350 vortec cam question
 

Roller mercury cam 811658 which is 430/450 and 197/207 @.50 109 lobe separation. I was thinking about installing 1.6 roller tip rockers should add about 28-30 on the lift. Won’t change duration. sound like an easy no brainer since the stock vortec heads max out 480-500. Adding air gap rpm intake, and a Holley 650 double pumper Has anyone done this? Only thing I was curious about is does mercury use a stock spring, or are they little stiffer spring?

On the rockers. Yup. Nada. Zilch. Zero.
And forget you heard about roller tip rockers. Many oiling problems from them. Ie: burnt up rockers. Have not seen oiling issues with roller rockers.

Have to go screw in studs for roller rockers.

I did a similar build but installed new Vortec heads, but Performer intake because I wanted to keep the Quadrajunk carb. I used a flat tappet Comp 212 / 218 if I recall correctly. I did not want to machine the valve guide bosses or use the expensive beehive springs so I just purchased the old style Z28 valve springs (like those used on early Z28's) and they are around 115 lbs on the seat. I did not use the inner springs, just the outer springs so they would fit fine. I had no issues in 10 years but it would only turn about 5100 rpm at the most. I had 1.5 roller rockers, you have to drill out the pushrod holes for 1.6 roller rockers so they do not scrape the head.

You have to cut the valve guides down to run .500. Also the press in studs have to be removed and the holes tapped for screw in studs.

Why I went with RHS Vortecs when they were available. Better flow than Vortecs and all setup for high lift.

I used Magnum 1.6 roleler tips. No issues...did a lot of WOT runs at 5500. About 55mph.

Vacuum Secondaries are great for boats. If you are running a double plenum intake...you'll flow better with a larger carb. Each bank only sees one primary and one secondary. The cut out on the Air Gap between plenums is a band-aid. Poor flow results from cross plenum air feed.

My perspective, is most marine engine use is at cruise RPM. Where we want efficiency as a focus. So an air gap style will remove some natural intake charge heating for better atomization...and efficiency.

David Vizard suggests that an 850 or even a 950 vac sec is the best option for a dual plenum 383. You may get away with a 780 3310 style on a 350.
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This is a bit of a revelation...as the only reason the dual plenum cut out helps out...is because it's under carbed.
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Makes sense...a Single Plenum pulls from all 4 tubes on a carb...so would need less carb CFM.

When measured with a Rimac ,350 scorpion heads 305 -260hp heads and vortec truck heads run the same springs , spring pressure