[URL][URL]http://victorylibrary.com/mopar/cam-tech-c.htm[/URL][/URL]
Case #1,
Anyone who has ever swapped a set of small chamber heads out, for a set with large chambers , with no other changes, can tell you that the bottom end went away.
I did this to an early 9.2Scr 318, installing X-heads.
Case #2,
anyone that ever swapped a 340 cam into a 318, with no other changes can tell you that, the bottom end went away.
I did this to a few Smogger 318s
Case #3,
anyone who ever swapped a set of large chamber heads AND a 340 cam, onto a smogger 318 can tell you that, you guessed it, the bottom end went away.
I did this more than once, like an idiot.
Case #4
anyone who ever installed early 318 heads and a 318 cam, onto/into a 340 can tell you what a tire fryer that makes.
I'm talking about, "bottom end". Rpm from idle to about 3000rpm.
If the changes were made to a car with an automatic transmission, the bandaid for this lost bottom end is a higher stall, to get the engine OUT of that soft bottom end. I get that.
With a manual trans, to get the car moving, in a timely manner, the only option is rev the engine higher and/or slip the clutch longer.
In both cases, low series gears that force the engine to stay in the soft zone, only make accelerating up to speed worse.
Now; as the rpm comes up, and the engine begins to manufacture torque, there comes a point in the rpm band where the engine no longer cares about it's Dynamic Compression ratio, which is only a mathimatically derived number, and moves towards Effective Cylinder Pressure. If the engine has a working overlap cycle, this will happen sooner than if it has not.
Generally, this will happen above 3000 rpm, and somewhere below the torque peak, depending on how well the parts are matched.
In my 50 plus years of modifying street cars, I have built examples of every kind of thing not to do. And in the early years I have pulled some serious boners.
Twenty odd years ago, I made my first foray into high cylinder pressure. I can tell you that this works almost like turbocharging.
Which pumps air into the cylinders, which, among other things, increases the cylinder pressure. Except that, if you start with say 195 psi, on the street, you'll never need the turbo.
Whereas at 116 psi, that low pressure is a very serious handicap.
And the solution for this is so simple, just reduce the total chamber volume, and/or, increase the swept.
As rpm crosses the threshold of switching from Dynamic to Effective pressure, TIME becomes a problem. Time to get air into the cylinder. If the cam is too small, it begins to restrict the ingestion of air, and the power begins to fall off. But if the cam is too big, then, peak-ingestion occurs at a higher rpm than may be suitable for street use, requiring too severe band-aids and/or sacrifices, to live with.
But the goal is always to ingest more air, which plumps up the falling cylinder pressure.
Making POWER; is all about making pressure, which makes heat, which drives the production of torque, which translates to power.
When an engine gets old, it loses the ability to make pressure, and we say she is getting tired. But have you ever done a compression test on a tired engine? It may only be down 10 psi or LESS, than when it was fresh. Already, the driver knows it's days are numbered.
Have you ever taken a trip over the mountains, and experienced your engine getting more and more tired on the way up? Well, part of that is the loss of low-rpm cylinder pressure.
No cam in the world can get you out of that situation. No super-flowing heads, not even more gears. The only way past that, is to figure out how to ingest more air, to increase the cylinder pressure.
I know you and I
@273 are often on different pages about things. So as a parting cookie; I can tell you that in 2004 my freshened 367 was making cylinder pressure of 185 or better. Some 125,000 miles or so later, she is down to 177 on the lowest one. I can tell you that she is, IMO, tired; that's a loss of only 8psi = 4.3%
Imagine going from 155 to 116, a loss of 25%, and that's with all new parts! How disappointing would that be!
OK sure, I get it, just install a 3200 stall or better and 3.73s and yur good to go. I get that, it works with an automatic. However, with a 3200 stall, whenever the roadspeed drops below about 26mph, the convertor will begin to slip, which will crush your fuel-economy around town, in the which, starts and stops and lowspeed driving is/are the norm.
And finally,
My alloy heads have supported up to 195psi on 87E10 with full timing.
OP already has alloy heads. Therefore, IMO, to not exploit that pressure potential, already having those alloy heads in his possession, if nothing else, is a crying shame. In a heartbeat I would figure out how to get the pressure up, to make this work.
If the KB107s really are .100 in the hole, then,
I see this as a simple piston swap to get the crowns up into the open chambers, and then taking the decks down to get the Q into the ballpark, then selling the current pistons to recoup whatever.
BTW
IMO, that engine builder is right to not want to slam this together as is.
Alloy heads, IMO need at the very least, a half a point of compression more than iron, just to break even in power production; and closer to a FULL point at part-throttle.
So then, with the pistons truly .100 down and the Scr predicted to be 8/1, that 8/1 is really gonna only feel like 7/1 around town.
But not only that, with the 272 cam in at 108, the Ica becomes 62*, and thus the actual pressure in the chamber I would expect to be even lower.
I know how disappointed I would be.
To OP,
if you did slap it together as is, you should expect to have to buy a higher than stock stall convertor, AND have to install some high 3.xx series gears ...... at added expense.
Whereas;
if you get the pressure up to even just 175psi , the stock 2200 stall will be fine as would any rear gear from 2.94 up; saving you money that you can put towards fixing this situation.
The 2.76s might require a couple of hundred more stall, but if they were already in the car, I would at least try them. I mean I ran 2.76s behind my hi-pressure 367, with a stinking manual trans. While doable, with a 3.09low, I can't say that I'd ever recommend it. Whereas the convertor will nearly double torque multiplication off the line.
I just do stuff because I have the junk, the tools, and at one time, the energy. And I have never been afraid to undo what has been done with poor results.
My advice for if you decide to leave the pistons down there;
is to leave the alloy heads in the box, and just install any old iron heads. At low-rpm and Part Throttle, iron heads will do everything that the alloys will, atta fraction of the cost. If you already have some closed chamber iron heads, so much the better.
