Rebuilding and Upgrading the Corvette L-48 Engine | Part 7

When I talked to Mark at Sunrise Automotive here in Orange City, Florida about having a local race shop rebuild the L-48 with a few performance parts a while ago, I mentioned to him that I'd also like to have a list of specs of what they did and/or added to the motor.

This is the complete report of what they did:

  • 3-Angle Comp. Valve Job
  • Surface
  • Install New Springs

  • Wash
  • Bore .030 Over
  • Install Cam Bearings & Brass Freeze Plugs
  • Remove and Replace Pistons
  • Resize Connecting Rods

  • Polish Main Bearings STD
  • Polish Rod Bearings STD
  • Disassemble and Assemble Engine

  • Pistons: .030 Over
  • Rings: .030 Over
  • Cam Bearings
  • Brass Freeze Plugs
  • Main Bearings
  • Rod Bearings
  • Full Gasket Set
  • Double Roller Timing Set
  • Performance Camshaft: Melling: Flat Tappet (Hydraulic)
    Intake Duration @ .050 214 | Exhaust Duration @ .050 224
    Adv. Duration: Intake 288 | Exhaust 298
    Valve Lift: Intake .443 | Exhaust .465
    Lobe Separation: Intake 107° | Exhaust 117°
  • Power Range: 2000-4500 | Idle: Fair
  • Comments: Good Low to Mid Torque | Noticeable Idle
  • Lifters
  • Oil Pump w/Screen and Pickup Retainer
  • Springs | Locks
  • Intermediate Shaft
  • Hi-Zinc Assembly Lube (4 oz)
  • Head Bolts

So what does it all mean?

My factory-built-and-installed engine rolled off the St. Louis, Missouri Corvette assembly line as a 350 cubic inch, 180 h.p. L-48 back in 1976.

After the aforementioned mods, it no longer qualifies as an L-48 or a 350, for that matter. Since it had to be bored .030 over, this added an additional 5 cubic inches, so it now is a 355 c.i. motor.

But how about horsepower? you may be asking.

According to Mark at Sunrise Automotive, the shop claims 310 hp at the crank. But as we all know, real-world (or "net") numbers are quite different.

So in order to get the "real" horsepower number, I had the car dynoed. 

But before we get to that I think it's important to clarify a common misunderstanding between gross vs. net horsepower.

Both gross and net horsepower is measured at the flywheel. The difference between the two resulting numbers is caused by engine accessories, such as water pumps, a/c compressors, alternators, etc., which "weigh down" the engine's performance.

Needless to say, when you factor in drivetrain components, horsepower keeps shrinking.

So I assume it is safe to say that the original L-48 powerplant that came in my car, had a rear-wheel horsepower output of—at best—150 horses.

Above: Parking lot burnout marks
let you know you're in the right place.
I should've had my car dynoed before and after, but I didn't. However, if my assumption is somewhat accurate for a stock L-48 motor, then at least we have a baseline to do a comparison.

Through a friend, I learned that C&H Motorsports in Sanford, Florida has a dyno, so I paid them a visit and spoke with Stephen Harper, the shop's owner.

I explained what I had done to my Corvette and that I would like to find out if the 310 horsepower claimed by the engine builder was an accurate number. Stephen said that if that number was correct, we should see around 250 to 260 horses at the wheels.

So I made an appointment to bring the car in today to have my car dynoed. I must admit that the dyno run results were not what I was expecting. At all!

Stephen was ready for me when I arrived at his shop, and he quickly drove my Vette up onto the rollers and secured it in place. He then made sure everything was safe and ready for the dyno pull.

Once everything is good to go, the run itself does not take that long, and this is footage I shot with my phone of the actual final pull.

Based on what the shop told us, I was sure the number was going to be in the mid 250s, so you can imagine my surprise and (mostly) disappointment when the printout showed 185.26 hp at 5170 RPM.


Stephen also indicated that the power curve wasn't constant or even, and he suspects a timing issue so I will be bringing the car back in for a tuneup.

From C&H Motorsports I drove straight over to Sunrise Automotive in Orange City.

Mark, the shop owner, knew that I was going to have the car dynoed, and so he was eager to hear the final number. 

When I told him 185 he thought I was kidding, but soon realized I was as serious as a heart attack. I gave him a copy of the paperwork since he said he was going to have a talk with the engine builder.

At this point, I am not sure I want to have them tear into the motor again, but I feel I was lied to as far as the expected horsepower. Not sure if a partial refund is going to be offered but I think that would be appropriate.

I am not holding my breath, however.

So I guess it is safe to say that my car now has an engine that is the equivalent of a 1976 L-82, which was rated at 210 hp (at the crank), which would translate into 185 to 190 hp at the rear wheels.

Nothing to write home about!

In hindsight, I think I would've done a lot better if I had gone the crate motor route, with either a 290 hp 350 Chevy engine or a Stroker 383, and had said "to hell" with the numbers-matching nonsense.

If you are on the fence trying to decide whether to have the numbers-matching motor rebuilt or to buy a crate engine, I hope my experience helps you plan and execute your engine swap a little easier and without so many pitfalls.

Also, if you decide to go the rebuild route, make sure you see the finished motor run on a dyno. Numbers don't lie. And asking for that at the beginning may prevent last-minute "misunderstandings" or confusion.

I will post any updates to this article as they become available.

Thank you for following.

UPDATE 1/12/2017

I stopped by Sunrise Automotive to see if Mark had spoken to the engine builder, and he confirmed that he had. He said to drive down to Sanford and talk directly with the guy in order to better understand the work and upgrades he had performed.

Since Sanford is just a short drive away, I drove straight to Daniel's shop.

He explained that he had used the same blueprint to build a 300 hp motor but admitted, at the same time, that he did not have an engine dyno to confirm the flywheel horsepower of the finished product, so he guessed.

When I asked him about the compression he—again—guesstimated approximately 9:1 compression and said that it would be OK to use regular gas.

Pistons? Just 30 over flat tops.

I then asked him about the camshaft, he said that even though the Melling cam he had chosen was a step above the factory stick, he had used that particular camshaft in order to maintain streetability.

Even though I felt the guy was on the up and up as far as trying to explain the work he had done, I really think he was surprised that I had actually paid someone to dyno the car, and when I explained that even though with a few tweaks and adjustments the car could be easily brought up to deliver 200 hp at the wheels, that it was quite far from the anticipated 250 horses everyone—especially me—expected.

I also explained that with the current setup, my engine was the equivalent of an L-82 Corvette motor, which even though far more powerful than the L-48, is a dog by today's standards.

Now granted, today's engines are fuel-injected, computer-controlled-and-designed, lightweight powerhouses compared to detuned motors of the mid-1970s. I added that, nevertheless, I was disappointed that I chose to rebuild the original motor instead of buying a crate engine. Daniel "sort of" agreed.

At that point, I realized that this was a battle that I would not win, and even if I got Daniel to pay for a "better" cam, that would not be the solution for my predicament.

My Corvette has PLENTY of torque; and when I step on it, it responds well and a LOT better than the original L-48.

My disappointment stems from the exaggeration of promised horsepower that never materialized. I would've rather been told a more realistic guesstimate of what to expect rather than a fish story.

This does not mean that I'm giving up on improving my Vette. Quite the contrary.

There are a few other projects that will take priority over performance, for the time being, but I plan to have a few things changed in the future. But for now, things such as new u-joints for the driveshafts, new rear wheel bearings, and seals, to name a few, will take precedence.

So stay tuned for Part 8 of this series sometime in the near future.

Thank you for following my '76 Vette Blog!

Product Links... (#sponsored)

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