Upon removing the already good flowing LT4 heads and intake manifold from my 1996 Corvette, I attempted to port them myself. I quickly concluded I could do more harm than good. So I called two reputable head-porting shops in Texas. Both recommended pocket porting since I was not planning any other modifications. I visited Trevor Johnson Cylinder Heads in Rockwall near Dallas, Texas. Trevor Johnson showed me some of his work while I outlined my objective which was improving low valve lift airflow while retaining the original LT4 camshaft and exhaust system. Since stock LT4 heads have relatively big intake ports, there is no reason to make them any bigger as that would hurt port velocity thus impact low RPM torque. He agreed to perform the work for a reasonable price with a quick one-week turn around. My heads received what is sometimes called a "Stage I" porting. This included pocket ported, combustion chambers polished and CC’d, intake port gasket matching to the new GM 12367777 LT4 gasket, head milled 0.016 inch for an exact 10.8:1 compression ratio, new valve stem seals, and results verified on the flow bench. The stock valves and springs were retained.
I decided not to spend the extra money to machine down the valve guide bosses or polish the entire ports. This may yielded a few more CFM at high valve lift, but would have little or no impact with the stock cam. It would also have blown my modest budget. I gasket matched the intake manifold myself using a Dremel tool with bits purchased at the local home improvement store.
Prior to the base Dyno run on July 7, I did the throttle body coolant by-pass and installed a K&N air filter. This almost stock LT4 produced 300 rear wheel horse power and 307 ft/lbs of torque. Rear wheel horse power increased by 22 and peak torque increased 15 ft lbs after the port work. However, torque over 5500 RPM increase about 20 ft/lbs. The engine idled exactly as before with no change in drivability. However, the power now stays strong until fuel cut-off at 6400 rpm. I now am planning to increase the fuel shut off to 6600 rpm.
This is a cut-away photo of the stock LT4 intake port. Note the path is not straight at the intake manifold/head area and the port narrows just before the valve seat.
Here is the stock combustion chamber. Look carefully at the area around the valve seats and rough finish. The modified combustion chamber relieved the area around the valve seats to promote better airflow at low valve lift.
Here is the finished intake manifold. The hole in the middle is for the fuel injector. Sorry but I did not snap any before photos. The gasket matching to the GM #12367777 removed .25 to .375 inch from the port roof and almost the same amount of material on the sides. The port floor was smoothed and radiused going back about 2 inches. The camera flash makes the port look rougher than it really is.
Here is the finished combustion chamber. Note the smooth finish and rounded corners. Look closely at the area around the valves and see how it was opened up to help increase flow at low valve lift. Before, this area was flat to the valve seat.
The Flow Bench showed a 20 CFM increase on the intake port and 10 CFM increase on the exhaust port in the important .200-.400 inch valve lifts region. This is exactly what I was looking to achieve.
Here is the finished exhaust port. Note the grinding marks way in the back to increase the diameter of the port just after the valve. I decided not to machine the valve guide bosses for budget reasons and since I planned to use the stock cam, it would not have made much difference. A 30 degree back cut was added on both intake and exhaust valves. From my research, this has a significant impact on low valve lift flow which was confirmed on the flow bench.
This is the finished intake port. Sorry for the poor depth of field. Note the grinding in the bowl area (back) and the gasket matching done for the intake manifold. No fancy port polish or valve guide machine work due to budget reasons. The short side radius received a lot of attention to remove sharp edges and smooth flow over the stock heads.
This is my LT4 intake manifold bottom with the heat shield removed. Note "LT4" casting located in the center of the photo. The blue around the port is the outline of new intake gasket (GM #12367777) prior to port matching.