GM L31HD and cam choice

[Stokes1114]

That cam is one of the options that came up on the cam quest program from CompCams website. Grind #XR258HR. Won’t their matching spring set need to have the heads machined? Their notes section mentions it I believe. I might be interested in it, I will let you know for sure in a few days.

No you don't have to have anything machined. The springs are stock size. Just much stronger. The machining is for the dual springs
.. Just remembered you have vortecs so the springs won't work. The cam and lifters are the same. As others have said, comps ramps are MUCH quicker and if that's not what your looking for then stock GM cams would be your best bet.

 

[Supercharged111]

Those images look like fake news to me. Seriously.

There is a LOT more to identifying a cam lobe than using the advertised duration vs. duration at 0.050", but it is intended to *help* determine the overall shape. What you're looking at are numbers to determine the lobe ramp intensity or "steepness" of the lobe. The GM cam has more of a gentle, gradual ramp, while the comp cam has a steeper ramp. Generally speaking the steeper the lobe the more power, but with a trade off for valvetrain noise and longevity. This is scraping the surface of camshaft design theory. If you want more explanation, let me know.

It's worth pointing out too that the Marine cam has more aggressive lobes than stock too. IIRC advertised duration for stock was higher than marine, but marine at .050" is clearly more. It packs all that under a smaller curve, but my valvetrain is quiet inside the truck even with Comp Pro Magnum rockers. You can kinda hear it under the hood, but it's not as loud as my dually.

 

[rangrayy]

I’m looking for the best cam possible without machine work on the heads. Although if I’m purchasing a crate motor it probably wouldn’t be too much more to pull the heads and have them machined, gasket matched. Or just buy a short block, if they still sel them, and buy better heads but that’s getting into more money than I’m ready to throw down on this pickup currently.

 

[slowburb]

It's worth pointing out too that the Marine cam has more aggressive lobes than stock too. IIRC advertised duration for stock was higher than marine, but marine at .050" is clearly more. It packs all that under a smaller curve, but my valvetrain is quiet inside the truck even with Comp Pro Magnum rockers. You can kinda hear it under the hood, but it's not as loud as my dually.

Yes. Mine's quiet too, with Scorpion 1.6:1 roller rockers.

I’m looking for the best cam possible without machine work on the heads. Although if I’m purchasing a crate motor it probably wouldn’t be too much more to pull the heads and have them machined, gasket matched. Or just buy a short block, if they still sel them, and buy better heads but that’s getting into more money than I’m ready to throw down on this pickup currently.

You can put more cam than you'd want in there without machine work. Gasket matching won't do anything for you.

 

[rangrayy]

You can put more cam than you'd want in there without machine work. Gasket matching won't do anything for you.

Good to know gasket matching would be a waste. What do you suggest the biggest cam I could get away with before machine work is required. Given 1.5 roller rockers and new springs.

 

[slowburb]

You don't understand. There are trick spring and retainer kits available that allow for well over .500" valve lift...some to as much as .550" valve lift. For what you are doing there would be no reason to alter the heads with machine work.

 

[rangrayy]

You don't understand. There are trick spring and retainer kits available that allow for well over .500" valve lift...some to as much as .550" valve lift. For what you are doing there would be no reason to alter the heads with machine work.

You’re right, I don’t understand. I know I don’t...hence the meaning behind the thread and asking questions. It was not a sarcastic response by saying it would be a waste; rather a genuinely good to know response.

Now...
time to rearch those kits and cams to match accordingly.

 

[slowburb]

Generally speaking, concerning aftermarket/performance camshafts, lift increases as duration increases. In other words, the "bigger" the cam, the bigger the lift and duration values will be. The biggest cams are for all out race applications. You would be approaching the territory of all out race grinds if you were looking at cams that exceed .550" of valve lift with 1.5:1 ratio rockers. Also consider that stock vortec heads can't handle more airflow than that which would be allowed by around .500" of valve lift.

 

[rangrayy]

Generally speaking, concerning aftermarket/performance camshafts, lift increases as duration increases. In other words, the "bigger" the cam, the bigger the lift and duration values will be. The biggest cams are for all out race applications. You would be approaching the territory of all out race grinds if you were looking at cams that exceed .550" of valve lift with 1.5:1 ratio rockers. Also consider that stock vortec heads can't handle more airflow than that which would be allowed by around .500" of valve lift.

Similar to the swirl port heads I have on my 87 350. They drop off around 4,500rpm. Not sure what lift that is but seen countless reports on the topic. Sticking to lift below .500” is key, at or below at least. Thank you for the information.

 

[slowburb]

Similar to the swirl port heads I have on my 87 350. They drop off around 4,500rpm. Not sure what lift that is but seen countless reports on the topic. Sticking to lift below .500” is key, at or below at least. Thank you for the information.

Aside from the obvious similarites (metallurgy, bolt patterns, etc.) there is nothing similar between swirlports and vortecs.