Replacement 68 340 four speed cams
I'm "cam"inally challenged, so to speak/slur. These were one of the "best " sb factory mopar cams correct? My 340 build will have oem 273 adjustable rockers and I would like to go solid flat tappet.
By doing this, how(if any) will this alter the cams "performance"? Will the cam specs need changing to a solid?
Thanks
They are not ONE of the best, they are THE best .... but only because MaMopar never installed a bigger or better cam, and as you know the 340 only ran from 68 to 73.
By today's standards, there are lots of better grinds; especially if you plan to run headers, and even more especially if you don't have the cylinder pressure to support these wide LSAs.
As to hydro versus solid;
Hydraulic lifters present to the valve springs as very heavy, and the higher you rev then, inertially they just get heavier and heavier. Eventually the springs give up. At the other end; the ramps have to be Looooong to accelerate the heavy buggers without squeezing the oil out of them. And to set them down easy without bouncing.
So when you look at the Advertised spec, you might see 268 degrees.
But, firstly, this number is ethereal; in that some grinders measure this at .008 tappet rise while others at .006 , and others at whatever they want. There are no standards for advertised numbers.
So then, at .008 Tappet rise, the valve is still open, say .012inch with a true rocker arm ratio of 1.50.
What that means is that, your engine cannot make cylinder pressure until that .012 disappears to zero. And unless you know where zero is, this could be 10/14/18 degrees later. So then, seat to seat, of a 268* cam could be double the run-off room (say 14 x 2=28) plus the advertised 268 =296 degrees... or more.
Why do you care?
Because like said, your engine cannot start making pressure until the intake valve closes. In a short stroke engine, with a long-period cam, the EFFECTIVE Stroke computed from the closed position of the valve, can be very very short.
For a streeter this is bad because in order not to suffer a huge loss of bottom end power/torque, you have to increase the compression ratio to get the cylinder pressure up to a useful level. But to use today's gas, you can only go so far; Especially with iron heads.
Solid lifters, on the other hand are much lighter, and so ALL ramps can be faster and YOU, by setting the lash, determine EXACTLY when the intake valve actually closes, and by so doing, have exact control of the Effective Stroke.
However, because of the lash, you lose an amount of lift equal to the lash.
Furthermore, by adjusting the lash smaller or greater, you are also determining the advertised duration..... AND the 0.050 simultaneously.
This means that if you stick a 268 hydro cam into your engine and run it with solid lifters and lashed;you actually end up with a cam that is smaller than what you thought. Some say, about one size smaller.
This means, you gotta order a bigger solid cam to equal the smaller hydro cam.
The up-sides of this are;
1) Principally, that the lighter solid lifters will rev to a higher rpm with the same springs. and
2) the cam can be ground with faster acceleration rates, for more airflow potential at any given position, and
3) YOU have some control of the actual installed cam size by varying the lash.
The downsides are;
1) periodic lash adjustment, and
2) possibly a shorter cam life.
Now here is my opinion/ my experience; having run three hydraulic cams, namely; 270/276/292 intake degrees, in my Hi-compression 367;
IMO for a streeter, if you have the luxury of high cylinder pressure; just run a hydro.
But if you are stuck with a low-pressure engine, a solid lifter cam may, I say may, get you a tiny bit more all-round performance . If you go too big, you will suffer the same low-pressure/low-rpm problem, and the difference, typically,
is only one cam size.
And finally, you can get some pretty good results, by running a too-big hydro cam, and lashing it, lol. As we saw above, a 268 advertised cam might actually be 296* seat to seat. but if you lash it up, you might turn it into a true 268, seat to seat. You will lose both lift and .050 duration, but you will gain a bunch of cylinder pressure; perhaps as much as 1.5 psi per degree. If you give up 7 degrees of ramp, that calculates to 10psi, which could bring your pressure up from a mediocre 145 to say 155, hmmmmmmmmmmmm
This is of course trading away top-end power, to get bottom end power. But you can easily get that power back by resetting your valvelash to zero or less.
The downside to this is that the hydraulic lifters are now crashing into the non-existent clearance ramps, so into the acceleration ramps; and one would surmise this to shorten the life of the cam/lifters. Maybe, but I've been waiting since about 2006 for mine to give up......
Again, if you can get your cylinder pressure up to 155/160 with iron heads or 180/185 with alloy heads, in a streeter, IMO, you might as well stick with hydros. You can use your 273 gear with decent springs, to minimize the preload and thus rpm your engine to however high you dare; are you feeling lucky today? lol.
Happy NewYear !