Ask Away! with Jeff Smith: Camshaft and EFI Advice for 540 Big Block Chevy

I’m building a street 540 big block Chevy with a mild hydraulic COMP Thumpr cam, rectangle-port heads, and 9.5:1 compression ratio that looks like a stock L88 to go in a 1967 Corvette Coupe.

I’m looking at the Fast EZ-EFI, Fi-Tech, and the Holley Sniper. Which one of these systems works the best overall? I’m not worried about spending a few more bucks up front. What I want is a system that works well, is reliable, and has good driveability but I’m not necessarily concerned with peak horsepower. — B.W.

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Jeff Smith: There are a couple of items to discuss.

The first is the choice of the Thumpr cam combined with a throttle body EFI.

I tried that exercise several years ago on a small-block Chevy with three different throttle body EFI systems and wasn’t happy with any of them. The issue was not with the EFI but rather the camshaft.

Here’s the issue. COMP Cams designed the Thumpr cam to offer the street enthusiast a competition-sounding idle by increasing exhaust lobe duration and tightening the lobe separation angle (LSA). This creates a significant increase in valve overlap where both the intake and exhaust valves are open at the same time.

Increasing overlap is what creates that lumpy idle.

The idle instability comes as a result of a reduced signal in the intake manifold. Unfortunately, when we add a throttle body EFI system, the first thing that happens is much of that idle instability is reduced. In other words, the idle is automatically smoother. So you tend to lose the impact of what the Thumpr cam was intended to produce.

Additionally, what produces that engine sound is a lot of overlap combined with compression — around 11:1.

Engines with a lower compression ratio, especially those at below 9:1, just sound flat or dull with a long-duration cam because the overlap greatly reduces the cylinder pressure at idle.

A knowledgeable technician from one of the EFI companies once told me that he wouldn’t recommend a throttle body system for any street engine that idles below 10 inches of idle vacuum.

One aspect you have going for you is displacement. A 540 c.i.d. Rat will idle better with the mild Thumpr cam than my 350 c.i.d. small-block because the duration and LSA numbers are the same between those two engine families. My engine barely idled at 9 inches of manifold vacuum.

The annoying aspect with the Thumpr in my small-block with a four-speed trans was the engine always exhibited an off-idle hesitation when we left from a dead stop in First gear.

The only way to get past that was to throw a bunch of fuel at it which really wasn’t right for the rest of part-throttle operation. In the cam’s defense, I did not try adding a bunch of initial timing to the engine nor did I try to bring the mechanical advance in sooner. Either of those would have likely helped the stumble issue.

If you wish to try this, I do have some experience with a Holley Sniper on a milder 468 c.i.d. Rat motor running a COMP XR282HR-10 camshaft hydraulic roller camshaft. The chart that follows will immediately reveal how close these two camshafts are — with the exception of the exhaust duration and LSA. The Thumpr adds 14 degrees of exhaust duration compared to the Xtreme Energy cam and the LSA tightens three more degrees from 110 to 107 degrees. This adds a ton of overlap to the Thumpr.

This 468 c.i.d. big-block engine has been running this combination for about two years with excellent street manners, decent highway mileage spinning a 4L60E overdrive and lockup converter. In fact, my friend measured 14 mpg on a freeway jaunt up the coast last year. In-town mileage is in the single digits because it’s a big-block in a 3600-pound El Camino. But nobody builds a 540 c.i.d. Rat if they are looking for fuel mileage.

If you want to run a throttle body EFI with a little more lumpy idle, I’d suggest the slightly longer duration XR-288HR-10 which is also in the chart.

It’s slightly longer in duration and will reduce idle vacuum. I built a 540 big-block for a magazine using this camshaft and it made 630 hp and 660 ft.-lbs. of torque. It idles around 11 to 12 inches of manifold vacuum with a carburetor and would make a great street engine. It won’t have the lumpy idle but everything else will be outstanding.

I think any of these TBI systems will work well, but like I said, I do have experience with the Sniper and that El Camino is driven about 2,000 to 3,000 miles a year in Southern California traffic with no issues.

An important consideration with any of these EFI systems is to have a well-built exhaust system with zero exhaust leaks. This is critical because all of these EFI systems are self-learning based solely on input from the wide-band oxygen sensor. These sensors are often called air-fuel ratio sensors but that is actually an incorrect label. These sensors detect free oxygen in the exhaust. Using that as input, they then calculate the air-fuel ratio and deliver that to the ECU.

If your system has a leak, pressure might force exhaust gas out but it also then pulls fresh outside air into the exhaust. If this leak is upstream of the O2 sensor, it will read this higher percentage of oxygen as a lean engine condition and increase fuel. After each driving session, the ECU then adds this “short-term” fuel to the main fuel map. You can guess what happens. The next time you drive the car the sensor reads the exhaust again as too lean and adds more fuel. After about a dozen startups — the engine is running extremely rich. But it’s not the fault of the EFI but rather as a result of the exhaust leak.

So the trick is to drive the car for a few days and if the engine runs well, then disable the EFI’s learning function. Then drive the car on multiple trips and evaluate how it runs.

Watch the air-fuel ratio readings and keep track of how well it runs. If it continues to run well, just leave it as the tune is probably very close. By disabling the learn function after the engine has made 90 percent of its learning, you will avoid this over-rich condition.

Hopefully this discussion has given you things to consider. In this position, I’ve learned that offering somewhat more conservative recommendations for the street is usually a good idea.

Ultimately, the choice is yours. Either way, a 540 c.i.d. Rat on the street will be lots of fun!