FLYER]

So my question still stands - if D-ports work on heads (which I readily admit it does) why don't we find the concept on header bends?

[glofria]

Because doing so will have adverse affect of what is intended. You want to keep the velocity at a constant rate throughout the header. Otherwise you have air that will get trapped before the bend and a reversion affect (reversal of air flow) will happen when both valves are off the seat and will go back into the cylinder. That is also why the opening of the header does not match the exhaust port runner; it acts like an air dam. Once the air exits the header there is a slight back-draft that goes back into the header.

I asked dad that question this morning on the way to breakfast and he also stated that air does not like to change directions. On the manifold side you want it to be as straight as possible because the air is heavier (mixed with fuel) and relies on the piston cycle to draw air in. On the exhaust side the air is obviously hotter and thinner, thus bends help with the exit velocity.

There are other benefits to bending zoomies as oppose to straight weed burners like propulsion and so on, but you can learn all that by watching the NHRA big show.

There is a lot more that can be explain, but I not going to type this all out. However, you get the gist of it.

[THE FRENCHTOWN FLYER]

Air would only get "trapped " before the bend if the cross sectional area is reduced at the bend, like cheap muffler shop pipe benders. But suppose the D shape does not have a smaller cross section by design. Then the advantages of a flat radius floor and resultant laminar flow (or at least reduced turbulence from the bend / change in direction of flow) from the D-shaped bend might actually help to maintain gas velocity, no?

Mismatch at the exhaust port / pipe juncture is a whole nuther discussion and is probably straying far afield of the original topic.

[cad500justin]

Guys thanks for the detailed responses!
Flyer, that was my next question! You should line up the top of the pipe with the top of the port right?

[THE FRENCHTOWN FLYER]

Generally speaking, yes.

If the ports are well designed I do not think pipe misalignment / mismatch is necessary or useful. My experience is with non-supercharged applications so others with blower experience may know better.