TBI 305 Budget Power Guide

Useful resources for professionals and DIY enthusiast!

Why Build a 305?

The 305 isn't anyone's first choice for a build so this guide is budget centered and makes use of factory parts, salvage yards, and DIY effort to keep the cost down while building one of the most common platforms in North America. Most of these mods, with the exception of the heads, are intended to carry over to a later 350 swap while still benefiting the current platform.

If you follow all of the steps you still won't be the fastest car at the drag-strip, but it should feel like a v8 and put a smile on your face whenever you press on the gas.

This is write-up is intended for those of you building a 305 TBI third gen Camaro or Firebird, though some of these mods will apply to anyone with a GM TBI or a 305cu, 5L engine:

These engines like between 1 and 4 degrees over base timing for best performance.

ACCEL Ignition Control Module 35370 or comparable should be used for these applications, avoid ACCEL 35362. They are both listed as compatible but 35370 is based on GM's performance module and 35362 is based on GM's economy module. These modules have different latencies that are taken into account by the ECM. Use of the economy module will result in very poor performance in these vehicles. Additionally, if you are upgrading the ignition system you need to gap the plugs wider to receive the benefit.

The best mod for the money on these throttle body units is to double up the gasket under the injectors - raising them above the bores and allowing more CFM's to enter the engine. Years ago there were "kits" with a double thickness gasket for this. Swapping to the 350 injectors is also a worthwhile mod. You will be able to feel these changes. Some have even bored intake to work with the 454 throttle body as well.

There are still short tube headers available for third gens. I like the 2460HKR by Hooker, but there are plenty of options. For those of you willing to loose ground clearance for better torque, there are several good long-tube choices as well. I would recommend replacing the factory manifolds and exhausts as soon as possible as it made the most noticeable difference on my, at the time stock, 305 TBI Firebird Formula. Just remember that if you are pushing for 350 power you need an exhaust to accommodate that - shop for headers for a 350cu engine.

The factory rocker arms have a tolerance of +.1 to -.1. Replacing them with literally any reputable aftermarket will improve performance, even if they are also stamped rockers.


The H.O. 305 heads should be considered for any third gen owner trying to get the most out of their 305 as they flow much better, even without porting and valve work. Look for castings ending in 416 or 601 at the scrap yard, they are the first gen of Vortec, have 55cc closed chambers, and will support 1.94 intake valves and 1.55 exhaust. They can often be found on mid 80's trucks with 305s. I do not recommend up-sizing the valves unless you are boring the block and de-shrouding the heads.


Most of these engines started with about a 9:1 compression ratio. Raising this to 9.5:1 - 11:1 is a good way maximize your power output and gain fuel economy. Just be aware that higher octane fuel is required at higher compression rates and you may have to retard the timing if pre-detonation occurs.

The H.O. 305 also featured the 929 cam that was originally released in the 300HP 327cu Chevelle if you like the retro sound and good low end to mid-range power. I've had good results from Melling's MTC-1 with .422 INT. lift, .444 EXH lift, 278/288 SAE duration. If you are looking at aftermarket cams don't go over .48 lift unless you have made major alterations or the valve will contact the cylinder wall.