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I'm getting ready to build a 'stock' class motor for one of my trucks (mud truck). What they use to determine 'stock' or 'modified' is the engine vacuum and at what RPM it is drawn at and the type of fuel it runs on.
Basically I want to get 400hp (or more) out of a 350 while still drawing 15 inches of vacuum at or below 1000 RPM on pump gas (91). I know you can get 400hp from a 350 just from looking at it right, the vacuum requirement is the only thing i'm not sure about.
Justin
Your cam choice has a lot to do with your vacuum. The more overlap, longer duration, the lower the vacuum tends to be.
Try to pick a cam with high lift, shorter duration.
Couldn't I also do one with a relatively high duration and wide lobe centers to keep overlap to a minimum. It'd make more power, but would limit the RPM range (which doesn't really matter that much, in fact, i'd like to make the power as low as possible), but don't know how it would affect vacuum. I've enver built a motor with vacuum as the primary requirement, so this is a new direction for me. A hit in power is acceptable to bring vacuum up.
I was thinking somewhere around .500 at the valve, 220/230 .050 duration, and a 112ish LSA. Run 9.75:1, single plane manifold (would a dual bring up the vac signal?), 750 CFM vacuum secondary 4bbl.
And this is where my background as a Ford guy shows through:
Heads are my really big stumbling block right now. What stock heads can support 400 in a useable RPM range? I'd really like to be able to get by without going aftermarket.
Justin
There is only one way to get that kind of idle and that kind of hp and thats a roller cam. Comp's Xtreme Energy series are the next best thing in a flat tappet. The faster you can lift the valves off the seat, the higher your idle vacuum will be, but flat tappet cams are limited on lift rate. You'll have to use a dual plane manifold to get any reasonable amount of vacuum. GM's Vortec heads can support 400hp in stock or near stock form. You may have to change out sprins in order to accomodate the lift you want. GM sells the heads with the cut valve guides and improved springs, too.
Do the rules allow variable duration hydraulic lifters such as the Rhoads units?
Anything goes so long as the motor runs on pump gas and draws 15 inches under 1000 RPM and is NA.
So Vortechs will support 400? I know that they have a lift limit at 500 without machining pockets for the valves, but that wouldn't be that big of a deal. Something I'll check is price, because a set of new edelbrock heads is about $1,000 and I know they support over 400 and don't need pockets cut.
There is also nothing that says I can't use a roller cam, so it's definitely an option. I want to stay away from a mechanical cam if at all possible, but if i can find a hydraulic roller that specs the way I need it to, it'd be great. I have nothing against adjusting valves once a week, I'd just like to avoid it if at all possible.
Keep it coming, this is the kind of stuff I'm looking for.
Justin
EDIT: What about aluminum LT1 heads? Would they work or are they setup for reverse flow? I can get a pretty good deal on a set, but don't know how good they are for my application.
i run a 350 sbc with comp cams XE268 224/230 duration .477/.480 lift. roller rockers. dart platinum 200cc 64 cb heads. approx. 10.5:1 compression. dual plane edelbrock AIRGAP rpm manifold. i get 15'' vaccum @ 950-975 rpm. it's 110 LSA.