Engine & Exhaust / Q&A

Ask Away! with Jeff Smith: How Much Horsepower Does My Car Need for 12-Second ETs

Small Block Ford Engine on Dyno

With a mild cam, decent heads, and a good carburetor and headers, 375 to 400 hp should be achievable. That may not sound like much today, but it is still 1.3 hp/cubic inch, which is pretty good. (Image/Jeff Smith)

 

I would like your opinion on my ‘61 Comet 2-door wagon. It’s a car that will be used for local cruise nights and Friday night street-legal drags. My question is how much horsepower would I need to get into the mid 12-second range?

I see all sorts of online estimators but quite frankly, they seem very optimistic. I think the car weighs around 3,000 lbs. The engine is a 331ci small-block Ford with a set of Edelbrock heads and a mild hydraulic roller camshaft with 224/232 degrees at 0.050 with 0.555/0.565-inch valve lift. The intake system is an Edelbrock Performer RPM with a Holley 670 cfm Street Avenger carburetor. The transmission is a C4 with a mild converter. The rear end is a 9-inch with 3.25:1 gears. The rear tires are drag radials at 225/50 R15 that are 24 inches tall. The engine was in the car when I bought it and I expect it will realistically make 375 hp. Thanks for your help.

G.W.

This sounds like a great car especially as a wagon. I think a great place to start would be to estimate the horsepower and then we can attempt to construct a reasonable torque curve. My friend Steve Brule’ at Westech Performance Group has devised a simple horsepower formula for mild street engines with 10:1 compression, decent aftermarket heads, and a good hydraulic roller cam.

Here’s the equation:

Displacement x 1.25 = Peak Torque

Peak Torque x 0.9 = TQ at max horsepower rpm

This formula assumes that peak torque will drop by 10 percent at the peak horsepower rpm point. If you look at a few street engine torque curves, you will see this is a very consistent event.

Multiply this torque times the peak hp rpm—let’s be conservative at 6,000 rpm and then divide that number by 5,252. This will produce estimated peak horsepower.

This uses the horsepower formula:  HP = TQ x RPM / 5,252

So 331ci x 1.25 = 413 lb.-ft. for peak torque

413 x .9 = 372 lb.-ft. at the peak hp rpm point

372 x 6,000 = 2,232,000 / 5252 = 425 hp 

Your estimate of 375 is probably not far off because our calculated numbers might be slightly aggressive. Let’s split the difference and make it 400 horsepower. Peak horsepower is only one number. The QUARTER Pro drag strip simulation program requires an entire torque curve that we estimated with the following curve.

Simulated Torque Curve

Engine RPMTorque
(lb.-ft.)
Horsepower
2,00025095
3,000310177
4,000345263
5,000370352
6,000350400

I loaded these numbers into the QUARTER Pro drag strip simulation program and it spit out a 2.05 60-foot time with a 12.99 at 107 mph. My atmospheric pressure estimate is probably high while my air temperature estimate was 80 degrees. In my experience, my inputs into the QUARTER Pro result in numbers roughly 0.20-second quicker and 2 mph faster than what my car runs so this would put you around 13.20 at 105 mph.

All of this is a SWAG (Scientific -Wild-A**- Guess) but probably not far off. The killer is the 2.05 60-foot. Your C4 has a decent first gear ratio at 2.46:1 but the little motor just doesn’t have enough torque to quickly accelerate from a dead start. Another guess is converter—I estimated the stall speed at 2,200 rpm which is low but not far off for a mild small-block.

The biggest improvement would be a manual transmission like a T-5 to help the car launch harder yet keep the revs in the power band. Or, either a deeper rear gear or a looser converter but neither of these will help the everyday driving so likely it will be difficult to get the car into the 12s without sacrificing something.

Of course, a mild 125-shot of nitrous with a simple plate will put your little wagon solidly in the mid 12’s. The nice thing about nitrous is that small displacement engines really benefit from the torque boost that nitrous offers.

If we assume a 125 hp shot at 6,000 rpm, this is equivalent to a little over 100 lb.-ft. of torque. Hitting the nitrous at 3,000 rpm would boost the torque from 310 lb.-ft. to 410 lb.-ft. which is an instant 33 percent boost in torque! Little motors like your 331 could easily pick up a half-second or more with a 125-hp shot hitting it about the 30-foot mark.

That actually sounds like a lot of fun! But even without the nitrous, a 13-flat run is certainly fun enough.

Of course, you know what’s going to happen. The moment you get that car running in the 12s, you will immediately resolve that it needs to run quicker and soon you’ll be chasing low 12s then high 11s.

That’s what keeps the performance industry in business! It’s funny but also very true. Enjoy your hot rod!

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3 Comments

  1. I was a contestant on PassTime. ( Drag racing game show, “Name the time, Win the cash”) I studied power to weight ratios for weeks and my formula for an ET of 12.50 came in around 10 lbs of car weight to one horsepower. Assuming that your car weighs in at about 3000 lbs, and your combination is estimated at 375 horse, I think you will have enough to crack in to the 12’s. Much of this is speculative though, I am also assuming optimal conditions (proper gearing, tire size, traction, driver experience/ skill level)
    Admitting that these were theoretical equations, I must say, they served me well, as I did happen to win on that show and while the purse was only $2k, it was still a win. Good luck with your Comet, wish I could see pictures, I will bet it looks Bad@$$!

    • Jeff,
      Sorry for the slow response but I was away. If you would send me your e-mail address, I can send you some pictures of the Comet.

  2. Hello Jeff, Can you shine some light on the reduce power problem with the C5 corvette. Not sure if any one else is having this problem.

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