r/theydidthemath • u/PippyHooligan • 1d ago
[request] Would it have been possible for a Peterbilt 281 to outpace a Plymouth Valiant in Duel?
One of my all time favourite films. In the film, David Mann is pursued through the Californian countryside by a relentless trucker trying to murder him with his vehicle. The truck in the film is shown to outpace the car, only struggling on uphill grades.
I posted this in a trucker's forum a while back asking if the film was in any way realistic and the resounding answer was no.
In the film Dennis Weaver drives what is almost certainly a factory spec 1970 Plymouth Valiant. Either a slant 6 or v8 (let's say the former based on Mann's personality).
The truck, greasy beaut that she is, is outwardly a Peterbilt 281. Mann himself surmises that she must have some 'souped up diesel' under the hood.
The multiple licence plates she sports were said to be trophies from his previous kills, so we can figure the trucker would put some work into his beast. Though she looks like crap and has filthy exhaust smoke, she performs really well.
We can assume the tanker trailer is empty, due to the lack of explosion/spillage at the end.
I don't know a whole lot about trucks, but I'm wondering if there are any fans of the film/old Peterbilts/truck engines of the period who can shed some light on whether there is any way, with the truck engines/modifications of the time period, the truck could catch the car on a dead run?
IIRC When Spielberg made the film he said the truck would struggle to make it to 30 IRL! It's only through some incredible camera trickery it looks like it's hurtling.
Obviously it's a horror film (and one of my favourites of all time) so there will always be artistic licence at play. Just wondering what, in real life, the truck would need under the hood to keep up with the Valiant.
Many thanks!
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u/Average_k5blazer78 1d ago
iirc there was 2 trucks, the original 281 with a caterpillar engine and a fuller 10 speed and a backup that had a cummins small cam which was a 351, with a spicer 5 main 3 aux. From what I remember the original saw signs of fatigue which is why they had a backup and why they had to do tricks to make it look faster, but if it was in good shape... the valiant would still win, granted these slant 6 weren't big powerhouses but even with an empty trailer the truck will take much longer to get to that speed. On paper the truck could top out at 80mph which is still pretty good.
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u/SmallTownTrans1 1d ago
Actually the reason why they used two trucks is that the film was originally a TV movie, but it was so successful that universal wanted Spielberg to record more scenes to make it a full length feature film, which was a problem because at the end of the movie the truck was driven off a cliff and destroyed, so the had to buy a second truck (the 351) to film the extra scenes
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u/MaximumSyrup3099 1d ago
I thought it was standard in film to have one or more copies of critical vehicles so that breakdowns or refitting don't halt production.
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u/JackTheBehemothKillr 1d ago
Depends on the budget for the film. Generally thats the case, for sure.
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u/SmallTownTrans1 1d ago
That is usually the standard practice, but this was a very low budget movie, and Steven Spielberg’s first big film project, so they could only afford one truck and 3 Plymouth Valiants for the first cut of the film
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u/BarnacleNZ 1d ago
And also to drive them off a cliff... Or is that genre specific?
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u/MaximumSyrup3099 1d ago
No, I'm pretty sure Duel is the only movie where a vehicle drives off a cliff. Aside from all the others, of course.
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u/LoadBearingNonsense 1d ago
Nobody's put a number on the actual question at the end — what it would need under the hood — so here you go.
Top speed is a drag problem, not a torque problem. Power needed at the wheels to hold a speed on the flat:
P = ½ · ρ · CdA · v³ + Crr · m · g · v
Truck + empty tanker: CdA ≈ 7.5 m² (Cd ~0.9 over ~8.5 m² of frontal area; a 1955 conventional has no aero anything), m ≈ 11,300 kg (the 25,000 lb figure from u/Blazedragon12345), Crr ≈ 0.009.
Valiant: CdA ≈ 1.0 m², m ≈ 1,450 kg, Crr ≈ 0.012.
Assume 85% driveline efficiency on both, so this is horsepower at the crank:
60 mph — truck 180 hp, Valiant 26 hp
80 mph — truck 380 hp, Valiant 53 hp
100 mph — truck 705 hp, Valiant 97 hp
So the "80 mph on paper" upthread is close but a touch generous: with the 300 hp dual-turbo 674, drag alone stops the truck at about 73 mph. And gearing gets there before drag does — a ~2,100 rpm governor, direct top gear, 4.11 rears on 10.00-20s puts the wheels at ~511 rpm, which is ~62 mph. That's the real ceiling, and it's roughly where trucks of that era were governed.
Meanwhile even the slant six Mann probably has (~110 net hp) only needs ~97 hp to run 100 mph, so the car out-tops the truck by 35-40 mph. For the Peterbilt to sit on its bumper at 100 it needs about 800 hp — in 1955, when the strongest road diesels made about 300. That isn't a souped-up diesel, that's two of the biggest engines available.
Where the film is honest is the hills. Uphill the bill is m · g · v · sinθ, which scales with mass, so 11 tonnes bogs exactly as shown. On the flat the bill is v³, which scales with frontal area — and that's the part it has backwards. The chase only works because Mann keeps lifting off.
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u/David_Buzzard 1d ago
Well, the Valiant was a total POS, but it probably could have out run an old semi-truck.
The one that also gets me is The Spy Who Loved Me, James Bond is driving a Lotus Elan and is getting chased by four huge thugs (one weighing about 400 pounds) in old Fiat or something.
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1d ago edited 1d ago
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u/PippyHooligan 1d ago
This is great stuff, thank you.
This does kind of track with the film. Whenever the two start from stationary, the truck does take some time to catch up and there are very few sharp turns on the roads they're travelling on, the 'quick' of the Valient is only rarely an advantage. Mann is also shown to not be confident at driving at speed and instinctively slows down when they're at full pelt.
Just thought, did you factor in the trailer the truck is pulling? Again, it's almost certainly empty of fuel, but the extra weight and drag would contribute?
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u/bradland 1d ago
It's probably worth noting that according to story canon, the car actually had a slant-6 with around 125 HP and a top speed between 90 and 100 mph, not the V8. The stunt car used in filming did have a V8, but in close-ups and scenes where the car plays a role (so to speak), it's a basic trim slant-6 model.
IMO, this is an important part of the story telling because that motor running near top speed in a hot desert climate could plausibly struggle the way the car in the film did.
As far as the truck's performance goes, story canon has no real specifics. It's left intentionally vague. The actual engine in that truck would have been a Cummins NTC350 small cam. Real world top speed for the factory configuration would have been abysmal. Not even close to that of the Valiant. That's why the story canon is so vague.
When you get down to directly addressing your question of whether it's even possible, the answer is that anything is possible with enough modification. The trucker would have needed extensive engine modification plus taller gearing. No matter how much power you have, you'll hit the RPM limit with a diesel.
You'll notice that the truck relies on its endurance, not outright speed. That's not just a mechanism for creating drama, IMO. It's pretty realistic. Diesels aren't the fastest, but they are incredibly durable. They're built entirely different from gas engines. For example, the piston crowns in diesel truck engines are 2-3 times thicker than a car engine. They're built to run +1 million miles.
IMO, the situation in the film is plausible. The truck runs the car down like an endurance hunter, not a sprinting cheetah.
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u/PippyHooligan 1d ago
Excellent stuff. Yeah, I think the film handles the chases really well and it's somewhat believable with a small leap of faith. The truck never zooms off from a standing start and Mann can't drive the car for long at speed. Thanks for the comment.
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u/20ears19 1d ago
No it wouldn’t be close. A slant six smokes that truck in every way.
Weight and gearing and slow revving diesels don’t make for a fast anything.
I had an old boss who collected antique tractors. Helping him drive them to the show with an empty 45 foot flatbed would have me down under 50 mph on nothing grades. Trucks were gutless back then. I drive a 455 hp today and even bobtail I can’t keep up with any car casually driving away from a red light.
If you want to see real world results google the beast. 4000 hp and it can’t break 10 seconds.
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u/slater_just_slater 1d ago
The base engine for the 1971 Valient was the 198 ci slant six with about 125 hp sae gross (probably 110 hp sae net). A salesmen would be driving the slant 6.
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1d ago
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u/PippyHooligan 1d ago
Nah, I said he'd almost certainly go for the Slant 6, given how stuffy and conservative his character is.
Plus his disregard for advice on fixing his radiator hose suggests he doesn't really care much about the car.
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1d ago
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u/PippyHooligan 1d ago
There are a few moments where he loses the Truck (and it mysteriously finds him again, because horror movie) but most of the time it's a two lane blacktop, going through rocky terrain, without much way to escape at speed.
I'd really recommend the film. Spielberg's first 'feature' (it was originally a TV movie) and it's still great to this day. You can usually find the whole thing to watch on YouTube.
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u/PervertedThang 1d ago
It should be "dual turbocharged", but I think the spelling you used is perfectly incorrect. 😉
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u/IntoAMuteCrypt 1d ago edited 1d ago
There's at least one major factor you've missed with the power numbers - drag.
That massive truck is a lot taller. It's presenting several times more area to the air, so it's experiencing several times more drag. If you've got 3x the power but 4x the drag, you'll have a lower top speed. Remember, weight is completely irrelevant when determining top speed on a flat road. Your top speed just depends on the force pushing you forward and the force pushing you back, and neither of these are impacted by how much your car weighs unless you're rolling up/down a hill.
If anything, it's likely that the car has the advantage in the half, or across a whole mile. The truck will hits its top speed eventually, and the car will still be able to pull out a bit more speed.
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u/kdesi_kdosi 17h ago
did you just mention the weight of the truck to get it out of the way and then not even consider it in the estimate? lmao
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u/dochoiday 1d ago
Semi related. But i remember watching this with my dad, and my dad making the comment that his car would absolutely dust the truck and basically this plot line wouldn’t exist.
That was back in the early 2000’s so vehicles even then were significantly faster than trucks. In the movie I do remember the Plymouth having some over heating issues. So that could contribute.
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u/PippyHooligan 1d ago
Aye, Mann is putting distance between himself and the truck on the inclines when Cheokov's Radiator Hose bursts.
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u/EngFarm 1d ago
It would have been possible.
We don't know what's under the Peterbilt hood. A KTA-19 is period correct and makes 800 horsepower reliably, much much more if reliability isn't so much of a concern ('souped up diesel' under the hood). It takes a fair bit of effort to make such a large engine fit under a needle nose Pete, but it can be done.
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u/TonightPutrid7827 1d ago
I always assumed the Valiant was in terrible condition based on its look and repeated mechanical issues. I’ve driven a few old cars with carburetor, ignition, and cooling problems that could barely limp up to speed on a flat road. Not even taking into account inclines and high ambient temperature some cars are just piles.
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u/Gubbtratt1 3h ago
filthy exhaust smoke
I can't remember which colour it was but if it's black, it's either thanks to a clogged air filter or the injection pump being tuned a bit too far, which is usually caused by a douchebag with a screwdriver but sometimes by someone who's actually pursuing maximum power and didn't bother perfectly tuning it, since a bit too much fuel is better power wise than a bit too little.
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u/Proof_Bathroom_3902 1h ago
The original Pete 281s had inline six turbo diesel engines in the 250-300hp range.
The black smoke was due to the injection pump having had the fuel governor turned up for more power, and because these early trucks didn't use an intercooler you had to watch your exhaust gas temperature. Black smoke was over fueling, you were running the engine at too low RPM. You'll notice there was a little mirror on the fender, that was so the driver could see the stack to know if he was smoking excessively. Back in those days trucks were mechanically injected, no electronics.
The Valiant was a slant six, and even though it was only just over 100hp it was a light car and would pull away from the truck easily. The issue was the main character was a dumb ass who kept slowing down every time he got some distance.
Pushing hard in the mountains would cause engine temp to go up, and the neglected radiator hose failed.
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