Opinions on a 340

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RustyRatRod said:
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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How would you go about getting a true 8.5:1 compression ratio on this 340? I think somewhere between 8.5:1 and 9:1 would be a good middle ground and would run well on pump gas and would do all I want it to. Would an early 340 piston with a thick head gasket get me in the 9:1 ballpark? Or is it better to keep the 8.5:1 pistons and mill the heads to get a true 8.5:1 compression ratio?
 
First you need to measure how far down the hole the piston is at TDC.
TDCpiston01.jpg

(you want to measure on the flat part of the dished piston around the edge and not in the actual dish itself)

Then you need to find out the part number for the piston and determine how much dish or dome the piston has in CCs.

You need to figure out how thick of a head gasket you want to run. The Mopar thin ones are .027 " thick. The typical head gaskets are .045" - .054" roughly.

You need to figure out the combustion chamber size on the heads you plan to use.

You will need the bore and stroke numbers for the engine.

Once you have all that you can plug the information into a compression calculator like the one found at Summit Racing:
http://www.summitracing.com/expertadviceandnews/calcsandtools/compression-calculator
 
The process is referred to as blueprinting. People throw around "balanced and blueprinted" a lot and few that use those terms really know what it means. They just want to sound cool and they usually end up soundin stupid. lol

Blueprinting means getting each engine spec in line with what the factory spec was. If the factory spec on the heads for example was 68cc's, then that means making sure the combustion chambers are at 68cc's through milling to make the chambers smaller, or machining the combustion chambers to increase their size.

Also with the deck height, Chrysler blocks were almost always way tall. Normally, the block will need to be decked to get it to the factory spec. You will also need to measure the piston head volume to see where it is so that can be used in the compression equation as well. All of this sounds complicated, but I promise, there is nothing more than grammar school math involved.

I don't see a problem with being able to get to 9:1 with proper blueprinting and maybe using a thin head gasket and maybe milling the heads a little bit to achieve it. That said, I never recommend head milling to raise compression more than about 1/2 point. Usually on a small block Chrysler, around .070" equals one compression point. That is ALOT to mill off. It will necessitate milling the intake to match. That gets costly. Usually, you can get by with around .025-.030" before you have to worry about milling the intake.

Remember, engines back in the 60s and 70s rated at 9:1 or more were considered premium fuel engines. Most people don't realize that, but it's true. You cannot go much over 9.2:1 on a non quench engine with iron heads and stay pump gas friendly. I don't care what you see on forums. It's just not true. It's also not necessary.
 
I can tell you that high compression is fun but a royal pain in the ***!! My 340 has 13 to 1 pistons and I love put also hate them. The car is a BLAST to drive but knocks like a bastard unless I run race fuel. Run lower compression and enjoy your pump gas car.
 
Some good pocket port / bowl work trumps compression, on combinations like this. A mild cam, and some good quality head work ,is more street friendly/ common sense useable than compression.
 
I never said one can't make power with low compression. But, it's easy to make more, and make it more driveable rather than peaky. Compression has a similar affect as stroke. It flattens power peaks and lowers the rpms at which the peaks occur. You can make a 350hp 340 with 7.5:1 static. But it's not going to have much for torque under 4K. Where you can make a 350hp 340 that has more torque off idle by adding pistons and raising the static ratio by two points. The side affect of that is a litle more peak power, but the real goal is low end torque. Nobody will EVER get low end torque with low static, as Jim's testing proved. 340s suffer from modest levels of torque in the best of circumstances.
 
I didn't see that Jim's test proved that at all. That's not what desktop dyno says either......or my experienced right foot. Is 400 pound feet at 2500 not "good" bottom end torque?
 
Shave the heads .040 block side and .038 intake side. Use mp thin gaskets .026 Keep the 1.88 intake valve. Bowl blend intake and exhaust. Check lifter preload. might have to shim rocker shafts. Run isky 270 mega cam. 221 at .050 465 lift 108 lc. Port match intake you have to heads. This cam had good low end for me and pulled strong to about 5600.
 
aaronk785 said:
I have also read hughes whiplash cams work very well in low comp. engines.
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I don't buy it. Every time I replace something that makes good power with a Whiplash in my dyno sym program, power goes down. They are ground pretty much like cams were 40 years ago. Short intake duration, long exhaust duration, tight LSA. All that adds up to a cam that needs a stall converter, gears and does not come on good until around 3K. Hughes claims they are ground to "optimize cylinder pressure n low compression engines" but I ain't buyin into it.

Now, I'm not sayin they don't work and cannot make some power. I am simpy sayin that IMO, I don't believe they work much better than anything else on the market. They are not a magic bullet to make a low compression run fast, however, there are ways to do it and one is simply not to over cam in the first place.
 
RustyRatRod said:
I didn't see that Jim's test proved that at all. That's not what desktop dyno says either......or my experienced right foot. Is 400 pound feet at 2500 not "good" bottom end torque?
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We share different opinions about Desktop Dyno and on Jim's test - neither of which should be news to anyone here. I saw nothing stellar about Jim's testing nor does DD impress me with it's output. I don't see 400lbs ft at 2500 from this engine without some good machine work and better parts regardless of what your simulator tells you.
 
moper said:
We share different opinions about Desktop Dyno and on Jim's test - neither of which should be news to anyone here. I saw nothing stellar about Jim's testing nor does DD impress me with it's output. I don't see 400lbs ft at 2500 from this engine without some good machine work and better parts regardless of what your simulator tells you.
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Since when were we limited in what parts or machine work we could choose? I didn't realize this was a competition. I bet we come closer to sharing the same opinion about desktop dyno, though. Remember, the more info that goes in, the better info that comes out.
 
I say go for it but do what you're doing, ask questions first esp if on a tight budget. My motorhead mentor saw a guy who got a vehicle with a killer 318 in it, don't know the parts but it ran like raped ape. After awhile he ditched it for a 360 and built it wrong, several times, thing never ran as good as his 318 did. So that's my captain obvious story, if it was me personally, I trust Rusty and A body bomber, think if you listen to those guys you won't go wrong. My motors not built yet but it's a 69 with .018 postive out of the top, am putting Magnum EQ heads on, I'll be asking how I can keep my motor on pump gas. Good luck with the 340!!!
 
Did you know that if you bore a 360 out to 4.04 you can call it a 340 stroker. And all your chebby friends will stop asking the age old question;" why didnt you build a 340?" Thats 26 cubes right there. And the extra stroke makes torque right where the 340 dont got it.Now you can dump the clutch just about right off idle with 3.55s and motor away,even with 232*@.050. Just try that with your open chamber 340.And with Silvolites,and closed chamber heads, it revs quite a bit faster, like it should.Dont get me wrong; Ive had several 340s. But Im not going back.And with 10* less cam shes a tire fryer, and still goes in the 12s. Can you see me smiling? Never going back, never, never, never, no not ever. Never going...... Ok sorry.
 
Do it the mad scientist way, build it, install it, see what it does and go from there. Start with what you have. You would be surprised at the results. you cant go wrong with the 4.04 inch bore 340. You can tweak and mod down the line as time and cash flow permits. I started out with 318s back in the day, as you learn more, you naturally gravitate toward the 340/360. it sounds like a natural choice to me. But don't get caught up in the bench racing, use your bottom end and build it as your budget dictates. Like I said, you may be surprised at the results.
 
A couple of days ago my 318 started acting up on me really badly. It was misfiring and down on power. Just not running right at all. Pulled the plugs and they actually looked pretty good. Ran a compression check and everything went to hell. Cylinder #1 is at 30PSI and #6 is just gone. Hooked up an air hose to cylinder 1 and could hear it leaking out the exhaust pipes. As for cylinder #6... I pulled the valve covers and saw a rocker wasn't moving when the engine turned over. With the possibility of a bent pushrod I pulled it out and checked it for straightness. It wasn't bent at all. Looks like the cam wiped a lobe. I was planning on keeping the 318 long block intact when I dropped in the 340 but now it'll need at least a new cam, bearings, and a valve job. I'm pretty upset but hey Sh*t happens.

As for the 340, I'd really like to keep the cost down as much as I can. Since I had a perfectly good running 318 I planned to take a while to do this build but with my car out of commission now I'd like to get the 340 together relatively soon so I can get back on the road. I called up the machine shop and it would be about $150 to mill the heads and $150 for the intake. and I'd only mill off about .025" so idk if its worth it especially if i'm going to change intakes down the road. Abodybomber suggested pocket port and bowl work on the heads. I think I could do that myself as long as I knew what all needed to be done to make them flow better. Anybody got tips for porting 915's? Also, what parts don't interchange between the 318 and 340 so I know exactly what I need? '68 teen '72 340. I talked to Schumacher and my current engine mounts will work but what about stuff like my harmonic balancer? I should still be able to use the same timing cover and water pump too, correct? I'd like to keep as much as I can to keep costs down. Stock bottom end, stock valve size heads with some porting and new springs, and then everything I can get from the 318 since it's free. Will I run into any issues with balancing? I'm running a 340 replacement flywheel already so I don't think i'll run into any issues there.
 
AJ/FormS said:
Did you know that if you bore a 360 out to 4.04 you can call it a 340 stroker. And all your chebby friends will stop asking the age old question;" why didnt you build a 340?" Thats 26 cubes right there. And the extra stroke makes torque right where the 340 dont got it.Now you can dump the clutch just about right off idle with 3.55s and motor away,even with 232*@.050. Just try that with your open chamber 340.And with Silvolites,and closed chamber heads, it revs quite a bit faster, like it should.Dont get me wrong; Ive had several 340s. But Im not going back.And with 10* less cam shes a tire fryer, and still goes in the 12s. Can you see me smiling? Never going back, never, never, never, no not ever. Never going...... Ok sorry.
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I'm sure 360's are great but my car was always destined to be a 340. A Dart Sport 340! My next project could very well have a 360 though since they're so easy to find and make great power.
 
Id trade that 340 short block away, for a complete 360 2bbl long/complete in a heartbeat.Thats if it was me. 26 cubes, long stroke, Torque, fuel mileage cheap.For a streeter, how can you go wrong? Never going back, no not.......
 
You don't have to mill the intake if you mill the intake side of the head. Then any intake will fit without modification. Just don't use the cork end gaskets.
 
BlueDream said:
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!
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Well.. I had a used 3 bill 360 short block. (78 version some bowl work/good valve job- no Serdi....(this is 96 ,mind you..) Pulled out a Comp 260h, Wanted a bigger cam ,not one on the shelf... Went with the Crower Baja Beast...( like the stock 340 cam , pumps up cyl psi. Ran in a 67 Cuda( 355 gears/road runner converter /street radials ( no drag stuff..). A best of 13:61 @ 102. Mind you, the car was built on a budget ,and brains....Ask for information.
 
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