340 Clean Slate - what to do

-
marksmopars

marksmopars

Well-Known Member
Joined
Nov 30, 2011
Messages
352
Reaction score
27
Location
Minnesota
We (my son and I) have been doing body work etc on our 70 swinger project for a little over a year and are getting to the point where it is time to start thinking motor. We have shortened a C body 8 3/4 for the rear with 3:921 sure grip and installed disks off a 73. We installed a hump for the 4 speed also, note that the car was a 198 with 3 speed column shift on arrival. We have assembled the makings of a 340, forged crank, J heads with 2.02/1.6 that are pocket ported and an edelbrock manifold. Also have a rocker arm set up off a 273 and a number of stock 340 rods. Our intention is a driver and we have been looking at what is readily available in rebuild kits. I have always been pretty good at the body stuff and can turn a wrench, but no deep knowledge on the drive train side. I see cast pistons versus forged, many cam options, etc. I have read a number of the discussions here and thought I would ask the group here for your thoughts on how you would approach the engine for this project. Note that we have crafted floor pans from sheet metal and scrounged parts from many member here as budget is a big issue. Please take a moment and offer us some of your wise guidance, it would be greatly appreciated.
 
A flat top Hyper/forged piston ,mocked up & cut to zero deck. A nice hydraulic cam.How much horsepower ,you want? 400 makes for a decent daily driver number. A good dual plane(air gap or not),beats a single plane here. Cam specs In the 220-230 @.050 range/.460-.500 lift/110-112 lobe centers. Headers are a pain,help the overall power band. Good luck.
 
Dont waste your money on the 340 , buy a roller cam magnum 360 and build a 408 stroker , it will be way more bang for the buck
 
Roller and valvetrain will set you back $750 easily. You got the mechs, go solid. A little valve adjusting never hurt anyone. Solid can do about 4 less in duration with same results. My slant has solids and I cant hear them at all.
 
marksmopars said:
Our intention is a driver
Click to expand...

Reading this, i'll stay on the conservative side.....A solid 300hp with compression in the low 9's is achievable with a fairly mild build that's pump gas friendly with the iron heads, and although that sounds low by today's standards, it will easily put a regulation weight A body deep in the 13's with a 4spd & your gear.

Piston wise, even though you may find a Badger/Speed Pro casting available, just go with a KB hyper. Way better quality for just a couple bucks more. With a stock oil pan, a standard pressure/volume pump should be fine to 6k as long as the bearing clearances are in the stock range. While we're on the short block, i'd recommend spending the money for balancing regardless of what piston you end up using. A lot of this will come down to the numbers your shop provides, meaning deck height, head gasket thickness, chamber cc, etc.

Cam wise, I'd stay with a hydraulic for a mild build. The 273 rockers are great for a hyd also since you can fine tune the preload. You can get pretty liberal on your cam choice with the 4spd & that gear, depending on whether the heads can take a dual spring or not?

With a dual plane intake, a 650 to 750 carb, and headers, you'll have a very fun street car to tool around in. :thumbrig:
 
I say go with the Comp 268 and hold the hell on.
 
9.5:1 hypers or forged, voodoo 268 or XE268 or 274 style cam, heads you have, intake (???) edelbrock is wide open, which is it? Torker340, throw it in the trash. RPM, Air Gap, LD 340 would be best choice. A Performer 318/360 with some clean up work would be good. 750 holley carb, dougs or tti headers.

That's a solid 330-360hp engine capable of mid 12's/105+ in a well set up car. Not bad for a somewhat well street mannered 340.
 
...guy at the junk yard gave me $10 for scrap value on the last intake I dropped off...I was ready to just pitch it
 
autopar3000 said:
Hey, now that's a pretty crappy thing to say. It makes a perfectly good door stop!!

:toothy7:
Click to expand...

Yep, My mistake. Just found pinion snubbers as better stops to keep shop door open and take up less space! :)

:cheers:
 
I'll jump on the bandwagon for the KB hyper flat tops-they're super light compared to the factory slugs they used to hang off those rods. I'll also second the notion for a comp XE268 or Lunati voodoo 268 or something like that. Headers would certainly help open it up, but if you want some simplicity, manifolds will work. Rpm airgap, Weiand stealth, or LD340 would be a great match for a mild streeter 340...

Jmo
 
The picture below is pretty close to what a factory 340 piston will look like. Actually, this is a JE piston that has shorter skirts and is made of different material. The difference between high compression 68-71 340 and the low compression 72-73 is the pin placement in the piston. There also seems to be some difference of the height of the piston cap (top ring to top of piston) between the low and high compression piston. The high compression piston sits .045 above the deck at TDC, while the low compression piston sits .054 below the deck at TDC.

Granted aftermarket pistons will be all over the place. 9.5:1 seems like a reasonable ratio on a street car to shoot for.

Given a 9.5:1 compression ratio, I recommend a cam that ranges 195º-214º duration @ .050". Couple that with no more than a 3.73:1 rear end. This is higher than I have ever recommended for a street car (3.55:1). If you want to keep the 3.92:1 rear gear, you're heading for race car territory; 10.5-11.0:1 compression, camshaft from 225º-240º duration @ .050". One of the problems IMO is that a 10.5:1 iron head LA won't run well on currently available pump gas.

View attachment 340 piston.jpg
 
The KB pistons will only be .018" above the deck if your actually at 9.6" deck height...your J heads are more than likely closer to about a 70cc volume unless they've been shaved or had aftermarket valves installed (or both). Depending on your head gasket choice and assuming .030" over, you'll actually be closer to ~9.7:1. That's still doable with iron heads properly chosen cam timing events and a reasonable ignition timing curve.

If you can spring for the forgings, I'll put in another plug for DSS slugs...they're competitive in price and they're pretty light. They come in at .020" under 9.6" deck and would certainly drop your static compression compared to the KBs.

http://www.dssracing.com/6300X_p/6300x.htm
 
Thanks for all the input, greatly appreciated. I doubt that I can get hands on the cash required to do a stroker, perhaps down the road. The manifold I have is a DB4 and I also have a Holley 750DP to go with it. I figure it should be touched to match the ports to he heads which we can do as it doesn't look like it would take much. I need to ask what are workable tolerances on pistons as the short block assembly I have includes pistons and with uncle sam taking 50 cents out of every dollar money is an issue. I would use the ones I have if possible figuring that upgrades down the road are likely easier once I have a street-able ride. I still need to get some body seals and a windshield and the interior is marginal as well. I would like to use pump gas so advice on how to identify the the compression on what I have is also something which would be very helpful.

Thanks!
 
A D4b or an LD4b? The D4b is for the early 273 bolt pattern. Bolt hole angle is a little different.

Cam choice will be a big factor in whether it will be pump gas friendly.
 
:happy1: I'm building from scratch as well and enjoying this thread ?

Curious why nobody is recommending the XE275HL yet...
 
crackedback said:
A D4b or an LD4b? The D4b is for the early 273 bolt pattern. Bolt hole angle is a little different.

Cam choice will be a big factor in whether it will be pump gas friendly.
Click to expand...

Definitely a D4B, though I have had it on a 340 before as I used it to break in my T/A motor, was worried about the six pack for breaking it in. I have heard before that the cam can have a big effect on this and definitely need a pump gas capable unit. Any guidance on this is welcome since it is a bit beyond me how to understand this aspect!
 
I just had a stroker 340 built not too long ago and was really surprised how cheap it was. It's a 73 model 340 with a factory 360 cast crank turned down mains and internally balanced, Eagle I beam rods and KB pistons. I decided to build it to run on E85 ethanol so I could bump the compression up and still drive it on the street. Compression is 12.5. Aluminum heads were in the plans at first but I went with some W2's that have been sitting on the shelf for at least a decade and had them worked as much as possible. The cam is just a Mopar .590. Nothing really special about the motor but It was really cheap minus the heads and rockers. It might be something to look into anyway.
 
You must log in or register to reply here.
-

Back
Top