Originally Posted by skinnysfd
I have a 1970 GMC that we are builiding and the motor is my question. How can i make the motor put out 300+hp and what should i have done to the heads, block, etc..
A 1970 engine has valves seats that have not been hardened for use with unleaded fuels. This is something you should consider either fixing the seats or replacing the heads.
Fixing takes the installation of hard seat inserts, these plus the expense of refurbishing the guides, replacing or refacing the valves, replacing old springs, etc. quickly gets into the cost of more modern heads that include all these features along with much improved combustion chambers. While an argument can be made for retaining the beefier 1970 castings if this is a hard working truck, the power and reliability improvement of modern heads is a significant argument for replacement.
It's easy to recommend the Vortec head but these require a unique bolt patterned intake. If you have a suitable 4 barrel intake, you might want to look at the DART Iron Eagle or World SR heads as these can be had with some options which include using conventional intake bolt patterns. All these heads include improved ports though they're really designed with porting in mind by the end user, and they include the all important features of the Ricardo Fastburn combustion chamber. You are better to use the small chambered heads, these being the 67 cc rather than 76 cc chambers as the tighter chamber has better flow swirl and shorter burn time characteristics than the larger chamber. Compression will be established by piston choice which I'll discuss with the block.
I'm assuming you'll have the block taken care of by a competent machinist so I won't be-labor the point of deck milling if necessary, alignment of the main and cam bearings, and fixes to replacement of the rods if the big ends are no longer in dimensional spec. The same goes for the crank, if it's in decent shape perhaps only a journal polish is required to regrinding them if they are worn beyond what you can make up for with a slightly oversized .001 thicker bearing insert. Boring and honing the cylinders will restore their surface. So lets get to parts selection.
Pistons, these days cast hyper-eutectic pistons are the choice for everything short of going racing. While factory pistons have a circular dish used to establish the static compression ratio, these are not ideal for effective use of unleaded fuel. Modern fuels need all the detonation resistance help they can get, to that end the combustion chamber and piston have much to do with developing what's called mechanical octane. To obtain the most mechanical octane you want the top of the piston and the section of the combustion chamber opposite the valve and spark plug pocket to close toward each other to within .040 to .050 inch when using a steel connecting rod. This configuration is easiest to achieve with a flat top piston, but this can result in too much compression. The solution is the use of a dish in the piston, but unlike the factory's circular dish that kills squish/quench, the hot rod industry has developed the D dish that is located under the valve/plug pocket. A common piston is the KB193 see it at this URL http://kb-silvolite.com/performance....tails&P_id=155
Of course you'll need to select a dish volume specific to the compression ratio you're trying to hit. For 92 octane a static CR in the range of 9.1 to 9.5 with an iron head is good, for 87 octane fuel 8.5 to 9.0. to do the latter you need a bigger dish which is available. For bore diameter, I prefer the minimum amount of boring as there isn't that much displacement to be had against how fast increasing the bore is weakening the cylinder wall. So about .030 inch is plenty and will make your 350 into a 355.
Your 300 horse objective for a 327 or 350 engine isn't too hard to get at with modern heads, a 4 barrel and headers connected to dual exhausts. The key is the camshaft. Chevy used a cam P/N 3896929 that specs 195/202 degrees, 112 LSA, with .390/.410 inch lift in the 300 horse 327 with 11 to 1 compression. While we can't run iron heads at 11 to 1 on today's fuels in spite of all the tricks we can do with mechanical octane, a cam of same/similar timing and lift combined with modern heads and electronic ignition along with a 4 barrel and headers to duals will easily match or exceed the power numbers of the 300 horse 327.