318 vs 340 Questions

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jthomas38

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I am new to the forum and will have many questions. I just bought a 1970 duster which was the first car that I owned as a teen and totaled just as I was getting good at driving a stick.. The one that I owned had a 318 with a holley 4 barrel, headers, 4 speed with a hurst shifter and a few other extras, seemed to have plenty of horse power to me. The car that I just got has a running 318 in it but also has a 340 block with crank and piston rods. I know that the 340 is a very reputable engine but its stamp date is 72. I have heard that after 1970 the compression dropped and so the horse power. Im not an engine guy so is that inherent to the block or something else. Basically am I better off putting money for horses into the running 318 or starting fresh with the 340. Also what if anything will direct swap from the 318 to the 340? Thanks in advance,
 
The compression change was a function of changes to the internal parts other than the crank and rods and block. So any of that can be restored by proper selection of pistons, heads, manifolds, carb etc, and you can easily go past what the original 340 had for power and torque. The 340 will certainly add resale value to your car. They are in essence the same engines with modifications to certain parts.

Things like the oil pan, oil pickup, timing cover, water pumps, and valve covers, charging system, engine senors, and ignition system can swap over. The heads CAN swap over but would not be a good idea. You should get new parts like an oil pump and timing chains and bearings; they are interchangeable but don't reuse those.

It will help if you lay out your objectives and general some indication of budget limitations. And how you want to approach a rebuild... have someone else do it?
 
jthomas38 said:
I am new to the forum and will have many questions. I just bought a 1970 duster which was the first car that I owned as a teen and totaled just as I was getting good at driving a stick.. The one that I owned had a 318 with a holley 4 barrel, headers, 4 speed with a hurst shifter and a few other extras, seemed to have plenty of horse power to me. The car that I just got has a running 318 in it but also has a 340 block with crank and piston rods. I know that the 340 is a very reputable engine but its stamp date is 72. I have heard that after 1970 the compression dropped and so the horse power. Im not an engine guy so is that inherent to the block or something else. Basically am I better off putting money for horses into the running 318 or starting fresh with the 340. Also what if anything will direct swap from the 318 to the 340? Thanks in advance,
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71 Was the last year for high compression 340s...the compression drop was achieved by changing the compression height on the piston from 1.84 inches to 1.74 inches...compression height is measured from the center of the wrist pin to the top of the piston ....
 
I'll take a low compression 340 over a 318 any day. If I were on a limited budget and had both options I'd put a set of flat top pistons in the 340 with the 318 heads on it until I could muster enough money fro a set of decent heads.
 
70aarcuda said:
71 was the last year for high compression 340s...the compression drop was achieved by changing the compression height on the piston from 1.84 inches to 1.74 inches...compression height is measured from the center of the wrist pin to the top of the piston ....
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^^this^^
 
While it is true that the "compression change" did occur, the difference is more in how the horsepower was measured, rather than physical changes to the engine. That's something that most people seem to forget.

Beginning for the 1972 model year, horsepower went from gross (1971 and prior) to net. That means even with no changes at all, the advertised horsepower rating would have fallen dramatically.

I don't think the gross horsepower fell much at all from 1971 to 1972. Maybe 15 HP. I have seen several 1971 and prior 340s and owned one and owned one 1973 340 car. The 73 car would have given any of the rest a run for the money. There just was not that much difference, other than the above described change in HOW the horsepower was measured.

I know the later 340 used different pistons, blah blah blah, but the fact is, they were still a high output engine, any way you slice it.

If I had the choice to make and already had a 340, I would use it. I have always wanted to blueprint a stone stock 71 340 to what the factory said and have some fun in a light car.
 
cosgig said:
I believe Feb 17th was the changeover date! I'm also doing just as you said, putting a 71 340 into my 66 Cuda, it should be breathing fire by spring!!
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Yes, but are you blueprinting it? There's a big difference.
 
Basically am I better off putting money for horses into the running 318 or starting fresh with the 340. Also what if anything will direct swap from the 318 to the 340? Thanks in advance,
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personally i'd sell the 340 to someone thats infatuated with those things and build a 360 and be money ahead... :)
 
RustyRatRod said:
Yes, but are you blueprinting it? There's a big difference.
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This motor would've been blueprinted to the exact specifications as it was being done my Compuflow, but now that Nick has passed away, my regular engine builder will be assembling the shortblock using the items that came back from Compuflow! It was destined to be a stroker motor, but will now be using TRW .030 overs, a 509 purple cam, J heads and an LD4B intake and my Carter carb, and will be going into an early A body with Spitfire headers, 8 3/4" with 3:55's, and a rebuilt 904!! Should do alright???
 
^^^ I thought about that but I suspect the 340 in a car adds more to the car value than what you could get out of the block, crank & rods. Something in those numbers and in that sequence makes some people slobber a lot.....
 
abodyjoe said:
personally i'd sell the 340 to someone thats infatuated with those things and build a 360 and be money ahead... :)
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I was going to say that in the earlier post...but figured it would start a **** storm..
 
cosgig said:
This motor would've been blueprinted to the exact specifications as it was being done my Compuflow, but now that Nick has passed away, my regular engine builder will be assembling the shortblock using the items that came back from Compuflow! It was destined to be a stroker motor, but will now be using TRW .030 overs, a 509 purple cam, J heads and an LD4B intake and my Carter carb, and will be going into an early A body with Spitfire headers, 8 3/4" with 3:55's, and a rebuilt 904!! Should do alright???
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Yeah that sounds great, but when I said blueprint, I meant stock Chrysler specs, right down to compression and cam.


I have always wanted to see what a blueprinted, box stock 340 would do in a light car like an A body. Remember, the deck heights were tall and chambers were big so they were not making the power they could have been.
 
I would trade 'em both for a good running 360. Those 20 cubes are hard to argue with,(cuz they represent about a bit better than one cam size),and allow a better cam selection before getting bottom-soft.. I just know I'll get flak for saying it but If you have to spend money on a rebuild You might as well get the most bang for your buck. I owned every size of SBM, and by far, the 360s are the most fun. Forget the cast-crank to steel stories you have heard. The 360, in a streeter, doesn't need to spin 7000Plus rpm. Although mine has for 16 years. A streetable cam in the sub 230* range, only needs to go around 6500 with a 4-spd to kill most comers. A 220*ish cam to give plenty of torque only needs to go 6000 to 6200.That 360 cast crank is one tough piece.

I did not like that 292/509 in a 360 Barracuda @ 3450 pounds(me included). The bottom end was pretty soft. My compression was 10.82Scr. It needed a starter gear of 10.5 to 11.5 for it to come off the line respectfully. It will be even softer in a 340, and especially so with a wider-ratio 904 behind it. Unless you put a pretty loose TC in it. And then it will be no fun at all on the hiway, cuz you are looking at 4.10s.Of course your car is a smidge lighter than mine; perhaps as light as 3300 pounds,car and driver. That's worth nearly a cam size, right there I guess.
Here's the thing; if you optimize the Scr for the 292/509 cam, and you find it not to your liking, it's very hard to back up the bus and fit a smaller cam in there cuz the engine may not run WOT on pumpgas with an earlier closing intake valve. If however you set it up with a smaller cam, and find it too small, A bigger cam can often be fitted with a thinner headgasket, and still run on pumpgas.
You may have heard it said that when presented with two camshafts, chose the smaller one. For street, this has been my experience.
 
70aarcuda said:
I was going to say that in the earlier post...but figured it would start a **** storm..
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**** storm be dammed. :D

What Joe said was EXACTLY what my first thought was.
A decent 340 as a core could fund a nice 360, but I'd run the heck outa that 318 till the 360 was ready for sure.

RRR, you're right about people forgetting the revamp of the rules in HP measurement.
I did until you mentioned it :D

(Did I hear correctly that the HP thing got changed because car insurance companies would insure for more if the car was classified as a performance car so they revamped the measuring system so they didn't classify as a performance car?)
 
Build the 340 while enjoying the 318. Of course you won't be able to swap over the parts that interchange, unless you do it at the last minute to minimize down time. Some stuff, like the heads, will be on the small side for a 340 anyway. But a set of properly done 360 heads will work very well. Definitely don't want an OEM 318 intake & carb on it.

The driver's side motor mount is wider for a 340, but can be spaced with washers and longer bolt.

I like the 360 idea too. The main difference would be the external balance which can be solved with a B&M flexplate for an LA 360. Top ends could interchange between all 3 engines. You'll be gaining another 20 cubic engines over the 340.
 
I went through close to the same thing. The 1st thing that I did and I would advise you to do would be to find a machine shop that has a lot of experience in working with Mopar engines. 2nd I would ask the shop to give you some phone #s or at least have them take your # and ask for some other people who had work done there call you and then ask these people what engine they had done, how they like the work etc. This way you will be making a educated decision.

73 Duster
96 Dakota 4 WD
2013 Dart.
 
TrailBeast said:
(Did I hear correctly that the HP thing got changed because car insurance companies would insure for more if the car was classified as a performance car so they revamped the measuring system so they didn't classify as a performance car?)
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I am not sure why it was changed. Insurance may have been it, but I don't know. I am not that smart.
 
Up here the 240 Net Hp put it below the radar, and allowed this 16 year old hell-raiser to get insurance at around $450Can/year,As I recall. At $1.35 an hour and still in high school, I worked a lot of hours for that steenkeeng car. Tires cost me a fortune. Good thing homework didn't interfere.
 
TrailBeast said:
(Did I hear correctly that the HP thing got changed because car insurance companies would insure for more if the car was classified as a performance car so they revamped the measuring system so they didn't classify as a performance car?)
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Nope. The HP ratings were too much marketing silliness and for any sort of high output engines, things would be done like remove the alternator, the fan, the belts to the water pump and PS pump, cool air, no air cleaner, and so on and on, just to get the max possible HP in a quick test before the engine overheated. It did not accurately what an actual operational engine would do as configured and really used. A good explanation is below.

http://www.secondchancegarage.com/public/201.cfm
 
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