I have a 89 305 with the dished pistons and stock heads

what kind of compression will I get with flat-tops and will I be ok with 92 octane

 

I think the '89 305 TPI motors had flat tops factory, and a 9.5:1 CR.

United Engine and Machine Co.


Your cylinder heads are probably: 58cc
flat tops pistons are usually 5cc or so
gasket thickness, factory, is .015", aftermarket ones are usually .039"
gasket bore is usually 4.166"
cylinder bore is 3.736 factory, if you have a .030 overbore, add .030
deck clearance stock is usually .025", unless you get the block decked, or the aftermarket pistons have a lower pin height than 1.56"
stroke is 3.48"


There, no you can find out for yourself, based on the info you have

 

that gave me a static compression ratio of 5.1
what does that mean

 

you screwed up, try again

 

ok I got it right now but it seem to be impossible to get to the 9.5 compression ratio that is supposed to be stock on the tpi motors not even 9:3 can be achieved

 

ok ok

according to:
https://www.thirdgen.org/tech-data

your '89 305 TPI motor had 9.3:1 CR, my guess is that GM spec'd the parts as in my attached .jpg file, to get that CR.




to answer your original question, I imagine you want new pistons since you have worn out rings, etc? I wouldn't bother getting a 305 rebored, when you could do a 350, but if you really had your heart set on it, then flat tops with 5ccs volume, and .045" deck height would probably be the result, along with a .030" overbore. Unless your cylinders are ok, then you could just run a bottle hone through, and put in stock size pistons and new rings.....

I'm just giving the solutions based on your question, i'm certainly not saying it's a good idea though, just answering your question

 

its already 30 over and all that has to be done is clean, bearings, pistons(optional) and rod bolts

 

ah, then i'd recommend flat tops, most have 5cc volume, try and find a set with 1.56" compression height, not 1.54", unless you want to deck the block.

if it's .030" over then pistons aren't optional, unless you have a set of .030" over that you're not sure about using....

 

I have some dished stock pistons and a set of flat tops are only about 150 with rings. I figure that that will be the best hp gain for the money while I have it apart.

 

The dished pistons in 305 TPI motors don'e have much volume in the dish. What you pick up in compression with flattops you often lose back to thicker aftermarket head gaskets. If you were to use factory replacement head gaskets with typical flattops you'll be right around 9.5:1. If it's bored 30-over, all else held equal, you'll be around 9.6:1

 

FPP-1094 Head Gasket, Rubber Coated Steel Shim, 4.100 in. Bore, .015 in. Compressed Thickness, Chevy Small Block

these should give me 9.975:1. right where I want to be