Jim G’s SLURP Project – Posting 10: DYNO charts, and I need YOUR suggestions!
This posting is one in a series. If you have not yet read the earlier postings in the series, you’ll need to, as this posting won’t make much sense without having done so! Just do a search using the word “SLURP”.
In my last posting, I described how Red’s actual road weight is, after 53 years, almost exactly what GM claimed for him as manufactured.
In this posting, we are going to take a brief, but very critical look at the actual GM-published dyno charts for 1950s era trucks. Then, we’ll see how Red’s actual dyno chart compares to the factory numbers. Then I’m going to ask for YOUR help.
I have to warn you: you’ll be surprised just how modest the actual power output of 1950s trucks was when viewed in the cold hard reality of a dyno chart. Back then, pickup trucks weren’t built to accelerate impressively. They were built to work, and to work mostly at low speeds. Truck drivers only cruised at 60 mph in their dreams; most cruising was done at 45 mph or less, as the road infrastructure was simply not good enough to support higher sustained speeds. In addition, our forefathers were practical people, and the power curves of our forefathers were weighted towards high torque directly off idle, so that trucks carrying heavy farm loads, on unimproved ground, could get underway even uphill, from a standing idle start.
With that warning, let’s take a look at the first dyno chart below, which compares a sampling of the most prevalent engines found in 1950s Chevy trucks:
The 1948 216 cubic inch inline 6 used when the Advance-Design series was introduced
The 1954 235 cubic inch inline 6 that was standard equipment for the last couple years of AD series truck production. Note that this engine, souped up via a 3-carburetor intake, dual exhaust, and cam change to 150 gross hp, and renamed “Blue Flame”, also powered the 1953 and 1954 Corvette!
The 261 cubic inch inline 6 used from around 1954 to 1962 in heavier model Chevy trucks (not in pickups), and that everyone who has an AD pickup with 216 or 235 engine lusts after (a 261 bolts right in – if you can find one in decent or rebuildable condition)
The 302 cubic inch GMC inline 6 used in heavier model GMC trucks (not in pickups). I am including this one to show you how little difference there was between this “big truck” engine and the 261.
Note that Chevy did not offer a V8 in its pickups until 1955 2nd edition (the new Task Force series). That was also when the Corvette got the V8.
Take a good look at the first dyno chart with the “crank horsepower” power curves. Note:
- The STRONGEST engine of the 4 makes only 150 crank hp (unrealistic gross crank hp in fact, by today’s standards), and the weakest makes only 90
- 3 of these engines make their peak power by 3500 rpm, and only the “hotrod” 261 peaks higher (at 4000 rpm)
- The graph does not show it directly, but if you run the numbers using the fixed mathematical relationship between horsepower and torque, all these engines make their peak torque by 2000 rpm
- All these engines make incredibly strong torque right from idle speed – typically at least 80% of peak torque even at 500 rpm
- The hotrod 261 makes 60 more peak hp than the 216, and at higher rpm the gap between them hits 70 hp! These two engines are WORLDS apart
- The 261 is also very much stronger than the 235 – much more than the 11% displacement difference would suggest
- The 302 GMC is stronger than the 261 by a few horsepower up to about 3250, but then runs out of air while the 261 keeps going. Hotrodders assuming that the GMC offers much more power than the 261, because it is 16% larger in displacement, take note
- Note that all of these engines, in their original factory usage, run much higher engine rpm at 60 mph than modern engines do. For example, on the AD pickup trucks, 2700 to 2850 rpm is typical at 60 mph, with either manual or automatic transmission. This puts the engine in the “meat” of its powerband at highway speed
http://i252.photobucket.com/albums/hh16/JimGnitecki/Stoveboltdynochart-crankhp.jpgNow take a look at rear wheel versus crank horsepower. Again, you might be shocked at how much lower these numbers are than in the “crank hp” chart. Chevy published detailed dyno charts (which I used to construct these composite charts), and included “crank” power curves (engine with NO accessories or exhaust restriction), and also “net” power curves (engine with standard accessories and simulated vehicle exhaust). What they failed to mention, except in section 10 page 7 of the “Chevrolet Truck Data Book”, subtitled “Specifications Catalog for Chevrolet Salesmen”, was that there was an additional 10% loss going through the transmission and rear axle – and that was the BEST case, where a manual versus automatic transmission was used.
Furthermore, the 10% drivetrain loss applies only to top gear where the drive through the manual transmission is 1 to 1. In 1st through 3rd gears, Chevy told the salesmen to use 15% loss.
With an automatic, the loss is of course far greater. On a modern automatic, the additional loss is about 9 to 10%, but that is with a transmission that has a modern torque converter. The early 1950s Hydra-Matic, like in Red, has far greater losses, due to a simple fluid coupling versus torque converter, and other shortfalls compared to a modern automatic.
This next chart shows the rear wheel power actually delivered, in top gear, with a manual transmission. Note that:
- NONE of the engines actually delivers more than 120 rear wheel horsepower, even with a manual transmission, and even in top gear. The 216 barely breaks 70 rwhp.
- The GMC 302 has proportionately less loss via accessories and drivetrain than the 261. I don’t know why this is so
http://i252.photobucket.com/albums/hh16/JimGnitecki/Stoveboltdynochart-rwhp.jpgAnd, here is the exciting next chart – the one showing Red’s actual rear wheel power curve superimposed in bright red onto the chart.
Your first question might reasonably be, “Hey, I thought Red has some “goodies”, like the dual Carter Offenhauser intake, the split Fenton exhaust manifold, true dual exhaust, and an aftermarket cam (The Bulldog cam). Why is he barely beating out the stock 261, and only in a narrow rpm range?”
The answer is simple: First, his cam was selected by his previous owner for its torque, not its peak power (although the torque peak is at a “hotrod” 2700 rpm or so!). More importantly, Red has that early “fluid coupling” automatic transmission, that is costing him at LEAST an extra 10% driveline loss, probably more. In fact, GM proactively upped the power of its engines in automatic-equipped cars in the early 1950s for precisely this stated reason.
There are also two more reasons, which we’ll discuss below.
Note that even with the penalty of the automatic and a cam that peaks 500 rpm lower, Red is still the power king in this humorous comparison. Sort of like saying that Dustin Hoffman is taller than Tom Cruise.
However, when we look at the power curve for a modern Chevy SSR – even one with the weaker 300 crank horsepower engine versus the 400 hp LS2 engine – even AD superstar Red starts to look really pale. See how that purple SSR curve absolutely dwarfs every other curve on this chart?
The numbers beside Red’s curve and the SSR’s curve are the APPROXIMATE rear wheel horsepower at 60 mph (The software would not let me select precise points that were not included in the original data array used to build the graph).
http://i252.photobucket.com/albums/hh16/JimGnitecki/Stoveboltdynochart-rwhpplusRedplusS.jpgAnd, next in all its glory, is Red’s individual dyno chart, complete with air-fuel ratio readings.
You can see that Red runs lean at full throttle. That 14 to 1 air-fuel ratio at full throttle should be more like 13 to 1 for best power and a more cool running engine. I’ll need to find a senior mechanic who remembers how to change the main jets in these Carter Webers and hasn’t yet retired . . .
I should also point out that the testing was done on a dyno that consistently tends to yield conservative numbers. For example, that dyno believes that the rear wheel horsepower of a 6-speed Corvette with LS7 427 cubic inch engine is only 407 rwhp STD or 400 SAE (the two standards differ due to different “standard” ambient condition assumptions), out of an engine that delivers 505 crank horsepower and has a manual transmission.
Disregard the text on the graph that says maximum torque was 203 rear wheel ft. lb. That was just the fluid coupling spiking when Daniel the dyno operator hit the gas. The real peak was 195 ft.lb. at the rear wheels, at a surprisingly high 2700 rpm.
Peak power was at 3700 rpm, but stays above 100 rwhp even at 4500 rpm, where we terminated the testing out of respect for the 53 year old drivetrain. This explains why Red can peg his 90 mph speedometer, while AD trucks with less generously endowed engines struggle at highway speeds.
http://i252.photobucket.com/albums/hh16/JimGnitecki/Reddynorun2008-01-09run2.jpgOk, so with results so underwhelming, why do these Advance-Design series trucks NOT feel really weak on the road?
The answer is in the shape of the power curves and in the gearing the designers specified.
The low rpm biased torque curve, with strong torque right off idle ensures that these trucks always have enough grunt to handle the load. The steep gearing – typically close to or even exceeding 3000 rpm at 60 mph – ensures that the trucks are right in the meat of the power curve at moderate highway speeds. In fact, Red runs 2825 rpm at 60 mph, and you can see he is making close to 111 hp at the rear wheels at that speed despite the automatic tranny.
To put that in perspective, ANY stock Chevrolet SSR – whether the 300 hp 03/04 version, or the 400 hp 05/06 version, with either manual or automatic transmission, and any recent Corvette, delivers far less than this kind of rear wheel power at 60 mph. The 300 hp 03/04 SSR version in fact delivers, as shown in the graph, only 64 rwhp at 60 mph (actually a bit less, as this 64 hp is at 2000 rpm, and the gearing results in only 1800 rpm at 60 mph). This is because the modern vehicles run very low engine rpm at 60 mph in a misguided CAFÉ-driven effort to squeeze out the best fuel mileage possible, even though the loss of mileage with 10 or 20% more rpm would be very modest (I have run the tests and have the data).
Red’s strong highway performance is evident on hilly 70 mph Highway 71 that connects our home in Spicewood with the city of Austin, Texas. There, Red is pretty oblivious to the steep hills. He NEVER downshifts from 4th to 3rd, even on the steepest hill where semi-trailer trucks crawl.
Keep in mind the historical perspective too. Remember, a 1953 or 1954 Corvette had a 150 (gross) hp version of the 235 cubic inch engine covered above, used a Powerglide automatic transmission, and was only 10% lighter (truly!) than a stock AD truck of the same model years. The Powermatic was a TWO-speed automatic, so it is conceivable that an enterprising hotrodder who installed a 261 in his AD pickup truck and kept the manual transmission, could have provided a heck of race to a Corvette owner!
Of course, the flip side of all this is that with his current powertrain, Red can’t really accelerate “briskly”. A weight to power ratio of 29 pounds per horsepower isn’t exactly a prescription for “lively” acceleration. In fact, I’m pretty sure that if I ever race my wife in her Honda Civic, she’ll win. 
Of course, we intend to fix that . . . via a new powertrain.
And here is where I need YOUR help. I need suggestions on what that new powertrain should be.
Now, every one of us is an individual, and places different emphasis on different factors when choosing a powertrain. In my case, I want to (a) honor the SLURP project parameters, and (b) I want to show some respect for both American automotive history and Red’s history. By (b), I mean try to come up with a powertrain that at least tips its hat to the approach that a 1950s or 1960s hotrodder might have taken, rather than simply saying “pick the first solution that looks like it will work regardless of how modern it might be”. A simple example of this is that we should prefer carburetion versus electronic fuel injection, and a cold air intake would look way wrong.
So, here are the ground preferences I’m starting with (subject to change of course if there are compelling reasons!):
- Simple and practical versus complex, with minimized maintenance and minimized opportunities for parts failure
- Lightweight within reason – a lighter powertrain not only reduces overall vehicle weight, but also would improve the current skewed weight distribution between front and rear
- NOT pretentious and not “bling”
- Retro superficial appearance within reason
- Carburetion would be preferred versus injection
- Automatic transmission for three reasons: simplifies the installation engineering, allows me to keep the priceless factory column shift (I just love it!), and enables seating 3 versus 2
- American engine and transmission – foreign is just plain wrong for this truck and this project
- GM versus Ford or Chrysler if possible, just because it’s sort of wrong to put a Ford or Chrysler into a Chevy!
- I’ve already determined via computer simulation that we need at LEAST 300 crank horsepower to attain the performance needed to qualify as a “street rod” in a modern street environment, even with Red’s low vehicle weight
- Reasonably small physical engine size, since I am trying to keep the original front crossmember, beam axle, steering setup, undented firewall, etc.
- Reasonably practical and minimized cost installation effort
- Choice of automatic transmission depends to an extent on power curve of the engine: overdrive (e.g. 700R4) if a “low revver”, and non-overdrive (e.g. Turbo Hydramatic) if a “high revver”, in order to prevent top gear “feebleness” at 60 mph
- MUST be suitable as a “daily driver”!!
- NO provisions needed for power brakes or power steering (both will remain manual), but we need to support air conditioning (hey, it’s Texas!)
- We need either more displacement or more rpm to make more power, and since displacement is cheaper than high rpm capability, higher displacement normally has a cost advantage. BUT the package must FIT into the vehicle honoring the above restrictions on size and ease of installation
- An engine type readily available in either salvage yards or in the used market is a big plus, since cost would be appreciably lower than an engine bought brand new, or one that needs to be specially built up. Budget constraints make a brand new engine a less attractive option
- Assume that Red will end up weighing about 3000 pounds or less. I say this with confidence, because I’ve already found 340 of the 490 pounds of reduction needed to hit 3000 pounds
Now, keep in mind that we don’t literally have to meet ALL the above preferences. We would like to meet as many as PRACTICAL, while keeping both cost and ease of implementation reasonable.
What do you suggest? Let me hear your ideas!
Jim G
p.s. Yes, I realize it is a pain to look at the analysis text while having to separately go to the graphs at Photobucket and print them out. This website's policy prohibits use of embedded graphics for bandwidth reasons. I need to abide by that limitation. jg
