Some Proof of the Pudding

Written by Don Stewart | 31 March 2008

Those of you who read the “Pony Tricks” column each month have heard me say that I rarely know from month to month where the next topic will come from. It might come from personal experience, an article I’ve read, a conversation, a TV program, or . . . . who knows what. Some months I wonder if I’ll find anything at all to write about. Well, the idea for this month’s pony trick jumped up and hit me right between the eyes. With uncanny timing, two articles in Mustang Monthly meshed perfectly with the “Pony Tricks” articles from February and March. The synergy was amazing. I’m calling this one “Some Proof of the Pudding” and it follows below.

Before we get to the trick, though, I’d like say thank you to the CRMC members who approach me from time-to-time to mention how much they enjoy reading Pony Tricks. Your positive comments motivate me to work hard to come up with new topics so that the series of articles can continue. It’s hard to believe that the column is nearly seven years old but I enjoy writing it so it will continue as long as topics pop into my head. Along those lines and to help keep the ball rolling, I could sure use your help in identifying interesting subjects.

Some of my articles are global in that they cover all Mustangs but mostly I concentrate on first generation cars. I’d be grateful for your help on any year or topic but especially for the Mustang II, the Fox, the SN-95, and the S197. You’re invited to write an article or if you give me enough information, I’ll gladly put it together for you. All I ask is that you check with me first to see if the subject has been covered in an earlier article.

I need to say one more thank you before we get to this month’s trick. Recently, MCA President Bill Johnson was kind enough to send an email applauding some of the technical content in “Pony Tricks.” And then in the February issue of Mustang Times, his “From the Driver’s Seat” column mentioned Carolina Regional Mustang Club with attention drawn to the same technical content. Bill, thank you very much for recognizing CRMC.

You should know that in the same February column, Mr. Johnson also recognized an article from the Mustangs Northwest club newsletter (greater Seattle area). It was about the importance of people in our lives and was very thought provoking, bringing to mind that it’s people who make up Mustang Clubs and the hobby in general. Mustangs are our passion but people are the most important part of the equation. The cars are merely the facilitators. Perhaps that’s the ultimate pony trick of all!

Now for everyone who hung with me through these protracted opening comments, I’ll take the heart off my sleeve, step down from my soapbox, and go directly to the April Pony Trick.

After completing “Pony Tricks” for the February and March issues of Pony Express and purely by a coincidence of timing, I read two articles written by Miles Cook, the Mustang Monthly technical editor. They covered an exhaust upgrade followed by a carburetor upgrade, placing them right on target as proof of the pudding for my two recent columns. Mr. Cook’s articles put actual numbers on the concept because he measured, on a chassis dyno, the step by step effect of the modifications. The car he used was a stock 1967 T5 C-code with 156k miles on the clock. If you’d like to read the Mustang Monthly articles, look for them in the January and February 2008 issues.

You’ll recall from February’s “Pony Tricks” that an exhaust upgrade was mentioned a couple of times as a good way to compliment a 4-barrel conversion. Right on cue, the first Mustang Monthly article covered the installation of shorty headers and a 2¼” dual exhaust in place of the stock cast iron exhaust manifolds and 2” single exhaust. Before installing the exhaust, Cook baselined the T5 and recorded 141 rear wheel horsepower (rwhp) at 3800 RPM with torque of 236 lb-ft at 2200. The exhaust upgrade pushed the numbers respectively to 154 at 4100 and 241 at 2700 while still running the 2-barrel.

The second article described the 4-barrel conversion. Cook wanted to retain a stock appearance so an original cast iron dual plane manifold was used along with an Autolite series 4100, 480 CFM carburetor. The carburetor came from Pony Carburetors so you know it was rebuilt to “better than new” specs. After the installation, the dyno measured 171 rwhp at 4400 RPM and 248 lb-ft of torque at 2800, another nice jump.

If you do the math, the exhaust upgrade bought 13 rwhp and a 5 lb-ft increase in rear wheel torque. The carburetor added another 17 rwhp and 7 lb-ft of torque for impressive total gains of 30 hp and 12 lb-ft. of torque delivered to the ground.

The change in rear wheel power, as impressive as it was, turned out to be the platform for a real surprise. Buckle up and listen to what happened when Cook decided to try a simple air cleaner change. The stock air cleaner was replaced with an open-element design and the car was put back on the dyno. Horsepower at the wheels jumped to 186 at 4700 and torque went up to 258 at 2800. Wow, was that engine ready to breath or what? Overall, rear wheel horsepower compared to baseline had increased by 45 (or 32%) and torque was up by 22 lb-ft (9%). I can only wonder what the numbers would have looked like if the engine had been freshened with a general rebuild.

The 480 CFM carburetor that Cook used was, as you’ll recall, the same size mentioned for an A-code in the February Pony Tricks. And you may also recall from that article that there was a caution against over-carbureting because “bigger is not always better”. As timely proof of that pudding, Pony Carburetors asked Cook to try a 600 CFM Autolite and check the dyno numbers. The results showed a drop of 2 to 3 rwhp across the RPM band when compared to the 480. Case closed and the moral is this: While it may be nice to brag about something like a 750 double pumper, an oversized carburetor actually steals power. That’s because the intake system can’t develop 1.5 inches of vacuum at WOT, resulting in compromised control of the air column.

So there you have it: proof of the pudding. A 4-barrel conversion is well worth considering especially when coupled with an exhaust upgrade and a new air cleaner.

If you look at the table that accompanies this article, you can see the results of the C-code dyno pulls from baseline to end-of-project. The numbers are net, net, meaning they take into account all parasitic losses from devices mounted to the engine (the first net in the equation) plus losses in the drive line (the second net). In other words, they measure the power available where the rubber meets the road. The final numbers are impressive but there’s no immediate way to relate them to — for example — A-code specs or K-code specs as published by Ford in 1967; the Ford specs were gross values for torque and brake horsepower (bhp) measured at the flywheel. But having said that, we can develop a reasonable basis for comparison.

Manufacturers used gross engine ratings prior to 1972 when new regulations required them to switch to net ratings. Gross meant that no parasitic power robbers were mounted to the engine during the dynamometer run, things like the alternator, cooling fan, exhaust manifolds, etc. On the other hand, net values included those devices. Parasitic losses vary from engine to engine but a ballpark estimate would be something like 15%. That means flywheel horsepower for a K-code would drop from Ford’s 271 gross (no parasitic devices) to 230 net (15% loss with parasitic devices in use) and an A-code from 225 gross to 190 net.

Regardless of rating system (gross or net), driveline losses were always present but manufacturers weren’t required to report them. Nonetheless, they existed as rotational and frictional siphons that absorbed power before it reached the wheels. Those losses usually ranged between 12% and 15% for a stick axle car like the Mustang. So if we use 12%, the horsepower reaching the ground in a K-code would be about 200, less if there was a 15% loss. That value is what a chassis dyno would measure. Using the same assumptions, an A-code at 12% loss would make about 165 rwhp.

Now we have numbers that give us an apples to apples basis for comparing Mr. Cook’s C-code rear wheel dyno numbers to a stock A-code and/or a stock K-code. And what we find is that without any internal modifications, the upgraded C-code’s 186 hp falls slightly north of midway between the A-code and the K-code. Quite impressive, I’d say.