292/508 question
not too concerned about 1/4 mile performance , never done it. just want a kickass idle for car shows and cruising, gas mileage,same. i seem to remember posing same question about the 284/484 cam,same response from alot of the same members.id sure like to know the end all and be all cam.apparently its out there, popular consensus seems be the 268xe.no thanks
The best cam?
Firstly is application specific,
then engine and powertrain specific,
and
tempered by budget.
If you just want the sound;and care about nothing else, get you a ThumpR. That might keep you happy for a summer.............. Or a small cam with a lotta overlap.
Overlap and good fuel mileage do not co-exist ..... normally.
The cam is the key to unlocking power at specific rpms.
The 292 for instance, peaks at around 5800 depending.
The 284 at about 5600
the 276 at 5400
the 268 at 5200
and so on.
With an automatic, to get the best from the conventional cam at the next shift, the rpm is usually buzzed up 800 to 1000 or more past the peak, depending. If your engine spends a lotta time past 6000, you need to do some oiling mods to keep it alive.
The problem comes when a guy wants to run a cruiser gear and the top of first gear ends up at around 60mph. Then the daymn cam doesn't start until 40/45 mph; but worse is the first 30 mph end up being so soft, that a sharp teener comes along and embarrasses you. So now you have an engine with a killer idle that pretty much makes a useless streeter.
Ok so the solution to that is a 3500... or more, TC. And that comes with a new set of streeter's problems, and it still doesn't change the fact that the 3.55s are peaking at 60 mph. So you are still looking at changing the rear gear.
So now that big lopey cam is costing you, additionally; a TC, a rear gear, many mpgs,and a lot of other things, AND it has turned your $30,000 dollar cruiser into a weekend-only bomber, and a city-only car: and all because you want the sound.
And we haven't even talked about what has to be done to the engine to accept that cam, nor how much it's gonna cost.
If you are on a tight budget, you might be tempted to just toss the engine together, and so it ends up at or near the factory compression ratio. And then that big lopey cam, with iron heads, has no bottom end at all.
And if a TC and gears are not in the budget either, then your soggy-to-35/40 mph engine, with it's killer idle, is gonna end up being a constant reminder of what not to do.
IMO, it's a combo. An integrated combo of budget, powertrain, compression and cam size. If you can't afford the supportworks (TC and gears) to run that big lopey cam, or the gas to drive it afterwards, or the supporting engine changes, Or the chassis upgrades, then......... just maybe it's the wrong cam.
This is where the little ThumpRs come in. You just gotta remember the design philosophy behind them.
Or the small, high-overlap cams. At least with these you can put the power at a useful street rpm/roadspeed.
Take the 268 cam, for instance. It normally peaks at around 5200. And with an automatic the 1-2 shift might want to be at 6200 to drop in at 3660, for a powerband requirement of a tic over 2500. So now instead of building power with rpm, you need to put the power into the heads, and into the efficiency of the engine. But of course you will lose the lopey idle of the big cam. What you get in return is a stouter bottom end and much more fuel efficiency. So maybe you get to keep your gears and maybe also a lot smaller TC.
But hang on, if you order a custom cam about the same size, but with a much tighter LSA, four things are gonna happen; 1)the overlap is gonna increase, and so the rpm of peak power is gonna go up a bit, with attending bit more power. At the same time, a little lope is gonna creep into the idle,and 2) the cylinder pressure is gonna rise, increasing the low rpm performance, and 3) power stroke is gonna lengthen, increasing steady-state fuel-economy, and 4) because of the first three, you are gonna be a much happier guy.
Here is a comparison
These are all in a 340, with the Cylinder pressure adjusted to about 160psi for open chamber iron heads, to run today's pumpgas.
OK;
First the regular oem
268/114 factory 340cam in at 110
Static compression ratio of 10:1.
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 7.98:1 .
Your dynamic cranking pressure is
159.36 PSI.
V/P (Volume to Pressure Index) is
126
Notice the VP is only 126, just slightly more than a 5.2 magnum.(124)
Ok so here it is with a modern
268/110 cam in at 106
Static compression ratio of 9.7:1.
Effective stroke is 2.66 inches.
Your dynamic compression ratio is 7.98:1 .
Your dynamic cranking pressure is
159.36 PSI.
V/P (Volume to Pressure Index) is
130
Notice the VP went now up to 130
And again with a
268/104 cam now in at 100
Static compression ratio of 9.35:1.
Effective stroke is 2.78 inches.
Your dynamic compression ratio is 8.00:1 .
Your dynamic cranking pressure is
159.88 PSI.
V/P (Volume to Pressure Index) is
137 .
Again less Scr, and more VP; 137 is getting somewhere. Fun starts around 140/144. But now we have gone about as far as we can with shrinking the LSa
Ok so now lets put aluminum heads on it and a solid lifter cam equivalent to the 268/104 cam. This would be close to a maximum effort.
The 268 hydro is about
[email protected], so we need a similar
[email protected] solid and I get a 260 and it might lose 3 degrees in the lash so let's gross it up to 263 and still a 104LSA, but I'm gonna put it in at split overlap to take advantage of the nice overlap-created, header-augmented, power peak.I get an install of 102.5 ok then
that cam comes in at
263/271/104 in at 102.5
Static compression ratio of 10.5:1.
Effective stroke is 2.78 inches.
Your dynamic compression ratio is 8.97:1 .
Your dynamic cranking pressure is
185.35 PSI.
V/P (Volume to Pressure Index) is
159 .
Holeeeeeee, do you see that VP is now 159. And the combo of aluminum heads and 185psi, means it runs on 87E10. 159 is like a 1969 440 Magnum........ from idle to about 3000/3600. You can keep your 3.55s or 3.23s or even 2.94s. And you can keep whatever stall is in there.
Of course it's still just a
[email protected] cam, so not making 430 hp. But the 104 Lsa is gonna push the power peak up some 300 or so higher than the oem 340 cam so now it's pushing 5400 or perhaps 5500 with those aluminum heads. If you also boosted the lift on that cam into the .550 range, then I guarantee you will never think about the 292 again.
Well except for the almost total lack of lope,lol. The above solid cam has overlap of just 59* compared to the 292/108 at 76* so bye-bye lope.
Well that's not entirely true. You can coax a little lope out of her with the tune; a low idle, retarded ignition timing, and a messy carb-tune. In my combo, I just installed a dash-mounted, dial-back, timing device, with a range of 15 degrees,lol. Hello rumpidy rumpidy.
Here are the other events of the above theoretical cam:
Compression is 126 degrees and power is 119; so not only will it burn 87E10, but now it has about 15 degrees more extraction on the power stroke, taking full advantage of the 185psi of cylinder pressure to drive many more miles per tank.
And finally, the 10.5Scr is easy and cheap to make, the engine practically falls together at that ,with proper pistons.
How many wins is that? I count ; plenty of power, a killer bottom end, great fuel mileage, burns cheap gas so drive it anywhere all the time,money saved in not buying gears or a TC with attending supportworks,nor an overdrive, windows down cruising because no stinky exhaust,smokes the tires to 50/60 mph,IDK whadImiss? Oh yeah; cheap/easy build.
How many losses? Maybe two; the loss of the killer lopey idle, downgraded now to mild lope, and the cost of the aluminum heads, which is offset mostly by the plusses..
Course with solids, I guess you could move up one cam size, cuz a VP of 159 is waaaay overkill. Ask me how I know..... ( I ran 162 for 4 years,currently 153 which is still way overkill)
Or you could go back to a hydro
Depends on where the Scr comes in at,
or how much you wanna spend on machining
....... lol
Happy HotRodding
