I've spoken with two machinists who have told me that enlarging the exh. valves in cast iron heads is a no-no because it removes the flame hardening of the seat and a pressed in seat can't be used because of the risk of cutting into the water jacket. But I see a lot of ads for the smog type heads offering the big valve option and I thought those were only available from the factory as 1.94/1.50 valve size. So that means somebody is doing something they shouldn't be doing, or someone is incorrect.
What is the truth?

 

Think you need a new machinist. Exactly which head casting are we talking about?

 

about any chevy head can be enlarged to the next size valve.when you do this the valve becomes shrouded and it can hurt flow unless you do some grinding in the bowls.

 

I had a lengthy discussion about this with my machinist on cast iron LT1 heads. He wasn't sure either. We each found out (through separate means) that the indution hardening of the valve seats is way beyond deep enough to support a 1.60" exhaust valve.

If you're really concerned, you can have them flame hardened after machining and before the final grind. F'Bird is right - maybe you should be talking to a different machine shop.

 

I'm not sure what the casting # is 'cause I don't have the heads handy right now but they're just your basic smog heads from the mid '70s.
The machinists that I've spoken to both told me that the hardening of the seats only goes to a depth of .030 and they've had bad luck with installing oversize valves.
Can someone refer me to a shop that's got a good success record with this mod?
Thanks

 

Umm, don't take this as a flame, but unless you want nothing but low RPM torque out of your finished product, you might be a little disappointed with the smogger heads unless you do some serious porting and milling to get the ompression up.

 

If they are mid 70s heads, more than likely they are garbage. Don't spend a dime on them because it will all be a waste. Throw them in the trash where they belong, and help erase the memory of the disgrace of 170 HP 350s from the collective memory of the planet.

For sure, before laying out even one penny, get the casting # and post it. All heads are not created equal, and they cannot all be made equal. The quality of the finished product can vary widely depending on what you start out with. Before you start spending your money trying to turning chicken s*** into chicken salad, be sure you know what you're starting with.

No machine shop has a good record with doing this. Sure, lots of them can stick bigger valves in, and then have a 100% success rate of cashing your check; but as far as making power.... it's a different matter.

Catch me some time when I'm in a worse mood, and maybe I'll tell you how I really feel about that type of heads.

 

I know what you're talking about with those anemic mid '70s 350s, I was there.
I've got to make a trip to my garage to get those casting #s and I won't be able to get back 'til tomorrow with that.
The question that comes to mind is this,
Is there any such thing as a factory 76cc head that is any good?

Back with more info tomorrow, I'm interested in finding out more.
Thanks for the reply.

 

The casting is #993
I became motivated to do some homework over the weekend and I found a TPIS book Insider Hints which contained a series of flow tests including the #993, flow numbers were on par with other stock 1.94/1.50 heads. I realize it's not all about flow, chamber shape has a lot to do with it, and the chambers on my heads look like they were cast around a section of 2x4. But these heads with 76cc chambers give me the CR that I need to work with the short cam that I'll be limited to with a TBI.
Feel free to double check my math,
350 +.060=360ci, flat pistons w/ 5cc valve relief, zero deck, .040 gasket thickness=8cc, 76cc chambers = 9.4/1 CR
I'd like to find a set of cutting edge heads that will work with my combo but it seems like they all give me about 10/1 CR or more.

whattya think now?

 

Aw, c'mon, RB. Tell us how your REALLY feel... don't hold back now.

That's why I mentioned it, too. You just added a lot more "OOMPH" to the statement.

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4X4,

4.060" bore + 3.48" stroke + 0.039" gasket + 5cc piston relief + "0" deck height + 76cc chamber volume = about 9.23:1 static compression ratio. I say "about" since we don't know the top ring depth , which affects the number a little bit. That same combo with 64cc heads would yield about 10.5:1. Your extra bore is going to be an issue if you are trying to stay under 10:1. It IS usually possible to open up the chambers to get more volume if you need it, especially if you want to unshroud the valves around the outer perimeter. That's usually good for about 3cc per side of the chamber with big valves, or 6cc total.

If you started with decent 64cc heads and opened them up to 70cc, that would yield about 9.82:1 stsic. You could open them more around the spark plug boss to get more volume if you really wanted to. Just try to avoid opening the quench area if possible.

Incidentally, I used 0.039" for a compressed head gasket thickness since that is a VERY common gasket.

 

Not that it's any big deal, because when all the nuts & bolts are torqued down and you're ready to turn the key, things is what they is in the real world, and all these calculations are just writing on paper.
Anyhow, I've noticed a lot of discrepencies in CRs and as far as I know there's only one formula.
I went home and measured everything and re calculated, there were some errors in my measurements, the valve reliefs were only 2cc not 5, the piston to head clearance is .035.

So correct me if I'm wrong.

350=717 cc/cyl. ( that's .7854 x 4"sq. x stroke)

350+.060=738.8 cc/cyl

76cc (chamber vol) + 2cc (valve relief) + 7.5cc (.035 piston/ head cl.) = 85.5cc

85.5 + 738.8 = 824.3 / 85.5 = 9.64 to 1

Whattya make of it? Am I making a mistake somewhere?

Any way you slice it I dont think 62cc vortecs would work.

And there's a load of difference between the CR of a 350 with .030 deck clearance and a 360 with zero deck.

Thanks for the reply

 

Ya forgot about the head gasket in your calculation.

If you include the head gasket volume in your calcs you get

8.82:1 with a felpro .04 gasket. The gasket is bigger than the bore size.

Get a compression ratio calc program and move out of the slide rule era.

you can get a good one from www.download.com

Go there and search for engine CR 1.0

Further more if you include the vol of the ring gap below
the piston top you'll find the stated relief vol of 5cc's quite acurate. Vortec heads are infact 64cc's I've cc'd them before.

Your motor with vortec heads would have a cr of 9.65:1
with a felpro .040 gasket. If that is too much use a .051 gasket
and get a 9.40:1 cr.
The fastburn high swirl design of the new vortec heads
makes them more octane tolerant then to older smog heads even with lower cr. The increased power/ torque and efficiency is well worth the effort.

Remember that because of the increased volumetric efficiency
that a vortec head will have over a smog headed TBI motor
your stock TBI computer program's tables may need adjustment.
or you may have to tweek the running fuel pressure to compensate for this @ WOT. But maybe not. Just so you're aware.
The vortec heads do not need quite as much ignition timing either. this also makes them more octane tolerant.
I think you'd be pleasantly suprised in the power improvement.

 

4X4,

This might save you a little calculation time: Engine Calculator . It is an Excel spreadsheet that is written in Quattro Pro 9, so it will open with either. It's about a five-second download.

Teh second page has some common Chevy engine values, but there are a lot of variations in engines and years, so they are not Gospel.

 

I entered most of what you said into this calculator and got 8.82:1 also.

That would be great for ~150 HP shot of juice.

 

That's pretty close. My calculator spit out 8.83:1 with those specs, but I presumed a top ring depth of 0.250". Close enough.

BTW - I think download.com took that down since I complained (I didn't give them authorization). The spreadsheet is faster than the VBA anyway.

 

Still, I'm getting 9.51/1. I didn't forget the gasket thickness, I milled out the deck clearance.
I'm astonished at the effect overboring and zero decking has upon the CR.
All this talk has led me to do some thinking about the actual effect created by stirring the fuel/air mixture around in the combustion chamber, the effect it has on detonation and why vortecs work.

Thanks everyone.