hartsmike

Decided to go ahead and build a motor for the Z... Researching putting the 305 heads on a 350 block and have read where it bumps up the compression sometimes too high for pump gas.. Who all has swapped the heads, how much of an impact did it make and were you satisfied with it?

Thanks,

camaronewbie

I've never read where it bumps the compression too high for pump gas. Might have to start using higher octane, but GM heads are GM heads, SBC parts are all designed to be swapped for a reason ... they all work with one another.

Given that most 350 blocks from the 1970's had the worst imagineable compression ever offered, you can't go wrong.

BUT - I'd do a nice port/polish job on them before I did it - easy to do on your own, and there's several writeups here with nice pics and such on how. I did a set a few years ago on my own, never had ever done anything like it before, but it was easy, kinda fun, and I ended up with a decent set of heads. Search "416" and you'll see posts on porting. Think the old one I followed was by a poster named Sitting Bull - maybe search his name and 416.

hartsmike

Thanks once again newbie!!!

hartsmike

Id doesn't matter that my heads on the 305 come from the non HO motor does it?

camaronewbie

I mean it could happen I guess - anything is possible - but I haven't heard of it myself.

Folks baulk at premium gas - but it's old school thinking ....

In say 2000, regular was say $1.00 a gallon, premium was $1.20 -
.20/1.00 = .200 = 20% ... that''s paying 20% extra for the premium per gallon! Rich man's gas huh?

Today, we'll say regular is $3.50, and premium is $3.70 a gallon -
.20/3.50 = .05714 = 5.7% ... now we're only paying 5.7% extra for premium gas per gallon. Anyone can afford an extra 5.7% increase for performance right?

PS - don't pass this logic on to the oil companies! They catch on and we're all screwed!

camaronewbie

No, don't think so. Get the casting numbers - should be 416, or maybe 081 (last 3 digits). The LG4 heads were decent heads, especially when you did a little port work to them. I mean there's alot better heads in the world, but consider that the LG4 heads are 1) free, 2) easily ported/polished with $100 worth of tools at home ... it's hard to go wrong really!

camaronewbie

I think ...

LG4 had like 9:1 compression. But it's a 305, so it's a smaller cylinder diameter. Put a 9:1 305 head on a larger 350 diameter cylinder (remember, 305 bore smaller than a 350, thus 350 has larger diameter bore, piston, etc.), and you will end up with less compression that what that head had on the 305 motor. Yeah, it will be more compression than whatever the equivalent 350 head originally was, but it will be less than what it was on the smaller motor. Make sense?

PS - I'm rambling - I had massive oral surgery this week, and I'm trying to keep my mind off the pain, so ignore me if I seem weird

hartsmike

No.. It does make sense.. Both equations... ON one of the web posts I'd seen they mentioned a certain type of 305 head.. I used to not believe in the 87-93 gas really made a difference but I became a BIG believer when I had my LT1.. Milage sucked and so did performance when 87 was run through it...

Apeiron

Your LT1 had a tuning issue.

camaronewbie

Yeah my LT1 seems to run fine on 87, although I usually use 89 (I don't know why).

But my old carbed 350 in the 1992 with the dome-top pistons does not like anything other than 93! But, since it's only costing me 5.7% more these days, I don't mind

hartsmike

Went back and looked again on the different web post and they said that the dome top pistons jettison the compression up way more than the flat top and that high compression was causing the cars to have more than pump gas run through them.

Benjam88

Higher octane gasoline is more resistant to predetonation. So when you have a motor that runs hot and bothered (High performance) or has a high compression, then the gasoline won't go off on its own when its compressed with hot air, the ignition spark sets it off.

hartsmike <- Your mileage probably sucked because your tuning was off and your computer would pull lots of timing on 87 octane, because it was detonating and trying to compensate.

hartsmike

thanks for the help guys!