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305 TPI to 350 TPI Conversion

If you are looking to convert your car from a 305 TPI to a 350 TPI setup, then you should find the information below rather useful. These are all the needed changes to do the conversion.

First of all, the TPI intakes (manifold, plenum, fuel rails, throttle body, runners) were exactly the same between 305 and 350 engines of the same year. There were no differences in these parts whatsoever between one engine size and the other during any year (there were however differences between the intake manifold and fuel rails between the Camaros and Corvettes).  For example, a 1987 intake was the same for both a 305 and 350. Please note that this does not mean all intakes are the same throughout the years, there are differences between a 1987 intake and a 1992 intake for example.

All your belt driven accessories, brackets, exhaust, etc... will transer over from the 305 to the 350 without any problems. Once you install the TPI intake on the larger engine, you will need to note the following items that need to be changed. These are the only things you will need to change or worry about to properly convert to a 350:

The size of the injectors was a very important difference between 305 and 350 applications. ALL 305 TPI engines came from the factory with 19 lb injectors. ALL 350 TPI engines came from the factory with 22 lb injectors. The Corvettes did not have injectors any larger than the Camaro, as I have people often asking. If you want to use 19 lb injectors on a 350, or 22 lb injectors on a 305, you can, but you MUST have the injector size in the PROM changed. Otherwise, you will run into fueling problems.

Knock Sensor
The knock sensor physically looks the same between the 305 and 350 version, but they do are not the same. This is very important, and should not be overlooked. The knock sensor is responsible for detecting detonation. Detonation produces a specific frequency that travels through the engine block. This frequency is different depending on engine size. In addition, it is important to note that the 350 knock sensors were different between the 85-89 TPI setups and the 90-92 TPI setups. You must use the correct one for your application. The 305 knock sensors were also different between 85-89 and 90-92.

ESC Module
The 85-89 TPI setups used an external ESC module which was mounted by the firewall. This module was different between 305 and 350 engines. Changing this is as simple as disconnecting the 305 module, and plugging in a 350 module. The 90-92 TPI setups did not have an external ESC module. Instead, the ESC circuitry is contained inside the PROM. For this and several other reasons, it is important that you use the correct prom for the engine size you are using.

Although there were several different ECMs used depending on what year TPI setup you have, the same ECM was used for 305 and 350 engines for a given year. The difference between the engines was the prom. In other words, the 86-89 350 TPI engines used the 1227165 ECM, and so did the 86-89 305 TPI engines. The 90-92 350 TPI F-Body engines (Camaro and Firebird) used the 1227730 ECM, and so did the 90-92 305 TPI engines. The 1985 305 TPI engines used 1226870 ECM, which is the same as the 1985 350 TPI ECM (350 was only available on the Corvette in 1985).

The Corvette used the 1227727 ECM on the 90-91 350 TPI engines. Although the Corvette was never available with a 305, the 1227727 ECM can be used on a 305 if desired.

This is probably the most important change that needs to be made. Regardless of what ECM you are using, the 305 prom is setup very differently from a 350 prom. Which prom you need will depend on what ECM you use, transmission type, engine size, and a few other details. Contact us if you need help finding the correct PROM for your application.

If you have a 90-92 ECM, and you are having your PROM reprogrammed, you must start with a PROM that was originally for a 350. The 90-92 PROMs have the ESC circuitry onboard, and that cannot be changed.

The 85-89 PROMs can be changed from one engine size to another without worrying about ESC within the PROM, since the module is external on those years.



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