Modifying the 305 SBC
The late 1970s series of 305 cubic inch engines are the limp wristed sister of the small block chevs (sbc), I have one. What
most people will tell you is that you are best off with popping in a 350
and starting from there if you want any power. They're right. That's the
cheapest, quickest way to get power out of the small blocks. If you are
foolish (like me) and want to get some streetable power from the 305,
here's a way that's going to cost a few more dollars versus taking a
350 and hopping it up.
What you will end up with is 334 cubic inches of displacement and a decent performing small block that
started off as a 305 and will easily get over 350 hp depending on how wild you go.
The poor flowing heads, compression, cam timing and crank problems of the 305 are solved.
Here's how to go about it:
- Stroke the engine with a 400 crank and 5.565" rods. Grind 400 crank's main
journals down from 2.65 to 2.45" (okay to do this but cost $$). Must use the 400 crank's
5.565" rods otherwise clearance problems on the block with the standard 5.7" rods.
This crank is commonly called a 383 stroker crank for a 350 cid. If you use a 400 crank, you should be aware that
the crank is externally balanced and you'll need the flywheel and harmonic damper. I don't know if the aftermarket
383 stroker cranks are internally balanced (nice) or externally (more hassle).
- Install 0.030" oversize pistons (about the limit for 305 blocks). These two will get you a displacement of 334 cid
- Go to higher compression pistons at Accelerated Motion's site. I plan on
making a street/highway cruiser and decided that 9.3:1 compression is a nice compromise between performance and available
high test gas. These pistons are rated with the poor flowing factory 60.5 cc head chamber heads. More on this next..
- If you replace the heads with 76 cc heads for better flow, put in
pistons rated at 11.2 to 1 compression from above site, you'll end up with 9.3 to 1 compression.
If you want higher compression, you're stuck with original heads or use 64 cc heads (hard to find and $$).
Rate the pistons roughly 1/2 compression point lower than indicated for stock 60.5 cc heads.
- Add a performance intake mainfold like the Edelbrock Performer.
- Add a nice 600 cfm 4 bbl carb (or 750 cfm quadrajet in my case) or better yet any fuel
- Add ram air flow and a good flowing air filter. Look at Ram Air Box for ideas. Make sure that your air filter housing leaves LOTS of room
between the top of the carb and the top of the air filter housing. I had a Chev 454 that was just a dog and couldn't figure out why
until I realized that the low profile performance air filter had only about 1" of space between the top of the carb and the
top of the housing. I stacked two air filters on top of each other and then that engine could breath! Cost one air filter and
one long stud.
- Make the exhaust breath: add headers with 1 1/2" primaries, 2 1/2" collectors (that's all you need for
334 cid), go for dual turbo exhaust and ideally, a 2" crossover pipe right after the collectors. If you are
tight for space between the engine and frame like I am, you can go to block hugger headers (lots of manufacturers) or
go with 62-65 Corvette Ram Horn exhaust manifolds (2 1/2" outlet) available from your local GM dealer or
60s Chev and pickup trucks (2" outlet).
How to identify your exhaust manifold (any make/model)
- Add a hot cam kit (Edelbrock Performer cam and lifters), new cam bearings, after market rockers (true 1.5 to 1 ratio),
new springs. For the most hp, go with roller rockers and cam.
- Recurve the distributer so that complete advance happens around 3500 rpm, get a high output coil.
- If you've done anything on the engine, put in a new or high output oil pump.
- Whatever other tricks you do to a 350 - porting, polishing heads,
intakes, blueprinting, balancing, etc..
- If off the line performance is your game, change your rear end gears to 3.42 rear end or higher and posi.
I have 3.08:1 gears which is a compromise towards highway driving and gas mileage. I don't plan on racing. I don't even
plan on putting a tach in the car.