I have a set of 68' 340 x heads that I am putting on a 318. I have heard that the compression drops when putting 340s on a stock 318. Was wondering what all I would have to do to get the compression right. I have read somewhere that a set of Keith Black pistons fixes the hole issue. Just wanted to know that you all think.

 

s -

I actually don't know what x heads are, but I ran 340 heads (from a 70 'cuda, IIRC) on my 74 318, and there was an overall boost in performance. If there was a CR drop, it was more than compensated for by the larger valves.

Archer

 

I would definitely try to up the compression or else you will be a dog off the line. KB pistons would work great and youll probably see the greatest increase in tq. Or you can go the route of milling the heads down to lower the chamber volume and increase compression. But if you mill too much off youll have to consider custom pushrods. It just depends on what you want your compression ratio to be in the end, you can do the math to calculate what your CR would be if you switch pistons or mill heads. Hope this helped

 

If you are planning to bore it ? yes get the KB pistons BE careful withe 340 heads on 318.. You may need to chamfer the cyl bore for valve clearance.. you have valve shrouding and if you have installed oversize 2.02 valves they may hit the cylinder wall SO CHECK clearances. A 030-.040 over bore you may be Ok Do not use on a 273 motor

 

Yes I do plan to bore. Atleast .30 maybe even up to .60 over. And the heads do have the 2.02 valves. I don't to have to do anything to the heads as they were expensive. But I don't care to spend it on the block to get the compression right.

 

If you are re boring it It doesn't cost any more to get the correct pistons

 

I don't care who builds your engine !! It needs a blueprint !!!!!

 

Honestly, sell the X heads to someone who wants them (some people think they are the beat all for small block heads) and get a decent set of 1970-71 J heads (3418915) put 2.02 intake valves in them, they flow better than the X's, port and polish them and you will be very happy with the end result. I did it to a '72 stock 318 in a Charger SE and it ran great.

 

I also have a 340 Air Gap intake with a 670 Street Avenger Holley set-up to go with the x-heads. I am definitely going to use the x-heads as that's what I want on the engine. Everything else is just extra to add to it. I want it to be a little 318 screamer.

 

You'd be better off with Magnum heads unless you have the X heads setup for unleaded gas

 

Depending on the year of 318 the compression will run from 9.2 - 8.6. You will lose .75 - .88 point of compression so at best 7.7 - 8.45 compression with a .025" thick gasket.

I did this long ago with a 284' .484" purple shaft cam and 2500 torque converter with 3.23 gear. It was a mutt off the line but when it hit 3500 rpm it was like someone turned a switch and went like hell up to 6500, dumb kid but lots of fun.

I say what the heck go for it and put in a better modern camshaft, maybe you'll get lucky like I was and burn a rod bearing then have to build a proper short block.

 

I plan on using a Comp Cams cam kit. Not sure which one just yet as I don't know what will be the best drivable cam for the motor yet. still researching that one. Would decking the block of milling the heads help keep the compression where I can still use pump gas?

 

Before deciding on decking or milling get familiar with piston compression height and block height (9.6in.) Most stock pistons run 1.74-1.77in. making the pistons 50-80 thousandths "down in the hole" with low compression and no quench. Many aftermarket pistons run in the 1.81" height raising compression and allowing a chance of quench with closed chamber heads and the correct thickness of head gasket.

If you have open chamber heads manufacturers are making "stepped" pistons which will give quench if matched correctly to the chamber, again with the correct thickness gasket.

Before you look at camshafts finalize your short block and compression ratio. Don't forget to look at piston valve notches and unshrouding the cylinder bore at the top to accommodate the 2.02in. valves. Wouldn't hurt to look at some block oil modifications too.

 

Before you look at camshafts finalize your short block and compression ratio. Don't forget to look at piston valve notches and unshrouding the cylinder bore at the top to accommodate the 2.02in. valves. Wouldn't hurt to look at some block oil modifications too.

340/2.02 heads will be a direct bolt on. No need for unshrouding/the cylinder bore on a 318. This operation is only necessary when putting 2.02 valve heads on a 273.

I also did this type of build with a 318. Used the "U" 340 T/A head, 284/484 MP camshaft and 6BBL.

 

Not unshrouding to prevent interference but to aid air flow around that side of the valve, 273's are an interference problem.

 

Sounds like I just need to let Coronet 500 and Drag Pak build this motor for me.