Q&A - March 2014

Kevin McClelland Feb 5, 2014 0 Comment(s)
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Swirl Tune Port

Q I purchased a new 12568758 GM crate engine. What can I do for a street-performance cam and heads? It has a Tune Port injection setup with the computer. It thinks it is a '92 Z/28 Camaro. Should I swap out the heads and cam, or just the cam? Any help would be greatly appreciated.

Rick Cross
Champaign, IL

A The GM crate engine you've listed is a replacement engine for 1987-95 Chevy and GMC trucks with VIN code K. This is the replacement engine up to 7,200 GVW. It's a solid short-block with a cast crank, powdered metal rods, dished hypereutectic pistons, 9.25:1 compression, and a high-volume oil pump. The short is rounded out with a hydraulic flat-tappet camshaft that specs out at a teeny 166/175 degrees duration at 0.050 inch tappet lift, 0.382/0.402 inch max lift and is ground on 112 centers. The best thing about this short-block is that it's machined for production hydraulic roller tappets if you wish to upgrade.

Now, this is where it goes a little downhill. These engines are equipped with "high swirl" cylinder heads. What creates the high swirl is a ramp that is casted into the roof of the exhaust side of the inlet port bowl. Essentially, it blocks off half the bowl, which kills the airflow potential. It gives you serious mixture motion, which boosts slow-speed torque, but kills the top end. These heads, with the really short cam and your TPI injection setup, probably give you enough torque to boil the rear hides! However, when the tach hits around 3,800-4,000 rpm, the fun is all over. Let's pick a couple of parts that will work well with your TPI injection.

First, let's look for some affordable aluminum cylinder heads with 58cc combustion chambers. With the dish pistons, thin head gaskets, and the small chambers, we can get you close to 9.8:1. After searching around we've even found a great choice for your little TPI runner using the TFS Super 23 175 aluminum cylinder heads. These heads were developed for small-bore small-block engines with a 56cc combustion chambers and 175cc inlet runners. This will put your squeeze right at the 10:1 point. These heads have TFS's "Fast As Cast" technology on its intake runners, which give you a cast-ported head. They feature 1.90/1.50-inch valves and come fully dressed with stainless valves, and 1.250-inch valvesprings that accept up to 0.480 inch max lift. They are topped with steel retainers and locks, and screw-in 3/8-inch rocker studs and guideplates. The fully dressed cylinder heads are sold under PN TFS-30310001 and you can pick up a set from Summit Racing. Install these heads using factory GM head gaskets PN 10105117, which come in at a very thin 0.028 inch.

Next, because your crate engine is equipped with iron swirl-port cylinder heads, and you state that you've got a (1992) Camaro TPI system, we're going to assume the center manifold bolts are at the skewed angle of 72 degrees. Only the factory Corvette engines with the aluminum L-98 cylinder heads had the standard 90-degree-to-inlet-surface bolthole angle. You will need to either pick up a Corvette TPI base, or you could go with an Edelbrock High Flo baseplate PN 3861. This base is designed to be used with '85-and-earlier small-block and aftermarket cylinder heads. One benefit of going with the Edelbrock base is that you've taken the largest restriction out of the TPI system by upgrading the base. Then you have the option of going with aftermarket runners in the future for a staged build-up.

Finally, you need a suitable camshaft that stays within the bounds that the TPI system sets, and would give you strong power to the mid-5,000 rpm range and outstand torque. The largest camshaft we would try to run in a TPI application would be the Comp Cams Xtreme 4X4. These camshafts have a very broad torque curve with a decent idle. Check out the grind number X4254H, which specs out at 210/218 degrees duration at 0.050 inch tappet lift, 0.447/0.462 inch max lift, and is ground on 111 separation angle installed at 107 degrees intake centerline. This camshaft will complement the cylinder head flow specs of the Trick Flow Super 23s, and the lift of this camshaft will work perfectly with the 1.250-inch performance valvesprings on the cylinder heads.

These upgrades will completely change the personality of your crate engine. You'll lose some of the very slow speed torque you currently have, but when the tach hits 2,800 rpm, hang on! It's going to hit you with a surge of torque that will keep you in your seat well in to the 5,000 rpm range. Enjoy your upgrades.

Sources: chevroletperformance.com, compcams.com, edelbrock.com, summitracing.com

Keep On Bumping

Q In the November 2013 issue there was a question, "What Was That Bump?" The reader had a van what wouldn't shift out of First gear. I once had an old (about 1970) Jeep Wagoneer with a TH350 transmission that sat in the yard all winter because it would not shift out of First until about 55 mph. Through a friend, I found out that the problem was a stuck kickdown switch located under the accelerator. After spraying some lube and working the plunger in and out a few times, it worked just fine.

Terry Hertz
Hopewell, VA

A. Thanks for the quick tech tip.

Back in the day, before Chrysler purchased Jeep, it would pull from many manufacturers' parts bins to build its vehicles. In an automatic transmission configuration, most Jeeps, especially the fullsize Wagoneer, used the TH400 gearbox. Just to clear things up, the TH350 transmissions have a kickdown cable, which attaches to the carburetor and controls the kickdown valve in the valve body of the transmission. As for your Jeep, it was equipped with a TH400 trans. The 400 uses and electric kickdown solenoid on the valve body to kickdown to a lower gear. GM used switches either in the engine compartment by the carburetor, or at the throttle pedal, to control the electricity to the kickdown solenoid.


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