[Craig Hendrickson]

A thread late last year discussed H-O Racing Specialties "Rocket Box" plenum spacer used on my 1974 T/A 455SD NHRA SS/KA national record holder. At the end of that thread I promised to disclose details of its design. I did so on April 1 at Pontiac Heaven during the tech session given by Kern Osterstock and I.

The purpose of this thread is to post that information and expand upon it as response queries occur.

Here's a scan of the installed "Rocket Box" out of the PHR article on my record setting TA:


Obviously, one cannot tell anything about the inside of it. Most people thought it was just an open plenum deal. But, no, it was not. More to follow...

[GOAT490]

Take it off Baby, take it ALL off!
Oh, you are the tease!!!

[Craig Hendrickson]

Originally Posted by GOAT490

Take it off Baby, take it ALL off!
Oh, you are the tease!!!

I'd guess you did not attend Pontiac Heaven?

[GOAT490]

Only in spirit!

[Craig Hendrickson]

As a point of reference, here's an approximate cross-section of a QJet on an open plenum spacer:

[Craig Hendrickson]

Here's one primary cross-section discharging into the open plenum (rotated 90deg). Therein lies the inefficiency.

[Craig Hendrickson]

QJet discharge into open plenum is similar to this: an orifice plate flow meter! An orifice plate flow meter has a "CD" (Coefficient of Discharge) of about 0.68 or 68% of maximum efficiency compared to a straight through pipe. A QJet venturi into a straight hole throttle plate into an open plenum spacer is not as "bad" as the orifice plate, but it's not all that good either.

[Tom Vaught]

Good Info here:

http://rothfus.cheme.cmu.edu/tlab/fluid2/fluid2.pdf


Pressure Recovery Factor
LOW-RECOVERY RATE: A design that dissipates a considerable amount of flow-stream energy due to turbulence created by the contours of the flow path. Consequently, pressure downstream of the valve vena contracta recovers to a lesser percentage of its inlet value than a valve with a more streamlined flow path. The factor does not vary with travel to any significant degree.

HIGH-RECOVERY RATE: A design that dissipates relatively little flow-stream energy due to streamlined internal contours and minimal flow turbulence. Therefore, pressure down stream of the vena contracta recovers to a high percentage of its inlet value. The factor of a high recovery design will vary with its design.

Tom Vaught

[SD421]

Craig, If Im following you correctly, Im very interested to see what you changed in the carb.
-Don

[johnta1]

It's changed in the 'box', not the carb I believe.



Craig, I wish I could have been there.
Sounds like a great time by all.

[GOAT490]

It will be interesting to see this. So should each entrance(carb) to and exit(intake port) from the plenum have a smooth/tapered mouth?

[Half-Inch Stud]

Seems to me the Primary CCA has a flow impedance ( velocity & mass-air flow) much higher than the Plenum's low impedance, that an impedance transform is needed.

If that was important (improvement to hidden flow, Performance, mixture) then I can think of a couple transitions:

1). Taper the Secondary outlets toward the Primary outlets to combine flow.

2). Taper the Primary outlets toward each other

3). "Turtle the flow" at Plenum's bottom to difuse the flow toward Intake runners.

Seems to me that a Turtle may be debated with a Flat-floor Plenum. Because the Flat-Floor Plenum has ability to increase flow at high velocity. Bernouli said so.
================================================== ===============
Finally, If a fella honors the CYL intake pull sequencing (firing order), then they would isolate the Cyl 7Path with a wall to pull from the PAssender-side Venturis. In effect 5 CYLs pull from the Right venturis while 3 CYLs pull from the Left venturi. I would be left guessing how a "Rocket Box" would attempt to blend the CFM pulls.

[Craig Hendrickson]

As Tom pointed out above, a Pressure Recovery Factor with a LOW-RECOVERY RATE is like a throttle bore dump into an open plenum, i.e., an orifice plate with CD of 0.68. A Pressure Recovery Factor with a HIGH-RECOVERY RATE is like a Venturi Flow Meter, which has a coefficient of discharge of about 0.96!

[Craig Hendrickson]

Because of my background with NASA Apollo, the Venturi Flow Meter reminded me of an Apollo Saturn V rocket F-1 engine:

[Craig Hendrickson]

Rocket engine exhaust nozzles are designed to be extremely efficient in order to extract the maximum thrust out of the combustion gases and the transition from combustion chamber to ambient atmosphere. Likewise, I reasoned, a rocket exhaust nozzle transition shape between the discharge from a carburetor throttle bore should be more fluid dynamically efficient than a simple open plenum.

One design question was: what shape? The well known (in rocket propulsion circles, anyway) 80% "bell nozzle" seemed like a good candidate.

[Half-Inch Stud]

Of Nozzles and Orifices...you appropriately chose to Nozzle

Factory also chose to Nozzle, while Open-Plenum Single-Plane Manifolds revert back to Orifice.

[Tom Vaught]

[/QUOTE]


Very nice explanation so far Craig, My comment in my sig still fits:

"Engineers do stuff for reasons"

Tom Vaught

[Craig Hendrickson]

Here's a scanned picture of the Edelbrock Torker I with the Quadrajet throttle body outlet holes overlaid. A modified Torker I was what I used at the time. Obviously, different intake manifolds were available and allowed in NHRA SS both then and now. The main point of this illustration is to show approximately (since I used a graphics program for the overlay) how the throttle body holes discharge into the open plenum from a top view.

As everyone probably now knows, the Edelbrock Torker I was a race-oriented open-plenum (360 deg) intake masquerading as a torque-oriented street manifold. I believe that the constrictions (shrouding flanges) at the 1:30, 4:30, 7:30 and 10:30 clock positions were to speed up the airflow for street operation. Also, the "volu-step" in the plenum floor was to account for the different flow velocities coming out of the primary and secondary throttle bores, as Edelbrock touted at the time. For my racing purposes, I removed those constricting flanges, removing the "speed up" impediments at the four corners. However, this had little to do with the "rocket box" technology soon to be applied.

[blueghoast]

HUH------- I only went to high schoooooo

[Cliff R]

Me too....but I've been on the dyno scores of times with the Q-jet, and have observed some DRAMATIC power differences with single plane intakes and various spacers used on them.

Several years ago, we did a back to back dyno run on a 455 using a stock Torker I and my ported iron intake, with my own 1977 Pontiac Q-jet. No spacer was used on either intake.

The stock intake made 57 MORE HP than the stock Torker I!!!

I know that this happens because of the basic design of the Torker I and the relationship between the Q-jet carb and the intake.

A few years ago we were on the dyno with a 505cid engine that should have been up near 600hp. We were using a Tomahawk intake with my own Q-jet bolted directly to it. We could BARELY get 500hp out of it.

After many pulls and much tuning, we swapped on a Victor intake and HP950 Holley and it went up around 610HP on the very next pull.

During the "down time", a custom spacer was made for the Tomahawk intake to use the Q-jet. About an hour later we were rewarded with a pull that rivalled the Victor/Holley set-up. The power numbers were 102HP greater than the previous pulls using no spacer at all!

Lacking all the scientific stuff as to what's going on here....I can just tell you that I'm not surprised in the least that a custom spacer would be worth a LOT of power on a single plane intake when using a Q-jet.......Cliff