from a 305 to a 350

[seawolf18]

Putting 305 heads on a 350 was a cheap way to get the compression up in the low compression 350's of the 70's and 80's. 416 305 heads are the HO heads or big valve heads. Most 305 head's had really small 1.72 intake valves. your 350 will run out of breath in the upper 4000 rpm range. You also have to watch your compression. Putting those small chamber heads on can up the compression to over 10:1 with flat top pistons. You would have to run race gas to drive it without it detonating.

The 350 I rebuilt in my 80 chevy pickup has pocket ported 305 HO heads on it and a torque "rv' cam. I used them mostly because they were a good set I had laying around, and it probably won't see 5k rpms ever. That and I wanted to build it on the cheap. They work quite well moving a heavy low rpm 4x4 pickup around, not a hotrod.

The 350 your looking at will probably need more than a rering job. Its probably got a gazillion miles on it and is tired. It more than likely has a nice ring ridge at the top of the cylinders, so it would need to be bored to get a new set of rings to seal properly. Your better off rebuilding a 350 with a good of heads such as vortecs and a matching cam. You will like the results much better than trying to cobble a bunch of 305\350 parts together.

 

[79cutlasssalon442]

We bought the motor. It hasnt been rani n a while, guy turned it over and it fired right up. $150 didnt smoke at all to be honest. We pick it up this weekend to tear it apart. If it has to be bored we have a friend with a shop to do all that stuff and he'll probably get us a good deal. More than likely just putting flat top pistons in it and a new cam. Stock 350 heads and my intake and carb. All new bearings and gasket all around and a new oil pump.

 

[GT_80]

Putting 305 heads on a 350 was a cheap way to get the compression up in the low compression 350's of the 70's and 80's. 416 305 heads are the HO heads or big valve heads. Most 305 head's had really small 1.72 intake valves. your 350 will run out of breath in the upper 4000 rpm range. You also have to watch your compression. Putting those small chamber heads on can up the compression to over 10:1 with flat top pistons. You would have to run race gas to drive it without it detonating.

I disagree- I have a set of ported and polished (by me) 416 heads - I had them on a .060 overbored 350, 10:3 compression, .500 lift "3/4 race" cam, stock crank and rods, and it ran up to 6000 rpm before is started to fall on its face-and I ran it on 93 octane with 41 degrees timing (locked out). those heads will flow better than most stock 350 and 400 sb heads from the 70's. They are made much better with better water ports as well, are just the "lightweight" casting so take care to keep them cool. I never had an issue with mine. They are on my mild 350 now (1980 corvette block wth 9.7:1 compression) and they run nice and cool, and the torque they produce is unreal on the 350. I have a set of 601 (54cc chamber 305HO heads) that I am saving for my next build. Either a stock bore 350 or a hot 305 I got off a friend.

The crank in your 305 could not be used in a 350 without rebalancing anyway because the 305 pistons are lighter than a 350, it will revv up like an SOB, but the rod and main bearings probably won't last too long... Ask me how I know 🙂

 

[Clone TIE Pilot]

Keeping a tight quench cuts down on spark knocking. With the correct qunench you should be able to run pump gas with 10-1 CR.

 

[olds307 and 403]

Man, smoked by a 3.9 Dakota, pretty awful. Mine is only barely accepttable with 3.92 gears, better air filter and muffler. I run 307 heads on the Olds 350 in my 94 4x4 truck. Great till 4000 rpm but then it falls off quickly. I plan on putting a better 350 in the 4x4 and plan keeping the 350 heads this time. You guys have so many heads cheap, don't waste a dime on 305 heads. The 350 sbc were poorly made those years, it will need rebored and new pistons anyways.

 

[King_V]

those heads will flow better than most stock 350 and 400 sb heads from the 70's. They are made much better with better water ports as well, are just the "lightweight" casting so take care to keep them cool. I never had an issue with mine. They are on my mild 350 now (1980 corvette block wth 9.7:1 compression) and they run nice and cool, and the torque they produce is unreal on the 350. I have a set of 601 (54cc chamber 305HO heads) that I am saving for my next build. Either a stock bore 350 or a hot 305 I got off a friend.

I'd come across similar info (but do NOT have personal experience) as well- that the mid-70s 350 heads were just so awful that the HO 305 heads are an improvement, performance-wise.

 

[seawolf18]

A tight quench does help with detonation. The problem with that these days is the rebuilder pistons that come in a lot of engine kits. The compression height on the piston is usually .020 shorter than the factory 350 piston. The reason is to keep the same compression stock when you deck the block, since the factory piston is about .020 down the hole with an undecked block. Most people don't deck their block unless it needs to be. now when you put it together, the piston is around .040 down the hole. Now add a .041 head gasket. Now you don't have any quench. Now add small 54-58cc chamber heads to your flat tops in your kit, you have a detationing motor that runs like crap.

I agree 305 HO heads are good for torque in a 350 due to the short runners that they have. They work well for that on my old pickup. They can be made to flow better by having them cut for 1.94 valves, and having a back cut added to the intake valves. Then buy a porting kit from summit, and get all the casting flash out of the intake and exhaust side. the exhaust side is what really needs help on those heads. You can port them farther, if your comfortable with doing it. I'm not comfortable having more than 9.5:1 compression on an old school 350 with iron heads. 10:1 with aluminum heads.

They can be made to work if all the parts are setup up properly. I had my block decked. i used dished pistons, and made sure they were factory height, and used the appropriate head gasket. My compression ended up being around 9.6 or 9.7:1. close enough for me. My quench was was in .040 range, where you want it. It runs really good for me.

I'm surprised GT_80's engine with a .060 overbore, and 10.3:1 compression with 305 iron heads is not having detonation issues. The large cam does help with bleeding off some of the compression. As well as a tight quench. My experience with they guys that have built them this way is that they have detonation problems, and have to pull timing way back to remedy it. They didn't properly set the engine up when they built it.