Engine Size vs. Weight - .7 Factor.....

[mbrauning1]

I would like to open a discussion forum for some of you gurus out there. Given there are numerous variables, I would like to hear opinions if you were given the following:

Engine size (in cc's) multiplied by .7 equals weight of car. For example: A 2000 cc engine x .7 = 1400lbs.

Constraints: 1500lbs minimum (1000 cc = 1500) 3500cc maximum

The question would be, if you were building a drag racing car, full out power, not to overanalyze the torques involved, gearing, etc., lets leave all that constant between the two, which would you choose and why?
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[GregLocock]

1) As big as possible. Aerodynamic drag will be a significant part of the resistance above 60 mph, and there will be less of that per cc with a bigger car.

2) As small as possible. Your traction is limited by the size of tyre, and, generally, you cannot afford to increase the size of the contact patch in direct proportion to the vehicle weight.

3)Somewhere in between. One of the laws of spacecraft design is that the optimum is rarely to be found at the extremes of a solution space.




Cheers

Greg Locock

 

[patprimmer]

I would be looking at engines and trying to pick the one with the greatest power density when optimised according to your rules.

I see a large 6 cylinder motor cycle engine, or maybe a Honda S2000 engine in your future.

Regards
pat pprimmer@acay.com.au
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[SMOKEY44211]

The scenario you described would suggest that a heavier car would be optimum. The increase in mass does not mean that the frontal area would have to increase. This would afford the luxury of being able to distribute the additional weight over the drive wheels in a more optimum fashion then would be possible with a lighter version.-------Phil

 

[patprimmer]

Power to weight is much more critical to drag racing than power to frontal area, as any gains made at launch carry through the entire race, where gains at the top end only influence the last few yards, or as drag racers say, a hundredth of your 60' is worth a tenth at the finish.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bobqzzi04]

Could you provide some more information? Body style constraints? (Production or "rail") Tire restrictions?Slicks or street tire? Front drive or rear drive? Any power adders allowed? Nitrous or turbo? Any restrictions on the actual engine build itself? Production Engine using stock parts or unlimited prep? Race engines okay? Money an issue?

I think all these factors couldimpact the choice.

Off the top of my head I'd say buy an F1 engine (a few years old, of course since they are north of 900 HP these days

 

[crysta1c1ear]

I'd choose the light one.
I believe that the tire coefficient of friction drops as weight increases, since increased weight can easily contribute to increased tire abrasion and tire deformation components of the tire's friction, but not to an increased tire adhesion component.

 

[Tmoose]

I'd pick an un-popular racing class with the softest/oldest record, so my (de)tuning and modest driving skills would be less likely to completely cancel out the benefits of modern tech stuff like tires, ignitions, cams....

Also the class where my ample weight was part of the total vehicle weight, and not a pure disadvantage.

Also a full bodied car so there was a place I could hide the NOS bottle.

And I'd never win a race by more than 0.002 seconds, like the Russian weightlifter Vasili Alexeev who broke 79 world records