Jebbysan

GM used to mill the whole plenum out square as much as they could and machine the divider clear out down to the floor.
Also, any radius work at the floor level where the upper and lower meet, or the "cliff" is helpful as well as any area where the mill leaves a sharp edge.
When I decide to put the sneaky Winters LT-1/DZ manifold on mine......this is what I will do.......

Jebby

69427

That's a "question" I've always been curious about. If the idea is to open the plenum sufficiently to allow both sides of the carburetor to feed all of the runners, why is/was Chevrolet doing one thing on the manifold (machining the wall completely out), and recommending something else in the Power book (leaving a quarter inch of wall remaining). And, once you get that close to the upper plenum floor, is that quarter inch wall doing anything productive?

BLUE1972

Machining the center was a way to get the open "chamber" single plane benefit in a dual plane. Dual plane manifolds are / were better on the street in low power / low RPM driving - less stumble.. etc.

Way back in the late 60's there was an article with tests to "determine" what worked best, and how much to machine out. (for street / strip cars)

Without getting too technical - it has to do with the reverse pulses and signal to the carb.

If you understand how a "tuned pipe" works on a chain saw or motorcycle you can understand the theory of machining the divider on the manifold. It's not the exact same but it has do with the waves and return waves from the closed valves...

It's quite complex and the math is not easy... + it's RPM dependent.

IT's like a Tunnel Ram on the street = looks cool but a PIA and great at the track.

7t9l82

I wonder when someone will try it on one of those junk L-82 aluminum intakes.everybody back in the day used to cut down the divider, because generally it worked.

v2racing

Actually they missed what is going on here. If you look at all the runs they all have a torque dip at the start of every run. This is an exhaust reversion induced torque dip from running open headers. I've seen it a thousand times if I've seen it once. It can be cured with anti reversion methods such as just putting a muffler on the header. What they are really missing is that when you have an open plenum with a larger area there is less signal to the venturis. The reversion waves are lessened in force by the large volume plenum so the backwards airflow produced by reversion waves do not have as big of an effect on the air fuel curve from double carburation. With the divided plenum there is less dampening of the reversion waves and they actually push air out backwards through the carb and then are drawn back through again making a way overly rich air fuel ratio. The whole test would have looked different if they would have had mufflers on the engine.

Mike

PAmotorman

that is because without the lip you will have fuel spill over as there is always raw fuel puddled in the floor of the intake

Jebbysan

Not sure.......but the LT-2 454 Test car was milled out and down as much as possible........this was 1970. I think that if you leave some divider in it....it would cause some turbulence at WOT when it is moving some real air.......the "divider" to me would just get in the way of flow left and right.
Of course at this point, the only real reason to run a dual plane like this is for looks........just step up to a single plane, it is going to signal the same but flow better......

Jebby

69427

I agree with you there. My problem is I just like the stock/sleeper look.

Jebbysan

cv67

Milling it down if you have overlap gets shared with all cyls...personally I think you eiher get a good dual or single plane setup you cant make one do both.

Can only get so much through those longer runners anways

BLUE1972

7t9l82

Well there's a few minutes of my life I won't get back. Boring! Sorry , just expected nothing else