What do you guys think about a stock profile cam of that of a 390GT/428CJ,SCJ? Someone told me that this is a great cam to use for mileage and all around street performance. Just looking for a cam that will run in a 9.5:1 compression 390, D2TE-AA heads, four barrel induction and headers in a small half ton 2wd truck.
I dont really know, maybe it would. Chances are it would work ok. However, there are many excellent alternatives from Crane and Comp that are 30 years newer. Unless you're looking for the absolute cheapest, safest way of doing things, and are sure the stock cam is in good shape, I'd look at a modern aftermarket cam and valvetrain.
rusty, maybe you should write books on this stuff cause that was first thing that I have understood all week
"Sorry for the book"
"sorry for writing something worth reading"
And didn't even think of Rusty's last reason, not to get a 260/260 "RV cam" for my stock 360FE (2100 2v, single straight thru-2+1/2" exhaust). Was just thinking in terms of power band when i went with the measly little ol' 252/252 "economy cam". :)
The fact that the 252/252 had bigger -bumps- on it than a new stock cam sounded good enough to me. :)
Also the fact that i couldn't even -buy- a transmission speedometer gear to fix the odometer shows that 3.25 gears and 31.5" 235/85-16's weren't inside anyone's RPM planning.
obey; the 390GT/428CJ cam is 270/290 w/194/204 @ .050", .481"/.490" lift, 46* overlap. Yes it's a old cam grind looking at the lazy .050" numbers, it doesn't slam the valves open like modern cams allowing for low valve train wear and high mileage. I'm over 249K, 3.54's w/33.5" 12.00's with a custom low stall converter, can turn tires at will plus 15 mpg if driven carefully.
A 410 build, to do it over again I would use another OEM CJ in a hot second as this is a daily driver not a Chevron profit maker.
My experience is in post #10, but maybe I should state clearly that my truck was a driver, not a racer, and did a lot of towing too. It was fun to drive with or without a trailer. It had good torque and good acceleration.
First, the adjustable FE rockers have a 1.76:1 ratio, as opposed to the non-adjustable 1.73:1 ratio. Now that's not a very big difference, and you'd likely never notice it, but you will get slightly more power with the adjustables.
Second, is the adjustability! But before we can appreciate that, we must know how hydraulic lifters work. Basically you have a plunger and a check valve. This can either be a ball or disc. There is then a spring which tries to push the plunger out of the lifter. This is what makes a lifter with no oil in it springy (you can compress it and it rebounds back). In a running engine, the lifters are exposed to oil pressure. The oil naturally flows into the lifter, displacing the air that was there. Thing is, oil isn't compressible like air. So when the lifter is on the base circle of the cam, the oil flows in behind the plunger, with the spring helping, pushing up on the plunger and taking up all the play in the valvetrain. This is what makes a hydraulic lifter motor so nice and quiet. Essentially, it's like a solid lifter at zero lash.
But there are things that can go wrong. The little check ball or disc can leak, causing a noisy lifter. Or, the lifter can pump up with oil at high rpm, leaving the valve off it's seat when it should be closed. This can have bad consequences. The reason this happens (IMO) is because of valve float. The valve spring at high rpm no longer has enough force to close the valve when it should according to the cam lobe, and the valvetrain becomes loose. The lifter sees this and expands to compensate, but instead is now expanded too far and leaves the valve open.
Another conecpt to understand is the relationship of preload to lifter pump up. Lifter preload is how far the plunger is pushed down in it's bore when the lifter is on the base circle of the cam. The more preload, the more force the spring in the lifter is exerting on the plunger, and the more the lifter wants to take up any slack. This is good for a low rpm motor that never sees any valve float. But on a higher rpm motor, you want a minimum amount of preload. Most of us with adjustable rockers go 3/4 to one turn after taking up all the slack in the valvetrain, to preload the lifters. Of course that number is up for debate, and some guys prefer to directly measure the preload. To each his own.
Which brings us back to adjustable rockers. In the non-adjustable system, you have no way of accurately setting the preload short of a custom pushrod for each valve. Basically the best you could do on a budget is measure all of 'em at zero lash, take an average, add your desired preload, and order your custom pushrods. But even then, you'll end up with some with too much and some with too little preload. It's a losing proposition. With adjustables, you simply get the rockers, your matching pushrods, and adjust as described in the last paragraph. Nothing custom, and preload set accurately on all lifters.
Some guys have had problems with over-oiling in the top end with adjustable rockers. I suspect this is because the rockers themselves are designed differently and let more oil out. So you need the 3 finger oil baffles to keep that under control.
Does that help?
Last edited by rusty70f100; 04-26-2007 at 11:40 AM.
Kurt, you've got this stuff down, brother. Lots of good info.
When I spoke of adjustable timing in post #37 I was referring to the timing gear and how someone like me with a daily driver can benefit from an adjustable timing gear set as opposed to a non-adjustable timing gear set.
Kurt, please do not over-react, I agree with 99% of your well-written post.
I wonder if the very light springs in the lifters seriously affect much in the valve train. I think the springs are there to keep the newly installed lifters in the extended position during assembly while the are filled with air.
The way I understand the principle of anti-pump-up lifters is that what you refer to as "preload" is set to the top of the lifter stroke so that pump-up is limited to little or none. The hot-rodder can live with running on the edge of having a little rocker noise as a trade-off for avoiding pump-up.
I have been told (perhaps incorrectly) that the main difference between anti-pump-up & stock lifters is that they have a more robust clip at the top of the lifter bore because it has to act as a stop to limit pump-up. (The clips in stock lifters really only are there to keep the lifter together until it is in the engine.)
I am ready to be educated if I have understood this incorrectly. I have never personally used anti-pump-up lifters, but I worked at an engine rebuilder and we did put them in some applications at customer request, so I have handled them many times. (There may be minor, but important internal differences, but the robust clip is the one obvious difference.)
P.S.: It occurs to me that anti-pump-up lifters could have lighter internal springs . . .
Correct me if I'm wrong here, but I seem to remember reading somewhere that adjustable lifters really come into play when you have a cam with a .533 and over lift. Not that they don't/won't work or be beneficial for valvetrains with smaller lift amounts - just not as critical? Am I right about this?
More is more & you get the gains of adjustibility. (The numerical increase will be bigger with a bigger lift, but everything will be gaining the same percentage.)
Anybody got info on the inertial differences? I always wondered if the fixed rockers had an rpm advantage. Not only is the rocker likely to be heavier, the cup end of the pushrods look like they might be adding to the valve-train mass.
You would benefit from an adjustable timing set by being able to correct for any advance / retard that may have been ground into the cam itself. If there is none, then of course there would be no benefit. But with a stock cam, I suspect that there is. At a minimum use a degree wheel and make sure.
Yes, the spring in the lifter helps with assembly too. I suspect that in a running engine the lifter would function just fine with no spring, as oil pressure would take over at some point. But in an idling FE with 10psi or less oil pressure, I think the spring there helps.
Lifters also do something called "bleed down" over time. What this does, is oil eventually escapes the lifter through the check ball / disc. This also helps with lifter pump up, as it will allow an over extended lifter to come back down to proper adjustment quicker. I think this is what they do with "anti-pump up" lifters. They just bleed back down quicker. They may also have a lighter internal spring, although I'm not entirely sure on that one.
I had suspected that bleed-down rates might be higher with the anti-pump-up. You do not mention it, but am I correct that the main point of anti-pump-ups is to run them at the top of their stroke?
P.S.: Good point on the 10 psi - it is acting on a pretty small area, so there might need to be some help. I agree with your description of how a floating valve allows the lifter to over-compensate, but it seems that the main defense is to limit this mechanically by working against the clip.