3forty
"Raw Power"
My '71 340 Duster has the stock factory cam and pistons. No problems running on 94 octane gasoline.
1970 340 has the 1973 low compression pistons
- Thread starter wolfhammer
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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For an early 340
to be at 10.5Scr, the Total chamber volume needs to be roughly
(340/8) x16.387 /(10.5 less 1) =73.3cc
The X-head chambers by themselves might be close to that, then add gasket, add eyebrows, subtract pop-up, add back a lil clearance volume; and what do you get?
>with pistons only .018 up out of the holes; I'll guess 84cc or so, and Scr falls to 9.29...........
> with pistons .045 up, I'll guess 78.3cc, so Scr of 9.89 ...............
But that stinking 114LSA factory-cam in at 110, drops the Dcrs of those to 7.43 and 7.89, at 900ft elevation, which is well into pumpgas territory.
The lower the Dcr goes, the more lazy the low-rpm is.
By the time the Scr is 8/1, the Dcr becomes 6.43, and I'ma thinking she will burn any old puke gas, but be turd-slow on the take-off no matter what gas is in the carb; 6.43 is pretty low, lower even than a Smoggerteen which being 7.10, also at 900ft.
EDIT
As you can see, I did not include any specific numbers for any of the players. My post was just to show what it takes to get to the rated Scr with a 340 and what happens after you install the cam.
to be at 10.5Scr, the Total chamber volume needs to be roughly
(340/8) x16.387 /(10.5 less 1) =73.3cc
The X-head chambers by themselves might be close to that, then add gasket, add eyebrows, subtract pop-up, add back a lil clearance volume; and what do you get?
>with pistons only .018 up out of the holes; I'll guess 84cc or so, and Scr falls to 9.29...........
> with pistons .045 up, I'll guess 78.3cc, so Scr of 9.89 ...............
But that stinking 114LSA factory-cam in at 110, drops the Dcrs of those to 7.43 and 7.89, at 900ft elevation, which is well into pumpgas territory.
The lower the Dcr goes, the more lazy the low-rpm is.
By the time the Scr is 8/1, the Dcr becomes 6.43, and I'ma thinking she will burn any old puke gas, but be turd-slow on the take-off no matter what gas is in the carb; 6.43 is pretty low, lower even than a Smoggerteen which being 7.10, also at 900ft.
EDIT
As you can see, I did not include any specific numbers for any of the players. My post was just to show what it takes to get to the rated Scr with a 340 and what happens after you install the cam.
Last edited:
aaronk785
Well-Known Member
With that .049 gasket you should be at about 9.6 comp. mine ran on 91 no problems. My buddy runs that cam in his 70 factory 340 no problems on 91. 87 is a no go.
Of course not. The ethanol raises the octane.
Put the figures in the Wallace DCR calculator to be sure.
You are so incorrect, it's not even worth the time to correct you.
Why not you blast Everyone's else post. You think you are the guru of everything Mopar. Last time i checked 1 point of compression on a STOCK engine was 10 hp . Ive seen it in action. 10 horsepower equates to 1/10 second, these are not hard fast rules but approximations.. Look it up for yourself.
In my opinion the HV oil pump is not required, put a standard volume pump on it and check the pump clearances.
I'm not debating that. But TO debate that, you don't know squat without a dyno.
My point IS the 340 was never "10.5" compression. It was RATED at 10.2. Rated. The actual MEASURED compression was "somewhere" around 9.5 because of Chrysler's tall decks and larger than advertised chambers. This is common knowledge across Mopsr circles from people who've actually DONE THE MEASUREMENTS and not just sat back being an armchair blowhard. You'd know that if you'd been paying attention to the right things instead of spouting nonsense.
wolfhammer
Well-Known Member
Jeeze guys, I didn't want to start an argument or pissing contest. I was just looking for experienced advise because I'm an old guy but new to Mopar small blocks.
When I got my Barracuda the P-O installed a Torker 340, headers, some kind of RV cam and a junk single point chinesium distributor on a stock 318 with a beat 904 and the original 7 1/4 no gear one tire fryer, ran like ****. I put the 8 3/4 3.55 in, a 2 plane intake rejetted eddy 600 and it woke right up.
I'm on a budget and just want to try to get as much as I can out of the 340 without disappointment, I don't have the funds to take it apart and re do it if it's not a stout runner.
I will respect any input and appreciate any input but please don't argue over others trying to help.
Thank you.
When I got my Barracuda the P-O installed a Torker 340, headers, some kind of RV cam and a junk single point chinesium distributor on a stock 318 with a beat 904 and the original 7 1/4 no gear one tire fryer, ran like ****. I put the 8 3/4 3.55 in, a 2 plane intake rejetted eddy 600 and it woke right up.
I'm on a budget and just want to try to get as much as I can out of the 340 without disappointment, I don't have the funds to take it apart and re do it if it's not a stout runner.
I will respect any input and appreciate any input but please don't argue over others trying to help.
Thank you.
K.O. SWINGER
Meeting in the alley since 1976
I have been running at 340 for the last 40 years straight, you will be fine with those Pistons if you keep the cam relatively mild the 268 and below sounds good. Yes there's some power to be gained with a true zero deck piston but with an iron head you will be on the brink of detonation with the timing where it should be. I ran the KB 243s with x heads and even with the 509 purple cam it would detonate with maximum timing on pump premium. But by backing the timing off just a few degrees it ran great. I also ran a 430 rear end gear and that brought it all together.
It's no problem. All "THAT" aside, I can tell you this for sho and for certain......
REGARDLESS of opinions, if you blueprint this engine to a measured ratio of somewhere north of 10:1 with iron heads and that small cam, you will very possibly end up with some detonation issues.
That's why I recommended the lower compression pistons.....well that and because you already have them. To me, on a street car, the almost two points of compression are not worth the possible detonation issues. .......PLUS with the lower compression pistons, 9.5:1 is within your grasp with some mild head milling. That saves dough from new pistons and keeps compression more friendly.
But hay, you seem intent on pushing the envelope, so push away! I wish you luck. You can do it. A lot of people do. Hell, I did it with my slant 6......but I won't do it again. LOL
wolfhammer
Well-Known Member
Thanks RRR, alot of my reasoning is that I don't want to deck the block or shave the heads too much or at all if I don't have to. I don't want intake fitment issues and being a stock bore block that's been sitting around since 1975 it shouldn't have many miles on it or core shift like when the cam journals get crooked.
Thank you for all the input, its very appreciated and like I said I am waiting to talk to the machinist who just might tell me the same as you bud. He is very respected and puts out some really awesome stuff. S&S Performance in Farmingdale, Maine, look them up on facebook and check out Steve Clukey's duster 340 he built, it's a screamer. A little bit pricey though lol...........Thanks again
Well, you said yourself you're on a budget...and you're talking about buying pistons. LOL Good luck with it, regardless of which way you go. It'll be cool either way!
In my opinion, as long as the whole package is tailored together, the low compression engine can be made to be a formidable performer. With today's rising fuel prices, and quality of fuel, a lower compression ratio with iron heads may be desirable. One thing I would do for sure is have a custom cam ground with a narrower lobe separation to fool the engine into thinking it has more compression by building more cylinder pressure.
If your stuff you have now is in good shape, use it. I certainly wouldn't be rebuying parts to change it. Make a plan based around a lower compression build, especially cam, and work with that.
Get in touch with a great engine builder and LISTEN to what he tells you. There are many theories and "experts" out there, be cautious who you follow. That said there are many very intelligent people on this forum, if you read a lot of stuff on here you will likely realize who to put stock into in short order. The builder of the Steve Clukey Duster is likely someone to listen to.
If your stuff you have now is in good shape, use it. I certainly wouldn't be rebuying parts to change it. Make a plan based around a lower compression build, especially cam, and work with that.
Get in touch with a great engine builder and LISTEN to what he tells you. There are many theories and "experts" out there, be cautious who you follow. That said there are many very intelligent people on this forum, if you read a lot of stuff on here you will likely realize who to put stock into in short order. The builder of the Steve Clukey Duster is likely someone to listen to.
And no, I don't mean a Whiplash cam. I mean one custom designed to your package requirements.
Elevation
CID
Compression ratio
Intake design
Carb size
Transmission type
Gear ratio
Weight
Tire size
Intended usage
Etc, etc, etc.
Elevation
CID
Compression ratio
Intake design
Carb size
Transmission type
Gear ratio
Weight
Tire size
Intended usage
Etc, etc, etc.
jos51700
Green Bearing thread connoisseur
OP say it with me now.....booooooooost.
I love statements like this, when half the factors that affect detonation are left out.
Timing? 10 degrees after top dead center.
Cam overlap? 110 degrees.
Chamber volume? 200cc's.
I love statements like this, when half the factors that affect detonation are left out.
Timing? 10 degrees after top dead center.
Cam overlap? 110 degrees.
Chamber volume? 200cc's.
Which is why I err on the conservative side of recommendations. People inadvertently just leave things out of their thought process. "Where" initial timing will really need to be, elevation, never considering how hot the car will get in stop and go traffic and on and on and on.
I like your boost idea, even though I'm a N/A guy. With boost, there's zero additional heat down low or at idle. It's just a low compression engine then. They'll idle in traffic all day long. Even naturally aspirated though, a low compression engine can make plenty of power with the right combination and right tune. I'm just not going to make a recommendation for someone to end up on the ragged edge and end up having detonation problems. If they make that decision, it's gonna be all theirs. lol
wolfhammer
Well-Known Member
Hahahahaha!!!! No boost, those days are over for me bud last one was a 68 Charger back in 2003 Back then that engine 440 with the Don Hampton 671 blower cost me about $6500 to build in my garage, I couldn't imagine the cost these days lol
33IMP
Well-Known Member
I haven't read the whole thread, but I'm sure it's been mentioned. Do your own measurements (might require buying a tool or two) and find out what your compression is now.
Until you KNOW what you have now, you can't make an intelligent decision.
Until you KNOW what you have now, you can't make an intelligent decision.
Post #21 .108 in the hole (about)
33IMP
Well-Known Member
Yikes!
I somehow missed this upon reading the post the first time. This could make for a sub 9.0:1 ratio, that is admittedly not ideal even for a low compression performance build.
jos51700
Green Bearing thread connoisseur
How come nobody told me that Dominic Torretto was on this board?
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