Need a judgment call...

I've got the 305 apart in the garage (been that way for like a year) and I'm looking to put it back together soon. If it matters, it's out of an '87 Iroc carb.

The question is, will I see any noticable gains from replacing the pistons with some aftermarket slugs. The originals are in pretty good shape. They have 4 valve reliefs and the tops are just slightly dished (not a true flat-top) but I believe that the CR was around 9:1 or even 9.5:1. I've had the heads surfaced witch might raise the CR a little bit and I'm thinking of having the block decked as well.

This question is of course useless if I decide to punch out the holes 0.030 over.

Also, what CR are you guy's running in your 305's? and what did you do to achieve this. I'm shooting for 10:1 to try to build a bit more power.

 

I don't think a half a point of compression is worth justifying the cost of new pistons.

but its not my money, nor my car

 

An overbore, flattop pistons, block decking and head milling might raise your CR 1 point.
Tightening up the quench clearance makes a difference too.
If you do some other mods while you're in there, you'll find a noticable gain.

http://kb-silvolite.com/calc.php?action=comp

You can figure it out more closely with this calculator.

 

yeah..and those are good pistons to go with if you are thinking about it. Those Keith Black series

 

i wouldn't replace pistons unless you were boring the block oversize.

 

i'd have to agree with ede.. and I wouldn't bore the block oversize unless it actually needed it. A lot of people do it for some unknown reason, but it makes about as much sense as pissing in the wind..

 

I'm not understanding the part about spending money on a 305 shortblock. I've done the math many times, it never comes up favoring the 305 vs. getting a 350 builder.

It's not like I'm a 305 hater, either. Or against making one make more power. I drive one to work every day. But, the economics of $'s/HP just don't work out once you start spending money on the 305 shortblock.

 

I'm not trying to be disagreeable. But as far as getting new pistons for a std bore block, it's not as if it can't be done. It's more a question of how much of a perfectionist you want to be, how worn out the original bore is, and how much $$ you really want to invest in your 305.
It might not make a lot of sense to some guys to do it, but then to some guys it doesn't make sense to rebuild a 305 at all.
Personally if I were building a 305, I'd do it as inexpensively as possible, and put any real money into parts that could be swapped to a 350 at a later time.
Considering that your engine is torn down already, and you're going to be investing in parts to reassemble it, a set of cast flattops might not add more than $120 to your total incl labor to press the pins in.

My main reason for telling you this is cause IMO the factory dished pistons are a "power disabling" piece of equipment, and to me it makes less sense to put together an engine with those things in there than to replace them with a set of std bore flattops.

I have done it, successfully

No flame intended... Just my opinion