1972 Demon 340 engine mods

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If your cap has terminals like that, then the last inch or so of the associated wires, if any kind of carbon core wire, will also be junk.

A 20psi boost in cylinder pressure is a flipping HUGE change,lol
 
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AJ/FormS said:
If your cap has terminals like that, then the last inch or so of the associated wires, if any kind of carbon core wire, will also be junk.

A 20psi boost in cylinder pressure is a flipping HUGE change,lol
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Yes the whole ignition system needed a rework. Keep that cap if it works; the brass terminals look nice. Hope it does not have any cracks.

20 psi in the cylinders? Yes, that is a whole point increase in CR! That will sure make it run harder down in the low RPM's.

BTW, at first I read the above as using 20 psi turbo boost....I wondered how THAT got started LOL
 
AJ/FormS said:
If your cap has terminals like that, then the last inch or so of the associated wires, if any kind of carbon core wire, will also be junk.

A 20psi boost in cylinder pressure is a flipping HUGE change,lol
Click to expand...

Yep only two wires were not junk.
 
nm9stheham said:
Yes the whole ignition system needed a rework. Keep that cap if it works; the brass terminals look nice. Hope it does not have any cracks.

20 psi in the cylinders? Yes, that is a whole point increase in CR! That will sure make it run harder down in the low RPM's.

BTW, at first I read the above as using 20 psi turbo boost....I wondered how THAT got started LOL
Click to expand...

Cap looks like this after serious brushing with wire brush on dremel. This cap looked really bad.
I will give it a shot but the other side needs work also.

Yes 20psi jump was expected after going to 63cc heads from stock J heads.
Test just confirmed that all is good in this area.
It’s probably around 9.5:1 now.

Shame we did not use thinner head gasket, 10:1 would be perfect.
 
on my 92 Magnum I switched from that female terminal style cap to the one with male STUDS- much better
I like Magwires
too many times I've had arcing in the middle of a carbon core wire, finely the opens space gets so big that the spark will not jump
now auxiliary gap is good up to .030 or so- champion used to make plugs with a built in auxiliary gap (they can't foul)
With the old insulation an arc inside the wire would melt the wire- with today's insulation you just can't tell- you just die and usually with Murphy involved
congrats on the compression change
 
OK so here is another update.
I finally got my vacuum gauge back and checked the vacuum on the 340...
Its around 6,5-7 range, so I'm not surprised I could not get it idling half decent with 6.5 power valve in it :) Secondaries never opened on the vacuum secondary carb.

Ignition updates and changes resulted in really nice spark, it also starts right up.

Another problem of this car is rust in the tank. I cut the fuel filter open and was amazed, it was half way full of rust. Now I do carb tuning with fuel from bottle until we clean the tank and put new fuel lines on it.
Please find photo of rear carb bowl after 3 days of playing with the carb. It was cleaned before install.

I will finish with initial carb tuning today if all goes as planned.
Need to work on jetting under load after we deal with the rust.
So far I have 4.5 pw, 64 jets in the front and 72 jest in the rear. Need to get smaller squirter for the rear...

Everybody that targeted the carb were right, but this car was neglected over longer time and now needs moren than expected. Whatever I touch needs some work...

2B4F7324-615B-4C57-B566-FDCA19737731.jpeg
 
Miszny said:
OK so here is another update.
I finally got my vacuum gauge back and checked the vacuum on the 340...
Its around 6,5-7 range, so I'm not surprised I could not get it idling half decent with 6.5 power valve in it :) Secondaries never opened on the vacuum secondary carb.

Ignition updates and changes resulted in really nice spark, it also starts right up.

Another problem of this car is rust in the tank. I cut the fuel filter open and was amazed, it was half way full of rust. Now I do carb tuning with fuel from bottle until we clean the tank and put new fuel lines on it.
Please find photo of rear carb bowl after 3 days of playing with the carb. It was cleaned before install.

I will finish with initial carb tuning today if all goes as planned.
Need to work on jetting under load after we deal with the rust.
So far I have 4.5 pw, 64 jets in the front and 72 jest in the rear. Need to get smaller squirter for the rear...

Everybody that targeted the carb were right, but this car was neglected over longer time and now needs moren than expected. Whatever I touch needs some work...
Click to expand...


Never set your power valve opening by idle vacuum. It was wrong when that stupid idea was first published and it's still wrong now.

If it's fat at idle, fix the idle circuit. If the power valve is making it rich at idle it's because the diaphragm is ruptured.

Set power valve opening by cruise vacuum

Why the idle vacuum is that low I have no idea. I have 10 inches of vacuum at idle and I have 255 at .050 on 340 inches.
 
yellow rose said:
Never set your power valve opening by idle vacuum. It was wrong when that stupid idea was first published and it's still wrong now.

If it's fat at idle, fix the idle circuit. If the power valve is making it rich at idle it's because the diaphragm is ruptured.

Set power valve opening by cruise vacuum

Why the idle vacuum is that low I have no idea. I have 10 inches of vacuum at idle and I have 255 at .050 on 340 inches.
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How does the carb know to use power valve or not? Is it regulated by vacuum or anything more than that?

I seriously doubt that whoever installed this cam bothered with degreeing it.

I will do all adjustments from scratch. Totally gave up on the vacuum secondary carb also because it has multiple vacuum leaks and needs total rebuild.
 
Miszny said:
How does the carb know to use power valve or not? Is it regulated by vacuum or anything more than that?

I seriously doubt that whoever installed this cam bothered with degreeing it.

I will do all adjustments from scratch. Totally gave up on the vacuum secondary carb also because it has multiple vacuum leaks and needs total rebuild.
Click to expand...


The carb doesn't know what power valve to use. You, as the tuner need to check the vacuum at cruise and then pick the power valve accordingly. Let's say you have 12 inches of vacuum at your cruise RPM (since your idle vacuum is so low I'm just guessing 12 inches) I'd pick an 8.5 or maybe even a 10.5 PV. Then you can trim down the T slots a touch more. You just have to keep working with it.
 
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So I left it sit for a day and went back to the beginning. Went thru what happened step by step... Sometimes thinking works better than doing ;-)

I figured out what was the problem!

It was all happening due to almost zero seat pressure due to too short and soft springs that we put on the Edelbrock heads in place of double springs that were on there. We installed springs from J heads. I went to quickly check this theory and I discovered that I could push the valve in by 1/4" with some push on the rocker arm.

I changed back the springs today and magically all works as it should!
Now it pulls 12 inches of vacuum. Gauge needle is steady as a rock. It idles at 850 with really nice pulse for this cam.
It revs like it should and reacts to gas as should. The power is also back.

Now its time to clean this fuel system mess and swap the cam.
Carb tuning with O2 sensor this weekend.

I hope it helps someone some day.

Thank you for helping out.
 
Update with the Demon!

So all fuel system is now new, tank, lines, pump, carb (850cfm duble pumper that I had sitting).
Cam is swapped with the howards mentioned earlier, we used EDM lifters, billet timing set, its all nice and fresh. This engine wants to rev like crazy, its incredible how it wakes up over 3000rpm.

When swapping the cam it turned out that timing set was shut, chain was so stretched that you could move the cam left to right by a lot.

There is one last thing I need to sort out. Timing curve in the distributor.
Can anyone recommend source for springs and how fast should it advance?
Its all in at 3500 rpm, which I think kills power in lower rpm under 3000.
At what rpm it would be best to have full advance?

Another thing, I set timing at 16 initial and it seems that it gives around 38 total? Is it possible? Anyway, the car idles really nice at 800 rpm, sounds very different from what it used to be. What total timing is best for 340?

I will be putting o2 sensor on the car to see afr values, but spark plugs look really good. The carb that we used I had tuned with o2 sensor on my car before I put it away. It was 15 cruise, 12.5 wot, 14.5 idle in gear (I have 727).

Thanks for all previous suggestions, this car is a new thing now.
 
Miszny said:
Its all in at 3500 rpm, which I think kills power in lower rpm under 3000.
At what rpm it would be best to have full advance?
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Generally, no.
But with no vac advance, you can bring it up much earlier - especially if drag racing - since the engine is relatively cold (compared to a long distance haul or a stock car or road race).
But even with drag racing, if the weight actually hits the stop too early, then timing can retard in the upper rpms.
Some links here about that
Mopar orange box retards timing?
 
34 or so total at around 2800 rpm without vac advance is fine but you can let detonation be your guide and 38 may work for your build.
I had my old Duster all in at 1800 rpm for a while but eventually brought it in a little slower.
 
I was thinking to advance it because my Duster is all in at 2200 but its 470 big block with over 11:1 compression...

This car now behaves as if it had two natures, its SLOW and gutless under 3000 rpm and wakes up over 3000. Really likes to rev high, it pulls to as much as you make it turn, I tried 7000 rpm gear shift and it felt great. I just dont know how far it can be pushed with those cast pistons, not too far I suppose.
 
Double check those timing suggestions
34 was what I ran for X-J heads
The late heads, magnums and the Aluminum may take much less (which is good)
what cam did you end up with
did you post a recent compression check? get the carb sorted out?
 
Wyrmrider said:
Double check those timing suggestions
34 was what I ran for X-J heads
The late heads, magnums and the Aluminum may take much less (which is good)
what cam did you end up with
did you post a recent compression check? get the carb sorted out?
Click to expand...

And they might like more. I have seen several eddie combo’s that ran best at 40. Saw B1BA aluminum heads that like 40 as well
My w5 heads ran best at 32. Its more chamber/combo
specific than anything else. Not the material.
 
Those pistons will work fine if no detonation
what cam did you end up with again- it's been so long I forget
and compression?
lucky that chain did not skip some teeth and cause real problems
if nylon and portions of teeth missing you may want to clean your oil pump screen
cheers
 
Wyrmrider said:
Those pistons will work fine if no detonation
what cam did you end up with again- it's been so long I forget
and compression?
lucky that chain did not skip some teeth and cause real problems
if nylon and portions of teeth missing you may want to clean your oil pump screen
cheers
Click to expand...

Yes the chain could have skipped, but nothing missing aside of wear and tretched chain.

Compression is minimum 9.5:1 I would need to check my notes but 9.7:1 number seems correct to me. It was fluid checked when motor was apart.
As for cam we went with Howards mechanical flat tappet
Duration at 050 inch Lift: 238 int./238 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.558 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.558 in.
Lobe Separation (degrees): 108

With 1.6 ratio rockers we have 0.595 in. lift which takes advantage of that flow of 285 cfm at .550 and 288cfm at .600 lift.

There is no detonation, just in case he uses premium gas in it.

For carb we have 850 cfm double pumper that was setup using o2 sensor. The plugs look really good so A/F mixture must be pretty good also.

I ordered light springs for the distributor. Will see how it changes the car under 3000 rpm.

I am also thinking that perhaps air gap intake would be better then victor that is on it right now. This could be next thing on the list of stuff to buy...
 
Also I had a chance to race two young guys in new 5.0 Mustang and still beat them with those 225 tires on 14" rimms spinning... I think this is not bad given 3 speed transmission being another "problem" of this car. Shift from 1st to 2nd feels like going straight to 3rd.

Car is not slow, it sounds very nice, idles perfect at 750 rpm, starts right up... Its just missing the low end which you would expect to have with v8. You need to rev it up and it really likes to rev up high. We had to put in chromoly .080 wall pushrods because steel .060 wall pushords all bent after few faster runs. Would be good to install some aftermarket rev limter to stay safe.
 
Miszny said:
Also I had a chance to race two young guys in new 5.0 Mustang and still beat them with those 225 tires on 14" rimms spinning... I think this is not bad given 3 speed transmission being another "problem" of this car. Shift from 1st to 2nd feels like going straight to 3rd.

Car is not slow, it sounds very nice, idles perfect at 750 rpm, starts right up... Its just missing the low end which you would expect to have with v8. You need to rev it up and it really likes to rev up high. We had to put in chromoly .080 wall pushrods because steel .060 wall pushords all bent after few faster runs. Would be good to install some aftermarket rev limter to stay safe.
Click to expand...
Whats your rpms roughly at 65mph with the 3:55 rear end?
 
Just small update.
Car runs great after proper carb tuning. No low end was too small front jets and wrong primary cam lobe. Low end is there and plenty of it.
This car on those 225 tires on 14” rims was able to beat 94 RT/10 Dodge Viper.
Very funny stuff to see this car in full stock looks beating car like that with tires spinning all the way
Hope the trans can take it...
It’s a total sleeper now easily beating cars like A45 AMG Mercedes or M3 BMWs...
New Mustang 5.0 is no match for it now.
Lots of fun!
 
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