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Mustang Engine Tuning With Speed Demon Carburetor - Demon Tweaking
We Look Over Demon Carburetion's Shoulder As It Fiddles With Both Road And Race Carburetors
Chuck Jenckes has exercised his engineering talents at several companies, including the major OEMs in Detroit, Trick Flow, and-now, through his consulting business-Demon Carburetion. Always in demand, Chuck says he lives "half the time in Charlotte, half the time in Atlanta, and in Philly with my wife." Testing the larger Speed Demon carburetor required more oomph from the engine
We've all bought parts from companies that we later learned did their research and development on the public. It's not a pretty experience, and so it was great to be invited to a dyno test being held by a manufacturer just to make sure what it was selling was working.
The manufacturer was Demon Carburetion, which was sending well-known engine engineer Chuck Jenckes to conduct the examination. The dyno facility was our usual test haunt at Westech, with main man John Baechtel on the Superflow's throttle lever. Even the test engine was familiar, being our own 302 short-block. It was run in two configurations: stock, and hopped up with a cam, heads, and larger intake. The idea was to check both the street-oriented Road Demon carburetors for stock or mild small-blocks and the Speed Demon for more seriously modified engines.
Specifically, Demon wanted to check the jetting and other variables in the carburetors it is shipping these days. Demon has a wet flow bench at Barry Grant which, combined with years of experience, gives an excellent idea of what sort of jetting that carburetor would want, but still, it's smart to check on an engine dyno.
To represent the bolt-on, stock engine crowd, our stock short-block was dressed with a stock 5.0 H.O. cam and standard-issue E7TE cylinder heads. This includes stock pistons, even a stock oil pump, pan-everything. For dyno duty, the usual set of Hedman 1-5/8-inch long-tube headers were fitted, along with an Edelbrock Performer RPM intake manifold. The carburetor was a BG 525 Road Demon.
As usual, the ignition was supplied by an MSD 6 box, with the timing ending up at 35 degrees after dialing it in. No alternator was fitted, and the water pump belt was set extremely loose. The fuel used was 91-octane Union 76 pump gas, supplied by Union in 55-gallon drums. The gasoline is the same good stuff sold from retail pumps, but having it on hand in large supply saves the monkey motion of running to the local service station to fill cans.
Testing the larger Speed Demon carburetor required more oomph from the engine. Thus the camshaft was switched to a Comp Cams 274 Extreme Energy unit, which gives 224/232 degrees of duration at 0.050 inch valve lift, and 0.555 inch of intake valve lift and 0.565 inch of exhaust valve lift. That's all on 110-degree centers. This is a healthy, workable cam that could get by on the street for the enthusiast, yet make a ton of steam in the midrange and top end.
Helping with the deep breathing was a set of highly regarded Air Flow Research 185 cylinder heads. These boasted the large, 2.05x1.60-inch valve package, which, while often requiring piston-fly-cutting, have a well-earned reputation as deep-breathing, big-power units. No other changes were made, so the ignition, intake manifold, and headers were left alone. The carburetor for this "big" engine was a 575 Speed Demon. As with the 525 Road Demon,it was a randomly selected example installed box-stock.
As dyno testing goes, this was a fairly simple session. The idea was not to tune the engine, but rather to verify the jetting supplied by Demon to its carburetors at the factory. With the engine cooperating and not throwing us too many curve balls, the tests went quickly and with little drama. Still, we had to satisfy ourselves the engine was running correctly, then hunt around with the jetting a bit to ensure we'd hit the peak of the mixture and power curves'
With the stock engine configuration, we lit the combination and warmed it up. It ran a bit rich but made 275 hp and 300-plus lb-ft way down at 2,500 rpm. Chuck decided to take out three jet sizes on the primary, moving from 60s down to 57s (Holley and Demon/Barry Grant jet sizes use identical numbering, for those keeping score at home). There was some concern about whether the secondaries were opening, and a visual check showed they were, but only at higher rpm, around 4,200.
Chuck offered that the carburetor might be a bit small for the engine, to which John suggested installing the dyno's air hat to see what sort of air the engine was pulling. So, we stopped to put the air hat on, which didn't take more than setting the hat on the engine, tying it down with some springs, and plugging in the air hat electrical lead.
Firing up again, John made another pull to check the airflow reading. As the data popped up on the screen, both Chuck and John mumbled "seems rich," which we guess we already knew. The numbers also showed only 300 cfm of air consumption, and a funky VE of 60 percent, which was far below the expected 90-percent value.
This led to lots of double-checking of dyno settings-moving readings around on the computer screen to get them on the same page, and so on-but finally Chuck decided he was still not convinced the secondaries were opening. So, donning a set of earmuffs, he had John make a pull while Chuck stood in the cell and manually held the sec-ondaries open (the things we do for science). This produced more likely fuel-flow numbers, so the conclusion was the secondaries were not opening until far up the tach. A smaller spring in the secondaries vacuum can was likely in order, Chuck figured, but then he realized the lightest spring available was already in the secondary vacuum can. At points such as these, you have to simply accept it's a small engine and doesn't need that much airflow.
We all noted a stumble at throttle tip-in, so the engine was definitely going lean on the primary side to start with. Power, by the way, was up to 285 hp at this point.
Also, the air/fuel ratio was looking a bit fat on the secondary side, so Chuck elected to take out "three numbers." The secondary jets were changed down three sizes, from 78 to a 76, while the primaries were bumped up a notch from 57 to 58. A bit more accelerator pump shot would also help the stumble, but Chuck figured moving up on the jet size was the way to handle the situation. He was obviously trying to even out the fuel flow between the primary and secondary sides of the carburetor at this point before chasing other issues.
Another run was made with Chuck holding the vacuum secondaries open. This brought on a 1hp gain, at 286 hp and 330 lb-ft, but it also showed the balance between the front and rear float bowls was still not perfect. Chuck once again went down on the secondaries, from 76 to 74 jets. Figuring he was getting close on the secondary jetting, Chuck said once he balanced the bowls to each other, he'd start working on the brake specifics. By that he meant he'd make even jet changes, front to rear bowl, to dial in the entire fuel curve.
John pulled the lever again, and once more we netted a single horsepower "gain" with a reading of 287 hp (this is effectively no change, as the dyno and engine won't repeat this accurately-warming oil could easily account for a single horsepower, for example).
Chuck wanted to rerun the test so he could watch the Horiba A/F meter readout from his station alongside the engine in the dyno cell. This made 288 hp on this run (another 1hp gain), but the A/F on the Horiba oxygen meter was only showing in the 11s. Therefore, Chuck wanted to go leaner on both bowls. We stopped for a jet change, moving down two steps to 56/72 jets.
This time, the Clark Kent 302 put up 12.3-12.5 A/F numbers on the Horiba. Chuck wanted to see it again, so we immediately reran this test while he watched the Horiba readout. There was no change, and as the A/F was right at the happy 12.5:1 peak power zone, the power was up to 290 hp and 336 lb-ft. No one was saying much-it was clear we had reached that point where everyone stands around trying to think of anything else to do. In other words, we were finished.
We won't bore you with the blow-by-blow of testing the "big" engine, as it was basically the same drill of hunting around with the jetting to find the optimum combination for that particular engine, and you can see the peak power figures in the sidebar.
All told, the preliminary results were fairly good, as it looked as though the Demons were coming out of the box as close as you'd want to perfect (leaned for max power), short of possibly a lighter secondary spring on the Road Demon. If anything, both carbs were a hair rich, which is definitely the safe side to be on. And now you know one company that isn't doing its R&D with your pocketbook.
Tracking Your Demons
There are four basic Demon carburetors. They are the stock-replacement Road Demon, the street/strip Speed Demon, the track-oriented Race Demon, and finally, the all-out King Demon for Pro Stock-type applications.
All these carburetors are avail-able in several cfm flow ratings (sizes), and some models have changeable chokes so the sizing can be varied. All are miles ahead of stock carburetors in build quality, quality of airflow, and power produced.
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Below are the two highest power dyno sheets for the stock and modified versions of our 302 dyno mule. There really is no comparison between the two configurations, as the entire top of the engine was changed. However, each sheet shows the potential of a high-flow carburetor intake manifold coupled with a good, power-oriented carburetor.
|Road Demon||Speed Demon|