These days racing formulae tend to tie the hands of the engine builder, but a notable exception is Dirt Late Model (DLM). In this wild world, any 16-valve pushrod V8 goes, provided it follows the pattern of a production original and is fed by a single four-barrel carburettor. There is no limit on displacement, no cap on rpm, and no restriction of compression ratio. These engines can be as large as 470 cu in (7.7 litres) and on racing gasoline can develop more than 900 bhp.
Cornett Racing Engines has evolved the 4.5-in bore spacing Chevrolet for Dirt Late Model racing over many years
The 2023 Dirt Track World Championship race at Eldora Speedway, Ohio, was won by Brandon Sheppard using an engine built by Cornett Racing Engines. RET profiled Jack Cornett’s Big Bore Chevrolet 12 years ago [RET 55 June/July 2011], so how has it evolved in response to the challenge of DLM?
The DLM environment
The Lucas Oil Late Model Dirt Series is still considered the premier DLM national touring series. As they did 12 years ago, the technical regulations for it simply say the engine must be a factory-design based V8 with the cam in the block, two valves and one plug per cylinder, a single distributor or magneto and naturally aspirated using a ‘conventional’ single four-barrel carburettor.
Aftermarket aluminium and steel blocks are permitted. There is no longer a weight penalty for the former, meaning that an aftermarket aluminium block is now de rigueur. However, the rules now state that “any new engine components and/or new engine configuration must be approved by the series director”.
That doesn’t imply that the engines now have to be homologated. Cornett says, “I guess that would happen if somebody brought in some kind of completely different engine. But so long as it was a pushrod engine that fits the rules I don’t think the rule makers would say anything.” As always, no form of electronic engine control is permitted, and the throttle has to be mechanical. Launch and traction control are outlawed, as is data acquisition.
In the past only racing gasoline was permitted but now ‘alcohol’ can also be used. Cornett says, “Nobody who does the tours runs methanol. The first reason is you need twice the amount of fuel in the car to burn off, the second is that methanol is extremely corrosive.”
It is not only the extra starting weight of concern here, the cars are restricted to 35-gallon tanks and even using gasoline that can be marginal for longer races. The World Championship event at Eldora was a 100-lap race on a half-mile track, so a 50-mile race. In winning, Sheppard ran out of fuel just ahead of the flag!
Today’s gasoline is still high-octane unleaded, as used in 2012, and compression ratio remains 14:1 for Cornett’s engines. However, he notes that Sunoco came out this past year with a new fuel called DXP, rated 119 RON and 113 MON. “We tested it and it worked really well,” he says. The alternative is VP Late Model Plus, which is rated 120 RON and 113 MON.
Typically DLM races are held on a half-mile (or less) oval and the condition of the dirt track will vary throughout the event. Given so much regulatory freedom, the engine builder is ultimately constrained in respect of power output only by the amount of grip the tube frame car’s rear tyres have in putting the power to the dirt.
This year the Lucas series regulations dictate a far more limited range of tyre compounds than before, and these are of a different construction. Also new for 2023 is a ‘Droop Rule’, which specifies using a chain to constrain rear body lift. Cornett explains, “Limiting how high the body can get up into the air limits downforce and cuts the traction down.
“When we were on the old tyres, which gave more grip, and without the Droop Rule, we couldn’t build enough power. Our customers could hook up everything we could give them, which is pretty unbelievable.
“But due to the 2023 tyres and the Droop Rule they can’t hook the engine up in the same way. A lot of guys have struggled this year with traction because of these two new rules. I have been asked by a lot of guys to not essentially kill power but to soften the acceleration rate. That makes it easier to hook up off the corner, makes it easier to drive.”
Clearly, in the absence of electronic engine control, this comes down to traditional tuning tools. “Basically, we’re trying to tune the engine via the cams and the porting of the cylinder head and the intake manifold to make it not accelerate quite as hard,” Cornett says.
“Softer acceleration is what we’re after because they can’t hook the power up! “Before this year we were asked all the time to ‘give me more’; ‘give me more acceleration; more power’. But then these two rules show up and now we have to change the configuration. But that’s the way it is. It’s constantly changing.”
Ford and Chevrolet
A key consideration in respect of engine architecture is the weight of the unit. DLM cars are down to 2350 lb [1066 kg]. In view of that and the crucial nature of weight distribution, Small Block engines have long been preferred over Big Blocks. It was way back in 1993 when Cornett started bringing the Small Block Ford into the DLM spotlight alongside the well-established Small Block Chevrolet. Ford Racing and later Dart provided a suitable aluminium block to carry aluminium heads that Yates had developed for NASCAR Cup. For a while, the Cornett DLM Ford had an edge over its sister Chevrolet, but as we noted in RET 55 the Chevrolet caught up again thanks to cylinder head improvements made around 2004.
Then came the Big Bore version of the Chevrolet, as described in RET 55, which exploits an increase in bore spacing from the Small Block’s time-honoured 4.4 in to 4.5 in – crucially without adding to the weight of the engine. This provided scope to gain 10-20 bhp in top-end power over Cornett’s established 4.4 in bore spacing Chevrolet and (4.38 in bore spacing) Ford Small Blocks.
More important, the Big Bore Chevrolet had more substance to its power curve, at no cost in terms of driveability. In RET 55 Cornett said, “I would like to see a 4.5 in Ford aluminium block, but none of the aftermarket manufacturers seem to have any interest at present. A Big Bore Ford would be awesome!”
In the intervening 12 years, Roush Yates has produced such a block, the RY45. “That is basically a [4.5 in bore spacing] Ford FR9 NASCAR engine block they have made in aluminium,” Cornett says. “We have built some of them and some guys are running them today. But in my opinion the RY45 is limited because the only thing available is the NASCAR Cup head. And that’s built to NASCAR rules whereas, as you know, we don’t have any such rules.
“So when you build one of these things it doesn’t really run any better than the old Ford D3 [4.38 in bore spacing] engine. They run about the same, but the RY45 is a lot more expensive to build and it’s a little heavier too. So we kind of gravitated back to the D3, and most of the other guys who build [DLM] Fords are doing that as well.
Jack Cornett prefers to use this All Pro cylinder head with its 10o valve angle and staggered valve layout
“I think the RY45 has potential if a new [DLM-dedicated] cylinder head was developed for it. But that is a lot of cost and nobody seems willing to step forward to do it.”
Engine displacement and performance
The DLM transmission is two-speed and clutched but there is no downshifting on track. “Because the low gear is a real low gear, you only use it in the pits,” Cornett explains. As such, driveability is crucial in DLM, together with the width of the power band. With the cars using a one-to-one gear throughout the lap, power can be required from as low as 2500 rpm on a restart all the way to 8000 rpm and more.
In RET 55 we saw that Cornett was then building either a 454 or a 431 cu in Big Bore Chevrolet (to DLM racer preference) whereas nowadays the most popular customer option is 440 cu in. That marries a 4.25 in bore to a 3.875 in stroke – midway between the 454 and 431 strokes – for 439.8 cu in.
We noted 12 years ago the 454 was giving 860 bhp at 7600 rpm, whereas now, Cornett says, “A new 440 makes, well, 880 is a good average number, at about the same rpm as before – around 7500/7600.”
Cornett continues to use 300M steel rods supplied by Dyer’s
At 680 lb-ft (922 Nm), maximum torque is comparable and, notes Cornett, “We have a very flat torque curve – the 440 has the same torque from around 5000 to 6500 rpm.” In addition, Cornett now offers a 470 cu in version, which marries the 4.250 bore to a 4.125 stroke [for 468.1 cu in].
“That uses a taller block, a longer rod and so forth,” he says. “Different porting, different cams. From the 470 we’ve seen 920 bhp and 720 lb-ft of torque. Peak power speed is a bit lower than for the 440, around 7300-7400 rpm.”
Why, we asked, doesn’t everybody want one of those more powerful engines? “Well, it is back to the Droop Rule and the current tyres; it’s hard to connect it to the ground. There’s really only certain places they will run this big motor now, mostly big racetracks. Places where they run a really low numerically but very high gear. That’s where the big motor works because it has the torque to pull the high gear.
Cornett’s Big Bore lifter, pushrod and rocker stand
“I do have one customer who runs the 470 all the time. But most everybody else has only one of them and will just run it occasionally.”
In respect of the 470 rev limit, Cornett remarks, “It runs best if guys turn it at about 8200-8300 rpm. You can turn it higher but then it’s just making noise. The 440 runs best if you turn it to 8500- 8800 rpm. The 470 needs to be kept in that 8200-8300 rpm range as you’re swinging that big old stroke and you have got a lot more cubic inches to feed. Above that rpm you can’t get enough air in it so the power drops off.”
Engine components
Most current Cornett Big Bore Chevrolets use a Brodix-supplied aluminium block fitted with cast iron liners. Cornett admits the bottom end is little changed from 12 years ago, although the crankshaft now runs in Daido bearings and has eight rather than six counterweights “to eliminate crank flex” in the interest of durability. It is still driven by crank-guided 300 M steel rods.
Cornett’s Big Bore Chevrolet manifold
The piston pin still runs in a bush in the small end of the Dyer’s rod and is still DLC-coated. “DLC coating the pin eliminates a lot of problems,” Cornett says. The piston is still a 2618 aluminium forging, which can be from either Mahle or JE and which carries three rings. Again the ring grooves are not hard anodised. “I feel that is a good solution but it is not necessary for what we’re doing.”
Eight lateral gas ports are still used, while the rings are a different package, still from Total Seal. Rather than a plasma moly-coated steel top ring an uncoated stainless steel top ring is now fitted. That is still not a gapless ring. “We have tested the gapless ring and it works extremely well,” reports Cornett. “But it is more expensive and is very tedious to install. So we just kind of moved away from it for those reasons. We seal up pretty tight with the conventional ring.”
Twelve years ago, the top ring was 1.2 mm in axial height; today’s similarly barrel-faced production is only 1.0 mm tall. The Napier-type cast iron second ring is also now 1.0 mm, whereas the old one was 1.5 mm. At the same time, the three-piece oil control ring has shrunk in height from 3.0 to 2.0 mm. All of these exploit ongoing piston ring technological development in the interest of friction reduction.
The Chevrolet block used by Cornett has a 4.5-in bore spacing
Cornett commented 12 years ago that whereas in NASCAR the engines have canted and splayed valves, he was using a pure wedge head, which he thought was better in the rpm band he was operating in. “These days we are using a 10o All Pro head. It isn’t a canted and splayed valve head but nor is it exactly a wedge head either, as the intake and exhaust valves are not in line with each other. However the valves are both straight up.
“Where the alternative 11o Brodix head is an inline valve wedge head, the All Pro head has staggered valves. I prefer the All Pro with its 10o layout but there are other [DLM] engine builders that are very successful in running the 11o Brodix head, so obviously both heads work.”
The current Cornett DLM engines still use the Chevrolet LS firing order. 12 years ago, Cornett quoted ignition timing of 30-32o. “With the current All Pro cylinder head, we run a bit less; it’s 28 to 30.”
Twelve years ago the camshaft was belt-driven, whereas as standard it is now gear-driven. “At one point we were having extreme trouble getting hold of belt drives,” Cornett says. “So we looked into the gear drive from RCD, who do a lot of stuff for the desert racing guys. It’s worked out very well.”
The lifter is still the Isky EZX, the pushrods are still from Trend, the rockers still T&D. However, notes Cornett, “The steel rocker we used 12 years ago had inside it a roller bearing for the rocker shaft. Now we have a bushing inside the rocker running on a DLC-coated rocker shaft, just like the piston pin set-up, and it literally runs forever.”
The valves are still titanium, but whereas they were previously chromium nitride-coated only on the intake head, now both valves have the coating from tip to foot. Cornett uses a steel intake valve seat and a copper beryllium exhaust. With the All Pro head the valves are 2.250 in intake versus 2.230 in 12 years ago and 1.650 exhaust versus 1.625.
At the same time, there has been an increase in valve lift from intake 0.790 in, exhaust 0.770, to “0.820-0.850 on both intake and exhaust, depending on the engine application.” The steel valve return springs are still made by PSI, now to the specification of supplier JHE. There are still dual springs with interference.
“But the spring we run now compared to back then is much smaller in diameter,” Cornett says. “The diameter is about 1.300 in; back then it was about 1.550 in.” And, we asked, what’s allowed you to get away with that? “Lighter valves. Better cam profiles. Stiffer rockers. Stiffer pushrods. That all allows you to get away with a much smaller amount of spring [force].”
Cornett adds: “there’s been a lot of changes in cam profile design in the last five years. The stuff we have now is a lot better than we had 12 years ago. Daylight to dark.”
Carburation
We saw 12 years ago that Cornett had had a long relationship with Braswell for carburettor supply. Now he works with Winning Edge, which is run by ex- Braswell employee Hank Elizalde. We noted in RET 55 that a recent innovation had been going from two to four circuits. However, notes Cornett, “The four-circuit carburettor for us kind of went away, so we’ve been running the standard two circuit for a lot of years.
“There’s been experimentation with three and four circuits but we haven’t really found anything that suits us yet. “Probably the biggest innovation that’s come through carburettor-wise is the Super Bowl set-up from Willy’s carburettors. We buy that as a kit from Willy’s.
“It does several things. First there’s no float in the float bowl. Second there’s no needle and seat in the float bowl. And third you can do away with the bypass pressure regulator. So you eliminate things that are constant headaches.
Brandon Sheppard has taken recent victories using Cornett Big Bore Chevrolet power (Photo courtesy of Josh James Artwork)
“There are no longer floats to adjust, no needles and seats to stick and go bad and no bypass regulator to stick and go bad. Basically it is a constant-flow setup. You have a barrel valve attached to the primary throttle shaft, so the more you increase the throttle opening the more fuel it lets into the bowl. And then, when you get out of the throttle, the fuel is completely shut off.”
Using the Super Bowl the fuel pump is either belt-driven or if it’s traditional mechanical it has to be capable of constantly supplying fuel at 15-plus psi pressure. Cornett adds, “It’s a system whereby the main jets are completely covered with the same level of fuel 100% of the time. It also eliminates fuel being spat out under deceleration or cornering, which makes the motor stumble. It’s a really neat system. And it works.”
Has the size of the carburettor changed? “I think 12 years ago we were probably running smaller venturi carburettors to get the motor to accelerate harder and restart better. We were willing to give up some top-end horsepower just so it would restart better.”
But now you don’t want that hard acceleration? “No, so we might gravitate towards bigger ones.” In terms of carburettor tuning by the team at the track, that has changed in so far as Cornett has installed a Computech weather station system. “Now, when we dyno an engine, we record its ADR [air density ratio] number. So, we dyno it here according to the weather we have that day, which we record. Then if the customer has the same Computech weather station at the track, they will have that as their baseline.
“Basically, you take the weather station to wherever you’re racing and take an air sample. The weather station then tells you exactly what jet change to make to keep it in the same state of tune, for the weather conditions. You can go anywhere on the planet like that, in any weather, and keep the engine in the exact same tune I gave it to you. “Most all the serious professional teams now have the Computech weather station, and they know how to use it. It’s pretty simple, it takes the guesswork out of changing weather conditions completely.”
Mileage and performance
The Cornett Big Bore Chevrolet runs in the region of 750 miles between rebuilds. Cornett notes that for some of his customers who own just one engine, that can be enough for an entire season of racing.
At the other extreme are professional teams who might do that mileage in one month. “They start racing in New Mexico in January. And they race through to the St Louis Dome indoor event the first week of December, so then there’s just one month off!”
We asked, does engine performance drop off very much between rebuilds? “A lot depends on how well they’ve taken care of it and kept the dirt out of it. If they have taken good care of it then it might lose 25 bhp. But if they haven’t looked after it, it’s gonna lose a lot more. “The main thing is stopping dirt coming in through the breather, and keeping the oil changed regularly and not getting the oil too hot and so forth. Most of them are pretty good about staying on top of it, but not all of them. Some are a lot better than others, maintenance-wise!”
So there you have it. In the absence of electronics these engines are straightforward for a customer to run while providing enviable scope for development from the perspective of the engine builder. Cost remains sensible though.
While over the past 12 years DLM engines haven’t changed fundamentally, the way they have evolved is fascinating. The challenge of DLM is to continue that evolution, not through the application of dollars but through knowledge and expertise.
The Cornett Big Bore Chevrolet has undergone subtle but significant modification over the past 12 years
Datasheet
Cornett Racing Engines’ 440 Big
Bore Chevrolet Dirt Late Model V8
90o V8
4.25 in x 3.875 in = 439.8 cu in
(7.2 litres)
Naturally aspirated
114 RON gasoline fuel
Aluminium block and heads
Iron liners
Five main bearings, plain
Steel crankshaft, four pins
Steel con rods
Light alloy pistons, three rings
Pushrod; gear-driven single camshaft
Two valves/cylinder, one plug
10o valve inclination
2.250 in intake valve, 1.650 exhaust
Analogue electronic ignition
Single carburettor
14:1 compression ratio
Maximum rpm, 8800
Some key suppliers
Block: Brodix
Heads: All Pro
Rocker covers: Ultra Pro Machining
Crankshaft: Bryant
Crankshaft: Callies
Cam cores: LSM
Cam cores: Comp Cams
Cam grinding: Comp Cams
Timing drive: RCD Engineering
Pistons: Mahle
Pistons: JE Pistons
Rings: Total Seal
Piston pins: Trend Performance
Piston pins: PPPC
Con rods: Dyer’s
Bearings: Daido
Lifters: Isky
Pushrods: Trend
Rockers: T&D
Valves: Del West
Valve springs: JHE/PSI
Gaskets: Cometic
Gaskets: Fel-Pro
Gaskets: Superformance
Ignition: MSD
Plugs: NGK
Plugs: Bosch
Plugs: E3
Carburettor: Winning Edge
Water pump: Stewart Components
Oil pump: Barnes
Exhaust:
Beyea Performance Fabrication
Dyno: SuperFlow
Honing machine: Rottler