i am porting a set of 113 vette heads for my 305 and was wondering if i should have a 3 angle valve job done, in addition to the porting, how much flow gain is there really with the seats cut at 3 angles?

 

There is no flow "gain". However there would be flow reduction if you don't do it, especially at low valve lift (the part of the time that the valves are less than .100" open).

The reason there's no "gain" is because that's the factory valve job. Anything less is a downgrade from stock. The only reason not to do it is if this is a truck motor or something, and you have to put it back on the road and don't care whether it runs good or not, so long as it runs soon for cheap. If you're even the least bit concerend about power, get it done right.

 

Yep 3 angle is the standard nowadays. Just a little fyi. Testing on indy cars have found that there are gains to be had all the way up to 17 angles. Makes a 5 angle look downright "old".

 

The guy I use for machining says the Serdi 5-angle is BS! They even had one in his shop for a while to test it out. He has lots of experience and does alot of race engine building.

I did not know that they were even able to do a 17 angle job.

Oh..btw...what are vette 113 heads? They fit a 305? Any specs?

 

There are cutters for any shape seat you can imagine. Some look like one continous curve. Its real tough to see where the actual seat is.

 

I have not heard anybody refer to the angle your final seal is going to be cut at. Truth be known that a 30 degree seal will produce more flow at low lift than a 45 degree seal.

 

The actual amount of difference it makes will depend heavily on the rest of the setup.

If you put in valves with undercut stems and backcut margins, you'll find that anything you do to clean up the seats will pay off. If you put in stock truck valves or something, like what comes in Vortec heads, with a 1/8" wide face and a sharp corner on the margin, it won't make near as much difference.

It's just one more little detail that's basically free these days, with the popularity of the multi-angle seat cutters that you can buy. One step, and you're done, instead of having to grind each of the parts 3 times with 3 precision stones. But IMHO even if you have to pay a couple of coins extra to get it done the old-fashioned way, it's worth it.

 

Good point about the reduced margins. Another thing to consider is HOW the margin is reduced. A radius from the face of the valve to the margin can also help with low-lift flows. It reportedly seems to make a more significant difference on the intake side than on the exhaust valves.

Just a general rule of thumb: Any sharp angle or corner is going to create localized turbulent flow and therefore restrict overall flow. Air molecules getting in their own way and in the path of other molecules aren't a good thing.

 

well i guess i am just all confused. these heads apparently were redone 10,000 miles ago. the valve seats look aweosme right now and i didnt hit them when i ported the heads. i can see a really narrow perfect place on the on the valve seat where the valve was sealing. the intakes all look perfect and the exhasut valve seats look good too. they look so good in fact that my dad said i could prolly lap them in by hand and be good to go. i didnt know a 3 angle cut on the valve seat was stock