my lg4 has 140,000 on it and im thinking of having it rebuilt, i know i could get a 350 cheaper but id rather keep the original. what kinf od horsepower would i be able to see if i bored it .30 and got a better non-peanut came. id like 275. if not what would have to be done to get it between 250-275?
thanks

 

SM,

Going over by 0.030" should net you somewhere around 312 CI when completed with a stock stroke. Incidentally, the 3.78" net bore will be the same as the '99 and later Vortec 325 CI (5.3L) engine, but don't consider using the pistons from the Vortec since the rod length/deck height is different.

The extra bore will certainly let you produce a little more torque, thus a little more horsepower. You'll have to make sure the valves are large enough and can open soon/far enough to feed the larger displacement.

The extra few thousandths of bore can give you another edge as well. You may be able to go larger than the 1.84/1.50 valves since you can unshroud them a bit more on the outer edges.



Measure carefully at the bore and valve faces, and you can get away with 1.94/1.56 valves depending upon your head castings (14014416 can work). You'll still have shrouding at the outer edges, but the larger diameter could offset the difference. Of course, larger valves will need larger valve bowls, and straighter, larger ports to keep the cylinders satisfied. Of course, if you don't increase the valve size, you'll always wish you did. If you don't have the correct heads, plan on purchasing and reworking old heads or buying new performance heads, like World's S/R Torquer 305s for about $800.00 a pair.

Since the engine will be moving more air, it will need more fuel to mix in. That is fairly easy with a carburetor, and you may get by with only experimenting with the secondary metering rod/cam to satisfy the setup.

With the tight chambered 58cc heads, you'll have a little higher compression. Even after opening the edges to accomodate the larger valves, compression should be up. This can help you make a little more power, but you'll have to make sure you use better fuel all the time.

When you're all done with the top end, you'll have new, heavier pistons, and will probably want to rebalance the rotating assembly.

By the time you've finished, you'll have a 305 that should produce decent power. The pistons will probably be fairly expensive, since they aren't as common as other oversizes. But you probably will never be satisfied with the power output. For all of your efforts, time, and money, you will have a 312 CID engine approaching the limits of its naturally-aspirated potential. For a few hundred bucks up front, you can purchase a 350 cylinder case and engine stand, slowly work to build the engine the way you want it, buy the parts from any number of sources at better prices, and have an engine that produces a lot more horsepower and torque. All of your accessories and wiring will transfer to the prepared engine, and the engine will still look just like a 305 from the outside.

A 305 is a worthy engine if you only want to freshen it a bit, but is probably not the better of the two choices for a performance build. However, if you were talking about a 302 versus a 350, that would be a tougher choice. Nonetheless, if you are stuck on the 305 for whatever reason, you can make some more power with it.

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Later,
Vader
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"Make Me Bad"
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getting 250 to 275 hp out of your 305 isn't that difficult. you could change heads to the 58cc chamber heads which would help and are in many junkyards. could also go with the small dome pistons from keith black. a decent cam along with some as many bolt on parts as possible will get you what your looking for. main thing is to make everything as efficient as possible.

 

Vader. heh. you just wrote a novel on 305 performance and rebuilding.. on a "boring 305lg4 Q" thats cool though.. only thing your missing is a 305 autobiography.

 

here are the plans for my lg4:

New heads, ported and polished, stock bottom end, with 158000 miles on it, new cam similiar to the compucam 2030, and a edlebrock air gap intake and a new carb with a better hanger rod setup, and headersas well as a cat back. i am figuering that this will net about 275 hp... i am using the botom end as is cuz the thing had good leak down numbers.. VERY GOOD leak down numbers... if i had looked at the numbers with out knowing the milage i would say no more than 50,000. plus it smokes on startup like a bitch, but that is from the heads.

steve

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88 Firebird Formula- deseased 2/9/99- 305 TPI, SLP 3 inch exhaust, SLP Intake runners, SLP cam, 3.45 BW disc brake posi, WS6, NOS, and all that good stuff, 13.~ sec 1/4 mile- hit by what else!?!? a FORD EXPLORER!!!!!!

86 Firebird, LG4, t-tops, slp 3 inch exhaust, catco cat, and a bunch of new/replaced stuff, open element, soon to come, 2 inch cowl hood, 3.42 posi disc rear from a 4th gen, and hopefully a 6 speed!
My Webpage- Battle of the birds, My own Thirdgen Page, still under construction, http://www.geocities.com/soares711/index.htm

 

A couple of engine buildups are described here.. http://www.hioutput.com/pete/tech/tech.html.

Pete

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Pete's Place
Now at HiOutput.com<P ALIGN="center"><IMG SRC="http://www.hioutput.com/pete/83SC.gif" TITLE="Pete's 83 SC">
My 83 SC...(originally V6) 196,500 mile LG4/5-speed from an 84 Z28, 3.42 one-tire-fryer, T-tops, PW, PH, gray interior from an 86 Firebird SE, seats from an 89 Formula, wheels from an 83 Z28, springs and rear bar from an 84 Z28, WS6 front bar, boxed LCA's, Hedman Hedders and y-pipe, Dynomax cat-back, .420" lift cam, Accel HEI Supercoil and foam air filter, custom air induction through 84 Z28 hood.....best ET on paper is 14.935@94.14 (corrected is 14.79@95.75)</P>