L31 Vortecs on a 305 block - Hot Rod Forum : Hotrodders Bulletin Board
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Old 10-10-2008, 02:36 AM
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L31 Vortecs on a 305 block

Need some guidance from you motorheads who have done this (or tried it or know someone who tried it). Putting parts ideas together for a friend who is bucks-down. He has a 305 short block that needs bored. I was thinking a 0.030" overbore and some cast flattop pistons with valve reliefs (5cc's) and a zero piston deck height with 0.040" gasket and 62cc chambers (cut the heads 0.012") would put the static compression ratio at 9.35:1, which should work great on pump gas with the proper cam and tight squish.

I played with this on the DynoSim and came up with a marine cam that seems to work the best. Comp 12-318-4, ordered with 0.440" lift to work with stock L31's. I know some of you will say that the Vortecs will take more lift than that, but I'm just working it on the low side for the software dyno run.

2000 325 124 73.4 157.8
2500 330 157 75.7 160.5
3000 353 201 81.2 171.4
3500 379 252 87.2 184.1
4000 393 300 92.1 191.2
4500 396 339 95.2 192.4
5000 386 367 95.8 187.4
5500 360 377 93.1 174.9
6000 319 364 87.7 154.9

Question is now, will the L31's bolt onto the 305 without something kissin' something? I'm wonderin' too, about shrouding the valves with that tiny little bore. I don't think the software takes that into consideration. Don't know for sure. I went through a search and saw the Car Craft build with L31's, but I'm not sure I would trust their word. Would rather hear from some of you guys. Already thought about the "shelf" between the head and block. Also talked to my buddy about a 350 block to no avail.


Last edited by techinspector1; 10-10-2008 at 02:55 AM.
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Old 10-10-2008, 06:49 AM
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Here is another article by popular hot rodding. They don't say how close the intake valve is to the cylinder wall but...it's gotta be close.

I would personally keep looking for a 350 block since he needs a bore job on the 305.
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Old 10-10-2008, 08:05 AM
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For the cost of the machine work and parts on the 305 you could buy three running 350s.

I know... bucks down... but that doesn't hold water on the 305/350 debate. If he's bucks down, seriously have him buy a running 350 shortblock, a set of vortec heads, and a used ebay intake and carb. He'll have 300 hp potential out of the box, and spend less than the machine work, parts, and heads on a 305. I have two 350s that I got for free, and one complete vortec marine engine used one season I got for $50 with a blown head gasket. I replaced the head gasket and it was like brand new. For the 305 you propose you're looking at tanking, cam bearings, boring and honing, blueprinting the crank centerline and decking the block, shaving the heads, and more tanking. That is serious money at a machine shop. Heck, for $1200 he can buy a reman 350 vortec longblock with no core charge, which is a lot less then you'll spend on buying heads and all that machine work.... and then you'll be left with a lot of work in a 305 that has very little value.

The Vortecs will work on the 305, but I might reconsider the benefits. Firstly, I've been told that you should shoot for a .040" overbore to prevent valves from contacting cylinder walls, and secondly; the major benefit to the Vortecs is their flow which will be seriously hampered by the tiny bores.

If he's truly bucks-down, then a completely blueprinted and customized 305 is NOT the cheap way to get things done. If he want's to buck the system and try to make a performer out of a redheaded stepchild with a blatant disregard for physics, then the 305 is his *****
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