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John Dougherty Dougherty Racing Cams 

09282005, 08:13 PM 

drag racing the short bus
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09282005, 08:48 PM 

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Hello all.
I entirely agree that closing up the gears can keep the engine in a narrower rev band: exactly as Walt has described. The drop in BHP below peak is therefore less both over and underpeak HP. But what dd and I are discussing is not closing up the rev band between changes: we are discussing simply changing the R+P. This has no effect on the revs drop between gears....only on the road speed at which those changes occur.. BTW, a lot of confusion arises with respect to terms like BHP, torque and thrust...all that really matters is rear wheel bhp at any particular road speed. Kind regards David 

09282005, 11:14 PM 

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Hello, John.
You asked about actual ratios. So I had a quick look at my charts. To take the specific case of Walt's stockish SC.. The stock box has an 8/31 R+P, ratios probably 18/32, 23/29, 26/26, 28/23. Looking at a chart for stock tyre sizes power curve, we can see that shifting at 6000 rpm we get shifts from second at 65mph, 4th at 91 mph, fifth at 115 mph, and run out at 140.. The HP at 6000 looks to be about 160. Revs on the new gear are 3rd 4500, 4th 4800, 5th 5000 So each shift gives HP of third 157, 4th 164, 5th 167 from the dyno curve. OK, now with the 7/31 R+P. The shift MPH are reduced: 65 to 57, 91 to 80, 115 to 101 140 to 123. But the revs and HP numbers at shift are the same of course.. Now, the really accurate way to calculate the effective power applied when going up through the gears is to integrate the HP curve against time...including of course the shifts when no power is applied, and the time taken for the engine to recover from the upset when the throttle is shut...but that is not really needed here, to answer our question.. "What will be the effect on acceleration of a lower R+P?" Answer, you say (and I agree) the acceleration in each gear will be greater...why?..The HP available at the wheels is the same .. Its because the drag (aero, tyre etc) are lower because the car is moving slower.. Yes, I say, but what I'm really looking for is acceleration at a particular speed... And the answer to that is to be found by plotting the HP against road speed with 7:31 , and putting that curve over the numbers with the 8:31. And obviously we will see that at some speeds the HP available is higher with the 7;31, and at others with the 8:31.. So, for example, at 80 mph the 8:31 gives 5,400 rpm, and about 168 hp, and 7:31 6000 and 160hp. While at 90 mph, the 8:31 gives 5900, 161HP ( 3rd gear) and ,while 7:31 is 5600 (in 4th gear) and About 167HP... Kind regards David 

09292005, 02:15 AM 

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Everyone is talking horsepower, but gearing is related to torque, not HP.
At a low gear you have lots O'torque. the longer it lasts (wide torque band) the longer you can accelerate hard. If it lasts to the high revs, you have lots O'horsepower. But that means absolutely nothing if the available torque is really low at lower revs and it only there from 5000 to 5600 rpm. If you can get the torque to come on early and stay all the way to the higher rpm range, you have a very good, very fast, very useable engine. That is not easy to do. In second gear, you have effectively reduced the torque to the rear wheels. If the second gear is lower, you reduced it less but also reduced the amount of time you will spend accelerating in the higher regions of the torque band. third gear reduces the torque to the rear wheels even further. the width of the torque band, the gearing, and the time it takes to shift, all determine if a lower overall ratio will benefit or hurt the rate of acceleration. 

10032005, 07:51 PM 

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