Upgrading Cylinder Heads On A L98 Chevrolet Corvette Motor - Heavy Breathing Deep Pockets

Cylinder Head Improvement Equals More Horsepower For The L98 Corvette

Richard F. Newton Aug 1, 2002 0 Comment(s)
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These are the aluminum heads as they arrived from the salvage yard. Everything was removed and the bare castings were hot tanked. After this cleaning, they were shipped to Extrude Hone. I really paid $300 for a set of bare aluminum castings. You might think of them as a canvas for an oil painting.

This adventure started with a $300 set of used aluminum cylinder heads from a trashed '87 Corvette. Several months later, not to mention about $8,000, I found just over 80 hp, and about 90 lb-ft of torque. In an effort to pass a Porsche 944 Turbo-S, I learned a whole lot about cylinder heads-as well as spending money. Anytime you can spend a pile of money that is roughly equal to the value of your car, you learn a few lessons. You might want to call this Cylinder Head Consumerism 101.

The extent of misinformation about cylinder heads, not to mention power increase, is staggering. Every manufacturer has a set of claims that would lead you to believe that they're the only one that can produce a quality cylinder head. It's virtually impossible to sort the good from the bad, not to mention the ugly. Unable to deal with this situation, I fell back on my old performance rule: Go talk to the guys that go fast. Real racers have to have the best equipment-they get paid to win. Since most of us just play with cars, and don't depend on speed for our income, we're not totally informed. Trust me, people who go fast for a living have a pretty good idea about what really works. They also have a pretty good idea about what it costs.

The cylinder head is the key to improving horsepower and torque in your Corvette. I'm not going to tell you that my final selection is the best combination. I really don't know what the best combination is. It would take several hundred-thousand dollars and months to do the necessary testing. When the NASCAR BUSCH series went to a new set of motor rules last year, most teams spent about a $500,000 on research to find the best combination. We simply don't have that kind of money here at Team VETTE. What we can do is help you wade through a little bit of the cylinder head hype.

Let's talk about why this cylinder head swap was necessary. The old L98 Corvette motor ('85-91) is a great torque motor. It's not so good at producing horsepower at high rpm; and by high rpm we mean anything over 4,500 rpm. This motor was designed in the late-'70s to produce adequate torque and clean emissions. The emphasis was on clean emissions. The torque came as a side benefit.

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One reason these heads only cost $300 was the small problem where the head was eroded away at the edge of the chamber. This is supposedly very common on aluminum cylinder heads. We welded it and then resurfaced the head. In hindsight it was, just as everyone told me, no big deal, but I did have a few moments of anxiety about my bargain heads.

Chevrolet installed over 150,000 of these L98 engines in the C4 Corvette. Every single one of them runs out of power at around 4,300 rpm. These are great motors for driving around town. They have tremendous torque and can move away from a stoplight with the best of them. My '85 can run the first 60 feet of the drag strip in less than two seconds. That's what bottom-end torque is all about.

Over the past few years I had installed a TPIS ported plenum and big runners. I had also installed a set of TPIS headers. The result of all this was a great street motor that simply ran out of breath above 4,500 rpm. The peak horsepower was occurring at 4,300 rpm. This isn't all that bad for a street car, but on the back straight at Sebring I was relegated to a parade of Porsches going past me. That's embarrassing.

I run a half-dozen track events at Sebring every year. Sebring is a fairly fast track, and you can actually use higher rpm. If I could pick up 1,000 rpm, not to mention some real horsepower, I could get down the backstretch without shifting into overdrive. I would need horsepower to develop this speed. Anytime you're over 100 mph you should look at the horsepower, not the torque. A total lack of horsepower at 5,000 was the problem. A different cylinder head, a new camshaft, and a new manifold seemed to be the solution. Visa would make all this possible.

The earlier changes allowed me to get a fair amount of air into the plenum and down to the manifold. Then all this air was simply backing up in the manifold and cylinder heads. The manifold and the intake passages were one huge bottleneck. This was confirmed by looking at the fact I only moved the horsepower curve up about 300 rpm with all the improvements I had previously made to the car.




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