I want to start aquiring parts for a rebuild on an L48 this winter. In selecting a cam what effect do these dimension have on performance etc. I know what they are by definition, but want to know what happens if I lengthen the duration as opposed to increasing lift etc or vise versa. Hope this isn't to stupid a question but would prefer to know what I am talking about. Thanks in advance

 

To achieve more lift, you need a higher duration.

Imagine that you're riding a bicycle. Flat ground, easy.

A hill is up ahead. How quickly the hill rises will determine how much shock you and the bike feel as a product of the elevation change. If it's just a sharp 3 foot rise with minimal ramp, you're gonna crash. If the 3 foot rise has 6 feet of rise and 6 feet of fall, it's gonna be a gentle ride.

If lift is aggressive and duration is low, you will wear out your cam and lifters. The various cam types (hydraulic, solid, and the roller permutations thereof) have different ramp capabilities. The roller variations can run more lift without longer duration.

 

When looking to "cam" an engine, be careful you don't end up with valve/piston clearance issues, or so much duration that your DCR falls to a point to where you wonder why your rebuild turned out to be a dog. With that in mind, a couple of things that might be worth consideration are 1.6:1 rockers or a roller cam to reduce the amount of duration required to achieve more lift. My $.02

 

The basis of this question is this. I have been given a new cam and lifters for an LT1 but was told the duration was to long for this motor and would be better with a higher lift. Why?

 

In way general more lift low end, more duration more top end. Though there are many factors that can change this effect.

 

Duration should be chosen based on the intended rpm capability, where lift should be chosen based on the head flow capability.

It doesn't do you any good to have duration so large that it can rev to say 8,000 rpm, if you'll never go above say 5,000 rpm. Duration changes, if larger enough, can and will have a dramatic affect on an engine's operating characteristics. So, if you still want to have a mild mannered street engine, you do not want enormous duration. And increases in duration also require valve to piston clearance checks, more so than simple lift increases. Because max lift does not occur when the valve is closest to the piston.

And excessive lift will do you little or no good if your heads run out of breath well below that level. Some extra lift can be a good thing, but too much lift only takes a larger toll on valve train parts for no reason.

A cam change needs to be well thought out in advance, considering everything involved.

 

Not sure what your plans for the motor are...but the LT-1 cam is a sweet little street piece. Being solid lifter it will like to rev. I used one in a '71 350 low compression motor with stock heads and it ran great with a 4 speed and 3.36 gears. I later put a set of killer ported aluminum heads on the same shortblock that raised compression and threw some 1.6 rockers on it. The sucker was killer. Drove great and still could easily spin 7000 RPM.

Need to base cam choices as folks described here. Select the cam based on all the other parameters. It's a big decision so you want to hit it right.

JIM

 

with 4.11 gears I can use the high RPM. It is running a standard 3 speed auto for now. What was the idle like on the small block? also what about stall speed with the converter. Thanks for all the help. Sometimes I feel pretty stupid, but if you don't ask questions it can get pretty expensive quick

 

The LT-1 cam is nothing radical at all. It has a nice little lope to it.

It will fit in the plans fine for a nice little street motor...but of course you know there are better ones out there. Is this a quick freshen up build, or something that will become a 383..or get new heads? What intake and carb?

Better to build a combo than to base a build around a cam just because it was free.

JIM

 

because of budget this is only first step. heads, intake and carb will come later. Any suggestions. I just wanted to make sure I could drive it while I replenish by funds

 

If it's going to stay as a 350 sized motor...or even if it isn't...I'd get compression to the 10-10.5 range. You can do this with different pistons or with different heads. None of the factory GM heads are really worth messing with for performance other than the later Vortec style. If you were going to buy heads and intake also, you can get some nice packages from Scoggin Dickey with the Vortecs or you might do some local searching for used ones. They were used on pickups/tahoes etc up through '98.

Past that you move into aftermarket heads with any number of choices. Before I would spend a penny on the original castings I'd go to almost anything from the aftermarket. I wouldn't spend extra on the Vortec either. If I found a used set with good guides I *might* do a valvejob..but likely would just check them because the factory did a real nice job on them and a cheap rebuilder type valvejob will mess them up. By the time you add in the price of a real performance valvejob, the cost of the castings, better springs and machine work to handle a better cam..you can just buy a new set of them or move to aftermarket.

Roller rockers aren't needed with that cam...but some stamped steel ones with a 1.6 ratio are nice. Have to get the right ones to match the heads though as far as whether they use guideplates or not. If you want roller rockers...make sure to get good ones. Stay away from anything that says Proform or CAT or are no namebrand ones.

We still need to narrow down your plans for the car. What accessories, RPM range, manners required etc.


Again, the LT-1 cam isn't a bad start. You can get 400-425HP easily.

JIM

 

You sir, are a great source of info and I thank you. The motor will stay a 350. This winter I will change the cam and bearings, and while I am there will put in new rod and main bearings. At that point I hope to be able to drive it during the summer. The following winter will do the heads. I had a Z28 small block that I had Dart heads on ( don't remember which ones now). They seemed to work well and will probably return to those. From there as time and money allows will go to the intake and carb. Eventually want to put a 5 speed in, but that will be a while. Again thank you for your help. I really appreciate it. If you have any suggestions, I am all ears