AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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- Jan 19, 2014
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What cam really hinges on your Scr and/or current cylinder pressure, or how much you are willing to do with or spend on, getting the pressure.
The higher the Scr is, the bigger the cam can be.
But; the higher it is, the less willing the engine is to accept a small cam before detonation sets in. For best results;the pressure and cam have to be co-ordinated.
For an A-body, and a DD,and the rising cost of gas, I think I'd be looking at a fast rate of lift FTH cam around 218 to 223 @.050, or a conventional rate cam one size smaller. This size cam can make really good torque from off-idle to 3500 rpm where most of the driving gets done with conventional rear gears.
For instance; with an automatic and 3.23s, normal driving will keep the rpm at under 3000rpm, so you might as well co-ordinate the power at those rpms. 3000 will get you nearly 50 mph in second gear and 65 is still under 4000 in second gear. So IMO, I don't see a good reason to have a 6500 rpm cam in there, which won't come to full power until say 5500@90 mph, still in second gear.
Furthermore big cams and low-rpm spell disaster for fuel economy. and the lower the cylinder pressure, the worse it is.
Take that 284/296/110 for instance. In at 106, and with an Scr of 8.5, the pressure is predicted to be under 127psi at sealevel, and dropping about 5.25 psi per 1000 ft elevation.That's bad news, because the low-rpm performance is already very poor at sealevel; about par with a slanty. But even worse is that power extraction with that cam is just 98*. So the piston is just over half way down the bore, on the power stroke, when the exhaust valve opens and dumps whatever pressure may still exist in the chamber. So the gas you are feeding it is not being properly compressed and not burning very long, so of course it gets terrible fuel economy. And I mean atrocious.
But now if you pump the Scr up to 10.5, then the pressure is predicted to be 161psi, and the low-rpm performance is predicted to be about par with a 5.2 Magnum.. up to around 3000 rpm. But that does very little for the power-extraction which is still just 98*.
So what you end up with, AFTER co-ordinating the cylinder pressure to the Ica, is a low-performance, 318-like power output at low-rpm, that picks up thru the midrange beginning at 3000/30mph, becoming a fire breather, spinning the skinny 255s to forever in first gear and peaking at 60 mph =6200; still in first gear.
Great, and we just hit second gear.
But recall that at low-rpm the performance is on par with a 5.2 Magnum. To get past that you gotta install a 2800 or better TC. And there goes still more fuel economy.
So that's the short version.
Depending on your Scr, you may want a small cam like above, the 223/230/110 say. With a fast ramp design this MIGHT be a 266/272/110. In at the recommended 106, this gets you an Ica of 59* and at 8.5Scr at Clarksville elevation of 823 ft, this is predicted to make just 135 psi, with the low-rpm performance being like a sick 318LA. So this cam is already too big for the stock TC and will not like the 3.23s at all. So we have to crank up the pressure.
Cranking it to 9.7 Scr, we see the pressure rise to about 160psi, and the low-rpm performance is now pretty Hot, about as hot as you can make a street-340, with iron open-chamber heads, and a 223 cam.
This hypothetical 223cam has a power extraction cycle of 110 degrees now, compared to the 98 of the 284 cam, and I know I can tune that to get hi 20s mpg in point to point travel with a manual trans and a low number hi-way gear. But you will not get that kind of result around town. However, with a bigger cam, it will only be worse. And you cannot put a smaller cam into that engine with keeping the Scr at 9.6, cuz you will detonate it to death; cuz the pressure is already maxed out at 160psi for premium gas. Unless...... you switch to a slow-ramp design, which is sorta counter-productive.
BTW, this hypothetical 223 cam described above, has an extra 6* tagged onto the exhaust duration, so will perform with factory heads and HO manifolds. But... the overlap period is 49*, so will do better with headers. This is about as small a cam as you might want to put headers on. The next size down drops the overlap down to about 42*, with an effective overlap down to perhaps 37*, and while headers are never a bad idea, 37 is not much to play with. The 340 cams had 44* and no headers. But as guys soon found out, headers really woke them up.
I'm not telling you to put a 223 cam in your 340. I'm just offering a primer in cam selection.
I won't tell you that many decades ago I mighthaveinstalled a complete 318LA2bbl top end, and cam, into/onto a fifty dollar junkyard hi-compression 340, and dropped it into a 65 Valiant wagon with nothing but fenderwell headers; and I had a blast with that car for a few years. I cannot imagine what the Scr was, but man did she bark. I doubt that combo would survive long on today's gas. It finally burned up the trans.
The higher the Scr is, the bigger the cam can be.
But; the higher it is, the less willing the engine is to accept a small cam before detonation sets in. For best results;the pressure and cam have to be co-ordinated.
For an A-body, and a DD,and the rising cost of gas, I think I'd be looking at a fast rate of lift FTH cam around 218 to 223 @.050, or a conventional rate cam one size smaller. This size cam can make really good torque from off-idle to 3500 rpm where most of the driving gets done with conventional rear gears.
For instance; with an automatic and 3.23s, normal driving will keep the rpm at under 3000rpm, so you might as well co-ordinate the power at those rpms. 3000 will get you nearly 50 mph in second gear and 65 is still under 4000 in second gear. So IMO, I don't see a good reason to have a 6500 rpm cam in there, which won't come to full power until say 5500@90 mph, still in second gear.
Furthermore big cams and low-rpm spell disaster for fuel economy. and the lower the cylinder pressure, the worse it is.
Take that 284/296/110 for instance. In at 106, and with an Scr of 8.5, the pressure is predicted to be under 127psi at sealevel, and dropping about 5.25 psi per 1000 ft elevation.That's bad news, because the low-rpm performance is already very poor at sealevel; about par with a slanty. But even worse is that power extraction with that cam is just 98*. So the piston is just over half way down the bore, on the power stroke, when the exhaust valve opens and dumps whatever pressure may still exist in the chamber. So the gas you are feeding it is not being properly compressed and not burning very long, so of course it gets terrible fuel economy. And I mean atrocious.
But now if you pump the Scr up to 10.5, then the pressure is predicted to be 161psi, and the low-rpm performance is predicted to be about par with a 5.2 Magnum.. up to around 3000 rpm. But that does very little for the power-extraction which is still just 98*.
So what you end up with, AFTER co-ordinating the cylinder pressure to the Ica, is a low-performance, 318-like power output at low-rpm, that picks up thru the midrange beginning at 3000/30mph, becoming a fire breather, spinning the skinny 255s to forever in first gear and peaking at 60 mph =6200; still in first gear.
Great, and we just hit second gear.
But recall that at low-rpm the performance is on par with a 5.2 Magnum. To get past that you gotta install a 2800 or better TC. And there goes still more fuel economy.
So that's the short version.
Depending on your Scr, you may want a small cam like above, the 223/230/110 say. With a fast ramp design this MIGHT be a 266/272/110. In at the recommended 106, this gets you an Ica of 59* and at 8.5Scr at Clarksville elevation of 823 ft, this is predicted to make just 135 psi, with the low-rpm performance being like a sick 318LA. So this cam is already too big for the stock TC and will not like the 3.23s at all. So we have to crank up the pressure.
Cranking it to 9.7 Scr, we see the pressure rise to about 160psi, and the low-rpm performance is now pretty Hot, about as hot as you can make a street-340, with iron open-chamber heads, and a 223 cam.
This hypothetical 223cam has a power extraction cycle of 110 degrees now, compared to the 98 of the 284 cam, and I know I can tune that to get hi 20s mpg in point to point travel with a manual trans and a low number hi-way gear. But you will not get that kind of result around town. However, with a bigger cam, it will only be worse. And you cannot put a smaller cam into that engine with keeping the Scr at 9.6, cuz you will detonate it to death; cuz the pressure is already maxed out at 160psi for premium gas. Unless...... you switch to a slow-ramp design, which is sorta counter-productive.
BTW, this hypothetical 223 cam described above, has an extra 6* tagged onto the exhaust duration, so will perform with factory heads and HO manifolds. But... the overlap period is 49*, so will do better with headers. This is about as small a cam as you might want to put headers on. The next size down drops the overlap down to about 42*, with an effective overlap down to perhaps 37*, and while headers are never a bad idea, 37 is not much to play with. The 340 cams had 44* and no headers. But as guys soon found out, headers really woke them up.
I'm not telling you to put a 223 cam in your 340. I'm just offering a primer in cam selection.
I won't tell you that many decades ago I mighthaveinstalled a complete 318LA2bbl top end, and cam, into/onto a fifty dollar junkyard hi-compression 340, and dropped it into a 65 Valiant wagon with nothing but fenderwell headers; and I had a blast with that car for a few years. I cannot imagine what the Scr was, but man did she bark. I doubt that combo would survive long on today's gas. It finally burned up the trans.
