1972 Demon 340 engine mods

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The low compression 340 pistons will have 4 valve reliefs similar to these:
http://www.forabodiesonly.com/mopar/attachments/340-duster-jpg.1714559471/

Also, run compression tests and see what the cranking compression is. That will also give some indication of the pistons.

I really enjoyed your video. Where is that road course? Is this the type of driving for which this car will be used?

I agree on the 318; a new build would be the best way to optimize performance. You could increase the compression ratio and then go bigger on the cam without killing the low RPM torque. My son and I built his 340 with 10:1 static compression, a 268 cam, 1.6 ratio rockers and lift at .500" and Edelbrock heads and Performer intake for good high RPM breathing. Dynamic compression ratio is about 8.2. It has torque at 2000 RPM that will spin tires at 1/2 throttle. We could have gone bigger on the cam but wanted to maintain some fuel economy. We live in the eastern mountains and the type of driving that you showed on your video is exactly what we do around here.
 
The road course is in Poland, its Tor Kielce. It's not great for big powerful cars, but still fun.
This is what new Demon owner wanted to do + sstreet driving, he did not want to build the suspension as radical as the Challenger in video, so just different sway bars and thats it. There are some cars that are less modified and handle in more "classic" way lets say.

Will check the pistons probably next week and report back. I hope ist HP as RK Motors claim, that would make things so much easier and more affordable I guess...

What kind of Mopar your son has?
 
His Mopar is a '65 Barracuda. I own the '62 Dart shown in my avatar. That road course looks like a lot of fun!

OK on the new owner's intended use; that confirms that you want to keep a wide torque band. His use is more like rallying.

If the engine has high compression pistons, then everything changes. The top of the original high compression pistons are above the block about .018" at TDC, instead of being about .080" down in the block for the low compression pistons. But the pistons could be different from either of the original stock types if the engine was rebuilt.
 
Ok time to refresh this topic!

Well the owner did some shopping ;-)

Thing is that low or high compression is still unknown but will be verified any day now.

Intake is also ported and matched to the heads.
Also there is a set of HS rockers set 1.6 ratio.
Carb is 750 double pumper with manual choke.

Is there any off the shelf cam that could work with manual and this setup? I’m not expecting low end power with this intake, but would be nice to have something...

Should he get something that puts him in .550 ish lift and under 250 duration?
I was suggesting sold flat tappet from Hughes with edm lifters to him, but perhaps there is something better?

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Quit thinking about cams till you know what your compression is and what rear gear he can live with
lots of good points above about the big gear spread with the three speed and you evidently have the 8 1/4 rear end with that 3.43 gear
did you ID the pistons yet?
if it is low compression here are some target cam keep your eye on the intake close point (early) and 200 duration (more)
Howard 1.6 1.5 lobe duration .006, .050 200
HM2063164A .506 .475 .316 [email protected] [email protected] 126@200
Hughes
504-512 473-480 325-320 206-210 113 lca -6 32/ 42 -12 -18 olap HUG SEH0610BL-13
Voodoo
VD .484 ,454,,.302 253 208 (stock 340 cam is 209 but really long seat timing kills the low end- this one gets the intake shut earlier to build dynamic compression
ones above do also
dual pattern voodoo
454/454 253/256 208/213 112 VD 10230700 54.5 [email protected] same intake close point as 256 comp cam
Jones 491 461 307 256 @ 202 This is a max area asymetrical grind fill this out Street Performance | Jones Cams and let mike run your build through his computer program once you figure out what you have and what you want feed mike those flow numbers let us know what he says- after you run that compression test
if you find out you have high compression pistons you would consider a different set of cams
There are solid lobes out there Comp cams MM6580 560 w 1.6 525 w 1.5 261 @.020 [email protected] 153@200 (more duration than any of the HYD shown above)
but you are going to need EDM lifters, more spring, etc IDK if it is worth it for a street ride In fact I would NOT do it for a street ride MM series are race cams
no 250 cams with 550 lift with 1.6 rockers
HR cam 260 you can get 523 and 264 you can get 544 but no use looking without all the details

 
Wyrmrider said:
Quit thinking about cams till you know what your compression is and what rear gear he can live with
lots of good points above about the big gear spread with the three speed and you evidently have the 8 1/4 rear end with that 3.43 gear
did you ID the pistons yet?
if it is low compression here are some target cam keep your eye on the intake close point (early) and 200 duration (more)
Howard 1.6 1.5 lobe duration .006, .050 200
HM2063164A .506 .475 .316 [email protected] [email protected] 126@200
Hughes
504-512 473-480 325-320 206-210 113 lca -6 32/ 42 -12 -18 olap HUG SEH0610BL-13
Voodoo
VD .484 ,454,,.302 253 208 (stock 340 cam is 209 but really long seat timing kills the low end- this one gets the intake shut earlier to build dynamic compression
ones above do also
dual pattern voodoo
454/454 253/256 208/213 112 VD 10230700 54.5 [email protected] same intake close point as 256 comp cam
Jones 491 461 307 256 @ 202 This is a max area asymetrical grind fill this out Street Performance | Jones Cams and let mike run your build through his computer program once you figure out what you have and what you want feed mike those flow numbers let us know what he says- after you run that compression test
if you find out you have high compression pistons you would consider a different set of cams
There are solid lobes out there Comp cams MM6580 560 w 1.6 525 w 1.5 261 @.020 [email protected] 153@200 (more duration than any of the HYD shown above)
but you are going to need EDM lifters, more spring, etc IDK if it is worth it for a street ride In fact I would NOT do it for a street ride MM series are race cams
no 250 cams with 550 lift with 1.6 rockers
HR cam 260 you can get 523 and 264 you can get 544 but no use looking without all the details
Click to expand...

Rear end is 8 3/4" 3.55 sure grip.

I will do compression test soon, this will tell me if im dealing with high or low compression engine.
The company that did full restoration on this car told me its a 71 spec 340 with 10.25:1 compression bone stock with steel crank.

Looking at balancer i can confirm its a steel crank engine, it has J heads on it also.
 
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I wanted to see what cranking pressure it generates.
If that says nothing we will pull the heads.
 
I'm going to be gone for a week but post up what octane gas you are going to use and how they measure octane in Warsaw
perhaps AJ then will calculate your Dynamic compression guesstemeate based on your cranking compression and can advise on a cam that will optimize for your build
that cam you mentioned is bigger than the stock 340 cam which is too big for a low compression motor (till you get it really reved up)
how are you going to use it?
did you say what exhaust?
sorry about the rear end- another thread has the 3.41 8 1/4
spend some time tuning the advance curve and let us know what you come up with
your current build should run better than you say it does
cam could be retarded
 
Wyrmrider said:
I'm going to be gone for a week but post up what octane gas you are going to use and how they measure octane in Warsaw
perhaps AJ then will calculate your Dynamic compression guesstemeate based on your cranking compression and can advise on a cam that will optimize for your build
that cam you mentioned is bigger than the stock 340 cam which is too big for a low compression motor (till you get it really reved up)
how are you going to use it?
did you say what exhaust?
sorry about the rear end- another thread has the 3.41 8 1/4
spend some time tuning the advance curve and let us know what you come up with
your current build should run better than you say it does
cam could be retarded
Click to expand...

As for gas we have 98 and 95, but also racing 100 octane.

If a fuel is 98 RON then it will be 93 PON -> 93 PUMP
If a fuel is 95 RON and 87 MON then it will be 91 PON -> 91 PUMP

My 11.3:1 big block with aluminum heads runs well without pinging on 98 gas.
340 has factory headers, its in all stock street driven Demon 340 3 speed.
 
i don't know about all the work and money these guys want you to spend.
I see it has 14" tires and probably 3.23 gears.With that i would suggest leaving the cam alone.I have driven a few stock 14" tire 340 A bodies and if well tuned they pull very hard on the street.
Maybe that thermoquad isn't working right and may have vacuum leaks.I'd start by recurving your distributor and putting on a new 600 carb.
Perhaps replace the three speed with a 4 speed and tune...tune...tune.
That car should run hard on the street and take no prisoners street light to street light
 
ir3333 said:
i don't know about all the work and money these guys want you to spend.
I see it has 14" tires and probably 3.23 gears.With that i would suggest leaving the cam alone.I have driven a few stock 14" tire 340 A bodies and if well tuned they pull very hard on the street.
Maybe that thermoquad isn't working right and may have vacuum leaks.I'd start by recurving your distributor and putting on a new 600 carb.
Perhaps replace the three speed with a 4 speed and tune...tune...tune.
That car should run hard on the street and take no prisoners street light to street light
Click to expand...

Rear end is sure grip with 3.55 gears, check above.
Wheels will be swapped for 15" rally (to keep the stock look) with 275/60/15 on the back and 205 or 225 front(dont remember).
Carb on it now is holley 650 with vacuum secondaries. There are no vacuum leaks or anything.
For me this car actually runs really nice and strong, but after he had a ride in my Duster with big block he wants more out of it.
The good thing is that he does not want to go all the way and only mods will be done to the car with parts that you bolt on. This is the only reason I am involved in this.
 
SLOPAR72 said:
Here is a recipe for a car just like this from back in the day....

Mopar 280 Purple Camshaft
273 Adjustable Rockers
Mild shift kit
4:30 gear

Everything else was left stock including factory exhaust to the tips. Car ran 12.90's with a sticky on the back. I am sure there are plenty of other recipes but I saw this one for myself and it flat worked....

It boils down to what the owners expectations are. Personally the original drivetrain would go in the corner and I would go over 400ci but then you will be changing other aspects of the car and once that train starts moving everything changes lol.... Post some pictures of it if the car is all original. Hard to find a demon like that these days lol...

JW
Click to expand...


Your experience differs greatly from what i have ever seen. Had a bone stock 70 340 Duster that needed way more than a cam and more gear to get into the 12’s........ way more. Your talking a car picking up a couple of seconds in ET. Great results friend
But its way off from anything like that i have ever experienced. I needed headers, and other boltons to get there, think i even had to use aftermarket convertor.
My little brother has a 71 Dart with 10.5 compression 340. 202 heads, comp flat tappet cam, 3500 convertor, holley 750, aluminum intake, 4.11 gears, headers, and it runs 12.70’s in good air. Well sorted out little combo
 
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you will probably notice the low end torque is less...exactly what you don't want on the street.
 
ir3333 said:
you will probably notice the low end torque is less...exactly what you don't want on the street.
Click to expand...
Another option is 295/50/15 but this car has stock rear suspension so I am not sure if this size gets there without rubbing.
Wish we had tires choice here that is in USA, not the case...
 
Miszny said:
The company that did full restoration on this car told me its a 71 spec 340 with 10.25:1 compression bone stock with steel crank.
Click to expand...
Just for information, the 10.25 is a factory number and is higher than reality. The early 340's were actually approximately 9.4-9.5 static compression ratio. It seems like all the of the Mopar factory compression ratio numbers were about .75 points high; I suspect they were based on combustion chambers that were milled to something like NHRA minimums. (And perhaps NHRA minimums were based upon some theoretical factory minimum chamber size that the factory used for marketing purposes.)

We all look forward to seeing the actual compression volume measurements. Let us hope that the correct early 340 pistons were used.
 
ir3333 said:
i don't know about all the work and money these guys want you to spend.
I see it has 14" tires and probably 3.23 gears.With that i would suggest leaving the cam alone.I have driven a few stock 14" tire 340 A bodies and if well tuned they pull very hard on the street.
Maybe that thermoquad isn't working right and may have vacuum leaks.I'd start by recurving your distributor and putting on a new 600 carb.
Perhaps replace the three speed with a 4 speed and tune...tune...tune.
That car should run hard on the street and take no prisoners street light to street light
Click to expand...


What does the size of the wheel have to do with anything. I just went through this with another guy. You can have a 14,15 or even a 16 inch wheel with the exact same OD tire. Tire circumference is what counts, north wheel diameter.
 
in my post i say stock 14" tires...which are around 25 1/2" diameter with about a 6" tread.
...should have been a little clearer.
 
the factory rated them at 10.5:1 so you could build to that ratio and be NHRA legal for stock class style racing.If you start at 10.5:1 and perform a stock rebuild...ie square the deck,machine the heads,bore .030 you would be over 10.5:1 and be illegal unless you make other adjustments to reduce the compression back to 10.5:1.
 
if the Demon had stock tires E70-14 the OD would be around 26 inches, if you go to a 27560R15 tire the OD will be 28 inches. That would make your gear ratio more like 3.00 instead of the original 3.23. Using a taller tire drops your gear ratio.

26"/28" = .92857142857
.92857142857 x 3.23 = 2.999 or 3.00 gear ratio

that's going to make your car slower off the line
 
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B3422w5 said:
Your experience differs greatly from what i have ever seen. Had a bone stock 70 340 Duster that needed way more than a cam and more gear to get into the 12’s........ way more. Your talking a car picking up a couple of seconds in ET. Great results friend
But its way off from anything like that i have ever experienced. I needed headers, and other boltons to get there, think i even had to use aftermarket convertor.
My little brother has a 71 Dart with 10.5 compression 340. 202 heads, comp flat tappet cam, 3500 convertor, holley 750, aluminum intake, 4.11 gears, headers, and it runs 12.70’s in good air. Well sorted out little combo
Click to expand...

It did not come overnite trust me.... Lots of rocker arm this, timing that, and of course working with a TQ. I don't even know that I could repeat the effort but this is what my dad did back in the day.... He didn't have much money and made the best of what he could afford....

JW
 
Are those aluminum heads?
Aluminum heads pull a ton of heat out of the chambers. You have to compensate for that, just to break even. You HAVE to either; A) up the static compression, or B) close the intake sooner, just to maintain the EFFECTIVE cylinder pressure. Failing to do that makes the bottom go soft. The good thing is that Aluminum heads allow very high Scr/Dcrs on even crap gas.
By the time you add up the costs of all the bolt-ons, you would be waaaaaaaaaaaaay ahead of the game, if you just brought up the compression in the first place. When that big-cammed, low compression engine falls off the pipe, your man is gonna be one unhappy customer. Nothing is worse than having to pull a shift due to running outta revs,and then having to fight your way back up the power curve. Well, almost nothing.
About your RON fuel
The best I have been able to figure out is that your RON gas is just a different way of measuring the same old gas we have here. When all is said and done, your three grades of gas are the same as our three grades of gas, as far as the anti-knock is concerned. Whatever will run here on our midgrade will run equally on your midgrade. That is the best I have been able to figure out. I COULD BE WRONG!!
SO
that means, with the aluminum heads, you can pump the crap out of your compression and run any cam you want to. Well within reason. But...... the cam has to make a wide enough powerband to work with your trans, and you have to put a rear-end behind the whole works, that puts the power at the speed you want to be at, in the gear you want to be in.
Otherwise your man is going home with a long face.
Or you can just stroke it, and leave whatever trans and gears are in there, in there,lol.
But if you run skinny 14/15 pizza cutters, just stay home.

But if he just wants to have a little fun, well that is usually a short-lived fuse. You run the course a time or two and then you are hooked.
But to all those guys that said to just throw some gears at it; with a small engine,there is zero comparison to having the right gear,at the right time,all the time. This is why we have small engines today with lotsa gears and VVT, doing what engines twice as big used to do. Power ain't everything, when it comes to just having a little fun.

But I just gotta ask; your customer doesn't seem to be hard-up for cash. And from outside the engine, you cannot tell the cubic inches nor the compression ratio. So my recommendation is just stroke it,and with aluminum heads, and with any old street cam and lots and lots and lots of cylinder pressure. I run 185psi on the lowest grade gas we have out here, namely (R +M)/2 =87E10.To keep the chamber temp up, I run the engine at 205*F. And to keep the exhaust temp down, I run dual 3 inch, full length,mandrel bent, pipes. And to keep the intake air temp down, I run fresh cold air into the 750DP/ AirGap. And to keep the underhood temps down, I ventilated it.
367 cubes was plenty for me;4.04x 3.58
Jeepers I was only gonna say a few words,lol.
 
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