Most all cams are installed 4 degrees advanced,
This Post is NOT a reflection on you, B3432w5, nor anyone else, nor to anyones understanding.
This post is just cuz this is what most of us were taught and believed.
(We used to be able to go into those quote-boxes and make changes, and if I still could, I would have erased your name from it.
I tried to delete the entire box and just use a quote, but the program wouldn't even allow that. So, please forgive me, I made a mistake.)
That said, everything hereafter is directed at a street car;
IMO,
a split-pattern cam, can be installed anywhere within about a 12* window from 4 retarded to 8 advanced. I mean it's hard for me to imagine needing a bigger window, and I think that 12* is really really generous.
Moving the cam does several things.
Number one;
Probably the most obvious is that advancing the cam increases the cylinder pressure, which generally increases bottom end torque. If you have a 3000 stall convertor, who cares about 5 or 6 psi. Put it wherever you want, in the window.
But, with a low-stall and a low cylinder pressure, every stinking psi is important and that is why guys advance the crap out of idle-Timing, namely, to get a few more ftlbs of torque.
If your engine already has a generous amount of cylinder pressure, then there is no need to advance the cam, or better said; you might find a better ICL.
IMO;
the number on the cam card only reflects the grinders opinion, based on the average joe stuffing a too-big cam into a too-low pressure engine, so the grinder wants us to have the best experience as is possible, and 4* could be it. I said could be, he's just protecting his azz, cuz he doesn't want to hear about your 95psi 318 falling on it's face.
Number two;
Moving the ICL, moves the overlap period. Overlap, from the time that engine vacuum peaks, has the potential, with headers, to build hp, one atta time, thru the torque peak, to the Power peak, and a bit beyond.
If you have log manifolds, overlap is just a lumpy idle, and you can kiss the power bulge goodbye..
Moving the overlap, tunes not just where the power peaks, but also how big it can grow. So a 4* advance, peaks a a lil sooner, and the nut behind the wheel with 3.23s is a happy guy.
But if you're running 3.91s , 4* advanced may not be optimal.
In fact, if your combo has plenty of compression, and is running headers, and big gears, those 4* advance may be limiting your top-end power; you know, 200 rpm thru the traps could be just what yur looking for.
Number three.
As to Fuel mileage on the hiway.
No matter what
streetable-cam that you install, when you add the compression degrees to the power degrees, yur always gonna get a number that varies somewhere between about 220 to 240. You the installer can choose, within that 12* window, how much to give either way.
Suppose you have selected a cam in the which this number is 225 degrees. Suppose this engine already has a generous DCR, and you don't need the typical 125 degrees of compression, but you installed it there anyway. This leaves just 100 degrees for the Power stroke, which is getting to be pretty small for a streeter. That works great going thru the power peak, cuz of the sheer number of power strokes.
But below the Torque peak, a lot of gas-pressure is going straight out the tailpipe. That is energy that could have been used to propel the vehicle, and so, your car is a gas-hog.
Now Suppose you move that cam, and trade away 5* from Compression and give it to Power. You still have 120 degrees compression ,but now also have 105* power!. that's plus 5%.
Yes you will lose Cranking cylinder pressure, so you may, I say may, want to compensate for the loss, it depends how much you started with.
Personal experience
I have played this game with a
292/292/108 single pattern cam, all summer, moving it all thru the 12* window, looking for torque and or mpg; I couldn't find what I wanted.
The Compression plus Power on that 292cam was 212. The engine was at 11.3. At 4* advanced, Compression degrees were 110, and Power was 102 . At 8*advanced, compression was 114 to power at 98* and lordy that was hard on gas. Lots of midrange tho.
This cam taught me about the Compression plus Power relationship.
I pulled that highly mismatched cam ,
and next I installed a
270/276/110, and hot-daymn !
Compression plus Power was up to 230 now. With 119 to Compression, that left 111 for power, and now she totally eclipsed the former crappy fuel mileage. and at 119 Compression, the bottom end was , dare I say phenomenal, compared to the 292 cam at 110. Cylinder pressure was way up!
The following winter I made some changes to decrease the very high cylinder pressure, and retimed the cam for a lil of that 292 top-end rush.
For me, this was now ideal.
I was very sad when that cam dropped lobes.
The current cam is 230/237/110, and it has a
Compression plus power of 219. Predictably the fuel economy sucks again. But it pulls pretty good at the top, and has more than enough at the bottom, however, I have been waiting for a long time, for it to wear out, cuz I already have my eye on a replacement cam;
In conclusion,
to me, now,
the ICL is only used to to trade degrees between Compression and Power, within that 12* window, and/or to keep the overlap more or less centered, if a little biased........
other notes;
> this not the whole of it, just the parts that matter to me.
>I have a clutch, so tuning for the chosen cam install can be a bit of a chore, not having a converter in which to bury mistakes.
>I would still be running the 270 cam, had it not dropped lobes a few years later.
>If you install an overdrive, onto an engine that is already spewing high pressure gasses from a short power stroke; then as the rpm decreases, this will allow ever more gasses to be spewed. Thus your hiway fuel economy may actually get worse with the reduced cruise rpm. To this I can attest.
>For a streeter; This all only works with a split pattern cam, so that the EFFECTIVE overlap remains similar, anywhere in the window. If you chose a single pattern cam, yur kindof stuck having the overlap centered within a degree or two, and just drive it, else you rapidly lose effective overlap, which is a power loss.
Ok so, as a famous man likes to say;
"drive thru."
