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305 Budget Power Guide: 5L Upgrades and Mods

Updated: Jul 7, 2023

With Notes on TBI, TPI, and Carburetor Based Builds!

A GM 305cu/ 5L engine with roller-rockers being built on an engine stand in a shed.

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 in this guide 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.


Table of Contents:


Who This Guide is For


This guide is intended for those of you building a 305 TBI third gen Chevy Camaro or Pontiac Firebird, though many of these parts and mods will apply to anyone with a GM TBI unit, TPI system, small cap distributor, or a 305cu/ 5L engine.


Throttle Body Mods


The throttle body is one of the biggest bottlenecks in the system as it only flows around 450 CFM from the factory, so a lot of power can be gained here. You will be able to feel these changes! Take this opportunity to rebuild your injectors if you are retaining them or sourcing used injectors.


Injector Pod Spacer


The best mod for the money on throttle body equipped platforms is to double up the gasket under the injectors, raising them above the bores and allowing more air to enter the engine. Years ago there were kits with a double thickness gasket or spacer.



Bigger Fuel Injectors


Swapping to larger injectors is generally a very worthwhile mod for anyone building an engine, and the TBI 305 is no exception. This calculator by fueltech.net will help you choose the right size injector for your build using the data below. Keep in mind that there could be as much as a +/-10% variance in flow rate from one set of injectors to another.


  • The 5.0L injectors GM part number 5235279, flow 54lbs./hr. at 12 - 13psi, up to ~173hp

  • The 5.7L injectors GM part number 5235206 flow 61lbs./hr. at 12 - 13psi, up to ~195hp

  • The 5.7L injectors GM part number 17084327 flow 65lbs./hr. at 12 - 13psi, up to ~208hp

  • The 7.4L injectors GM part number 17084304 flow 74lbs./hr. at 12 - 13psi, up to ~237hp

  • The 7.4L injectors GM part number 5235231 flow 80lbs./hr. at 12 - 13psi, up to ~256hp

Check out gearhead-efi.com for more!



More Fuel Pressure


A typical GM TBI fuel system operates at 9 - 13psi, but most quality TBI injectors can operate at much higher pressures for greatly increased flow! For example: The 5L injectors that only flowed 54lbs./hr. at 12 - 13psi could flow as much as 72lbs./hr. at 22psi - a 33% increase! There are a few methods for increasing the fuel pressure, all involve modifying or replacing the fuel pressure regulator.


The Washer Method


Washers may be placed under the spring inside the fuel pressure regulator to increase fuel pressure. Be sure to select washers that will not react with fuel. While this is the cheapest method it requires you to disassemble and reassemble the TBI multiple times to test and make adjustments until you reach your desired fuel pressure and offers a limited range of adjustment.


Replace the Spring


There are many online options for "TBI regulator springs" ranging from 12 - 28psi. Washers can be used to farther increase your fuel pressure after installation if needed.

Use an Adjustable TPI Fuel Pressure Regulator


Several companies, such as South Bay Fuel Injectors offer adjustable fuel pressure regulators for tuned port applications, with a little work this can be made to function on a throttle body as well! Dissemble your throttle body and use a die-grinder to remove some material from the back of throttle body, below the fuel pressure regulator, to allow clearance for the adjustment screw. Be sure to remove enough material that you can get a wrench on the adjustment screw with the throttle body installed.


Boring Your TBI for More Airflow


This guide by TBIChips will explain the process of modifying your TBI unit to support approximately 500 CFM, or send it to SPRperformance.com and they will bore your TBI to support 600+ CFM . Some have even bored intake to work with the 454 throttle body, thought this reportedly results in poor idle quality.



By doing these simple and cheap mods, you can increase the airflow and fuel delivery to your engine, resulting in more power and responsiveness.



Ignition System Upgrades and Tips


Another area where you can improve your engine performance is the ignition system.


  • If you are upgrading the ignition system with a better ignition module, coil, plugs, and wires you need to gap the plugs wider to receive any real benefit. I generally recommend a gap of .058" or .060" with an upgraded HEI ignition system, though you can keep increasing it until you stop seeing additional gain.

  • When choosing a module I recommend ACDelco's ignition control module part number D1943A, ACCEL's 35370, or equivalent. Avoid ACDelco part number D1984A and ACCEL 35362 unless you have the following engine codes: L86, LS0, LR0. They are both listed as compatible but D1943A is GM's performance module and D1984A is 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 on any vehicles it did not come equipped on from the factory.


By upgrading your ignition module, coil, plugs, wires, and gaping your plugs, you can improve your spark quality and timing, resulting in better combustion and efficiency!


Setting the Timing


These engines like between 1 and 4 degrees over base timing, depending on stock tune and elevation, for best performance. To set the base timing, you need to follow these steps:

  1. Disable the computer advance by plugging in the two connectors with a single orange wire by the passenger side airbox, near the firewall, on most TBI vehicles.

  2. Connect a timing light to the number one spark plug wire and point it at the timing mark on the harmonic balancer.

  3. Loosen the distributor hold-down bolt and rotate the distributor until the timing mark lines up with the desired degree on the timing tab.

  4. Tighten the distributor hold-down bolt and reconnect the computer advance.

Next, let’s talk about the exhaust system and how it affects your power output!


Exhaust Upgrades


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.


Long Tube Headers

  • For those of you willing to lose ground clearance for better torque, there are several good long-tube choices.

  • Long tubes aren’t the easiest install in a third gen, but they offer the best performance gains and sound quality.

  • Some examples of long tube headers are Hedman 68460, Hooker 2465HKR, and Pace Setter 70-1326.


Short Tube Headers

  • If you want to keep your ground clearance and have an easier installation, you can opt for short tube headers.

  • Short tube headers don’t offer as much performance gains or sound quality as long tube headers, but they are still better than stock manifolds.

  • Many short tube headers like the Hooker 2460HKR bolt directly to the factory “Y” pipe, but there are plenty of options.


Just remember that if you are pushing for 350cu power you need an exhaust to accommodate that - shop for headers sized for a 350cu engine.



Rocker Arm Upgrades


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.

  • Rocker arms with a ratio of 1.6" are generally not much more expensive than the standard 1.5" replacements and will exaggerate your cam profile for a noticeable bump in performance. This can be a good choice for those not wanting to swap their factory cam or who want to split the difference between two different tiers of aftermarket cams.

  • While it would be ideal to burn a new chip, upgrading to 1.6" rocker arms should be within the scope of what the ECU can correct for. It is good practice to always check valve to piston clearances whenever you do anything that effects valve lift or cam timing.


Scrapyard 305 Head Upgrades and Mods


The H.O. 305 heads should be considered for any third gen owner trying to get the most out of their 5L as they flow much better.

  • Even without porting or valve work they will add around 30hp over the swirl port TBI heads. Look for castings 14014416, 14022601, and 14101081. These are commonly identified by the final three numbers in the casting: 416, 601, and 081.

  • These castings can be found on mid 80's square body C-10's trucks and F bodies, Camaros and Firebirds, at the scrap yard. These have 58cc closed chambers, come equipped with 1.84 intake valves and 1.5 exhaust, and will support 1.94 intake valves and 1.55 exhaust.

  • I do not recommend up-sizing the valves unless you are boring the block and de-shrouding the heads or it can actually hurt flow.

  • After working the gen 1 H.O. heads they are good for around a 40hp gain over the stock TBI heads.

  • The 305 Vortec heads, casting numbers 12552520 and 12558059, can be found on GM vehicles manufactured from 1996 - 2002. These beat the flow of most worked H.O. heads from the previous generation and are good for around a 50hp gain over the TBI heads, however there is little room for additional improvement.

Before you install your new heads now is a good time to find a machine shop to check your heads for defects, replace the valve seals, springs, keepers.



Notes About Compression


  • Most first generation GM small blocks started with approximately a 8:1 - 9:1 compression ratio depending on year and application.

  • You can ask your machinist to deck the heads to raise the compression higher.

  • Raising compression to 9.5:1 - 11:1 is a good way maximize your power output and gain fuel economy.

  • Higher octane fuel is required at higher compression rates and you may have to retard the timing if pre-detonation occurs.

  • Typically engines with a compression of 10:1 - 10.5:1 can run on 87 octane, though mid-grade or premium may be required depending on platform and configuration.


Cam Selection


  • If you block is from 1987 or newer you can easily upgrade from a flat tappet to a roller cam.

  • I don't recommend going over .450" lift as there is little benefit without port matching, polishing, larger valves, de-shrouding, and removing .050" from the top of the valve guides as well as clearance checks.

  • Unless you have made certain that the valve will not contact the valve guides and cylinder wall do not exceed .480" of lift!

  • While I recommend buying a matched set of valve springs from the same company selling your camshaft, the Z28 springs are a great budget minded upgrade.

  • Always check to make sure that your valve springs do not fully compress when installing a new camshaft.

  • If you aren't familiar with how camshafts functions or the terminology used check out Camshafts Demystified.


Fuel injected 305 Cams


If you are retaining the factory TBI or TPI injection system or are just looking for a for friendly tuning experience with your aftermarket fuel injection a cam designed for a computer controlled system is a must.

  • For a flat tappet street build I recommend Comp Cams 12-249-4 with .434" INT. lift, .444" EXH lift, a duration of 206/212, and lobe separation of 112 degrees.

  • For a roller lifter based street build Comp Cams 12-407-8 with .472" INT. lift, .480" EXH lift, a duration of 200/206, and lobe separation of 110 degrees could be an interesting choice.


Carbureted 305 Cams


The H.O. 305 also featured the 929 cam that was originally released in the 300HP 327cu Chevelle.

  • While the 929 camshaft isn't suitable for a factory TBI build, with .390" INT. lift, .410" EXH lift, 319/320 SAE duration, 112 of lobe separation, if you are considering a carburetor and like the retro sound and good low end to mid-range power it could be a good choice.

  • I've had good results from Melling's MTC-1 with .422" INT. lift, .444" EXH lift, 278/288 SAE duration, 112 of lobe separation on carbureted builds as well.



Engine Computer Options and Mods


If you have followed along to this point it would be a good idea to tune you ECM.

  • Very few tuning shops still offer services for factory TBI and TPI engine computers, but that doesn't mean you don't have options! Luckily DynamicEFI.com, RedDevilRiver.com, and BoostedNW.com have you covered with all the equipment and software you need to burn your own chips.

  • DynamicEFI offers the EBL P4 Flash System. This allows you to replace your OBD1 chip with an add-on board with a modern USB interface capable of on the fly tune updates, data logging, and Windows interface!

  • DynamicEFI also offers the EBL Flash-II System. For this option you can send in your ECM or order a computer with it preinstalled!

  • BoostedNW is now taking pre-orders for their replacement for the Moates Ostrich2. The BoostedNW RealTime Chip Emulator will allow you to replace your OBD1 chip with a modern USB interface capable of on the fly tune updates and data logging.

  • For those with TPI engines FiTech offers four different options for drop-in replacements for the factory ECM. The FiTech 38352 and 38353 also double as transmission controllers, for 4L60E and 4L80E transmissions. In addition to being supported by most tuning shops this unit changes the system from batch fire to sequential injection by using the distributor's hall-effect sensor as a cam sensor as noted in the FiTech Ultimate TPI Kits Instulation Manual.


Final Notes


I hope this guide has been helpful for anyone who wants to build a 305 TBI engine on a budget. Many of these mods have worked well for me and I’m happy with the results. If you have any questions or feedback, please leave a comment below. If you are looking for more 305 specs, information, and mods check out CrankshaftCoalition.com and HarrisTuning.com. Happy building!


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Third Gen Firebird/ Camaro Maintenance and Mods YouTube Playlist


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