Originally Posted by xbcoupe
Apologies if this is a completely stupid question but here goes ...
Why does a dual plane manifold suit an automatic transmission and a single plane, a manual ?
(ready to be labelled a dumby now !)
Not a dumb question at all. In fact, these are generalizations. Usually, for a modestly built street engine, an automatic transmission and a dual plane inlet are "better" than a single plane with the auto trans. The reason is because a dual plane inlet separates the plenum into two parts. One part feeds all 8 cylinders and is connected to one-half of the carburetor. The other part feeds all 8 cylinders independently, usually from the other half of the carburetor.
If you think of the carburetor as an air metering device in that it actually restricts air flow through the engine as a function of its throttle valves (and physical size, in some cases), then you can see how one-half of the carburetor feeding the engine will result in the low-pressure signal (some call this a vacuum signal) being concentrated across a half of the carburetor rather than the entire carburetor as in the case of a single plenum inlet. This usually results in greater "responsiveness," as the air to fuel mixing capability of the carburetor has more "signal" to work with.
The dual plenum inlet, due to its design of separating the feeding of the engine into two separate paths, also takes more space in the area underneath the carburetor, resulting in a smaller plenum volume. That means that the "primary" side of the plenum in a dual plane has less total volume (space) than the open plenum inlet. Smaller volumes of air react more quickly to changes in pressure than larger volumes of air. The dual plenum inlet usually increases lower RPM velocity of the movement of the air/fuel mixture inside of the manifold.
The open plenum inlet is basically a big, open space underneath the carburetor. At lower RPM, the air tends to react more sluggishly, which results in generally poorer low RPM engine responsiveness. Responsiveness is basicially related to the rate at which an engine will gain RPM at the introduction of increased throttle opening. The time that it takes for the cylinders to fill at low RPM is less for a dual plane than a single plane manifold.
At some point in the program (RPM), the dual plane refuses to flow more volume and the separation of the two "sides" of the carburetor become a liability rather than an asset. This is typically around 5500 RPM for most dual planes. However, a well-designed street engine on pump fuel is likely to see most of its duty cycle at RPM well below 5500 RPM. The dual plane inlet is a system that has two distinct "flow quality" properties. One at idle to about 3000-3500 RPM the other occurs at RPM above about 3500-4000, depending on many factors of the entire engine/transmission/vehicle combination. Some may argue that it has a 3rd flow quality property that is WOT (wide open throttle), but really that tends to be a mixed quality of the transitory state between the first and second properties.
A single plenum inlet works well at an RPM range usually starting at around 2800-3000 RPM. The single plane is considered to be an "RPM extender" type of manifold, meaning that you bolt one one when you want to increase the RPM where maximum work is being done in your engine. The dual plane tends to see its maximum work being done at RPMs between ~1500-2800 and 3000-5000. The single plane tends to be something more like 3000-6500+. As one can see, it practically does no meaningful work at the first set of RPM ranges that is done by the dual plane. This is because the engine is "overwhelmed" by the volume of air in the plenum and must catch up (in RPM) before it can make good power and effectively enrichen the fuel mixture by producing enough "vacuum signal" (really, a strong enough low-pressure area) to cause fuel to flow in the desired quantity through the carburetor venturii.
If we think of burning fuel as making power (brake specific fuel consumption), then we can correlate the amount of fuel burnt to power produced. The properties of the single plenum inlet cause the air to move more slowly at lower RPMs, however, at higher RPMs, it allows the air to move in volumes exceeding the capability of the dual plenum. For this reason, the single plane is suited for "extending" the RPM where the engine does its work (burns more fuel).
The manual transmission is a design that uses less engine power than the automatic transmission design. The automatic has a fluid pump, similar to a water pump, that takes power to operate. The automatic has a torque converter/multiplier that has efficiency "issues" based on its design and how it operates. Basically, the power need to turn a manual transmission versus an automatic transmission differ by a generalized factor of about 15 (or more) percent. In other words, if you had an engine making 15 horsepower tied to a manual transmission, at the tailshaft you'd get roughly 15 HP minus about 1.5% for turning the transmission. On the automatic transmission, you'd get 1 HP at the tailshaft as all of the power would be consumed by just turning the transmission. This amount is a generalization considering the typical transmissions associated with factory Clevelands and do not account for the variety of locking converters, overdrive transmissions and other more modern "advances" in transmission and torque converter design.
When we think of what makes a good street engine, we know that most of its life is going to be putting around at speeds well below 3000 RPM. A lot of its life is going to be spent idling and on highways. When we think of what makes a good drag engine, we know that most of its life is going to be spent well above 3000 RPM and rarely idling or driving on highways.
A manual transmission is a better choice for economy for nearly every purpose. However, in racing, automatics have been extremely popular because of their very precise shifting qualities. No one misses a shift with an automatic or shifts into 3rd instead of 5th. Automatics are also generally more easy on drivetrain components.
Part of the idea of a manual transmission in racing is that you spin the engine up and drop the clutch. You burn up a bit of clutch disc material and/or pound the hell out of the rear end/tires and launch like a super stocker on steroids. In my younger years, I used to build high RPM engines and I would drop the clutch at 7000 RPM with manual transmissions and small block Fords.
Over the years, I got lazier and decided not to replace parts nearly so often, and began using automatics in everything. There is nothing quite like a 9000 RPM alky Cleveland banging through the gears at 160 MPH in the quarter. There is also nothing like not having to replace the clutch after every 6-8 launches. Soft enough to launch, hard enough to hook on the top end. I had an open account at 10,000 RPM Racing in those days. I probably rebuilt my entire suite of 10 or so top loaders every 3 months. I used to be able to change clutches and set pressure plates in my sleep. Frankly, I got tired of getting under the car and getting covered in 90w and asbestos clutch dust.
Back to the point, the dual plane makes a lot better low speed torque on just about any engine while the single plane makes better high RPM horsepower on just about any engine built to spin those RPM. Another generality, it is usually always (oxymoron) better to turn less RPM and make more low RPM power/torque than spin the engine faster.
The short answer is that a dual plane makes better low RPM power/torque than the single plane. The dual plane can not flow enough volume to allow most engines to spin well above about 6000 or so RPM. A couple of years ago, I built a 383" stroker using alloy heads and a dual plenum inlet with a 750VS carburetor. It made something like 440 HP and 360 TQ and ran on PULP just fine at 9.5:1 compression. It pulled very strongly to about 6000 RPM afterwhich, it "laid over." My 393 stroker makes over 600 HP, but doesn't do anything until about 4000 RPM. A 4000 RPM car on the street is a waste. You can't really appreciate it, though there is something to be said about the "pain/pleasure" of driving a strip beast on the street. An alky Cleveland with a 4.88 gear set in a lightweight Mustang never gets out of 2nd gear and never sees 1st, 3rd or 4th on the street...unless the highway gets involved. You just drive around everywhere in 2nd and, if the local officials seem to mind, then you upshift into 4th to keep it quieter while going by them. A 16:1 alky engine with a single plenum, good alloy heads, Ron's Terminator and a decent solid roller cam makes 680 HP and 600 TQ with a launch RPM of 4500 RPM from a +.030" 351C. Combined with a 2600# car, a 4-speed or better, a Richmond 5 speed and 4.88 gears (3.90 with the 5-spd) and you have a car that nothing on the street that can catch it for any distance up to about 1500-2000'. However, a good C4, 302C heads, pump petrol engine with a good converter and exhaust with a Weiand Xcelerator dual plane will be much more fun to drive, especially between refueling stops. There is something to be said about pulling the front wheels 5-feet in the air whenever desired, too!