GM 350 Crate Engine Build II - The Goodwrench Quest, Part II

Pocket-Ported Heads And A Bigger Cam Pump 335 Hp Into Our Budget 350

Jeff Smith Oct 1, 1999 0 Comment(s)

Step By Step

Fel-Pro makes a great rubber-embossed steel shim gasket that is only 0.015-inch thick, which increases the compression on engines like this one. This is a great gasket that seals well and also improves power by reducing the quench area above the piston.

We cc’d the heads before and after to determine the actual combustion chamber volume to work out the static compression ratio. Milling the heads only reduced the chamber volume by 3 cc.

As the graph reveals, the combination of a pocket-ported head and the camshaft increased torque and horsepower dramatically. The short torque curve in the upper left indicates what the low speed torque should have been had we repaired the Q-jet’s fuel curve problem. The dip in the torque curve is purely a fuel curve situation.

Our quest for more power started with the cylinder heads. Todd McKenzie did some minimal pocket-port work (right side) on the stock iron castings and improved the port flow, especially on the exhaust side. This is easy work that you can do at home in roughly eight hours. We retained the stock 1.94/1.50-inch valves just to see how much we could improve without spending money on bigger valves. A stock intake port is on the left.

If you choose to do your own home porting, the best place to start is with a Standard Abrasives Deluxe Engine Porting Kit. This provides everything you need, except for the die grinder. If you want to speed the process, a carbide cutter would also be a good investment.

McKenzie did back cut the exhaust valves with a 30-degree angle (right) just inside the 45-degree seat. This dramatically improves port flow at low lift numbers, helping scavenge the cylinder when the valve first opens.

The hot ticket is to purchase the Comp Cams K-kit when you buy the cam. The kit comes with the cam, lifters, retainers, keepers, springs, Magnum rocker arms, and a timing set.

The factory spring tested at 75 pounds of tension at the installed height of 1.70 inches. The Comp Cams spring tested at a much better 110 pounds on the seat at the same installed height.

Budget horsepower is the name of this game. If you followed the exploits of our GM Goodwrench budget 350 provided by Scoggin-Dickey Performance Center from last installment, then you know that we pumped a few steroids into the stock OEM combination in the form of a set of headers and an Edelbrock Performer intake. In this configuration through mufflers, we made a respectable 265 hp and an even stronger 350 lb-ft of torque. While that’s not bad for a couple of simple bolt-ons, we’re not nearly satisfied. We know this pedestrian 350 is capable of much more power.

The simplest path to increased power is to methodically remove the restrictions that prevent the engine from breathing more deeply. We started that process last time with the headers and intake. Now the cork in the system was the cylinder heads. While we could just bolt on a set of better heads (which we will do next time), we thought we’d first try improving the existing cylinder heads. This was also the cheapest way to go.

Our first step was to remove the cylinder heads and do a little measuring. Chevy claims the engine squeezes a mean 8.1:1 compression, but somehow, in a fog of mathematical befuddlement, we claimed last time that the Goodwrench motor measured 8.4:1 compression. We were wrong. After we rechecked our ciphering, Mr. Goodwrench proved to be much lazier. The actual compression figures out to be a menial 7.8:1! Based on this complacent squeeze factor, we decided that increasing compression and improving airflow would be a good idea.

Flow To Go

As you might imagine, stock 1.94/1.50-inch castings are weak when it comes to air flow. In the name of budgetary restraint, we decided to stick with the stock valves so we could evaluate the effect of the porting. McKenzie Cylinder Heads in Oxnard, California, performed the cylinder head porting for this story, but per our instructions, Todd McKenzie spent very little time on these heads. Better yet, you can duplicate this work yourself with little more than a porting kit from Standard Abrasives, a small air compressor, and a hardware store ¼-inch die grinder. The plan was to perform a simple pocket port job that would generate the most results for the least effort.

If this is your first attempt at cylinder head porting, you may want to practice first on a junk cylinder head. While the work looks intimidating, it really is easy to do. In fact, once you get comfortable, the hardest part is to stop working after blending the seat area 1 inch past the bottom of the valve seat. The key is to stay conservative and not hog out the port in search of more power.

Another area worth investigating is back-cutting the valves. Flow bench testing has proved that placing a 30-degree back cut on both intake and exhaust valves improves the low- and mid-lift flow of both the intake and exhaust ports. This back cut is placed just inside the 45-degree face of the valve and acts much like the three-angle radius of a valve seat to improve flow. The 30-degree back cut can be accomplished by any competent machine shop and will improve cylinder head airflow even if pocket porting is not included. We have found that back cutting both the intake and exhaust works, but McKenzie decided to back cut only the exhaust side for this test.

Before the heads were reassembled, McKenzie also treated them to a simple milling operation that reduced the chamber volume from 76 cc to 73 cc. The effort was aimed more at ensuring that the deck surface was flat than at attempting to dramatically increase compression. However, we also decided to eliminate the stock GM composition gasket that measured a stout 0.039-inch thick in favor of a much thinner 0.015-inch-thick gasket from Fel-Pro. After measuring everything again and doing accurate ciphering, we came up with a slightly better 8.4:1 compression. While this is hardly killer squeeze, we figured every little bit helps.

Port Testing

While all the theory supports these efforts, the only way to know for sure is to test these stock heads before and after and report on the results. Typically, pocket porting is worth around 20 hp, with most of the power gains seen above 4,000 rpm. While we did improve the torque, as usual the bulk of the improvements occurred above the 4,000 rpm mark. Keep in mind that all we did for this first test was yank the heads off, do the pocket porting, and stuff them back on the Goodwrench grist mill. From last month’s best numbers of 265 hp at 4,400 and 350 lb-ft of torque at 3,600, the combination of the pocket-porting work and the compression ratio increase pushed our Goodwrench Quest 350 to a solid increase of 26 hp at 5,300 rpm. The cork was now the miniscule camshaft. It was time for a hotter cam.

The stock cam was downright wimpy, (see the Cam Specs Chart) and now that we had a set of cylinder heads that were up to the task, it was clearly time to take advantage of the heads with more duration and lift. There’s always a tradeoff when swapping camshafts. The stock cam produced a smooth idle but didn’t make great power. The Comp Cams Xtreme Energy 268H cam we chose bumped the duration at 0.050 inch by a massive 30 degrees on the intake side and 28 degrees on the exhaust. Even more amazing is the lift comparison, where the intake valves now jump 0.094 inch higher and the exhaust gains 0.079 inch! After we installed the cam, we turned Duttweiler loose on the dyno and pushed the Goodwrench 350 to a thumpin’ 336 hp at 5,300 rpm, with peak torque jumping up to a killer 377 lb-ft at 3,800 rpm. These are excellent numbers for a basic 350 with a few simple modifications.

Conclusion

Are we satisfied with the power? Hardly. We’ve pumped it up from a measly 236 hp to a righteous 336—a massive 100 hp over the bone-stock baseline. Next time, we’re all set to try a set of Corvette L98 aluminum heads from GM Performance Parts, and then we’re going to pocket port them to see what they can do. After that, we’ve got a set of those trick iron Vortec heads to try. And then maybe a bigger camshaft and then…well, even we haven’t gotten that far yet.

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