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Discussion Starter · #1 ·
Without getting into any debate about the real life accuracy of Dyno2000 type programs, I have been doing desktop simulations of different cams against my 351C combo in a relatively lighter 2820lb car .

In comparing cams that maximise horsepower or ones that maximise torque, it seems that in every instance the LOWEST ET invariably occurs with a high torque or torque biased cam. More specifically, it seems to me to be what you should be looking for is the greatest area under the torque curve for your shifting rpm range.

If this is "real world" experience why would we ever have any interest in HP. For a road race circuit, acceleration (out of the corner) is probably just as relevant as the strip, so other than the banked ovals big HP numbers would seem to have little relevance in a motor that has a rpm limit of 7000.

Maybe it would be different for a 800hp 1200lb F1 racer or perhaps where lack of traction made the big tq unusable. Put me straight on this guys
 

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Software simulations can give you a rough idea of what is happening, but there is nothing like "seat of the pants" feel backed up by time and speed in the 1/4 mile. Its been a long time since engine class, but I seem to recall that HP is dependent on RPM. The more rpm you can turn, the higher the HP number will be. That's how those F1 4 cylinder cars can generate 1000 hp out of a 1 ltr motors. They can go up to 10,000 - 14,000 rpm. Generally, those same motors have about 1/2 ft./lb of torque at idle... you are just comming off idle at 2,500 rpm, so its a trade-off. Have a motor that can spin high numbers, gear it right, and build it to hold together, or do a higher displacement that will pull a stump out of the ground from 3 counties away and leave 2 black streaks from 1500 rpm on up. Its all determined by the designers idea and what he wants to accomplish. Like a friend used to say, "Turn them big motors slow!"

Sounds like you are trying to decide what cam to put into your 351. Personally, I'd go with a torque grind for a street car, you'll be a lot happier with it.
 

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If the car is not set up for drag racing (stall converter, low rear end gear) then torque will do you more good than peak HP because you spend so much time just coming off the line and climbing through first gear. If you are set up for drag racing, say with a stall converter and/or rear gear that can keep you up in the RPM range where you're making good HP right off the line, then the most HP under the curve in your racing RPM range will make the fastest car.

HP is the product of torque and RPM... and is the engine's ability to make torque AT THE REAR WHEELS. To make this more clear, compare a mild big block making 350hp @ 4500rpm and 450ft/lb to a small block making 400hp @ 6500rpm and 350ft/lb. Put them both in identical street cars, and the big block will of course waste the small block off the line, and the small block MIGHT catch it at the end of the track, revved out but only in 2nd gear. But then simply change the rear end gear in the small block car so that both cars redline at about the same speed (say the BB car has 3.20 gears, and you swapped to 4.56 in the SB car) your effective torque at the rear wheels in the small block car, from the gear change, increased from 350ft/lb to 500ft/lb... and the small block produces this at a high enough RPM that it isn't bouncing the rev limiter before it crosses the 1/4 mile mark. THAT is why in a street car, where you are restricted in what gear ratios you choose and the RPM range you want to cruise at, torque is more important... but for a race car HP is more important.

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<font size=-1>[ This Message was edited by: Motorhead on 4/9/06 1:52am ]</font>
 

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Discussion Starter · #6 ·
If I have understood your informative posts, given the constraints Torque IS far more important than HP.

For a Street car/Sunday racer (which is what the majority of us really are), we need a realistic diff ratio, our cars are middle to heavy weights and building an engine to regularly rev beyond 7000 rpm is unrealistic both from a cost and durablity standpoint.

I suspect its equally important if you are are going to run a street car on a road race circuit unless it is all very fast corners.

So we should be preoccupied with maximum torque area under the curve rather than maximum HP area.
 

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Horsepower is a direct function of torque - and torque is a function of nothing but the twisting force made by the engine acting on the rear wheels, like Motorhead said.

To define Horsepower more accurately, it is a unit of power, like watt (yes, on your lightbulb) Horsepower is a measure of doing a given amount of work in a certain time. One horsepower is 33,000 ft-lb/min. That is, 33,000 pound feet of torque delivered over the course of one minute.

The Horsepower output is determined by the following:
HP=RPMxTQ/5252

Horsepower equals Revolutions/minute multiplied by ft-lb divided by the constant (and hence no units) 5252. The resulting number gives you the units of horsepower, a ft-lb/min. If you look at any dyno readout, the HP and TQ lines will cross at 5252 - when, by definition, HP=TQ

So in a nutshell - there are only two things you can look at when dealing with power output in an engine - the torque produced, and the speed of the engine when that torque is produced. If you can create 450 ft-lbs or torque at 2000RPM, you have a certain torque acting 2000 times per minute and get a HP of 171. If you can maintain that ouptut of 450 ft-lbs from the engine, but can increase the RPM to 7000 (basically, keeping the same engine efficiency at 7000 that you had at 2000) you will have 600HP acting at the wheels.

So, horsepower is a measure of engine efficiency throughout the engine band - because efficiency of an engine varies with the speed it is turning, HP can give you an accurate measurement of the actual power that is being delivered to the wheels.

If you think about Torque mattering more off of the line than HP, here is why. If you are sitting still, the wheels are not turning at all - if your motor can generate an instantaneous torque of 600 lb-ft, but your car isn't moving, you get no HP. The force isn't acting through a distance. So, the torque that is applied to the wheels creates the initial acceleration.

Once the torque can act through the rotations of the wheels, the motor is effectively given a mechanical advantage, since it can spin faster and deliver the torque to the wheels at an increasing rate. This is the basis of HP.

And if that makes no sense, then I've begun rambling again.
 

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What theengineerofazle said. As a more pratical matter based on experience in 1/8 mile bracket racing - the better your performance in the first 300 or so feet, especially the first 60~100', the better your ET wil be. Torque and the right converter gets you better 60's times and the "more efficient" the car is at the start, the lower the ET (quicker) will be. Examples:

Me - 1.77 60', 7.90 ET @ 86 MPH.

Hot "street" Firebird - 8.14 ET, 92 MPH. 92 MPH should be 7.40ish for a "real" drag car. The 'bird is a slug out of the hole but makes big HP at higher RPM, so the ET is not what it could be but the MPH is way up there. This car likely has 60~100 more HP than I do.

Friend's Monte Carlo - 1.50+ 60', 7.60 ET @ 88MPH. This car leaves the line at 2200, yanks the wheels and ETs quicker than my 351C but doesn't have much more HP - maybe 20~30 more or so. The difference - it's a big block Monte. Has low RPM torque out the wazoo.

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<font size=-1>[ This Message was edited by: ckelly on 4/10/06 11:38pm ]</font>
 

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...or you could be like my mustang. Not a whole lot of torque (just a little 302). 60's in the 1.60 range with 4.86 gears and 28x9 slicks. 7.40s at 99 mph in the 1/8. We run 1000' track normally. Just before the 1/8 mile, its like a turbo car getting up on boost. It just starts to come alive (around 6000 RPM). Weird. But, its neat because a lot of the competetors in the same class run pretty close ET's...but at a LOT less MPH. Mine runs 9.20's/113 mph closest competitor is 9.25/106. I've broken out more than a few top end racers.

My Maverick...totally opposite. Good torque down low (415" windsor on alky), GREAT torque converter and a powerglide. Runs 6.0's (1/
..but only at 112 mph where it really should be up around 115 or so. 1000' times 7.90's 125 mph and it outta be up around 130. Needs more HP for better MPH. I think the heads are pretty well maxed out (box stock Victor Jrs)

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<font size=-1>[ This Message was edited by: mavman on 4/11/06 10:45am ]</font>
 

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Other note - HP is "good" for bracket racing because if you're gaining speed fast on top it makes it really hard for your opponent to judge the stripe.
 

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A good way to look at things is that hp and RPM work hand in hand, meaning the more RPM you can turn efficiently the more hp is made.

Now combine that with gear ratios and torque values and you can start to see the trends. Your torque value used to accelerate the car is not what your seeing at the flywheel, its the that value multiplied by the overall gear ratio (trans gear, rear gear, tire diameter).

For example:

You have Engine A that has 300ft-lbs of torque average from 4000-6000RPM.

You have engine B that averages 275ft-lbs from 4000-7000RPM.

Engine A has an average torque advantage, Engine B has an RPM advantage (and thus hp advantage). I am keying on averages because its averages that determine how fast you are, not the peaks.

Now lets say both of these cars have overall ratios of 12:1 in 1st gear, and 7:1 in second gear.

For simplicity sake lets say they both launch at 4000RPM.

Engine A will be averaging 3600lbs of force to accelerate the car in 1st gear through its RPM range, when it reaches 6000RPM it will be shifted into 2nd gear and will start to average 2100lbs of force.

Engine B however will be averaging 3300lbs of force to accelerate the car in 1st gear, and will be able to hang onto that 1st gear ratio %33 longer.

So, for this time period (both cars from launch up to the point where Engine B will have to shift), average acceleration becomes:

Engine A = (3600lbs * %66) + (2100lbs * %33) = 3069lbs of force average

Engine B is simply 3300lbs average force because it does not need to shift gears

Now this is a hugely simplified situation, but it should show the general idea and if the cars are geared the same way in the other gears this trend will continue. It should also be noted that swapping the gears around to favor the higher RPM's attainable would show even more benefit.

Engine B will be faster simply because it averages more torque output, even though the engine technically has a 25ft-lb torque deficit. It's all about the usable RPM range, average torque values, and overall gear ratio's.
 
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