94-99 3100 Large Port Swap

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Small port 3100 owners can gain a considerable amount of power and gas mileage from doing the large port manifold swap. The intake gasket kit for instance, needs to be a hybrid type for your 94-99 heads and the large port manifolds. The heads, while compatible, have a different pin locator for the intake gasket on the head. 94-95 owners will need to use an EGR adapter to use the newer manifolds, and 96-99 owners will need a new EGR as there is no adapter for this. The electrical connector is the same for 96-99 as 00+, and good used EGRs are available on ebay or a local recycling yard. For off road use applications, an EGR delete will take care of the situation.

1994 small port upper intake manifold on the left, and the 2000 large port 3100 upper intake on the right. This is the easiest way to tell what manifolds you have.
The lower intake manifolds are compatible with al 3100 and 3400 cylinder heads.
Here is what the stock small port manifold looks like compare to the stock gasket.
This is just the top of the lower intake. The rest of the runner is much different between these manifolds. That is a 2000+ 3100 (all 3400) intake gasket.
A lot of the 94-99 3100 (if not all) are threaded here for the coolant return from the heater core. There is a fitting that threads into the manifold and then the tube is quick release style.
The large port manifolds have a press in coolant fitting. You can either use the newer tube, or the manifold can be threaded to accept the stock coolant line setup. .
1994-1995 EGR style. An adapter to use the 2000+ manifold with your stock EGR is available (shown below).
1996-1999 EGR style. This is shown for informational use only. If you have this style EGR, you will need to get a newer EGR.
2000+ style EGR. This is what you will find on the best manifolds to date.
Here is the EGR adapter setup for 94-95 EGR, made from billet aluminum by TCE. (No longer available)
2000+ on the left and the 96-99 EGR on the right. Same connector.
Different location for the egr tube to the rear exhaust manifold.

For 1994-1995/early 1996 owners, the rocker arms will also be important for the large port swap. The older style stamped/pivot ball rockers won't clear the newer manifolds and need to be clearanced to fit. Please specify that you need this clearancing done if you are not using roller rocker heads.

Roller Rockers
Non Roller Rockers
Rocker to Manifold Contact

Once the manifolds and EGR have been taken care of, its time to decide what route to take with the injectors/fuel pressure regulator. With no tuning options,1994 and 1995 owners, either a more subtle upgrade path should be used, with stock injectors and rail being reinstalled. This will support the manifold swap no problem and present no drivability issues on the stock computer.

OBD2 owners can use HPTuners to tune for this upgrade. We also have the ability to reflash these PCMs. Tuning allows more upgrades to be done and drivability remain along with improve performance. Multec 2 injectors are 22.5# vs 19# for the older injectors, and they are superior in not clogging in comparison. The newer injectors run on a 55psi rail instead of the 43psi of 99 and older. This means an adjustable regulator or stock 00+ regulator should be used to keep the spray pattern at its best. To run the newer injectors, your injector wiring harness will need to be modified to change the injector connectors.

Multec 1 (left) vs Multec 2 (right).
Injector connector difference.

This last part could be the first mod on a small port 3100. The stock 52mm throttle body will flow approx 281cfm at 1.5Hg. A 56mm TB can be found on any 96-05 3400, and while it is not a direct bolt on due to the linkage, it is as simple as swapping the linkage for that drop on modification. This unit flows approx 327cfm at 1.5Hg.

Since this is the stock size for the large port manifolds, it should be considered the minimum upgrade on throttle body size for this swap. 56mm is still a bottleneck for the intake however, which is why the 62 and 65mm throttle bodies are popular. A 62mm unit flows approx 400cfm at 1.5Hg and 440cfm at 1.5Hg for the 65mm. There is a point where the throttlebody becomes no more effective, but considerably harder to drive on the street, and is the reason 65mm is the largest recommended for this swap.

The throttle body on the left is the 94-96 style, if not more years included. The middle is a 99 alero 3100 style, which is also a 52mm plate. The unit on the right is a 99-05 grand am 3400 56mm TB. The linkages are closest in the picture
This is a 52mm plate on top of a 56mm plate.


If you use a 2nd coolant temp sensor (single wire) in the rear head, you will need to either relocate this sensor by drilling and tapping into a coolant passage, relocating to a block location, or using a newer 3 wire coolant sensor in the lower intake and wiring it up for both sensor connectors. A and B represent the stock 2 wire, and C goes to the single wire connector.

Product CodeArticleLPS

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