Hey guys, I'm not all that familiar with valves and headwork yet. I'm rebuilding a set of 081 305 heads and am planning on getting 1.94/1.50 Race flo valves. First, I know these valves say they're undercut, but is that the same as back-cut? I've read a few books from Vizard and wanted to get the 30* back-cuts he recommends. I only know of one reputable machine shop and I asked them to do back-cuts and they said they don't get into that stuff. Do the seats need to be cut differently for these Manley valves? Help straighten me out a little, thanks.

Jim

 

No, not the same thing.

Imagine taking a 3-angle valve job, and adding another cut, behind the 3 angles; at a 30° angle. That's a "back cut".

One little detail that makes a VERY MEASUREABLE difference in flow, especially at low lift, is radiusing the "margin" of the valve. Which is, imagine looking at a cross-section of the valve, lengthwise; starting at the stem tip and working your way down toward the head, you'd see the stem, then the undercut (which is a narrowing of the stem right above the head.... less stem volume in the port.... instant larger port), then the head, then the 30° back cut, then the 3 angles of the valve job (42°, 45°, 48°, or the like); then finally the outer edge of the valve would be at 90° where it meets the chamber surface of the head of the valve. Radius that corner to a .030" or so radius, with a very fine grinding stone or something. It can be worth as much as 10 CFM at low lifts (<.100").

Likewise, if the shop can cut your seats in with a Serdi or similar instead of grinding them, you can get a smooth radius transition from the throat of the port, to the angles of the seat. There's another 5-10 CFM or so, at all lifts.

This sort of thing is the difference between a backyard & corner machine shop hack job, and "racing" head prep.

 

Ok, thanks RB, I thought it was just an undercut stem but figured I'd ask. So, would you suggest getting these race flo valves and finding a good hi-po machine shop that can back cut and radius the valves as you described and make sure they cut the seats with a Serdi? Or should I just get all that machine work done on a cheaper set of valves?

edit: Forgot to mention the combo, so far a 305 that's in my sig, with the home ported 081's and a ZZ4 cam with springs pulled from a Fastburn 385.

 

The valves aren't very expensive.

I sure wouldn't pour a bunch of money into some stock crap valves or something like that. If you're going to work something up, start out with something good to begin with; otherwise you have crap with gobs of money tied up in it, instead of good stuff with gobs of money tied up in it. Kind of like why it doesn't make much sense to rebuild a 305 short block if you're looking for max power.

As far as machine shops, find out who build heads for the circle-track guys in your area THAT WIN. That's who you want doing yours. Machine work is one of the purest domains of getting no more than what you pay for. It takes time (lots of hours) by a machinist with experience and the right tools ($$$ per hour). You NEVER EVER want to get your machine work done by the low bidder. "Gee boss I always wanted to hog out a set of them thar Brodix Track 1s...."

 

Cool, I definitely hear you on that. Sounds like a plan then, just have to find a shop like that in the Chicago area, I'm sure there are a decent number of them, just need to find one. Thanks RB