1972 Demon 340 engine mods

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so what are the combustion chamber size on the new heads? get the old heads off and see what the pistons look like and measure piston to deck height. I ran a 1970 340 Duster with a 3 speed manual transmission, not bad as many have said
 
Yes the heads are aluminum Edelbrock Performer RPM ported, I posted the numbers earlier. Also intake is matched with the heads. All set with rockers was bought as a really nice package deal.

Gas is the same, just different way of measuring. Numbers look better 95 vs 91 and 98 vs 93 but it’s all the same thing more less.

I do this stuff for free, Demon owner is a younger guy that bought his first Mopar not long ago. I’m just trying to help him out and also protect nice car from getting intro wrong hands. Tons of hacks here that don’t think what they are doing.
I prefer to ask you guys as this forum really made me learn a ton.

Money part is not easy, you see all the stuff costs us almost double to what you pay when we add shipping and customs. This makes things a bit less accessible. We constantly hunt good deals and this set of heads with rockers was just that.
Stroker build is a no go. If we had better access to stuff at USA prices I’d probably be done with building 408 for him with 4 speed or some aftermarket box by now. Sadly it’s not the case.
I want to make it work well and look good also.
New heads are 63cc (I checked with plugs in) so there will be a few extra hp from compression bump also.
Will update tomorrow about what psi it cranks and heads will be off a week later.
 
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OK I checked what it cranks out and its a low compression for sure as it cranks 125 psi all around.

Now its probably close to 8.5:1 so small chamber heads will bring it up to around 9.5:1... Thin gasket would probably make it closer to 10:1 but no quench or swirl advantage of aluminum heads what so ever...
What is standard J head chamber size? Is it around 73cc? Also what is the thinnest gasket I can use to bump compression as high as possible?
I will take the heads off and cc the piston in tdc to see what exactly this thing will be.

It is what it is...
 
How about this cam?

I prefer solid cam if possible, wanted to use Hughes EDM lifters.

54F017F7-9B0F-4572-B23F-4AE4685B7F48.jpeg
 
I plugged your numbers in, and at 330ft elevation, to make 130, the Scr needs to be 8.6
To make an 8.6Scr, the Total chamber volume needs to be about 91.5cc..
To make that, with 71.8cc heads, an 8.8 gasket and 5cc eyes, leaves 5.9 in the decks which calculates to .024 pistons down.
You can put closed chamber heads on that with an .028 gasket for a Q of .052, that's pretty good. So then adding the 63 and the 6.5 and the 5 we get a new Total chamber volume of 74.5cc and a new Scr of 10.33.
Ok now with 10.33 and targeting 185psi for aluminum. the ICA would like to be 48* for a Whoppingly strong VP of 164. Unfortunately a cam like that is 318 2bbl cam, so that will never fly.
The calculator wants an exact Advertised ICA to be accurate but with a solid cam that's always a guessing game especially from Hughes who doesn't publish the advertised numbers. So I'll have to use a best guess on that [email protected] cam. First I'll gross it up by 38* to get a 272*. then I'll crank in 3 more cuz Hughes doesn't measure at .006 but rather .008, for a gross of 275*. Then I'll lose 3 in the lash and end up with an estimated 272* net. Ok from that and the 104 install, that calculates to an ICA of 60*. Now plugging that into the Wallace, with the 10.33, I get Dcr of 8.49, and pressure of 167@ a VP of 137, about 3 points better than a stock 1969 9.2Scr teener.
This will burn el-cheapo gas with those aluminum heads.
This will make good power after 4000
The bottom will be soft in comparison; it will feel like a good 318 below 3000.
The power band is not the best for a 3 speed stick, but the 108 LSA will bring in good numbers. For a streeter it should be fun
It will probably needs gear in compensation for the low VP, and the large splits.

Now remember, this is all a guessing game right now, because
A) the 130psi could be because of an 18% leakage off an actual 160psi engine.
B) I used flat-tops in the holes, with 5cc eyebrows
C) I guessed at the cams ICA
D) I guessed at the head volume
E) I guessed at a lotta numbers.
F) Warsaw is only at 330ft
 
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Any of you guys have experience with using stamped rockers and stock pushrods with iron to edelbrock rpm heads swap? I am tempted to do just this first but not sure if I need any different pushrods?
I'm tempted to get the heads off, swap it all but camshaft, use stock rockers, lifters and cam to get my measurements before we order the cam that it needs. I hate this guessing game.
Also wanted the car to be running in the mean time. I need to wait about a month for parts to get here.

Another factor is my SHITY cheap cylinder pressure gauge that I just use to see if all cylinders check the same and don't really trust what it shows.
 
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AJ/FormS said:
I plugged your numbers in, and at 330ft elevation, to make 130, the Scr needs to be 8.6
To make an 8.6Scr, the Total chamber volume needs to be about 91.5cc..
To make that, with 71.8cc heads, an 8.8 gasket and 5cc eyes, leaves 5.9 in the decks which calculates to .024 pistons down.
You can put closed chamber heads on that with an .028 gasket for a Q of .052, that's pretty good. So then adding the 63 and the 6.5 and the 5 we get a new Total chamber volume of 74.5cc and a new Scr of 10.33.
Ok now with 10.33 and targeting 185psi for aluminum. the ICA would like to be 48* for a Whoppingly strong VP of 164. Unfortunately a cam like that is 318 2bbl cam, so that will never fly.
The calculator wants an exact Advertised ICA to be accurate but with a solid cam that's always a guessing game especially from Hughes who doesn't publish the advertised numbers. So I'll have to use a best guess on that [email protected] cam. First I'll gross it up by 38* to get a 272*. then I'll crank in 3 more cuz Hughes doesn't measure at .006 but rather .008, for a gross of 275*. Then I'll lose 3 in the lash and end up with an estimated 272* net. Ok from that and the 104 install, that calculates to an ICA of 60*. Now plugging that into the Wallace, with the 10.33, I get Dcr of 8.49, and pressure of 167@ a VP of 137, about 3 points better than a stock 1969 9.2Scr teener.
This will burn el-cheapo gas with those aluminum heads.
This will make good power after 4000
The bottom will be soft in comparison; it will feel like a good 318 below 3000.
The power band is not the best for a 3 speed stick, but the 108 LSA will bring in good numbers. For a streeter it should be fun
It will probably needs gear in compensation for the low VP, and the large splits.

Now remember, this is all a guessing game right now, because
A) the 130psi could be because of an 18% leakage off an actual 160psi engine.
B) I used flat-tops in the holes, with 5cc eyebrows
C) I guessed at the cams ICA
D) I guessed at the head volume
E) I guessed at a lotta numbers.
F) Warsaw is only at 330ft
Click to expand...


If you have any other cam suggestions, please let me know.

I also like this:
Howards Cams Mechanical Flat Tappet Camshafts 710862-08

Cam Style: Mechanical flat tappet
Operating RPM Range: 2,400-6,200
Intake Duration at 050 inch Lift: 238
Exhaust Duration at 050 inch Lift: 238
Duration at 050 inch Lift: 238 int./238 exh.
Advertised Intake Duration: 266
Advertised Exhaust Duration: 266
Advertised Duration: 266 int./266 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.558 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.558 in.
Valve Lift with Factory Rocker Arm Ratio: 0.558 int./0.558 exh.
Lobe Separation (degrees): 108

I have Howard's billet cam in my big block and quality is great.

BTW I talked to Hughes and they said their cam is around 262/266 advertised.
 
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OK so I took the heads off and found strange flat top pistons with 4 big valve reliefs at zero deck.
Pistons are not completely flat and reliefs + area around bore gives me bit over 10cc. This added to 8.8cc for felpro 1008 head gasket +63 cc heads gives total 81.8cc so 9.5:1 compression.
Piston will be head gasket away from the head so 0.039" and I guess I don't have much room here.
Another discovery is Mopar purple camshaft .484" lift and 241 duration @.050 which added to no low end power nicely I guess... We will be using 1.6:1 rockers so lift will be just over .516". J heads from the car have small intake valves.

Cam swap coming soon.
 
Miszny said:
OK so I took the heads off and found strange flat top pistons with 4 big valve reliefs at zero deck.
Pistons are not completely flat and reliefs + area around bore gives me bit over 10cc. This added to 8.8cc for felpro 1008 head gasket +63 cc heads gives total 81.8cc so 9.5:1 compression.
Piston will be head gasket away from the head so 0.039" and I guess I don't have much room here.
Another discovery is Mopar purple camshaft .484" lift and 241 duration @.050 which added to no low end power nicely I guess... We will be using 1.6:1 rockers so lift will be just over .516". J heads from the car have small intake valves.

Cam swap coming soon.
Click to expand...


Why change cams? If you need more cotton add some compression. That cam is not big. BTW, the piston needs to come out of the block. Factory MINIMUM is plus .017 and you can stick it out more.
 
Yes but keep in mind he bought the close chamber eldebrocks, so anything sticking out of the hole will not work with the new heads.
 
Cam-swap.....
or step-dome pistons
or bigger TC and maybe gears.
9.5 with aluminums is kindof cheating yourself. They will support over 185psi on pumpgas. With that 284 cam, 11/1 is not too much. The problem is getting it with a 3.31 stroke .Your looking at a total chamber volume of about 69cc. I can't see that happening and still maintaining a minimumQ. You would need specialty pistons,more swept, or an earlier ICA.
So IMO, you are probably on the most logical track.
 
yellow rose said:
Why change cams? If you need more cotton add some compression. That cam is not big. BTW, the piston needs to come out of the block. Factory MINIMUM is plus .017 and you can stick it out more.
Click to expand...

These are not factory pistons for sure. Its something aftermarket.
They look identical to the ones in the photo.

EEFD007A-9AF4-4C83-A3B6-9C6BA6D4DDCF.jpeg
 
These have an offset in the pin, hence the 4 eyebrows. IIRC, the late 340 pistons had 4 valve eyebrows but were about .080" below deck.

If those pistons above truly are at 0 deck, then they may be the Silvolite cast ones; they look right per all the pix I have ever found. The Silvolite number is 1267 and have a CH of 1.824" which would put them around .002" above a stock LA deck. (That Silvolite number crosses to Federal Mogul/Sealed Power 428P, per the Silvolite catalog, which have a .005" higher CH.)
 
If you have above deck pistons you can run a thick gasket or a shim+gasket to avoid a teardown
Obviously you could have a better combustion chamber with the new heads and some new pistons that match
At TDC it looks like those will have a dead set of valve reliefs with closed chamber heads
 
nm9stheham said:
These have an offset in the pin, hence the 4 eyebrows. IIRC, the late 340 pistons had 4 valve eyebrows but were about .080" below deck.

If those pistons above truly are at 0 deck, then they may be the Silvolite cast ones; they look right per all the pix I have ever found. The Silvolite number is 1267 and have a CH of 1.824" which would put them around .002" above a stock LA deck. (That Silvolite number crosses to Federal Mogul/Sealed Power 428P, per the Silvolite catalog, which have a .005" higher CH.)
Click to expand...

Photo is off internet, I did not take it. Also I did not do detailed measurement. At TDC I put transparent plexi to measure cc over the piston and saw contact with it along with contact to the block. Seemed flat but 0.002" is something I wont deny. I'm sure even thick plexi will adjust to this. Frankly when I saw these pistons I was disappointed... To the point I did not bother taking a nice photo. Those valve eyebrows have more volume than I would like to see, but it could have been worse I guess?

Car runs much better with these heads, more power everywhere. When the owner gets some more money to spend I am sure right cam + carb will make this Demon run really nice.
I will measure cylinder pressure tomorrow just to see what it shows.

For now it is what it is. I don't like hydraulic cams, Never saw a car with all lifters to be hard as they should be. In this case 4 lifters were soft so I am not fully happy with end result.

Cant wait for next step to finish this one up.
 
This is what it looks like now.
Also apparently there are different shades of corporate blue paint. I used mopar color but it did not match the block and accessories.

7ED493B3-CC42-45E2-94C9-FD63DE19A5AB.jpeg
 
With cam swap I will try work on other small stuff like crazy fuel line with one million clamps on and stuff gas/clutch pedal working with too much resistance, not working speedo...
Overall its a great car, just needs some time to be more fun.
 
OK, with the piston contact with the plexiglas, sounds like you have the Silvolite or Sealed Power pistons. They are plain cast pistons so keep that in mind when you go hammering on the engine. Your 10+cc measurement seems right.

I think Wym's point may be that there is now another space with fuel-air in it that needs to be burned that may slow down the overall combustion process.

BUT the good news is that, with the 1008 gasket, you will have a decent quench gap with the closed chamber heads. That is a definite advantage. Your 9.5 SCR number is right; you indeed would be much worse off with the original late 340 pistons. A bit more would be nice with that purple cam, IMHO. DCR is in the low 7's if you stay with that purple cam; that is a part of the reason for the low compression numbers that you measured.

Whether that is a good cam or not depends on how the car will be used. IMHO, the 3 speed manual trans means better low RPM torque and a wider usable torque range is a good idea (which I am sure has been said by others). That Howard's cam is very interesting; it would put your DCR up be around 7.6-7.7 and widen the torque band some. (Note: I computed the DCR above with the duration set to 278 since the advertised duration for a solid will be with the tappet at .015" lift or thereabouts.)
 
nm9stheham said:
OK, with the piston contact with the plexiglas, sounds like you have the Silvolite or Sealed Power pistons. They are plain cast pistons so keep that in mind when you go hammering on the engine. Your 10+cc measurement seems right.

I think Wym's point may be that there is now another space with fuel-air in it that needs to be burned that may slow down the overall combustion process.

BUT the good news is that, with the 1008 gasket, you will have a decent quench gap with the closed chamber heads. That is a definite advantage. Your 9.5 SCR number is right; you indeed would be much worse off with the original late 340 pistons. A bit more would be nice with that purple cam, IMHO. DCR is in the low 7's if you stay with that purple cam; that is a part of the reason for the low compression numbers that you measured.

Whether that is a good cam or not depends on how the car will be used. IMHO, the 3 speed manual trans means better low RPM torque and a wider usable torque range is a good idea (which I am sure has been said by others). That Howard's cam is very interesting; it would put your DCR up be around 7.6-7.7 and widen the torque band some. (Note: I computed the DCR above with the duration set to 278 since the advertised duration for a solid will be with the tappet at .015" lift or thereabouts.)
Click to expand...

It's a street car, the owner likes cruising but from time to time it just needs to eat these new 5.0 mustangs for breakfast ;-)
For now this Howards is our target. Purple cam needs to go, it was previous owner mistake that needs fixing.
 
new 5.0 mustangs have to rev up to make power
you can eat their lunch for a block or two pretty easy and still have a streetable car
don't try and match them at high RPM- use the advantage you have and work on the chassis/ traction
 
Now its time to do something about carb on the Demon.

Car now has 80508-2 Holley which is vacuum secondary 750cfm. I finally had a chance to drive it a bit longer and adjust this carb idle etc.
To put it short, I hate how it works. This 340 should like to rev and should have nice throttle response, but has none of that. Its sluggish and there is really slow throttle response, 4000 rpm feels like an effort...
This carb as used with spacer or previous intake, I don't plan on using it.

I have Holley 0-4779C on the shelf and am tempted to use it. It has 75 jets on all corners which is probably way to much of this car right now. What jetting would you guys suggest to start with? I was thinking 72/75 or 69/72?
Will also check vacuum to see what power valve to use... Boy this will be fun. I will put my o2 sensor on the car to make my life easier.
 
Miszny said:
Now its time to do something about carb on the Demon.

Car now has 80508-2 Holley which is vacuum secondary 750cfm. I finally had a chance to drive it a bit longer and adjust this carb idle etc.
To put it short, I hate how it works. This 340 should like to rev and should have nice throttle response, but has none of that. Its sluggish and there is really slow throttle response, 4000 rpm feels like an effort...
This carb as used with spacer or previous intake, I don't plan on using it.

I have Holley 0-4779C on the shelf and am tempted to use it. It has 75 jets on all corners which is probably way to much of this car right now. What jetting would you guys suggest to start with? I was thinking 72/75 or 69/72?
Will also check vacuum to see what power valve to use... Boy this will be fun. I will put my o2 sensor on the car to make my life easier.
Click to expand...


Use it. Vacuum secondary carbs are boat anchors.

Flame on but I detest VS carbs. If you bother to check I'd bet the secondaries either never fully open or come open so late it's worthless.
 
yellow rose said:
Use it. Vacuum secondary carbs are boat anchors.

Flame on but I detest VS carbs. If you bother to check I'd bet the secondaries either never fully open or come open so late it's worthless.
Click to expand...

Yes, I really regret reusing what was there in the 1st place. Biggest waste of my time in the whole process. Just plan to put electric choke from the carb to double pumper to make life easier.
Was hoping that the Demon owner can live with it till cam swap but this is pathetic.
 
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