gonna run 187 heads on a 350, probably 1.6rr, and im wanting to run the xfi 268 cam. will this combo work or am i gonna have a lift problem. do i need to machine the heads for clearance? i dont have the money to buy new heads right now.

 

You'll end up with a stellar 2000-4000RPM powerband or so. Low RPM heads, mid-high RPM cam.

And yea, you will also have a lift problem. You need to machine the guide bosses down on the heads.

 

the heads have been pocket ported and polished. how much will i have to machine down?

 

.050" or so. Mock it up and see how much clearance you have, and machine what you need to.

 

let me get this straight so i dont screw things up. i pretty much put the motor together on a stand. rotate the camshaft, and see when the pushrods hit. and from that point, just cut enough area out to stop the pushrods from rubbing. correct?

 

No, that's pushrod slot clearance, and may be an issue but not the biggest one.
There's a few ways to do this, but here's one:

1) With cylinder heads assembled, on a bench, measure between the bottom of the retainer, and the guide boss. If your valve lift is .520", then add .060", so .580". If you only have .530" or so, then you need to grind .050" off the guide boss.

 

im really gonna need to study up on the heads before i start cutting at them. i consider myself a beginner in engine building and learning as i go. because im not 100% on what the guide boss is. is that the area the valve sits in? basically the whole that the valve goes up and down in? if that is true then i think im finally understanding what you are saying.

so, looking at the cam spec card for the xfi 268, valve lift is .570" with 1.6 rr.

if i measure from the bottom of the retainer, to the top of the guide boss and that comes to lets say .520" then i will need to cut .060" off to get me over the .570" mark.

after i do this, this will prevent me on having any lift valvetrain problems?

 

Those heads and that cam is a recipe for disappointment. Being "pocket ported" doesn't mean squat. Those heads won't flow any better at .52" lift than they will at .45" lift.

Either get a milder cam, or get better heads - especially on a 350. Upgraded Vortec heads from sdparts.com would work well with that cam. 1.6:1 rockers are a waste unless the cam was designed for them.

 

I thought the 268 was a flat tappet cam, but .570 lift (even with 1.6'ers) screams roller. Holy cow that's a ton of lift.
I agree with Five7, you won't like that combo.

Yea, the "volcano" where the valve slides in is the guide boss. You need to give that volcano a haircut (slice a bit off the top).

 

sonix thanks for the help. the 268 cam is a roller cam, and on the comp cam website and summit its says to use 1.6 rr with this cam. i already have 1.6rr on my 305 right now. i have a set of 1.5 rr in the attic. as for heads, yeah i know i need to replace them. but right now i dont have the money for a good set of aluminum heads. hopefully when the income tax season rolls around and the check rolls in ill get enough to afford some good aluminum heads like afr heads for the 350. also that is why im looking at the 268 cam is because i know im gonna replace the heads down the road. and also i like the sounds of that cam and from what ive heard people have been getting good results with that cam when the engine is built correctly for that cam. my ultimate goal is i would like to squeeze 300 hp at the wheels. actually i just want to smoke my dads stang. the sad thing is he has beaten ls1 f-bodies with his stang. the only time he lost to an ls1 f-body is when the one was supercharged and pushing 9 lbs of boost and the other one had noz.

 

I think there's a Comp 268H, but this is the XFI268HR-113. Its a roller.