340 buildups

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Daves67Cuda

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I have a 67 Barracuda with a 318 and 4 speed. Are there any suggestions for a 340 buildup nothing to expensive or anything, and a good runner, like 13's or so. I would have the 340 block and magnum heads, will these work on that? Thanks
 
Thats an easy one.

340 with a good set of flat top pistons as close to zero deck as you can get them. If they are 68-71 replacement pistons you'll either need to shave them down about .020" or run a thick gasket to keep them from kissing the Magnum heads closed combustion chambers. You should end up with alittl eover 10:1 compression.

Leave the heads stock except for a good valve job and new valve springs.
For a cam Id look in to something like a Comp cams 270H or XE268

Edlebrock makes a air gap rpm intake for the mag heads or you can drill the heads to use a LA intake, Holley 750 double pumper will work good with the 4 speed but would work alittl ebetter with some 3.91s. You could use a 3310 750 Vac secondary if you dont want a mech secondary.

1-5/8" headers and a 2.5" exhaust.
 
Solidly built 340 short, good Magnum heads (converted to std bolt pattern to run any intake), and the Comp XE262 would be my idea. The Magnums run 1.6 rockers, so it would be easier to overcam a street engine with them. Prob run around $3400 around here (CT) if you have the cores. I think it's a great idea, but keep in mind, those heads are made for a good controlled burn, and low speed torque. They will give up a little on the high end, and I would keep a shift point of 5500 or lower with them.
 
I just built a 360 with magnum heads. It's has zero deck KB107 flat top pistons, and 0.039 head gasket which results in 10.6:1 compression. I am running a Comp Cam XE268 cam, with a Professional Products CrossWind dual plane manifold (copy of an Edelbrock RPM Air Gap). I have 1 5/8 headers with a 2.5" dual system with a x-pipe and Flowmaster 50 series mufflers. I am currently running a holley 600cfm vacuum secondary carb. The engine is in a 68 Barracuda with a A833OD 4spd and 3.55 geared sure-grip rear end. I now have a little over 1500 miles on the car since installing this engine. Below are my impressions;

* The engine is very strong. It has instant throttle response off idle. Nailing the throttle at any rpm above 1000 to 3000 in first gear and the back tires go up in smoke. A hard shift to second and the back end steps out with another cloud of smoke. The engine does not lack for low rpm punch :)

* The engine continues to pull hard right up to 5000 rpm but starts to roll over at 5500. I believe a bigger carb will help but both the cam and intake are geared to make power from 1500-6000 rpm. I have been looking to borrow something in the 700 to 750 range to try but I don't want to loose the crisp response on the bottom end where I drive the car.

* The Comp XE268 cam is a hydraulic but it is noisier than is normal from a hydraulic cam. The brain trust here as well as Comp feel that it's a nature of the very aggressive lobe profiles. Most people ask if it's a solid cam.

* I have no detonation problems with the 10.6:1 compression as a matter of fact it runs well on 89 octane. Gas mileage was not a consideration when I built the end but with $3 gas I have been checking and it's getting about 15 mpg around town. The 600 cfm carb is jetted a littel rich so I hope to find the time this weekend to do some jetting and maybe squeak a littel more out of it.

A 340 will not have the low end grunt the 360 has but you can expect pretty much the same characteristics from a similar 340 build.
 
Thanks for the advice, I'm gonna look into it and see what I can do. Wouldnt it be better with the dome pistons? but i'm not sure since you said they'd have to be shaved down for the magnum heads, so is that why I should look into flat top ones? Thanks again.
 
i am not sure of how much smaller the chambers are on the magnum heads but i am pretty sure that a domed 340 piston will hit the chamber or create to much compresion(never thougt i would ever say that there is such a thing as to much compresion ;) )
yeah check out the flatops you will be fine with dose i believe!
 
The Magnum heads have a closed chamber and wont work with a domed piston. They also have a 60-62cc chamber for more compression instead of the 68cc chamber on other 340-360 heads.
 
If I get the block and heads, I'm thinking of maybe boring it out, cuz I'll have to get new pistons and stuff, what would you guys say on that? 30 or 60 over or what? Thanks alot again, my dads helping and stuff, but we dont really know alot about the magnum heads. Thanks again.
 
I don't know how set you are on a 340, but I might suggest the Mp 10:1 360 short block. You can get them for around a grand. I got one of those, put a .450 Mp cam, stock x-heads, edelbrock performer rpm intake, 1 5/8 headers, 2.5" exhaust. That coupled with 3.21 gears (8 1/4 rearend), I ran a best of 13.19 @ 106mph.
 
Only go as far as you have to when boring the cylinders. Less heat and room for next time. Plus I don't think you gain that much power from 3 or 4 cubes. Just my $.02
 
Daves67Cuda;

I read this thread through. Your getting very good advice here. You'll want to run flat top pistons for several reasons. There cheaper to start. You won't have to make mods to fit the domes into each head chamber, and comp ratio adjustment is handled by head gasket thickness changes.
If it's a zero deck build, the MoPar .020 thick gasket is out. to close. I suggest a .039 Fel-pro or equal.
Keep overboreing down to a min. The thicker the cyl. wall is, the straighter it will stay down the road. This equals power and long life.
Adams cam suggestions are fine. They'll get the job done. But, if you decided on a different one, choose the cam as a total package to the build. It should work in concert with the style of driving rear gears/tire size.
Over camming a engine is duration based problem. Not lift. Though to much lift is a short coming of ethier valve to piston clearance or a head problem. In the valve guided clearance or coil bind.
(Coil bind is a problem with big cams and/or wrong springs. Theres more to it, but I'm not getting into it since the cam you'll use for the target time slip is small. Piston to valve clearance is a near nil problem since the cam is small and modern pistons have great valve reliefs for monster cams.)
 
Just another note. If you use the Magnum heads you need to keep total lift (remember they use 1.6 rocker not 1.5s) under .520" or you'll need to cut the guides down for retainer clearance.
 
So I'm picking up the 340 this weekend I think, but its not gonna have the magnum heads, just the stock ones, but I'm not sure what the valve sizes are yet. So any more suggestions or just stick with alot of this advice? Thanks
 
If the 340 has 1.88 intake valves in the heads, I wouldn't cry about it, with the specs given.
In fact, the smaller valve is going to work better for street usage, on a 340, than a 2.02 valve will, anyway, especially with the cam choices and compressions I've seen in this thread, so far.
1.88s should handle 6,000 RPMS on a non-stroked engine (Under 380 cubes), easily.

Mark.
 
If some one put magnum heads on the 340 the stock size valves are 1.92 I and 1.62 E.
 
Thanks Adam. I could not remember max lift on a stock Magnum head. But, at .520, that should cover most bases on a street and light race engine.
 
I picked up the 340 today and stripped it all down. It had one 340/360 head, then the other was a 318 one. Then the blocks blue but the date on it is 6-15-71 and it has a steel crank (72-73 maybe?). I think the cam is a 268H. We also brought home the magnum heads, but we're not sure if we're gonna buy them($300 to buy them, or same price to get the other ones cleaned up and stuff), or maybe go for aluminum ones or something. It's gonna need to be bored .30 over and hopefully its all good, but hey, we got it for free so if its bad, I guess its not too bad.
 
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