350 upgrade suggestions

Stock Vortec L31 springs/retainers are lift limited to 0.450", not 0.475". At lifts over 0.450", the bottom of the retainer crashes into the valve seal. Also, the springs are too weak to consider any cam change over bone stock.

The valve guide boss is thick on these heads, preventing use of a performance spring with the stock 1.250" O.D.

There are a couple of fixes. The one I like best is to cut the valve guide down to 0.530" so you can use a good stock-diameter spring with damper and use PC valve seals. Not that there is anything wrong with the stock seals. They work great. It's just that with the thick guide, there isn't enough room for a good spring and damper with the stock spring diameter.

The other fix is to leave the guide and seal alone and cut the spring pockets for a larger diameter valve spring with a higher installed height so that the retainer won't interfere with the stock valve seal.

For cutting the guides, use a CompCams cutter (SummitRacing part # CCA-4726 $39.25) and a CompCams arbor (SummitRacing part # CCA-4732 $12.00)
www.summitracing.com

Finish off with a set of Z28 springs and retainers and 0.530" PC seals from Scoggin-Dickey Chevrolet (part # KITSD1004 $66.98 for the springs/retainers/keepers) and (part # SD1711 $19.20 for 0.530" seals). These springs install at 1.735" with 100 lbs on the seat. Open pressure at 1.210" is 285 lbs. They coil bind at 1.160" and the max theoretical valve lift to be used is 0.525".
www.sdparts.com

The choice of cam will be determined by the static compression ratio of the motor and that will be determined by the stack of parts used, the block deck height, piston deck height, gasket thickness and chamber volume. Please, please do not just reach out there into thin air and choose a cam without going through the numbers and choosing it scientifically. If you don't know how, PLEASE ASK. I or any of the others on this board will be pleased to help you.

Unless you cut the rocker stud bosses and install screw-in studs with guide plates, you will be forced to use rail rockers like those that come stock on the L31 heads. There are aftermarket rail rockers in 1.5 or 1.6 ratios. Scorpion makes a nice roller rail rocker for a reasonable price. If you use a 1.6 rocker, pay attention to the lift on the cam and don't exceed the 0.525" max for the valve springs. Also pay attention to the pushrod clearance where it comes through the head.

An RPM high-rise intake manifold will show 30 to 60 hp improvement over a stock type Performer low-rise intake manifold, but you may not have room for it on your Vette.

I think the 060 castings are the gmpp vortecs? which are rated at .480 lift. Thats what summit told me when I called them, and is what it says in the summit mag. I would go with a 600cfm carb or a 650. A 600 is more than sufficeint for a mild 350. And im not sure if you are planning on putting a different cam in, but if you were I would go with either a voodoo 262 or comps version xe262 (you wont have to change the springs with the xe). Get an eddy rpm manifold for a vortec application, NOT the air gap. I dont see a need for the gap. Since your putting new berrings and such in look at the summits engine kit, comes with main, rod, and cam berrings, timing chain, full gasket set, Keith black pistons which with a 64cc chamber which the vortecs are you get a 9.6 compression ratio from what I remeber, I'm not to sure I havent looked at the numbers in awhile (theres more to the kit also), but the engine kit cost 398.95 with the keith black hyperutectic pistons, you get a choice of cheap cast, or forged. many different options on this kit. Ill try to find the part number for it and give it to you if you are interseted. I too am going through with a 355 vortec build so I'm looking at all this stuff too. I got 062 vortecs for 150 bucks which I deffinetly will have to put different springs in for the cam I running. Im going with voodoos 268 cam. good luck!

12558060 is the part number for new heads at the Chevy dealer. They are lift limited to 0.450" with the stock springs/retainers, but dealers such as Scoggin-Dickey offer upgraded heads with different springs/retainers.

Part number SD8060A2 from S-D will get you a 12558060 head with Z28 springs and retainers installed, same as I outlined above, with a theoretical valve lift limit of 0.525". This upgraded head will be about $70-75 over the base price of the stock head.

Part number SD8060AGP2 from S-D will get you a 12558060 head with Z28 springs/retainers and machined for screw-in studs and guide plates. Studs and plates not included. The advantage of guide plates is that you can run conventional rockers instead of the rail rockers. This upgraded head will be about $160 over the base price of the stock head.

S-D also has Vortecs altered for roller cam applications with theoretical valve lifts of 0.600" and open pressures of 350 lbs.

12558062 is the production casting number for these heads, not a part number. It's the same head.

In the wild, these heads have two different casting numbers. 12558062 and 10239906. The 906 heads supposedly have pressed-in exhaust valve seats. Some heads with the 906 casting number may have gotten through production with induction-hardened seats instead of the press-ins. Although the word has gotten out that the pressed-in seats will flow a little worse than the induction-hardened seats, I've seen as many credible people say that this isn't so. They say both heads flow the same.

For those of you following along at home, here are the flow figures that are generally accepted in the industry as being credible, although there are a few head shops that say they flow less than the published data would suggest. Irregardless, they are jewelry in the cylinder head world in my opinion. These heads are only 170 cc's on the intake runners, yet they flow better numbers than larger volume heads. Just goes to show you, it's the SHAPE of the port, not the SIZE that's important.

Lift Intake Exhaust
0.100" 70 48
0.200" 139 101
0.300" 190 129
0.400" 227 140
0.500" 239 147

I'm pretty sure there are aftermarket companies reproducing their version of "Vortec" cylinder heads. The problem I have with them is that they produce no flow figures that I can find. Now, I know flow figures can be misleading, but we have to rely on something. In my opinion, if I wanted Vortec L31 heads, I'd buy the GM heads with one of the two casting numbers in place, cast in the head.

a 77 350 should have 8.2:1 cr with 76cc heads. Changing from 75 to 64cc heads will raise compression to 9.2:1.

with 9.2:1 cr, an auto trans, and 3.08 gear with a stock converter, then I would run a cam with 260 to 265 on the intake and another 8 degrees on the exhaust if no headers will be used. Otherwise (with headers) use a single duration cam (intake and exhaust the same). Furthermore, use a LSA of 110 to 112.

If you have a Q-jet in good shape then use it. Would not use an edelbrock carb on anything. If you need a new carb then use a Holley.

The rpm intake is better but will not fit under a stock vette hood. Use the performer intake if you are changing intakes.

Vortec heads are good but require vortec rockers, vortec valve covers, and a vortec intake.

If you want to keep the stock look of the engine then get some 180cc iron eagles and use the stock manifolds, carb, valve covers, springs, and rockers. Then install a compcam 260H. iron eagles flow very well on the exhaust so they don't need extra duration on the exhaust side.

The iron eagles heads go for 1000.00. You will end up spend that much on the vortecs plus all the other stuff for the vortec heads.

Look into some better mufflers (dynomax super turbo). I also found that a H-pipe at the rear of the vette exhaust really does a good job to reduce drone in the cabin.

Stock hp is 180hp. You should be able to pick up 75 to 100hp with a good set of heads, a mild cam while using the stock manifolds.

headers and aftermarket intake would be worth another 30 to 40hp on a mild build like this.

finally, use oil additive with your new cam. www.zddplus.com