BIG DUKE CYLINDER HEADS: ALKY OR GAS?
Story by: John DiBartolomeo
Drag Racing Action Magazine
The fuel car classes are certainly one of the most technologically advanced in regards to making horsepower. Let’s face it. Three-hundred miles per hour and estimates of seven to eight thousand horsepower are nothing to turn your nose up at. However, when it comes to the “trickle-down” theory and actually attempting to utilize any of their horsepower advances on your “bracket bomber”, chances are slim you’d be able to. That all changes, though, when it comes to the Pro Stock class. And it’s probably why that category is one of the most popular amongst sportsman/bracket racers.
Many of the ideas that have evolved in that class have actually trickled down into the sportsman ranks. Tunnel ram manifolds, planetary transmissions and four-link suspensions are just some of the things that are now standard-issue amongst weekend-warriors.
What seems like now eons ago, Pro Stock engine builders experimented with moving intake runners within the confines of a cylinder head in an effort to straighten out the port and flow more air. The experiment was a success and speeds increased while elapsed times dropped. The downside to it was the increased costs involved with the modifications. Today, CNC (Computer Numerical Control) machines can take a raw casting and duplicate a good flowing head in hours. These Big Duke cylinder head castings are noted by the separation of the intake ports, moving them outward of one another in an effort to end up with a better flowing port.
The introduction of CNC (Computer Numerical Control) machines has revolutionized the manufacturing industry and allowed what took, in the past, one person weeks to produce, to now be completed in just a few hours. The actual process is seemingly simple. A “good” head is scanned into a computer program and a CNC milling machine “duplicates” it, alleviating much of the human aspect, and in this case, allowing Pro Stock-type cylinder heads to become economical enough for even the lowest budget. It’s the reason we have powerplants that now “average” 900-1000 horsepower.
Brodix’s Big Duke version cylinder heads for big block Chevrolet engines have been extremely popular amongst “big speed” bracket racers, but some have questioned their use with alcohol as a fuel. Because of its ability to run more consistently from morning to night, along with its cooling capabilities, alcohol is used by a great many bracket racers. Those choosing to run their Big Duke cylinder heads with alcohol, though, have noticed they don’t seem to run correctly, sometimes even popping or missing.
It used to be that a switch from gasoline to alcohol netted you an increase in horsepower. Nowadays that’s not as true, however it’s not that alcohol has lost some of its punch, but rather that advances in gasoline combinations have increased. Most serious engine builders, as well as camshaft and carburetor manufacturers, have spent a considerable amount of time perfecting gasoline powerplants. The proof is in the pudding as to their success, but today, roughly 50-percent of bracket- and super-class racers run alcohol as their fuel of choice.
Greg Brotherton of Brodix says, “In the past three years or so, we’ve received a lot of calls from our customers with questions on whether to run their Big Duke engines on gas or alcohol. This is mostly due to rumors they will not run correctly on alcohol. It’s almost like the things you heard in the 1980’s when people would say that if you run alcohol in your engine it would blow up.”
With more racers wanting the performance advantage of the Big Duke cylinder head, Brodix teamed up with John Kyle at Advanced Product Design (APD), a carburetor specialist and engine builder, to do some extensive testing with those heads on alcohol verses gasoline.
Kyle said, “We sell quite a few alcohol carburetors, as well as produce a number of race engines a year, and we’re concerned when a customer mentions that one of them doesn’t run properly on his Big Duke engine. We had some theories as to why this was happening and discussed it at great length with Brodix. We didn’t feel the problem was our carburetor, but we also didn’t believe it was a cylinder head problem.”
John Kyle and his group of employees at APD (Advanced Product Design) are noted for their big inch alcohol engines, but the problems with the Big Duke heads caused them to delve deeper and locate the source of the issue. Countless hours were spent on the dyno finding answers.
The 1800-series cylinder heads are the Brodix Big Duke versions and are available in a number of forms, starting with the 1802 being the “as-cast” version. The casting utilizes rectangular intake ports and is contoured to flow great amounts of air and fuel right out of the box.
Kyle says, “We started our testing with a 565-cubic inch engine that utilized the 1802 head. The engine had what most would consider a standard cam for the application with 286-degrees of intake duration and 298-degrees on the exhaust side. The lobe separation the camshaft was ground on was 112-degrees and the engine ran good on gas. However, on alcohol it lost power and wouldn’t run clean.
“With the engine in a 2050-pound dragster on gas, it produced a 7.39 at 183-mph. Switching it over to alcohol, though, only produced a 7.52 at 178-mph. We then took the engine out of the car and stuck it on the dyno for some heavy-duty testing. Fooling with the alcohol carburetor first, we found very little improvement.”
One major difference between a gasoline- or alcohol-fueled engine, (other than the choice of fuel), is the fact that you’ve got to feed the engine roughly twice as much alcohol as you would gas. This means that while the air/fuel ratio for a gasoline race engine might be in the 12.9 to 14.0 to 1 range, an alcohol engine needs to be somewhere in the 5.5 to 6.5 area.
Kyle said, “We thought—and what it really came down to was a guess—that with that much alcohol being pumped into the engine, could it have been possible that on the exhaust side of the cycle not all the burnt gases were being forced from the cylinder? We sort of theorized that even though the air/fuel mixture is in a burnt form at that point, there still was double the amount of alcohol than gasoline. Again, this was all just a theory, but it was an area we wanted to pursue.”
The first step was to simply move the camshaft timing in relation to the crankshaft. “We started to notice little things,” Kyle says, “and I thought that just maybe we ought to make a big change and see what happens.”
The “big” change was to swap out the camshaft with one that had much more exhaust duration. The cam chosen had the same intake lift and duration; however on the exhaust side the new lobe had 310-degrees of duration as opposed to the 298 that the engine originally tested with. In addition, the lobe separation that the cam was ground on was opened up to 116-degrees versus 112.
Lobe separation is the distance in crankshaft degrees from the center of the intake lobe to the center of the exhaust. By making that number larger, in essence, you have “delayed” the actual opening time of the exhaust in relation to the intake. In this case, it allowed the exhaust to stay open for a longer period of time, not only because of the extra duration, but also longer in relation to crankshaft and piston location. The hope was to allow more of the burnt exhaust to exit.
“With the new cam in,” Kyle says, “we tested the engine first on gasoline and noticed that, while horsepower remained close to before, it now produced peak power at 7300-rpm versus 7000. We then switched it over to alcohol, and while the peak power was made at a lower 7100, the engine made remarkably more power in the 5500 to 6700-rpm range.”
Because alcohol-burning engines require roughly twice the amount of fuel than gasoline, the air/fuel ratio must be that much richer. Whereas a typical gasoline engine runs best with an air/fuel ratio of roughly 12.9 to 14.0 parts of air to one part of fuel, an alcohol engine runs best in the 5.5 to 6.5 range, as evidenced by this dyno sheet from a 582-cubic inch engine built by APD.
Knowing full well that we don’t race dynos, the proof would obviously be the time slip with the engine back in the car. But nonetheless excited over the change, Kyle and crew couldn’t wait to get back to the track. On the car’s first pass with alcohol in the tank and the weather conditions roughly close to the previous test days, the scoreboards lit up with a 7.29 at 186-mph. A back-up run netted a 7.30 at the same mph. No missing, no popping, just as clean as could be from one end of the track to the other. An obvious increase from what the engine ran like the last time, but now what did it do to the gasoline combination?
Switching the engine over to run on gasoline is a relatively easy thing to do, and on the first pass, the car ran a 7.34 at 185-mph. “At that point, we were pretty pleased with the results,” says Kyle, “but we wanted to back it up on another engine as well.”
Brodix 1803 cylinder heads are the oval port versions of the Big Duke head, and in the as-cast variety, they tend to flow slightly more air than the rectangular versions do. APD built a 582-cubic inch engine with the same carburetor and camshaft used in the 565 previously tested. Kyle found that the 1803-head motor made more power throughout the entire rpm range than the 1802. However, at peak speed it only made ten more horsepower. The increases are apparent at all other rpm ranges, due in part to the extra cubic inches, but also to the increased breathing of the cylinder heads. Switching to alcohol, it produced roughly the same results as the 565 did, and sounded as crisp and clean throughout the entire rpm range tested. Test two complete, Kyle still wasn’t through.
“We were pretty sure we had answered some questions, but I wanted to try it one more time to make sure,” Kyle says. “We used a 598-cubic inch short block and I ordered a set of 1802 Big Duke heads with 2.450” intake valves and 55-degree valve seats. The combustion chambers and ports were CNC-machined and the bowls of the intake and exhaust ports were blended to match the rest of the ports. All of this work was performed by Brodix. I called a major cam company to see what the most common cam being used for this combination was and they said intake duration from 284 to 288 and exhaust duration from 300 to 304 with a 114-degree lobe separation. What I ordered was a 287-degree intake lobe and a 304-degree exhaust with 114-degree lobe separation. This was what was considered to be on the big side of the ‘so-called’ normal.
“The motor ran good on gas, but when we put it on alcohol it sounded like it was dropping cylinders at an idle,” Kyle continued. “When we made a dyno pull it started misfiring at 6500-rpm and we had to abort the pull at 7000-rpm because it sounded like it was going to blow the headers off of it.”
Obviously disheartened, Kyle tried fooling with the carburetor jetting and air bleeds, but in each case it hurt power. Having a cam lying around that was the same as the model installed on the other test engines, Kyle’s thought was “Why not try it?”
Some of the problems associated with the use of alcohol as a fuel on Brodix Big Duke cylinder head-equipped engines were missing and popping at relatively high rpms. John Kyle of Advanced Product Design said, “We thought—and what it really came down to was a guess—that with that much alcohol being pumped into the engine, could it have been possible that on the exhaust side of the cycle, not all the burnt gases were being forced from the cylinder?” Kyle’s guess proved right when he swapped in a camshaft with a larger exhaust duration.
As simple as it sounds, the new cam made an immediate improvement, picking up roughly ten horsepower and more importantly, sounding smooth throughout the range. With that motor installed in a 1990-pound dragster, during the course of a 15-hour race day, it made several runs from a 7.169 to a 7.177 at over 189-mph, proving not only power but consistency as well.
Do Big Duke cylinder heads work with alcohol? In each case what was learned is that these cylinder heads do require a cam with more exhaust duration and a wider lobe separation. And do they make power? The proof is always the time slip.
Sources (Click to link to company website):
Brodix Cylinder Heads
Advanced Product Design