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Can a Corvette perform even better? You bet it can!
By Joe Mondello
The 1998 Corvette looks fast, sounds fast and is a downright great performance car — but we were asked recently to make one perform even better.
This particular project involved a 1998 Corvette LS-1 350 Chevy engine upper end tune up. In this "Hot Heads" column I’ll be discussing what we did to the engine – and if you pay attention, you’ll pick up some extremely valuable and usually well-guarded performance secrets!
The project involved the fast-burn chamber Vortec aluminum cylinder heads with oversize Manley valves, Comp Cams roller hydraulic profile high lift cam, lifters and valve springs plus Crane aluminum 1.8:1 roller-rocker ratio rocker arms. The headers and a new chip for the EFI completes the package.
We received the heads from the customer and began cleaning them up. We flow-tested the heads (after cleaning) in stock form before any machining was done so we could know the before and after results.
Then we started to do our performance porting, reshaping and polishing of the heads. After cleaning the heads, we installed K-Line bronze guide liners and machined the valve bowls and seats with our Serdi 3.1 cylinder head machining center to accept the 2.02˝ Manley Pro Flo Intake Valves and 1.575˝ Manley exhaust valves.
After the machine work was done, we started to do our clean up, reshaping and porting the intake and exhaust ports, not exceeding stock sizes. I do not believe in gasket matching because port matching gives you better airflow. If the ports are enlarged too much — especially the intakes — a funnel effect takes place, causing turbulence, which decreases velocity and maximum air flow. There are large gains to be made just by blending, shaping and smoothing the air flow into the valve guide and bowl area. Of course, tear-dropping the valve guide boss is very important as well. In most cases, the valve bowl should be blended up to the bottom of the seat insert for both intake and exhaust.
If your flow numbers are a little low, I suggest using a 7/16˝- or a 1/2˝-round ball carbide cutter to cut a .030˝ to .040˝ deep groove about .060˝ to .080˝ wide just below the intake valve seat insert. This includes partially cutting into the insert 180° around the seat just opposite the short side radius. This allows the air to accelerate over the seat like a ski jumper, increasing velocity and CFM of air flow. This is a well-kept secret among good head porters.
The exhaust port needs very little work because its stock shape and size are really well designed. Most people grind way too much material out of the intake and exhaust ports and ruin the flow and velocity. Remember: the shape is the most important thing, not the size. The tear-dropping of the exhaust valve guide is very important.
If the exhaust valve seat insert is smaller than the valve bowl, it is not necessary to grind and match the seat area to blend it into the bowl. In most cases, this actually hurts velocity and reduces pressure in the exhaust port. The blending, rolling and smoothing of the short side radius on both intake and exhaust valve bowls is critical.
If you remove too much on the Vortec head you will ruin it, so leave as much short-turn in the head as possible. If properly shaped, this will always give you better low- and mid-lift flow numbers, which develop great power and torque.
Do not remove any material from the floor of the intake and exhaust ports, just blend and smooth them into the short side radius. The raising of the intake and exhaust roof – when you can – will always increase airflow numbers. However, this cannot always be done because of intake and exhaust alignment and sealing, so check this very carefully.
The use of too large of an exhaust valve is not needed and usually decreases airflow and power. Remember, velocity and pressure are more important than CFM of air flow on the exhaust side for maximum power and torque.
On EFI heads, I like to finish the intake port surface with an 80- to 100-grit finish and the exhaust with 120- to 150-grit. This helps to slow down carbon build up.
The only thing left now is to shape and finish the fast burn combustion chamber shape. I only blend the sharp edges of the floor of the chamber and blend it into the side and back wall just below the spark plug and fast burn bump. I do this with a 60- or 80-grit 1-1/2˝ disc with my 2010 porting and polishing lubricant. I then polish the sidewalls of the chamber with an 80-grit cartridge roll.
I leave the area just below the deck surface and spark area about 1/2˝ to 3/4˝ rough as cast for better fuel and air swirl. This allows better total flame travel and burn in the cylinder. I like the chambers a little rougher than highly polished except for long distance and high mileage street or race cars.
In addition, the area just below the rocker arm stud area in the roof of the intake port should be filled with a good quality epoxy. I offer such an epoxy, called Splash Zone A-788, from my tech school. It won’t come loose and fall out, nor does fuel bother it.
I offer everything needed in my porting & blueprinting catalogue to port, polish and fully prepare these heads. Catalogues are free for the asking.
I hope this will help you gain more power in your LS-1 or vortec cylinder head and engine modifications.
Parts used for Project
Camshaft: Comp Cam "Extreme Energy" hydraulic roller camshaft, p/n 54-412-11;
Intake Lift: .522˝;
Exhaust lift: .529˝;
Intake Duration at .050˝ lift: 212°;
Exhaust lift: 218°;
Lobe separation: 114°;
Lifters: High Energy Hydraulic Roller Tappets, p/n 875-16;
Stock size high-energy valve springs: p/n 26915 (works with stock OE valve spring retainers and valve locks);
Valves: Manley Race Master valves with hard chrome stems and hard tips, swirl polished for maximum air flow; both valves use OE valve locks.
Intake: p/n 11362-8; 2.020˝; .3133˝ stem; 10° angle; 5/16˝ radius; NK-842 stainless material Pro-Flo design;
Exhaust valve: p/n 11363-8; 1.575˝; .3136˝ stem 20° angle; 7/16˝ radius; XH-426 stainless material, standard stem with no undercut.