Our cars, usually with modified engines, present a minor problem for me, and maybe you. Just how much horsepower does it have? I’ve been asked this question a few times. Not nearly so much as the “Is it real?” question. In fact most people just want to pay me compliments on the car. Yet the Hp question has been asked.

I usually answer “I don’t really know, but it compares nicely with one of the 425 Hp versions”. I actually thought it did, but have no idea any more where I saw specs that made me think that. Anyway, it makes them happy because it meets that “it’s easy to 1 Hp per cubic inch” philosophy that is quite prevalent.

Actually, David Vizard in his book “How to Build Horsepower, Vol. 1”, states that one should aim for anywhere from 0.8 to 1.0 Hp per cubic inch as a realistic goal for a street engine.

My engine is actually an over-bored 410 Ford FE with a total displacement of 418 cubic inches, so I guess I should be happy with anything from 334 to 418.

Quite a while back, someone here started a thread about how much Hp one of our cars should have. Thinking I had something close to 425 Hp and knowing I can’t use that much power I recommended about 350 Hp as a good goal to achieve. Turns out I might have been more correct than I thought, but not quite for that reason.

Although Ford’s rating of the 428 is probably very much underrated at only 335, it has been said that only a very skilled driver could tell the difference between the 428 and 427(425 Hp the most common) by driving them. I suspect that person is correct, but I’m getting ahead of myself.

The question of how much Hp my engine can produce has occupied my thoughts many times over the years. In fact back in the late 1990’s I bought a G-Tech meter, a small device that measures, amongst other things, an engine’s horsepower. You input the weight of your car with driver and fuel, then accelerate and the little box does the rest based on readings from its internal accelerometer.



The new ones use GPS and can actually plot Hp and torque curves. Very nice, but possibly useless for our cars as a full throttle run might not be possible depending on what gear you're in.

Although I played with my G-Tech on other cars, I never tried it on the Cobra as I didn’t have a cigarette lighter socket to plug it into. So I bought one and let it hang down under the dash after I fed the wires through the firewall.

Time for the test. This was last October and we had quite the cold snap, so I waited for a warm day and finally was rewarded with one hitting a whopping 50degF.

Now for the next problem; trying to find a long level straight stretch on public roads with little to no traffic. Not easy, but I managed to make a couple of runs.

My list of troubles were:

1/ My chicken quotient. Cold pavement, 24 year old hard, cold tires, unexpected arrival of traffic are real worries. So is speed.

2/ I can’t use full throttle in second gear. In third, maybe, but the speed would be excessive.

3/ Not focusing on what I was doing well enough and short-shifting through habit.


Nevertheless, I managed some runs, although not at full throttle; only about 1/3 throttle in second gear.

My maximum rear wheel horsepower cam up as 252. Hmmm…. A tad low, or so I thought.

For that particular run, I know I only hit 4500, although I can’t remember if I “accidentally” short shifted, or traffic showed up and I let off the gas. Anyway, at 4500 rpm I would have been doing 53.5 mph in second gear and I‘m certain the Hp was still climbing.

At the tires, engine power appears to be used in three main ways:

1/ vehicle acceleration, which the G-Tech measures at the wheels and converts to rear wheel Hp which is the 252;

2/ air resistance, which I can calculate for the 53.5 mph and works out to 18 Hp

3/ Rolling resistance, basically from the deformation of the tires, which I can also calculate for 53.5 mph and works out to 6 Hp. I’m not sure if a portion of that attributed to the rear tires should be used or not, but I will as it wouldn’t be much of an error.

Adding these up yields 276 Hp at the wheels and then allowing 15% for drive train losses, comes out at 325 Hp at the flywheel.

Now as that's only at 4500 and part throttle, it would be interesting to predict the peak power. It's really not possible due to the throttle not being open all the way, although comparing it to a 425 Hp 428CJ using the same intake manifold as mine, an 11% increase could be expected. I got this from a dyno-chart in Jay Brown’s book “The Great Intake Comparo”. The end result is 361 Hp and that‘s still at part throttle.

So what about software programs to predict the peak Horsepower? An expert on the FE engine site has his own software and came up with only 347 Hp. When I originally called Crane Cams about a choice of camshafts I wanted as much performance as I could get while retaining a decent idle. They plugged some numbers in their computer and came up with an incredibly optimistic 480 Hp. On line calculators have yielded anywhere from 361 to 408 Hp with many in the 370 Hp range.

OK, so with my prediction at 361 Hp with only part throttle, a peak ought to at least be 370 to 380.

So why not a lot more? According to the experts on the FE Engine forum, my Offenhauser dual-carb 360 single plane manifold is a dog, even though that Cobra Jet 428 did hit 416 Hp with the same manifold. My cylinder heads don’t flow particularly well either. So if the airflow is restricted mostly by the intake and cylinder heads, going to full throttle might not achieve all that much even with those twin 600cfm Holleys wide open. Apparently my cam choice could be more aggressive too.

To summarize the result, if I am putting out 375 Hp, that works out to 0.9 Hp per cubic inch, right in the middle of David Vizard’s recommended range with lots of room either way in case I’m off a bit. Also, as you can see, if I can’t use all the Hp my engine can put out, then 350 Hp sounds like enough to get that original Cobra experience.

One other thing I wish to emphasize; my car is dangerous, especially in low gears due to the massive torque at the wheels, and of course in higher gears due to the speed. The tires don’t help, BUT although I intend to replace them this year, it won’t be with those atrociously expensive and short lived Avons. I only drive on the street, not the track and it’s a rare occurrence when I push the car really hard due to the fear factor and worry about crashing something I built and worked on for so many years.