Does anyone know what size the stock piston CC is on a LO3 305? Are they 5cc flat tops or are they larger dishes? I searched around the forum and wasn't able to find this information.

 

they are dished. I know GM claimed you have 9.1:1, but it's really 8.8:1, and your craptastic LO3 heads are about 55 cc.

 

Actually I think they claimed 9.3:1, but what does that matter anyway? So what do you recon? 9cc dish or so?

 

It's more of a dish than that, plus the huge chamfer around the circumference gets added to the central dish. It's more like 17.5 cc according to my calculator.
Speed Pro H534NCP pistons are 5cc flat tops, which, unlike most cast 305 flattops on the market, don't have a huge chamfer, thank goodness.
Assuming cleaned-up 081 LB9 heads at 54 cc, and Fel-Pro 1094s, with a 0.060" over-bore, That'd get compression up to 10.68:1
Switch to Vortec 350 heads lightly cleaned up to 60 cc, that's 9.88:1.
Keith Black has some 12cc 305 pistons. Use them with the LB9 heads and you'd have 9.76:1
Stroke it with KB 10cc pistons and top with lightly cleaned up TFS 175 heads at 55 cc and get 10.13:1, assuming ROL steelshims because of the pistons.

 

Yeah, what the hell is with 305 pistons and the retarded chamfer? I don't get it, why is it there. For the record, KB also makes flat tops without the stupid chamfer. They also make a hyper that's a -0.5cc dome, but they're $340 for a set, rather than the more usual $240-260. For $50 you can get a -2.8cc dome made for 6 in rods.

Anyway, you seem to have guessed I'm drawing up what-if plans. Seems the pistons are gonna hafta go and that means new rotating assembly as reusing the stock rods is retarded and I might as well go for a new cast crank while I'm in there. So I'm thinking SpeedPro flat tops plus standard vortecs and a tight .040" quench. It should bring the compression to about 9.1:1. Not massive, but it'll keep me out of NOx territory for emissions. I'd mill the Vortecs but I hear they're prone to cracking. At least it'll run on regular.

I actually setup this plan based on your 400hp carbed vortec 350 build. I think I should be able to make almost 400 from a 305 by hanging on until 6000 rpm. Also, I'd use either a HSR or a Edelbrock EFI single plane. It'll lose torque down low, but it should still have enough.

 

Wait, WHAT?!?!
There's no problem with reusing the stock rods, even for running 6500 rpm.
If you're gonna buy a crank, there's no excuse to not go stroker. But if you're buying both of those, stop and just find a good used '87-up 350 short block assembly.
You didn't specify what heads you're thinking of using. I can't think of any that'll give you 9.1:1 with flattops and still get you 400 HP at 6000 rpm. And I know every SBC head that exists.
the HSR would be my choice for an intake if you do go for a rev-happy 305.

 

Ok, so stock rods are ok as long as I get good rod bolts? I figured with the cost of reconditioning and new rod bolts it would just make more sense to get a new set of forged I-beam rods for $250. Same with the crank, have to get it machined to reuse, and a new cast eagle crank is sub $200. I really have no idea what the machine work costs, I'm just trying to get an idea of what to expect.

As for heads, I implied a set of L31 Vortecs. Those have 64 cc chambers, correct? With a tight quench the math says it should have a 9.1-9.2 with flat top pistons on a 305. With a good tune and a 6000 rpm redline it should be able to just scrape 400 hp. I think...

 

You can get stock rods reconditioned and resized with ARP bolts for under $125 from nearly any automotive machine shop in America.
Vortec heads are advertised as 64cc, but are really closer to 61cc, and usually need a slight mill, so figure 60cc.
Assume your pistons are 0.023" in the hole at top dead center, and assume Fel-Pro 1094s, with H534NCP60s your compression will be 9.90:1, which is about perfect.
Now, if you use the GMPP HOT cam, then 400 HP might be possible. If you try a COMP XE268H10, you're definitely looking at about 375 HP. And a worse idle with worse MPG.
Use the HOT cam, redline it and upshift it at 6200 rpm, and have much fun!

 

Thanks for the info on the rods. So will my stock 89 rods be good with ARP bolts? Or would it be worth my while to pick up a cheap set of used powdered metal rods to recondition? I have a same question with the crank, I understand the later model nodular steel cast crank that came in around the time of the LT1 to be nicer and stronger than the older cranks. Is it worth it to source one of these or just balance the stock piece? I have a similar question about main caps. Obviously a 305 is a 2 bolt bolt, but is it worth it to source better main caps or should I just bolt on the stock pieces with ARP bolts?

You must have a different compression calculator cause I only get 9.6 (9.7 with an overbore). Oh well, close enough. Now to get off my butt and finish my transmission swap so I can actually build this.

 

Pm rods w/ ARP hardware are fine past 7000 rpm in a 305. Your stock rods w/ ARP are fine past 6500.
The nodular crank is wise if you'll be doing sustained running above 5500 rpm, or regular brief blasts to 6500+.
Mains aren't a weakness, so just go ARP, then have the shop determine if a line-hone is needed.
For the compression, I don't use any online calculator. I do it the old fashioned way: by hand. It's precise, and easy to double check. Remember your school teachers saying: "show your work" ? They were right.
Electronic calculators never forget to carry the one, but they are forced to repeat the wrong assumptions of whomever programmed them.
Things like crevice volume above the top ring. You can't add that in because once the pistons warm up, there isn't any.

 

Thanks for all the miscellaneous SBC info, it's been very informative for me.

As for calculators, I was never satisfied with not know the assumptions made by others so I made my own calculator in a spread sheet. I triple checked it at the time, but I may be going through it again just to be sure.

 

Maybe the discrepancy is from you assuming a smaller bore. Atilla mentioned a 60 overbore on the 305, and he's always advocated always boring all 305 blocks 60 over, every time, no matter what, because it gives more cubes, it unshrouds the valves a bit, and these blocks are so easy to replace, just transfer your 60 pistons to your next 305 block. Crank too, if you bought a stroker.

 

Yep, 9.90:1 at 60 over.

 

Ahhhh, yes, it makes sense now. Perfect 9.9:1. Not sure how I missed that in retrospect.

60 over, really? You're not worried about the cylinder walls being too thin and affecting the ring seals? Do 305s have thicker cylinder walls than a standard 350?

 

The walls aren't dangerously thin at 60, and even a stroker 305 with a 125 shot of nitrous doesn't cause any problems with a 60 bore.
Back in the old days when I was a newb, I read about everyone wanting to bore them to a 350, and how you can't do that, so I got a large journal 307 crank, and bored a '76 305 block to 3.875". It drove fine, so I put a 125 shot on it, and killed the pistons after a month, but still didn't crack any cylinders. This was basically a <350 horse combo even with the nitrous, as I just used the 307 heads with the small valves, and a stock 175 hp 350 c.i. cam and Q-Jet intake.
I knew I was pushing my luck, but I had to know. IDK if it ran warm, I didn't think to use a real temp gauge, but the idiot light never came on, and it never outright overheated.
Then again, I also bored a early '70s 350 block to 4.125", and it drove fine, too. I didn't try nitrous on it, having learned from the 305 turned 307 experience. It was also a low-horse build. I wanted a 400, and I'd heard of people doing 383s. This was before you could get a Scat 383 crank. This one did run warm, It mostly stayed between 200 and 225, but also never outright boiled over.

 

Well the issue isn't cracking necessarily (obviously that's the most extreme problem that can occur) but more the cylinders flexing and deforming to the point where they ruin in the pistons. The Subaru EJ25 turbo engine is rather notorious for this problem.

My issue is longevity, as we know thin cylinder walls can lead to piston problems. If I'm going to do a rebuild, I want it to last 150K. It needs to last until I get bored and want something else. I don't want something hastily built to get 3% more displacement that'll die in 3 months. I want a different engine or car because I feel it's time not because my engine is shot.

Mathematically I need to build a 305 short block capable of a maximum of 6200 rpms for less than $1200. If I can't do that then it's not worth the money and I should just buy a 1-pc rear 350 short block.

 

The L69 I built for myself with H534NCP60s got sold to a friend, who I e-mailed last night. He says he's put 70,000 miles on it since I sold it to him, with no problems of any sort. I believe him.

 

Nice, good to know.