The Ultimate Hot Rod? 1957 Kenworth Thunderhead

I’ve seen enough vehicles that it takes a lot to make me go completely slack-jawed. This incredible vehicle, however, succeeded! What you are looking at has parts from several different trucks and some other things tied together with a custom frame, although the builder goes by the highly modified cab, which makes this at least partially a 1957 Kenworth. It’s listed here on craigslist in North Denver, Colorado for $50,000.

Of the pictures in the ad, I think this one expresses the sheer scale of this beast the best. The seller says they haven’t worked on it in at least three years and it’s time to move it along. The frame is totally custom, with the rails being 35 feet long each. The front suspension, which is new, is normally used in motor homes and buses. The rear drive set up is from a 2007 Kenworth (so we have at least 50 years separating the Kenworth parts) and the wheel base ended up being a massive 335 inches!

The engine is a 12V71 V12 Detroit Diesel removed from a stationary generator. On top of the 12 cylinders are two 6-71 superchargers. The custom 2-pass aluminum radiator was sized for 1,400 horsepower. Naturally, there’s a huge Allison transmission installed and a spare one that goes with the vehicle.

I suppose theoretically you could actually use this to pull a trailer, although I’m not sure why anyone would? However, I did fantasize for a moment showing up at the race track with this pulling my also hypothetical race hauler. Wow!

Here’s an artist’s conception of what the Thunderhead (named by the builder’s son for the sound it makes) would look like once completed. If you have a desire to own a truly unique, one of a kind vehicle, I think you have found one that would fit the bill! I only hope I get to see it on the road once it’s finished. Let us know if you are the one that buys it!


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  1. Mark

    Very impressive vehicle to say the least, the sound of it running in an enclosed space has to be deafening! You have to really wonder why the person who is building it wants to sell it? For all the work that had to go into it, it sounds like a very reasonable price however.

  2. DAN


  3. Francisco

    The seller probably realized it’s never going to happen, so he’s trying to recoup some of his investment.

  4. JW

    I can not see any useful purpose for this truck but show, just my honest opinion !!!

  5. JW

    I would rather have my mailman’s wheels.

    Like 1
    • mark

      Looks like what Herman Munster would drive if he worked at the Post Office.

  6. Martin Sparkes

    One of my friends rebuilt a hall scott indistrial engine for fun. It was massive. And pointless. He fired it up a few times and we drank some beer and watched, and it is still in the corner of the shop 30 years later. This is kind of like that.

  7. Dave Wright

    Misleading ad, this is a normally aspirated 12V71 Detroit. The super chargers are what allow the engine to run as a 2 stroke, all they do is scavenge the exhaust gas out of the cylinders eliminating the need for a 4 stroke cycle. They are modified and used as power inhansers in hot rod gas engines but not in this. The horsepower rating is controlled by the size of injectors installed, normally 12V71 generator engine is set to about 250KW @ 1800 RPM, or 450 HP. These engines are built to a TTA specification with twin turbochargers and large injectors mostly for marine use to over 1000 Hp but cooling in a stationary or vehicle application is too limited to really make that spec useful. A few old long hood semis were ordered with 12V71 N (71 cubic inches per cylinder) engines but cooling was always a problem. I love these engines and have many in my workboats, they look impressive and are the largest mono block small bore Detroit engine built. Larger railroad, generator or marine engines with 645 cu inches per cylinder come up to 20 cylinders and make several thousand horsepower. I think this build might have been more fun with a 701 cubic inch GMC V12 gas engine. They like to acellerate better then these diesels that prefer medium constant RPM. A generator engine has a constant speed governor set at 1200 or 1800 RPM so is mostly useless in a vehicle in any event. High RPM on this engine in a vehicle application would be no more than 2500 or so……

    • 68 custom

      agree, with out the turbos this 12/71 is gonna be way down on power. I helped build the US army’s HEWATT which is basically a fire truck with eight wheel steer and drive plus the 8/92 Detroit Diesel and Allison transmission, suckers were loud! would roll over most anything!

      • Dave Wright

        I have owned some USN 36 foot LVCPL boats that the guys had modified to race as Captains gigs. Evidently it is great sport for the ships crews to race other ships with them. I had one with an 8V92 TAA engine with 120 injectors that made close to 1000 hp. It was chromed out and an incredible piece of machinery. The problem was they couldn’t get enough prop under it to take advantage of the HP. There wasn’t enough clearence between the shaft and hull. Fun stuff. Today’s modern diesels are in another universe. My new……10 year old……semi makes 550 HP bone stock and gets 7MPG at 80,000 lbs. it makes power so easily it feels like an electric motor.

    • Jamie Palmer Jamie Staff

      Dave, thanks as usual for your expert opinion and knowledge :-) I appreciate what you add to our community.

      • Jay E.

        Jesse, Posts like that are why we need the thumbs up back. I have nothing to add, but want to thank the poster who contributed so knowledgeably. It is these sort of posts that make this site so interesting!

    • grant

      I came here to say what Dave said, but he did it much more eloquently than I. I see no point to this although it is cool. Not sure why the sleeper cab is included, it would make a cool dragster with a little less length.

    • Myron

      I am not sure that the superchargers are even working. It appears that they are just for show.

      • Dave Wright

        If the engine runs……..they have to work

  8. Grid Member

    Dave: your knowledge of the Detroit is excellent. However, as a mobile marine mechanic, let me pass this on from 50 years’ experience: never underestimate the willpower of a guy with a beer in his hand. One of the books I had published last year (Boneheads and a Bully Pulpit 2) has a true story about where Boneheads come from.My subject certainly would have been waist-deep in the KW if his wife turned her head! Available as an e-book (or paperback) from Amazon.With royalties averaging $4/month, feel free to buy hundreds of copies!

    • DrinkinGasoline

      Cheers !

  9. sunbeamdon

    Mr Grid, if I may be forgiven for a moment of reverence, I shall venture over to Amazon and secure one copy of your book!

    This vehicle is an unbelievable monster, deserving to be showcased at the nearest tractor pull!

    Dave – keep the knowledge flowing – it’s tax time and I need all the smiles I can muster (aka: CPA Employment Act)

  10. sunbeamdon

    OK – Now I done the deed – I bought both versions; enjoy the royalty (I know you’ll just get two sips of a lite beer out of Amazon, but at least it is something!)

  11. Woodie Man

    Slackjawed! Over the build AND Dave Wright’s knowledge! Hell I’ve forgotten less than he wrote! Just amazing.

  12. Grid Member

    Thanks Sunbeamdon! I’ve enjoyed watching folks think before they do, in fear of seeing themselves in the next Boneheads! Just ordered a lite from Amazon; should be here Wednesday, foaming.

  13. Howard A Member

    Calm down now. Not sure what the intention was with this. I hope they weren’t planning on pulling a wagon with that 5th wheel setup. Very little makes sense here, from a class 8 tractor point of view, much less legality. The “Buzzin’ Dozen” may look impressive, but like 68 sez, without at least one (big) turbo, gonna be more noise than anything. As most know, don’t care for Detroit’s. Takes 12 cylinders to match the power of 6 in a Cat, and even then. Looks like a total flop. Here’s what they butchered.

    • 68 custom

      while I wish it was me it was in fact Dave that first pointed out that the 12/71 was sans turbos. the HEWATTs I helped build were equipped with the 8/92s straight from Detroit Diesel with turbos and the smallest off road exhaust ever! at idle those things carried almost no oil pressure!

    • Dave Wright

      Howard, the first real Cat truck motor came in the late 60’s in the form of the old 425 hp 1693. It was a modified dozer engine originally called the 343. The 71 series Detroits were designed and built in the late 30’s when most Cats had under 50 hp. The first over 400 hp small bore Detroits came in 1957 over 10 years before the first Cat truck motors. Detroit was probably the most common engine used by our military in WW2 powering most boats 200 feet and smaller, trucks, pumps and generators. You can rebuild a Detroit for 30% the cost of a Cat, they are easier to start, 40% fewer moving parts. They were the engineering masterpiece of the time. The early Cat truck engines had more low end torque making it a favorite of lazy drivers that didn’t like to shift, the 70s and 80s 8V92 TA engines I ran in my truck company were rated at 400-475 hp and were indestructible by the worst drivers, Cats would pull sleeves quickly when driven in properly requiring a major tear down and huge expense.. The next common Cat was a 3406 that had a long manufacturing run and was improved over time but still, Detroit is the biggest selling heavy truck engine on the planet. There were other Cats in heavy trucks like the big 3408 V8′ but they built them for such a short time they were unable to refine the quirks out of them.

      • Howard A Member

        Hi Dave, we’ll never agree on this. Clearly,, your experience with Detroit’s, or “Rocky Mountain Hummingbirds”, differs greatly from mine.( we had a bunch of names for these, mostly derogatory) We never had 400 hp Detroits. The in-line 6-71 and 8V-71, 238 hp and 318 hp, respectively, were the engines I’m familiar with. They had a hard time hitting the redline on a “dry wind”, much less trying to pull 40K lbs. And I drove many dump trucks in the 70’s and 80’s, Detroit’s were very popular BECAUSE they were cheap to rebuild and they just couldn’t hold up, mostly because they were driven too hard. Many of the “fuel converters” I drove, ( converts diesel fuel into noise) unless you held “the needle against the pin”, it would drop off pretty fast, sometimes requiring skipping a gear on a downshift. I can’t argue these engines were great for boats or stationary equipment, but for the road, give me a Cummins or Cat ( or Series 60 Detroit) anyday. You seem to be adamant about how nice these motors were, I hope you paid your drivers by the hour. :)

  14. RJ

    Cool in the front. From the back it’s blah. In my opinion it is too long from the cab on back. Would take creative planning to turn it on a city block.

  15. CJ

    Two words to describe: BAD ASS!!!!!

  16. DrinkinGasoline

    Screams Jay Leno….I look forward to the YouTube update.

  17. bill

    hit one small bump and that frame is going to snap like a twig.

  18. DrinkinGasoline

    At Our age…..would any of us take this seriously ? C’mon…
    Really ?

    • DrinkinGasoline

      Resistance is futile. There will come a time when You will be forced to relent.

  19. dogwater

    Hey People,
    Can we make your comments a little shorter it like reading the bible

    • Jason

      Bye, troll!

  20. David J David J

    This belongs on a “Mad Max” set, for sure.

  21. JimmyinTEXAS

    “The frame is totally custom, with the rails being 35 feet long each.” I suppose that would be better than one 35 feet long and the other something else….lol, sorry, I’ll go back to sleep now..

  22. G.P. Member

    It’s fun to read about all the knowledge that people at barn finds have. Doesn’t matter what the subject is, the year it is, or what kind of vehicle it’s about.

  23. Dairymen

    A cool build but it couldn’t drag a turd out of a litter box!

  24. JW454

    I wonder what it’s turning circle is? I guess this is what the Willis Brothers were talking about regarding that 40 acres thing.

  25. Joe Haska

    It seems I am always on the wrong side of the comments. I know about this build , and even saw it when it was started. I think two things happen when you see something like this, one being getting all hung up on why its a waste of time and it will never run and its going to break in two if you even try to drive it!
    The other side comes along (like me), and goes WOW, how creative is that, I don’t see the mechanical problems, I see a rolling piece of art, and a very creative person, who is thinking and dreaming way outside of the box. Its a statement of wouldn’t this be cool, not this is the most state of the art form of transportation you will ever find. Its done to be fun creative and make people think, I could have never thought of that.

    • Howard A Member

      Hi Joe, I don’t think anyone here will argue with you how creative it is. It certainly is. My issue with it, is it’s done to excess ( more must be better) I see someone, whatever the motivation, had access to a nice shop and all the fixin’s ( and money) to put this together, and kind of found out this may or may not work, and we’re not talking about a go-kart here, or even a pickup. This is all heavy duty expensive stuff here and they’re asking what I could buy a small house for in N. Wis. and it doesn’t even work yet. In a cost no object situation, I’d go for plausible results, and maybe even haul a load with it, but this is just too much.

      • Nova Scotian

        …You can buy a small house for $50,000.00 ? Is that plumbing excluded? Or windows optional? Lol

      • Doug Towsley

        Nova scotian, Yes, you CAN buy a small house for $50k. It just depends on ……………..sorry, but Location location location.

        We built a small house/cabin up in the hills with a lifetime 99 year lease for $30k. Looking to retire there. We looked at a lot of places. Here in Portland Oregon, Seattle or the Bay Area Calif prices are going up, up, up… But out in the boonies prices are low. If there is no work in the area or a means for income it really puts a damper on sales prices. Do some research.

  26. BradL

    I wouldn’t try to cross any railroad tracks in that rig.

  27. Stuckin2nd

    Yea, it looks like it belongs on the sequel to Mad Max, Fury Road. You can just see it being surrounded and chased down.

  28. Doug Towsley

    Well its interesting, that I can say,, but the high point of this posting was Daves knowledge being shared, and the ADD/ADHD weirdo complaining too much info hurts his pea brain and is a pain to scroll on his crackberry can go work the wash rack until he learns some manners/

  29. John P

    Cool build–well thought out and engineered.. I doubt any of the concerns are truly valid which were tirelessly typed above here.. This is one persons dream which for whatever reason is being abandoned.
    I’d like to see this one finished-that’s for certain.

  30. Mark S

    I’m with Howard what a waist of a vintage kenworth. This is even more useless than an old fire truck and storage is always going to be a problem. I say the seller would do better to part it out. This is a classic example of just because you can doesn’t me you should. Just plain silly!

  31. Russell

    I love it! I would buy it, but it doesn’t have parking assist and trying to navigate that into a tight space would be a challenge.

  32. Grid Member

    At 72, with some past regrets, I say “If ya wanna, and ya can, ya oughta!”

    • Jim M

      Right on!

  33. Art M.

    I admire the ingenuity and I like it. I also appreciate the comments Dave made, a lot of knowledge. I learned a lot about Detroit engines that I did not know. Thanks Dave.

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