Ok, before you start typing that there will be no benifit, ect. ect. I know all that. But my friends 350 cracked a head, and I have some 305 heads laying around. he cant afford much, so im gonna give these to him, but first. How much should I expect compression to raise. He has a GM goodwrench crate motor with about 8.5:1 compression. 2nd, do I need to use a special head gasket, or one for 350, or one for a 305? thats all. thanks.  

Hi, I performed this swap myself in an effort for more compression on a budget, and to spend the last few bucks I had left to complete my project... Yes folks, there is a theory to my "wasting time with 305 heads..." First off the 58 cc chambers with my micro-dished (4cc? + valve reliefs..)gave me about 9.75/10.0:1, now with the larger cam and the overlap it had, has yielded no detonation problems (272/282 deg. exh. duration..). It may require higher octane in your case though.. so be prepared to use premium fuel if it wants it..., food for thought...  Yes, they have "small" valves in them... 1.84/1.50, BUT they are used in a large, heavy car with a maximum rpm top of about 4500. The car has 3.42 gears and a 4L60 trans, so 99% of the time it is operating below 4000, so I think there is no big losses due to valve size, and the 600 AFB seems to work fine also, in this application. There are however a few things you need to know about the heads... Be aware the center bolts on some of the newer 305 HO heads are canted, thus requiring a special intake.. (or you can just slot the holes on your stock intake, like I did... (scary, but it worked for me...) The valve sizes are smaller and some say they are also prone to cracking.. just get them checked out by your local machine shop guy.  No special head gasket needed, but you might run 350 head gaskets to avoid the 305  smaller bore gasket overhang into the bore. In my case it worked out and sometime I'd like to find a good set of heads for my 350. For the money I have in mine I can't complain. Good luck with your project! Mike  

you need the 350 head gaskets so you don't over hang the bore with the gasket.he should get a nice seat of the pants feel with the new found compression,but,if his engine is tired the new compression is going to find the path of least resistance,so i would expect some blow by and dirtier oil in between changes.good luck  

89LXHatch:
Hi, I performed this swap myself in an effort for more compression on a budget, and to spend the last few bucks I had left to complete my project... Yes folks, there is a theory to my "wasting time with 305 heads..." First off the 58 cc chambers with my micro-dished (4cc? + valve reliefs..)gave me about 9.75/10.0:1, now with the larger cam and the overlap it had, has yielded no detonation problems (272/282 deg. exh. duration..). It may require higher octane in your case though.. so be prepared to use premium fuel if it wants it..., food for thought...  Yes, they have "small" valves in them... 1.84/1.50, BUT they are used in a large, heavy car with a maximum rpm top of about 4500. The car has 3.42 gears and a 4L60 trans, so 99% of the time it is operating below 4000, so I think there is no big losses due to valve size, and the 600 AFB seems to work fine also, in this application. There are however a few things you need to know about the heads... Be aware the center bolts on some of the newer 305 HO heads are canted, thus requiring a special intake.. (or you can just slot the holes on your stock intake, like I did... (scary, but it worked for me...) The valve sizes are smaller and some say they are also prone to cracking.. just get them checked out by your local machine shop guy.  No special head gasket needed, but you might run 350 head gaskets to avoid the 305  smaller bore gasket overhang into the bore. In my case it worked out and sometime I'd like to find a good set of heads for my 350. For the money I have in mine I can't complain. Good luck with your project! Mike

If it is a stock 350 crate engine, your compression should increase to about 9.5:1. You will need 91 octane or higher if this happens to prevent detonation. Older designed engines "prefer" higher octane gasolines anyways and running 87 or lower will cost you more in the long run from fouled plugs, carbon buildup, etc. You should have dished pistons, which I think have more of a concave design than micro-dished, or flat-top pistons. Don't hold me to that as a quote. Yes you will need to use 350 head gaskets. As far as your "small-valves," don't even worry about them. 305's tend to flow poorly not from the small valves, but from lack of displacement(3.736in bore). It is an unwritten law that older engines with pushrods and 2 valves per cylinder, that are under a 4in. bore, flow about as well as molasses in the middle of Antartica. Chevy Corvettes with the 283's made decent power from the "PowerPack" heads they had in them.(1.72in intake, 1.48in exhuast) Those were a stock cast head too. My 267 in my Malibu has the same valves, and I have gained around 35-40 HP from just bolt-ons.That is almost a 33% gain over stock. Also if you use too large of a valve on a smaller bore engine, it'll do more damage than good. The valve will take up so much space in the cylinder that it'll shroud part of the cylinder and you'll lose flow, or power. So don't believe the hype around large valves and what gains they have over small ones. Smaller vavles on your friends engine will decrease his High RPM HP, but it'll increase his low RPM torque and drivability. You will most likely gain back lost HP from smaller valves from the higher compression. Just remember that more compression puts more stress on your seals and gaskets, and raises combustion temperatures. This will cause higher emissions, and decrease the longevity of the engine overall. You risk craking your piston rings, blow-by, possibly damaging your valve seats and seals, not too mention your head gaskets. Use a reputable name brand or performance head gasket. Don't use aftermarket stuff that you can get for like 20 bucks.  

Question.
"Older designed engines "prefer" higher octane gasolines anyways and running 87 or lower will cost you more in the long run from fouled plugs, carbon buildup, etc."  - How so? Why?  

The amount of time a plug lasts would be much more dependent on tuning than a couple of points of octane. If you don't have a vacuum advice, precisely metered fuel at low speeds, and a fuel system in great condition these things would contribute more to carbon buildup and plug fouling. Saving money in the long run? The amount of money saved would be negligible.

I would agree that older engines would more prefer more octane since "technology" has brought about much better detonation resistance for the amount of compression used but it doesn't matter if the engine in question has 8.0:1 compression! Even if it is a smogger 350 with bathtub chambers and .060" "quench" clearance it will still run fine on 87 octane with such low compression. Now if you're talking about a 300 horse 327's then it's a whole other story...  

Basically what I wanted to point out is what determines if a plug will foul has to due with the tune of the engine, not the grade of fuel.  I can foul plugs just as easy on 103 octane as I could on 84.  My Centurion has a healthy diet of 84-87 Octane gas and I've never seen a fouled plug on it. Just changed them after 5 years and 40,000miles just because. With the points ignition they still looked practically new.  Octane only is the measurement of it's detonation/pinging resistance.  

I completely agree with Randall's argument. But I did forget something very important, how could I not mention the condition of the engine. Poor rings not only can allow oil into the burn but drops in cylinder pressure contribute to "weaker" combustion. This slower flame front is more apt to have more "leftovers" that make the combustion "dirty" for the next cycle.

Octane is highly overrated, especially when you're trying to say that 4 points make a significant difference in how frequently you change plugs in your car. I can't see 4 points effecting the amount of money in my pocket significantly unless maybe there is a slight mileage increase...  

I wouldn't say octane is highly overrated, it is highly misunderstood. Older engines from the 60s, 70s, and maybe early 80s, "preferred" higher octane. Some examples would be like Corvete engines which includes the 327. Those older V8's were designed when Hi-Test was still sold at Sunoco's gas pumps. You can run 87 or lower on an 8.5:1 compression engine, but not a 350 with a set of 305 heads on it. I agree that engine condition does affect directly how fast a spark plug will foul, but poor quality fuel that doesn't consistenly burn or burns slower, as with 87 octane versus 93 octane, will foul a plug faster. 6 points of octane may not seem like much, but when I worked for my grandpa at his auto recycler yard, I would strip core engines. I could always tell which engines were ran on cheap gas as opposed to higher quality or higher octane fuels. 87 octane burns slower and cooler than 93 octane. While this cuases lower emissions and saves you 20 cents a gallon at the pump, it also increases the amount of carbon and other deposits left in the cylinder. 93 octane burns hotter and faster, and can help keep your combustion chamber cleaner. I know from personal experience. I had a 1992 Oldsmobile Achieva with a DOHC Quad-4 that I bought for 500 bucks with 151000 miles on it. I decided to try and put some octane booster in my enginge to clean it out by burning hotter. When I did it the first time the car ran better and I was avg around 29 MPG combined city and highway. I even drove to Indianapolis and back to north east Ohio on one tank of gas, which was around 746 miles total. When I tried it a second time, about 15000 miles later, I burned a hole in my #4 piston. On that fill up I used NOS brand of "Off-Road" use only octane boost combined with Sunoco Ultra 94. When the car blew up, my mechanic had only 50-60 psi compression in my #4 clyinder. So basically you can run your car on 93 all day long and it'll help maintain cleaner combustion chambers, or you can run lower octanes and just throw a tank of premium in every so often. I run premium in my 1992 Honda Accord and I have had no engine related problems since I bought the car and put plugs in it. I run premium in my Malibu also, my plugs come out a beautiful pancake brown with 9 dgrees advance at idle, and 6% leaned out main metering for cruise mode on my 1406 Performer series carb.  

Octane rating only changes the gas's resistance to detonation.  Differences in quality are only between suppliers, so if the supplier makes a good premium then their regular is good. It's a myth that higher octane burns faster or hotter.  The only reason it would burn hotter is because of engine compression, not because it's high octane.  Octane needs are determined by compression, timing, fuel mixture, combustion chamber design and carbon build up, irrelevant if the engine is older or newer. Computer controlled ignition cars can sense higher octane fuel via the knock sensor and change the tune of the engine for better performance, not because it's a higher quality gas but because the engine changed it's tuning to take advantage of the only difference, higher octane.  If you burned a hole in your piston it was because you were running lean for too long, if you lost the rings it was probably because the rings had been bad for a good while and carbon build up was sealing the combustion chamber and the additives cleaned it off.  My motorcycle service manual expressly says not to clean the carbon around the sides of the pistons for this reason.  And again my Centurion the whole time I've owned it has ran only 84-87 Octane (8.5:1 Compression 455) and I only changed the plugs because it had been over 40,000 miles (and five years) even though they still looked new (with a slight good burn red/brown).  

From the Wiki -  "In other words, the flame speed of a normally ignited mixture is not directly connected to octane rating." " while engines perform best when using fuel with the octane rating for which they were designed and any increase in performance by using a fuel with a different octane rating is minimal or even imaginary, unless there are carbon hotspots, fuel injector clogging or other conditions that may cause a lean situation that can cause knocking that are more common in high mileage vehicles, which would cause modern cars to retard timing thus leading to a loss of both responsiveness and fuel economy." - Which when you put high octane fuel in your car it advanced timing recovering some of the mileage and power.  Any cleaning of the carbon off the pistons would result in longer term results.

http://en.wikipedia.org/wiki/Octane_rating  

Ok well I stand corrected. It is still my opinion to run premium gasoline, regardless of compression ratio.  

No harm in an opinion.