Tech Tips
305 CID TPI Engine & Camshaft

Courtesy of 3rd Gen Member ~ Vader

Your original 1986-1992 Chevrolet TPI engine should has the following characteristics:

Automatic transmission

1986 LB9 V8 9.5:1 compression, 5.0L(305CID), 2.73 standard rear axle ratio Horsepower-195@4000 RPM Torque- 295@3200 RPM TPI induction system.

1987 LB9 V8 9.3:1 compression, 5.0L(305CID), 2.73 standard rear axle ratio Horsepower-190@4000 RPM Torque- 295@3200 RPM TPI induction system.

Manual transmission

1987 LB9 V8 9.3:1 compression, 5.0L(305CID), 3.08 standard rear axle ratio Horsepower-215@4400 RPM Torque-295@3200 RPM TPI induction system.

All 305 TPI (LB9) engines (1985-1992) came with one of two basic cam grind profiles. An easy way to identify the type of cam on an LB9 car is to look at the tach: the lesser cams have a yellow line at 4500 and red line at 5000; the hotter cams have a yellow line at 5000 and red line at 5500. All camshafts 1987 and later use hydraulic roller lifters.

Cam info on 3rd Generation F-bodies

Model

Tach

GM P/N

Lift

Duration

Lobe

Yelw

Red

In.

Ex.

In.

Ex.

Sep

In.

Ex.

1986-87
305 AT (LB9)

4500

5000

10088155

.350

.384

179

194

109.0

108

-112

1987
305 5spd 350 AT
(LB9-L98)

5000

5500

14093643

.404

.415

202

207

114.5

115

-114

305 LM1
Police Pack.

   

14093640

.383

.404

191

202

112

108

-116

1988
1989
305 AT

4500

5000

10088155

.350

.384

179

194

109.0

108

-112

305 5spd
350 AT

5000

5500

10066049

.415

.430

207

213

117.0

116

-118

1990-1992
305 5spd
305 AT

4500

5000

10088155

.350

.384

179

194

109.0

   

305 G92 5spd
350 AT

5000

5500

10111773

.413

.428

202

207

114.5

116

-116

All varieties from 1985-1991 used MAF sensors instead of speed density sensing for fuel metering control. Fortunately for you, the MAF is more accurate. Unfortunately, it is more costly to repair and maintain, and the costly MAF sensor is susceptible to destruction from vibration and backfire.

The engine uses self-aligning rocker arms with a stock ratio of 1.5:1. Performance and economy can be improved by installing 1.6:1 ratio rocker arms. These must be of the self-aligning type, and if you are going to the trouble of replacing the rockers, you should probably spend a few extra bucks for roller rockers. The OEM valve springs can be retained with 1.6:1 rockers if the original camshaft is used.

Another shortcoming of the design is poor flow through the intake air plenum. More improvement can be realized by clearing the excess material from the throttle body area floor and matching the ports to the runners than by installing a larger throttle body or air foil. And the modification only takes time and a few gaskets, not a lot of aftermarket parts.

You should also check the PROM installed in the ECM. This is the chip that programs the brain. GM upgraded many PROMS after initial release of the vehicle, and the upgrades address many driveability and performance issues discovered by owners.

There is information in the GM TSB database, and
on this website regarding upgraded PROMs, and your local dealer service department should be able to help you. I installed a Hypertech PROM in my 86 because of some difficulties I had. I later discovered and installed the GM replacement (purchased for less than half of the Hypertech chip) and found even better performance than the aftermarket PROM. And the GM PROM retains 50-state emissions legality guaranteed by GM.


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