View Full Version : poor man hp?

04-07-2014, 09:08 PM
iv been putting my x and j 360 heads on my 318s for a while now for off road butt kicking wide open fun lol. i was wondering if i was to drop some 318 heads down on a 360 would that bump up the compretion for more hp or just pop some heads rear quick. :jawdrop:

04-08-2014, 12:40 AM
318 heads have small ports and valves. Your better off with Magnum heads or Indy RHS heads. Indy heads allow you to use LA rockers while having big ports, big valves, and closed chambers. They are what I'm going to be running on my next project.

Madman Stephan
04-08-2014, 08:01 AM
Shave your existing heads. It's not that expensive. Then, you can do a light porting job on your intake, gaskets, and heads. It's time consuming and dirty, but not expensive.

04-08-2014, 11:37 AM
if you like the whiplash feeling and instant torque use the 318 heads ,they have tons of bottom end torque ,I did use them when racing street stocks on the 360 .the ford 351 w truck heads flow less then the 302 heads stock from the mid 80's up.

04-08-2014, 08:37 PM
i understand the hole valve differance ect. dont want to spend any more than the gasket set to do this was just wondering if the 318s would still let it breath good . but also sqeeze some compretion in ther

04-08-2014, 10:08 PM
I don't see how you figure putting small valve heads on a big engine will help give you better results than putting big chamber, big valve heads on a 318 will give better results on that small engine.

04-09-2014, 08:23 PM
with the big valve 360 heads on my 318 ,lets face it its a big air pump.so with the big valves isnt my compretion lowerd ? so i thought why couldnt i do the reverse . or am i over thinking it. im not trying to pick up ALOT of hp. just wondering

Madman Stephan
04-09-2014, 09:06 PM
No, the size of the valves and the valve chambers don't affect your compression since they're not in the compression chamber. Can someone load up some graphics? What it does allow, is quicker release of exhaust and quicker entry of gas / air mixture.

The 318 heads will only "choke" your engine. What size of carb and headers are you using? Have you played with the timing a bit yet?

04-09-2014, 10:00 PM
Exactly, your engine is an air pump. The smaller valves and ports are going to choke your engine down.

318 heads came in both open and closed chambers. So the only way to increase your compression is to switch from open chambered heads to closed chambered heads.

Now switching to magnum heads. Closed Chambers, Large Valves, Better Flow.

04-10-2014, 08:28 PM
OK thank u thats wanted to know lol. um carb is a gm 2 barrel. headers are 4 into 1 headers out of hood. iv turned cam gear drive a little ruff to start but good snap. but now im confused. if my valves arnt in the cumbution chamber were is all the magic happing?

04-10-2014, 09:09 PM
The valve heads are in the combustion chamber.

All the magic happens where the best flow is created.

A large engine with small port heads is like trying to run a race breathing through a straw.

Superchargers, Turbo Chargers, Ethanol, NOS, etc. are all power adders because they all get more oxygen into the engine. More oxygen in equals more power out. It's that simple.

318 small valve, closed chamber #302 castings flow around 172CFM max in stock form.

360 J heads flow around 200 CFM max in stock form.

Magnum heads flow in the 210 CFM range max in stock form.

Keeping your compression ratio in the 9:1 to 10:1 range while maximizing flow is what you want to do.

04-11-2014, 08:32 AM
I think everybody has explained almost everything. Except that only the bottom surface of the valve heads are part of the roof of the combustion chamber. So the valve has almost nothing to do with the compression ratio. The compression ratio is the volume of the whole cylinder with the piston at bottom dead center divided by the volume of the combustion chamber. The valves don't have much to do with the volume of the combustion chamber. If you mill the head, you decrease the size of the combustion chamber. Then, when you divide the new combustion chamber volume into the whole cylinder volume you get an increase in compression ratio.
The 318 heads have a smaller combustion chamber than 360 heads. So putting the 318 heads on would give you a greater compression ratio. But, as has been said, you will be sacrificing power because of less flow. So there is no quick and dirty and cheap way. As has been said, just milling the 360 heads will give more compression ratio. The other thing that will do it is to install new pistons but that costs some real bucks.