Small Port vs Large Port 3100 Manifolds

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GM introduced the Generation III 60V6 3.1 liter V6 in the last half of 1993 with the Oldsmobile Cutlass. In 1994 these engines replaced every Multi Port Injection 3.1 liter in GM's lineup, other than the Cavalier and Lumina which used the MPFI motors for one more year. Although the upgrades over the Generation II were significant to say the least, there is little reason to port these manifolds. Referred to as "Small Port" , these will never perform as well as "Large Port" manifolds found on all 3400 engines, and the 2000 and newer 3100 engines. A stock large port easily outflows and outperforms the best of the small port manifolds.

Small PortLarge PortGasket Comparison

If you own a 1993-1995 3100 engine, you may want to check your rocker arms before upgrading to the large port manifolds. The non roller rockers have an interference issue and require clearancing on the lower intake. In the pictures below you will see the roller and non roller rockers. Unfortunately, not many non roller rocker heads have the notches in the rocker mounting boss to accept the roller rockers found on 96 and newer motors (don't assume, even some 96 3100s have non roller rockers). Check out the cylinder head section for more information on why upgrading your cylinder heads to even a newer stock part could benefit you.

Roller Rockers
Non Roller Rockers
Rocker to Manifold Contact

 

To better illustrate the significance of the large port manifolds over the small port, check the following flowsheets.

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Product CodeArticleSPvsLP
ConditionNew

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