Recommended 305 TBI mods

Not very many 305 trucks out there but other than cam selection most of the same issues apply.  The trucks use a flat tappet cam and if you must mess with the 305 in the trucks then there are recommended flat tappet cams on the 350 V8 modification page although the truck blocks do have all the roller cam provsions and so I would grab the roller stuff out of one of the car engines as that is 10-15 free horsepower for roller cams.   there are 5 general horsepower killers on the Camaro TBI. (exhaust, airfilter, intake, cam and heads).  I tried to put these in order of importance.  Also, if you want to make over 210hp, don’t mess with the 305. Pickup a used 350 preferably a 1996+ vortec 350 and build on it while you drive around in the 305 and then just swap over.  The cost for those extra 45 cubic inches isn’t very much and worth every penny.  If you need assistance finding one, do a search on  or check out the great deals on new GM L31 vortec 350 engines listed on the recommended engines tab.

1. The exhaust. Change the WHOLE thing from the engine back.  Well honestly the exhaust manifolds are fine up to 250hp builds but the Y pipe is TERRIBLE and the single 2.5″ exhaust is too small.   I recommend getting a flowmaster Y250300 collector from favorite vendor and cut it down pretty short and where the NASTY T looking y section of the factory y pipe is have that cut and new pipes bent to connect to that flowmaster y.  Then have 3″ pipe bent around to a matching 3″ cat if you need one.  You can get the 3″ catalytic converter. You can get them for about $50 on ebay just search for THUNDERBOLT 3 CATALYTIC and you should fine one.   Then a 3″ pipe back to the muffler will dramatically help that little 170hp 305 get another 15hp and its mandatory for any decent engine upgrade.

Exhaust     is the most important change for these cars.  Check out the black magic project truck     to see the affects the exhaust had on its performance and Camaros have an even worse     exhaust design than the trucks.

2. The air cleaner.     The snorkel on the TBI camaro is too small for a 2.8L V6 much less a 5L V8. Easiest fix is     to replace it with a 3″ tall 14″ diameter open element filter with a flat base.     You can drill a hole in the base to put your IAT sensor in. Also toss the choke collar     that is between the TBI and the air cleaner assembly. Good for around 5hp on a 305 car and again mandatory for any decent engine upgrade.

Another nice setup can be seen here     for colder air input.

Here are the Summit racing air filters that I recommend and use.

SUM-239421     14 in. diameter, chrome, flat base, 2in. element, air cleaner assembly for tight clearance stock hoods

SUM-239431     14 in. diameter, chrome, flat base, 3in. element, air cleaner assembly

3. Modify the throttle body. This one pretty cheap but does require removing the throttle     body.      Good for a couple horsepower  and its free just time invested.

With 350’s I typically recommend going 46mm bored throttel body from depending on the head and cam combination your planning on using with the 350.

4. Cam upgrades,  Stock cam is awful.  At least the cars did get a roller cam which helped them make the 170hp GM claimed they would make but the trucks don’t even come close.  lucky to get 150hp from the flat tappet cam truck 305 engines.  You can snag a used vortec L31 cam (same thing as 350 B Body LT1 cam)  or even better is a used 1994-1997 LT1 Z28 cam.  All can be bought used for under $100 and with a nice stage 2 chip can get you to 200hp or a bit better.

NOTE: 91-93 B-Body cars (Caprice,     Roadmaster) have a larger distributor hole in the intake than any other GM car.  The     distributor thus has a larger base on it and won’t fit down a standard intake distributor     hole.  This was done so the distributor would have more room to go in at a steeper     angle due to the motor sitting so far back in the engine compartment.  If your going     to use an aftermarket intake, take that intake and distributor to a machine shop and have     that hole enlarged to match your distributor.

If your using a 350 that has a flat tappet cam then check out for cam choices.

5. The Heads. The TBI heads are the worst flowing heads GM ever released. Honestly, I don’t recommend modifying a 305 engine to this level.  However, should you feel its absolutely necessary then find yourself a set of 416 casting or 081 casting 305 heads.  Those are the last 3 digits of the casting number on top of that head under the valve cover.  These heads flow as well as pretty much any other 305 compatible head out there and respond very well to some porting and good for nearly 30hp over the TBI 305 heads.

6. The TBI unit. The stock 305 and 350 TBI unit has 2 (1 11/16″) bores which flow 450 cfm of air. Up to 230hp engines that air flow is fine.  you can do the ultimate TBI mods to the stock 42mm TBI unit and get that air flow up to 500cfm without increasing that bore diameter which is plenty for any 305 engine that you can run with TBI.

7. Injectors.  Really don’t mess with changing injectors much on the 305 engines but have lots of customers that run the LT1 Z28 cams in the Camaro 305 engines and like to use the 350 truck injectors and I program the chip to run with those injectors.  On stock engine, don’t mess with the injectors and never need more than 350 truck injectors with any TBI compatible 305 build.

Also,  a new fuel pump, typically don’t mess with it on any 305 build but if it fails replace it with a 1996 vortec AC Delco (not cheap aftermarket crap) EP381.  I have been selling GM AC Delco part number 19237634 on my ebay store for about 2 years now.  Good strong reliable pump that can support about any engine setup you can run with the 2 injector TBI injection system.

8. If your engine has serious mileage on it (over 150k) or you want more than 230-240hp then you need to look at better replacement engines.  I have a recommended engine page now that recommends 3 different vortec 350 engine configurations which are affordable and makes at least 300hp with bored throttle body and good exhaust setup.

9. The chip. Depending on how many of these modifications you do the more the need for a custom chip or chips.  No matter what you do you are going to need a chip of sort (the stock chip is just too wimpy).  Whether it is my base stage1 1 HOT chip, stage 2 chip or a custom burned chip for those major modifications.

10. GEARS.  The automatics came with the 2.73 gear ratio and the 5speeds 3.08.  Just terrible for overdrive transmission sports cars.    You can change to 3.42 gears and preferably posi at the same time and your car will be MUCH quicker and will only cost you at most about  2-3mpg and runs great on the highway.  I wouldn’t recommend the 3.73 unless you have at least gotten as far as the cam swap in your modifications and that mileage drop will be closer to that 3 number.  The stock power band is just too low in the rpm band to take advantage of the 3.73 gear. For the automatic cars go to     to get the gears to recalibrate your speedo.

11. Torque converter for you automatic lovers.  The stock converter stalls at 1400-1600 rpm which is very low.  The 95+ S10 4.3L V6 torque converter stalls  2000-2200 rpm.  That 600 rpms will help keep your engine in its powerband and thus  make it faster.  If you go with a pretty much any cam upgrade then a higher stall would be advised for most applications. The 2000 rpm stall  is a good upgrade that won’t screw up your gas mileage, burn up your trans, break the budget, or be unbearable on the street.  I have been running that stall in my personal Camaro for years.  Still run one with my 450hp 383 LT1 transplant currently.

12. Oil pumps, I have decided to add this on the list as TOO many people when building engines put high volume oil pumps in their motors and don’t do this for a few reasons. 1. you would need to run a 7qt oil pan or you will suck the stock pan empty with any spirited use.  2. It takes 20hp to turn a high volume pump at higher rpms over a std volume pump. 3. Unless you prep the block properly you will end up sending too much oil to the hydraulic lifters and pump them up too much causing vacuum issues and poor performance.  I use a std volume, high pressure pump and its better than stock TBI oil pump and will save us both a LOT of headaches. 

None of this stuff if done correctly will ruin your gas mileage.  Properly tuned you shouldn’t get less than 23 mpg on the highway with cruise set (no lead footing).  My personal car with 305 automatic and 3.42 gears with most of the mods listed got 26-27 on the highway.  Still gets 25 with the 450hp 383.   In town however gas mileage is directly affected by the lead foot attached to the go pedal.  my car averaged 21-23mpg in my mixed in town driving and still does with new engine.  All that new found power may adversely affect your gas mileage as you flex your muscle.

Some good competition Cams.  Choose one that provides the rpm range that you want     your engine to run in.  Whether you have a flat tappet or roller cam in your     car/truck any of those options will run well in your TBI engine.  All 87+ cars got roller cams but the trucks didn’t get one until 1996 (Vortec).  The trucks have all the provisions for the roller equipment but just didn’t get the parts.

WARNING: 305 engines have tiny cylinders and I don’t recommend over .480 lift on that intake valve as you could shove the valve into the side of the cylinder.  Also, most stock GM iron heads can’t run more than .475″ lift anyway before the valve locks hit the valve guides and requires expensive machine work to fix that issue which is why I recommend the production LT1 Z28 cams so much as their lift is perfect for TBI heads.


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