340 Build

[LEVOHS]

I know this has probably had many post.Starting to build 340,block bore is .020 now but needs cleaned up.Have new in the box mopar P4876785 commando 340 heads 2.02/1.60 these are 60cc combustion chamber heads.Forged crank turned .020 undersize.Will be running 4-speed/391 gears.Was thinking of KB-243 pistons,Scat I-beam rods 6.123,Hyd. cam .508/292 not sure on intake and carb.Would this make a strong street and 1/8 mile car?

 

[rumblefish360]

I'm not sure where the piston wll sit in the cylinder and it's over ratio will be. But if it is pump gas freindly, then IMO, your off to a great start.

The rest of the parts so far suggest a 12 second 1/4 mile car.

Find the thread, "Post your 12 sec. combo's"

 

[70Hardtop]

A Victor 340 single plane (Edelbrock) would make a very good inlet i think, if you havn't found one yet. Or Edel 340/360 RPM AirGap.

 

[racerbob]

My engine is very similar to that :

.030 over 340 Forged 10:1 pistons, stock crank and rods, 292/.509 MP hyd cam, 2.02/1.6 "J" heads, wieand Single plane intake , hooker comp headers, 3600 turbo action converter, 727 with manual valve body. 4.10 gears with 26" tall slicks

Pushes my 3340 lb dart sport down the 1/4 in 11.92 seconds at 111.91 mph

I trailer it to the track , but the car could easily be street driven if I wanted to.

Bob

 

[70aarcuda]

your commando heads are closed chamber heads...

you will need to see how far out of the bore the KB pistons will stick out...

you can use a thickner head gasket to get the necessary clearance between the pistons and the heads...

but you need to figure out first where the pistons are ......how far out out fo the deck to they stick up

 

[72bluNblu]

I think you're going to have a problem with the closed chamber heads with 243's.

I just built a '68 340 with KB 243's and can attest that they are .018" over the deck, as KB specifies on their website. I even measured it on my engine, they really are a full .018" out of the holes. This will probably give you a clearance problem, not to mention a compression ratio of 10.5:1.

Edelbrock LA (60769) heads have the same problem (closed chamber), which is why they offer a "340" (60179) version that has been machined for clearance (open chamber, 65cc volume).

I looked at both the edelbrock heads and the commando's because I wanted to go with closed chamber heads, but there just isn't room with the 243's. Because I had already sent the pistons to the machinist, but didn't have the heads yet, I ended up going with 308's with my build. They're open chamber, but if you have your closed chamber heads cut for clearance they won't really act like closed chamber heads anymore anyway. Plus, depending on how its done, the chamber shape may suffer.

So far, my 340 is set up like this:

KB 243's .060 over
Stock forged crank, .010" under and fully balanced with the rotating assembly
Stock rods, re-sized and with ARP bolts.
Lunati 60404 (Advertised Duration 276/284, Lift .513/.533)
308 heads, 2.02/1.60, 66 cc chambers, stage 3 port, flow 264 cfm @ 0.500"
.040" head gaskets (~9.85:1 compression ratio)

Not in the car yet, but it will also get Hooker super comps (1 3/4"), Edelbrock air gap and a street avenger 770. Should be between 400-450 hp, depending on who you ask.

My recommendation would be to go with a different piston, since you already have the heads. Unfortunately, as I'm sure you're discovering, piston choices for a 340 are a little limited, especially if you have to go bigger than .030". At that point you have stock cast (ugh!), the KB 243's, or you're on to forged pistons.