Basically the intake manifold connects the carburetor or throttle body to the intake ports of the cylinder head and in some cases like the small block and big block Chevy engine, it seals the lifter valley and provides a means of mounting the distributor and water neck.
But that is just the tip of the iceberg. The intake manifold design has a lot to do with how the engine will perform and can radically influence the engine output and the shape of the power band. Matching the intake type to the engine and vehicle specs is an important part of making a balanced combination, and the choice of what intake best suits the circumstances should not be taken lightly.
Factory intakes have run the gambit from simple log intakes with no concern for efficiency or high flow, to aluminum high rise intakes designed for multiple carbs (tri power and dual 4 barrels).
There have been "oddities" like Ford using a Q-jet on the 429 Cobra Jet, seen below:
 Small block Chevy intake bolt patterns
From 1955 until about 1986, all SBC engines had the same 12-bolt intake bolt pattern. In about 1987, GM changed the center 4 bolts to 72 degrees. This was used (except on Corvette aluminum heads, which retained the early bolt pattern) until the 8-bolt Vortec head bolt pattern took over in about 1996.
The early intake can be made to fit later 72 degree heads by slotting the bolt holes and fabricating (or buying) shims to correct the angles. Using a stock TBI Intake Manifold and this Hotrodders forum thread has the details.
Neither of the earlier intake bolt patterns are compatible with the Vortec 8-bolt cylinder head, nor can a Voterc intake be used on either of the earlier bolt pattern heads.
 Types of intakes
 Dual Plane
 High rise
As of 2012 the Edelbrock Performer RPM/RPM Air-Gap intake manifold is considered to be the best of the breed. It consistently makes as good if not better power over a wider range than any of it's competitors. In fact, it rivals the open plenum intakes like the Edelbrock Vic/Vic Jr.- even at the top of the powerband, unless the cam and cylinder head intake volume are up there pretty good. It will always make more bottom end than the open plenum intakes but that is to be expected. But what is surprising is how well the RPM performs at the top of the power band.
The original Weiand Stealth p/n 8016, which has been discontinued and replaced by p/n 8150, is considered by some who are "in the know" to be an equal to the Edelbrock RPM.
The original Holley p/n 300-36 was possibly the best of breed in its day, and it isn't made any longer either. The replacement Holley p/n 300-36S is NOT the same manifold.
The Edelbrock RPM 7101, Weiand Stealth 8016 and Holley Street Dominator 300-36 were all patterned after the dual plane, high rise intake first used on the Z-28 and LT-1 in the late '60s and early '70s. Those original aluminum Chevy manifolds were cast by the Winters Foundry and will have the Winters "snowflake" cast into the aluminum.
Professional Products makes a manifold of this design also, called the Typhoon, p/n 52021 for satin finish. Ad copy on the various Professional Products intake manifolds can be read here.
 Air gap
The so-called "air gap" intake concept has been around since the late '40s-early '50s. It has been used by various auto makers through the years, like Pontiac, as original equipment. Air can circulate beneath the plenum; the plenum is separated from the hot motor oil being splashed onto the bottom of the intake where it seals the lifter valley.
Only because the Chevy V8 uses the intake bottom to seal the lifter valley is this considered to be an improvement. Other engine that use a separate plate to cover the lifter valley have had "air gap" intakes all along. But with the SBC engine, especially if using Vortec or aftermarket heads that lack a heat crossover beneath the plenum, the engine may tend to be cold-blooded until the intake comes up to temperature from engine heat.
That means the Edelbrock Air-Gap intake will require a longer warm-up. In cold weather this can lead to drivability problems, almost like a tunnel ram. The air/fuel mixture- if tuned for a fully warmed up engine- will be lean until the engine is up to temp. This can be compensated for by the choke, but if using the choke, spirited driving is out until the choke is fully open.
As far as cutting the RPM like the AG, because the carb height and runners are basically the same between the two intakes, that mod will make their top end output equal for all intents and purposes. Not saying that's the thing to do in every case- just saying that if the same powerband as the AG is wanted, it can be had by notching the divider. For the right engine and vehicle combo (low gears, higher stall speed, light weight, more track than street, etc.) notching the plenum (or adding an open spacer) can help more than it hurts.
 Single plane vacuum loss
A single plane intake will have about 4 in/Hg less vacuum than a dual plane due to the design, all else being equal. Obviously this is not written in stone; other factors like carb size and cam specs will have a large effect on the amount of vacuum the engine has.
 Multiple carb intakes
 Dual 4 barrel
 Tri power
 Ram induction
 Cross ram
 Individual runner
 Tunnel ram
One of the more iconic intake set-ups is the tunnel ram. First used in competition in drag racing, it was made a household word by driver/engine builder/tuners like Grumpy Jenkins (RIP). He used a 331 SBC with a tunnel ram/dual Holley 4 barrel carbs against engines ~100 ci larger than his in A/Stock- and won.
 Cross ram
 Carb adaptors
Adaptors have been used since the beginning. There are adaptors to mount dual carbs to a single carb intake, to mount 4 barrel carbs to 2 barrel intakes and vice versa, there are adaptors to mount a squarebore carb on a spreadbore intake and vice versa, there are adaptors to mount a squarebore carb on a Dominator intake, etc.
One of the more useful adaptors (if it can be called that; Edelbrock does) is the Edelbrock p/n 2732. It is a simple flat steel plate and a pair of gaskets used to prevent vacuum leaks at the secondary side of the intake when mounting a squarebore carb onto an intake having a carb flange designed to accept either a squarebore or spreadbore carb.
 Carb spacers
Carb spacers in their various designs are often used as a tuning aid and for helping to make power in the range where it's needed.
 Open spacer
Tend to boost output in the upper range and will increase plenum volume. On an intake like the Edelbrock RPM (non Air Gap) dual plane intake, using an open 1" spacer will let the intake make identical power to the cut down plenum divider of the Air Gap.
 Divided spacer
These will retain the dual plenum low-to-midrange benefits while possibly raising the power peak. Tends to make the runners seem longer.
 Notched plenum divider vs. spacer vs. single plane
The 1" x 3" notched plenum divider of the RPM AG falls in between an intact plenum divider of a dual plane and an open spacer on a dual plane. It allows some of the benefits of a single plane, but w/o all of the downsides. It has been observed that using an Edelbrock C3B- with its cut out for the secondary throttle blade of the Holley 3 barrel extended out to a little over 3.5"- it performs right in between an open spacer and no spacer or "notched" divider of a dual plane. This also seems to be the trend w/the Edelbrock RPM vs. RPM Air Gap. An unmodified C3B is shown below, left. An RPM AG below, right.
An open spacer on a dual plane will increase plenum volume- something that a notched plenum doesn't do, and this is part of the reason why an open spacer/dual plane often falls in between an open plenum and a standard dual plane without an open spacer in performance. Also there's no added carb height when using a notched plenum divider like there is using a spacer. The added height of the spacer can help allow the air/fuel to make the turn into the runners with less deflection of the A/F stream when it hits the plenum floor at high rpm.
 Also see:
- Carb spacer thread on Hotrodders forum.
- Racing Technology Tuning with carburetor spacers from NHRA.com
- Here's a dyno flog by Car Craft involving 23 different intake manifolds.....
 Marine intakes
 Vacuum ports/fittings
 Vortec heads
L30/L31 Vortec engines use fasteners made specifically for the plastic Vortec intake and intake gasket. To prevent over torquing the fasteners and crushing the intake gaskets, the bolts come with a longer shank having a non threaded bottom portion so as to bottom out before the gasket or intake sealing flange/bolt hole is compromised. Below the OEM bolt is on the left, an ARP bolt, right:
 Using early intake on later (non Vortec) heads
On the 1986-back heads and intakes, the center four bolts were the same angle as the other bolts. 1987-up use a different 72 degree angle except for Corvette aluminum heads. To compensate, spacer/washers can be made or bought to allow the intake to fit.
 Using an early style intake on an aftermarket Vortec-type head
Many aftermarket Vortec style heads feature dual bolt patterns allowing the use of the early 12-bolt intakes. This has a very limited use.
Because of the height of the Vortec intake port, only the single plane 'raised port' race intakes have enough height to actually mate to the Vortec port- but even then the ports may be too wide or have other port fitment issues. The commonly available dual plane intakes do not have enough metal above the port to be ported to match the Vortec port, w/o having a vacuum leak or razor thin sealing surfaces above the ports.
 Intake installation
Generally speaking, using RTV on intake gaskets acts as a lubricant until it's set up; using it alone can let the gaskets slip out of position when tightened. Most gasket makers caution against using RTV, especially on the side of the gasket that has an RTV bead applied at the time of manufacture. Instead of using RTV over the entire gasket surface, use 3-M weatherstripping cement or Gasgacinch on one side of the gasket to hold the gasket in place. These are able to hold a gasket in position almost immediately if applied by the directions. Nothing is needed on the other side in most cases, although a light application of RTV (following the directions for skim time) or one of the non-hardening sealers mentioned above can be used around the water crossovers on both sides of the gaskets, in conjunction with the cement.
RTV should be used on the ends of a Chevy intake instead of the cork or rubber seals supplied with many gasket sets. Apply a 1/4" thick bead of RTV across the seal surface at each end of the block. If using an O2 sensor be sure to use sensor safe RTV.
 Intake heat shield
Many OE intake manifolds came equipped with a sheet metal heat shield. The purpose of the heat shield is to protect the bottom of the intake manifold from hot oil being splashed up onto it. This wasn't to keep the intake cooler, it was to keep the splashed oil from carbonizing on the hot exhaust heat crossover that runs beneath the plenum to provide carb heat.
That said, if the heat crossover has been blocked off, the shield would tend to help keep the hot oil off the intake. But after the engine has heat soaked, the underside of the intake will be close to the temperature of the rest of the engine, anyway.
 Removing the heat shield
During use, the heat shield can become occluded with built up carbon. This can be cleaned out by removing the heat shield.
Use a sharp, gradually tapered 1/4" chisel to get up under the edges of the rivets (you might need to grind a chisel down to get the best tip. Don't grind until red hot or the temper will be lost). The rivets used on Chevy intakes usually have spiral ridges on them that help hold them in position, so they tend to turn as they come out.
If a rivet gets broken off in the removal process, do not worry. Leave the broken piece in the intake. Move a little to one side of the hole where the broken rivet is located and center punch and drill a new hole through the heat shield into the intake to receive a replacement rivet. A depth stop can be used to prevent the hole from being drilled too deep.
 Replacing the heat shield
The shield should be replaced after cleaning. The intake can be tapped for screws or the rivet holes peened/staked to close them some so the rivets will have a good hold. The original rivets may be OK to be reused if they weren't damaged during removal, or new rivets can be bought to replace them. When replacing the rivets use locktite to help hold them in place.