Chevy/GM Cylinder Heads - Breathing On A Budget

Making Gm's Vortec Heads Work Better For Less

Mike Petralia Sep 1, 2002 0 Comment(s)
Sucp_0209_01_z Chevy_gm_cylinder_heads Polish 1/17

Today's technological advancements are great. We've got products rolling off assembly lines that can make more power and will last longer than even the most highly modified parts could a few years ago. With advancements in technology there may also come a reduction in cost. Cylinder heads have benefited tremendously from today's advancements, but there's still something that can be gained from a set of stock heads in the hands of an experienced head porter. Stock, in this case, means GM's iron Vortec small-block heads, and by experienced head porter, we mean Joe Mondello.

For those of you who might be unfamiliar with Mondello's work with Chevrolet, he was porting small-block heads long before he became famous for his Oldsmobile stuff. Back in the '60s Mondello worked closely with GM engineers on refining the cast-iron heads of the day, and many of his modifications were incorporated into the factory castings that made more power. Even the heart-shaped chamber of the Vortec heads was something Mondello worked on over 30 years ago, so we thought it'd be best if we asked him to show us how to make a set of Vortecs flow.

Why'd we choose the Vortec heads? Cost is the reason. A complete set of iron Vortecs (GMPP PN 12558060) will cost you just $477.50 plus freight from Scoggin-Dickey Chevrolet. These out-of-the-box castings are only good for cams with about 0.450 inch lift, which is okay for mild everyday street cars, but if you want something that can take more camshaft and breathe better, Scoggin-Dickey also sells modified sets of Vortec heads for a little more cash.

Sucp_0209_02_z Chevy_gm_cylinder_heads Valve_job 2/17

The stock valve job can be improved on Vortec heads. Mondello got more cfm at high lift after performing his own 35/45/60-degree valve job and 75-degree throat cut, but this sacrificed some flow at lower lift (compare: Intake tests 1 and 3).

Experience Pays Off
As we mentioned before, Mondello's experience in porting Chevy heads has allowed him time to fine-tune the process. It's also given him the opportunity to test practically every modification in the book and determine what works on the Vortec heads. Of course, it also helps to have a flow bench handy when you're trying to find out which porting modifications work best. But, since most of us don't have one, we asked Mondello to show us all the details of what he thought would be the best porting modifications for Vortec heads, street or strip.

He took a set of Vortec right out of the box and clamped them down to his Superflow bench. With 230-cfm max flow, these heads are actually pretty good, except when you notice that peak flow is at .700 inch valve lift, and no street cams are that big. What Mondello tried to do was mainly concentrated around gaining more flow at lower lifts. And when you compare the flow figures below .500 inch lift from each test, you'll see that some things worked, while others didn't. That's the truth about porting.

Not all modifications work on all heads. And there's really no way to tell ahead of time if the mods you plan to make are going to help or hurt. Pay close attention to the test notes listed in the charts to see what mods Mondello tried for each step. Then, take a look at the photos to see details of some of the modifications he performed. Basically, if you compare tests 4 and 1, you'll see the first definite improvement at almost all lifts-after Mondello performed a valve job and then blended the bowls into the 75-degree throat cut above the seat.

Then, for later tests Mondello went all out and performed what would equate to about an $800 porting job on the heads to see how they would flow. Tests 8/4 and 10 reflect those results. It's plain to see that full porting has some advantages. But for the cost, you'd be better off buying a set of aluminum heads that'll flow those figures out of the box. All in all, for any stock small-block head to flow 262 cfm is outstanding, especially for a head that started out costing under $250 each!

INTAKE PORT TESTS (CFM @ 28 inches)
Intake Lift Test 1 Test 2 Test 3 Test 4 Test 5/1 Test 6/2 Test 7/3 Test 8/4 Test 9 Test 10
0.100 61 62 60 63 62 61 63 62 61 58
0.200 127 130 124 132 129 131 131 133 131 125
0.300 185 188 180 187 182 182 183 174 180 177
0.400 224 225 217 223 215 212 219 214 212 215
0.500 224 221 242 225 238 237 242 244 237 243
0.600 228 219 230 229 230 234 238 260 238 262
0.700 230 221 234 232 231 228 241 255 230 256
*The second digit in tests 5/1, 6/2, 7/3, & 8/4 refers to actual testing on the chambers we numbered 1 thru 4.

Sucp_0209_09_z Chevy_gm_cylinder_heads Hole_deburring 9/17

Be sure to deburr the area around the spark plug hole after polishing.

Intake Testing Notes
Test 1: Stock outboard Vortec port as shipped (i.e. cylinder No. 1, 2, 7, or 8).
Test 2: Stock inboard Vortec port as shipped (i.e. cylinder No. 2, 4, 5, or 6).
Test 3: Performed 35/45/60-degree valve job and 75-degree throat cut on outboard port.
Test 4: Same as #3, added back-cut valve, outboard port.
Test 5/1: Same as #4 with hand-blended throat area above valve seat, outboard port.
Test 6/2: Same as #5/1 with fully blended valve bowl up to guide boss on inboard port.
Test 7/3: Inboard port with same porting as #6/2 but also blended intake port opening.
Test 8/4: Outboard port with same porting as #6/2, but with reshaped valve guide boss, totally smoothed port walls, and filled rocker stud hole with clay.
Test 9: Same as #6/2 with just rocker arm stud hole filled on inboard port.
Test 10: Same as #8/4, but using Manley valve.

Exhaust Testing Notes
Test 1: Stock outboard (i.e. cylinder No. 1, 2, 7, or 8) Vortec port as shipped.
Test 2: Stock inboard (i.e. cylinder No. 2, 4, 5, or 6) Vortec port as shipped.
Test 3: Performed valve job with radiused cutter and throat cut, outboard port.
Test 4: Same as #3, added back-cut valve, outboard port.
Test 5: Performed valve job, but no throat cut or back cut on stock valve, inboard port.
Test 6: Performed valve job, but no throat cut, added back cut valve, outboard port.
Test 7: Same as #6 but without back cut, outboard port.Test 8: Same as #6, inboard port.
Test 9: Fully ported with back cut valve, outboard port.
Test 10: Same as #9 with Manley valve.

Sucp_0209_10_z Chevy_gm_cylinder_heads Exhaust_port_polish 10/17

Once you've got the intake ports done, you can clean up and polish the exhaust. These ports are short and easy to work on, but take your time and don't enlarge them. Just straighten them out and clean them up.

Explaining The Valve Job
Discussing valve jobs can be confusing even when you're talking to a pro. Most of the confusion revolves around the words "above" and "below" when used in relation to the valve. The problem is that when the heads are on the motor, the valve cuts, throat cuts, and valve pocket are all "above" the valve. But on the valve seat machine they're "below" the valve because the head sits upside down. For accurate reference here, we'll refer to everything as if it was on the engine. So the pockets, valve guide boss, valve seat, and throat cuts are all "above" the valve.

The typical three-angle valve job is what we're all used to hearing about. In actuality, only one angle, the 45-degree cut, really contacts the valve, and the rest are there to smooth airflow around it. These additional cuts are especially helpful at lower lifts. There are also what are called "back cuts" on the valves that help improve flow, but they don't come in direct contact with the cylinder head.

The first angle, which is actually exposed to the chamber and therefore the only one that's actually "below" the valve, is a 35-37-degree radius cut that smoothes the transition from the rough chamber wall to the valve seat. Next is the 45-degree valve seat. This is the area that creates the seal and is also responsible for taking heat away from the valves; this cut is usually wider on the exhaust to absorb more heat. The next cut is a 60-degree angle that smoothes the transition from the rough pocket wall to the 45-degree valve seat. Finally, there's the 75-degree throat cut, which is not always performed but can help airflow if you're won't be hand-blending the bowl area above the seat. This angle offers the smoothest transition for the air entering the bowl.

Sources

Standard Abrasives
Simi Valley, CA 93063
805-520-5800
Mondello Technical School
Paso Robles, CA 93446
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