The Role Of Mechanics In Drag Racing – When it comes to the Chrysler Hemi-based engines used in Top Fuel dragsters and Funny Cars, you’ve probably heard the common reference to 11,000 horsepower that those engines can produce. That’s 1,375 horsepower per cylinder! That level of performance is pretty insane, but did you also know these drag cars knock on the door of 340 mph, and come very close to 300 mph in the eighth mile? Or how about the fact that they burn about 15 gallons of fuel in a single run, and at a max pull of about 6,800 rpm, they flow 90 gpm of fuel?! These engines also create so much horsepower that many engine components are only good for one to five runs, and in the case of pistons, the amount of power can even reduce the dome to some extent!
This is the pinnacle of drag racing, and the engine builders, crew chiefs and teams that keep these cars running at peak performance all season look at every single area of the engine and the car to make it as fast as possible on the track. We wanted to highlight some of the characteristics of the 426 Hemi-based engines used in Top Fuel and Funny Car, as well as what really goes into each run and the thought process of an NHRA team.
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To do that, we spoke with NHRA driver Joe Morrison and NHRA crew chief Rob Flynn about their experiences in both Top Fuel and Funny Car classes.
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As mentioned, the engine used in Top Fuel and Funny Car is loosely based on the 426 Hemi. NHRA rules state that it can’t be more than 500 cubic inches, and camshafts can only be as big as 54mm, but other than that, teams can generally build the engine however they want and use whatever components they want have.
“It’s all based on the Hemi,” said Rob Flynn, an NHRA crew chief with a background in both Top Fuel and Funny Car classes since 1994. “The blocks are all billet aluminum and the heads are also billet -aluminum. There are no water passages in any of those things. The bore and stroke are a 4.500″ bore. That’s basically what the NHRA rules allow. a few people who were into a little shorter stroke and a bigger bore, because the stock Hemi bore size is 4.252″. When all of us finally got our engines bigger, they limited the cubic inches to 500 cid, so the engines are around 496 cid for both classes.
Since these engines are loosely based on the Hemi, they differ from a stock 426 Hemi in a few ways, according to manager Joe Morrison.
“I say loosely because a lot of modifications were made to increase the airflow from the intake and exhaust ports,” said Joe Morrison, a Funny Car driver for Herzhhauser Racing and a former Top Fuel driver for Leverich Racing. “Also, the big difference from a stock 426 Chrysler Hemi would be that we have two spark plugs per cylinder as opposed to just one. This is for two reasons. Number one, methane is explosive when compressed, but not super flammable, so it takes more to get it to light. The second reason is that we burn an enormous volume of fuel.”
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Thanks to the angle of the zoomie-style headers on Funny Car engines in particular, the headers help produce downforce and thrust for the car.
We’ll get more into fuel in a moment, but for internal engine components, the NHRA doesn’t dictate which teams are allowed to use, but aside from different brand names, the components are generally similar. Teams run aluminum connecting rods, forged aluminum pistons and different compression ratios and different ring packages.
“Teams can do whatever they want when it comes to internal components,” says Morrison. “You can change your connecting rod length and your compression height of the pistons. Typically, because of the intense heat, the ring pack is further down on the piston.”
According to Flynn, a Dykes top ring was commonly used in the past and the second ring was also a compression ring, and some people used it as a scraper, but mostly now most people use a compression ring with pistons with gas ports.
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“We use the same thrust ring in the first and second groove and then we have an oil ring, and the oil ring takes a lot of abuse, so we run one on it and throw it away,” says Flynn. “A tremendous amount of parts are run through these engines. The engine comes apart every time, so everything is either replaced or overhauled. In our situation we will take the top ring and put it in the second groove and install a new top ring and then we will replace the oil ring.
“With the bearings, the upper rod bearing takes a lot of load on it and we actually measure it every run. These are usually replaced every run as there is quite a bit of clearance to begin with and you don’t really need more. The main bearings, mainly the lower half, are replaced or certainly inspected every time.
“The valvetrain components have a certain amount of runs that they’re typically good for, like camshafts and lifters. Everything is inspected every run because of the kind of power we’re making. It puts a lot of stress on the internal components.
“After every run we measure the bearings, the pistons, how much the pistons sink, how much the bearing pressures, if the rings have too much heat and lose tension. The piston can knock off the ring land. Every run the pistons get a little bit of sink in the dome and the center of the dome, so we measure that and there’s a certain number where we don’t consider that piston satisfactory anymore. It could be one to five runs.”
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As mentioned, the camshaft in these engines cannot be larger than 54mm, but it is in the standard position with pushrods and overhead valves and two valves per cylinder. Something many teams do is change the cam firing order to get better harmonics.
“Instead of a stock Hemi firing order, we’re using the Chevy LS firing order,” says Morrison. “We fire the outer cylinders first and then the inner cylinders.”
Flynn elaborates by saying the standard firing order is similar to a small block Chevy, but the majority of the class run what is called a 4-7 swap fire order, but some use an LS fire order, which is a 4-7 , 2- is. 3 exchange.
“It’s a preference thing,” Flynn admits. “I don’t know that either makes more power, but there is some discussion that the engine runs a little smoother with certain swaps. One thing I noticed on the LS swap is that the cam is a is a bit stronger structurally. I don’t think either one makes a big difference in performance. It’s just a personal preference that people have.”
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These Hemi engines also feature a Roots-style 18-71 supercharger, which typically sees 50-60 lbs. of boost and can dictate a lot when it comes to these engines’ compression ratios.
In addition to a block and cylinder heads, these Hemi-based engines have an 18-71 supercharger that typically sees 50-60-lbs. of boost.
“The compression ratio on most teams will be 6.5:1 to 6.8:1, and it varies according to the atmospheric conditions,” Flynn points out. “On a cool, dry day in Florida, you’re going to run 6.5:1, and then on a hot, humid day in Topeka, KS, you’re probably going to be in the 6.7:1 or 6.8:1 range. There are some people who have different setups that might be lower or higher than that.”
Final compression ratio is based on barometric pressure, and that adjustment is made once you cc’d your compression heights and your combustion chamber, but that final compression ratio decision is also based on your head gasket thickness, which for these classes is typically between .070”-.080” and up to .120”.
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As for oil, these engines run nitro 70, and the reason for this is to help fight blow. The nitro dilutes the oil because of all that cylinder pressure and that volume of fuel. The heavier engine oil acts like a cushion.
“When we start the car when it’s cold, we’ll see up to 300 psi of oil pressure,” says Morrison. “As I set the car up after the burnout, after the engine warms up a little bit, we’d like to see 120 psi.”
To make the horsepower levels that Top Fuel and Funny Car do, it’s no secret that a lot of fuel is required. From starting the car to the burnout and backup, the crew chief’s adjustment, the run-up to the shutdown of the car, a Top Fuel or Funny Car will go through in the neighborhood of 15 gallons of fuel.
“The fuel line is about 3″ thick, and the fuel pumps are flowing somewhere in the neighborhood of 100 gpm,” says Morrison. “We’re idling between 3.6-3.8 gpm. It’s burning