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#1
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mill heads
Does anyone know how much I can mill off 318 heads to increase performance abit.
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#2
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There will be many others here who will know more than I do on this front, but there are two points coming to mind:
Which 318 heads (casting number? from a Poly?) Don't forget that you can also usually mill a bit off the block, and it's a good way to go if pistons don't reach the top of the bores. |
#3
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#4
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The 318's are pretty low CR engines, and there is not much you can do with milling. Changing pistons is the way to go. The 318 I built had a calculated CR of 7.5:1., if I remember correctly the pistons were about .080" in the hole at TDC. I milled the heads .090", making them pretty much "closed chambers". That raised the CR to about 8.7:1. However, it's quite a lot of work and not very cost effective; you also have to do somtehing to the intake manifold fit, the end rails propably need trimming too, and you better do something for the lifter preload too.
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#5
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It all depends on what heads he is using. If it's a LA 318 and if he's using 302's then he almost has 8.5:1 already (probably more like 8.6:1). My pistons on my 318 are 0.066" down in the hole. So using that number. Milling the heads to 58cc and using a small bore head gasket (4.060") with a 0.028" compressed thickness like mopars P4120094. That will put him right at 9.46:1. If they are down 0.080" then he'd have about 9.1:1.
If it were me I would leave the stock valve size and just do some clean up porting in the bowls and gasket match the heads. And if I wanted to go to 1.88" valves I'd have them ground to 30 degrees along with the seats instead of 45 degrees and get some better low lift flow. |
#6
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If it were me I would go with the piston change it would be the same cost or cheaper & less headache. The best way to get the performance you want is to shop around & pick up a good used 4bbl. 360 on the cheap.
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#7
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rule of thumb is heads are the best single place to gain power, so why not a good port job?
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#8
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I would only change the inlet valve to 30* and then only if you use the right valves and increase the head dia. I would leave the exhaust at 45*
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#9
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The early 318 LA (and 273) head NHRA legal minimum was 57.3 cc. The heads can be safely milled to 51.3 cc though. Piston to valve clearance should be 0.080". You must mill off 0.0065" per cc of chamber. For each 0.010" removed from the head, remove 0.012" from the intake surface of the head. Do not sink the valves.
With standard valves the seat angles should be 45 degrees with seat widths 0.050" to 0.070" and kept as close to the outer edge of the valve as possible. The seat approach angle should 70 degrees. For larger valve use 360 valves, 1.60" exhaust and 1.88" intake. Exhaust should be faced 45 degrees, valve seats 45 degrees, with a width of 0.050" to 0.070". Dress the seat bottom with a 70 degree stone. Top dress with a zero degree stone. Use a hand grinder for a smooth transition from the 70 degree bottom-dress to the port. Intake valve should be faced 30 degrees. The calve seats should be 30 degrees, with seat width 0.050" to 0.057". Dress the seat bottom with angles of 40, 50, 60 and 70 degrees. Each bottom dress should be 0.070" to 0.090" wide. Top dress with a zero degree stone to provide some clearance between seat and the cast chamber. Use a hand grinder to smooth the transition from the 70 degree bottom-dress to the port. The large radius approach to the intake valve seat provides a greater power gain than opening the up the throat below the valve seat and using a tighter radius approach, keep the valve seat as close to the outer edge of the valve as possible. Remove the intake port guide boss inside the intake ports. |
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