Reading this again since I posted this question 2 years ago
Completely stock 340. 4 speed car 3:23 gears non sure grip
Thinking about long tube headers vs stock 340 manifolds.
Car will be tuned if header installation moves forward.
Realistically, how much gain can I expect?
No other modifications planned.
Asking all the Mopar experts on here.
I'm no expert.
The stock cam is 268/276/114, and boasts a paltry 44* overlap.
This is not a performance cam.
The Chrysler 292/292/108 performance cam boasts 76* overlap
This is a performance cam.
Chrysler also offered a 292/292/114 cam with just 64* overlap.
You need to examine ALL the events to understand the big picture.
Here is a quick comparison of the 292 to the 268, in a 10/1 340 at 500ft elevation, assuming headers, except as noted.
The numbers are intake/compression/power/exhaust/Ica/DCR/predicted cylinder pressure/ and VP
292/110/102/292/76/ Ica of 70/150psi/7.60112VP this is the 108 LSA cam
292/106/ 94/292/64/ Ica of 74/143psi/7.33/103VP this is the 114 LSA cam
268/116/104/276/64/Ica of 64/160psi/7.98/127VP
Notice that the 340 cam has the earliest Ica, the highest pressure, the highest DCR, and the highest VP. Hands down it will be the strongest street cam.
But notice also, that the overlap is a very modest 44*. This leaves the headers very little to work with. AND, the factory logs kill what little scavenging could occur. Thus, when the power peaks, it has a long flat broad subdued peak.
But notice the 292/108 cam has a big 76* of overlap, but also notice that it is 10psi down on pressure. Thus, this cam will lose power throughout the rpm band to the pressure drop, but gain peak power, as the overlap cycle gains momentum.
Next up is the 292/114 cam. Notice the pressure has now dropped 17psi, and the overlap is now 64*. This cam will loose even more throughout the rpm band to the pressure loss, but gain some of it back to overlap.
Headers are still almost always better throughout the rpm band, but .Log manifolds kill the overlap cycle.
So the take away here is, that the pressure needs to be restored.
And you can see that in the V/P numbers, which keeps pace with the DCR numbers.
The conclusion I come to is that the 340 cam, altho short on overlap, was still a good street cam with log manifolds, but only made modest power with headers , due to the modest overlap. If it would have had more overlap then it would have had an earlier Ica, which would have made More Pressure, which would have made more power thru-out the rpm band, with a greater power peak with the additional overlap.
That it made 275 or more hp, with the factory tune, has got to be a testament to the Chrysler engineers.
Having said that, every guy I knew in hi-school who bought a 340 car, couldn't get headers on fast enough, ........ but my stock 340 car still wasted them from zero to 60, and easily up to 85mph. How did I do it? Well think about it. These were 3.23/3.55 cars, that never hit the powerpeak to until 70 mph or more, and with that 44 measly degrees of overlap, they weren't making much more power than I was, if any....... lol.
But as soon as they ditched the factory cam, for one on a 108/110, I was toast.
To catch up, I swapped out my 3.55s for 4.10s , and installed slapper bars, and the widest/tallest , tires that fit in the 70-Swinger tubs. This put me ahead again, until they figured out what I had done.
The point is this, as already said;
Every combo is different, and if you want headers for power, you are gonna need a "power" cam; but perhaps just as importantly, is if you want power at lower rpms, where like 90% of your driving will be, then you gotta keep the pressure up.
I always say;
pressure makes heat, and heat makes torque, which at rpm, makes power.
Would I install headers on a stock-cammed 340, for street?
notta chance for $900. I'd swap out that cam first, then immediately install headers.
But if my pressure tanked during the swap, or was just low to begin with, I'd fix that first.
Now; if you look at the PowerStrokes of these cams, you will see them vary between a low of 94 and a high of 104 degrees. Which means that the exhaust valves are opening very early, which works well at high rpm; BUT at typical street rpms, this means a lot of energy is STILL in the chamber when the valve opens, which is energy that could have been used to propel the vehicle, which then speaks to poor fuel economy.
During overlap, at more modest rpms; as the exhaust cycle is ending, the low-pressure in the tuned headers, "PULLs" on the intake plenum, to get the air-fuel charge moving into the chamber; aaand if the headers are tight, and sized right, there is lots of time for that tuned pipe, to yank that plenum charge right across the piston and out the port it goes, unburned.
This too speaks to poor fuel economy. However, this can be mitigated by cruising at a high enough rpm where there is no longer time for this to occur; whereas you can't do anything about the early opening exhaust valves.
Now look at the overlap periods of these cams, and see that at 44 degrees, the stock cam has the shortest period and by this reasoning, of these three examples, it should return the best fuel economy at the lowest rpm
But as most of us who have ever had a 340 car can attest to, they were hard on gas anyway. and that points back to the short powerstroke. So even if you put logs on the 340 car and kill the overlap cycle, that short powestroke of just 104*, is still making itself known.
Now if it was possible to regrind that 340 cam on a 107LSA, the new numbers would be
268/120/114/276/58/Ica of 60/167psi/Dcr of 8.22/136VP
Notice that the Powerstroke is now 114*, and Overlap is up to middle of the road, 58*; Nice cam for headers! if you keep the cruise rpm up. Too low and it would again be a gas-hog.
Put log-manifolds on it to kill the overlap, gear it down, and watch the fuel-economy soar!
