which heads on a 318
As guys have suggested if you want to keep the HP in the 300-325 range your best bet is a set of 302 heads. I would go for the 1.88 & 1.60 valves and have a good valve job and bowl cut/blend done. Gives best compression bump and flow for the $ spent. Match what you stated to a 340ish cam and 3.23 - 3.55 gears.
Magnum heads would be second choice. Higher compression + larger valves and flow from the factory. Cost is the issue, after you purchase these heads you will also need lifters (AMC) and pushrods for through the pushrod oiling of the valve gear, Magnum valve covers, Magnum valve gear, Magnum head bolts and a Magnum intake (Performer/Performer RPM). Starts to add up.
340/360 heads would be my choice if you wanted more power than your stated goal. The problem with these heads is the combustion chamber volume, they are all large: +/- 74ish cc. To really take advantage of these heads on a 318 you need pistons to raise the compression to an appropriate
level to support HP levels in the 350 plus range. They would need quite a bit of milling and that causes other issues that have to be addressed.
I may be wrong but I don't think stock 318 heads will support your HP goal without lots of work.
The only draw back to 318's is torque. This is why most would steer you towards a 340/360. More cubic inches = more torque. To make HP you need air flow and an appropriate level of compression. The more HP you want out of the 'teen the more you need to increase the rpm to get the flow volume & velocity up. At low rpm power dies because of the big intake track volume but slow velocity. Think sewer pipe vs garden hose. To keep the engine in a happy rpm range and driveable on the street you have to increase the converter stall, rear end gearing, exhaust system, ect. You get 400 HP braggin rites but your stuck with a 3000 stall converter and 4.10 gears. Your "street rod" just became a "track rod".
The key to a powerful 318 is in the "packaging". The whole car has to work together in harmony. Here's my combination, it's snappy at low rpm, cruises around 3000 rpm at freeway speeds and is just fun to drive everywhere: '65 Cuda, '89 318 police motor (360 heads but with pistons for increased compression), 650 Holley, LD 340 intake, spitfire headers into 3" single exhaust, electronic distributor and orange box, 999 trans (904 with additional clutches and 2.76 low gear set), 2300 stall converter, 3.55 rear gear, 60 series 15" tires. Its all balanced and works together.
Stock 318 heads flow 165'ishcfm stock, that 165 x 2 =330hp capability.
Yes but Its not an issue of velocity with lil motors making bigger hp, it's simply more rpm's cause they have to do more work to get the same accomplished and torque starts coming from more gear out back....more rpm does not need as much velocity as does certain low rpm/off idle app's, it just needs the proportional flow, with increasing flow in a cylinder head comes more volume, it's a by product.
fwiw ...stock 675-124cc int port, after some porting to the 200cfm range that same port=132cc
The mag heads are uni sex between the 318/360, which IS the reason why those mag 318's are really soft, if you've owned/driven one, you'd know.
Velocity...Thats the problem with a lot of stock heads, there is too much velocity...so much they have trouble supporting the potential flow that passes through , the air no longer flows with the port and just separates from the port shape and starts crashing around dragging the what flows left with it therefore decreasing flow.They/some need more cross section imo.
Too much is a bad thing.
I own a flow bench and I have 302's,675's,974's,915's,894's,906's,slant heads new and old.....So I know what they flow.
The 302 head flows the same as the 675 head or any other 318 heads for that matter 'as stock'
Granted there is a change in air speed between the 340 vs 318 port, this is where we need to start figuring out what rpm range he will be driving in, the cam....the whole combo.
Are you gonna be driving below 2000 rpm all the time?
Do you race under 2000rpm?
Have the OP considered the gear he will use and where 'with tire size' it put him in the range, he may find that the 3.91 gears & 26'' tire has him cruising 2200+ rpm.
Like you say and give example with your own combo=you can use 675 or a 302 head or even a 340 head...you just have to build around it, like with your low gear tranny,high comp pistons and stall convertor..
The only diff between the 675 & 302 is the closed 64cc chamber of the 302 vs the 68cc open of the 675.
One minor detail is the fact that unless he rebuilds the 318 and installs flat top zero deck pistons..the closed chamber does nothing in the area of quench=detonation resistance & faster burn, it just raises the comp a lil more which is still good, but instead of buying new 302 heads that flow the same as the heads he has, he could just mill his 675 heads down .020 to 64cc and slap them back on with whatever other machine work deemed necessary.
With only milling .020, there'd be no issue with intake fit.