Piston recommendation for 340

-
DFootCuda

DFootCuda

Well-Known Member
Joined
Jun 30, 2011
Messages
283
Reaction score
16
Location
Friday Harbor, Washington
I am getting ready to start rebuilding my 340 short block , it has to be bored .030 over and machine shop wants to have pistons before doing job . Just wanted to know if any one has recommendation ? The 340 is out of ( # match ) 1969 340'Cuda . I only have the short block so far but am planning on no more than 9.5 to 1 compression . Also need recommendation for heads since I have none . Looking for know more than 350 horsepower engine but need it to last since engine goes with the car . If you need more info let me know ?
 
You got to pick heads first so you know what piston to run to get your CR. any stock 340/360 head will get you 350+hp but EQ or RHS are probably the best deal and good for 450hp OOTB but are closed chamber gonna need a dish piston, here's a link to a CR calculator too play around with. http://www.kb-silvolite.com/calc.php?action=comp
I did a quickly on it 62cc chambers, .04 gasket, zero decked you'll need a piston with about 12cc dish for 9.5CR
 
With a '69 340 you have to keep in mind that pistons patterned after the stockers will be around .018" proud of the deck. This makes closed chamber heads a little entertaining, not only because of the compression ratio you get from the small chambers but also because of the valve/piston clearance.

I built up a '68 340 that's about to go into my Duster, I've got KB243's in mine, which are in fact .018" over the deck. I run a set of 308 heads (open chamber) on it that have been ported to get the flow I wanted. With 65cc heads, a .039" gasket and the 243's over the deck .018" I ended up at 9.8:1. (The 243's being over the deck mean there's some extra calculations on the piston volume side, I got my numbers with some help from KB tech).

You definitely want to pick your heads first on this one. Keep in mind that if you do go closed chamber, you may need to spend some extra cash getting a piston that will work for you. For a 350 hp build, I would suggest just staying with the stock heads and having a little work done to them- 2.02/1.60 valves if they aren't already, maybe a little clean up/smoothing work on the ports and a good valve job. That will be more than enough for 350 hp, and you'll be able to use off the shelf KB pistons and not worry too much about valve clearance.

Here's a link to some 360 build ups. Gives a decent idea of where you need to be with a 340, figure on a little less HP and torque based on a few less cubes and shorter stroke. But on the builds that are close to 1 hp per cube, you'll be in the ballpark for 350 hp. Notice that on the builds that are less than 400 hp most of the heads are basically stock, just the 2.02/1.60 valves and a little clean up work.

http://www.mopar1.us/dyno.html
 
I rebuilt mine 340 using Edelbrock 60779 heads with Ross racing forged aluminum pistons (flat tops). Ended up with ~11:1 comp ratio but with aluminum heads it dont matter.
 
jimjimjimmy said:
whatever specs you decide on get forged
Click to expand...

Totally unnecessary for a 350 hp build. Are they more durable? Yes, absolutely. Are they better if you're going to keep adding HP down the road? Yes, keeps your options open. But, if you're not planning on nitrous or boost its just an extra $200-300 down the tubes. Plenty of folks here have had great success with cast and hyper's. My KB243's are hypers, and I'm going to end up with quite a bit more than 350 hp. I'm not worried.
 
I didn't catch how you plan to use the car (road/track). If it's going to be a road car, cast pistons will do well. If you're trying to keep the CR low in a road car, suggest the 72-73 stock 340 piston. If it's going to see regular appearances at the track, forged pistons are recommended.

For heads, I recommend the Edelbrock 340 heads (Al), the Indy LA-MAXX (Fe), and the Mopar J (Fe) in that order. Because the aluminum (Al) heads dissipate heat better, it is possible to bump the CR by ~ a point and not have detonation issues. All of the heads recommended are big valve heads supporting a 2.02 intake valve.
 
Pay close attention to the weight of the piston. The KB hypers are light. A regular stock replacement piston will be heavy. Heavy pistons means slower revs and more stress on bottom end. And hypers pistons will be plenty strong for what you are doing. You could probably get the .018" shaved off the top of the pistons to zero deck them if you want to go with closed champber heads. But J heads or any other 360 stock heads will get you what you want. Here is an article on a nice 340 build:

http://www.compcams.com/information/Articles/Type/Chrysler/CC_MM0201-001.asp
 
J heads run about 72cc's give or take 2cc's. with that info you can read the sites info or piston specs to what the c-ratio should be with the piston your looking at. Keep in mind what gasket there using and the overbore size of the piston.

350 HP is mearly adding a small cam & headers to a stock 340 and a careful tune of the engine and carb.
For added punch, I suggest the upgraded 340 cam.
 
Diamond Pistons. Chose camshaft first then heads and then pistons. The bigger the cam the more compression needed, I run a SB428 with 12-1 with a 700 lift roller and can run pump gas on the street. When It makes traction and the motor is loaded is when it needs race fuel. 829 at 7900@ the crank. 1050 quick-fuel and 2 inch tubes to 3.5 exhaust sees 9300
 
You must log in or register to reply here.
-

Back
Top