Opinions on a 340

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BlueDream

BlueDream

Aspiring Mopar enthusiast
Joined
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Location
Fayetteville, Arkansas
I came across a 340 locally that I can pick up for a fair price. For under $600 I'd get freshly machined '72 340 block with new cam bearings and freeze plugs, with the crank, rods, and new pistons. I also have a set of 915 J heads that I'd rebuild and put on the 340 if I got it. My car currently has a mild 318 in it with a COMP High Energy cam, Edelbrock Performer intake, Edelbrock 1405 carb, Hooker Headers, and a dual 3" exhaust (yes I know it's too big), a230 trans, 8 1/4 rear with 3.21's. My plan was to buy the 340 and build the long block with the parts I buy and the heads I have then I'd take what I could from the 318 like the oil pan, carb and intake, timing cover and front accessories, COMP 901-16 springs, true roller timing chain, rockers, and the valve covers. Is this a decent plan? Is it worth the trouble? And with the information I've given what would be a good cam to go with? I've been looking at the Thumpr line of cams. I want something that goes lumpity lump but still performs good. I'd like to know what you guys think. Thanks!
 
What pistons are you using? What is your compression ratio going to be?
 
gtsdude said:
What pistons are you using? What is your compression ratio going to be?
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I believe the pistons that come with the motor are stock compression. So 8.5:1 for the 72 340. If I had to I could buy higher compression pistons but if the motor would make good power without them then I'd just leave them.
 
For every point rise in compression you will make approx. 3% more power.
More importantly, the larger cam installed will run more efficiently rather than like a lame dog down low.
A lot depends on the intended target of the cars performance and power level sought.

What is it you want to do with the car and how it performs.
(Do not shoot for a HP level as this normally leads to a long list of issues old trying to get a car to perform and be enjoyable for what you want to do with the car.)
 
BlueDream said:
I believe the pistons that come with the motor are stock compression. So 8.5:1 for the 72 340. If I had to I could buy higher compression pistons but if the motor would make good power without them then I'd just leave them.
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Why? That compression is dead perfect for a street car. Don't get caught up in all the compression hype. You WANT lower compression in a street car. Gas IS gonna eventually go back up. Build it to run on 87 and be happy. You can make way more than enough power to put a smile on your face and get you in trouble.
 
rumblefish360 said:
For every point rise in compression you will make approx. 3% more power.
More importantly, the larger cam installed will run more efficiently rather than like a lame dog down low.
A lot depends on the intended target of the cars performance and power level sought.

What is it you want to do with the car and how it performs.
(Do not shoot for a HP level as this normally leads to a long list of issues old trying to get a car to perform and be enjoyable for what you want to do with the car.)
Click to expand...

My end goal is for the car to throw me back in the seat when I mash the gas. I also want it to have enough power to turn two 295's into a cloud of smoke at will. I'm a couple hours away from a drag strip so It'll be a street car 99% of the time.
 
RustyRatRod said:
Why? That compression is dead perfect for a street car. Don't get caught up in all the compression hype. You WANT lower compression in a street car. Gas IS gonna eventually go back up. Build it to run on 87 and be happy. You can make way more than enough power to put a smile on your face and get you in trouble.
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So what would my cam options be like with 8.5:1 compression? I want more than what I have in my 318 but I still want it to be streetable.
 
ir3333 said:
..stock 340 style cam with 2400 stall converter.
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My 318 has a cam that's close to 340 spec with 268 duration and .454' lift. I feel like the 340 can handle a lot more cam than what it had when it left the factory. I haven't put any thought into the converter since I've got three pedals and a pistol grip... :burnout:
 
One of the smaller VooDoo cams would be good. The Comp 275DEH is another good choice.
 
would like to chime in here my old motor 340 30over pop ups 915s with 208in 160ex 528purple solid 3'' ex sounded wicked burned 275x60x15s to the ground 410s helped idled at 1000rpms the 915s have a large chamber i got away with 93 octane and octane booster.imo use flat tops and zero deck the block then u can run 93 or premium fuel all day long
 
With 8.5 and open chambers and a cam to make you smile? It will never happen. It will make maybe 5% hp more than your 318 and the bottom will be soft.
-I would ditch those slugs.
-Like was said; zero deck flat tops and closed chamber heads. That will get you closer to 10.5 c/r.Then you can cam it.That will get you close to or into the 400+ hp. And thats where the smiles start.
-If you have to stick with 8.5s, then Id put some 318 closed chamber heads on it with a 360 2bbl cam and a smallport dualplane intake and a TQ to top it off. You will be smiling for 6 days or 300 miles.Then park it for a week.Repeat. Every time you drive it after the resting, you will again smile. Ask me how I know.
-If you fail to park it for the resting week, then sooner or later the smiling will end,and you will be be wishing you had put the flat tops in it,and the closed chamber big port heads, and a real cam, because even though its making plenty of torque, its way soft on top.
-It would actually run pretty decent with your plan, including the current 318cam.It will probably be a mid 14 second car.Mid 14s made me smile when I was 16. Big time.Its addictive. I quit spending money at mid 12s. It gets expensive in a hurry from there.
-I wouldnt want to be in your shoes, making this decision.
-And, like Rusty said, 8.5 will work in a DD. Just be really careful with cam selection.
-Too small is better than too big.No lumpity-lumps at 8.5. Well, let me rephrase that; I would stay away from the lumpity lumps at 8.5. Id be trying to keep the Dynamic c/r up.

Happy Wrenching
 
BlueDream said:
Is it worth the trouble? I'd like to know what you guys think. Thanks!
Click to expand...

This question is a lot harder then it looks on the surface? You have a combo that's just about perfect for a nice street car, with some power.

The "J's" and some tweaking with the cam will make a little more power with the 340 vs the current teen, but is it worth the time, effort and $$$? idk.
 
AJ/FormS said:
With 8.5 and open chambers and a cam to make you smile? It will never happen. It will make maybe 5% hp more than your 318 and the bottom will be soft.
-I would ditch those slugs.
-Like was said; zero deck flat tops and closed chamber heads. That will get you closer to 10.5 c/r.Then you can cam it.That will get you close to or into the 400+ hp. And thats where the smiles start.
-If you have to stick with 8.5s, then Id put some 318 closed chamber heads on it with a 360 2bbl cam and a smallport dualplane intake and a TQ to top it off. You will be smiling for 6 days or 300 miles.Then park it for a week.Repeat. Every time you drive it after the resting, you will again smile. Ask me how I know.
-If you fail to park it for the resting week, then sooner or later the smiling will end,and you will be be wishing you had put the flat tops in it,and the closed chamber big port heads, and a real cam, because even though its making plenty of torque, its way soft on top.
-It would actually run pretty decent with your plan, including the current 318cam.It will probably be a mid 14 second car.Mid 14s made me smile when I was 16. Big time.Its addictive. I quit spending money at mid 12s. It gets expensive in a hurry from there.
-I wouldnt want to be in your shoes, making this decision.
-And, like Rusty said, 8.5 will work in a DD. Just be really careful with cam selection.
-Too small is better than too big.No lumpity-lumps at 8.5. Well, let me rephrase that; I would stay away from the lumpity lumps at 8.5. Id be trying to keep the Dynamic c/r up.

Happy Wrenching
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I need to stick with the 915 heads because it's what I have available to me. I scored a complete pair for $25 a few months ago. I'm thinking about ditching the stock 8.5:1 pistons and going with the 1971 pistons instead to up compression some. I talked to the guy who I'm buying the motor from and they're stock compression pistons for 1971 or 1972. He was going to build a '71 340 but ended up with this 72. So the pistons could be for either, I'll know for sure when I get my hands on the 340 soon and can measure them. I was going to swap in a 340 long block in place of the 318's and keep my carb and intake and all my accessories. I want to keep this on a budget because I'm a college student and don't have a ton of money to make this happen. But considering I can get a machined 340 block, pistons, rods, and a crank plus some other stuff for $500 and some change I think it's a good buy. The only other 340 I can find around here is for sale for almost $2,000 and it's not even a runner, it's just in pieces. The 1971 340's came with 915 heads if I remember right and they made good power so I'll probably use the 71 style pistons. Lets say that the pistons that come with the 340 are the 71 replacement ones. To get the short block together I'd just need the cam and lifters (bout $200), main and rod bearings ($50), gasket set ($60), rings ($40). I'd use the pan, oil pump/pickup, and timing set from the 318. The local machine shop will do the heads for $250. Thats $600 plus the cost of the initial purchase and $1,100 in total for the 340 long block. Then theres the cost of fluids and little miscellaneous nuts and bolts, I figure $100. Just swapping my induction and ignition over to the 340 long block should work right? I talked to Schumacher on the phone today and the 340 will work with their 318 mounts thats in my /6 K frame. Getting a 340 in my Dart for under $1,500 sounds like a good thing to me, and if I up the compression and choose my cam accordingly it sounds like it should run pretty good too. If I had more money I'd go all out with aftermarket heads and a different carb/intake setup and all kinds of other goodies. I'm just on a budget. I don't blame you for not wanting to be in my shoes making this decision, I only get one shot at getting this right. For now at least. I've been trying to hunt down a 340 for over 4 years now and to finally have one within my reach is a good feeling.
 
OldmanRick said:
This question is a lot harder then it looks on the surface? You have a combo that's just about perfect for a nice street car, with some power.

The "J's" and some tweaking with the cam will make a little more power with the 340 vs the current teen, but is it worth the time, effort and $$$? idk.
Click to expand...

How much more is a little more? My 318 is running stock '68 pistons and heads. I know those 318 heads aren't the best so that's got to be restricting how much power it's making. From what I've seen it doesn't look too hard to get a 340 close to 375-400hp. Shooting for a specific number would mean setting myself up for disappointment but I feel like a 340 would make my butt dyno happier than the 318. I've always wanted a 340, so I built the 318 to use until I got a 340. I just had one fall into my lap sooner than expected so this teen has really low miles on it, so I'll stick the long block in the corner and use it on a future project. As for it being worth the time and effort, I really enjoy building engines and working on my car so I don't mind putting a lot of time into this 340. I'm hard headed and dead set on having a 340, for better or worse. The money is kind of an obstacle but I've gotten good deals on some parts and can use a lot from my 318. I kind of outlined my plan in my last post but I think I can get it all done for under $1,500. We'll see how it goes... I just need to figure out pistons and cam. What pistons come with the motor will play a big role in how my budget is going to work. Hopefully the guy I'm getting the 340 from bought the 71 pistons.
 
375-400 hp out of a 340 is easy. I have a (hyper pistoned) 9.8:1 340 w 2:02 J heads. Stock forged crank and rods. Rods were lightened and ARP bolts installed. Some minor port and bowl work on heads. Voodoo 60403 .494/.513 cam Good old 273 rockers. LD340 Holley 750 18 initial w 36 total on timing and it made 398 HP on dyno. Not shabby for mild cheapish build.
 
I don't understand why everyone insists on giving you bad info. It most certainly CAN be done with 8.5:1. You simply have to know how to do it and which parts to use. More importantly, which parts to AVOID. 350 plus HP is easily within reach of a low compression engine, with ZERO chance of having trouble with pump gas. It's simply not true that everything has to have 9.5 or more to make reasonable power.
 
Simply not true, but given the reality of parts vs manual stated compression, that 340 is closer to 7.8 than 8.5:1. It's a weak point. Add to that the lack of any valve reliefs it makes camming up later if he wanted to a problem.
If it were me, I'd run the hypereutectic pistons as Swinger mentioned, have it rebalanced and the heads and deck set up to give an honest to goodness 9.5:1, run the rest of the stock parts with the exception of the cam and valve springs, and will get 375-400hp from it on any pump swill you find.
 
Ok. I am out of it. If yall don't think you an the kinda power figures I spoke of out of an 8.5:1 340 then yall are crazy. Look what Jim Laroy did with the low comp 440 build and yall don't think a 340 can do 350 or so HP with 8.5? Yall are nuts. Keep on feedin the guy horse feathers, cause that's what you're doin.
 
RustyRatRod said:
Ok. I am out of it. If yall don't think you an the kinda power figures I spoke of out of an 8.5:1 340 then yall are crazy. Look what Jim Laroy did with the low comp 440 build and yall don't think a 340 can do 350 or so HP with 8.5? Yall are nuts. Keep on feedin the guy horse feathers, cause that's what you're doin.
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You are correct, if you can obtain a true 8.5:1. Moper's point is he's not likely going to get 8.5:1 unless things are checked and corrected to get 8.5:1. We all know the stock "10.5:1" 340's we more like 9.something:1 from the factory.
 
RustyRatRod said:
I don't understand why everyone insists on giving you bad info. It most certainly CAN be done with 8.5:1. You simply have to know how to do it and which parts to use. More importantly, which parts to AVOID. 350 plus HP is easily within reach of a low compression engine, with ZERO chance of having trouble with pump gas. It's simply not true that everything has to have 9.5 or more to make reasonable power.
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I've balanced and blueprinted more than a few "low" compression 340's, stock everything except milling .020 off the J heads, 1.88 intake valves, including cam intake and carb. They fly. And you can use pump gas all day long. The next engine in our 67 Barracuda will be just that. As for being 5% more than a 318, dream on.
 
If I change pistons, how well would the stock 71 slugs work in the 72 block with the 915 heads if I don't mill them down? What kind of compression would that get me and how well would it run on pump gas? I'm not planning on a massive amount of power but I want the engine to pull good to 6,000 RPM on occasion without turning into a 340 cubic inch grenade. As far as valvetrain stuff goes I'll be using the stock stamped rocker arms for now, 273 rockers down the road if I come across a good pair. My COMP 901-16 springs only have about 1,500 miles on them so I'll use those and they're good to about .500" lift. My biggest problem with low compression is the cam choices. Maybe it's just my inner 16 year old talking but I absolutely love the cool factor of a nice healthy cam. One of the most fun experiences I've ever had was riding in a Dart with a 408 stroker that ran 10's and it had a Purpleshaft cam, .508" I believe. Man that thing would move and just pin you to the seat. It would shake the ground and I could feel the thump in my chest from 30 feet away. Can a 340 thump anywhere close to that with low compression? If I decide to up the compression I think it's definitely better to decide that sooner rather than later. I don't want to get too crazy with compression but I want to be able to run a nice cam without killing performance. RRR suggested VooDoo cams and I like them but I think if I went with a small VooDoo i'd be wishing I put a bigger cam in down the road. Especially If I end up raising compression which would open up more cam choices.
 
dano said:
You are correct, if you can obtain a true 8.5:1. Moper's point is he's not likely going to get 8.5:1 unless things are checked and corrected to get 8.5:1. We all know the stock "10.5:1" 340's we more like 9.something:1 from the factory.
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I understand his point. Thanks. But I am not talking about the compression he HAS. I am talking about a true 8.5:1. I never once said anything about what compression he has.

I understand completely how Chrysler engines are. They had higher than advertised deck heights, larger than advertised combustion chambers. Those rated at 8.5 were luck to be an actual 8.0. I realize that. I have blueprinted really more engines than I care to remember. lol I was merely saying an honest 8.5 engine is plenty capable in its own right, and I'm stickin with it.

There's simply no reason to build a STREET engine with compression that will push the limits of pump gas wen they will make respectable power with god parts choices.
 
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